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Influences of geometry and flow pattern on hydrogenseparation in a Pd-based membrane tube

Wei-Hsin Chen a,*, Wei-Ze Syu b, Chen-I Hung b, Yu-Li Lin c, Chang-Chung Yang c

aDepartment of Greenergy, National University of Tainan, Tainan 700, Taiwan, ROCbDepartment of Mechanical Engineering, National Cheng Kung University, Tainan 701, Taiwan, ROCcEnergy and Environmental Laboratories, Industrial Technology Research Institute, Hsinchu 310, Taiwan, ROC

a r t i c l e i n f o

Article history:

Received 22 August 2012

Received in revised form

18 October 2012

Accepted 21 October 2012

Available online 26 November 2012

Keywords:

Palladium-based (Pd-based)

membrane

Concentration polarization

Hydrogen separation and recovery

Geometric size

Baffle

Numerical simulation

* Corresponding author. Tel.: þ886 6 2605031E-mail address: weihsinchen@gmail.com

0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2012.10.0

a b s t r a c t

The abatement of concentration polarization in a membrane tube is of the utmost

importance for improving the efficiency of hydrogen separation. In order to enhance the

performance of hydrogen separation, the characteristics of hydrogen permeation in a Pd-

based membrane system under various operating conditions and geometric designs are

studied numerically. The effects of Reynolds numbers, shell size, baffle, and pressure

difference on hydrogen mass transfer across the membrane are evaluated. The predictions

suggest that a larger shell deteriorates concentration polarization, stemming from a larger

H2 concentration boundary layer. Baffles equipped in the shell are conducive to disturbing

H2 concentration boundary layer and reducing concentration polarization at the retentate

side, thereby intensifying H2 permeation. The more the number of baffles, the less the

increment of improvement in H2 permeation is. The installation of one baffle is recom-

mended for enhancing H2 separation and it is especially obvious under the environments

of high pressure difference. Within the investigated ranges of Reynolds number at the

permeate side and the retentate side, the feasible operating conditions are suggested in

this study.

Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction is the most abundant element in the universe [7], but most of

Hydrogen is important raw material and fuel in industry [1].

On the raw material aspect, hydrogen has been extensively

consumed for petroleum refining, production of ammonia and

methanol, desulfurization, hydrogenation of liquid oils, and

pharmaceutical products [2,3]. On the fuel side, hydrogen can

be used in liquid rocket engines, space shuttles, gas turbines,

and internal combustion engines as the source of power. On

account of much progress in low-temperature fuel cells in the

last decade [4e6], hydrogen production for power generation

from fuel cells has received a great deal of attention. Hydrogen

; fax: þ886 6 2602205.(W.-H. Chen).2012, Hydrogen Energy P68

the hydrogen is stored in water, hydrocarbons, and biomass

[8,9]. To gain pure hydrogen, fuel processing from the afore-

mentioned materials followed by hydrogen separation is thus

required.

In contrast to pressure swing adsorption and cryogenic

distillation, hydrogen separated from hydrogen-rich gas by

means of membranes, especially via Pd-based membranes,

seems to be more flexible for prospective hydrogen economy

[10e12]. This arises from the fact that hydrogen separation

through membranes can be achieved in small-scale facilities

which are suitable for fuel cell applications [5,13]. Besides,

ublications, LLC. Published by Elsevier Ltd. All rights reserved.

Nomenclature

Ac cross sectional area, m2

Am surface area of membrane, m2

C volumetric molar concentration, mol m�3

CPI concentration polarization index, dimensionless

cp gas mixture specific heat, J kg�1 K�1

D diffusion coefficient, m2 s�1

DH hydraulic diameter, m

Di inner diameter, m

Do outer diameter, m

F flux, mol m�2 s�1

K permeance, mol m�2 s�1 Pa�n

k thermal conductivity, W m�1 K�1

Mi molar mass of species i, kg kg mol�1

P pressure, Pa

Pw wetted perimeter, m

R universal gas constant, 8.314 m3 Pa K�1 mol�1

Re Reynolds number, dimensionless

S source term, kg m�3 s�1

T temperature, K

V velocity, m s�1

wi mass fraction of species i, dimensionless

xi mole fraction of species i, dimensionless

z axial coordinate, m

Greek letters

m viscosity, Pa s

r density, kg m�3

f binding factor, dimensionless

Ud diffusion collision integral, dimensionless

c volume of source domain or sink domain

Subscript

atm atmosphere

H2 hydrogen

i species i

in inlet

j species j

m membrane

p permeate side

r retentate side

total total pressure at the retentate side

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 1 4 5e1 1 5 61146

membrane separation possesses other advantages, such as

more compact system, lower energy consumption, and higher

purity of hydrogen [13,14]. In Pd-based membranes, hydrogen

separation is characterized by a solution-diffusion mecha-

nism [15] and local hydrogen flux across a membrane has

been extensively described by

FH2¼ K

�pnr;H2

� pnp;H2

�(1)

In the preceding equation, the pressure exponent n usually

ranges from 0.5 to 1 [16] and the ratio between the hydrogen

flux and the pressure difference is a constant, namely, the

permeance. Normally, the partial pressures of hydrogen at the

retentate side and the permeate one are expressed in terms of

the inlet conditions, based on the concept of continuous

stirred tank reactor (CSTR) [1,17]. The higher the hydrogen

partial pressure difference, the better the hydrogen flux. In

realistic situations, however, concentration polarization may

be encountered [17] and it is highly related to the performance

of membrane [18,19]. While hydrogen permeates through

a membrane, the partial pressure of hydrogen along the

membrane surface at the retentate side is reduced, whereas it

is increased at the permeate side stemming from hydrogen

accumulation [20]. This results in two mass transfer resis-

tances so that the behavior of hydrogen permeation departs

from the concept of CSTR. Currently, there is a trend to

develop high-permeance membranes. The transport resis-

tances due to concentration polarization will become more

obvious when high-permeance membranes are employed.

To minimize concentration polarization, two different

methods have been developed. The first route is introducing

a sweep gas into the permeate side of a membrane [20e24].

The sweep gas is able to move permeated hydrogen away

quickly so as to lower the partial pressure of hydrogen at the

permeate side. Considering the directions of sweep gas and

feed gas, two different flow patterns with one the counter-

current mode and the other the co-current one can be adop-

ted. The second mean is to change the flow field at the

retentate side of a membrane using baffles [1,25]. When the

flow field is disturbed by baffles, the hydrogen concentration

boundary layer along themembrane surface will be destroyed

to a certain extent. As a result, it yields a more uniform

hydrogen concentration field and the hydrogen partial pres-

sure along the membrane surface at the retentate side is

enlarged, thereby enhancing hydrogen separation efficiency.

Though a number of studies concerning the formation of

concentration polarization and its improvement have been

carried out, more detailed investigation is still required. For

instance, the information of concentration polarization

affected by scaling up a membrane module in order to

approach industrial applications is absent in past studies. In

addition, the combined effect of sweep gas and baffles on the

performance of hydrogen permeation under various operating

conditions has yet to be studied. To provide a comprehensive

recognition of hydrogen separation influenced by various

geometric designs and operations in a membrane system, the

present work aims to utilize a two-dimensional numerical

method to simulate the phenomena of hydrogen separation

and recovery from a gas mixture. The concentration polari-

zation and hydrogen separation efficiencywill be examined in

detail.

2. Mathematical formulation and modeling

2.1. Permeation system and governing equations

The membrane system consists of a membrane tube and

a shell, as shown in Fig. 1(a). Three different sizes of shell are

considered and their geometries are shown in Fig. 1(b). Steam

is adopted as the sweep gas and it is sent into the membrane

50 mm 100 mm

150

mm

400

mm

25 mm

a

b

8 mm

2

Fig. 1 e Schematics of (a) permeation system and

computational domains as well as (b) three different shell

sizes.

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 1 4 5e1 1 5 6 1147

tube from the center of the system (i.e. the tube). Alterna-

tively, a hydrogen-rich gas serves as the feed gas and it is

transported into the shell. Steam and the feed gas flow in the

membrane system in the form of counter-current pattern. It is

Table 1 e Governing equations and boundary conditions.

Governing equation

Continuity equation

Momentum equations

Species equations

Equation of state

Boundary condition Retentate

Inflow V!¼ �Vin;r n

! and w

Outflow vV.

vz¼ Vwi ¼ 0 and p

Tube centerline e

Membrane surface V. ¼ Vwi ¼ 0

Wall V. ¼ Vwi ¼ 0

assumed that the hydrogen permeation system is isothermal

(350 �C) and the mass transfer process is in steady state. The

flow fields in the tube and the shell are laminar and axisym-

metric. Regarding the membrane, its permselectivity to

hydrogen is infinite but it is zero to other gases. The body

forces of all gases in the permeation system are neglected and

the gas mixture in the system abides by the ideal gas law.

For hydrogen separated from a hydrogen-rich gas in the

membrane system, the physical phenomena are governed by

the conservative equations of continuity, momentum, and

species as well as equation of state. Meanwhile, the boundary

conditions are made up of the upstream inflow, downstream

outflow, tube centerline, membrane centerline, and wall in

the shell and the tube. The governing equations and boundary

conditions are tabulated in Table 1.

In this work, the concept of conjugate hydrogen perme-

ation is adopted to approach hydrogen permeation process;

that is, fluid dynamics and mass transfer at the both sides of

the membrane are fully simulated and hydrogen permeation

in the membrane is simultaneously considered. To deal with

hydrogen permeation process easier, the membrane is

conceived to be composed of a hydrogen-sink region and

a hydrogen-source one [1,20]. Accordingly, the entire perme-

ation system is partitioned into four computational domains

(Fig. 1(a)), consisting of a retentate domain, a permeate

domain, a source domain, and a sink domain. The computa-

tional domain in the membrane at the retentate side is

referred to as the hydrogen-sink domain, whereas it is spoken

of as the hydrogen-source one at the permeate side. The

source and sink terms obey Sieverts’ law (n ¼ 0.5) locally.

Details of the source and sink terms are listed at Table 2 where

the notations c and Am stand for the volume of source

domain or sink domain and the surface area of the Pd

membrane, respectively. The values of c and Am can be ob-

tained from the geometries shown in Fig. 1(b).

2.2. Properties of gas mixture

In the permeation system, four different gases of H2, CO, CO2,

and H2O are simultaneously considered to account for the

V$ðrV.Þ ¼ S

rV.$VV

. ¼ �Vpþ V$½mðVV. þ ðVV.ÞTÞ�

V$

� rwi

Pj

Dij

�Vxj þ ðxj �wjÞ

Vpp

�þ rwi V

!1A ¼ Si

p ¼ rRTPNi

1xiMi

Permeate

i ¼ wi;in;r V!¼ �Vin;p n

! and wi ¼ wi;in;p

¼ ptotalvV.

vz¼ Vwi ¼ 0 and p ¼ patm

VV. ¼ Vwi ¼ 0

V. ¼ Vwi ¼ 0

V. ¼ Vwi ¼ 0

Table 2 e A list of source and sink terms in governing equations at different domains.

Domain Continuity equation Species equation

Retentate S ¼ 0 Si ¼ 0 ði ¼ CO; CO2; H2O; H2ÞSource

S ¼ þMH2Am

cKðp0:5r;H2

� p0:5p;H2Þ Si ¼ þMH2Am

cKðp0:5r;H2

� p0:5p;H2Þ ði ¼ H2Þ

Si ¼ 0 ði ¼ CO; CO2; H2OÞSink

S ¼ �MH2Am

cKðp0:5r;H2

� p0:5p;H2Þ Si ¼ �MH2Am

cKðp0:5r;H2

� p0:5p;H2Þ ði ¼ H2Þ

Si ¼ 0 ði ¼ CO; CO2; H2OÞPermeate S ¼ 0 Si ¼ 0 ði ¼ CO; CO2; H2O; H2Þ

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 1 4 5e1 1 5 61148

hydrogen permeation process. Therefore, Wilke semi-

empirical correlation [26e28] is adopted to calculate the

viscosity of gas mixture and the correlation is expressed as

m ¼XNi¼1

ximiPnj¼1 xjfij

(2)

where x, M, and f are the molar fraction, molecular weight,

and binding factor, respectively. The thermal conductivity of

gas mixture is also estimated by Wilke semi-empirical corre-

lation [26] as the following

k ¼XNi¼1

xikiPnj¼1 xjfij

(3)

When calculating the binary mixture diffusivity of gas

mixture under an isothermal condition, the Chap-

maneEnskog equation [27,28] is considered and it is written as

Dij ¼ 1:881� 10�3 �T1:5

�1

Miþ 1

Mj

�0:5

ps2ijUd

(4)

where T, p, sij and Ud are temperature, absolute pressure,

interaction value for binary mixture and diffusion collision

integral, respectively. The diffusion collision integral Ud is

correlated by

Ud ¼ A2B

þ CexpðD2Þ þ

EexpðF2Þ þ

GexpðH2Þ (5)

Table 3 e A list of properties of four gases (at 350 �C) and other

H2

Viscosity (Pa s) 1.47 � 10�5

Thermal conductivity (W m�1 K�1) 0.322

Heat capacity at constant pressure (J kg�1 K�1) 14,579

Binding factor fij (�) j ¼ H2

i ¼ H2 1

i ¼ CO 0.223

i ¼ CO2 0.195

i ¼ H2O 0.323

si (�A) 59.7

3i/k (K) 2.827

Parameters of correlation for calculating Ud

A B C D

1.06036 0.15610 0.19300 0.47635

2 ¼ kT(6)

3ij

The parameters sij and 3ij are obtained by the following

equations of pure compounds

sij ¼si þ sj

2(7)

3ij ¼ ffiffiffiffiffiffiffi3i 3j

p(8)

Detailed properties of pure gases (i.e. H2, CO, CO2, and H2O),

such as viscosity, thermal conductivity, and heat capacity at

constant pressure, as well as the values for the properties of

gas mixture are listed in Table 3. The permselectivity of the

membrane to CO, CO2, and H2O is assumed to be zero in this

study, implying that H2O does not mix with CO and CO2 in the

membrane system. Therefore, the binding factors for COeH2O

and H2OeCO2 are not included in Table 3.

2.3. Concentration polarization index (CPI) and Reynoldsnumber

To realize the polarization extent on the membrane surface,

a set of concentration polarization index (CPI) is defined as the

following [20,29]:

CPIr ¼ 1� CH2 ;m

CH2 ;in¼ 1�

CH2 ;m

Ctotal

CH2 ;in

Ctotal

¼ 1� xH2 ;m

xH2 ;in(9)

values for calculating the properties of gas mixture [27,28].

CO CO2 H2O

1.75 � 10�5 2.7 � 10�5 2.15 � 10�5

0.025 0.045 0.044

1043 1095 2039

j ¼ CO j ¼ CO2 j ¼ H2O

2.624 2.332 1.982

1 0.999 e

0.981 1 e

e e 1

91.7 195.2 809.1

3.690 3.941 2.641

E F G H

1.03587 1.52996 1.76474 3.89411

Fig. 2 e (a) A schematic of grid system as well as the

comparisons of (b) average H2 flux and (c) velocity

distribution at various grid systems with large shell

(Rer [ 30, Rep [ 160, and Dp [ 9 atm).

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 1 4 5e1 1 5 6 1149

CPIp ¼ xH2 ;m (10)

CPI ¼ CPIr þ CPIp2

(11)

where the subscripts r and p denote the retentate and

permeate sides, respectively, and xH2 ;m designates the mole

fraction of H2 in the gas mixture. CPI represents the non-

uniformity of hydrogen concentration on the membrane

surface. A higher CPI stands for more serious concentration

polarization. When a feed gas flows into the shell, the Rey-

nolds number in the shell is defined by

Re ¼ rVDH

m¼ rVðDo � DiÞ

m(12)

where DH (¼ 4Ac/Pw) is hydraulic diameter and in the shell it is

given by

DH ¼ 4Ac

Pw¼ 4� 0:25p

�D2

o � D2i

�pðDo þ DiÞ ¼ Do � Di (13)

2.4. Numerical method

The commercial software COMSOL Multiphysics 4.0a was

utilized to solve the governing equations in association with

boundary conditions in which SPOOLES solver was used. An

orthogonal grid system for the purpose of reducing numerical

truncation error was employed to construct the physical

geometry. Meanwhile, to provide a flexible grid system, the

entire permeation system was divided into a number of

blocks, as shown in Fig. 2(a). In the figure, only the gird

number in the sink domain rather than in both source and

sink domains is included. However, it should be illustrated

that the grid number in the source domain is equivalent to

that in the sink domain. Three different grid numbers

Table 4 e A list of operating conditions and properties ofmembrane.

Gas mixture 65.2% H2 þ 0.2%

CO þ 34.6% CO2

Sweep gas Steam

Pressure at the exit of permeate

side (atm)

1

Temperature (�C) 350

Pressure difference (atm) 5e30

Mass flow rate of feed gas

at m0 (mg s�1)

66.87

Mass flow rate of sweep gas

at Rep ¼ 20 (mg s�1)

2.70

Smallshell

Mediumshell

Largeshell

Rer (at m0 and DP ¼ 10 atm) 98.18 55.86 30

Membrane Pd membrane

Membrane thickness (mm) 20

Permselectivity to H2 Infinite

Permselectivity to CO,

CO2, and H2O

Zero

Permeance

(mol m�2 s�1 Pa�0.5)

1.83 � 10�3

Fig. 3 e Distributions of (a) average H2 flux and (b) H2 recovery at various operating conditions and geometric sizes of shell

(Dp [ 9 atm).

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 1 4 5e1 1 5 61150

(Fig. 2(a)) were tested and compared with each other. The

profiles of average H2 flux across the membrane (Fig. 2(b)) and

gas velocity along the r-direction at the outlets of the tube and

the large shell for a gas mixture flowing through the shell are

sketched in Fig. 2(b) and (c), respectively. The compositions of

the tested gas mixture are listed in Table 4 which was ob-

tained from water gas shift reactions [30], and the Reynolds

numbers at the retentate side (Rer) and the permeate side (Rep)

Fig. 4 e Distributions of H2 concentration contour in the

permeation systemwith (a) small, (b) medium, and (c) large

shell (feed gas [ m0 and Dp [ 9 atm).

Fig. 5 e Distributions of H2 partial pressure along the

membrane surface (feed gas [ m0, Rep [ 160, and

Dp [ 9 atm).

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 1 4 5e1 1 5 6 1151

are 30 and 160, respectively. The distributions shown in Fig. 2

suggest that that the difference between the second and the

third grid systems are almost imperceptible. As a conse-

quence, the second grid system is adopted for simulations

because of satisfying the requirement of grid independence.

The developed numerical method has been validated in

a previous study [20] by comparing the predicted H2 recovery

and gas concentrations to the experimental results [30]. It

indicated that the numerical predictions were in good

agreement with the experimental measurements.

Fig. 6 e A schematic of baffle location and operating

conditions at various cases.

3. Results and discussion

In the present work, a Pd-basedmembrane serves as the basis

of the study. Detailed properties of themembrane are listed in

Table 4. The membrane thickness was 20 mm, and the thick-

nesses of source and sink domains (Fig. 1(a)) are equally

distributed by 10 mm. Themembrane permeance (¼ 1.83� 10�3

m�2 s�1 Pa�0.5) was determined from the measurement of

a practical pure Pd membrane tube in our laboratory. Four

important factors of Reynolds number, shell size, baffle

number, and pressure difference across the membrane are

taken into account. The Reynolds number in the tube (i.e. the

permeate side) and the shell (i.e. the retentate side) are in the

ranges of 20e2000 and 20e800, respectively. Three different

shell sizes, as shown in Fig. 1(a), are investigated. Four baffles

are installed in the shell to evaluate their effect on concentra-

tion polarization. For a pure Pd membrane, the operating

temperatures are usually controlled between 300 and 400 �C to

avoidhydrogenembrittlementat lower temperatures (<300 �C)and material damage at higher temperatures (>400 �C) [30].Therefore, it is assumed that the system temperature is at

350 �C. With regard to pressure difference between the reten-

tate side and the permeation side, it is between 5 and 30 atm;

that is, the total pressure at the retentate side ( ptotal) ranges

from 6 to 31 atm.

Fig. 7 e Distributions of (a) CPIr, (b) CPIp, and (c) CPI along

membrane surface with large shell under various numbers

of baffles (Rer [ 30, Rep [ 160, and Dp [ 9 atm).

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 1 4 5e1 1 5 61152

3.1. Influence of shell size

Fig. 3 first demonstrates the distributions of average H2 flux

and H2 recovery at various operating conditions under the

three different shell sizes where the pressure difference (Dp)

at the two sides of the membrane is fixed at 9 atm. In other

words, the total pressure at the retentate side ( ptotal) is 10 atm.

As can be seen in the figure, increasing Reynolds number at

the permeate side (Rep) intensifies both the average H2 flux

and H2 recovery. When Rep is larger than 1000, it is noted that

the increment in H2 flux and recovery due to increasing Repbecomes slight, regardless of what the shell size is. This

reflects that the flow rate of sweep gas should not be too high.

An increase in the flow rate of feed gas is also able to enhance

average H2 flux, but it reduces H2 recovery. For instance, with

the flow rate of feed gas at m0, the H2 recovery is larger than

70%. Once the flow rate is lifted to 8m0, the H2 recovery is less

than 20%, implying that a lower flow rate of feed gas gives

a better H2 separation efficiency. With regard to the influence

of shell size on H2 permeation, a smaller shell is conducive to

the performance of H2 separation. This arises from the fact

that the H2 boundary layer in a smaller shell is thinner and it is

easier for H2 moving to the membrane surface, even though

the residence time of feed gas in the large shell is longer.

However, once the flow rate of feed gas is as high as 8m0, the

H2 recovery is insensitive to the variation of shell side

(Fig. 3(b)).

To provide a clearer observation of H2 permeation, the H2

concentration contours under various shell sides and Rey-

nolds numbers of sweep gas are displayed in Fig. 4 where the

H2 concentrations at both the lumen and shell sides have been

non-dimensionalized in terms of the individual maximum

concentrations. As can be seen in the figure, the larger the

shell size, the thicker the H2 concentration boundary layer,

resulting in lessened H2 concentration gradient along the

membrane surface. This further lowers H2 permeation across

themembrane so that the concentration of H2 at the exit of the

shell is higher. Fig. 4 also depicts that increasing the Reynolds

number of sweep gas at the lumen side (i.e. Rep) facilitates the

mass transfer of H2 in that the H2 concentration at the exit of

the shell is lower. In examining the distributions of H2 partial

pressure along the membrane surface at various shell sizes,

overall, Fig. 5 indicates that the small shell at the retentate side

has a higher profile of H2 partial pressure at the membrane

upstream. It follows that its concentration polarization at the

retentate side is less severe compared to those of the medium

and large shells. A higher H2 partial pressure leads to a higher

H2 permeation rate at themembrane upstream. This results in

a lower H2 partial pressure at the membrane downstream.

This is the reason that the profiles of H2 partial pressure at the

membrane upstream have a different trend as compared with

those at the membrane downstream.

3.2. Influence of baffle

The preceding results suggest that the membrane tube with

large shell gives a lower efficiency of H2 separation. However,

it should be pointed out that the membrane tube with large

shell has a potential to separate more H2 from the feed gas if

its flow rate is large. Accordingly, in the following study

Fig. 8 e Distributions of H2 concentration contour in the permeation system with large shell under various numbers of

baffles (Rer [ 30, Rep [ 160, and Dp [ 9 atm).

Fig. 9 e Profile of H2 recovery in the permeation system

with large shell under various numbers of baffles (Rer [ 30,

Rep [ 160, and Dp [ 9 atm).

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 1 4 5e1 1 5 6 1153

emphasis is placed on the influence of baffle installed in the

shell upon H2 separation. Fig. 6 presents the detailed locations

and geometric size of the baffles and Fig. 7 shows the distri-

butions of CPIr, CPIp and CPI under the conditions of Rer ¼ 30,

Rep ¼ 160, and Dp ¼ 9 atm. When the first baffle is installed in

the shell, the curve of CPI at the retentate side, CPIr, declines to

a certain extent (Fig. 7(a)), implying that the concentration

polarization is lessened. By virtue of more H2 separated at the

leading portion of the membrane, this results in less H2

retained at the trailing portion and, thereby, the curve of CPIris higher compared to that without installing the baffle. If

more baffles are equipped, the curve of CPIr can be further

reduced. However, the improvement in concentration polari-

zation becomes less pronounced. For example, the difference

in the distributions of CPIr between installing 3 baffles and 4

baffles is slight. In regard to the CPI at the lumen side, namely,

CPIp, the curves are almost independent of the installation of

baffles (Fig. 7(b)). This can be explained by the tiny influence of

flow patter by the baffles on the distribution of H2 concen-

tration at the lumen side. In view of the counter-current

pattern adopted in the present study, the trend of CPI at the

lumen side of the membrane is contrary to the other side.

When the CPIr and CPIp are simultaneously considered, the

profiles of CPI almost remain horizontal (Fig. 7(c)).

Upon inspection of H2 concentration contours shown in

Fig. 8, the H2 concentration boundary layer in the vicinity of

themembrane surface is obviously exhibitedwhenno baffle is

installed. Once a baffle is situated in the shell, the H2

concentration boundary layer is disturbed in a significant way

and more H2 is pushed toward the membrane surface.

Besides, a recirculation bubble is triggered behind the baffle.

This facilitates the mixing of H2 and other gases and entrains

H2 toward the membrane surface. In view of the two factors,

the profile of CPIr is reduced (Fig. 7(a)) under the situation of

one-baffle installation, in contrast to that without installing

baffle. When two baffles are mounted in the shell, the H2

partial pressure along the membrane surface adjacent to the

baffles is further enlarged. It is also noted that more H2 is

trapped in the space between the two baffles. This is the

reason that the profile of CPIr is further lowered. When more

baffles are installed, more recirculation bubbles develop and

the movement of H2 toward the membrane surface is more

notable. As a consequence, the concentration polarization is

diminished with increasing baffle number (Fig. 8(a)).

The profile of H2 recovery versus number of baffles is

plotted in Fig. 9. As a whole, increasing the number of baffles

enlarges the H2 recovery. When a baffle is placed in the shell,

the value of H2 recovery is promoted from 77.4 to 80.6%,

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 1 4 5e1 1 5 61154

accounting for 3.2% improvement in H2 recovery. When two

baffles are installed, the H2 recovery is increased to 81.8%,

yielding only 1.2% of improvement in H2 recovery in

comparison with that with one baffle. When four baffles are

included, the H2 recovery reaches 84.5%. From the profile, the

installation of one baffle is recommended in that more than

one baffle will increase the capital cost markedly. But the

improvement of H2 recovery is not linearly proportional to the

cost.

3.3. H2 recovery in the absence and presence of baffle

The three-dimensional profiles of average H2 flux on the

membrane surface in the absence and presence of one baffle

are shown in Fig. 10(a) and (b), respectively. The Reynolds

numbers at the retentate side (Rer) and the permeate side

(Rep) are in the ranges of 20e800 and 20e2000, respectively.

Fig. 10 depicts that the H2 flux is sensitive to the variations of

Rer and Rep when their values are low, especially for Rer.

When Rer and Rep are larger than 160 and 400, respectively,

the growth of H2 flux tends to slow down, regardless of the

installation of the baffle. An increase in Rep means more

sweep gas (steam) and thereby energy sent into the

Fig. 10 e Three-dimensional distributions of average H2

flux for the permeation system with large shell equipping

(a) 0 and (b) 1 baffle (Dp [ 9 atm).

membrane tube. Alternatively, increasing Rer makes H2

recovery go down (Fig. 3(b)). From operation point of view, the

conditions of Rer ¼ 160 and Rep ¼ 400 are recommended for H2

separation.

The three-dimensional profile of improvement in H2

separation is plotted in Fig. 11. The parameter of improvement

in H2 separation under the effect of one baffle is defined as

Improvementð%Þ ¼ FH2 ;b � FH2 ;0

FH2 ;0

� 100 (14)

where FH2 ;0 and FH2 ;b stand for H2 fluxes in the absence and

presence of one baffle. Within the investigated ranges of Rerand Rep, the improvement is located between 2.2 and 7.6%.

The maximum improvement occurs at Rer ¼ 800 and

Rep ¼ 2000, whereas the minimum one develops at Rer ¼ 20

and Rep ¼ 2000. Varying Rep has a slight effect on the

improvement. This can be explained by the flow field at the

permeate side hardly affected by the baffle which is installed

at the retentate side. Once Rer is larger than 80, the change in

the improvement becomes slight. This observation is consis-

tent with the foregoing illustration where Rer ¼ 160 and

Rep ¼ 400 are recommended for H2 separation.

The concentration contours of H2 at three different values

of Rer (i.e. 20, 160, and 800) in the absence and presence of the

first baffle are provided in Fig. 12. The H2 concentration

boundary layers are clearly exhibited in the shell under the

situation of without baffle (Fig. 12(a)). With the installation on

the baffle, the flow fields are significantly disturbed and more

H2 is pushed toward the membrane surface. This leads to

thinner H2 concentration boundary layers (Fig. 12(b)). In view

of the promotion of H2 concentration along the membrane

surface at the retentate side, the concentration polarization is

abated. In contrast, the concentration contours of H2 at the

permeate side are hardly affected by the installation of the

baffle.

3.4. Effect of total pressure difference

A higher pressure difference between the two sides of

a membrane is conducive to H2 flux, as expressed in Eq. (1).

The profiles of average H2 flux in the absence and presence of

Fig. 11 e Three-dimensional distribution of H2 flux

improvement in the permeation system with large shell

(Dp [ 9 atm).

Fig. 12 e Distributions of H2 concentration contours in the

membrane permeation system with large shell equipping

(a) 0 and (b) 1 baffle (Rep [ 800 and Dp [ 9 atm).

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 1 4 5e1 1 5 6 1155

a baffle are displayed in Fig. 13 to evaluate the influence of the

baffle in improving H2 flux under various pressure differences.

Obviously, the higher the pressure difference, the higher the

average H2 flux. Increasing pressure difference also intensifies

the difference of average H2 flux between the two difference

situations, thereby enhancing the effect of the baffle in

separating H2. Specifically, when the pressure difference is

5 atm, the improvement is 5.4%. Once the pressure difference

is as high as 30 atm, the improvement reaches 15.7%,

accounting for 3 folds in improvement. It is thus concluded

that the function of installing baffle will be intensified if the

pressure difference is larger.

Fig. 13 e Distributions of average H2 flux and H2 recovery in

the permeation system with large shell with/without

equipping a baffle (Rer [ 800, Rep [ 2000, and

Dp [ 5e30 atm).

4. Conclusions

Hydrogen separation in a permeation system under various

geometry designs and flow patterns have been examined via

a numerical method. In the method, H2 permeation through

a membrane is approached by introducing a sourceesink pair

in the governing equation, and a multi-block in association

with orthogonal grid system is employed. The parameters of

Reynolds numbers at the both sides of the membrane, shell

size, baffle, and pressure difference are taken into consider-

ation to account for their effects on hydrogen separation and

recovery. The numerical results indicate that an increase in

Reynolds number at the permeate side intensifies average H2

flux and H2 recovery. However, the performance of sweep gas

upon H2 separation tends to become ignorable if its Reynolds

number is not smaller than 1000. Increasing the flow rate of

feed gas is also able to enhance average H2 flux, but it reduces

H2 recovery. Though a large shell has higher potential for

industrial applications of H2 separation, the H2 concentration

boundary layer is larger compared with that in a small shell

under the same flow rate of feed gas, thereby causing more

serious concentration polarization. Installation of baffles in

the shell can disturb H2 concentration boundary layer so that

the concentration polarization at the retentate side is reduced

and H2 permeation is intensified. The more the number of

baffles, the less the increment of improvement in H2 perme-

ation is. From the viewpoint of capital cost, the installation of

one baffle is recommended in enhancing H2 separation. The

function of baffle is especially obvious when the pressure

difference is higher. If the membrane tube with large shell is

utilized, the conditions of Rer ¼ 160 and Rep ¼ 400 are appro-

priate for H2 separation whether one baffle is installed or not.

Acknowledgments

The authors would like to thank the financial support from

Bureau of Energy, Ministry of Economic Affairs, Taiwan, R.O.C.

r e f e r e n c e s

[1] Coroneo M, Montante G, Paglianti A. Numerical andexperimental fluid-dynamic analysis to improve the masstransfer performances of PdeAg membrane modules forhydrogen purification. Ind Eng Chem Res 2010;49:9300e9.

[2] Lin YM, Rei MH. Separation of hydrogen from the gasmixture out of catalytic reformer by using supportedpalladium membrane. Sep Purif Technol 2001;25:87e95.

[3] Rahimpour MR, Mazinani S, Vaferi B, Baktash MS.Comparison of two different flow types on CO removal alonga two-stage hydrogen permselective membrane reactor formethanol synthesis. Appl Energy 2011;88:41e51.

[4] Majlan EH, Daud WRW, Iyuke SE, Mohamad AB,Kadhum AAH, Mohammad AW, et al. Hydrogen purificationusing compact pressure swing adsorption system for fuelcell. Int J Hydrogen Energy 2009;34:2771e7.

[5] Chen WH, Lin BJ. Effect of microwave double absorption onhydrogen generation from methanol steam reforming. Int JHydrogen Energy 2010;35:1987e97.

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 1 4 5e1 1 5 61156

[6] Chein RY, Chen YC, Chang CS, Chung JN. Numericalmodeling of hydrogen production from ammoniadecomposition for fuel cell applications. Int J HydrogenEnergy 2010;35:589e97.

[7] Adhikari S, Fernando S. Hydrogen membrane separationtechniques. Ind Eng Chem Res 2006;45:875e81.

[8] Chein RY, Chen YC, Chung JN. Thermal resistance effect onmethanol-steam reforming performance in micro-scalereformers. Int J Hydrogen Energy 2012;37:250e62.

[9] Taghvaei H, Shirazi MM, Hooshmand N, Rahimpour MR,Jahanmiri A. Experimental investigation of hydrogenproduction through heavy naphtha cracking in pulsed DBDreactor. Appl Energy 2012;98:3e10.

[10] Lattin WC, Utgikar VP. Transition to hydrogen economy inthe United States: a 2006 status report. Int J Hydrogen Energy2007;32:3230e7.

[11] Chen WH, Chiu IH. Transient dynamic of hydrogenpermeation through a palladium membrane. Int J HydrogenEnergy 2009;34:2440e8.

[12] Chen WH, Hsu PC, Lin BJ. Hydrogen permeation acrossa palladium membrane in a varying pressure environment.Int J Hydrogen Energy 2010;35:5410e8.

[13] Sjardin M, Damen KJ, Faaij APC. Techno-economicprospects of small-scale membrane reactors in a futurehydrogen-fuelled transportation sector. Energy 2006;31:2523e55.

[14] Peramanu S, Cox BG, Pruden BB. Economics of hydrogenrecovery processes for the purification of hydroprocessorpurge and off-gases. Int J Hydrogen Energy 1999;24:405e24.

[15] ChenWH, Hsu BJ. Hydrogen permeation measurements of Pdand PdeCu alloy membranes using dynamic pressuredifference method. Int J Hydrogen Energy 2011;36:9355e66.

[16] Chen WH, Syu WZ, Hung CI. Numerical characterization onpolarization of hydrogen permeation in a Pd-basedmembrane tube. Int J Hydrogen Energy 2011;36:14734e44.

[17] Coroneo M, Montante G, Paglianti A. Modeling the effect ofoperating conditions on hydrodynamics and mass transferin a PdeAg membrane module for H2 purification. J MembrSci 2009;343:34e41.

[18] Mourgues A, Sanchez J. Theoretical analysis ofconcentration polarization in membrane modules for gasseparation with feed inside the hollow-fibers. J Membr Sci2005;252:133e44.

[19] Zhang J, Liu D, He M, Xu H, Li W. Experimental andsimulation studies on concentration polarization in H2

enrichment by highly permeable and selective Pdmembranes. J Membr Sci 2006;274:83e91.

[20] Chen WH, Syu WZ, Hung CI, Lin YL, Yang CC. A numericalapproach of conjugate hydrogen permeation andpolarization in a Pd membrane tube. Int J Hydrogen Energy2012;37:12666e79.

[21] Basile A, Tosti S, Capannelli G, Vitulli G, Iulianelli A,Gallucci F, et al. Co-current and counter-current modes formethanol steam reforming membrane reactor: experimentalstudy. Catal Today 2006;118:237e45.

[22] Gallucci F, Basile A. Co-current and counter-current modesfor methanol steam reforming membrane reactor. Int JHydrogen Energy 2006;31:2243e9.

[23] Gallucci F, Falco MD, Tosti S, Marrelli L, Basile A. Co-currentand counter-current configurations for ethanol steamreforming in a dense PdeAg membrane reactor. Int JHydrogen Energy 2008;33:6165e71.

[24] Iulianelli A, Longo T, Basile A. CO-free hydrogen productionby steam reforming of acetic acid carried out in a PdeAgmembrane reactor: the effect of co-current and counter-current mode. Int J Hydrogen Energy 2008;33:4091e6.

[25] Liu SX, Peng M, Vane LM. CFD simulation of effect of baffleon mass transfer in a slit-type pervaporation module. JMembr Sci 2005;265:124e36.

[26] Falco MD, Paola LD, Marrelli L. Heat transfer and hydrogenpermeability in modelling industrial membrane reactors formethane steam reforming. Int J Hydrogen Energy 2007;32:2902e13.

[27] Caravella A, Barbieri G, Drioli E. Modelling and simulation ofhydrogen permeation through supported Pd-alloymembranes with a multicomponent approach. Chem Eng Sci2008;63:2149e60.

[28] Catalano J, Baschetti MG, Sarti GC. Influence of the gas phaseresistance on hydrogen flux through thin palladiumesilvermembranes. J Membr Sci 2009;339:57e67.

[29] Bhattacharya S, Hwang ST. Concentration polarization,separation factor, and Peclet number in membraneprocesses. J Membr Sci 1997;132:73e90.

[30] Chen WH, Lu JJ. Hydrogen production from water gas shiftreactions in association with separation using a palladiummembrane tube. Int J Energy Res 2012;36:346e54.