Low temperature catalytic reforming of heptane to hydrogen and syngas

Journal of Saudi Chemical Society (2012) xxx, xxx–xxx

King Saud University

Journal of Saudi Chemical Society

www.ksu.edu.sawww.sciencedirect.com

ORIGINAL ARTICLE

Low temperature catalytic reforming of heptane

to hydrogen and syngas

M.E.E. Abashar *

Department of Chemical Engineering, College of Engineering, King Saud University, P.O. Box 800, Riyadh 11421, Saudi Arabia

Received 24 July 2012; accepted 25 September 2012

*

E

P

13

ht

PJ

KEYWORDS

Heptane;

Hydrogen;

Hydrogen membrane;

Reforming reactions;

Syngas

Tel.: + 966 1 4675843; fax

-mail address: mabashar@k

eer review under responsibili

Production an

19-6103 ª 2012 King Saud U

tp://dx.doi.org/10.1016/j.jscs.

lease cite this article in pournal of Saudi Chemical

: + 966 1

su.edu.sa

ty of Kin

d hostin

niversity

2012.09.0

ress as:Society

Abstract The production of hydrogen and syngas from heptane at a low temperature is studied in

a circulating fast fluidized bed membrane reactor (CFFBMR). A thin film of palladium-based mem-

brane is employed to the displacement of the thermodynamic equilibrium for high conversion and

yield. A mathematical model is developed to simulate the reformer. A substantial improvement of

the CFFBMR is achieved by implementing the thin hydrogen membrane. The results showed that

almost complete conversion of heptane and 46.25% increase of exit hydrogen yield over the value

without membrane are achieved. Also a wide range of the H2/CO ratio within the recommended

industrial range is obtained. The phenomena of high spikes of maximum nature at the beginning

of the CFFBMR are observed and explanation offered. The sensitivity analysis results have shown

that the increase of the steam to carbon feed ratio can increase the exit hydrogen yield up to

108.29%. It was found that the increase of reaction side pressure at a high steam to carbon feed

ratio can increase further the exit hydrogen yield by 49.36% at a shorter reactor length. Moreover,

the increase of reaction side pressure has an important impact in a significant decrease of the carbon

dioxide and this is a positive sign for clean environment.ª 2012 King Saud University. Production and hosting by Elsevier B.V. All rights reserved.

1. Introduction

The energy sector has been faced with major challenges ofcrude oil prices, diminishing of fossil fuel reserves, environ-mental quality, and the rapid growth of the industry. These

challenges have imposed a strong demand for alternative fuels.Hydrogen had been recognized as an important alternative

4678770.

.

g Saud University.

g by Elsevier

. Production and hosting by Elsev

19

Abashar, M.E.E. Low tempera(2012), http://dx.doi.org/10.10

clean fuel that produces no pollution or greenhouse gasesand has the highest energy content (Abashar, 2004; Abashar

et al., 2007). The conversion of syngas (H2,CO,CO2) to cleanliquid fuels free from sulfur such as gasoline and diesel is an-other promising route to alternative fuels (Mark, 1999; Olusola

et al., 2010).Hydrogen can be produced by different processes such as

thermochemical and electrolysis processes. The thermochemi-

cal processes release hydrogen in the molecular structure ofdifferent resources such as natural gas and biomass, whilethe electrolysis processes release hydrogen from the molecular

structure of water. Steam reforming of methane has beenknown as the main industrial thermochemical process that iswidely used to produce hydrogen and syngas (Abashar,1991; Elnashaie and Abashar, 1993).

ier B.V. All rights reserved.

ture catalytic reforming of heptane to hydrogen and syngas.16/j.jscs.2012.09.019

mailto:[email protected]

http://dx.doi.org/10.1016/j.jscs.2012.09.019


http://www.sciencedirect.com/science/journal/13196103



Nomenclature

Ac free cross-sectional area of the reactor for catalyst

circulation, m2

Ar Archimedes number, [-]dH2 diameter of hydrogen membrane tube, mdc diameter of catalyst particle, m

d�c dimensionless diameter of catalyst particleFj molar flow rate of component j, kmol/hg gravitational acceleration, m/s2

ki rate coefficient of reaction iK2, K4, equilibrium constant of reactions (2) and (4), kPa2

K3 equilibrium constant of reaction (3), [-]

L reactor length, mNH2 number of hydrogen membrane tubesP total pressure, kPaPj partial pressure of component j, kPa

QH2 permeation rate of hydrogen, kmol/hri rate of reaction i, kmol/kgcat hR gas constant, kJ/mol K

Rep particle Reynolds number, [-]T temperature, K

Tf feed temperature, K

uo superficial gas velocity, m/su* dimensionless gas velocityz dimensionless length of the reactor

Greek letters

dH2 thickness of hydrogen membrane, lme void fractionlg viscosity of gas, kg/m.s

qc catalyst density, kg/m3

qg gas density, kg/m3

Superscriptp permeation side

r reaction side

AbbreviationCFFBR circulating fast fluidized bed reactorCFFBMR circulating fast fluidized bed membrane reactor

2 M.E.E. Abashar

The conventional processes of methane steam reforming arevery expensive due to severe external heating and also suffer

from destructive elevated temperatures for the catalyst andthe reactors. For example, the production of syngas that wasused as a feedstock for the gas to liquid processes (GTL) such

as the Fischer–Tropsch (FT) process may cost about 70% ofthe capital and running costs of the total plant (Mark, 1999).This high cost and other technical factors have stimulated

the research arena to develop cost-effective technologies ofhydrogen and syngas processes and also to examine differentaspects of improvement with respect to the thermal efficiency,catalyst properties, operations and new designs (Kaihu and

Hughes, 2001; Venkataraman et al., 2003; Levent et al., 2003).Recently, Elnashaie and co-workers have shown that circu-

lating fast fluidized bed membrane reactors (CFFBMR) are

versatile and efficient type of reformers for hydrogen produc-tion (Chen et al., 2003a,b; Prasad and Elnashaie, 2003;Abashar et al., 2007, 2008; Abashar, 2012). Among the advan-

tages of circulating fluidized bed reactors are: effective gas–solid contact, uniform temperature distribution, eliminationof high diffusion limitations, high gas throughputs per unitcross-section, and high steam-to-carbon ratio (Brereton and

Grace, 1993; Kunii and Levenspiel, 1997, 2000). Moreover,the application of palladium-based membranes in theCFFBMR has a significant impact on shifting the thermody-

namic equilibrium and separation of hydrogen (Shu et al.,1994, 1995; Dittmeyer et al., 2001; Hughes, 2001; Abashar,2004; Abashar et al., 2007).n-Heptane is used by many inves-

tigators as a model compound for higher hydrocarbons suchas gasoline and naphtha (Pant and Kunzru, 1996; Seiseret al., 2000; Puolakka and Krause, 2004; Ran et al., 2004).

High hydrogen and syngas yields have been obtained by steamreforming of heptane (Chen et al., 2003a,b, 2004). Also, the re-quired heat per carbon atom of heptane is less than methaneand the heat duty will be less (Rostrup-Nielsen et al., 1998).

Chemical kinetics of higher hydrocarbon such as n-heptanehas been developed and used by many researchers (Phillips

Please cite this article in press as: Abashar, M.E.E. Low temperaJournal of Saudi Chemical Society (2012), http://dx.doi.org/10.10

et al., 1969; Rostrup-Nielsen, 1973, 1977; Tottrup, 1982;Christensen, 1996; Nah and Palanki, 2009; Rakib et al.,

2010). Surprisingly, there are only limited published studiesin modeling and simulation of steam reforming of n-heptanein the CFFBMR (Chen et al., 2003a,b, 2004). This preliminary

study is an extension to Elnashaie and co-workers (Chen et al.,2003a,b) to investigate the potential of production of hydrogenand syngas at a low temperature as an alternative to the costly

conventional high temperature steam reforming processes.Also a thin layer of composite hydrogen membrane of 5 lmthickness is employed in this study to enhance the displace-ment of the thermodynamic equilibrium for further perme-

ation of the hydrogen through the membrane, instead ofrelatively thick hydrogen membrane used by Chen et al.(2003a,b). Moreover, the factors affecting the H2/CO ratio

have been studied to achieve the desired recommended indus-trial range for the syngas suitable as a feedstock of the GTLprocesses.

2. Reactions kinetics

The main reactions of steam reforming of n-heptane over a

nickel based catalyst are represented by (Chen et al.,2003a,b; Christensen, 1996; Nah and Palanki, 2009):

Heptane steam reforming reaction

C7H16 þ 7H2O! 7COþ 15H2 ð1Þ

Methanation reaction (reverse of methane steam reformingreaction)

COþ 3H2�CH4 þH2O ð2Þ

Water gas shift reaction (WGS)

COþH2O�CO2 þH2 ð3Þ

Methane overall steam reforming reaction

CH4 þ 2H2O�CO2 þ 4H2 ð4Þ



Low temperature catalytic reforming of heptane to hydrogen and syngas 3

The rate expression for the heptane steam reforming reaction

(1) is given by (Tottrup, 1982; Nah and Palanki, 2009):

r1 ¼k1PC7H16

1þ 0:2487PC7H16

PH2

PH2Oþ 0:077

PH2O

PH2

h i2 ð5Þ

where

k1 ¼ 8:0� 103 exp � 67:8

RT

� �ðkmol=kPa:kgcat:hÞ; ð6Þ

Reactions (2)–(4) represent a general reaction network devel-oped by Xu and Froment (1989) for methane steam reformingreactions and their intrinsic rate equations are given by:

r2 ¼k2

P2:5H2

PCH4PH2O �

PCOP3H2

K2

!=DEN2 ð7Þ

r3 ¼k3PH2

PCOPH2O �PCO2

PH2

K3

� �=DEN2 ð8Þ

r4 ¼k4

P3:5H2

PCH4P2

H2O�PCO2

P4H2

K4

!=DEN2 ð9Þ

where

DEN ¼ 1þ KCH4PCH4

þ KH2PH2þ KCOPCO þ KH2OPH2O=PH2

ð10Þ

k2 ¼ 9:49� 1016 exp � 240:10

RT

� �; ðkmolkPa0:5=kgcat:hÞ ð11Þ

k3 ¼ 4:39� 104 exp � 67:13

RT

� �; ðkmol=kPa:kgcat:hÞ ð12Þ

k4 ¼ 2:29� 1016 exp � 243:90

RT

� �; ðkmol kPa0:5=kgcat:hÞð13Þ

KCH4¼ 6:65� 10�6 exp

38:28

RT

� �; ðkPa�1Þ ð14Þ

KH2O ¼ 1:77� 105 exp � 88:68

RT

� �; ðdimensionlessÞ ð15Þ

KH2¼ 6:12� 10�11 exp

82:90

RT


KCO ¼ 8:23� 10�7 exp70:65

RT


K2 ¼ 10266:76 exp � 26830:0

Tþ 30:114

� �; ðkPa2Þ ð18Þ

K3 ¼ exp4400:0

T� 4:063

� �ðdimensionlessÞ ð19Þ

K4 ¼ K2K3; ðkPa2Þ ð20Þ

3. Model development

Fig. 1 a shows a schematic diagram of the circulating fast fluid-ized bed membrane reactor (CFFBMR). Fig. 1 b shows a com-posite metallic hydrogen membrane tube in which an effective

very thin layer of Pd–Ag alloy is deposited on the surface of aporous thermostable substrate. The reactor model equationsare derived on the basis of following basic assumptions:

1. Ideal gas behavior.2. Steady state isothermal and isobaric conditions.


3. Plug flow with negligible radial gradients.

4. Negligible diffusion limitations due to the fine catalystparticle use.

5. Hydrogen flux is uniform through the membrane.

In the reaction side, the components’ molar balance givesthe following differential equations:

dFC7H16

dz¼ �qcAcLð1� eÞ½r1� ð21Þ

dFH2O

dz¼ �qcAcLð1� eÞ½7r1 þ r2 þ r3 þ 2r4� ð22Þ

dFCO

dz¼ qcAcLð1� eÞ½7r1 þ r2 � r3� ð23Þ

dFH2

dz¼ qcAcLð1� eÞ½15r1 þ 3r2 þ r3 þ 4r4� �QH2

ð24Þ

dFCH4

dz¼ �qcAcLð1� eÞ½r2 þ r4� ð25Þ

dFCO2

dz¼ qcAcLð1� eÞ½r3 þ r4� ð26Þ

and in the permeation side, the molar balance for hydrogengives:

dFpH2

dz¼ QH2

ð27Þ

where QH2 is the permeation rate of hydrogen through thecomposite membrane and is given by (Shu et al., 1994):

QH2¼ 7:21� 10�5

pNH2dH2

L

dH2

� �exp �15700:0

RT

� � ffiffiffiffiffiffiffiffiPr

H2

q�

ffiffiffiffiffiffiffiffiPp

H2

q� �ð28Þ

where d is Pd–Ag alloy’s film thickness, and ðPrH2Þ and ðPp

H2Þ

are the hydrogen partial pressures (kPa) in the reaction andpermeation sides, respectively. The total hydrogen yield is de-

fined as:

Hydrogen yield ¼ðFH2

þ FpH2Þ � Fo

H2

FoC7H16

ð29Þ

4. Hydrodynamics

The CFFBMR operates in the fast fluidization regime. Thedimensionless catalyst particle diameter (d�c) and gas velocity

(u*) are used to check the fast fluidization regime in the gener-alized map of gas–solid contacting system given by Kunii andLevenspiel (Kunii and Levenspiel, 1991). The dimensionless

catalyst particle diameter (d�c) is given by:

d�c ¼ Ar1=3 ð30Þ

u� ¼ ReP

Ar1=3ð31Þ

where the particle Reynolds number (Rep) and Archimedesnumber (Ar) are given by:

ReP ¼dcuoqg

lg

; Ar ¼ d3cqgðqc � qgÞg

l2g

" #ð32Þ

and uo is the superficial gas velocity.



Sweep gas + H2

Sweep gas

Sweep gas+H2

Feed gas

a

b

Porous support

Thin Pd-Aglayer

H2

H2

H2

H2

Syngas

Cyclone

Catalyst

Figure 1 (a) Schematic representation of the CFFBMR; (b) a composite metallic hydrogen membrane tube.

Table 1 Data used for simulation.

Reactor

Length (m) 2.00

Inside diameter (m) 0.10

Outside diameter (m) 0.12

Catalyst

Diameter (lm) 186

Density (kg/m3) 2835

Solid fraction of reformer 0.20

Feed flow rate (kmol/h)

C7H16 19.20

H2O 10.0

H2 0.1

CO 0.1

CO2 0.0

Hydrogen membrane tube

Outside diameter (m) 0.01

Flow rate of sweep gas stream (kmol/h) 1.00

Pressure of sweep gas stream (kPa) 100.0

4 M.E.E. Abashar

The system of initial value differential Eqs. (21)–(27) is

solved by a Fortran subroutine called Dgear from the IMSL(International Mathematics and Statistics Library), with auto-matic step size, double precision and input relative error boundof 10�14. The data used for the simulation are given in Table 1.


5. Results and discussion

Since there is no experimental data of heptane reforming at alow temperature in the CFFBMR in the literature, this study is

solely a numerical simulation.

5.1. Effect of membrane

The hydrogen membrane has been used to shift the thermody-namic equilibrium for reversible reactions such as reactions (2)–(4) to achieve high conversion and yield. Moreover, the reac-

tions and hydrogen separation can be achieved in one unit.Fig. 2 shows the heptane conversion profiles as a function ofthe dimensionless reactor length of the CFFBMR at a low tem-perature of 350.0 �C. The figure compares the performance of

the CFFBMR without hydrogen membrane and two mem-branes with different thicknesses, a relatively thick membrane(20 lm) used by Chen et al. (2003a,b), and a very thin layer

of composite membrane (5 lm). Techniques such as magnetronsputtering, chemical vapor deposition, solvated metal atom andelectroless plating have been successfully employed to deposit

very thin palladium films (2–10 lm) on mechanically stable sup-ports (Gobina et al., 1995). As it can be seen the thin membranehas a substantial improvement in the heptane conversion. The

reactor without hydrogen membrane achieved 79.02% conver-sion of heptane, while implementation of the thick and thinmembranes gives 87.11% and 97.62% heptane conversion,respectively. The corresponding hydrogen yield is depicted in



0.0 0.2 0.4 0.6 0.8 1.0Dimensionless reactor length

0.00

0.20

0.40

0.60

0.80

1.00H

epta

ne c

onve

rsio

n

= 5.0 m= 20.0 mμ

μ

without H2 membrane

Tf = 350.0 oCP = 15.0 (bar)S/C = 3.5 NH2 = 40 tubes

δδ

Figure 2 Effect of hydrogen membrane on heptane conversion.


0.00

4.00

8.00

12.00

16.00

20.00

Hyd

roge

n yi

eld

= 5.0 m= 20.0 m

without H2 membrane


μμδ

δ

Figure 3 Effect of hydrogen membrane on hydrogen yield.

(a)

(b)


0.00

50.00

100.00

150.00

200.00

250.00

H2/C

O ra

tio

= 5.0 m= 20.0 m

without H2 membrane



0.00

1.00

2.00

3.00

4.00

5.00

H2/C

O ra

tio= 5.0 m= 20.0 m

without H2 membrane


μμ

δδ

μμ

δδ

Figure 4 (a) Effect of hydrogen membrane on H2/CO ratio; (b)

Enlargement part of Fig. 4 a.


0.00

0.10

0.20

0.30

Met

hane

yie

ld

= 5.0 m= 20.0 m

without H2 membrane


μμδ

δ

Figure 5 Effect of hydrogen membrane on methane yield.


Fig. 3. In Fig. 3, the hydrogen obtained by the CFFMBR with-

out hydrogen membrane, thick membrane and thin membraneare 11.87, 13.27 and 17.36, respectively. The thin membranegives a significant increase of hydrogen yield of 30.82% overthe thick membrane and 46.25% higher than the exit hydrogen

yield obtained by the reactor without membrane. Fig. 4a showsthe H2/CO profiles along the dimensionless reactor length ofthe CFFBMR. This ratio is very important for the quality of

the syngas used for the gas to liquid processes (GTL). The rec-ommended optimum value of this ratio lies between 0.7 up to3.0 (Fernandes et al., 2006). All profiles show high spikes of

maxima at the entrance of the reactor. This phenomenon isdue to the small amount of carbon monoxide that has been pro-duced in the beginning of the reactor. Fig. 4b shows an enlarge-ment part of Fig. 4a. It has been shown that the reactor without

hydrogen membrane does not satisfy the recommended practi-cal industrial range of the H2/CO ratio. The thick and thinmembranes produce H2/CO values within the recommended

limits. However, the thin membrane gives lower values of theH2/CO ratio which are reliable in a wider reactor length.Fig. 5 shows the corresponding methane profiles vs. the dimen-

sionless reactor length of the CFFBMR. It is clear that themembrane has suppressed the formation of methane and theeffect of the thin membrane is profound. The suppression of

methane is desirable because this indicates that the removal


of hydrogen has displaced the thermodynamic equilibrium of

the reaction (2) to the left and that of the reaction (4) to theright for more production of hydrogen. The corresponding car-bon dioxide profiles are shown in Fig. 6. The membrane favorsreactions (3) and (4) to the right for more production of hydro-

gen and simultaneous production of carbon dioxide as clearlyreflected in carbon dioxide profiles shown in Fig. 6. The abovediscussion shows that employing a thin layer of membrane

enhances significantly the performance of the CFFBMR.Therefore, in the following analysis we focused on the other




0.00

4.00

8.00

12.00

16.00

20.00

Hyd

roge

n yi

eld

Tf = 350.0 oCP = 15.0 (bar)

= 5.0 mNH2 = 40 tubes

S/C= 3.5

S/C= 2.0

S/C= 5.0δ μ

Figure 8 Effect of steam to carbon feed ratio on hydrogen yield.

(b)

(a)

2.0

4.0

6.0

H2/C

O ra

tio S/C= 2.0S/C= 3.5S/C= 5.0

Tf = 350.0 oCP = 15.0 (bar)

= 5.0 mNH2 = 40 tubes


0.0

100.0

200.0

300.0

400.0

H2/C

O ra

tio

S/C= 2.0S/C= 3.5S/C= 5.0

μδ


0.00

0.40

0.80

1.20C

arbo

n di

oxid

e yi

eld

= 5.0 m= 20.0 m

without H2 membrane


μμδ

δ

Figure 6 Effect of hydrogen membrane on carbon dioxide yield.

6 M.E.E. Abashar

parameters that influence the performance of the CFFBMR

with a thin film membrane.

5.2. Effect of steam to carbon feed ratio

The flow of steam to heptane in the feed is defined as steam tocarbon feed ratio (S/C). In steam reforming processes excessivesteam is required to favor reversible reactions such as reactions

(2)–(4) for more production of hydrogen and to minimize thedeposition of carbon over the catalyst to prevent the catalystdeactivation. Fig. 7 shows the influence of different steam tocarbon feed ratios (S/C = 2.0, 3.5, 5.0) on the heptane conver-

sion profiles along the dimensionless reactor length of theCFFBMR at a low temperature of 350.0 �C and a thin mem-brane of thickness 5 lm. The exit heptane conversions ob-

tained by these ratios are 60.49%, 97.62% and 99.16%,respectively. Both steam to carbon feed ratios of 3.5 and 5.0give high exit heptane conversions. However, using the steam

to carbon feed ratio of 5.0 has attained an almost completeconversion of heptane at a short reactor length of about79.5% of the reactor length. This reduction in the reactor

length has a significant impact in lowering the reactor cost.The corresponding hydrogen yield along the dimensionlessreactor length of the CFFBMR for various steam to carbon


0.00

0.20

0.40

0.60

0.80

1.00

Hep

tane

con

vers

ion

Tf = 350.0 oCP = 15.0 (bar)

= 5.0 mNH2 = 40 tubes

S/C= 3.5

S/C= 2.0

S/C= 5.0μδ

Figure 7 Effect of steam to carbon feed ratio on heptane

conversion.


0.0

Figure 9 (a) Effect of steam to carbon feed ratio on H2/CO

ratio; (b) Enlargement part of Fig. 9a.


feed ratios is shown in Fig. 8. The results show that exit hydro-gen yields of 9.31, 17.36 and 19.45 are obtained by these ratios.

This indicates that increasing steam to carbon feed ratios by75% from S/C = 2.0 to 3.5 increases the exit hydrogen yieldby 86.47% and increasing steam to carbon feed ratios by

150% from S/C = 2.0 to 5.0 increases the exit hydrogen yieldby 108.92%. The steam to carbon feed ratio of 5.0 gives thehighest hydrogen yield due to the fact that the increase of

steam has shifted the thermodynamic equilibrium of reaction(2) to the left and of reactions (3) and (4) to the right for more




0.00

0.10

0.20

0.30

0.40

0.50M

etha

ne y

ield

Tf = 350.0 oCP = 15.0 (bar)

= 5.0 mNH2 = 40 tubes

S/C= 3.5

S/C= 5.0

S/C= 2.0δ μ

Figure 10 Effect of steam to carbon feed ratio on methane yield.


0.00

0.40

0.80

1.20

1.60

2.00

Car

bon

diox

ide

yiel

d

Tf = 350.0 oCP = 15.0 (bar)

= 5.0 mNH2 = 40 tubes

S/C= 3.5

S/C= 2.0

S/C= 5.0δ μ

Figure 11 Effect of steam to carbon feed ratio on carbon dioxide

yield.


0.00

0.20

0.40

0.60

0.80

1.00

Hep

tane

con

vers

ion

Tf = 350.0 oCS/C = 5.0

= 5.0 mNH2 = 40 tubes

P= 25.0 bar

P= 15.0 bar

P= 10.0 bar

δ μ

Figure 12 Effect of reaction side pressure on heptane

conversion.


0.00

5.00

10.00

15.00

20.00

25.00

Hyd

roge

n yi

eld

Tf = 350.0 oCS/C = 5.0

= 5.0 mNH2 = 40 tubes

P= 25.0 bar

P= 15.0 bar

P= 10.0 bar

δ μ

Figure 13 Effect of reaction side pressure on hydrogen yield.


hydrogen production. Fig. 9a shows the corresponding H2/COratio profiles. Qualitative similar trends of sharp spikes arealso presented for all ratios. The enlargement part of Fig. 9a

shown in Fig. 9b shows that the exit H2/CO ratios for all steamto carbon feed ratios of 2.0, 3.5, 5.0 are almost the same andwithin the recommended industrial range. Fig. 10 shows thecorresponding methane yield profiles. As it can be seen that

the increase of steam to carbon feed ratio has a positive impactin the substantial decrease of methane yield. This effect couldbe due to the displacement of thermodynamic equilibriums of

reactions (2) and (4) in the positive direction of more methaneconsumption and hydrogen production. The consequence of alow methane yield at high steam to carbon feed ratios is asso-

ciated with a high carbon dioxide production as reactions (3)and (4) suggest and is shown in Fig. 11.

5.3. Effect of reaction side pressure

The influence of the reaction side pressure is very complexsince it affects the thermodynamic equilibriums of the revers-ible reactions and the permeation of hydrogen through the


membrane and these effects are interrelated in a very complexmanner. According to Le Chatelier’s principles the thermody-

namic equilibrium can be shifted by addition or removal ofheat, addition or removal of reactants and products andincreasing or decreasing the pressure. The increase of the pres-

sure favors reactions with decreasing number of moles such asreactions (2) to the right and (4) to the left at the same time en-hances the permeation rate of hydrogen i.e. removal of hydro-gen for reactions (2)–(4). This complex interaction affects the

partial pressures of heptane, hydrogen and steam and affectsimplicitly the rate of heptane steam reforming reaction (1) asshown in Eq. (5), which has an overall positive effect on hep-

tane conversion as shown in Fig. 12. At side reaction pressuresof 10.0 bar, 15.0 bar and 25.0 bar the corresponding exit hep-tane conversions are 91.57%, 99.16% and 99.67%,

respectively. At the side reaction pressure of 25 bar, almostcomplete conversion of heptane occurs at about 74.31% fromthe reactor length. It is shown in Fig. 13 the corresponding

profiles of hydrogen yield along the dimensionless reactorlength of the CFFBMR for various side reaction pressures(10.0 bar, 15.0 bar, 25.0 bar). The exit hydrogen yields forthese pressures are 14.18, 17.36 and 21.18, respectively. The



(b)

(a)


0.00

2.00

4.00

6.00

H2/C

O ra

tio P= 10.0 barP= 15.0 barP= 25.0 bar

Tf = 350.0 oCS/C = 5.0

= 5.0 mNH2 = 40 tubes


0.00

100.00

200.00

300.00

400.00H

2/CO

ratio P= 10.0 bar

P= 15.0 barP= 25.0 bar

Tf = 350.0 oCS/C = 5.0

= 5.0 mNH2 = 40 tubesδ μ

δ μ

Figure 14 (a) Effect of reaction side pressure on H2/CO ratio; (b)

Enlargement part of Fig. 9 a.


0.00

0.04

0.08

0.12

0.16

Met

hane

yie

ld

Tf = 350.0 oCS/C = 5.0

= 5.0 mNH2 = 40 tubes

P= 25.0 bar

P= 15.0 bar

P= 10.0 bar

δ μ

Figure 15 Effect of reaction side pressure on methane yield.


0.00

0.50

1.00

1.50

2.00

2.50

Car

bon

diox

ide

yiel

d

Tf = 350.0 oCS/C = 5.0

= 5.0 mNH2 = 40 tubes

P= 25.0 bar

P= 15.0 bar

P= 10.0 bar

δ μ

Figure 16 Effect of reaction side pressure on carbon dioxide

yield.

8 M.E.E. Abashar

increase of the side reaction pressure from 10.0 bar to 15.0 bargives 22.43% increase exit hydrogen yield, while the increase ofthe side reaction pressure from 10.0 bar to 25.0 bar gives49.36% increase in the exit hydrogen yield. It is clear now that

the high reaction side pressure has a great impact on the per-formance of the CFFBMR. The corresponding H2/CO ratioprofiles for side reaction pressures of 10.0 bar, 15.0 bar and

25.0 bar are shown in Fig. 14a. As it can be seen that the phe-nomena of spike maximum appear for all pressures after whichthe profiles drop drastically to lower values as shown by the

enlargement part in Fig. 14b. It is shown in Fig. 14b that allthe profiles can satisfy the recommended industrial range ina wide range of the dimensionless reactor length of theCFFBMR. However, the high pressure profile gives lower val-

ues of the H2/CO ratio. The effect of several reaction side pres-sures on the methane yield is shown in Fig. 15. It is interestingthat the exit methane yield decreases with the increase of the

reaction side pressure. To explain this interesting trend, letus follow the forces affected by increasing the reaction sidepressure. The increase of the side reaction favors the reaction

(2) to the right for more production of methane due to the de-crease of number of moles at the same time favors the reaction(2) to the left by removal of hydrogen from the reaction media

by the membrane for more consumption of methane, also theincrease of the reaction side pressure favors the reaction (4) tothe left side of decreasing number of moles for more produc-tion of methane and to the right for more consumption of


methane by increasing the hydrogen permeation rate throughthe membrane. It is clear that in reactions (2) and (4) theincrease of side reaction pressure is acting to enhance two

opposing forces and the dominant one determines the directionof the thermodynamic equilibrium. Here, we have three possi-bilities. The first possibility is the removal of hydrogen that is

dominant for reactions (2) and (4) and shifting both thermody-namic equilibriums in the direction of hydrogen productionand methane consumption i.e. left and right, respectively.

The second possibility is the increase of the reaction side pres-sure that favors the reaction (2) for more production of meth-ane and favors the reaction (4) for more consumption ofmethane and the consumption of methane is more than its pro-

duction. The third possibility is the increase of the reaction sidepressure that favors the reaction (2) for more consumption ofmethane and the reaction (4) for production of methane and

the consumption is higher than formation. The correspondingcarbon dioxide yield in Fig. 16 shows that the increase of thereaction side pressure decreases the carbon dioxide yield. The

sources of carbon dioxide formation from the reaction schemeare reactions (3) and (4). The increase of the side reaction pres-sure favors the reaction (3) at all time in one direction for moreformation of carbon dioxide due to the boosting of hydrogen




permeation driving force, while for the reaction (4) can goeither ways. In this case we have only one possibility that isthe increase of reaction side pressure that favors the consump-

tion of carbon dioxide by the reaction (4) and the rate of con-sumption of this reaction is higher than the rate of formationby reaction (2), which has resulted in a drop of carbon dioxide

yield by increasing the reaction side pressure. As it can be seenthe picture is very complex.

6. Conclusions

In this preliminary study, modeling and simulation have beenutilized to predict the performance of the CFFBMR for pro-

duction of hydrogen and syngas by steam reforming of hep-tane at a low temperature. The advantages of a thin layerhydrogen membrane coupled with the circulating fast fluidiza-

tion hydrodynamics are this novel reformer is capable toproduce hydrogen and syngas at a low temperature achievingalmost 100% conversion of heptane and a very high hydrogenyield with syngas characteristics within the recommended

industrial limits. The ultra-clean hydrogen produced is suitableas a feed for fuel cells and the syngas as a feedstock for theGTL processes. This investigation has shown that keeping

the steam to carbon feed ratio and reaction side pressure athigh levels have profound effects in the improvement of theCFFBMR performance. The study has proven that this unit

can be a cheap alternative to the expensive conventional steamreforming units. Despite the basic theoretical understandinggiven by this study, future experimental results are requiredfor the proper design of this novel unit.

Acknowledgements

This project was supported by King Saud University, Dean-ship of Scientific Research, College of Engineering Research

Center.

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Low temperature catalytic reforming of heptane to hydrogen and syngas