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Transportation fuels from co-processing of wastevegetable oil and gas oil mixtures

Bharat S. Rana, Rohit Kumar, Rashmi Tiwari, Rakesh Kumar,Rakesh K. Joshi, Madhukar O. Garg, Anil K. Sinha*

CSIR-Indian Institute of Petroleum, Mohkampur, Dehradun 248005, India

a r t i c l e i n f o

Article history:

Received 18 April 2012

Received in revised form

26 April 2013

Accepted 27 April 2013

Available online

Keywords:

Hydrotreating

Gas oil

Biofuel

Waste soya-oil

Catalysis

* Corresponding author. Tel.: þ91 1352525842E-mail addresses: asinha@iip.res.in, anils

0961-9534/$ e see front matter ª 2013 Elsevhttp://dx.doi.org/10.1016/j.biombioe.2013.04.

a b s t r a c t

Hydroprocessing catalysts, sulfided NieW (on mesoporous silicaealumina) and NieMo (on

mesoporous g-alumina), under typical hydroprocessing conditions, can very effectively

produce liquid fuel from mixtures of waste vegetable oil and refinery gas oil. The acidity of

the catalyst controls the relative amount of diesel range (straight chain) alkanes and

cracked lighter products. The yield of diesel range (250e380 �C) product varied between 60

and 90%, while kerosene (jet) range product varied between 10 and 35% depending upon

the reaction conditions and type of catalyst used. The hydrodeoxygenation pathway for

oxygen removal from triglyceride seems to be favored over the NieMo catalyst, while

decarboxylation þ decarbonylation pathway is favored over the NieW catalyst and the

respective pathways becomes more dominant with increasing vegetable-oil content in the

feed. Vegetable oil conversion does not adversely influence hydrodesulfurization of gas oil

indicating viability of co-processing. The activation energy for overall S-removal is much

lower than that for overall O-removal. Density and acidity (TAN) of the products meet the

required specification and cetane number is better than that for pure diesel.

ª 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Increased demand for cleaner fuel due to environmental

concern and depleting petroleum reserves coupled with

deteriorating quality of the crude oil have led to a surge in

world-wide quest for renewable and clean fuel sources [1e4].

One of the renewable sources is biofuels from vegetable oils

[5], specifically, non-edible and used oils such as waste

restaurant oil [6], jatropha oil [7], algae oil [8], etc. Prior to use

in engines, these oils originating from vegetables and animals

need to be converted into suitable fuels by processes that can

lower their viscosity and oxygen content, and improving their

atomization and lubricity [9].

Bio-diesel, which is Fatty Acid Methyl Esters (FAME), is

produced by transesterification of fatty acids in triglycerides

.inha65@yahoo.com (A.K.ier Ltd. All rights reserved029

making it suitable as fuel. However, new biodiesel plants

require a large capital investment [10,11] and large quantities

of byproduct glycerol needs to have suitable market. To use

neat bio-diesel requires some modification in engine and

additionally it gives poor performance in cold weather and

poor emission. An attractive route that offers engine

compatibility and feedstock flexibility using the existing

petroleum refinery infrastructure, is the conversion of

renewable oils into hydrocarbons which have much higher

cetane value than conventional diesel fuel. This process in-

volves conversion of fatty acids in triglycerides into normal

and/or iso-alkanes. This may be obtained by hydro-

deoxygenation, decarbonylation, decarboxylation, isomer-

isation and hydrocracking or a combination of two or more

thereof [6,7,12e23].

Sinha)..

b i om a s s a n d b i o e n e r g y 5 6 ( 2 0 1 3 ) 4 3e5 244

Hydroprocessing is used in the petroleum refinery to crack

larger molecules and/or to remove S, N and metals from pe-

troleum derived feedstocks such as, gas oil and heavy oil [24].

Hydroprocessing has now become a well reported [6,7,12e23]

and an established process [25e28] to produce straight chain

alkanes from fatty acid triglycerides of animal fat, tall oil, and

other vegetable oils.

A ten month on e road test showed that engine fuel

economy was improved by a blend of petrol diesel with

hydrotreated tall oil [18]. The advantage of hydrotreating over

trans-esterification are compatibility with current refinery

infrastructure, engine compatibility and feedstock flexibility

[18]. Neste oil corporation and UOP have developed technol-

ogies for producing diesel fuel from vegetables oils in re-

fineries using modified hydrotreating process [27,28]. Glycine

max (Soya), being a food-crop and richest, cheapest and

easiest source of best quality proteins and fats and having a

vast multiplicity of uses as food and industrial products,

cannot be used as fuel feedstock being. Waste vegetable oil

such as G. max (soya) is produced,mainly from industrial deep

fryers, snack food factories, restaurants and hotels. More

recently, waste oil has become known for its ability to be

refined into biodiesel fuel. The waste oil which generally re-

quires proper disposal could be an additional non-edible oil

source for fuel. It can be hydroprocessed after simple filtration

to remove any food particles remaining in the oil after cook-

ing. It is necessary to study in detail how the vegetable oils

could be hydrotreated with petroleum e derived feedstocks

such as gas oil in the existing petroleum refinery infrastruc-

ture to produce diesel fuel with better product properties and

maximum yield with little modification in the existing con-

ditions in refineries to make the process economically

attractive. We report the results for hydrotreating mixtures of

gas oil and waste vegetables oil, by varying different process

parameters in the range varied in a typical diesel hydrotreater

(DHDT) unit, and discuss strategies as to how gas oil and

vegetables oil could be hydroprocessed in the same reactor

within a petroleum refinery.

2. Material and methods

2.1. Feeds and their properties

Gas oil was supplied by Mathura Refinery, India and its

properties are reported in Table 1. Waste restaurant G. max

Table 1 e Properties of feeds used in the study.

Feed mixture Density @ 15 �C(g cm�3)

S (mL/L) N (mL/L) TAN m

Gas oil Soya oil

100 0 0.8583 1940 125 (63.2)a 0

0 100 0.9148 41.6 n.d. 0

90 10 0.8599 914.3 n.d. 0

75 25 0.8696 884.3 n.d 0

60 40 0.8767 689.5 n.d. 0

a Basic N.

(soya) oil was used for this study after filtration to remove

solid residues and was characterized by various techniques

described later in Section 2.4, and the properties are listed in

Table 1. The species were harvested in the Indian sub-

continent (geo-coordinates: 22�N, 72�E). The oil is extracted

from the beans (w18%yield) and refined. The entire oil milling

process generally includes: cleaning, sieving, hulling, separa-

tion, cracking, particle making, frying, softening, flaking,

drying, and expelling oil materials. Extraction of oil from the

prepared material is done with the aid of a food-grade solvent

hexane. Distillation is done to remove the solvent from the

extracted oil. Final step is recovery of solvent, which is reused.

The commercial refined oil (Ruchi Soya Industries Limited,

India) was used for cooking in a typical restaurant for deep-

frying. The used oil, after it was declared unfit for cooking in

the restaurant, was filtered (Whatman No. 41) and used in this

work. The oil had composition of 10.8% palmitic acid (C16:0),

3.8% stearic acid (C18:0), 28.7% oleic acid (C18:1), 49.9% linoleic

acid (C18:2) and 6.8% linolenic acid (C18:3). The acid value

(TAN) and viscosity of fresh soya oil was 0.50 and 4.15

respectively, while that of waste cooking oil has slightly

higher TAN (0.9) and higher viscosity (4.45). Slight difference

in the properties makes both the kind of oils equally suitable

for processing. Moreover, hydroprocessing is not sensitive to

free fatty acid (FFA) content. Usingwaste oil has the advantage

of being a cost-effective raw material and sidesteps food

versus fuel issue.

2.2. Catalysts and their characterization

Waste G. max (Soya) oil and gas oil mixtures were processed

in a fixed bed reactor with sulfided NieW/SiO2eAl2O3 and

NieMo/Al2O3 catalysts listed in Table 2. The catalysts were

prepared by conventional impregnation of the support using

an aqueous solution of (NH4)6Mo7O24 and Ni(NO3)2. The

support was mixed with the impregnation solution and

after stirring for 1 h it was dried at 100 �C and calcined in an

air stream at 400 �C for 1 h. N2 adsorptionedesorption using

a Micromeritics ASAP 2010 instrument over samples evac-

uated at 350 �C for 4 h was used to determine specific BET

surface area (SBET) and pore volume. Pore size was calcu-

lated from the desorption branch of the adsorp-

tionedesorption isotherms by the BarretteJoynereHalenda

(BJH) method. Ammonia TPD was used to determine the

acidity and oxygen chemisorption was used to determine

the oxygen capacity.

g (KOH) g�1 Pour point

(�C)IBP-250 �C 250e380 �C 380 �C-FBP

.45 12 12.6 86.1 1.3

.9 �6 e 0.20 99.8

.5 15 3.7 68.5 27.8

.6 18 5.4 71.8 22.8

.8 18 7.5 71.9 20.6

Table 2 e Physicochemical properties of catalysts.

NieW/ SiO2eAl2O3 NieMo/Al2O3

Surface area, m2 g-1 250 262

Total pore volume cm3 g-1 0.29 1.03

Mean pore radius 23 �A 30 �A

Surface acidity, NH3 0.77 mmol g�1 (200e400 �C) e

O2 capacity, cm3g�1a 5.64 2.1

Chemical composition (% mass fractions

of the dry catalyst)

29.38(SiO2), 27.09(Al2O3), 26.7(WO3), 9.8(NiO) 3.3 (NiO), 14.7(MoO3) 81.97(Al2O3)

a At STP (298 K and 91.73 kPa).

b i om a s s a n d b i o e n e r g y 5 6 ( 2 0 1 3 ) 4 3e5 2 45

2.3. Catalytic activity studies

The catalysts (1.75 g, 2.5 ml) diluted with SiC (2.0 g, 2.5 ml) to

ensure sufficient catalyst-bed length and to improve the

reaction-heat transfer, were loaded into a stainless steel

tubular reactor (1.3 cm I.D and 30 cm in length) and the

experiments were carried in a commercial bench-top micro-

reactor (Autoclave Engineers’ BTRSeJr�) with single-zone

tubular furnace, for vapor phase catalyst evaluation in

continuous down-flow mode. Hydrogen pressure was

controlled by a back pressure regulator (TESCOM), gas flow

was controlled by a mass flow controller (Brooks), tempera-

tures of the catalyst bed were controlled by temperature

controllers and registered by two thermocouples. A high

pressure liquid metering pump (Eldex) was used to maintain

desired liquid flow. The gaseliquid reaction mixture passed

through the pressure gaseliquid separator to separate

gaseous fraction (containing hydrogen with small quantitities

of CO, CO2, propane and other lighter hydrocarbons) from

hydroprocessed effluent. Gaseous products were released to

atmosphere by a gas-meter and analyzed. Liquid product was

drained to the atmospheric separator in order to remove trace

amounts of gases.

The catalysts were presulfided using a mixture of dimethyl

disulfide and gas oil at atmospheric pressure and 350 �C for

9 h. The reaction condition for catalytic hydrotreating exper-

iment were as follows : temperature 340e380 �C, pressure

5 MPa, LHSV 2, 4 h�1, and the volume ratio of H2 to liquid feed

was 500:1 (at 301 K, and 91.73 kPa). The carbon yield reported

in this paper are based on the simulated distillation results.

2.4. Feed and product analysis

The liquid products were withdrawn after stabilization of re-

action conditions (6 h) in 2-h intervals (at each temperature)

and analyzed by off-line gas chromatography (GC) after sep-

aration of the water phase. We analyzed the liquid products

thrice during stabilization period by GC to monitor constant

activity, stabilization of process parameters were also

confirmed in terms of reactor hydrogen pressure, temperature

as well as hydrogen and liquid flow. Two samples were

collected at each reaction condition and analyzed by GC for

constant activity to ensure that there was no observable

catalyst deactivation during an experiment. The reaction

gases were analyzed using a Varian 3800-GC equipped with a

flame ionization detector (FID) and a thermal conductivity

detector (TCD). Liquid feed compositions and product samples

were analyzed with Varian 3800-GC using CP-Sil Pona CB

column for detailed hydrocarbon analysis (ASTMD5134-9), vf-

5ms column for hydrocarbons, free fatty acids, triglycerides

and Varian Select� Biodiesel column for free fatty acids, tri-

glycerides. Simulated distillation of the hydrotreated products

were carried out using a Varian 3800-GC according to the

ASTM-2887-D86 procedure. It was assumed that the area of

the each distillation fraction in the stimulate distillation re-

sults were proportional to amount of carbon in that fraction.

This assumption is valid for experiments in this paper which

were at complete triglyceride conversion. The product yields

are reported as % of volume fraction. Internal standard

(eicosane) was used for quantification. The triglyceride

quantitative results from GC analysis were compared and

matched with results from HPLC analysis for the same after

derivatization to confirm that there was no error in analysis.

GC Injector port temperature of 340 �C was used for a reliable

and direct quantification of fatty acid and triglycerides

without chemical derivatization. The temperature program

was set as: from 35 �C to 150 �C (rate: 3 �C min�1 and holding

time for 5 min), then increase to 300 �C (rate: 12 �C min�1 and

holding for 5 min) and then increased to 320 �C (rate: 15 �Cmin�1 and holding time for 15 min). The yield fractions were

calculated on a relative basis considering the entire range of

products formed as 100% and then evaluating individual

percentage of each fraction. Complete mass balance was

made over the reactor which was always >99%. Deactivation

during the runs were estimated by repeating the same

experiment again after 2 days of continuous operation to

confirm less than <2% change in activity otherwise fresh

catalyst was loaded and used. Experiments of a similar vari-

ation, say space velocity or variation of H2/feed ratio, or

pressure were performed in the same control limit of 2 days

and thus influence of deactivation in the data sets of similar

variation was avoided. The key experiments were reproduced

and assured that <2% variation in activity and yields of vari-

ation fractions was observed.

The concentration of sulfur in the feed and liquid products

were determined by XRF analysis (OXFORD Lab X model 3000

sulfur analyzer) following ASTM D-4294 method. A set of

diesel samples with known low S concentration of 0.2 mL L�1,

5 mL L�1, 10 mL L�1, 15 mL L�1, 20 mL L�1, 100 mL L�1, and 200 mL L�1

were mounted into the system and the data were collected to

obtain calibration curves. The hydrodesulfurization conver-

sion (HDS conversion) was calculated by subtraction the sul-

fur in the feed minus the sulfur in the product and divided by

the sulfur in the feed.

b i om a s s a n d b i o e n e r g y 5 6 ( 2 0 1 3 ) 4 3e5 246

Total acidity number (TAN) was determined following

ASTM D974 method. TAN is defined as the g of KOH per kg of

oil. This procedure involved titrating the oil with a KOH

standard. Nitrogen analysis was done according to ASTM

D5291 standard test methods for instrumental determination

of carbon, hydrogen, and nitrogen (CHN). Product density

(@15 �C) was determined according to ASTM D4052 method.

Cetane number was determined according to ASTM D613

method. Pour-point was determined according to ASTM

D97e12 method.

3. Results and discussion

The physicochemical properties of catalysts on mesoporous

supports are presented in Table 2. NieW/SiO2eAl2O3 catalyst

has a typical composition of a hydrocracking catalyst and has

considerable acidity. Second catalyst NieMo/Al2O3 is a typical

hydrotreating catalyst with mesoporous alumina as the sup-

port. Mesoporous supports are specifically needed for the

transformations of bulky triglyceride feedstock for better

diffusion of reactants and products and lower deactivation by

pore blockage by coking and by waxy product intermediates

formed during prolonged use of the catalysts. Using micro-

porous supports rapid deactivation of catalyst was observed.

3.1. C15eC18 paraffin yield

Gas oil and mixtures of vegetable oilegas oil were hydro-

treated at reaction temperatures ranging from 340 �C to 380 �Cat two different space velocities and the results for C15eC18

paraffin yield are shown in Fig. 1. GC analysis showed that all

the vegetable oil in the mixture was converted into hydro-

carbons at the studied temperatures (>340 �C) and space ve-

locities (2, 4 h�1). In the first instance, each triglyceride

molecule would break down into three hydrocarbon mole-

cules and one propane molecule during hydrotreating. The

majority of products produced from vegetables oil hydro-

treatment are CO, CO2, methane, H2O, propane and the

straight chain alkanes ranging from C8 to C18.

Propane, CO2, CO and methane are major gaseous prod-

ucts (Fig. 2) observed in the effluent gas mixture which

Fig. 1 e C15eC18 paraffin yield at WHSV 2 hL1 (left) and WHSV

(:), 10% soya oil (-), 25% soya oil (C) and, over sulfided NieW/S

gas oil.

primarily contains hydrogen (>90%). The amount of CO

formed during co-processing was much higher (nearly 4e6

times more) than CO2. More CO and CO2 was formed over

NieW than that over NieMo catalyst which may be attributed

to more cracking over former than that over latter below

(Fig. 2). Much higher CO þ CO2 content of the cracking cata-

lyst implies decarbonylation and decarboxylation pathways

as more favored over it. This is in fact supported by the

product hydrocarbon composition as discussed later in Sec-

tion 3.3.

The C15eC18 hydrocarbon yield for the soya oilegas oil

mixtures is a function of both reaction temperature and

concentration of soya oil. The yields of n-C15, n-C16, n-C17,

n-C18 from hydrotreating the pure gas-oil feed are between 45

and 50% as shown in Fig. 1. The desired diesel range hydro-

carbon (C15eC18) yield (Fig. 1) increased and reached as high

as 70%with increasing soya oil ratio in the feed from 10 to 40%

which is as expected due to increased contribution of C15eC18

hydrocarbons coming from the conversion of the triglycerides

to the corresponding hydrocarbons. The C15eC18 yield for the

feed mixtures at 2 h�1 space velocity does not show any

considerable change with temperature variation from 340 to

380 �Cwhich implies that complete vegetable oil conversion is

achieved at lower temperature of 340 �C and there is little

further cracking of hydrocarbons as well as the gas oil com-

ponents with increasing temperature up to 380 �C. The

increment in C15eC18 hydrocarbons with higher concentra-

tions of soya oil in the feed is more pronounced at higher

space velocity. For example, at higher space velocity of 4 h�1

the yields of major products C17 and C18 are higher than that

at 2 h�1. It can be observed from Fig. 1 that 10% mass fraction

soya oil containing mixture showed about 5% higher C15eC18

yield at 4 h�1 than that at 2 h�1 and similarly, 25% mass

fraction soya oil containing mixture showed about 10% higher

C15eC18 yield at 4 h�1 than that at 2 h�1. Thus, by increasing

the space velocity of the feed it is possible to increase the yield

of C15eC18 hydrocarbon as at 340e380 �C all triglycerides are

completely converted to hydrocarbons with negligible

cracking. These results indicate that there is a widewindow of

flexibility in terms of temperature and ratio of vegetable oil-

egas oil mixtures, when operating such feed mixtures in re-

finery hydrotreaters.

4 hL1 (right) over sulfide NieMo/Al2O3 catalyst with gas oil

iO2eAl2O3 catalyst with 25% soya oil (;), 40% soya oil (A) in

350 355 360 365 3700.0

0.1

0.2

0.3

Temperature (ºC)

Yiel

d (v

ol%

)

350 355 360 365 370 375 3800.0

0.5

1.0

1.5

2.0

Temperature (ºC)

Yiel

d (v

ol%

)

Fig. 2 e Gaseous product profile for propane (:), methane (-), CO (C) and CO2 (;), over sulfided NieMo/Al2O3 catalyst (left)

and over sulfided NieW/SiO2eAl2O3 catalyst (right).

b i om a s s a n d b i o e n e r g y 5 6 ( 2 0 1 3 ) 4 3e5 2 47

3.2. Diesel-range carbon yield

During hydrotreating, the product yield in the 250e380 �Cdistillation range (which is the fraction corresponding to the

diesel rangeeC15 to C18 hydrocarbons) based on simulated

distillation (SIMDIST) results vary between 80 and 90% at

different reaction temperature and different space velocities

for the NieMo based hydrotreating catalysts as shown in

Fig. 3. The yield of alkanes in the 150e250 �C cut is less than

20% over the NieMo catalysts. It implies that 340e380 �C is the

optimum temperature range for obtaining products with

minimal cracking at complete conversions of the vegetable oil

triglycerides. At temperature <340 �C considerable amount of

unreacted vegetable oil is observed in the product while at

temperature>380 �C considerable increase in cracked product

yield is observed. At higher space velocity (4 h�1) the diesel

range product yield shows variation between 80 and 90%

depending on the reaction temperature and the maximum

yield of the 250e380 �C fraction is obtained at 360e370 �C. Itcan be clearly noted from Fig. 3 that the diesel range product

yields for the vegetable oilegas oil mixtures is clearly 1e3%

lower than that for pure gas-oil at most of the reaction con-

ditions which might be attributed to higher acidities of the

reaction mixtures during the hydrogenolysis of vegetable oil

triglycerides to hydrocarbons via corresponding fatty acid

[22,29] which may contribute to slightly higher cracking

335 340 345 350 355 360 365 370 375 380 38560

65

70

75

80

85

90

95

Car

bon

yiel

d (%

)

Temperature (ºC)

Fig. 3 e Diesel range (250e380 �C) carbon yield from simulated d

sulfide NieMo/Al2O3 catalyst with gas oil (:), 10% soya oil (-), 25

with 25% soya oil (;), 40% soya oil (A) in gas oil.

activity for the feed mixtures than that for pure gas-oil. Free

fatty acids (high TAN)were observed in the productmixture in

large quantities at lower (<300 �C) temperatures as observed

from GC analysis, similar observation being also reported in

literature [22,29].

Using a sulfided NieW catalyst on acidic silica-alumina

support the diesel range (250e380 �C cut from SIMDIST)

product yield is much lower than that over sulfide NieMo/

Al2O3 catalyst, about 15e20% lower at 2 h�1 and about 2e10%

lower at 4 h�1. The lower diesel yield for the NieW catalyst

than the NieMo catalysts is due to more cracking of hydro-

carbons over the former due to its higher acidity. The yield of

the 150e250 �C distillation fraction (kerosene range) is nearly

30e35% for the hydrotreating of vegetable oilegas oil mixtures

at 2 h�1 and about 15% at 4 h�1 over NieW/SiO2eAl2O3 catalyst

but it is always <15% over NieMo/Al2O3 catalyst. The results

indicate that by varying the catalyst properties such as acidity

it is possible to obtain higher yields of desired products like

kerosene and diesel, as done in hydroprocessing of petroleum

fractions [30]. The fact that hydrocracking catalysts tend to

form lighter than diesel products has been identified by the

earlier works [6,12,14]. Commercial hydrocracking catalysts

were used in these works, whose compositions were not re-

ported. Studies compared three different commercial cata-

lysts: a mild-hydrocracking, a max-naphtha hydrocracking

and a max-diesel hydrocracking catalyst [12]. The max-

335 340 345 350 355 360 365 370 375 380 38560

65

70

75

80

85

90

95

Car

bon

yiel

d (%

)

Temperature (ºC)

istillation at WHSV 2 hL1 (left) and WHSV 4 hL1 (right) over

% soya oil (C) and, over sulfided NieW/SiO2eAl2O3 catalyst

b i om a s s a n d b i o e n e r g y 5 6 ( 2 0 1 3 ) 4 3e5 248

naphtha catalyst exhibited the highest conversion (w65%) as

well as the highest naphtha selectivity (w50%), as expected. In

all cases over 95% of the contained vegetable oil was con-

verted into lighter and more useful molecules (naphtha, gas-

oline and diesel range) [12]. We observe that the yield of the

250e380 �C fraction (which is the fraction containing C15 to

C18) also clearly increases with increasing soya oil content

over NieW/SiO2eAl2O3 catalyst which is as expected, due to

increased contribution of C15eC18 hydrocarbons coming

from the conversion of the triglycerides to the corresponding

hydrocarbons.

In brief, for the NieW catalyst the 250e380 �C fraction is

much lower than that over NieMo catalyst which is attributed

to themore cracking of this fraction into lower components due

to higher acidity of the former catalyst. Very high yield

(80e90%) for 250e380 �C fraction is obtained usingNieMo/Al2O3

catalysts (Fig. 3) due to very little cracking ability of this catalyst.

Thus these results indicate that it is possible to have

desired throughput during co-processing by varying the space

velocities and reaction temperatures at the same time having

complete triglyceride conversion, which is very important

when the quality of product cannot be compromised in terms

of sulfur content, density, pour point etc.

3.3. Oxygen removal pathways

It is necessary to analyze the effect of reaction conditions,

hydrocarbon chain length and vegetable oil ratio in the feed,

on the relative rate of different oxygen removal pathways. It is

now accepted that deoxygenation of these compounds can

follow three different reaction pathwaysdhydrodeoxygena-

tion, decarbonylation and decarboxylation.:

CnH2nþ1COOHþ 3H2/Cnþ1H2nþ4

þ 2H2O : Hydrodeoxygenation pathway

CnH2nþ1COOH/CnH2nþ2 þ CO2 : Decarboxylation pathway

CnH2nþ1COOHþH2/CnH2nþ2 þH2O

þ CO : Decarbonylation pathway

The distribution of hydrocarbons in the products is affected

by the way of oxygen elimination [31,32]. Hydrocarbons with

one carbon atom less than the original fatty acid chain i.e. C17

and C15 alkanes are the product of oxygen removal from tri-

glyceride by decarboxylation þ decarbonylation (with CO and

CO2 as the side-products), while C18 and C16 alkane hydro-

carbon, with the same number of carbon atoms in the mole-

cule as the corresponding fatty acid chain, from which it was

formed, is the product of hydrodeoxygenation reaction (with

water as the side-product). The extent of both pathways and

the effect of reaction conditions on them can be consequently

elucidated from the liquid hydrocarbon distributions. C17 and

C18 hydrocarbons make up more than 89% of total n-alkanes

from triglycerides with composition of 10.8% palmitic acid

(C16:0), 3.8% stearic acid (C18:0), 28.7% oleic acid (C18:1), 49.9%

linoleic acid (C18:2) and 6.8% linolenic acid (C18:3). We have

taken the C17/C18 ratio of major product alkanes i.e. n-hep-

tadecane (C17) and n-octadecane (C18), as a measure of rela-

tive ratio of decarboxylation þ decarbonylation versus

hydrodeoxygenation and the results are shown in Fig. 4. The

ratio is substantially affected by the nature of the catalyst and

reaction temperatures. Different extents of two different

pathways have been observed and reported as a result of

temperature variation, feed-ratio variation and due to

different catalysts used (NiMo, Ni) [22]. In terms of hydrogen

consumption, hydrodeoxygenation is more demanding than

decarboxylation þ decarbonylation and is therefore preferred

at higher hydrogen pressures. Moreover, more gaseous prod-

ucts are formed during decarboxylation þ decarbonylation

and it is consequently favored at lower reaction pressure. The

hydrogen consumption is an important issue from the in-

dustrial point of view; a process with lower hydrogen de-

mands would be more economical. The selectivity to

decarbonylation/decarboxylation and hydrodeoxygenation

products varies with temperature, type of catalyst used and

with Soya oil content. Since in case of only gas oil the reaction

pathways are completely different from the pathway of Soya

oil, therefore the difference between C17/C18 of gas oil from

its mixture would give an idea about the influence of soya oil

content in gas oil on the reaction pathways. The C17/C18 ratio

for NieMo catalyst is more than 1 at lower vegetable oil con-

tent (10% mass fraction) and rapidly decreases with reaction

temperature reaching towards lower value of 1, the trend

being similar to that for pure gas oil. But at higher vegetable oil

content (25% mass fraction) the C17/C18 ratio is lower than 1

(and is also lower than that for pure gas oil) and increaseswith

increasing temperatures. This implies that hydro-

deoxygenation seems to bemore favored over NieMo catalyst,

and this is clearly observed at higher vegetable oil content in

the feed. Also, with increasing reaction temperature, both

reaction pathways seem to be equally favored at high reaction

temperatures (380 �C). Thus, to have lower hydrogen con-

sumption using NieMo catalyst it would be more favorable to

have lower reaction temperatures during co-processing (as

well as lower vegetable oil content). The same is true at

different space-velocities of 2 and 4 h�1 (Fig. 4). Thus, it may be

possible for a typical diesel hydrotreater to easily co-process

low amounts of vegetable oils (�10%) with their existing

conditions with nominal increase in hydrogen consumption.

There is difference in the C17/C18 ratio for the two types of

catalysts, hydrodeoxygenation being more favored over the

NieMo catalyst (C17/C18 ratio being lower, Fig. 4) than that over

NieW catalyst. Hence NieW might be a better catalyst for

reducing the hydrogen consumption because decarbonylation/

decarboxylation pathway which consumes less hydrogen (as

discussed earlier) is more favored (C17/C18 ratio being lower,

Fig. 4). C17/C18 ratio is higher than that for pure gas oil and

increases with the increasing soya oil content from 25 to 40%

mass fraction in the feed over the NieW catalyst while it is

opposite for the NieMo catalyst at 25% mass fraction soya oil

content. Thus it may be concluded that, the hydro-

deoxygenation pathway which is favored over the NieMo

catalyst becomes more dominant with increasing soya-oil

content in the feed, while decarboxylation þ decarbonylation

pathway which is favored over the NieW catalyst becomes

more dominant with increasing soya-oil content in the feed.

In order to check if the hydrocarbon chain length has some

influence on the oxygen removal pathways we also evaluated

C15/C16 ratios for the hydrocarbons formed by the

335 340 345 350 355 360 365 370 375 380 3850.70.80.91.01.11.21.31.41.51.61.71.8

Deca

rbon

.+De

carb

ox.

Hydr

odeo

xyge

n.

Temperature (OC)

C17/

C18

335 340 345 350 355 360 365 370 375 380 385

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Temperature (OC)

C15/

C16

Hydr

odeo

xyge

n.De

carb

on.+

Deca

rbox

.

340 345 350 355 360 365 370 375 380 3850.5

1.0

1.5

2.0

Hydr

odeo

xyge

n.De

carb

on.+

Deca

rbox

.

Temperature (OC)

C17/

18

NiMoVO10 NiMoVO25 NiMoGO

Fig. 4 e Decarboxylation D decarbonylation versus hydrodeoxygenation measured as C17/C18 ratio (at WHSV 2 hL1, left;

WHSV 4 hL1, right) and C15/C16 ratio (WHSV 2 hL1, bottom) over sulfide NieMo/Al2O3 catalyst with gas oil (:), 10% soya oil

(-), 25% soya oil (C) and, over sulfided NieW/SiO2eAl2O3 catalyst with 25% soya oil (;), 40% soya oil (A) in gas oil.

b i om a s s a n d b i o e n e r g y 5 6 ( 2 0 1 3 ) 4 3e5 2 49

deoxygenation of the palmitic acid (16:0) component of the

soybean oil. The C15/C16 ratio shows similar trend as C17/C18

ratio over NieW catalyst (Fig. 4). But there is slight difference

in the two ratios over NieMo catalyst. With increasing reac-

tion temperature the two reaction pathways tend towards

being equally favored (the C17/C18 and C15/C16 ratios tend to

reach 1.0), but the temperature at which the C15/C16 ratio

tends to reach 1.0 (i.e. when the hydrodeoxygenation and

decarboxylation þ decarbonylation pathways are equally

favored) is lower (360 �C) than that for C17/C18 ratios (380 �C),but this difference is observed only for high (25% mass frac-

tion) vegetable oil content in the reaction feedmixtures. These

results clearly indicate that the carbon chain length influences

the oxygen removal pathway at least over the NieMo catalyst

340 345 350 355 360 365 370 375 380

82

84

86

88

90

92

94

Temperature (OC)

HDS%

Fig. 5 e Hydrodesulurization (HDS%) at WHSV 2 hL1 (left) and WH

oil (:), 10% soya oil (-), 25% soya oil (C) and, over sulfided Nie

and is clearly observed at high vegetable oil content in the

feed.

3.4. HDS performance

Inhibition of deoxygenation and desulfurization might be ex-

pected when both types of compounds (S- and O-containing

compounds) are present at the same time [33]. Fig. 5 shows the

HDS activity of different Soya oil and gas oil mixtures. The

addition of soya-oil does not decrease the HDS activity for gas

oil at 100% triglyceride conversion. This indicates the deoxy-

genation of soya oil and hydrodesulphurization are not

competing reactions and may occur on different catalytic sites

or the number of active sites are large enough for both the

335 340 345 350 355 360 365 370 375 380 385

65

70

75

80

85

90

95

Temperature (OC)

HDS%

SV 4 hL1 (right) over sulfide NieMo/Al2O3 catalyst with gas

W/SiO2eAl2O3 catalyst with 25% soya oil (;) in gas oil.

340 350 360 370 3800

102030405060708090

100HD

S/HD

O %

Temperature (oC)

NiMo25VOHDO NiMo25VOHDS NiW25VOHDO NiW25VOHDS

Fig. 6 e Hydrodesulfurization (HDS%) and

hydrodeoxygenation (HDO%) over NieMo and NieW

catalysts with 25% vegetable oil and 75% gas-oil in feed

mixtures.

b i om a s s a n d b i o e n e r g y 5 6 ( 2 0 1 3 ) 4 3e5 250

reactions to occur simultaneously. Importantly soya-oil addi-

tion does not decrease sulfur removal ability of the catalysts,

but in fact there is slight increase in the HDS rate at certain

temperatures which could be due to a dilution effect for the

gas-oil feed in the presence of vegetable oil and also due to

increased local acidities that might favor HDS reaction [21,29].

The sulfur conversion increases from 86 to 93% as the reaction

temperature increases from 340 to 380 �C for NieMo catalyst at

2 h�1 (Fig. 5 left). In comparison, the conversion of triglycerides

to alkaneswas close to 100%at 350 �C and above indicating that

HDO is amore favorable reaction than HDS (Fig. 6). S removal is

better over NieMo catalyst than that over NieW catalyst.

3.5. Reaction kinetics

Hydrocracking catalysts tend to form lighter along with diesel

products [6,12,14]. A recent catalyst evaluation study for used

oil hydroconversion [34] comparing three different commer-

cial catalystsea hydrotreating, a mild-hydrocracking and a

hydrocracking catalysteshowed the diesel yields over 94%

and >99% saturation, in all cases. The hydrotreating catalyst

was concluded to be the best catalyst for waste cooking oil

hydroprocessing for diesel production [34]. But it would be

necessary to use a hydrocracking catalyst if kerosene is the

desired product. Catalyst composition optimization and

detailed kinetic study is required when targeting different

productsegasoline, kerosene or diesel.

A more recent co-hydroprocessing study for gas-oil with

used cooking oil mixtures [35] using NiMo/Alumina catalyst

Table 3 e Kinetic and thermodynamic parameters for S-removahydrotreated products.

kHDS (s�1, 325 �C) kHDO(s�1, 325 �C) EHDS

25% Veg-oil 4.83 4.253 15

10% Veg-oil 4.66 4.114 17

Gas oil 4.78 e 16

kHDO ¼ rate constant triglyceride conversion. kHDS ¼ rate constant for sul

exponential factor for sulfur removal. EHDO ¼ activation energy for trigly

indicated that the heteroatom removal is favored by increasing

reaction temperature. The cooking oil content in the feedstock

favored conversion. Temperature had no significant effect on

conversion, but hydrogen consumption increased proportion-

ally with hydrotreating temperatures [35].

The activity of the catalysts in deoxygenation can be

described by looking at the conversion of triglycerides, i.e.

evaluating the rate of disappearance of triglycerides which

has been obtained by using the pseudo-first order kinetics to

fit the relevant experimental data for NiMo catalyst at

different reaction temperatures. The catalyst activity in

hydrodesulfurization can be described by looking at the

removal of sulfur containing feed components, i.e. evaluating

the rate of disappearance of S-compounds, using the pseudo-

first order kinetics to fit the relevant experimental data. The

results are presented in Table 3. In order to quantify the dif-

ferences in different activities, the relative pseudo-first order

rate constants have been calculated at 325 �C (Table 3).

The difference between the rates of disappearance of tri-

glycerides (kHDO ¼ 1.24 s�1 at 300 �C) and disappearance of

sulfur (kHDS ¼ 3.86 s�1 at 300 �C) clearly indicates that sulfur

removal reactions are much faster than oxygen removal from

triglyceride at lower temperatures (300 �C). The apparent

activation energy for overall S-removal is nearly 9-times lower

than that for overall O-removal. Near hydrotreating condition

(325e350 �C) the differences in rates of sulfur removal and

triglyceride conversion are not high.

3.6. Other bulk properties of products

3.6.1. DensityDensity of vegetable oil (920 kg m�3) is much higher than that

of the gas-oil (858 kgm�3). After hydrotreating, densities of the

products decreased (Fig. 7) due to hydrodesulfurization,

hydrodeoxygenation and partial hydrogenation reactions. The

density of hydrotreated gas-oil was higher than that for the

hydrotreated vegetable oilegas oil mixtures. The decrease in

densities, was more for higher vegetable oil content feeds and

also at higher hydrotreating temperatures. Densities of all the

products were in the standard range specified for diesel fuel

i.e. 820e860 kg m�3 (at 15 �C).

3.6.2. TAN and cetane numberThe acidity of the various product defined as total acidity num-

ber [TAN,measured as KOH values (g kg�1) in the oil] reduces to

zero after reaction as free fatty acids are completely converted

into alkanes and consequently the TAN reduces to zero even at

reaction temperature as low as 340 �C. This indicates that this

l and triglyceride-conversion alongwith cetane numbers of

kJ mol�1 AHDS (s�1) EHDO kJ mol�1 Cetane no.

.0 (�2.0) 87.36 136.6 (�4.0) 67

.9 (�3.0) 91.8 140.2 (�5.0) 54

.1 (�3.0) 105.24 e 43

fur removal. EHDS ¼ activation energy for sulfur removal. AHDS ¼ pre-

ceride conversion.

335 340 345 350 355 360 365 370 375 380 3850.825

0.828

0.831

0.834

0.837

0.840

0.843

0.846

0.849

Dens

ity (g

cc-1)

Temperature (OC)335 340 345 350 355 360 365 370 375 380 385

0.825

0.828

0.831

0.834

0.837

0.840

0.843

0.846

0.849

Dens

ity (g

cc-1)

Temperature (OC)

Fig. 7 e Densities of hydroprocessed products at WHSVs 2 hL1 (left) and 4 hL1 (right) over sulfide NieMo/Al2O3 catalyst with

gas oil (:), 10% soya oil (-), 25% soya oil (C) and, over sulfided NieW/SiO2eAl2O3 catalyst with 25% soya oil (;) in gas oil.

b i om a s s a n d b i o e n e r g y 5 6 ( 2 0 1 3 ) 4 3e5 2 51

process is suitable for production of hydrocarbons from vege-

table oil feeds containing fairly high level of free fatty acids

which also get converted to corresponding alkanes along with

the triglycerides. But very high TAN (high acidity) feeds with

largeamountof free fatty acidsmight bean issue for the long life

of catalyst as well asmight affect reactor metallurgy.

The product produced by hydrotreating of vegetable oil-

egas oil mixtures has much better cetane number than that

for hydrotreated gas oil (Table 3). With a 10% mass fraction of

vegetable oil in the gas oil mixture the cetane number

increased from the base value of 43e54. Further addition of

25% of vegetable oil to gas-oil increases cetane number of the

diesel product to 67.

4. Conclusion

Mixtures of waste soya oil and gas-oil can be easily converted

into hydrocarbons with high cetane value and acceptable

density and acidity under hydroprocessing conditions used in

refineries using typical hydroprocessing catalysts. NieW/

SiO2eAl2O3 catalyst gives considerable jet range product due

to cracking while NieMo/Al2O3 catalyst has high selectivity

for diesel range product due to minimal cracking. The

hydrodeoxygenation pathway for oxygen removal from tri-

glyceride seems to be favored over the NieMo, while

decarboxylation þ decarbonylation pathway is favored over

the NieW catalyst. The reaction pathways are temperature

dependent. The activation energy for overall O-removal is

higher than that for overall S-removal.

Acknowledgments

The authors thank Analytical Sciences Division, IIP for

analytical services. DST, India is also acknowledged for partial

research funding. BSR acknowledges CSIR, India for Senior

Research Fellowship.

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