CHAPTER ONE

1.0 INTRODUCTION

Currently, the concept of biodiesel as a source of

energy has been receiving great attention among the

futurists and the world policy makers (Kusdiana & Saka,

2001; Lu et al., 2007). This is so because of its

renewability, biodegradability, and better quality of

exhaust gas emissions. The idea of vegetable oil based

fuel to run diesel engines has been on the world stage

over a century ago. The discovery of the potential of

vegetable oil to serve as fuel was made by one of the most

famous scientists of the nineteenth century called “Rudolf

Diesel”. Rudolf Diesel, in 1912 stated that ‘‘the use of

vegetable oils for engine fuels may seem insignificant

today. But such oils may become in course of time as

important as petroleum and the coal tar products of the

present time’’(Avinash, 2007). Lack in interest to

develop the technology of biodiesel fuel over the years

was due to availability and low cost of petroleum products

(Antunes et al., 2007). However, the renewed and

increasing interest in the growth and development of

1

biodiesel fuel is driven mainly by its potential to solve

three main challenges confronting the global economy.

These include amongst others: how to attain energy

independence, reduce environmental impact and achieve fuel

of affordable prices that can compete favourably with the

conventional diesel fuel. Biodiesel fuel in large

quantities could be achieved if certain technologies are

developed. Some of these technologies include: (1)

Establishment of a scheme for the production of low cost

feedstocks. (2) Development of sound technology for the

purification of crude biodiesel. (3) Development of

suitable catalysts that can give higher yields of

biodiesel with less separation and purification

difficulties. (4) Establishment of polices that can

enhance the production of biodiesel fuels. (5) Exploration

and exploitation of biodiesel production systems with the

aim of minimizing energy and water utilization. (6)

Development of technologies to covert by-product glycerol

to added value chemicals (Lu et al., 2007).

In addition environmental concerns that have

drastically increased globally over the past decade due to

global warming effects are among the greatest challenges

2

facing the globe, particularly after the Earth Summit ’92

(Avinash, 2007). Thus, the most viable approach to meet

this rising demands, decrease greenhouse gas emissions and

minimize the effects of fossil fuel depletion is by

exploring alternative renewable energy sources (Guo et

al., 2010; Dias et al., 2012). Biofuels, particularly

biodiesel is such a fuel that shows great potential to

replace petro-diesel (Kafuku & Mbarawa, 2010; Leung et

al., 2010). Biofuels are commonly known to offer several

advantages over fossil fuels such as sustainability,

biodegradability, lower greenhouse gas emissions, regional

development, social structure and agriculture development,

and fuel security supply. Further, replacing petro-diesel

with biodiesel fuel could reduce the accumulation of

greenhouse gases such as CO2 in the atmosphere (Bhatti et

al., 2008). Also biodiesel fuel has been commonly found to

offer engine performance comparable to that of petro-

diesel fuel, whilst reducing engine emissions of

particulates, hydrocarbons and carbon monoxide (Haas et

al., 2006).

Biodiesel fuel is usually produced from virgin and

used vegetable oils and animal fats. Presently several

3

efforts are made to produce biodiesel from microalgae.

Microalgae clearly offer some advantages among other

feedstocks. These advantages include: much higher biomass

productivities than land plants (doubling times may be as

short as 3.5 hrs), some species can accumulate up to 20–

50% triacylglycerols, while no high-quality agricultural

land is necessary to grow the biomass, and even no land at

all, offshore microalgae farming could be a reasonable

alternative (Amaro et al., 2011). Biodiesel fuel can be

produced via direct blending with normal diesel,

microemulsion, pyrolysis and transesterification (Naik et

al., 2010; Zabeti et al., 2009). However,

transesterification reaction is the most adopted

technology for the transformation of fats and oils into

biodiesel fuel (Bhatti et al., 2008). Usually the

triglycerides of fats and oils are transesterified using

short-chain alcohols such as methanol and ethanol in the

presence of alkali catalysts. In addition, acid catalysts

can be used for the transesterification reaction (Harding

et al., 2008; Montefrio et al., 2010).

Recently enzymes are applied as biocatalysts for the

production of biodiesel. Both virgin and used oils and

4

animal fats can serve as feedstock sources for the

production of biodiesel fuel. Biodiesel has been

conventionally produced using batch reactors, plug flow

reactors, and continuous stirred tank reactors. However,

the recent introduction of membrane reactor has remarkably

provided excellent conversions of fats and oils to

biodiesel fuel (Cao et al., 2008a; He et al., 2012). The

membrane reactor is very significant in the production of

biodiesel for its ability to completely block the passage

of unreacted triglycerides to final biodiesel product.

This phenomenon has the benefits of providing high-quality

biodiesel fuel (Cao et al., 2008a). Generally, production

of biodiesel with full conformity to biodiesel

international standard specifications (EN 14214 or ASTM

6751- 07) is technically difficult, especially meeting the

requirements of both biodiesel standards of 99.7wt% methyl

esters (Sandra & Dejan, 2009).

At the end of transesterification, biodiesel is

mostly separated via gravitational settling or

centrifugation. The crude biodiesel is then purified and

dried to meet the stringent international standard

specification provided by EN14214 (Vicente et al., 2007).

5

The production of biodiesel using alkaline catalysts can

provide higher biodiesel yield (>98%), but the process of

biodiesel refining is difficult (Balat & Balat, 2008;

Sharma et al., 2008). This is due to soap formation

associated with alkaline catalysts. Further, the formation

of soap decreases biodiesel yield obtained after the

clarification and separation stages. As well, the

dissolved soap increase the biodiesel solubility in

glycerol, an additional cause of yield loss (Leung, et

al., 2010). However to mitigate the problems faced with

the use of homogeneous alkaline and acid catalysts,

heterogeneous catalysts such as solid and enzymes

catalysts have been developed and used during biodiesel

production process (Chew & Bhatia, 2008).

1.1 Research Problem Statement

Purification of crude biodiesel is necessary so as to

make biodiesel suitable for diesel engines consumption and

other applications. However it was clearly evident from

the literature reviewed that the problems with biodiesel

purification process is associated with the nature of

catalysts, alcohol to oil molar ratio, water and free

6

fatty acid contents. Several attempts have been made in

producing biodiesel using heterogeneous catalyst to

overcome the problems associated with homogeneous catalyst

such as difficulties in biodiesel purification, but so

many issues are yet to be addressed such as reaction rate,

conversion, catalyst leaching, catalyst regeneration, and

reuse. The purification of crude biodiesel is usually

achieved via two notable techniques; wet and dry washings.

Conventionally wet washing is the most employed technique

to remove impurities such as soap, catalyst, glycerol and

residual alcohol etc. from biodiesel. Conventional

purification of these impurities poses challenges. The

major disadvantage in the use of water to purify biodiesel

is increase in cost and production time (Berrios &

Skelton, 2008). Besides, separation of biodiesel phase

from water phase is difficult and produces large amount of

wastewater. For each litre of biodiesel, close to 10

litres of wastewater is produced (Saleh et al., 2010). In

addition biodiesel purification alone accounts for 60 to

80% of the process cost (Tai-Shung, 2007). As a result,

dry washing technique (ion exchange resins and magnesol

powder) was introduced to substitute water washing to

7

remove biodiesel contaminants. This technique is also

employed in commercial plants to purify biodiesel (Cooke

et al., 2003). However, the understanding of the chemistry

of dry washing substances is still skeletal (Mike, 2011).

Furthermore based on the published literature

reviewed, it was found that biodiesel separation and

purification through conventional processes have presented

high-quality biodiesel that can be used on diesel engines

without necessary engine modification. However, issues

such as longer settling time, cost of equipment, high

water and energy consumptions, and lack of regeneration

and reuse of adsorbents have been the main problems

confronting the application of these techniques. These

problems have resulted to the exploration and exploitation

of membrane technology for the purification of biodiesel.

Biodiesel purification via membranes have so far provided

promising results, in addition to less water utilization

(Low & Cheong, 2009.). Membrane biodiesel purification

processes are very good in providing high-quality

biodiesel fuel (Baroutian et al., 2011). Besides membrane

processes are more economical compared to classical

processes. Further, a large portion of membrane separation

8

processes are carried out under reasonable temperature and

pressure conditions, and also the implementation of their

scale-up are less cumbersome. Additionally, membranes are

generally most preferred in the refining processes for the

following reasons: low energy consumption, safety, simple

operation, elimination of wastewater treatment, easy

change of scale, higher mechanical, thermal and chemical

stability, and resistance to corrosion. Use of ceramic

membranes in biodiesel production, separation and

purification is fast growing due their stability in

organic solvents. Also, apart from the ability to

withstand the reaction conditions, chemical and thermal

stability also allows the ceramic carbon membrane to be

cleaned more proficiently when unrefined feedstocks such

as used/waste cooking oils are employed (Cao et al.,

2008b; Ochoa et al., 2001; Sarmento et al., 2004).

In all the previous researches conducted, the lowest

membrane pore size studied was 0.05µm. In addition most of

the studies focused only on single impurity retention. To

the best of our knowledge there is no research so far

conducted on the purification of crude biodiesel using

membrane with pore size of 0.02µm. In addition no

9

optimization study was carried on both membranes (0.02µm

and 0.05µm). Also no research conducted on the comparison

between the efficiencies of water washing, dry washing and

membrane separation for the purification of crude

biodiesel.

1.2 Research Approach

Presently separation and purification of biodiesel

using membrane technology is new. Nonetheless, current

investigations have shown the effectiveness of membrane

technology for the production of biodiesel (Cao et al.,

2008a; Dubé et al., 2007). Membrane separation technology

has been effectively employed in numerous applications,

such as gas separations, protein separations and water

purification. This technology has not being investigated

to a great extent in the separation and purification of

biodiesel. Membrane technology presents a great propensity

toward providing solutions to environmental problems by

recovering valuable products as well as treating effluents

and minimizing their harm to the atmosphere. The most

broadly employed membrane processes are membrane micro-

filtration (MF) and ultrafiltration (UF) pressure-driven

10

processes capable of separating particles in the

approximate size range of 0.1–10 µm and 1–100 nm,

respectively (Montefrio et al., 2010). Typically, UF will

remove high molecular weight substances, colloidal

materials, and organic and inorganic molecules (Saleh et

al., 2011). Membrane separation system is defined by the

nature of its material and type. Further, optimum membrane

performance is influenced by choice of adequate membrane

material. The membrane separation processes are made by

different kinds of membranes which could either be

polymeric or ceramic. The polymeric membranes include

amongst others: polyvinylidene fluoride (PVDF),

polysulfone, polyamide, and polycarbonate. Some of these

polymers can appreciably influence the efficiency,

maintenance characteristics and membrane performance. In

this study, ceramic membrane was selected because of its

numerous advantages over polymeric membrane. These

advantages include; structural stiffness and enhanced

mechanical strength, chemical and thermal resistance,

corrosion resistance, stability of operating

characteristics, resistance to bacterial attack and

chances of numerous regenerations. These defined

11

characteristics make separation of solutions under adverse

conditions such as corrosive media, higher pressures (1–10

MPa), higher temperatures and wide range of pH values (0–

14) to be possible. Additionally, ceramic membranes are

made from materials such as oxides of zirconium, titanium

and aluminum (Komolikov & Blaginina, 2002).

In this study, ceramic membranes with pore sizes of

0.02µm and 0.05µm were used to purify crude biodiesel

without water wash steps. As well as studying the effects

of process operating parameters such as transmembrane

pressure, flow rate and temperature on the performance of

the membranes. Further, rigorous optimization using

central composite design (CCD) coupled with Response

Surface Methodology (RSM) was separately performed on the

membranes for the simultaneous retention of glycerol, soap

and catalyst. In addition comparison between the membrane

process, water washing (acidified water), and dry washing

(magnesol) for the removal of free glycerol, soap, and

catalyst (potassium) from crude biodiesel toward biodiesel

quality were also carried out.

1.3 Aims and Objectives of the Research Study

12

The main aims and objectives of this research work

are:

i. To set up a laboratory scale membrane system for

the purification of crude biodiesel.

ii. To study the influence of process operating

parameters on the membrane performance during

separation and purification of crude biodiesel.

iii. To optimize the membrane process parameters in

terms of biodiesel purity using the Design of

Experiments approach.

iv. To compare the performance of the developed

membrane system in terms of the quality of

biodiesel product to the conventional biodiesel

purification processes such as biodiesel wet

washing and dry washing.

1.4 Thesis Outline

The thesis consists of the following chapters:

1.4.1 Chapter one: This chapter describes the

introductory aspect of the work comprising of the

research problem statement, research approach and the

research objectives.

13

1.4.2 Chapter two: This chapter discusses the reviewed

literature comprising of biodiesel production

processes, purification processes and the

friendliness of biodiesel to the environment.

1.4.3 Chapter three: This chapter provides the

methodology adopted. It comprises biodiesel

production process, membrane purification process and

the techniques used to analyse the initial and the

final concentrations of the contaminants in

biodiesel.

1.4.4 Chapter four: This chapter describes the results

obtained from biodiesel water washing, dry washing

through magnesol and purification through membrane.

The effects of process parameters on membrane

separation for the purification of biodiesel are

discussed. Furthermore the optimization of the

process parameters is also reported.

1.4.5 Chapter five: This chapter discusses the

conclusions derived from the research work conducted

and the recommendations for the future work.

1.4.6 Appendix: This part presents the journal

articles published, articles under review process and

14

the papers presented at national/international

conferences.

15

CHAPTER TWO

2.0 LITERATURE REVIEW

2.1 Biodiesel as Diesel Engine Fuel

Biodiesel is an important alternative vehicular fuel.

It has excellent properties as diesel engine fuel, so it

can be used in compression-ignition diesel engines (Kent

Hoekman et al., 2012). Biodiesel can be derived from

several different vegetable oils or animal fats feedstock.

Vegetable oil and animal fat direct use as fuel in diesel

engines is limited due to two main factors; low volatility

and high viscosity. Traditional processing involves an

alkali-catalyzed process, but this process is difficult

for lower cost high free fatty acid feedstock due to soap

formation. Pre-treatment of the feedstock with high free

fatty acid using strong homogeneous acid catalysts such as

sulfuric acid have been shown to provide reasonable

conversion rate, higher yields and high-quality biodiesel

final products. These technologies have played a vital16

role in ensuring the production of biodiesel from

feedstock like soap-stock that are normally regarded as

waste. Biodiesel is now mainly being produced from

rapeseed, cottonseed, soybean, canola and palm oils

(Demirbas, 2008; Demirbas and Demirbas, 2011). Furthermore

Demirbas (2009) stated that the higher heating values

(HHVs) of biodiesels are relatively high. The values of

HHVs of biodiesels ranged from 39–41 MJ/kg and are

slightly lower when compared to those of gasoline (46

MJ/kg), petro-diesel (43 MJ/kg), or petroleum (42 MJ/kg),

but greater than coal (32–37 MJ/kg). Further almost all

types of vegetable oils can be used to replace the diesel

oil; however the rapeseed oil and palm oil can be the most

suitable vegetable oils which can be used as diesel fuel,

additive or diesel fuel extender. Biodiesel termed as

clean fuel does not contain carcinogenic substances and

its sulfur content is also lower than its content in

petro-diesel. The ability of biodiesel to be highly

biodegradable and its superb lubricating property when

used in compression-ignition diesel engines makes it to be

an excellent fuel. Also its renewability and similarities

in physicochemical properties to petro-diesel, revealed

17

its potential and practical usability as fuel for the

replacement of petro-diesel in the nearest future. More

so, few other physical and chemical properties of

biodiesel fuel are of great concern and require to be

enhanced to make it fit for use in clean form (i.e. 100%

biodiesel). These properties include among others; engine

power, increase in calorific value, reduced emission of

nitrogen oxides (NOx), and low temperature properties

improvement. Furthermore methyl esters can improve the

lubrication properties of the diesel fuel blend. Biodiesel

decrease long term engine wear in compression-ignition

diesel engines. Biodiesel is a good lubricant and is about

66% better than petro-diesel. Its oxidation stability

improvement is also important to prevent it from

deteriorating when stored over time. Currently biodiesel

is compatible in blended form with petro-diesel in the

ratio 20 (biodiesel): 80 (mineral diesel). Biodiesel has

also being in use in many countries such as United States

of America, Malaysia, Indonesia, Brazil, Germany, France,

Italy and other European nations (Demirbas, 2009).

Biodiesel as an alternative to diesel fuel could only

be successfully used in compression-ignition diesel18

engines, if its physical and chemical properties conform

to the international standard specifications of biodiesel.

These standards (ASTM 6571-3, EN 14214) describe the

minimum requirement that must be met before biodiesel is

used as a pure fuel or blended with petroleum-based diesel

fuel. Biodiesel fuels are characterized by their cetane

number, density, viscosity, cloud and pour points, flash

point, copper corrosion, ash content, distillation range,

sulfur content, carbon residue, acid value, free glycerine

content, total glycerine content and higher heating value

(HHV) etc. The viscosity values of vegetable oils

decreases sharply after transesterification reaction.

Further the flash point values of fatty acid methyl esters

(FAME) are significantly lower than those of vegetable

oils. Also high regression between the density and

viscosity values of vegetable oil methyl esters and a

considerable regular relationship between viscosity and

flash point of vegetable oil methyl esters has been

established (Balat & Balat, 2008; Demirbas, 2009).

Transesterification reaction converts triglycerides

into methyl or ethyl esters and reduces the molecular

weight to one third that of the triglyceride and decreases19

the viscosity of vegetable oils by a factor of about

eight. The viscosities of biodiesel fuel from animal fats

such as lard and tallow show the same trends as

temperatures, higher than the soybean and rapeseed

biodiesel fuels. Virgin and waste vegetable oils can be

used as fuel for compression-ignition diesel engines, but

their viscosity is much higher than that of common diesel

fuels and this requires major diesel engines

modifications. Also the burning of vegetable oils in

diesel engines is not clean resulting to the formation of

unwanted materials such as acrolein and organic acid.

These materials lead to significant negative effects on

the performance and longitudinal engine durability.

However vegetable oils can be converted into their fatty

acid methyl esters by transesterification reaction and can

be convertibly used as fuels for diesel engine

applications without major modifications (Atapour &

Kariminia, 2011; Balat & Balat, 2008; Canakci & Gerpen,

1999; Alexandre et al., 2012).

2.2 Biodiesel Economic and Environmental Analysis

2.2.1 Biodiesel Economic Analysis

20

Biodiesel can reduce country’s over-reliance on the

importation of crude oil imports. It also supports

agricultural activities by providing a new labour and

market opportunities for domestic crops. And it is broadly

accepted by vehicle manufacturers (Demirbas, 2003). The

statistic of world biodiesel production revealed, 42% of

the global total in 2006 was accounted for by Germany,

where market conditions were favourable. Other countries

with major biodiesel markets in 2006 included United

States, France, Italy, and the United Kingdom (Balat &

Balat, 2008). Currently, soybean oil, alkaline catalyst

and short-chain alcohol, methanol are used to produce

biodiesel, however the huge quantities of less cost fats

and oils such as animal fats and restaurant wastes that

could be converted to biodiesel have lessen the problems

faced in using soybean oil as feedstock for biodiesel

production. Furthermore the costs of biodiesel from

oilseed or animal fats have been projected by economic

feasibility. The costs ranged from US$0.30–0.69/l,

including meal and glycerine credits and the assumption of

reduced capital investment costs by having the crushing

and/or esterification facility added onto an existing

21

grain or tallow facility. Biodiesel cost from waste grease

and vegetable oil are roughly projected to be US$0.34–

0.42/l and US$0.54–0.62/l respectively (Wen et al., 2009).

Presently, biodiesel is not economically realistic,

therefore more development in research and technology will

be required because of pre-tax diesel priced at US$0.20–

0.24/l in some European countries and US$0.18/l in the

United States (Demirbas, 2009). More so, if less expensive

feedstocks are used to produce biodiesel, a 25 per cent

reduction in cost production is possible (all other

production costs kept equal). Also, approximate

projections of the cost of biodiesel fuel produced from

vegetable oil and waste grease are US$0.54±0.62/l and

US$0.34±0.42/l respectively. Further, biodiesel from palm

oil costs around $0.66/L or 35% higher than petroleum

diesel. This suggests converting palm oil to biodiesel

fuel adds about $0.14/L to the price of the oil. As well

for biodiesel from palm oil to be competitive with petro-

diesel, the price of palm oil should not go beyond

$0.48/L, assuming no tax on biodiesel. By means of similar

equivalence, a considerable target price for microalgal

oil is $0.48/L, for algal-derived diesel fuel to be cost

22

competitive with petroleum diesel ( Canakci, 2007; Balat &

Balat, 2008; Demirbas, 2009; Demirbas & Demirbas, 2011).

Furthermore Haas et al. (2006) carried out economic

analysis to produce biodiesel by using low value

triglycerides as feedstocks. It has been suggested that

vegetable oil soap-stock used in producing biodiesel would

nearly cost US$ 0.41/l, a 25% reduction relative to the

estimated cost of biodiesel produced from soy oil (Fan and

Burton, 2009). Biodiesel production cost was reported to

vary inversely and linearly with variations in the market

value of glycerol, increasing by US$0.0022/l ($0.0085/gal)

for every US$0.022/kg ($0.01/lb) decrease in glycerol

value. The model is flexible in that it can be modified to

calculate the effects on capital and production costs of

changes in feedstock cost. So also the changes in process

chemistry and technology, the changes in the type of

feedstock employed, and changes in the value of the

glycerol co-product (Haas et al., 2006). Besides the cost

of feedstocks which contributes to high cost of biodiesel

compared to petro-diesel, the major limiting factor to

biomass use is the technology development for the

separation, purification, and its transformation into

23

biochemicals and biofuels (Tai-Shung, 2007). Consequently,

purification of biodiesel via membrane is envisaged to

benefit from low cost membrane process.

2.2.2 Biodiesel Environmental Consideration

Presently great attention is given to production of

biodiesel due to its environmental friendliness, providing

low emissions compared to non-renewable petro-diesel

fuels. Over a century ago, vegetable oil was tested by

Rudolf Diesel on his combustion engine. Due low prices of

fossil fuels interest in research on vegetable oils was

low at that time. The current fossil fuel price hike and

increase in environmental concerns due to greenhouse gas

emissions (e.g., carbon dioxide) have reignited scientists

and Engineers to concentrate on the development of

vegetable oils and their derivatives as substitute fuel

sources. The use of vegetable oil and its derivative has

the ability to reduce the concentration of pollutants and

carcinogens in our environment (Fukuda et al., 2001; Ma &

Hanna, 1999). Further, in 28 days, biodiesel can be

biodegraded by 95% compared to petro-diesel that can only

be degraded by only 40%. As well, compared to petro-

24

diesel, biodiesel combustion emissions, such as unburned

hydrocarbons, carbon monoxide, particulates, SOx emissions

and soot are much lower. In addition a slight increase in

NOx emissions can be positively influenced by delaying the

injection timing in engines. Similarly, a substantial

reduction in the total particulate matter (32%), SOx (8%),

and CO (35%), relative to petro-diesel was reported by

Demirbas (2009).

2.3 Biodiesel Production

Alkali-catalyzed transesterification is the most

adopted technique for producing biodiesel, this method

usually requires refined feedstocks containing less FFAs

content otherwise it will result to much soap formation.

Figure 2.1 presents transesterification reaction of

triglycerides to fatty acid alkyl esters (FAAE,

biodiesel). Whereas Figures 2.2 and 2.3 depict the

structures of triglycerides used as the main feedstock for

the production of biodiesel fuel (Bhatti et al., 2008).

25

For alkali-catalyzed transesterification, if the

feedstocks contains high amount of FFAs then it has to

undergo pre-treatment steps before transesterification is

carried out (Leung et al., 2010). Ramadhas et al. (2005)

recommended the acid value of feedstocks to be less than

4.0mg KOH/g before performing alkaline

26

Figure 2.1: Transeterification of vegetable oil with

methanol

transesterification. Although Canakci & Gerpen (1999)

stated that before alkali-catalyzed transesterification is

performed, the acid value of a feedstock has to be 2.0 mg

KOH/g. Nonetheless, the use of heterogeneous catalysts in

biodiesel production has reduced the effects of using low

quality feedstocks, especially enzymes catalysts that has

the potential of converting FFAs into biodiesel, besides

high purity by-products.

2.3.1 Feedstocks for Biodiesel Production.

Biodiesel production is achieved via different kinds

of feedstocks. The nature of feedstock used is dependent

on the geographical position and climate of the place. For

instance Europe employs sunflower and rapeseed oils, palm

oil predominates in tropical countries, soybean oil in

United States and canola oil in Canada (Cao et al.,

2008a). The major feedstocks employed in producing

biodiesel are cotton seed, palm oil, sunflower, soybean,

canola, rapeseed, and Jatropha curcas (Singh & Singh,

2010). Additionally, Zhang et al. (2003) remarked that

employing feedstocks such as waste frying oils, non-edible

oils, and animal fats, as feedstocks could be useful in

27

producing biodiesel. Even though, Banerjee & Chakraborty

(2009) stated that FFAs contents in the waste cooking oil

should be kept within certain limit (<0.5<3) for reaction

involving both acid- and alkali-catalyzed

transesterification reactions. Otherwise these substances

may cause severe difficulties in the refining of biodiesel

products. The cost of feedstocks decreases as FFAs

content increases. In case of industrial biodiesel

production, there is need for low-cost (high FFAs)

feedstocks such as used cooking oils, waste cooking oils,

and non-edible vegetable oils to be used, since biodiesel

fuels from refined oils are costly when compared to petro-

diesel fuel. Besides, application of such feedstocks in

biodiesel production could minimize competition between

demand of edible vegetable oils and cost of biofuel

(Zabeti et al., 2009). As well, application of vegetable

oils as sources of biodiesel requires great efforts to

either develop more productive plant species with a high

yield of oil or to increase oilseeds’ production.

Furthermore, many studies have reported microalgal oil as

feedstock for producing biodiesel. Demirbas (2010) noted

that microalgal oil is the only feedstock that can meet

28

the global demand for transport fuels. The author also

reported that soon microalgal oil will become the most

important feedstock for biofuel production. In addition

microalgae feedstocks are receiving great attention as

sources of energy because of their quick growth potential

coupled with reasonably high lipid, carbohydrate and

nutrients contents. In another study, Demirbas & Demirbas

(2011) highlighted that microalgae possess much quicker

growth-rates than terrestrial crops. The author noted the

per unit area yield of oil from algae is estimated to be

from 20,000 to 80,000 l per acre, per year. In deed this

is 7–31 times more than the next best crop, palm oil. Also

to highlight the significance of microalgae as feedstock

for biodiesel production, Tran et al. (2010) have reviewed

biodiesel production via microalgal oil. The authors noted

that catalysts such as acid, base and zeolites catalysts

can be used in catalyzing transesterification involving

microalgal oil as feedstock. Table 2.1 presents biodiesel

production through different feedstocks.

29

Table 2.1: Biodiesel production through transesterification reaction

Feedstock Catalyst Reaction time

(hr) Reaction temp (oC) Yield ((w/w%) Molar ratios

-Waste tallow H2SO4 2450 99.01 ± 0.71 1:30 (chicken)-Palm fatty acid H2SO4 270 99.6 7.2:1 -Waste cooking oil ZS/Si -200 98 1:18 -Soybean oil CaO 1- 93 - -Sunflower zeolite X -60 95.1 - -Refined soybean oil Sn(3-hydroxy-2-methyl-4-pyrone)2(H2O)2 360 93 - -Waste bleaching earths Rhizopus oryzae 9635 55 1:4 - Sunflower oil KOH - 70 96 - - Jojoba oil-wax Sodium methoxide 4 60 55 7.5:1 -Brassica carinata KOH - - 98.27 - - Canola oil KOH 0.33 25 86.1 6:1 -Cooking oils - 0.167 250–300 95 - - Soybean Na/NaOH/ɣ-Al2O3 2 60 96 9:1

30

-Crude palm kernel oil SO42−/ZrO2 1 200 90.3 6:1 - Soybean oil lipase 6.3hr 36.5 92.2 3.4:1 - Jatropha curcas KOH 1 30 92 - -Cottonseed oils Sodium hydroxide 1 55 >90 - - Roselle oil potassium hydroxide 1 60 99.4 8:1 - rubber seed oil NaOH 1 60 84.46 6:1 (Hevea brasiliensis) -Mahua oil NaOH 2 60 - 6:1 (Madhuca Indica) -Mahua oil H2SO4 /KOH 1 60 98 6:1 - Soybean oil ETS-10 zeolite 24 125 90 - -Sunflower frying oil KOH 0.5 25 - 6:1 -Tobacco seed oil NaOH 1.5 55 - 3:1 -Rice bran oil Sulfuric acid <8100 98 - - Palm oil CaO/Al2O3 565 94 12:1 -used frying oil KOH 235–78 74.2 6:1–12:1 -Waste cooking oil H2SO4/NaOH 350 90.6 3:7

31

2.3.2 Biodiesel Production via Homogeneous Catalysts

2.3.2(a) Biodiesel Production via Homogeneous Alkaline

Catalysts

Application of alkali-catalyzed transesterification

reaction provides faster rate, nearly 4000 times faster

than that catalyzed by the same amount of an acid catalyst

(Fukuda et al., 2001). Some of the alkaline catalysts used

for the transesterification reaction include among others;

NaOH, KOH, and sodium methoxide. Other alkaline catalysts

include; sodium ethoxide, potassium methoxide, sodium

propoxide, sodium butoxide and carbonates. Based on

biodiesel yield, CH3ONa or CH3OK are better and more

suitable catalysts than NaOH and KOH. Thus, CH3ONa and

CH3OK are more suitable due to their ability to dissociate

into CH3O- and Na+ and CH3O-and K+ respectively. Besides, the

catalysts do not form water during transesterification

reaction (Sharma et al., 2008). Transesterification of

refined oils with less than 0.5 wt% FFAs via chemical

catalysts could lead to high-quality biodiesel fuel with

better yield within short time of 30–60 min (Wang et al.,

2007). Furthermore, Vicente et al. (2004) have compared

different basic catalysts (sodium hydroxide, potassium32

hydroxide, sodium methoxide and potassium methoxide,) to

produce biodiesel fuel using sunflower oil. The reactions

were conducted at a temperature of 65oC, methanol to oil

molar ratio of 6:1 and basic catalyst by weight of

vegetable oil of 1%. They achieved 85.9 and 91.67 wt%

yield of esters for NaOH and KOH and 99.33 and 98.4 6 wt%

yields of esters for CH3ONa and CH3OK respectively. The

authors recorded 98 wt% yields of esters for methoxides

after separation and purification steps were completed.

Further, less yield losses and negligible ester

dissolution in glycerol were observed with methoxides

compared to hydroxides. Rashid et al. (2008) used alkali

catalyst to produce sunflower oil methyl esters. They

reported notable yield of 97.1wt% at 60oC. In addition,

alkaline catalysts concentrations ranging from 0.5–1wt%

could yield 94–99wt% conversion of vegetable oils to alkyl

esters. However, increase in catalyst concentration above

1wt% does not increase the conversion but could add to

extra costs of production. Since, it is essential to

remove the catalyst from the products after the reaction

is completed (Avinash, 2007).

33

2.3.2(b) Biodiesel Production via Homogeneous Acid

Catalysts

The most notable acids commonly employed in

transesterification reaction include among others;

sulfuric acid, sulfonic acid, hydrochloric acid, organic

sulfonic acid, and ferric sulphate (Leung et al., 2010).

Among these acids, hydrochloric acid, sulfonic acid and

sulfuric acid are usually favoured as catalysts for the

production of biodiesel. Brønsted acids preferably

sulfuric acid or sulfonic acid is used to catalyze the

reaction. Although the catalysts give high yield of

biodiesel, but the reaction rates are slow. The alcohol to

oil molar ratio is the main factor influencing the

reaction. Therefore addition of excess alcohol speeds up

the reaction and favours the formation of biodiesel

products. The steps involved during acid-catalyzed

transesterification are: (1) initial protonation of the

acid to give an oxonium ion (2) The oxonium ion and an

alcohol undergo an exchange reaction to give the

intermediate (3) and this in turn can lose a proton to

become an ester. Reversibility of each of the above step

is possible but the equilibrium point of the reaction is

34

displaced in the presence of excess large alcohol, by

allowing esterification to advance to completion

(Demirbas, 2009). Leung et al. (2010) reported that

esterification by acid-catalysis makes the best use of the

FFAs in the animal fats and vegetable oils and converts

them into fatty acid alkyl esters. The authors noted that

one-step esterification pre-treatment may not reduce the

FFAs efficiently, if the acid value of the oils or fats is

very high. This is because of the high content of water

produced during the reaction. In this case, addition of

mixture of alcohol and sulphuric acid into the oils or

fats three times (three-step pre-esterification) is

required. The time needed for this process is about 2hrs

and removal of water is necessary by a separation funnel

before adding the mixture into the oils or fats for

esterification again. Additionally, Soriano Jr et al.

(2009) studied transesterification of canola oil to

produce biodiesel via homogeneous Lewis acid (AlCl3 and

ZnCl2) as catalyst. The reaction occurred in a round bottom

flask submerged in an oil bath equipped with a reflux

condenser, temperature controller and a magnetic stirrer.

The authors reported use of variable parameters such as:

35

reaction time (6,18,24hr), methanol to oil molar ratio

(6:1, 12:1, 24:1, 42:1 and 60:1), reaction temperature

(75,110oC). And tetrahydrofuran (THF) as co-solvent (1:1

methanol to THF by weight in runs with THF), and catalyst

(AlCl3 or ZnCl2). In all the runs, the catalyst amount was

kept at 5% based on the weight of oil. 1H NMR monitored

converting canola oil into fatty acid methyl esters. Thus

the best conditions with AlCl3 were 24:1 molar ratio at

110oC and 18hr reaction time with THF as co-solvent

provided a conversion of 98%. AlCl3 was far more active

compared to ZnCl2 due to its higher acidity.

2.3.3 Biodiesel Production via Heterogeneous Catalysts

Despite the problems surrounding alkali-catalyzed

transesterification, the process is still favourable in

producing biodiesel fuel. The main reason is due to their

faster kinetic rate and economic viability (Wang & Yang,

2007). Several researches were conducted on heterogeneous

catalysts with the aim of finding solutions to the

problems caused by using homogeneous catalysts in

producing biodiesel (Chew & Bhatia, 2008). As a result a

good number of heterogeneous catalysts were explored and

many of the catalysts have displayed very good catalytic36

performances. Some of these catalysts include among

others; oxides, hydrotalcides, and zeolites. Currently,

majority of heterogeneous catalysts used in producing

biodiesel are either oxides of alkali or oxides of

alkaline earth metals supported over large surface area

(Zabeti et al., 2009). In addition, biodiesel is

commercially produced using heterogeneous catalyst through

the Esterfip-H process. This biodiesel process is

commercialized by Axens. The process requires neither

catalyst recovery nor aqueous treatment steps, which are

major bottlenecks from the current homogeneous catalytic

processes. Additionally the Esterfip-H process displays

high biodiesel yields and directly produces salt-free

glycerol at purities exceeding 98% compared to 80%

glycerol purity from homogeneous catalyzed process (Semwal

et al., 2011).

Moreover, Semwal et al. (2011) reported that many

studies on solid acidic catalysts for producing biodiesel

were carried out, however lower reaction rates and

unfavorable side reactions have limited their use. The

authors noted that a good number of investigations on

basic heterogeneous catalysts were also conducted but

37

their activity gets degraded in the presence of water.

They stated that acid–base catalysts are among the most

promising catalysts to employ in biodiesel production;

this is because they can catalyze both esterification and

transesterification simultaneously. Lee & Saka (2010)

reported simultaneous esterification and

transesterification of waste cooking oil using solid

catalyst ZnO– La2O3, which combines acid (ZnO) and base

sites (La2O3). Although the process provided high

conversion of 96% in 3hrs, but like zirconia, lanthanum is

a rare and expensive metal, therefore cost of the catalyst

would prohibit it high use in the production of biodiesel.

Furthermore gelular resin catalysts having covalently

bound sulfonic groups are developed and employed to

simultaneously esterify and transesterify variety of

feedstocks including beef tallow and soybean oil of high

acid number. The authors noted that the key factors

influencing the feasibility and performance of solid

catalysts are durability, catalytic activity and cost of

production. Therefore the main challenges to R&D in using

solid catalyst in the production of biodiesel are

exploring the impacts of those chemical properties of

38

supports on the catalytic activity, and designing

specifically tailored deactivation–prevention and/or

renewal techniques for each catalyst and developing less

expensive supports.

2.3.3(a) The Effects of Solid Alkaline Catalysts on

Biodiesel Production

In recent times there is a great development in the

preparation of solid alkaline catalysts for producing

biodiesel. Solid alkaline catalysts such as CaO provides

many advantages for instance higher activity, long

catalyst life time, and could run under moderate reaction

conditions (Kouzu et al., 2008). Antunes et al. (2007)

noted that like homogeneous catalysts, solid alkaline

catalysts present more catalytic activity than solid acid

catalysts.

Kouzu et al. (2008) produced biodiesel via solid

base-catalyzed transesterification at a reaction time of

1hr and obtained esters yield of 93wt% for CaO, 12wt% for

Ca(OH)2, and 0% for CaCO3. The authors stated that CaO will

39

provide good productivity as NaOH as well as easy recovery

of products and environmental benignity. They used CaO to

transesterified waste cooking oil with an acid value of

5.1 mg-KOH/g. The yield of esters was above 99% at a

reaction time of 2hrs. But a portion of the catalyst

transformed into calcium soap. Thus, solving this problem

will entail removal of FFAs before transesterification

reaction. The authors also noted that due to

neutralization reaction of the catalyst, concentration of

calcium in the produced biodiesel increased from 187 ppm

to 3065 ppm. Conversely, Lim et al. (2009) noted that

transesterification reaction involving CaO requires longer

reaction time. But the benefits gained from the process

such as elimination of neutralization process, less waste

generation and prospect of catalyst reusability

compensates the delay. The authors also achieved biodiesel

purity of 98.6 ± 0.8% within 2.5hrs. Besides for CaO

catalyst, the process generated 90.4% biodiesel yield

compared to biodiesel yield of 45.5% for NaOH and 61.0%

for KOH catalysts. Furthermore, it was observed that

compared to biodiesel yield of 80% under anhydrous

conditions using CaO, addition of 2.03 wt.% water into the

40

reaction medium of 8 wt.% catalysts, 12:1 alcohol/oil

molar ratio and at 3hr of reaction time, could provide

biodiesel yield above 95%. Besides the catalyst active

sites were found not to leach and the activity of the

catalyst was stable after 20 cycles of the reaction.

Moreover, Zabeti et al. (2010) optimized biodiesel

yield produced using transesterification of palm oil via

CaO/Al2O3. They used a 150 ml glass jacketed reactor

equipped with a water-cooled condenser and a magnetic

stirrer (1000 rpm) to perform the experiments for 5hrs.

The estimated optimal reaction conditions achieved were;

reaction temperature of 65°C, catalyst content of 6 wt%

and alcohol/oil molar ratio of 12:1, and achieved a

corresponding yield of 98.64wt%. The authors reported that

the values achieved from ICP-MS showed little leaching of

CaO/Al2O3 active species into the reaction medium and the

catalyst was successfully reused for two successive

cycles. To further examine the potential of solid alkaline

catalysts, different heterogeneous catalysts such as

K2CO3/γ-Al2O3,Na2CO3/γ-Al2O3,LiOH/γ-Al2O3,NaOH/γ-Al2O3, γ−Al2O3,

and KOH/γ-Al2O3 were used providing biodiesel yield of

89.40%.

41

Furthermore, hydrotalcites materials are receiving

great attention because they can be applied as precursors

and as catalyst. Hydrotalcite-like compounds (HTlcs) are a

category of anionic and basic clays referred as layered

double hydroxides (LDH) with the formula

Mg6Al2(OH)16CO3.4H2O. Thus production of biodiesel was

experimented using different hydrotalcites such as

activated Mg–Al hydrotalcites. Xie et al. (2006) have

synthesized fatty acid methyl esters from vegetable oil

with methanol catalyzed by Li-doped magnesium oxide

catalysts. A number of Li-doped MgO samples were prepared

by the incipient wetness impregnation with the Li/Mg molar

ratio in the range of 0.02–0.15. The catalyst weight was

varied in the range of 3–15 wt% (methanol/oil molar ratio

of 12, temperature of 338 K, residence time of 2 hrs). The

results showed that the formation of strong base sites was

principally promoted by adding Li. This results to an

increase of fatty acid methyl esters synthesis.

In addition, Suppes et al. (2004) have

transesterified soybean oil with zeolites (ETS-10 zeolite,

NaX faujasite zeolite) and metal catalysts. The

transesterification reaction was performed at 6:1 molar

42

ratio of alcohol and temperatures of 60, 120, and 150oC.

The conversion to esters increases from 60-150oC with an

average conversion of 90% achieved at 125oC. Thus, ETS-10

gave better conversions of triglycerides than zeolite-X

type catalysts. This was attributed to the higher activity

of ETS-10 zeolites and larger pore structures that

improved intra-particle diffusion. The catalyst activity

was not affected after reused. However, FFAs loadings in

excess of 25% quench the catalyst activity.

2.3.3(b) The Effects of Solid Acid Catalysts on Biodiesel

Production

The replacement of homogenous catalysts by the solid

acid catalysts is useful for green chemistry. Solid acid

catalysts have strong capacity to substitute liquid acids,

thus wiping out separation, corrosion and environmental

problems (Jacobson et al., 2008). The authors evaluated

different solid catalysts for the production of biodiesel

from high FFAs feedstocks (waste cooking oil). The

catalysts investigated include; Zinc stearate/SiO2,

MoO3/ZrO2, WO3/ZrO2, WO3/ZrO2–Al2O3, MoO3/SiO2, TPA/ZrO2, and

Zinc ethanoate/SiO2. They stated that zinc stearate

43

immobilized on silica gel was most active and stable. The

catalyst recycled several times at optimized conditions of

reaction temperature of 470 K, 1:18 molar ratio of oil to

alcohol, and 3wt% catalyst loading without being

deactivated. The authors recorded a maximum ester content

of 98 wt%. Generally, catalyst recycling reduces biodiesel

processing cost. In another study, Ngo et al. (2010) have

esterified FFAs in greases (12–40 wt% FFA) using a

diphenylammonium triflate acid catalyst immobilized onto

two robust and highly porous solid silica supports(MCM-48

and SBA-15). The authors evaluated the catalytic

activities of the catalysts. The catalysts were reported

to be effective for the esterification of FFAs in greases

with a conversion to biodiesel of 95–99%, resulting in a

pre-treated grease with a final FFAs content of <1 wt%. As

well Furuta et al. (2004) studied biodiesel fuel

production via transesterification using soybean oil and

methanol with solid superacid catalysts; sulfated tin

oxide (SO4/SnO2), zirconium oxide (SO4/ZrO2) and tungstated

zirconia (WO3/ZrO2) at 200-300oC. And conducted

esterififcation of n-octanoic acid with methanol at 175–

200oC in fixed bed reactor under atmospheric pressure. The

44

authors noted that out of the catalysts prepared,

tungstated zirconia–alumina catalyst (WZA) showed high

performance, yielding over 90% conversion for

esterification processes. Although detail analysis for the

acidity of WZA has not yet been determine.

In addition Jitputti et al. (2006) investigated

transesterification of palm and coconut oils using solid

catalysts such as KNO3/ZrO2, KNO3/KL zeolite, SO42-/ZrO2,

SO42-/SnO2, and ZnO. The authors noted that

transesterification of crude palm kernel oil using

SO42−/SnO2 and SO4/ZrO2 catalysts provided highest yield of

methyl esters (90.3 wt%). The purities of the esters were

95.4% for SO42−/SnO2 and 95.8% for SO4

2−/ZrO2, respectively.

However, for ZnO the highest content of the esters was

98.9wt%. As well, owing to the availability of enough acid

site strength, solid acid catalysts for example sulfated

zirconia, tungstated zirconia and Nafion-NR50 were

selected to catalyze transesterification.

Furthermore, zeolites are used as a catalyst for

esterification and as a support material for

transesterification. Zeolites are microporous crystalline

solids with well-defined structures and that they contain

45

aluminum, silicon and oxygen in their framework and

cations. As catalysts, unlike the compositionally

equivalent amorphous catalysts, zeolites demonstrate

substantial acid activity and shape selective features

(Chung et al., 2008). Moreover, Chung, et al. (2008)

employed different Si/Al molar ratio to remove FFAs in

waste frying oil by esterification with methanol using

different zeolite catalysts. The catalysts used include

mordenite (MOR), faujasite (FAU), beta (BEA) zeolites,

ZSM-5 (MFI), and silicalite. The pore structure and the

acidic properties of the zeolites were particularly useful

in the removal of FFAs. These properties influenced the

catalytic activity in FFAs removal. High conversion of

FFAs was comparatively induced by strong acid sites of

zeolites. The MFI zeolite induced an improvement of the

FFAs removal efficiency by cracking the FFAs in its pore

structure owing to its constricted pore mouth. Converting

FFAs on HMFI and HMOR zeolites provided conversion of 80%

at a reaction temperature of 60oC.

2.3.3(c) The Effects of Enzymes Catalysts on Biodiesel

Production

46

Transesterification reaction can be catalyzed with

enzymes catalysts such as Candida Antarctica lipase,

Pseudomonas cepacia, candida sp. 99–125, Pseudomonas

fluorescens, Rhizomucor Miehei, Chromobacterium viscosum

and Rhizopus oryzae lipase (Lara Pizarro & Park, 2003).

The immobilized candida antarctica lipase was explored by

Watanabe et al. (2000) for the continuous production of

biodiesel fuel from vegetable oil. The transesterification

of vegetable oil was conducted using 4% immobilized

Candida lipase as a catalyst at 30°C in a 20- or 50-mL

screw-capped vessel, shaken at 130 oscillations/min. The

authors noted that the activity of candida antarctica

lipase was not affected in a mixture of vegetable oil and

more than 1:2 molar equivalent of methanol against the

total fatty acids. They discovered inactivation of the

lipase to be eliminated via three consecutive additions of

1:3 molar equivalent of methanol. The authors then

developed a three-step methanolysis by which over 95% of

the oil triacylglycerols (TAG) were transformed to their

corresponding esters.

Additionally, biodiesel have been prepared by Shah et

al. (2004) from jatropha oil using lipase catalyst. The47

authors have screened pancreas porcine, candida rugosa,

and chromobacterium viscosum in a solvent-free system for

the production of biodiesel. They employed a screw-capped

vial, and jatropha seed oil (0.5 g) and ethanol were taken

in the ratio of 1:4 (mol mol-1). Also 50 mg of enzyme

(tuned or immobilized) was added and incubated at 40°C

with constant shaking at 200 rpm. The immobilization of

lipase (Chromobacterium viscosum) on Celite-545 improved

yield of esters from 62% to 71% by free tuned enzyme

preparation with a process time of 8hr at 40°C. In

addition to explore more information on the use of enzymes

for the production of biodiesel, Tan et al. (2010)

reviewed biodiesel production using immobilized lipase.

The immobilized lipase as biocatalyst drew great interest

because the process is environmentally friendly. The

authors noted different techniques for lipase

immobilization such as covalent bonding, cross-linking,

encapsulation, entrapment and adsorption. Lipase

immobilization technique is commonly used to increase the

stability of lipase in biodiesel production. They stated

that for biodiesel (fatty acid methyl esters) preparation,

at least a stoichiometric amount of methanol is needed for

48

the complete conversion of triglycerides to their

resultant fatty acid methyl ester. But, methanolysis is

reduced considerably by adding >1/2 molar equivalent of

methanol at the commencement of the enzymatic process.

Usually, the polar short-chain alcohols causes

inactivation of enzymes and this is the major obstacle for

the enzymatic-transesterification reaction. Therefore to

overcome this difficulty one of these options is

suggested: acyl acceptor alterations, solvent engineering

and methanol stepwise addition. The authors reported that

biodiesel yield of 97wt% was obtained after 24 hrs at

temperature of 50°C with a reaction mixture containing

13.5% methanol, 32.5% t-butanol, 54% oil and 0.017 g

enzyme (g oil)−1. With the same mixture, a 95% biodiesel

yield was achieved using a one-step fixed bed continuous

reactor with a flow rate of 9.6 ml h−1 (g enzyme)−1. The

authors concluded that low cost of immobilized Candida sp.

99–125 lipases is rather competitive for industrial use.

Also Fukuda et al. (2001) noted use of enzymatic-catalyzed

transesterification to avoid problems associated with

homogeneous catalysts. The authors reported conversion of

high FFAs feedstocks to biodiesel using immobilized

49

antarctica lipase (Novozym-435) with ease of separation

process.

2.4 Issues Governing Biodiesel Production Processes

Homogeneously catalyzed transesterification in a

stirred tank reactor is most favoured technique for

biodiesel production. Homogeneous catalysts such as NaOH,

KOH, CH3ONa and CH3OK are well established to provide

excellent catalytic activities in the production of

biodiesel. However issues such as difficulties in

biodiesel refining, cost of refining process and

wastewater generation etc, have being the major problems

of this technology. Therefore to overcome these problems

heterogeneous catalysts are explored and exploited, these

catalysts can be easily removed with a resultant high-

quality biodiesel product ( Leung et al., 2010; Saleh et

al., 2011). Despite the current development of

heterogeneous catalysts to circumvent problems associated

with conventional biodiesel production, biodiesel is still

associated with impurities that need to be removed to make

it suitable for diesel engine consumption.

2.4.1 Conventional Reactors used for Biodiesel Production

50

The non-membrane reactors that have been employed for

the production of biodiesel include among others: batch

reactors, plug flow reactors, continuous stirred tank

reactors (CSTR), fixed bed reactors, helicoidal reactor,

transport riser reactor and Oscillatory flow reactor.

Using batch reactors, a good number of researchers have

reported higher conversions of triglycerides to biodiesel

fuel (Helwani et al., 2009).

2.4.2 Challenges in Producing Biodiesel using Conventional

Reactors

Commercial production of biodiesel fuel via batch

reactors is mostly discouraged due to their tedious mode

of operations (Helwani et al., 2009), but batch reactors

are the most widely used. Besides, cost of biodiesel

production due to labour. These problems have led to the

use of plug flow reactors, CSTR and fixed bed reactors for

the production of biodiesel. These reactors have the

potentials of being used for the production of biodiesel

fuels, but problems ranging from lower conversion

efficiencies, difficulties in processing low quality

feedstocks, mass transfer limitation, non-uniform product

51

distribution, low quality biodiesel products, poor

biodiesel yields, higher reaction conditions, labour and

reactor facility degradation due to chemical attacks have

discourage their commercial applications. In addition,

Helwani et al. (2009) reported that the main drawback of

the continuous stirred tank reactors or tubular reactors

is that the temperature of the reaction is narrowed to the

boiling point of alcohol; 65°C for methanol, if the

reactor is to operate at atmospheric pressure. The authors

noted that for industrial size reactor, significant mass

transfer resistance is expected even when higher shear

mixing is employed. These problems have eventually led to

the exploration and exploitation of membrane technology

for the production of biodiesel fuel. Additionally, the

technical difficulties such as immiscibility of

triglycerides and alcohols and reversibility problems

commonly encountered with conventional reactors can be

overcome via membrane reactor (Cao et al., 2008a; Dubé et

al., 2007).

2.5 Application of Membranes for the Production of

Biodiesel Fuel

52

Membrane system exploits the inherent characteristics

of high selectivity, high surface area per unit volume,

and their potential for controlling the level of

components mixing between the two phases (Catherine,

2006). The application of membranes for biodiesel

production is usually designed in to two phases, one phase

is the membrane reactor to transesterfy fats and oils to

biodiesel, and the other phase is the separative membrane

to separate the crude biodiesel from its impurities such

as: catalysts, soap, glycerol, and alcohol etc, without

necessarily using water, acids, organic solvents and

absorbents etc. Membranes are usually classified into

organic, inorganic or combination of the two. Organic

membranes are mostly avoided for processes involving high

acidic and basic environments due to their less

resilience. However, inorganic membranes such as metallic,

ceramic, zeolitic or carbon-made are mostly preferred

because of their ability to withstand harsh conditions

such as higher temperatures, high acidic and basic

environments (Saracco et al., 1999). The term ceramic

membranes are porous fine ceramic filters sintered from

aluminia or titania, zirconia oxides under ultra-high

53

temperature and usually have an asymmetrical structures

with porous support active membrane layer. The macro-

porous support ensures the mechanical resistance whereas

the active layer allows microfiltration, ultrafiltration

separation process. Also, ceramic membranes always runs at

a across flow filtration mode (Jiangsu Jiuwu Hitech CO.,

2010). In the cross flow mode, the fluid to be filtered

flows parallel to the membrane surface and permeates

through the membrane due to a pressure difference

(Catherine, 2006). This characteristic of ceramic

membranes reduces fouling effects and improve high

filtration rate (Jiangsu Jiuwu Hitech CO., 2010).

The development of a membrane reactor and its

successful application in producing biodiesel has renewed

the strong interest to develop alternative renewable and

sustainable fuel to replace petro-diesel fuel. Membrane

reactors can serve different purposes such as intensifying

the contact between reactants and catalyst, selectively

remove the products from the reaction mixture, and control

the addition of reactants to the reaction mixture. These

reactors can be employed to avoid the equilibrium

conversion limits of conventional reactors. Besides, the

54

reactors can efficiently improve the maximum achievable

conversion of reversible reactions and the general

reaction pathways (Dubé et al., 2007). Furthermore,

membrane reactors can provide higher selectivities and

yields in many different processes as well as being safe

and more environmentally friendly (Coronas & Santamarı́a,

1999). A novel membrane reactor was developed by Dubé et

al., (2007) which enabled both acid- or base-catalyzed-

transesterification of canola oil as well as separation of

unreacted canola oil from reaction products. The membrane

reactor consisted of membrane pore size of 0.05 µm, inside

and outside diameters of the membrane were 6 and 8 mm,

length of carbon membrane tube of 1200 mm and a surface

area of 0.022 m2. The membrane reactor was charged with

canola oil (100g) and sealed. Following circulation time

of 10min, the reactor was operated continuously at a

pressure of 138kPa with feed (mixture of methanol and

acid) pump flow rate of 6.1 (mL/min). The heat exchanger

was switched on to achieve temperatures of reaction (60,

65 and 70oC) which was monitored by a thermocouple. After

starting the heat exchanger a stable reaction (+ 0.1oC)

time was achieved with 30min for 60oC, 40min for 65oC and

55

45min for 70oC. The experiments were all conducted for 6

hrs. The authors remarked that additional experiments were

also performed to study the effects of methanol/acid

catalyst, and feed flow rate on conversion for both acid-

and base-catalyzed transesterifications. The flow rates

were 2.5, 3.2 and 6.1 mL/min. Similarly Cao et al. (2008a)

transesterified a number of vegetable oils such as canola,

soybean, palm, and yellow grease lipids via a membrane

reactor. The authors noted that despite the wide range of

feedstocks used, the membrane reactor presented a

moderately consistent performance at one set of operating

conditions and enabled the production of high-quality

biodiesel fuel which was confirmed by gas chromatography

(GC) analysis based on the ASTM D6584 standard. The

biodiesel from all the feedstocks met the ASTM D6751

standard. Also, the glycerol content of biodiesel produced

using a membrane reactor was significantly lower than that

produced via a conventional batch reaction. Another study

conducted by Cao et al. (2008b) compared three different

recycling ratios for the production of biodiesel via

membrane reactor: 100%, 75% and 50% by volume, for

instance, 75% recycling entails that every 0.75 L of polar

56

phase was mixed with 0.25 L methanol with 1 wt.% (by

weight of oil) NaOH catalyst and pumped into the reactor

circulating loop at a feed rate of 3 L/h, while the feed

rate of canola oil was also kept at 3 L/h. The authors

noted that the catalysts and glycerol were also recycled.

Furthermore, to maintain biodiesel-rich non polar phase

containing 85wt.%, the permeate was consistently removed

as well as methanol/glycerol polar phase. Consequently, at

maximum recycle ratio, the fatty acid methyl esters

concentration ranging from 85.7 to 92.4 wt.% was found in

biodiesel-rich non-polar phase. The overall molar ratio of

methanol:oil in the reaction system was significantly

decreased to 10:1 while maintaining a FAME production rate

of 0.04 kg/min. Also in biodiesel-rich non-polar phase no

triglycerides (TG), monoglycerides (MG) or glycerol were

observed. The authors noted that despite the samples not

being water washed prior to analysis, high purity

biodiesel product free of non-saponifiable materials was

achieved. The yield obtained via homogeneous catalyst in

membrane reactor was below the value prescribed by EN14214

standard (Cao et al., 2008a). Therefore, to circumvent the

problems associated with use of homogeneous catalyst,

57

which despite use of membrane reactor still poses

difficulties. A novel continuous packed bed membrane

reactor (a tubular ceramic (TiO2/Al2O3) membrane) was

developed by Baroutian et al. (2011) to produce biodiesel

fuel using solid alkaline catalyst (potassium hydroxide

catalyst supported on activated carbon). The membrane

reactor comprised; length, inner diameter, outer diameter

and pore size of the membrane 40 cm, 1.60 cm, 2.54 cm and

0.05 µm, respectively. The filtration surface area for the

entire membrane was 0.0201 m2. The authors noted that

during transesterification the membrane reactor was able

to block the triglycerides, but biodiesel and by-product

glycerol alongside methanol passed through the membrane

pore size due their smaller molecular sizes. As discussed

earlier the ability of a membrane reactor to block the

triglycerides provided high-quality biodiesel fuel.

Conversion of 94% was obtained at 70 ºC reaction

temperature, 157.04g catalyst per unit volume of reactor

and 0.21 cm/sec cross flow circulation velocity. The

properties of the biodiesel produced at the optimum

conditions were within the ASTM standard.

58

The overall membrane performance is strongly

dependent on the membrane selectivity. Further membrane

performance is mostly affected by several numbers of

parameters such as; membrane composition, temperature,

pressure, velocity of flow, and interaction between

components of the feedstocks with the membrane surface.

Similarly, the performance of membrane separation process

can be characterized by permeate flow rate yielded by a

given membrane device which can be, in the simplest case,

calculated by the product of constant permeate flux and

total filtration area. The higher the selectivity factor

the better the membrane performance. The selectivity

factor is sometimes independent of temperature but

dependent on the nature of material used in the membrane

preparation. In addition, higher temperatures do not

necessarily produce higher selectivities. Thus extensive

researches must be carried in the near future using

various types of materials to form membrane with better

selectivity factor (Low & Cheong, 11-13 march, 2009; Lu et

al., 2007). In another study, Saracco et al. (1999) noted a

change in interest from improving the equilibrium limited

reactions through membrane separation of one of the

59

products, to the increase in selectivity through

membranes. Additionally the retention coefficient (%R) of

glycerol or other related substance can be calculated as:

%R = [(Cal − Cper) × 100]/Cal.

Where: Cal and Cper are the concentration of those

components, in the feed and permeate respectively.

Although current membrane technology has provided

promising results toward biodiesel production, but the

biodiesel produce through membrane reactors require

further purification process so as to achieve the

stringent specifications provided by ASTMD6751-03 and

EN14214. Thus, use of separative membranes has

proficiently provided high quality biodiesel that has met

the international standards specifications stipulated by

ASTMD6751-03 and EN14214 (Wang et al., 2009; Gomes et al,

2010; Gomes et al., 2011).

2.6 Biodiesel Separation Technologies

The first step usually employed to recover biodiesel

after transesterification reaction is separation of crude

biodiesel from the by-product, glycerol. The fast

separation of biodiesel and glycerol is as a result of

60

differences in their polarities and also significant

difference in their densities. The density of biodiesel

and glycerol are about 0.88 gm/cc and 1.05 gm/cc

respectively. The density of glycerol is dependent on the

amount of water, catalyst and methanol present in it. This

density difference is sufficient to employ simple gravity

separation technique to separate biodiesel phase from

glycerol phase (Van Gerpen et al., 2004). However, the

separation process between biodiesel and glycerol can be

difficult in the presence of soap formation, which in most

cases solidifies and form a semi solid substance.

2.6.1 Effects of Catalyst on Biodiesel Separation

Heterogeneous catalysts are simply estranged from the

crude biodiesel product and recyclable. Several

researchers have tried heterogeneous catalysts to

circumvent the problem of time and water consumption

encountered during alkali-biodiesel refining process

(Sharma et al., 2008). Table 2.2 presents reduced

biodiesel water washing process using heterogeneous

catalysts in biodiesel production (Fukuda et al., 2001). A

review was conducted by Zabeti et al. (2009) on biodiesel

production via heterogeneous catalysts (solid catalysts).61

The authors noted that heterogeneous catalysts are not

dissolved or consumed during transesterification, hence

they are easily separated. This characteristic limits

impurities and reduces cost of final biodiesel product

separation. Additionally, the catalysts can be regenerated

and reused, and as an added advantage, the catalysts are

environmentally friendly since the need to use acids,

solvents and water during separation stage is minimized.

Also reviewed is the enzymatic alcoholysis for biodiesel

fuel production (Shimada et al., 2002). Production of

biodiesel via enzymes could alleviate separation

difficulties commonly encountered with alkaline catalyst.

Application of enzymes also minimizes large quantity of

wastewater generated via use of homogeneous catalyst. As

well increase biodiesel purity above 90%.

62

Table 2.2: Comparison between alkaline catalysts and heterogeneous catalysts on the purification of biodiesel (Fukuda et al., 2001).

Variable Alkaline catalysis Enzyme catalysis Solid catalyst

Reaction temperature oC 60-70 30-40 453-493

Free fatty acids in raw materials Saponified products Alkyl esters very Low

Water in raw materials Inhibit transesterification insignificant insignificant

reaction

Yield of methyl esters Normal Higher Normal

Recovery of glycerol Difficult straightforward straightforward

Purification of methyl esters Repeated washings None straightforward

Production cost of catalyst Not exorbitant Relatively exorbitant potentially cheap

63

2.6.2 Effects of Alcohol to Oil Molar Ratio on Biodiesel

Separation

The application of higher alcohol to oil ratios

during transesterification could enhance the rate of

biodiesel production. However, this process complicates

removal of alcohol and increase the cost of biodiesel

purification (Behzadi & Farid, 2009).It was reported by

Avinash (2007) that conversion of vegetable oils to

biodiesel is effected by alcohol to oil molar ratio

ranging from 1:1 to 6:1, and noted conversion of 93–98% at

a 6:1 molar ratio. The author stated that alcohol to oil

molar ratios greater than 6:1 do not improve the yield of

biodiesel above 98–99%, but could hinder the separation of

glycerol from the product mixture.

Also, to simplify biodiesel separation, production of

biodiesel through non-catalytic transesterification of

vegetable oil with supercritical methanol was proposed by

Kusdiana & Saka (2001) and Demirbas (2003). Kusdiana &

Saka (2001) observed that post treatment of crude

biodiesel produced via conventional methods causes severe

separation difficulties, since several steps are required

to remove the catalyst, glycerol, alcohol, soap, and

`64

glycerides etc. They adopted supercritical methanol method

under the following conditions; temperature of 350 and

400oC, pressure of 45-60 MPa, and molar ratio of 1:42 of

the rapeseed oil to methanol with the aim of circumventing

problems associated with the common method. They noted

that supercritical methanol method for biodiesel

production consumes less energy, and present less

separation difficulties. Furthermore, supercritical

methanol technique using methanol to oil ratio of 24,

temperature of 280oC, CO2 to methanol ratio of 0.1, and

pressure of 14.3 MPa can result to biodiesel yield of 98%

within 10 min. This process is practically viable in the

industry, safer, and less costly (Han et al., 2005).

However the process requires higher temperatures and

pressures ranging from 252–402oC and 35–60 MPa respectively

(Demirbas, 2009).

2.6.3 Effects of Water and Free Fatty Acids

Water and free fatty acids (FFAs) in oils and fats

can pose a great problem during transesteri cation. Whenfi

water is present, especially at elevated temperatures, it

can hydrolyze the triglycerides to diglycerides and form

`65

FFAs as shown in Figure 2.4. Further, the presence of

water at average temperatures leads to formation of

excessive soap formation. When an alkali catalyst such as

sodium or potassium hydroxides is present, the FFAs will

react to form saponi ed product. The formation offi

saponi ed products of saturated fatty acids tend to befi

strengthened at ambient temperatures and the reaction

mixture may gel and form a semi-solid substance that is

very difficult to recover. The negative effects of

excessive soap formation include amongst others;

consumption of the catalyst, reduction of catalyst

effectiveness, difficulty in glycerol separation, and

prevention of crude biodiesel purification. Demirbas

(2009) reported that even a little amount of water (0.1%)

in the transesteri cation reaction will reduce the methylfi

ester conversion from vegetable oil. At the same time the

presence of water has a signi cant effect in the yield offi

methyl esters when methanol at ambient temperature is

replaced by supercritical methanol. In conventional

catalyzed methods, the presence of water has bad effects

on the yields of methyl esters (Demirbas, 2009).

`66

Further the separation between biodiesel and the by-

product, glycerol is primarily achieved through different

techniques such as gravitational settling (Chongkhong et

al., 2009), centrifugation (Zabeti et al., 2010),

filtration (Semwal et al., 2011), decantation (Gomes et

al., 2010) and sedimentation (Suppalakpanya et al., 2010).

It was remarked that separation of biodiesel from glycerol

via decantation is cost effective (Gomes et al., 2010).

However, the process requires a long period ranging from

1-8hrs to achieve good separation. Therefore to speed up

products separation process, centrifugation technique is

mostly employed. The process of centrifugation is fast,

but the cost involved is considerably high (Van Gerpen et

al., 2004.).

`67

2.7 Biodiesel Purification Technologies

The purification of the crude biodiesel can be

technically difficult thereby contributing to the increase

in biodiesel production cost. The purity of biodiesel must

be high and generally have to conform to the international

biodiesel standards specifications provided by American

standard for testing materials (ASTM) and the European

Union (EU) standards for alternative fuels. Otherwise the

contaminants could reduce biodiesel quality and affects

engine performance as shown in Table 2.3. According to

the European Union (EU) standard specifications for

biodiesel fuel; water content, free fatty acids, and free

and bound glycerine must be kept to a minimum level and

the purity of the fuel must exceed 96.5%. The crude

products of transesterification reaction consist mainly of

fatty acid alkyl esters (biodiesel), and other secondary

products such as soap, diglycerides, monoglycerides,

glycerol, alcohol, and catalyst etc in different

concentration levels. The main objective in the

purification of crude biodiesel is to remove the fatty

acid alkyl esters from the mixture, and maintain lower

`68

cost of production and also ensure a highly purified

biodiesel product (Karaosmanogˇlu et al., 1996).

`69

Table 2.3: Negative effects of contaminants on biodiesel and engines

Contaminants Negative effectMethanol Deterioration of natural rubber seals and gaskets, lower flash points (problems in storage, transport, and utilization etc.), Lower viscosity and density values, Corrosion of pieces of Aluminum (Al) and Zinc (Zn).

Water Reduces heat of combustion, corrosion of system components (such as fuel tubes and injector pumps etc) failure of fuel pump,hydrolysis (FFAs formation), formation of ice crystals resulting to gelling of residual fuel, Bacteriologicalgrowth causing blockage of filters, and Pitting in the pistons.

Catalyst/Soap Damage injectors, pose corrosion problems in engines, plugging of filters and weakening of engines Free fatty acids (FFAs) Less oxidation stability, corrosionof vital engine components. Glycerides Crystallization, turbidity, higher viscosities, and deposits formation at pistons, valves and injection Nozzles. Glycerol Decantation, storage problem, fuel tank bottom deposits Injector fouling, settling problems, higher aldehydes and acrolein emissions, and severity of engine durability problems.

`70

`71

Glycerol, considered as a major secondary product of

the transesterifcation reaction in its purest form can be

sold to various commercial manufacturing industries such

as cosmetic, food, tobacco and pharmaceutical industries

etc for different applications. In order to make biodiesel

production cost effective removal and resale of glycerol

is mandatory. The remaining product mixtures containing

other by-products such as alcohols also need to be

recovered through either distillation or evaporation

process. However achievement of high conversion rate

results in the immediate formation of distinct two liquid

phases, with also sharp solid phase when heterogeneous

catalyst is employed. The bottom phase of the product

consists of glycerol and the upper phase contains fatty

acid alkyl esters and alcohol. For cases whereby the

reaction could not attain complete conversion the

unreacted triglycerides and bound glycerol will form solid

substance at the bottom phase posing severe difficulties

in the separation and purification of crude fatty acid

alkyl esters. It was reported that a higher molar ratio of

alcohols to vegetable oils greater than 5.67 creates a lot

of difficulties in the separation of glycerol from

`72

methanol. Refined vegetable oils tend to ease the

difficulties encountered during separation and

purification of the transesterified products (biodiesel)

and provide biodiesel with better physicochemical

properties such as viscosity, flash point and densities

etc. However the use of unrefined vegetable oils as raw

materials in the production of biodiesel poses great

difficulties in the purification processes, leading to low

quality biodiesel fuel (Kusdiana & Saka, 2001). It was

also stated that poor-quality vegetable oils may

inactivate the basic catalysts or even enzyme catalysts,

lowering the yield and rendering purification of fatty

acid methyl esters difficult (Casimir et al., 2007). In

another study, Srivastava (2008) stated that vegetable oil

moisture content was removed by subjecting the oil to a

temperature of 110oC in an oven for 1hr before starting the

transesterification reaction. They stated that for

effective use of vegetable oils as raw materials for the

production of biodiesel, both free fatty acids and the

water level must fall within the stipulated standard

specifications. Free fatty acid must fall within 0.5-3%

and the water content to be less than 0.6% respectively.

`73

2.7.1 Biodiesel Wet Washing Technologies

Production of biodiesel is usually followed with soap

formation and water production especially when low quality

feedstock and alkaline catalysts are used as shown in

Figure 2.5.

The water content, free fatty acid level, and

saturation level are the main differences between

feedstocks. Thus, feedstocks should be dried to control

water content which causes hydrolysis of fats and oils to

FFAs. The presence of FFAs leads to soap formation, thus

interfering in the product purification process (Van

Gerpen et al., 2004). For alkali-catalyzed

transesterification, the yields of fatty acid methyl

esters and purification process are negatively affected by

the presence of water. It was remarked that during

transesterification reaction, water content is a more

`74

Figure 2.5: Formation of Soap and water.

critical variable than FFAs (Ma & Hanna, 1999).

Consequently, feedstocks with high amount of water and

FFAs molecules could easily interfere with the

transesterification reaction resulting in soap formation,

thereby affecting the purification of crude biodiesel and

lowering the yield of alkyl esters as shown in Figure 2.6

and 2.7.

`75

Figure: 2.6 Graph of yield of methyl esters against water content intransesterification (Kusdiana & Saka, 2001))

Figure 2.2: Graph of yield of methyl esters against watercontent in transesterification [Extracted from ref. [38]

Alkali catalyst

Thus to meet international standard specifications of

high purity requirements for biodiesel fuel as provided by

European standard (EN 14214) and the American standard for

testing materials (ASTM D6751) as shown in Table 2.4

(Demirbas, 2009), it is necessary to extensively purify

crude biodiesel. Traditionally until recently the

commonest effective technique to remove glycerol and

methanol from biodiesel product mixture is by water

washing, since both glycerol and methanol are highly

soluble in water. It was reported that use of hot water

`76

Figure: 2.7 Graph of yield of methyl esters against FFAs in transesterification (Kusdiana & Saka, 2001)

Alkali

washing can provide ester yield of 86% and high purity of

99%. However to meet either EN 14214 or ASTM D6751-07,

biodiesel should contain 96.5wt% Fatty acid methyl esters

(Sandra & Dejan, 2009).

`77

Table 2.4: International Biodiesel Standard Specifications (Demirbas, 2009).

Properties Units ASTM Method EN14214

Ester content % (m/m) - 96.5

Flash point OC 130 min. >101 Water and sediment vol.% 0.050 max. 0.05 Kinematic viscosity, 40 OC. mm2/s 1.9–6.0 3.5-5 Sulfated ash % (m/m) 0.020 max. 0.02 Sulfur mg/kg – <10 S 15 grade ppm 15max. - S 500 grade - 500 max. - Copper strip corrosion rating No.3 max. class1 Cetane - 47 min. >51 Cloud point OC Report - Carbon residue 100% sample % (m/m)

`78

0.050 max, - Acid number mg KOH/gm 0.50 max. 0.50max.

Triglyceride % (m/m) 0.20max 0.20max

Free glycerin % (m/m) 0.020 max 0.02max. Total glycerin % (m/m) 0.240 max. 0.25max. Phosphorus content mass% 0.001 max. 0.001 max

Methanol content (%m/m) 0.20max

Distillation temperature, atmospheric

equivalent temperature, 90% recovered OC 360 max. - Sodium/potassium ppm 5 max. combined 5max

Max: Maximum

Min: Minimum

`79

Biodiesel wet washing technique involves addition of

certain amount of water to crude biodiesel and agitating

it gently to avoid formation of emulsion. The process is

repeated until colourless wash water is obtained,

indicating complete removal of impurities. Wet washing

processes usually require a lot of water (Nakpong, 2010),

approximately water wash solution at the rate of 28% by

volume of oil and 1 g of tannic acid per liter of water

(Demirbas, 2003). The use of large quantity of water

generates huge amount of wastewater and incur high energy

cost (Wang et al., 2009). Jaruwat et al. (2010) reported

that Thailand is producing about 350,000 (l/day) of fatty

acid esters (biodiesel), resulting to production of no

less than 70,000 (l/day) of contaminated wastewater. The

authors noted that the wastewater disposed is at a high pH

due to significant levels of residual KOH and hexane-

extracted oil, high solid content and low nitrogen

concentration, besides higher concentration values of BOD,

COD, oil and grease etc as shown in Table 2.5. These

components were found to inhibit the growth of

microorganisms, making it difficult for the wastewater to

naturally degrade (Jaruwat et al., 2010).

`80

`81

Table 2.5: Chemical and physical properties of raw biodiesel wastewater.

Parameters Thailand Standard Values of raw biodiesel wastewater. Biodiesel waste water

pH 5.5-9 9.25-10.76 6.7

COD (mg/l) < 400 312,000-588,800 18,362

BOD (mg/l) <60 168,000-300,000 -

Oil and grease (mg/l) <5 18,000-22,000 -

TKN (mg/l) < 100 439-464 -

Conductivity (Scm-1) - 1119TSSb (mg/L) - 8850VSSc (mg/L) - 8750MSSd (mg/L)

`82

- 100

TKN: Total kjeldahl Nitrogena Chemical oxygen demand.b Total suspended solids.c Volatile suspended solids.d Mineral suspended solids.).

`83

Wet washing is mostly conducted through washing with

dionized water, washing with acid and water and washing

with organic solvent and water as discussed below:

2.7.2 Dionized Water Washing Technology

Water washing has been traditionally used to purify

crude biodiesel after its separation from glycerol. It was

reported by Demirbas (2003) that air was carefully

introduced into the aqueous layer, while gently stirring

the mixture of crude biodiesel and water. The process was

continued until the ester layer became cleared. In

addition, after settling the aqueous solution was drained

and water alone was added at 28% by volume of oil for the

final washing process. In addition after

transeterification, crude biodiesel and glycerol can be

phase separated within the first 10min and a complete

separation could be achieved in 2hr after stirring is

stopped. Also alcohol can be removed through distillation

and evaporation and that care must be taken to ensure zero

water accumulation in the recovered alcohol stream (Balat

& Balat, 2008). Fangrui et al. (1998) noted that washing

two times is enough to get rid of impurities from the

methyl esters. After washing, the product was then dried`84

at 80°C for 10 min to remove traces of moisture and the

methyl ester yield was found to be 97-99%. Chongkhong et

al. (2009) stated that after transesterification, 10.24wt%

of 3M NaOH-H2O solution was used to neutralize crude

biodiesel product containing residual FFAs of about 1.4wt

%. The crude biodiesel and the solution of NaOH-H2O were

mixed and stirred at a temperature of 80oC for 20min. The

biodiesel phase was removed from the top of the separator

whilst soap was taken from the bottom. The final biodiesel

product was then heated in an evaporator to remove the

residual water in the product.

Furthermore, Suprihastuti & Aswati (2007) reported

that to achieve low glycerol content in biodiesel as

stipulated by ASTM D6751 and EN14214 standards, the

washing should be done in multistage process. The authors

noted that water washing could significantly affect the

extraction of glycerol. They added that during water

washing process, higher temperatures gives more glycerol

extraction. The authors experimented different washing

times and remarked that for washing time more than twenty

minutes and at room temperature and by using esters to

water volume ratio of 1:3, the glycerol content in esters

`85

was reduced from 0.9331 % to 0.0423 % and the pH was down

to 7.3. Even though, the glycerol value obtained was

higher than the allowable value for international

standards.

2.7.3 Acids and Dionized Water Washing Technology

Acids such as phosphoric acid, sulfuric acid and

hydrochloric acid are mostly used in the purification of

crude biodiesel. This process is followed with use of

distilled water to completely remove biodiesel impurities.

For the purpose of immediate use on diesel engines and

long term storage, purified biodiesel is properly dried.

It was noted by Leung et al. (2010) that after one-step

transesterification reaction, the crude methyl esters

produced were purified with hot water at 70oC, and 5% H3PO4

(aq) at 50oC. The authors reported drying methyl ester

layer in a vacuum and checking with ceric ammonium nitrate

for glycerol removal. Hass et al. (2006) stated that water

have to be reduced to a limit of 0.050% (v/v) to meet the

ASTM D6751 standard specification. The authors washed

biodiesel using water with pH of 4.5. The process helped

in neutralizing the catalyst and converting the soap

formed to FFAs, thus reducing its emulsifying tendencies.`86

Further, vacuum dryer was used to reduce the residual

water from the initial value of 2.4% to the final value of

0.045%. The water removed via drying was recycled into

washing operation. As well, to reduce the cost of

production, the glycerol produced was also refined to a

concentration level suitable to the market value (80w/w%).

The refining of crude biodiesel via neutralization with

Sulfuric acid (1:1) was investigated by (Karaosmanogˇlu et

al., 1996). Two processes were explored: use of catalyst

in solid form and use of catalyst dissolved in methanol.

In the case of refining technique for catalyst in solid

form, decantation was used to remove the catalyst and the

product was transferred into a separatory funnel to

separate biodiesel from glycerol. Sulfuric acid (1:1) was

then applied to reduce the pH from the initial 11.92 to a

pH of 7.0. For the case of catalyst dissolved in methanol

containing pH of 13.07, the pH was reduced to neutral pH

value of 7.0 with similar refining approach as that of

catalyst in solid form being adopted. In addition Faccini

et al. (2010) thoroughly washed crude biodiesel using 10%

acid water at 55 ºC. The acid water was prepared by adding

2% (v/v) H3PO4 to distilled water. This washing step was

`87

conducted in the same transesterification reactor with

constant stirring over 5 min, maintaining the temperature

at 55 ºC. Subsequently, the mixture of wastewater and

biodiesel was separated using separatory funnel. The

wastewater (bottom layer) was removed and the biodiesel

was washed three times with portions of 10% (v/v) hot

water (55 ºC). The upper layer, containing the purified

biodiesel, was dried and stored for further analysis.

Furthermore, HCl of pH 1 was used to treat crude

biodiesel at room temperature (20oC). The product was then

washed twice with deionized water at a volume ratio of

1:1. The final biodiesel product was placed over heated

Na2SO4 (10% of the amount of biodiesel) for a period of

12hrs to remove biodiesel water content and the product

was filtered (He et al., 2006) . Similarly Atapour &

Kariminia (2011) used 35 ml of hot distilled water to wash

the biodiesel produced. In order to neutralize the

residual catalyst and decompose the soap formed, the

product was treated with 35 ml of HCl (0.5%). Further the

product was washed three times with 35 ml of hot distilled

water. They observed that successive rinses successfully

removed contaminants such as methanol, residual catalyst,

`88

soaps and glycerol. Finally the biodiesel obtained was

then dried using manganese sulfate and filtered under

vacuum conditions to eliminate manganese sulfate crystals.

As well Van Gerpen et al. (2004) noted that to eliminate

magnesium and calcium contamination and neutralize the

remaining base catalysts, softened water (slightly acidic)

is usually applied. Similarly, copper and iron ions

removal eradicates the sources of catalysts that decreases

the fuel stability and minimize the tendency for the fuel

to be out of specification. Finally, the refined biodiesel

is dried using vacuum flash technique, sent to storage

unit and made available for diesel engine consumption.

2.7.4 Organic Solvents Washing Technology

Organic solvents such as petroleum ether have been

used to purify crude biodiesel. This process is usually

followed with the use of large amount of demineralized

water to remove residual soap and catalyst. Further fatty

acid methyl esters were distilled under vacuum (40+5 mmHg)

at 180oC. When the temperature reached 240oC (40+5 mmHg),

the distillation was assumed to be completed. The crude

esters were separated after acidic transesterification and

`89

then purified with petroleum ether and washed with hot

water (50oC) until the washing water reached a neutral pH

(Sharma & Singh, 2009). n-Hexane was also used for the

extraction of crude biodiesel at a 1:1 ratio at room

temperature. The mixture was washed three times using

distilled water and the final yield obtained was 93.0 wt%

(Wang & Yang, 2007). Also after transesterification, the

residual alcohol and the tetrahydrofuran (THF) were

removed via vacuum distillation followed by extraction

with petroleum ether. The residual catalyst was then

removed by filtration process. The final biodiesel product

was achieved by using vacuum distillation (He et al.,

2006).

Furthermore, Fangrui et al. (1998) reported that

methyl esters were washed with petroleum ether, and

glacial acetic acid was added to adjust the pH to 7. The

authors revealed that the products obtained were further

purified by washing three times with water. The products

were then dried over anhydrous magnesium sulfate, filtered

and the solvent removed by evaporation. Karaosmanogˇlu et

al. (1996) used petroleum ether to refine crude biodiesel.

The process was employed after biodiesel and glycerol were

`90

separated via decantation. The catalyst was removed in

solid form from the reacting vessel while rotary

evaporator under vacuum was used to remove the methanol.

The crude biodiesel was then poured into a separatory

funnel and then petroleum ether and distilled water were

added, the pH of the mixture was adjusted by adding acetic

acid. To further purify the product, water washing was

repeated three times and the final biodiesel was heated

via Na2SO4 overnight and separated. The separation of

petroleum ether was achieved via rotary evaporator under

vacuum.

2.8 Dry Washing Technologies

The dry washing technique commonly employed to purify

crude biodiesel is usually achieved through the use of

silicates (Magnesol or Trisyl), ion exchange resins

(Amberlite or purolite), cellulosics, activated clay,

activated carbon, and activated fibre etc. These

Adsorbents consist of acidic and basic adsorption

(binding) sites and have strong affinity for polar

compounds such as methanol, glycerin, glycerides, metals

and soap. This technique is followed with the use of a

filter to enable the process to be more effective and`91

efficient. Dry washing is usually carried out at a

temperature of 65oC and the process is mostly completed

within 20-30min (Van Gerpen, 2008). Therefore during

washing process, the amount of glycerides and total

glycerol in crude biodiesel are lowered to a reasonable

level. Besides, the process has the advantage of being

waterless, strong affinity to polar compounds, easy to

integrate into existing plant, significantly lower

purification time, no wastewater, total surface area

coverage of wash tank is minimized, solid waste has

alternate uses, saves space, and improves fuel quality

(Cooke et al., 2003). The process of dry washing technique

was discussed by Dugan (2010) to purify biodiesel, and

noted the process to decrease production time, and lower

cost of production. The author stated that dry washing

provides high-quality fuel and since water is not added,

it is possible to achieve less than 500ppm water content

as stipulated by ASTM D6751. However in wet washing, the

water content of the fuel is usually above 1,000 ppm,

which makes its removal difficult, time-consuming and

costly. The use of magnesol, ion exchange resin, and other

`92

adsorbents such as activated clay, activated carbon, and

activated fibre are discussed as follows:

2.8.1 Magnesol

Water washing is been substituted by dry washing

(magnesol powder or an ion exchange resin) to neutralize

impurities (Cooke et al., 2003). The molecular formula of

magnesol is C6H12MgO10S. Further Figure 2.8 presents the

structure of magnesol.

Figure 2.8: Molecular Structure of magnesol

Cooke et al. (2003) reported the adoption of both dry

washing techniques in industrial plants. The treatment of

crude biodiesel with magnesol, a synthetic magnesium

silicate, requires 1.5-3wt% of biodiesel and need to be

thoroughly mixed. The mixture is filtered using cloth

filter of size 5μm and 1μm, nominal filter is used to

conduct final filtration process. The final product is`93

polished through a filter with sizes of 0.45μm or 0.55μm

before being used as fuel. The process of magnesol

biodiesel purification was experimented and the results

obtained were comparable to those provided by ASTM D6751

and EN14214. In addition, Bryan (2010) experimented use

of magnesol on both soybean and grease biodiesels and the

physicochemical properties met both EN 14214 and ASTM

D6751. The author stated that megnesol has a strong

affinity for polar compounds, thereby actively filtering

out metal contaminants, mono and di-glycerides, free

glycerin, and excess methanol as well as free fatty acids

and soap. In another study, Berrios & Skelton (2008)

reported that a vacuum filtration using a Büchner funnel

and water ejector was employed to separate the final

product. As well, a centrifuge was used to remove the

intermediate product. The authors noted that since

magnesol is hygroscopic the bag was opened with care and

re-sealed as tightly as possible. They suggested use of

mask before handling magnesol, since the powder is very

fine. Further the major limitation about use of magnesol

is that little is known about the process, its catalytic

efficiency and performance intricacies.

`94

2.8.2 Washing with Ion Exchange Resins

Ion exchange resin is an insoluble matrix (or support

structure) normally in the form of small (1-2 mm diameter)

beads, usually white or yellowish, fabricated from an

organic polymer substrate. The application of ion exchange

resins as a dry washing agent is being promoted by the

resins manufacturers; Purolite (PD206) and Rohm snd Haas

(BD10 Dry). Purolite (PD206) is a dry polishing media

specifically formulated to remove by-products remaining

after production of biodiesel. Although being sold as ion

exchange materials, but none of the suppliers advocates

its regeneration because of being acting as adsorbents.

The effects of ion exchange resins on the purification of

crude biodiesel were studied by Berrios & Skelton (2008).

The authors reported that the feed were passed through a

column of resin supported in a glass tube and metered pump

was used to control the flow, and restricted outlets were

employed to ensure a liquid head above the resins. They

noted that initial loading and flows of the resins were

based on the recommendation of R&H trade literature. The

authors analyzed the samples at interval of 2hrs for

`95

methanol and glycerol and demonstrated that ion exchange

resin has the capability to reduce glycerol to a value of

0.01wt% and considerably remove soap, but could not

successfully remove methanol. They obtained methanol

content of 1.14wt%, which is far above EN14214 standard

specification. More so, the adsorption of little soap

indicates a constraint for feed containing high soap

content (Cooke et al., 2003). Additionally, ion exchange

resins offers good performance and provide cost benefits

in the removal of glycerine and water, removal of salts,

soap, and catalyst and also eradicate water washing.

However, it has less effect on the removal of methanol.

2.8.3 Washing with Other Dry Washing Agents

Dry washing agent such as activated carbon is

commonly used to remove biodiesel excess colour. Thus for

effective dry washing of crude biodiesel, the adsorbent is

channeled into a paddle type mixing tank and thoroughly

agitated. Purification of biodiesel was experimented by

Hayafuji et al. (1999) using activated fibres, activated

carbon, activated clay and acid clay. Further, glycerine

was also used as a solvent to wash impurities. The authors

noted that clay; especially acid clay treated with`96

sulfuric acid is preferable, which is superior in the

aspects of dealkaline effect, deodorant effect and

decoloring effect. Also clay grain size ranging from 0.1mm

to 1.5mm is more suitable for effective biodiesel

purification. They stated that clay with smaller grain

size provides superior purification process, but

separation after the purification treatment is more

difficult. However, when the clay grain size is larger,

separation after the treatment becomes easier, but

purification process is inferior.

2.9 Biodiesel Membrane Separation and Purification

Technology

Membranes are semi-permeable barriers that separate

different species of a solution by allowing restricted

passage of some components in a selective manner. A

membrane can be homogenous or heterogeneous, symmetric or

asymmetric in structure, solid or liquid, and can carry a

positive or negative charge or be neutral or bipolar.

Transport through a membrane can be affected by convection

or by diffusion of individual molecules, induced by an

electric field or concentration, pressure or temperature

gradient. Membrane based separations are well-established`97

technologies in water purification, protein separations

and gas separations. However, commercial applications of

membrane technologies are limited to separations involving

aqueous solutions and relatively inert gases. Thus the use

of membranes to treat non aqueous fluids is an emerging

area in membrane technologies. Membrane separation is

primarily a size exclusion-based pressure-driven process

(Dubé et al., 2007). Therefore, different components are

separated according to their particle sizes and shapes of

individual components or molecular weights. The mode of

components’ operation is somewhat dependent on their

interactions with the membrane surface and other

components of the mixture. Also, performance of membrane

separation is affected by membrane composition, pressure,

temperature, velocity of flow and interactions between

components of the feed with membrane surface.

Operations involving membrane technologies in the

last years have shown their potentialities in the

rationalization of production systems. Membrane

performance is usually governed by: selectivity or

separation factor and permeability. In the absence of

defects, the selectivity is a function of the material`98

properties at given operating conditions. The productivity

is a function of the material properties as well as the

thickness of the membrane film, and the lower the

thickness, the higher the productivity.

Permeability, Lp = Qfiltrate-------------------------------------------------------2.1. A∆P

Selectivity, α = Flux ofimpurity-------------------------------------------------2.2. Flux of product

Membrane reactors intrinsic characteristics of

efficiency, operational simplicity and flexibility,

relatively high selectivity and permeability, low energy

requirements, good stability under a wide spectrum of

operating conditions, environment compatibility, easy

control and scale up have been confirmed in a numerous

variety of applications and operations, as molecular

separations, fractionations, concentrations,

purifications, clarifications, emulsifications, and

crystallisations etc (Erinco, 2004).

2.9.1 Organic Membranes

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The membranes used for the pressure driven separation

processes, are microfiltration (MF), ultrafiltration (UF)

and reverse osmosis (RO). Initially most of such membranes

were cellulosic in nature. These are now being replaced by

polyamide, polyvinylidenfluoride, polyacrylonitrile,

polysulphone, polycarbonate and several other advanced

polymers. These synthetic polymers have improved chemical

stability and better resistance to microbial degradation.

Additionally, It was remarked by Salahi et al (2010) that

polyacrylonitrile (PAN) is porous and asymmetric membrane

which combines high selectivity with high permeation rate.

However in organic solvents, polymeric membranes may

swell, resulting to instant and/or long-term pore-size

changes. Consequently, polymeric membranes in solvent

applications have shorter operating lifetimes (Dubé et

al., 2007).

2.9.2 Ceramic Membranes

Ceramic membranes have great potentials and represent

a distinct class of inorganic membranes. Much attention

has been focused on inorganic membranes for their

superiority than organic ones in thermal, chemical and

mechanical stability, high porosity, high flux, long life`100

time, resistance to microbial degradation, increased

resistance to fouling, and a narrower pore size

distribution. Thus, porous inorganic membranes (e.g. Al2O3,

TiO2, ZrO2, SiC) possess some practical advantages over the

polymeric ones such as higher mechanical strength, thermal

and corrosive resistance among others. In addition to

membrane material, pore size influences membranes and

small pore sizes give more stable membranes. Porous

ceramic membranes are normally prepared by sol–gel or

hydrothermal methods, and have high stability and

durability in high temperature, harsh impurity and

hydrothermal environments (Lu et al., 2007). Also using

asymmetric multilayer configuration, ceramic membranes

with high performance parameters such as permeation flow

and mechanical resistance can be achieved. The development

of such a multilayer configuration includes: shaping of a

suitable support material, formation of a microfiltration

interlayer and synthesis of an ultrafiltration (UF) top

layer. Multilayer asymmetric membranes usually consist of

permselective material as a thin film on one or a series

of porous supports, which provide the required mechanical

stability without dramatically reducing the total flux.

`101

Further, one indicator of molecular size is molecular

weight, i.e. a molecular sieving effect. Therefore, the

interaction between solutes and membrane appears to be

important. The effect of interaction between solutes and

membrane surface would be more pronounced in

nanofiltration membranes having pores approximately 1nm

than for ultrafiltration and microfitration membranes

having much larger pores (Tsuru et al., 1998).

2.9.3 Prospects of Biodiesel Membranes Refining Technology

The criteria for selecting membranes are difficult

and dependent on the application. Important considerations

on productivity and separation selectivity, as well as the

membrane’s durability and mechanical integrity at the

operating conditions must be balanced against cost issues.

The relative importance of each of these requirements

varies with the application. However, selectivity and

permeation rate (permeances) are clearly the most basic

properties of a membrane. The higher the selectivity, the

more efficient the process, the lower the driving force

required to achieve a given separation. The higher the

flux, the smaller the membrane area is required. Further

`102

the driving force is often pressure or concentration

gradient across the membrane. Additionally, an

authoritative outline of basic concepts and definitions

for membranes is obtained in a report of International

Union of Pure and Applied Chemistry (Lu, et al., 2007).

Thus, Table 2.6 compares polymeric and inorganic membranes

features. Thus, inorganic membranes favour applications

under higher temperature and chemical conditions, whereas

polymeric ones have the advantages of being economical.

The successful application of membrane technology to

purify crude biodiesel has boosted the interest in the

struggle to develop commercial biodiesel production.

Contrary to both wet and dry washing techniques, membrane

biodiesel purification process does not require both water

and adsorbent. Membrane processes are usually based on the

theory that higher permeates fluxes are followed by lower

selectivity and higher selectivity is followed with lower

permeates fluxes (Tsuru et al., 1998; Lu et al., 2007).

Figure 2.9 shows ceramic membrane biodiesel purification

process.

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Table 2.6: Comparison of polymeric and inorganic membranes (Lu et al., 2007).

Membrane Advantages Disadvantages Current status

Inorganic - Long term durability - Brittle (Pd - Small scale applications

- High thermal stability (>200 ◦C) - Expensive - Surface modifications to

improve hydrothermal stability

- Chemical stability in wide pH - Some have low

- High structural integrity hydrothermal stability.

Polymeric - Cheap - Structurally weak, - Wide applications in aqueous

not stable, temp. Limited. phase, and some gas

- Mass production (larger scale) - Prone to denature & be

contaminated separation.

`104

- Good quality control - short life

`105

Removal of glycerol/ alcohol/other contaminants

Consum ption/storage

Inorganic M embrne

Crude fatty acid alkyl esters (Biodiesel)

Purified Biodiesel

Purified glycerol for sale to other industries

Alcohol for storage/recycling

Recovered catalyst

Figure 8: Inorganic membrane for biodiesel purification

Recently, membranes are applied to separate and

purify crude biodiesel. The process seems to be promising

providing high-quality biodiesel (Low & Cheong, 2009),

besides being energy efficient. The application of

membrane reactor to produce biodiesel fuel was

investigated by Dubé et al. (2007). The membrane reactor

performed well for its ability to retain the unreacted

triglycerides, and provide high purity and quality

biodiesel fuel. The authors noted that one of the

advantages of triglycerides free fatty alkyl esters is the

simplification of the often onerous downstream

purification of crude biodiesel. Further maintaining a

`106

Figure 2.9: Inorganic membrane for biodiesel purification

separate lipid phase is a key factor to assure high-

quality biodiesel production with the membrane reactor

system (Cao et al., 2008a). The membrane used had a 300kDa

MWCO. This property provided an exceptional means of

retaining emulsions formed. Table 2.7 compares glycerine

purification results obtained from membrane and

conventional refining technologies. In the case of batch

reaction, the total glycerine and free glycerine were

approximately twice that obtained from the membrane

system. The use of inorganic (ceramic) membrane to purify

biodiesel from impurities such as; glycerol, soap,

catalyst etc was examined by Wang et al. (2009). The

removal of glycerol was less difficult due to the

formation of reverse micelle with soap forming molecule

size of 2.21μm which was analyzed by zeta potential

analyzer and showed to be bigger than that of biodiesel

molecule, and therefore was easily removed by the

membrane. However, great caution is required during

membrane refining process to achieve biodiesel with

glycerol content of 0.02wt%. They also showed that ceramic

membranes can considerably reduce biodiesel metal contents

as depicted in Table 2.8.

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Table 2.7: Comparison between different refining techniques for the removal of glycerine from biodiesel

Lipid feedstock Biodiesel from membrane reactor (w%) Biodiesel from batch reaction (w%)

Total glycerine Free glycerine Total glycerine Free glycerine

Canola 0.0712 0.00654 0.131 0.0124

Yellow grease 0.0989 0.00735 0.685a 0.0234a

Brown grease 0.104 0.0138 0.797a 0.0171

Palm oil - 0.0152±0.0074 - 0.0179±0.0067

canola oil - 0.013a - -

soybean oil - 0.04 ± 0.004a - -

- - - - 0.03a,b

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- - - - 0.03a,c

a Does not meet the ASTM standard for glycerin in biodiesel (0.02wt%).bIon exchange resincMagnesol

`109

Table 2.8: Contents of metals in the permeates and the retentate by membrane (Wang et al., 2009).

Component Metals content (mg/kg)a

Potassium Sodium Calcium Magnesium

Feed biodiesel 160±19 8.98±1.52 1.45±0.36 0.33±0.12

The permeate (0.6 μm): 4.25±0.37 0.68±0.15 0.70±0.24 0.25±0.11

The permeate (0.2 μm): 2.20±0.42 0.88±0.23 0.55±0.25 0.26±0.16

The permeate (0.1 μm): 1.70±0.31 1.36±0.34 0.95±0.38 0.15±0.07

Biodiesel by water washing: 2.46±0.41 1.41±0.35 0.64±0.29 0.18±0.08 a Results are means±S.D. (n=5).

Furthermore He et al. (2006) conducted experimental studies

on membrane extraction using hollow polysulfone and

polyacrylonitrile fiber membranes and the conventional extraction

techniques to purify crude biodiesel. The biodiesel obtained from

polysulfone fibre membrane gave purity of 99%, besides properties

such as kinematic viscosity (3.906 mm/s2) density (0.876 g/cm3)

and water content (0.042 wt%) met the ASTM D6751 standard`110

specifications. The authors remarked that, these results were

possible due to the absence of emulsion formation, zero density

difference between fluids for hollow fiber membranes, and high

interfacial area. The membrane purification provided better

results in terms of low water requirement, zero emulsion and less

wastewater discharges than the conventional methods such as water

washing, which produce significant amounts of wastewater

containing impurities. This results in an economic gain and the

avoidance of a serious environmental disposal problem (Saleh et

al., 2011). In another study, Low & Cheong, 2009) used polymeric

membrane system to purify crude biodiesel and reported improved

biodiesel yield and less water consumption. They have

experimented different types of membranes such as: PAN membrane,

hydrophobic polypropylene 0.2μm, 0.2μm polyethersulfone membrane,

flat PVDF 0.2μm membrane, 0.2μm ceramic tube membrane, 0.45μm

polysulfone membrane, and 0.2μm flat mixed cellulose acetate

membrane. The water discharged from the membranes was slightly

basic and only contained trace of oil content.

2.9.4 Biodiesel Membrane Separation Processes

`111

Membranes are selective either by pore size (porous

membrane) or because of their chemical affinity for permeating

component. Ceramic membrane in biodiesel separation and

purification technology is fast growing due to their stability in

organic solvents. Further over the last 30 years, membrane

separation processes have enjoyed great popularity and is widely

becoming a promising technology. This is due to stable effluent

quality, small area and zero chemicals needs. And in addition

membranes are becoming cheaper, fabrication technologies is

improved to allow for excellent control of pore size (Sarmento et

al., 2004).

The introduction of membrane technology in the field of

biodiesel production is remarkable in that both organic and

ceramic membranes have shown high prospects for the refining of

crude biodiesel products. Furthermore, pressure-driven membrane

technique provide permeate of excellent quality. Membrane

technology can be useful to industries in simplifying the entire

separation process, reducing energy consumption and decreasing

wastewater production. It is also worth mentioning, the

significant reduction in energy cost associated with membrane

`112

processes (Pagliero et al., 2007). As an added advantage, one is

required to consider the total adequacy of this new technology to

environmental and safety issues, since explosion risks, for

example, are minimized. Membrane separation process is usually

classified based on the size range of material to be separated

and driving force used for the separation. This separation

process is mainly a size-exclusion-based pressure-driven process.

The two most often used membrane processes for the separation of

crude biodiesel are the micro-filtration and ultra-filtration. In

addition ultra-filtration (UF) is a pressure-driven membrane

separation process used to separate different molecules in a

liquid mixture (Cao, 2008; Xu et al., 2010).

2.9.4 (a) Biodiesel Separation through Ultra-filtration Process

In comparison to the conventional treatments, ultra-

filtration method is environmentally friendly and the separation

process of contaminants from crude products is economical. Thus

to continuously obtain biodiesel of high purity, a membrane

separator integrated with liquid–liquid extraction for the oil–

FAME–MeOH system was studied by Cheng et al. (2009). The authors

`113

used porous ceramic disc membranes (47mm in diameter) with an

effective membrane area of 13.1cm2, the pore size of 0.14µm, and

the molecular weights cut off (MWCO) of 300 kDa. The membrane

active layer consisted of zirconia oxide supported on carbon. The

analysis of biodiesel used showed that the actual methyl oleate

(C18:1) content was 58.89 wt.%, and the overall content of C

(C16:0–C18:3) was 98.68% with TG, DG, MG and free fatty acids of

0.017%, 0.029%, 0.037% and 0.376%, respectively. The experiments

were conducted in a total recycle mode at 20oC, 40oC, 60oC, and at

transmembrane pressure range of 0–1000mmHg. After the permeate

began to be collected, each run took another 60 min for its

completion. At first, the ultrafiltration experiments were

conducted at three transmembrane pressures (600mmHg, 500mmHgand

400mmHg) and three feed flow rates (300ml/min, 400ml/min and

500ml/min). Furthermore, the runs were performed over a range of

composition of oil:FAME:MeOH (20:30:50wt%, 20:40:40wt%,

20:65:15wt% and 20:75:5wt%) at constant transmembrane pressure

and feed flow rate. The samples of permeates were taken every 10

min and the compositions of permeate and retentate were examined.

The authors demonstrated that a two-phase system is necessary for

`114

the successful operation of the membrane separator for biodiesel

purification. They further stated that the TG-free permeate

obtained was as a result of the feed bulk concentration being

controlled within two-phase zone such as oil:FAME:MeOH of

20:30:50 wt%, FAME and the methanol contents of the permeate were

about 14 wt% and 85 wt% respectively, regardless of the operating

pressure and the cross flow rate. The authors noted that the

tested modified UNIFAC models are not adequate for simulating the

phase behaviour of the oil–FAME–MeOH system and envisage

exploration of different models using the results of liquid-

liquid extraction (LLE) obtained.

Moreover Saleh et al. (2011) investigated the ability of

membrane processes to efficiently remove free glycerol particles

from FAME, without using a water wash step, as well as the effect

of different materials such as water, soap, and methanol on the

final separation performance. Modified polyacrylonitrile (PAN)

membrane; with 100 kD molecular weight cut-off were employed in

all the runs to remove free glycerol from biodiesel, which was

conducted at a temperature of 25oC and pressure of 552 kPa. For

FAME only the removal of glycerol via membrane was extremely

`115

difficult. Consequently different percentages of water, methanol

and soap were added to the FAME to facilitate the eradication of

glycerol. For FAME containing 1% methanol, the separation was

very low indicating that methanol does not lead to the formation

of a dispersed phase but solubilises glycerol in the FAME, thus

ASTM level was not reached. But for the case using FAME + 0.1 and

0.2 mass% water, excellent separation results were obtained.

Results of the tests showed that the addition of water in small

quantities (0.06 mass%), improved the % separation. The complete

miscibility of glycerol and water caused the formation of larger

particles and thus, two immiscible phases were formed: a water

and glycerol phase, and a FAME phase. This phenomenon eased the

separation of glycerol from biodiesel. Furthermore, the ASTM

level was reached without methanol when 1% soap and 0.06% of

water was added. The DLS measurement also indicated that the true

phase separation occurred between 0.06 and 0.1 mass% as the

particle size increased 10-fold in this range. The authors used

gas chromatography according to ASTM D6584 to analyze the free

glycerol content in the feed, retentate and permeate of the

membrane system. The results obtained revealed that low

`116

concentrations of water had a great effect in the removal of

glycerol from biodiesel even at 0.08wt%. The authors concluded

that the size of the distributed glycerol phase was increased by

water in the untreated biodiesel resulting to its ultra-

filtration membrane separation. They found that application of

membrane technology for the removal of free glycerol from

biodiesel used 2.0 g of water per liter of treated biodiesel

(0.225wt% water) against the current 10 liter of water per liter

of treated biodiesel. The process had avoided formation of

emulsion with great decrease in loss of esters. The authors

recorded excellent results in the removal of glycerol from

biodiesel via membranes compared to conventional glycerol removal

from biodiesel.

In a similar study, Saleh et al. (2010) investigated the

effect of different levels of the components such as water,

methanol, soap, and glycerol of the FAME phase (i.e., water,

methanol, soap, and glycerol) on particle size of

polyacrylonitrile (PAN). The addition of water to FAME mixture

served to decompatibilize the glycerol and increased its particle

size. At the same time, the addition of methanol acted in the

`117

opposite manner by enhancing the solubility of glycerol in FAME

and reducing the particle size. The addition of soap caused a

reduction in particle size by stabilizing more particles and

thus, resulted in smaller particles. The authors noted that the

addition of small amounts of water was found to improve the

removal of glycerol from FAME, and a glycerol content as low as

0.013 mass %, well below standard of 0.020 mass %, was achieved.

Similarly, Cao et al. (2008a) noted that the final biodiesel

product obtained via membrane system met the ASTM D6751 standard,

which compels the free glycerol content to be less than 0.02wt%

and the total glycerol content to be less than 0.24 wt%

respectively. Furthermore, Low & Cheong, 2009) stated that

membrane filtration is a method that may improve the process of

separation and purification of crude methyl ester by increasing

biodiesel yield, reducing water consumption, and generating less

waste materials. The authors reported improved biodiesel yield

with less water use. Thus, development and application of

membrane technology to refine crude biodiesel process could be

environmentally valuable, lower the energy consumption and

decrease losses of alkyl esters.

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2.9.4(b) Biodiesel Separation through Micro-filtration Process

Wang et al. (2009) developed a ceramic membrane separation

process to refine biodiesel. The crude biodiesel produced from

refined palm oil was micro-filtered by ceramic membranes of the

pore sizes of 0.6, 0.2 and 0.1 μm. The membrane pore size of

0.1μm was most suitable and was used at different transmembrane

pressures between 0.05 and 0.20 MPa and temperatures of 30, 40,

50, 60, 70°C. The authors micro-filtered ten kg of crude

biodiesel using membrane pore size of 0.1μm at transmembrane

pressure of 0.15 MPa and at a set temperature of 60°C. The pump

was stopped when the volumetric concentrate factor reached 4. The

flux of permeate was automatically recorded by the flow meter at

every 3 min. After each run the membrane was rinsed by the

methanol. For the process of biodiesel microfiltration by

membrane pore size of 0.1μm, the flux was quite stable over 300

Lm−2 h−1 because of low contents of soap and free glycerol in the

crude biodiesel. The contents of free glycerol, potassium,

sodium, calcium and magnesium in the permeate were reduced to

0.0108% (Below ASTM6751 and EN14214 stipulated value (0.020%)),

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1.40 mg/kg, 1.78 mg/kg, 0.81 mg/kg and 0.20 mg/kg (below EN14538

specifications), respectively. The authors obtained high-quality

biodiesel product and noted decrease in use of water required in

the conventional water washing process. In another study, Gomes

et al. (2010) stated that to produce biodiesel fuel, separation

of free glycerol is ranked among the most critical factors to

consider. The authors studied the effectiveness of

microfiltration with ceramic membranes to separate biodiesel and

glycerol. They experimented tubular Al2O3/TiO2 ceramic membranes

with filtration area of 0.005m2 and average pore sizes of 0.2,

0.4, and 0.8µm and the crude biodiesel was microfiltered at

transmembrane pressures of 1.0, 2.0, and 3.0 bar and temperature

of 60oC. They reported that the quantity of molecular glycerol

and free glycerol dissolved in biodiesel is a vital factor in the

quality control of biodiesel. Therefore free glycerol in

biodiesel fuel must be less than 0.02%. The preliminary results

showed the prospects of membrane technology to improve the

process of biodiesel separation. The authors highlighted the

importance of transmembrane pressure during biodiesel refining

process.

`120

Similarly, Murphy et al. (2010) employed micro-filtration

technique to determine the stability and effectiveness of

different polymeric membranes for the removal of residual

glycerol and water from biodiesel. These membranes include among

others: polycarbonate (GE osmonics), fluoropore (millipore),

SUPOR-200 (Gelman science), polypropylene (GE osmonics) and GS

filter type (millipore). They observed that the recovery of

biodiesel in the permeate could possibly be due to interaction

between the hydrophobic membranes and non-polar biodiesel. Both

fluoropore (millipore) and GS filter type (millipore) provided

biodiesel with glycerol less than 0.02% (ASTM limit). The authors

stated that transmembrane pressure was instrumental for the

separation of water from biodiesel and preliminary results showed

that biodiesel can be efficiently purified via polymeric

membranes. However, Choi et al. (2005) reported that micro-

filtration membrane process is more easily fouled than the ultra-

filtration membrane.

2.9.4(c) Biodiesel Separation through Pervaporation Process

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Pervaporation is also used for the purification of crude

biodiesel. This membrane process is useful in effectively

removing alcohol from biodiesel fuel. Shao (2007) reported that

pervaporation is an efficient membrane process for liquid

separation. The authors reviewed the opportunities, and the

prospect of pervaporation in membrane separation. In addition,

Van Gemert & Petrus Cuperus (1995 ) reported that polymeric

pervaporation membranes sometimes show great variety in

performance when they are alternately used for different solvent

mixtures. Further, one of the most promising uses of

pervaporation is the removal of ethanol from transesterified

products. The authors noted that ethanol forms azeotropes with

many esters, so driving out ethanol also removes a reactant. As a

result, separation of these azeotropic mixtures via conventional

distillation and evaporation is difficult. Therefore polymeric

membranes with low degree of linkages can be engineered which

will preferentially permeate ethanol. Similarly, Nick (2010)

noted that pervaporation, a membrane process is a promising

technique in removing ethanol from alkyl esters.

2.9.5 Parametric Effects on Biodiesel Membrane Separation`122

Biodiesel membrane separation is usually affected by a

number of parameters. Thus, in this study, the parameters chosen

to optimize the membrane process for the separation of

contaminants from biodiesel using membrane filtration process

include: transmembrane pressure (TMP), flow rate and temperature.

The choice of these parameters was made based on the literature

reviewed on membrane biodiesel separation process (Alventosa-

deLara, 2010; Gomes et al., 2011; Gomes et al., 2010).

2.9.5(a) The Effects of Transmembrane Pressure

Transmembrane pressure is referred to as the average feed

pressure minus the permeate pressure or the membrane pressure

gradient. The gauging of feed pressure is often done at the

initial point of a membrane module. Nonetheless, this pressure is

not equivalent to the average feed pressure, since hydraulic

pressure losses could occur due to flow through a membrane

(http://www.lenntech.com/membrane-systems-management.htm).

Application of a high pressure may lead to forceful permeation of

free glycerol via the membrane pores. Thus retention of free

glycerol is reduced when the TMP is increased; besides a

transmembrane pressure above 2 bar could result in the reduction`123

of permeate flux. The forceful permeation of the molecules

through the membrane pores leads to the reduction of filtration

area because of the blocking of the membrane pores, thus

decreasing the permeate flux. The application of TMP proved to be

an exceedingly a vital variable in the filtration of biodiesel

(Gomes et al., 2010). Similar behavour was accounted by Wang et

al. (2009) for the ultrafiltration of oil–water emulsions by

means of membranes consisting of pore sizes ranging from 0.1–

0.2 μm. The authors noted decreases in both permeate flux and oil

retention rate for pressures above 2.0 bar, thus demonstrating

the passing of oil through the membrane pores.

Abadi et al. (2011) remarked that based on Darcy's law,

increase in transmembrane pressure increases permeate flux.

Nonetheless, increasing transmembrane pressure can be a

compensated fouling layer compression. At lower transmembrane

pressures, permeate flux is directly proportional to

transmembrane pressure. Higher transmembrane pressures causes

droplets to rapidly permeate through the pores of the membrane,

hence more oil droplets accumulate on the membrane surface and in

the membrane pores, leading to membrane fouling. Furthermore,

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high TMP could lead to a more rapid flux decline and lower steady

state flux due to more compact cake formation and greater in-pore

plugging when tubular ceramic membrane are used (Muthukumaran et

al., 2011). Gomes et al. (2010) reported that even though the

stabilized fluxes at 2.0 bar were very close for membranes with

pore size of 0.2 and 0.4 μm, the higher concentration of glycerol

in the permeate for the membrane with pore size of 0.4 μm at

transmembrane pressure of 3.0 bar implies that membrane with a

smaller pore size have to be utilized to achieve a better quality

permeate. The authors noted that the best performance was

achieved with membrane pore size of 0.2 μm at transmembrane

pressure of 2.0 bar, giving glycerol retention of 99.4% and a

steady-state permeate flux of 78.4 kg/h m2. Further Hua et al.

(2007) investigated the microfiltration of oil emulsions with

ceramic membrane with pore size of 0.5 μm. They remarked that

operating membrane at a transmembrane pressure of 2 bar could

considerably reduce membrane fouling.

2.9.5(b) The Effects of Flow Rate (Cross Flow Velocity)

In ultrafiltration, it is essential to balance retention

with flow rate to achieve best performance. With ultrafiltration

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membranes, flux is the term that is more frequently used. The

reason that flux is used with ultrafiltration membranes is the

need for scalability. A membrane’s flux is defined as flow

divided by the membrane area. Ultrafiltration membranes are

usually utilized in the refining of valuable biomolecules. Prior

to scale up to larger volumes in a production setting,

separations are studied on a small scale in the laboratory.

Membrane separations are characterized on the basis of flux which

makes it easier for a laboratory scale study to be converted to a

production scale process (http://www.mem-teq.co.uk/page002.html).

The difference in flow rates could be attributed to the

differences in thickness, porosity, and pore architecture.

2.9.5(c) The Effect of Temperature

Temperature plays a vital role during membrane separation

process. Pagliero et al., (2007) reported that an increase in

temperature leads to a decrease in the kinetic constant, i.e.

there is a lower fouling. This effect is due to a decrease in the

solution viscosity. Further Abadi et al. (2011) noted that

permeation flux is increased with increased in operating

temperature. Temperature has dual permeation flux effects;`126

increased in temperature decreases viscosity, and as a result

increases permeation flux. From another point of view, increased

in temperature increases osmotic pressure and this decreases

permeation flux. Therefore, an optimum temperature must be

specified, because of the bilateral effects of temperature. Xu et

al. (2010) reported that higher feed temperature causes lower

viscosity of feed and increase solubility of some feed

constituents. The same as well decreases the concentration

polarization and intensifies transport of solvent through the

membrane pores, providing a higher permeates flux. The authors

also reported that increase in feed temperature usually increases

the energy (operational) cost and the potential of scaling, and

decreases the durability of membrane system despite the

superiority of the thermal stability of ceramic membrane compared

to polymeric membrane. It would, therefore, be practical to

operate membranes at ambient temperature unless a greater flux is

required at the expense of operational costs.

Wang et al. (2009) investigated the effect of temperature on

the permeate flux. The study was conducted by the cross-filtering

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the biodiesel through a membrane pore size of 0.1 μm at

temperatures of 30, 40, 50, 60, 70°C. The permeate flux was

increased with increasing temperature. The authors noted that the

permeate flux of the membrane (0.1 μm) was reasonably stable when

micro-filtering the crude biodiesel at the temperature of 60°C

and the transmembrane pressure 0.15 MPa. Gomes et al. (2010)

stated that a rheological study of glycerol showed that an

increase in the temperature leads to sharp decrease in the

viscosity, which becomes less accentuated above 60°C. They

concluded that for their system, 60oC is the optimum temperature

in terms of permeates flux.

2.10 Selection of Membrane for the Separation and Purification

Process

Different considerations are made in the selection of a

suitable membrane for biodiesel separation process. Some of the

considerations made include: configuration of membrane module,

membrane pore size, membrane stability and membrane material etc.`128

2.10.1 Membrane Module Configuration

Membrane module is the key component of the reactor system.

Module configurations include among others: tubular, hollow

fiber, spiral wound and rotating devices and flat plate. Tubular

modules are commonly applied where it is beneficial to have a

turbulent flow regime; for instance, in the concentration of high

solids content feeds. The membrane is cast on the inside of a

porous support tube which is often housed in a perforated

stainless steel pipe. Individual modules contain a cluster of

tubes in series held in a stainless steel permeate shroud. The

tubes are generally 1–6 m in length and 10–25 mm in diameter.

Tubular modules are cleaned without difficulty and a good deal of

operating data exists for them. Their major disadvantages are the

fairly low membrane surface area and their high volumetric hold-

up (Ghasem, 2007). Figure 2.10 presents different types of

tubular membrane modules

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Figure 2.10: Different types of tubular membrane modules (Jiangsu

Jiuwu HITECH CO., LTD, 2010)

2.10.2 Membrane Material

Membrane materials are characteristically ceramic or

polymer-based. Proper membrane material selection is critical in

achieving desired results. When a high throughput is required,

polymeric spiral membranes are usually used. For processes which

are frequently cleaned, polymeric tubular membranes are

preferably used. These processes include fluids with suspended

materials, highly viscous products, and low-maintenance

operations. However ceramic membranes are usually selected in

processes involving high levels of solvents, hostile

environments, wide pH ranges, and other process considerations.

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Typically this technology uses an alumina or zirconia coating

that is applied to the inside surface of a ceramic support. The

capital cost of conventional polymeric membranes is much lower

compared to the ceramic membranes. Presently the cost of ceramic

membrane is becoming more and more affordable, besides ceramic

membranes often provide a longer operational lifetime (Pearson,

2012).

2.10.3 Membrane Pore Size for Biodiesel Separation and

Purification

The membrane pore size plays a significant role in the

separation and purification of crude biodiesel. It is important

to estimate the minimum particle sizes in the oil-alcohol

emulsion for efficient refining process. Since separation of

crude biodiesel is to some extent dependent on the molecular size

of the constituents comprising biodiesel mixture. Cao et al.

(2007) investigated the effect of membrane pore size on the

performance of a membrane reactor for biodiesel production. The

average pore size for an oil emulsion was determined to be 44µm

with lower and upper limits of 12µm and 400µm respectively. The

oil droplet was found not to pass through the membrane pores`131

because of their large molecular size relative to the membrane

pore size. The membrane provides a barrier to the passage of

oleophilic substances in lipid feedstock. This introduced

inherent reliability in the production of biodiesel that

parallels the use of distillation in petroleum processes. It was

found that the use of four carbon membranes having different pore

sizes of 0.05, 0.2, 0.5, and 1.4 µm, with four different initial

methanol volume fractions of 0.29, 0.38, 0.47, and 0.64 to

produce biodiesel fuel. The authors reported the canola oil used

was retained by all the four membranes in the reactor.

2.10.4 Module Operation Mode

The two standard modes of operation are dead end and cross-

flow configurations as shown in Figures 2.11 and 2.12.

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Figure 2.11: Cross-flow Filtration

Figure 2.12: Dead-endFiltration

In a cross flow filtration, the fluids to undergo filtration

process flows parallel to the surface of the membrane and

permeate through the membrane pores because of the difference in

pressure. The cross-flow process decreases the formation of the

filter cake and keeps it at a low level. Most membrane separation

systems used are operated in a cross-flow feed configuration

where the concentrate passes parallel to the membrane surface as

opposed to perpendicular flow used in dead-end filtration. The

application of solute on the membrane surface is decreased and

the subsequent loss of permeate flux due to increased

hydrodynamic resistance at membrane surface is minimized by

cross-flow induced by hydraulic turbulence (Komolikov &

Blaginina, 2002). The cross-flow mode of filtration reduces the

effect of concentration polarization which induces mass transfer

limitations (Tansel et al., 2009).

Cross-flow filtration is a continuous process in which the

feed stream flows parallel (tangential) to the membrane

filtration surface and generates two outgoing streams. A small

portion of feed, called filtrates or permeates, separates out as

purified liquid passing through the membrane pores. The remaining

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portion of the feed is called retentate or concentrate. The

separation is driven by the trans-membrane pressure or the

pressure difference across the membrane. The parallel flow of the

feed stream, combined with the boundary layer turbulence created

by the cross-flow velocity, continually sweeps away particles and

other material that would otherwise build up on the membrane

surface (Sondhi et al., 2003).

2.11 Application of Acidified Water on Membrane Separation

Process

The solubility of glycerol in the esters is reduced by

producing crude biodiesel using a molar ratio of 1:7.5 and the

microfiltration temperature of 50°C.  The addition of water

presented a much higher permeate flux. Nonetheless, a sharp drop

in the permeate flux occurred to a mean value of 6.9 kg/h m2,

which remained constant until the end of the run. The results

obtained demonstrated the significance of adding water for the

retention of glycerol, thus the glycerol mass content was lower

than 0.02% demonstrating the efficiency of the methodology used

(Gomes et al., 2011). Saleh et al. (2011) reported that after

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addition of water, improved membrane performance in the

separation of glycerol and esters was observed. The authors noted

that addition of water in the range of 0.06 to 0.2wt% led to the

retention of glycerol by the formation of an aqueous phase

dispersed in the esters, which was removed by ultrafiltration.

2.12 Membrane Fouling and Cleaning Process

Membrane fouling and cleaning process are key variables in

the membrane filtration process and these variables are discussed

as follows:

2.12.1 Membrane Fouling

One of the key problems in pressure-driven membrane

processes is reduction in permeate flux far below the theoretical

capability of the membrane due to fouling of the membrane

(Abbasia et al., 2012). The fouling of membrane manifests itself

as a decline in permeates flux with time of operation. The

problem of membrane fouling is the sole most significant reason

for the relatively slow membrane technology acceptance in its

early days. Fouling is due to the deposition and accumulation of

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submicron particles on the surface of membranes and/or the

crystallization, precipitation of smaller solutes within the

pores of membrane and the formation of a slowly thickening layer

on the surface of the membrane, thereby causing continuous

permeates flux reduction during the first hours of operation

(Basso et al., 2006; Komolikov & Blaginina, 2002). Membrane

structure has an important influence on improving permeate flux.

If the sizes of the solute molecules are smaller than the

membrane pores, these particles/oil droplets may enter the pores

of the membrane causing irreversible fouling. When the size of

the particles/oil droplets present in the feed solution are

larger than the membrane pores, these particles/oil droplets

accumulate over the membrane surface causing the formation of a

cake/gel layer and/or pore sealing (Salahi et al., 2010).

On start-up of a process, a reduction in membrane permeation

rate to 30–10% of the pure water permeation rate after a few

minutes of operation is common for ultrafiltration. Such a quick

decrease may be even more intense for microfiltration. This is

often followed by a more steady decrease throughout processing.

The degree of membrane fouling is dependent on the properties of

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the process feed and on the nature of the membrane employed.

Therefore membrane fouling is firstly controlled by careful

choice of membrane type. Secondly, a good choice of module design

that offers appropriate hydrodynamic conditions for the

particular application. Equally, process feed pretreatment is

also essential. When membrane fouling occurred, the permeation

rate can be substantially restored through back-flushing of the

membrane. Nevertheless, this is rarely totally effective, thus

chemical cleaning is eventually required. The process of membrane

cleaning entails disruption of the membrane separation process.

Due to the extensive nature of cleaning required, quite

substantial time losses may result. Therefore, a typical cleaning

method would necessitate: flushing with filtered water at 35–50oC

to displace residual retentate; back-flushing or recirculation

with cleaning agent, probably at higher temperatures; and rinsing

with water to remove sterilizing solution. More recent approaches

to the control of membrane fouling include the use of more

sophisticated hydrodynamic control affected by pulsated feed

flows or non-planar membrane surfaces, and the application of

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further perturbations at the membrane surface such as continuous

or pulsated electric fields (Ghasem, 2007).

2.12.2 Membrane Cleaning

During membrane cleaning process, components are removed

physically, chemically or hydraulically. A module is temporarily

out of order, when the cleaning process is performed. As a

result, dead-end management is a discontinuous process. The

length of time that a module performs filtration is called

filtration time and the length of time that a module is cleaned

is called cleaning time. In practise one always tries to apply

the lowest possible cleaning time and make filtration time last

as long as possible. In addition, ceramic membranes are ideal for

in-place chemical cleaning at high temperatures, while using

hydrogen peroxide, chlorine, caustic, steam sterilization and/or

ozone and strong inorganic acids. These membranes can also be

back-pulsed, which is essentially a permeate flow reversal

technique to decrease fouling and to increase filtration

efficiency. Back-pulsing is an in situ technique for the cleaning of

membrane by periodically reversing the permeate flow by applying

`138

pressure to the filtrate side. In this manner, permeate liquid is

forced back through the membrane to the feed side. This permeate

flow reversal dislodges deposited foulants, which are then

carried out of the membrane module by the tangential flow of

retentate, or which may re-deposit on the membrane surface later

on (Sondhi et al., 2003). The cleaning process is more efficient

when using transmembrane pressure of 0.45 bar. It was suggested

that an efficient way to clean and recover permeate flux of a

ceramic membrane that is submitted to crude soybean oil

ultrafiltration, is by using only hexane. In this way it can help

to the understanding of the behavior of the cleaning process of

deguming of crude oil using ceramic membranes (Basso et al.,

2006).

2.13 Membrane Stability

The remarkable physical and chemical stability of ceramic

membranes permits them to provide reproducible performance over

long service lifetime, which is well established in many

industrial installations. It was demonstrated that the capability

of ceramic membranes to increase yields, recover valuable

products and concentrate process streams, makes them a preferred`139

method of filtration and cost-effective (Sondhi et al., 2003;

Komolikov & Blaginina, 2002).

2.14 Biodiesel Purity via Membrane Refining

The use of membranes to refine crude biodiesel product has

been remarkable, producing biodiesel purity of 99% (He et al.,

2006). This is so because blockage of unreacted triglycerides

during membrane biodiesel production enabled achievement of low

impurity biodiesel (Cao, 2008) as shown in Figure 2.13. Moreover,

Cao et al. (2008a) stated that the glycerol content of biodiesel

produced using membrane reactor is significantly lower than that

produced through a traditional batch reaction. Also, membrane

extraction technique was found to provide about 90% purity of

biodiesel (Leung et al., 2010). Further Cao (2008) reported the

biodiesel produced was of excellent purity without any

confirmation of the presence of particulate matter.

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Figure 2.13: Cross flow filtration configuration (Lu et

al., 2007)

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= Triglyceride = Biodiesel mixture