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Evaluation of two gas membrane modulesfor fermentative hydrogen separation

J.E. Ramırez-Morales a,*, E. Tapia-Venegas a, N. Nemestothy b,P. Bakonyi b, K. Belafi-Bako b, G. Ruiz-Filippi a

a Escuela de Ingenierıa Bioquımica, Facultad de Ingenierıa, Pontificia Universidad Catolica de Valparaıso,

General Cruz 34, Valparaıso, Chileb Research Institute on Bioengineering, Membrane Technology and Energetics, University of Pannonia,

Egyetemut 10, 8200 Veszprem, Hungary

a r t i c l e i n f o

Article history:

Received 19 June 2013

Received in revised form

14 August 2013

Accepted 21 August 2013

Available online 18 September 2013

Keywords:

Biohydrogen upgrading

Polymeric membrane

Selectivity

SAPO-34 zeolite

PDMS

* Corresponding author. Tel.: þ56 32 2273919E-mail addresses: juan.ramirez.m@mail.p

0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2013.08.0

a b s t r a c t

The ability of (dimethyl siloxane) (PDMS) and SAPO 34 membrane modules to separate a

H2/CO2 gas mixture was investigated in a continuous permeation system in order to decide

if they were suitable to be coupled to a biological hydrogen production process. Permeation

studies were carried out at relatively low feed pressures ranging from 110 to 180 kPa. The

separation ability of SAPO 34 membrane module appeared to be overestimated since the

effect concentration polarization phenomena was not taken into consideration in the

permeation parameter estimation. On the other hand, the PDMS membrane was the most

suitable to separate the binary gas mixture. This membrane reached a maximum CO2/H2

separation selectivity of 6.1 at 120 kPa of feed pressure. The pressure dependence of CO2

and H2 permeability was not considerable and only an apparent slight decrease was

observed for CO2 and H2. The mean values of permeability coefficients for CO2 and H2 were

3285 � 160 and 569 � 65 Barrer, respectively. The operational feed pressure found to be

more adequate to operate initially the PDMS membrane module coupled to the fermen-

tation system was 180 kPa, at 296 K. In these conditions it was possible to achieve an

acceptable CO2/H2 separation selectivity of 5.8 and a sufficient recovery of the CO2 in the

permeate stream.

Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction on hydrocarbons, reduce carbon dioxide liberations and

Since 20th century the world has continuously faced a rapid

economic growth at the expense of high energy demands and

environmental pollution. In order to deal with these two

major problems intense research has been focusing on eco-

friendly energy alternatives from renewable sources and

depleting CO2 emissions derived from fossil fuels. Hydrogen is

one of themost attractive alternatives to alleviate dependence

.ucv.cl, jeramir3@gmail.c2013, Hydrogen Energy P92

mitigate other environmental concerns. Themain advantages

of hydrogen as a clean energy carrier are found during its

transformation to electricity through fuel cells, where it is

possible to achieve high exergy efficiencies and produce only

water as product [1e3]. There are various ways to produce

hydrogen from fossil fuels among which thermocatalytic and

gasification processes operated at high pressure and temper-

ature are currently dominant, accounting for 96% of global

om (J.E. Ramırez-Morales), gonzalo.ruiz@ucv.cl (G. Ruiz-Filippi).ublications, LLC. Published by Elsevier Ltd. All rights reserved.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 4 0 4 2e1 4 0 5 2 14043

production. Only the remaining 4% is produced via water

electrolysis [4].

In recent years, hydrogen production from renewable re-

sources and particularly by biological process has taken great

interest due to the possibility of using organic wastematerials

and no need of high energy inputs. One of these processes is

based on the fermentative step of anaerobic digestion, which

has presented high hydrogen productivity when it is operated

in continuous mode [5,6]. The biogas produced during this

process is composed mainly of H2 and CO2 in different pro-

portion depending on the operational conditions. This gas

mixture is not feasible to directly be used in fuel cells, inwhich

the hydrogen purity is primordial for obtaining the highest

power production [7]. Furthermore, in the fermentative pro-

cess the continuous and selective removal of H2 is critical,

since the increasing concentration (partial pressures) of this

component affects the process performance. There are

different studies reporting on diverse strategies to reduce the

H2 partial pressure, among them gas sparging and vacuum

application are themostly used [8,9]. However, thesemethods

consume remarkable amount of energy or produce a diluted

product in the outputs of the system. Therefore, it is necessary

to establish an efficient fermentative hydrogen generation

process and obtain electric energy trough fuel cells. To attain

this aim a separation technology capable to separate the

gaseous mixture of H2/CO2 and at the same time able to

selectively remove hydrogen from the biological system is

required, which is schematically shown in Fig. 1. Membrane

technology is an excellent alternative to achieve this purpose,

offering the opportunity to be connected to the fermentative

process, establishing thus a hydrogen extractive membrane

bioreactor (HEMB). Compared to the conventional hydrogen

separation processes which are highly energy intensive,

membrane technology shows great potential since it is

compact, portable, environmentally friendly and exhibits

higher energy efficiencies and simpler mode of operation [10].

The gas separation process takes place through a thin film

(membrane) driven by pressure difference, where gas mixture

is separated by porous or non-porous membranes. Two

important parameters in the performance and efficiency of

Fig. 1 e Scheme of the integrated system for fermentative

hydrogen production, separation and utilization.

membranes are measured in terms of permeation rate (per-

meance) and selectivity toward the components of the

mixture. Hydrogen selective membranes can be classified as

polymeric (organic), inorganic (metallic and non-metallic),

supported-liquid/facilitated transport and mixed-matrix

(hybrid) membranes. Generally, inorganic membranes show

superior gas separation performance in terms of selectivity

and/or permeability, excellent chemical resistance and ther-

mal stability. However, these membranes are brittle, less

commercially available, costly to fabricate and require high

amounts of energy to operate [10]. In this respect, metallic

membranes (e.g. made of palladium) are usually utilized in

steam reforming process where wateregas shift membrane

reactors (WGS-MR) promote thermodynamically favorable

conditions to continuously remove the hydrogen produced.

For this reason and taking the advantage of the high tem-

perature and pressure achieved in this process, these mem-

branes are justified [11]. Nevertheless, reactions in biological

processes occur close to ambient conditions and therefore

metallic membranes are impractical. In contrast, other ma-

terials such as polymer and zeolite based membranes have

been tested for the separation of H2/CO2 mixture under less

extreme circumstances with promising results [12e15]. This

opens the possibility to use them in the continuous extraction

and separation of fermentative hydrogen. Most of the studies

about permeation tests for membrane characterization were

performed on permeation cells under extremely controlled

conditions and special designs. However, the permeation

studies on membrane modules for practical application are

rare. The investigations on the use ofmembranes in biological

hydrogen purification have been developed with dense poly-

meric membranes in contact with CO2 liquid absorbents

(contactors) [16,17], supported ionic liquidmembranes (SILMs)

[18] and porous or non-porous polymeric membranes [19,20].

Nevertheless, these likely suitable membranes have not been

considered for the selective removal of hydrogen in order to

reduce its negative effect on the biological production process

which is certainly another important purpose.

In this study two membrane (polymer and zeolite based)

modules were subjected to permeation tests in order to eval-

uate the viability to separate the binary mixture of H2/CO2 at

different pressures and reveal their appropriateness for being

coupled to a biological hydrogen production process. Perme-

ation tests were performed at ambient temperature and at

different low trans-membrane pressures, taking into account

the practical and achievable conditions to be used in an in-

tegrated lab-scale extractive fermentation system in a future

work.

2. Materials and methods

2.1. Membrane modules

The two lab-scale membrane modules evaluated in this study

and their structural characteristics are presented in Fig. 2 and

Table 1, respectively. The first module consisted of hollow

fiber of commercially available siliconeerubber membrane,

Permselect PDMS-XA 2500 (MedArray Inc., USA), Fig. 2(A). This

dense membrane was made of poly (dimethyl siloxane)

Fig. 2 e Lab-scale membrane modules utilized and made of

PDMS (A) and SAPO-34 (B) materials.

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(PDMS) with an area of 2500 cm2, 3200 fibers and had an inner

and outer diameter of 190 and 300 mm, respectively, resulting

in a wall thickness of 55 mm. The shell and fitting materials of

this membrane module were polycarbonate and the potting

material was polyurethane. The secondmodulewith a tubular

shape consisted of a silicoaluminophosphate (SAPO 34)

microporous zeolite membrane. The membrane had an

effective surface area of 22 cm2 and the length of the ceramic

support tube was 3.5 cm. The module shell was made of glass

and epoxy was used as potting material.

2.2. Gas permeation tests

Mixed gas permeation tests were evaluated using a contin-

uous flow setup. Fig. 3 shows the experimental scheme.

The separation ability of each membrane was investigated by

feeding a binary mixture composed of 10% (V V�1) of H2 and

90% (V V�1) of CO2. Different trans-membrane pressures

ranging from 10 to 80 kPa were examined in order to evaluate

the membranes properties such as permeance, permeability

and selectivity (separation selectivity). All experiments were

carried out at room temperature. Feed flowwas introduced on

the lumen side of membranes and the trans-membrane

pressures were controlled with a backpressure regulator.

Pressure in the retentate and permeate was measured by pre-

calibrated pressure sensors located in the respective streams

and connected to an online data acquisition (DAQ) system

with LabView software. All process lines had rubber septums

for gas sampling. The composition of feed, permeate and

retentate was analyzed by injecting the sample into a gas

chromatograph (PerkineElmer Clarus 500) equipped with a

thermal conductivity detector (TCD) and a Haysep Q 80/100

column (Alltech Co., USA). The temperatures of the injector

and detector were kept at 353 and 393 K, respectively, while

the temperature of the column began at 303 K with a ramp up

to 353 K. Helium was used as a carrier gas at a flow rate of

15 ml min�1. Permeate and retentate flows rates were

measured in two water displacement systems adapted with

an electronic balance, Fig. 3. Thewater volume displaced from

the inverted cylinder to the container was collected in a vessel

placed on an electronic balance. The flow rates were calcu-

lated by dividing the volume (obtained from themass of water

and its density of 1 g cm�3) over a determined time. All mea-

sures of composition and flow rates were taken into account

Table 1 e Structural characteristics of lab-scalemembrane modules studied.

Item Membrane modules

PDMS SAPO-34

Area 2500 cm2 22 cm2

Inner diameter 190 mm NA

Outer diameter 300 mm NA

Thickness 55 mm NA

Number of fibers/tubes 3200 1

Length of fiber/tube NA 3.5 cm

Shell Polycarbonate Glass

NA: Not available.

for the determination of the permeation parameters once the

continuous system reached the corresponding steady-state.

To characterize the separation properties of the mem-

branes various parameters can be determined from the com-

positions, pressures and flow rates measurements. For non-

porous dense materials, the permeability of a gas compo-

nent i through the membrane can be calculated as:

Pi ¼ Qpyi

Dpln;iAl ¼ Ni

Dpln;il (1)

where Pi (Barrer) is the permeability of a gas component i, l

(cm) the thickness of the dense layer, Qp (cm3 s�1) the gas flow

rate in the permeate stream, yi the mole fraction of compo-

nent i in the permeate, A (cm2) the effective permeation area

and Ni (cm3 (STP) cm�2 s�1) is the steady-state flux of the

component i. A log-mean pressure drop (DPln,i, cmHg) was

used as the driving force according to a cross-flow design of

the modules studied. It was defined as:

Dpln;i ¼�pf;i � pp;i

���pr;i � pp;i

�

lnh�

pf;i � pp;i

�.�pr;i � pp;i

�i (2)

where pf,i, pp,i and pr,i (cmHg) are partial pressures for compo-

nent i in feed, permeate and retentate sides, respectively. The

permeability was expressed in units of Barrers

(1 Barrer ¼ 10�10 cm3 (STP) cm cm�2 s�1 cmHg�1 ¼ 3.35 � 10�16

mol m m�2 s�1 Pa�1). Alternatively, when the thickness is

difficult to be defined the permeance of a gas component i (Pi/l)

is used. In this study the permeance (mol m�2 s�1 Pa�1) was

used to evaluate the performance of the zeolite based

membrane.

Fig. 3 e Continuous flow setup used for mixed gas permeation tests.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 4 0 4 2e1 4 0 5 2 14045

The gas permeation across dense polymer membranes is

based on the solution-diffusion transport mechanism and the

permeability of a gas is given by:

Pi ¼ Di � Si (3)

where Di (cm2 s�1) and Si (cm3 (STP) cm�3polymer cmHg�1)

represent the diffusion and solubility coefficients of compo-

nent i, respectively.

The measure of the effectiveness in membrane separation

is given by the selectivity. The ideal selectivity, aideal,i/j is

usually used in single-gas permeation tests. This factor is

defined as the ratio of the individual permeances (perme-

abilities) of each pure gas:

aideali=j ¼ Pi

Pj(4)

However, during mixed gas experiments a second compo-

nent can alter the transport properties into the membrane

material leading to wrong measures of the membrane per-

formance. In these cases, the separation selectivity, asep,i/j

gives a rather accurate measure of the membrane selectivity.

For a gas mixture of two components i and j, the separation

selectivity can be defined as the ratio of the permeabilities or

permeances as [14,21]:

asepi=j ¼ Pi

Pj¼

�Ni=Dpln;i

��Nj=Dpln;j

� (5)

Themembrane stage-cut, defined as the fraction of feed gas

that permeates the membrane, is a measure of the degree of

separation required. This parameter was calculated as:

q ¼ Qp

Qf(6)

where q is the membrane stage-cut. Qp and Qf (cm3 (STP) s�1)

are the gas flow rates in the permeate and feed stream,

respectively.

3. Results and discussion

3.1. Effect of feed pressure on separation performance ofthe two membranes

The effect of feed pressure onH2 and CO2 permeability andH2/

CO2 selectivity was investigated using silicone rubber (PDMS)

and SAPO-34 zeolite based membranes once the permeation

process reached the steady-state. Temperature was set to

296 K, and the different feed pressures ranged from 110 to

180 kPa, maintaining the permeate pressures at approxi-

mately 101.3 kPa. For all tests, the composition of feed binary

mixture of H2/CO2 was 10/90% (V V�1). Silicone rubber mem-

branes have been utilized for different gas separation pur-

poses and some studies have assessed the performance of H2

and CO2 in synthetic mixtures [22]. In addition, some studies

have evaluated the effect of wide ranges of temperature and

pressures for SAPO-34 zeolite membranes [14,15]. However, in

the present work permeation tests were conducted under

mild conditions (using low pressures in a narrow range),

considering the objective of practical membrane coupling in a

lab-scale fermentative process. The concentrations of H2 in

retentate and permeate for each membrane at the different

pressures evaluated are presented in Fig. 4.

For PDMS membrane, hydrogen concentration increased

with increasing feed pressure in retentate, while in permeate

an opposite behavior was observed. Therefore, the results

indicated that H2 was enriched in the retentate stream,

reaching a final concentration of 14.68% (V V�1) from the

Fig. 4 e Hydrogen concentrations at the different pressures evaluated in retentate (C) and permeate (B) for PDMS (A) and

SAPO-34 (B) membranes.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 4 0 4 2e1 4 0 5 214046

initial 10% (V V�1) of feed concentration under a feed pressure

of 180 kPa. This relationship with a degree of proportionality

between concentration and pressure applied (that is, while

higher pressure is applied to the membrane, higher is the

concentration of the different species permeated selectively)

is typically found in rubbery dense polymers [23]. From a

practical standpoint, this relationship could involve increases

on hydrogen concentration managing the operational pres-

sure of the membrane system. However, implementing an

effective hydrogen process obtained from anaerobic digestion

not only implies high product purity, but also involves a

considerable product recovery from the biogas generated in

the process, see next section. In SAPO-34 membrane, it was

expected that H2 concentration in permeate was getting

higher than the corresponding feed concentration, since it

was observed that its concentration has decreased in reten-

tate. This behavior could be explained based on permeation

mechanisms occurred in this membrane material and taking

into account permeate and retentate flows established in each

pressure condition, see next section.

The effect of feed pressure on CO2 and H2 permeability for

PDMS membrane and CO2 and H2 permeance for SAPO-34

Fig. 5 e Effect of feed pressure on H2 (:) and CO2 (,) permeabilit

membrane are shown in Fig. 5. As it is illustrated in Fig. 5(A),

the pressure dependence of CO2 and H2 permeability is not

significant and only an apparent linear decrease is observed

for CO2 and H2. Similar results were found by Sadrzadeh et al.

[24], who explained this behavior by interaction between three

factors; plasticization, hydrostatic pressure and penetrant

solubility. Taking into account the low pressures reached,

where plasticization is negligible, the slight decrease of CO2

andH2 permeability is explained because of the polymer could

be compacted or compressed with increasing pressure,

resulting in affected diffusivity of these molecules. Further-

more, this behavior and results obtained may be attributed to

the solution-diffusion transport mechanism, as it is described

in Eq. (3), where permeability is the product of diffusion and

solubility coefficients in the membrane material. Although in

the separation process of H2/CO2 mixture applying the PDMS

membrane the diffusivity of H2 is higher than that of CO2 due

to its small kinetic diameter, the solubility effect is the main

phenomena affecting the general permeation process. In this

case, the sorption capability of CO2 is greater in comparison to

H2 because of its higher critical temperature and consequently

it is more easily condensed. For this facilitated sorption of CO2

y and permeance in PDMS (A) and SAPO-34 (B) membranes.

Fig. 6 e Comparison of the kinetic diameters of different

gas molecules and the pore sizes of SAPO 34 and ZIF-7

zeolites.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 4 0 4 2e1 4 0 5 2 14047

and especially due to its high solubility in rubbery polymers,

CO2 permeability is higher as it is for H2, which is displayed in

Fig. 5(A). The mean values of permeability coefficients for CO2

and H2 were 3285 � 160 and 569 � 65 Barrer, respectively. This

remarkable difference between permeabilities creates a high

potential for PDMS membrane module as an excellent candi-

date to separate H2 and CO2, commonly generated during the

fermentation process. Furthermore, this permeability values

are in good agreement with those reported in the literature,

even during pure gas permeation tests or different gas

mixture compositions, which were performed on permeation

cells under extremely controlled conditions and special de-

signs. Permeability of pure CO2 and H2 has been determined

by Merkel et al. [22], where the values published were as 3200

and 950 Barrer, respectively, at 296 K and 138 kPa of feed

pressure. For a pure CO2 gas permeation, Jha et al. [25] re-

ported permeability value of 2645 Barrer at 295 K and DP of

100 kPa. The similarity of these values with pure gases to

those demonstrated in the presentwork indicates that there is

not a strong competition between CO2 and H2 under the

conditions (temperature and pressures) focused. For the case

of a research working with a H2/CO2 mixture (20% CO2, 20% H2

(V V�1), balance Ar), Barillas et al. [12] reported CO2 and H2

permeabilities values of 2848 and 813 Barrer, respectively.

The apparent influence of feed pressure on CO2 and H2

permeances for SAPO-34membrane is appeared in Fig. 5(B). In

this figure it can be seen that in general higher pressure

increased the CO2 and H2 permeance, except the value for H2

at 120 kPa feed pressure. At the different DP applied, the H2

permeances were higher than CO2 ranging values from

2.3 � 10�7 up to 5.0 � 10�7 mol m�2 s�1 Pa�1. In case of CO2,

permeance increased from 0.7 � 10�7 to

1.7 � 10�7 mol m�2 s�1 Pa�1. Similar results have been found

by Li [26], who investigated the separation of CO2/CH4 mixture

and reported CO2 permeance values of

1.6� 10�7molm�2 s�1 Pa�1 at 297 K and DP of 84 kPa. However,

different results were obtained for the separation of a CO2/H2

mixture in a non-modified SAPO-34 membrane by Das et al.

[15]. The authors found CO2 and H2 permeances of 12 and

10.2 � 10�7 mol m�2 s�1 Pa�1 at 303 K and feed pressure of

200 kPa, respectively. From Fig. 5(B), it seems to be substantial

difference between permeances of CO2 and H2, implying an

apparent good separation ability of SAPO-34membrane under

all conditions tested. The increased H2 permeance value could

be explained from its smaller kinetic diameter and adsorp-

tionsmechanisms. The permeation throughmicropores of H2/

CO2 gas mixture in SAPO 34 membranes is a competitive

adsorption and diffusion process but their combination de-

terminates the separation efficiency. The diffusion rate

decrease when the kinetic diameter of the gas is larger than

the size of the zeolite pores and a molecular sieving effect is

observed. As it can be seen in Fig. 6, the kinetic diameters of

CO2 and H2 are 0.33 and 0.29 nm, respectively. These sizes are

smaller than the pore size of SAPO 34 zeolite, which corre-

sponds to 0.38 nm, so a complete molecular sieving mecha-

nism is not possible, as can occur in other zeolites like ZIF-7

which has a pore size of 0.30 nm [27]. However, in SAPO 34

zeolite, H2 have a smaller kinetic diameter and permeate

faster than CO2. In case of H2/CO2 mixtures under increased

feed pressure, adsorption mechanisms become important. In

SAPO 34 zeolite, CO2 is adsorbed more strongly than H2,

resulting in an increased H2 permeance value [15]. Although

the high potential of these materials for hydrogen separation,

most of these were prepared on planar disks, which are not

well suited to large-scale applications. Under a practical point

of view, modules with tubular or hollow fibers supports must

be evaluated in order to prove their applicability on the

fermentative process. Recently, Pan et al. [28] reported success

in preparing ceramic hollow fiber-supported ZIF-8 mem-

branes, which opens the possibility to develop more realistic

permeation tests in order to implement a future real

application.

The permeabilities and permeances values shown in Fig. 5

must be discussed along with other factors like the different

stage-cut established under the feed pressures used. Further-

more, permeation tests were not performed under perfect

mixing conditions since these were carried out in hollow fiber

and tubular membranes and not in a permeation cell. Discus-

sion about the performance of the membrane modules along

with the stage-cuts values is given in the next section.

3.2. Comparison and evaluation of the membranemodules based on membrane configuration and operationalconditions

Mixed gas measurements are commonly performed on

permeation cells. These special devices have a particular design

that ensures goodmixing in the gas phase above and below of a

flat membrane. Furthermore, high feed flow rates are used to

improve mixing efficiency on the feed side preventing con-

centration polarization. Permeability and selectivity values are

then obtained under ideal and fixed conditions. Taking into

account the practical and achievable conditions of an inte-

grated lab-scale extractive fermentation system, permeation

studies should directly be performed on membrane modules

under the corresponding operational requirements. Thereby,

permeation parameters of membranes could be affected by a

specific mode of operation (i.e., considerable permeation flow

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 4 0 4 2e1 4 0 5 214048

rates) [29]. The comparison and evaluation of the membranes

tested must be based on membrane configuration and the

operational conditions used.

Membrane selectivity described by the separation selec-

tivity (Eq. (5)) and stage-cut (Eq. (6)), are also relevant factors to

determinate or explain the performance of a membrane gas

separation and process design. Fig. 7 shows the established

separation selectivities and the stage-cuts for the two mem-

branes at different feed pressures. As shown above, PDMS

membrane was CO2-selective and the observed CO2/H2

selectivity reached a maximum value of 6.1 at 120 kPa of feed

pressure (Fig. 7(A)). Despite, in the next two feed pressures

applied, this factor was slightly affected attaining a final CO2/

H2 selectivity of 5.8 at 180 kPa feed pressure. In Fig. 7(B), the

established stage-cuts under the different feed pressures

focused for the two membranes are presented. The stage-cut

is a system attribute that characterizes the performance in a

gas separation process. Small stage-cuts in membrane oper-

ation yields pure permeates but small fractions of product

flows, while large stage-cuts yields a high permeate flows

fractions with reduced purity [30]. During mixed gas mea-

surements performed on permeation cells, very small stage-

cuts (w0.001) are used to improve mixing efficiency. Howev-

er, modules (i.e., hollow fiber membranes modules) operated

in perfect mixing mode are not common in gas separation

applications [31]. For this reason, and considering the mem-

brane technology application for upgrading fermentative

hydrogen, the established stage-cuts values shown in Fig. 7(B)

were higher.

Although the selectivities and stage-cuts have a modest

variation under the different pressures in the PDMS mem-

brane, its performance seems to be more accurate at feed

pressures of 120 and 140 kPa. These sets provided the highest

CO2/H2 selectivity values where the corresponding stage-cuts

were close to 0.5, while lower selectivities were coupled to

stage-cuts of approximately 0.6. However, taking into account

the trade-off in gas membrane separation between recovery

and purity, the most suitable operation condition in terms of

feed pressure is 180 kPa with a stage-cut of 0.57. This value

would appear to ensure significant recovery of the CO2 from

the feed stream, which would correspond to the gas outlet of

Fig. 7 e Separation selectivities for SAPO 34 (:) and PDMS (-) m

used for SAPO 34 (>)and PDMS (C) membranes (B).

the fermentation reactor. Furthermore, themembrane system

was able to reach a quite high purity of CO2 (94.3%VV�1) in the

permeate stream (data not shown, but inferred from Fig. 4(A)).

These results indicate the positive effect of pressure on the

performance of themembrane considering the achievable gas

purity and the slight variation of the stage-cuts close to 0.5. On

this occasion, feed pressure has a direct effect on the CO2

concentration within membrane. Therefore, according to the

results obtained and pressures range evaluated, the separa-

tion performance of the PDMS membrane is determined

mainly by the pressure ratio (described by the ratio between of

feed pressure (Pf) to permeate pressure (Pp)) across the mem-

brane, which is positive with respect to operational aspects

and performance of the extractive separation system. In this

case, managing pressure around the evaluated values and

establishing stage-cuts around 0.5, it could be possible achieve

a desired composition without considerable selectivity

changes. However, to achieve this, an appropriate pressure

and flow control strategies must be implemented since

fermentative process could present instabilities due to

external disturbances.

For feed pressures of 120, 140 and 180 kPa, the established

stage-cuts in the SAPO-34 membrane were distinct to 0.5, the

region where it is expected to have a considerable recovery

factor and a reasonable purity. However, as it is shown in

Fig. 4(B), composition of H2 in the permeate side seems to be

constant. This behavior occurs due to the high permeation

flows according to the high stage-cuts established, making it

difficult an increase in the H2 concentration. In this case, the

feed and permeate sides have similar H2 concentrations. As

described above, the separation capability of the SAPO 34

membrane seems to be satisfactory due to differences be-

tweenH2 and CO2 permeances (Fig. 5(B)). Consequently theH2/

CO2 separation selectivities established have a considerable

value (Fig. 7(A)). However, it is evident from Fig. 5(B) that the

SAPO-34 permeances are related to its stage-cut profile

(Fig. 7(B)). In this case, because of the high flows established in

the permeate side and the tubular module configuration used,

concentration polarization could take place. Furthermore,

with the increase of stage-cut, concentration polarization

would become much severe. This phenomena has been

embranes (A). Stage-cuts established underfeed pressures

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 4 0 4 2e1 4 0 5 2 14049

studied byWang et al. [31] in a CO2/CH4 mixed gas permeation

through hollow fiber membranes. The authors studied the

influence of pressure drop within the hollow fibers, non-ideal

gas behavior in the mixture and concentration polarization.

Considering these behaviors into a mathematic model, they

could provide more accurate results in the characterization. If

the effect of polarization is not taken into consideration, they

conclude that the calculated permeance would be under-

estimated for CO2 and overestimated for CH4. In our case, an

underestimated CO2 permeance and an overestimated H2

permeance could take place because concentration polariza-

tion was not considered in the permeance estimations.

Therefore, H2/CO2 separation selectivities would be over-

estimated, too. For the feed pressure of 120 kPa the effect of

stage-cut on concentration polarization would be less and

consequently CO2 and H2 permeances yields similar values

compared to literature. As a result, a H2/CO2 separation

selectivity value of 1.68 was obtained. For a practical appli-

cation of this type of membrane, it is necessary the adequate

design ofmodules to reduce concentration polarization effect,

given that considerable permeate flows should be obtained for

an efficient hydrogen production from a fermentative process.

In Table 2 different selectivities found in literature for the

separation of H2/CO2 mixture through PDMS and SAPO 34

membranes are listed. For PDMS membrane, the reported

CO2/H2 separation selectivities values in literature are lower

with those found in the current study. The discrepancies lie in

the membrane performance estimation, since selectivity

literature values were obtained during tests on permeations

cells under ideal conditions. As discussed above, due to the

configuration of the PDMS membrane module (hollow fiber)

and the stage-cuts established, the effect of concentration

polarization would be taken into account during permeability

estimation, avoiding a possible overestimation of CO2/H2

separation selectivity. In the case of SAPO 34 membrane,

similar consideration must be taken. In SAPO-34 membranes,

the selectivity data found in literature for the separation of H2/

CO2 under similar pressure and temperature conditions

correspond to pure gas permeation tests (ideal selectivities).

Das et al., 2012 [15], evaluated the separation of H2/CO2

mixture through SAPO 34 membrane on non-modified and

chitosan modified clay-Al2O3 support and found that the

application of cationic polymer chitosan as an intermediate

layer helped in the formation of uniform seeded layer. With

this modification the H2/CO2 selectivity increased from 0.85 to

5.88. In addition to the improvements on the formation of

Table 2 e Comparison of selectivity values for the separation of

Membrane material Temperature (K) Pressure (

Poly (dimethyl siloxane) (PDMS) 296 138

Poly (dimethyl siloxane) (PDMS) 310 e

Poly (dimethyl siloxane) (PDMS) 296 180

SAPO 34 303 200

SAPO 34/clayealumina 303 200

SAPO 34 296 120

PDMS/zeolite 4A 308 700

homogenous SAPO 34 membrane layers, recently, other

studies reported on the development of mixed matrix mem-

branes enable one to achieve increased selectivity values

[32,33]. However, modules of these newmembranes should be

fabricated with the purpose of validating in practical

applications.

Comparing the performances of the two membranes used,

the PDMSmembrane reflects more suitable characteristics for

the separation of the H2/CO2 binary mixture and moreover, it

is easier to be operated than the SAPO 34 membrane. The

PDMS membrane showed a considerable CO2/H2 separation

selectivity for all the feed pressures set, allowing one to obtain

a retentate rich in H2 and a significant recovery of the CO2 in

the permeate stream, under low overpressures and tempera-

tures commonly found in the anaerobic process. Under these

same operational conditions established, SAPO 34 membrane

seems to overestimate its separation capability due to con-

centration polarization phenomena. In addition, during SAPO-

34 permeation tests there were operational difficulties to

establish constant feed pressures and reach a steady state in

gas flows and compositions. This could be explained due to

some unfavorable aspects related to membrane quality,

commonly presented in SAPO-34 membranes. In literature

several of these aspects have been studied and some include

material defects or presence of non-zeolitic pores due to the

lost of stability under certain storage conditions [26]. Addi-

tionally, water adsorption blocking SAPO pores is also an

important issue [34]. However, due to the potential application

of this type of membranes in H2 gas separations from other

light gases, more studies are needed. It is necessary to eluci-

date possible defects during formation of the zeolite layer on

porous tubular supports, storage conditions and humidity

effect on the material. Also, other operational conditions like

temperature and higher pressures could be evaluated. As

mentioned above, it is important to consider in the method-

ology of permeability estimation, the influence of pressure

drop within tubular configurations, non-ideal gas behavior in

the mixture and concentration polarization. A method to

decrease the effect of polarization boundary layers near the

zeolite membrane at high pressures were performed by Avila

et al. [35]. When teflon spacers were inserted inside the

membrane tube they increased the CO2/CH4 selectivity by

about 160%. Advances in module designs will play an impor-

tant role during the implementation of membrane technology

for processes with non-extreme operating conditions, like

fermentation cultures.

H2/CO2 mixture of the twomembranes to literature values.

kPa) CO2/H2 separationselectivity

H2/CO2 idealselectivity

Reference

3.36 e [22]

3.5 e [12]

5.8 e This study

e 0.85 [15]

e 5.88 [15]

1.68 e This study

e 3.41 [32]

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 4 0 4 2e1 4 0 5 214050

3.3. Application of membrane technology forfermentative hydrogen separation

Similar biogas upgrading techniques such as absorption,

adsorption and cryogenic separation could be applied for

fermentative hydrogen upgrading, since the mayor need in

both processes is to remove CO2. Although these technologies

have considerable efficiencies yielding a high pure product,

they require significant amounts of energy and large size

equipments. Membrane technology could be able to address

these drawbacks. Compared to conventional technologies,

membranes based-process for methane separation has lower

yield efficiencies and product purity [36]. However, the

development of newmaterials and optimal process design are

playing a great role to compete with the conventional process.

This fact is boosting membrane technology for being applied

in various biogas producing plants [37]. A similar situation

happens with hydrogen membrane separations due to ad-

vances and developments in new materials with high selec-

tivities. Investigations on technologies for biological hydrogen

have been focused on separation of mixtures containing H2,

CO2 and N2 using different types of membranes. Comparison

between different membrane technologies used for biological

hydrogen separation is shown Table 3. In terms of recovery

efficiency, membrane contactor technology demonstrated

that can provide effective recovery of hydrogen with 90% pu-

rity [16]. However, more studies are needed to elucidate the

stability of the system when the device is operating in

continuous mode and fed with a realistic biogas stream.

Implementation of membrane technology to fermentative

hydrogen separation will depend on proper selection of

membrane materials, an adequate module (configuration and

design) and integrated process design, taking into account the

operational conditions technically and economically achiev-

able. Compared to SAPO 34 membrane and due to the more

reliable separation ability and operation performance in the

continuous permeation system, PDMS membrane can more

potentially be coupled to a biological hydrogen production

process, in order to partially separate the biogas produced,

composed of mainly H2 and CO2. From the results obtained, it

is demonstrated that PDMS membrane module could divide

the feed stream introduced from the biogas production line (to

be more specific from the fermentative system) into two

streams. The first stream is a retentate with an increased H2

content compared to the feed, with the possibility to be

enriched in this gas in later separation steps for fuel cell

Table 3 e Membrane technologies used for biological hydroge

Membrane separation technology Advantages

Membrane contactor Gas compressor or vacuu

pump are not used

High hydrogen recovery

Supported liquid membranes Selective carrier transpor

Non-porous polymeric membrane Membrane modules

commercially established

applications. The second stream represents permeate, with a

relatively increased CO2 concentration in comparison to the

initial gaseous mixture to be separated. This stream can be

recirculated to the gas phase of the fermentative culture to

dilute the H2 present and decrease the H2 partial pressure.

This could lead to the suppression of its negative influence on

the hydrogen producing bacteria. Furthermore, permeate

recirculation helps to enhance H2 recovery, becoming a more

efficient process. From the above mentioned, PDMS mem-

brane (material and module selected) is able to be imple-

mented into the membrane separation system mentioned in

the conceptual process design (hydrogen extractive mem-

brane bioreactor, HEMB) and showed in Fig. 1. Considering the

practical and achievable conditions to be used in a future in-

tegrated lab-scale extractive fermentation system, the oper-

ational feed pressure of 180 kPa was found to be feasible for

the PDMS membrane at 296 K. At these circumstances it is

possible to achieve acceptable CO2/H2 separation selectivity

(5.8) and recovery (stage-cut of 0.57) of the CO2 to be contin-

uously recirculated into the bioreactor.

The next focus research must be addressed to the contin-

uous lab-scale separation of fermentative hydrogen. In this

case, it is necessary to take into account the presence of

different components in the biogas produced. Depending on

the biological process performance and existence of com-

pounds in the liquid phase (i.e. detergents, proteins, sul-

phates), considerable amounts of volatile siloxanes, NH3, CO

and H2S could be present in raw biogas. In addition, biogas is

saturated with water vapor when it leaves the fermentation

process. This group of substances has a negative effect on the

membrane process itself and also may cause corrosion or

catalyst poisoning during the subsequent fermentative

hydrogen utilization processes (i.e. engines, fuel cells).

Regarding membrane process, water content increases the

mass transfer resistance of the membrane, may induce

change in the transport of the components present in the

mixture and provide support for biofilm formation. The

presence of H2S affects CO2 permeation and it may cause

changes in the polymeric structure of the membrane,

decreasing its performance and shortening its lifetime.

Considering the case of PDMS membranes, Scholes et al. [39]

found a decrease of CO2 permeability caused by exposure to

CO, H2S due to competitive sorption of these gases into the

polymeric matrix. The removal of these interference compo-

nents could take place during adsorption process on activated

carbon or absorption inwater or chemicals. Thewater content

n separation.

Disadvantages Reference

m Need of heaters and coolers

Increase in mass transfer resistance

due to membrane wetting

[16,17]

t Low stability at high pressures

Low membrane lifetime

[18]

Plasticization (at higher CO2

partial pressures)

Sensible to H2O and H2S

[19,20,38]

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 4 0 4 2e1 4 0 5 2 14051

can be removed using condensationmethods or adsorption on

components dryers like silica. Furthermore, membranes have

also been employed as removal methodology too. A PDSM

membrane module (as we used in the present study) was

evaluated by Ajhar et al. [40]. They demonstrated success in

siloxane removal using ambient air as sweep gas. Taking into

account the pressure requirements and removal of unwanted

components in raw biogas, the continuous lab-scale extrac-

tive fermentation (Fig. 1) must be implemented with water or

chemical traps, feed pressure pumps and effective pressure

control strategy to avoid vacuum or overpressures in the

headspace of the biological reactor.

The applicability of the gas extractive fermentation pro-

cess based on this PDMS membrane should be further

assessed under versatile operational conditions and by

various membrane module configurations e.g. multi-step and

recycle designs, in order to optimize the overall hydrogen

generation system. From the view-point of large scale appli-

cations, the extractive apparatus should be operated near to

the typical conditions required for the fermentative step

letting not large amounts of energy to be consumed and the

spending on expensive pumps. For this case, the advances

and improvements in new membranes with high selectivities

like modified polymers [10] or mixed matrix membranes [41]

also should be considered to achieve a process technically

and economically viable.

4. Conclusions

Through permeation tests it was possible to evaluate the gas

separation performance of two membrane modules for the

separation of H2/CO2 binary model mixture. The separation

ability of the SAPO 34 membrane seems to be overestimated

since the effect concentration polarization phenomena was

not taken into consideration. Operational difficulties during

SAPO-34 membrane permeation tests could be explained due

to material defects. Presence of non-zeolitic pores, lost of

stability under certain storage conditions or water adsorption

blocking SAPO pores could provide explanation to this issue.

In addition, further experiments should be carried out on this

membrane module to elucidate possible defects during for-

mation of the zeolite layer on porous tubular supports. The

PDMS membrane showed higher potential to separate the

binary mixture and easier to operate compared to SAPO 34

membrane, reaching a maximum CO2/H2 separation selec-

tivity of 6.1 at 120 kPa of feed pressure. However, considering

the practical and achievable conditions to be used in a future

integrated lab-scale extractive fermentation system, the

operational feed pressure for the PDMSmembrane found to be

more adequate is 180 kPa, at 296 K. Under these circumstances

it was possible to attain a reliable CO2/H2 separation selec-

tivity of 5.8 along with sufficient recovery of the CO2 in the

permeate stream, allowing to continuously return it to the

bioreactor for the dilution of H2 present in the headspace and

consequently, decrease the H2 partial pressure for the more

efficient run of the biological system. In the retentate stream,

the H2 can possibly be more enriched in later separation steps

for fuel cell applications. From a practical point of view, the

continuous lab-scale extractive fermentation based on

membranes must be implemented with adequate removal

units for water and chemicals separation (present in raw

biogas). Furthermore, feed pressure pumps providing

adequate driving force and the implementation of an effective

pressure control strategy to avoid drawbacks with over-

pressures or vacuum must be addressed.

Acknowledgments

This work was financially supported by FONDECYT 1090482

and 1120659, and by the Janos Bolyai Research Scholarship of

the Hungarian Academy of Sciences.

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