Steam reforming of n-heptane for production of hydrogen and syngas

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Steam reforming of n-heptane for production of hydrogen andsyngas

M.E.E. Abashar*

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

a r t i c l e i n f o

Article history:

Received 3 August 2012

Received in revised form

3 October 2012

Accepted 15 October 2012

Available online 27 November 2012

Keywords:

Heptane reforming

Hydrogen

Membrane reactor

Modeling and simulation

Syngas

* Tel.: þ966 1 4675843; fax: þ966 1 4678770E-mail address: [email protected].

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

Simultaneous production of hydrogen and syngas from the catalytic reforming of

n-heptane in circulating fast fluidized bed reactors (CFFBR) and circulating fast fluidized

bed membrane reactors (CFFBMR) is investigated. This paper presents modeling and

simulation approach for the analysis of these reformers. Complete conversion of heptane

(100%) is attained at high steam to carbon feed ratios and shorter reactor lengths by both

configurations. However, the CFFBMR is very efficient in hydrogen production and can

produce exit hydrogen yield up to 473.14% higher than the CFFBR. It was found that

operating the CFFBMR at the optimal conditions results in a minimum value of hydrogen to

carbon monoxide ratio (H2/CO) within the recommended practical range for the syngas

used as a feedstock for the gas to liquid processes (GTL). The results of the sensitivity

analysis conducted for the CFFBMR has shown that the reaction side pressure and the feed

temperature have significant effects on increasing the heptane conversion (up to 100%) and

the temperature effect is stronger than the reaction side pressure effect. Considerable

improvement in the hydrogen to carbon monoxide ratio (H2/CO) has been achieved by

increasing the reaction side pressure, while the high feed temperature has negative effect

on this ratio. It seems that the practical range of H2/CO ratio can be achieved by controlling

the reformer length and the right combinations of the operating conditions.

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

reserved.

1. Introduction Hydrocarbons steam reforming has been widely used

The interest of research and industry in efficient production of

hydrogen and syngas has increased in recent years. Hydrogen

has been known as the promising clean fuel of the future [1,2].

Presently, fuel cells have been utilized hydrogen as fuel. A

good example is hydrogen-powered vehicles. Moreover,

hydrogen has wide applications in many essential industries

such as ammonia, methanol and refinery. The growing

attention to the production of syngas (H2, CO, CO2) is due to its

conversion potential to clean liquid fuels free from sulfur such

as gasoline and diesel fuels [3,4].

.

2012, Hydrogen Energy P81

for production of hydrogen and syngas [CnHm þ n

H2O ¼ nCOþ(n þ m/2)H2]. The expensive and most cost-

effective industrial thermochemical process available now

for production hydrogen and syngas is the catalytic steam

reforming of methane [5,6]. This conventional process has

series problems of diffusion and thermodynamic limitations,

catalyst deactivation and high energy cost due the huge

furnaces used. For example, the production of syngas that

used as a feedstock for the gas to liquid processes (GTL) such

as the FischereTropsch (FT) processmay cost about 70% of the

capital and running costs of the total plant [3]. All these factors

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

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http://dx.doi.org/10.1016/j.ijhydene.2012.10.081



Sweep gas + H2

Sweep gas+

H2

Porous support

Thin Pd-Aglayer

H2

Syngas

Cyclone

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 ) 8 6 1e8 6 9862

have stimulated the research and industry to develop cost-

effective technologies for efficient production of hydrogen

and syngas [7e9].

In the last decade, much attention has been paid to the

palladium-alloy membranes due to their permselectivity and

high permeability of hydrogen [10e17]. Furthermore, the

thermodynamic equilibrium of reversible reactions can be

displaced by the membrane to achieve high conversion and

yield. The palladium is an attractivemetal because can absorb

hydrogen of about 600 times its volume at normal tempera-

ture and is not liable to form a refractory oxide film that can

tremendously decrease the permeability of hydrogen [17]. The

introduction of alloys such as silver overcomes the phase

change of palladium from a to b [17]. Hughes [17] has

summarized the characteristics of a good hydrogen

membrane as follows: high hydrogen permeability, high

hydrogen selectivity, resistant to poison, reliable at severe

operating conditions such as elevated temperatures and it can

be adequately sealed in the reactor. He also pointed out the

preparation methods for composite membranes as follows:

chemical vapor deposition, electroplating, liquid impregna-

tion, electroless plating, magnetron sputtering, pyrolysis and

micro-emulsion techniques.

In recent years, there has been growing attention for

production of hydrogen and syngas fromhigher hydrocarbons

such as heptane [18,19,21e27]. Elnashaie and co-workers have

reported that the novel CFFBMR is a promising efficient

reformer for clean hydrogen and syngas production

[1,2,18,19]. Surprisingly, there are only few published studies

in modeling and simulation of steam reforming of heptane in

the CFFBMR [18,19]. This preliminary numerical simulation

study is an extension to the work of Elnashaie and co-workers

[18] to investigate further the potential of steam reforming of

heptane as an efficient alternative production route. In this

study we implemented a high and efficient flux composite

hydrogenmembrane of thin layer (3 mm) of palladium alloy on

a porous support. This configuration increases the clean

hydrogen permeability through the membrane and reduces

the capital cost of the CFFBMR. Moreover, this study has

a special emphasis on the parameters affect the H2/CO ratio to

achieve the desired recommended range (0.7e3.0) for the

syngas used as a feedstock for the GTL processes such as the

FT processes.

Sweep gas

Feed gas

a

b

H2

H2

H2

Catalyst

Fig. 1 e (a) Schematic representation of the CFFBMR; (b)

a composite metallic hydrogen membrane tube.

2. Reactor model

The circulating fast fluidized bed membrane reactor model is

implemented in this study to minimize the serious problems

of the conventional steam reforming processes of low effec-

tiveness factors (10�2�10�3), catalyst deactivation due to

carbon formation and the thermodynamic limitations

[1,2,5,6]. The main attractive features of this model can be

summarized as follows [2,18e20]:

1. The catalyst can be easily regenerated by smooth circula-

tion of the solids between the reactor and another catalyst

regenerator.

2. The diffusion limitation is eliminated (effectiveness factors

are almost equal to unity) by using very fine particles.

3. The application of hydrogen membranes has significant

impact on shifting the thermodynamic equilibrium for

higher conversion and hydrogen yield and reduction of the

elevated operating temperatures.

4. Good gasesolid contacting and the gas throughputs per

unit cross-section is very high.

5. Good control of circulating catalyst.

The main difference between the CFFBR and CFFBMR is

inserted hydrogenmembrane tubes in the CFFBMR. The tubes

inserted in the circulating fast fluidized bed decrease the free

cross-sectional area of the reactor for the catalyst circulation,

giving higher circulating velocity and improving the solid

circulating rate. Fig. 1 shows a simplified schematic drawing

of the CFFBMR. Fig. 1b shows an effective composite hydrogen

membrane tube made of a film of PdeAg alloy deposited on

a porous support [28]. Steam is a suitable sweep gas because

its separation from hydrogen is easy. The syngas and the

catalyst are separated in the cyclone. The recycled catalyst

maintains constant amount of catalyst inside the reactor.

The kinetics of heptane steam reforming reactions over

a nickel based catalyst is represented by following scheme of

reactions [18,25,26]:

C7H16 þ 7H2O/7COþ 15H2 (1)

COþ 3H2#CH4 þH2O (2)

COþH2O#CO2 þH2 (3)

CH4 þ 2H2O#CO2 þ 4H2 (4)

The rate expression for the heptane steam reforming reac-

tion (1) is given by [24,26]:



i n t e r n a t i o n a l j o u rn a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 8 6 1e8 6 9 863

r1 ¼ k1PC7H16� �2 (5)

1þ 0:2487PC7H16

PH2

PH2Oþ 0:077

PH2O

PH2

The rate equations for reactions (2)e(4) are given by [29]:

r2 ¼ � k2

P2:5H2

PCH4PH2O � PCOP3

H2

K2

!,DEN2 (6)

r3 ¼ k3

PH2

�PCOPH2O � PCO2

PH2

K3

��DEN2 (7)

r4 ¼ k4

P3:5H2

PCH4P

2H2O

� PCO2P4H2

K4

!,DEN2 (8)

where:

DEN ¼ 1þ KCH4PCH4

þ KH2PH2

þ KCOPCO þ KH2OPH2O=PH2(9)

and Pj, ki, Ki are partial pressure of component j (kPa), rate and

equilibrium constants of reaction i, respectively. Reactions

(2)e(4) are reversible reactions and the direction of the ther-

modynamic equilibrium depends on the conditions prevail in

the reactionsmedia. The reaction rate parameters are given in

Table 1.

The following are the main assumptions used for the

model formulation [1,2]:

1. Steady state conditions.

2. Negligible radial gradient.

3. Fine catalyst particles are used to eliminate the high

diffusion limitations.

4. The circulating fast fluidization approaches the plug flow

i.e. closeness to a pseudo homogeneous model.

5. The membrane is not catalytic.

Table 1 e Kinetics parameters [1,24,26,29].

Reaction rate constants ki ¼ Ai exp(�Ei/RT )

Parameter Ai Ei (kJ/mol)

k1 (kmol/kPa kgcat h) 8.00 � 103 67.80

k2 (kmol kPa0.5/kgcat h) 9.49 � 1016 240.10

k3 (kmol/kPa kgcat h) 4.39 � 104 67.13

k4 (kmol Pa0.5/kgcat h) 2.29 � 1016 243.90

Reaction adsorption constants Ki ¼ Ai exp(�DHi/RT )

Parameter Ai DHi (kJ/mol)

KCH4 ðkPa�1Þ 6.65 � 10�6 �38.28

KH2Oð�Þ 1.77 � 105 88.68

KH2 ðkPa�1Þ 6.12 � 10�11 �82.90

KCO (kPa�1) 8.23 � 10�7 �70.65

Reaction equilibrium constants

K2ðkPa2Þ ¼ 10266:76 expð�ð26830:0=TÞ þ 30:114ÞK3ð�Þ ¼ expðð4400:0=TÞ � 4:063ÞK4 (kPa

2) ¼ K1K2

6. Hydrogen flux is uniform through the membrane. This

means that the structural characteristics (porosity, tortu-

osity, thickness) of the membrane are uniform giving

a uniform hydrogen flux in the radial direction at each

point along the length of the reactor and varies in the

radial direction due to change of the hydrogen flux driving

force.

7. The pressure is kept constant in the permeation and reac-

tion sides.

8. The reactor operates at isothermal conditions. The iso-

thermality in the reactor can be achieved by many

configurations and their combinations such as the reac-

toreregenerator configuration in which the regeneration

supplies the necessary heat for the endothermic reactions

in the reformer, in situ heat integration by coupling the

endothermic reactions with exothermic reactions using

well mixed or bifunctional catalysts and insertion of heat-

ing tubes.

The components molar balance in reaction side gives the

following differential equations:

dFC7H16

dz¼ �rcAcLð1� 3Þ½r1� (10)

dFH2O

dz¼ �rcAcLð1� 3Þ½7r1 � r2 þ r3 þ 2r4� (11)

dFCO

dz¼ rcAcLð1� 3Þ½7r1 � r2 � r3� (12)

dFH2

dz¼ rcAcLð1� 3Þ½15r1 � 3r2 þ r3 þ 4r4� � QH2

(13)

dFCH4

dz¼ �rcAcLð1� 3Þ½�r2 þ r4� (14)

dFCO2

dz¼ rcAcLð1� 3Þ½r3 þ r4� (15)

where Fm, z, rc,Ac, L, 3, ri are flow rate of componentm (kmol/h),

dimensionless reactor length, catalyst density (kg/m3), cata-

lyst circulation area (m2), length of the reactor (m), void frac-

tion, reaction rate of component i (kmol/kgcat h),

respectively.In the permeation side, the hydrogen mole

balance gives:

dFpH2

dz¼ QH2

(16)

The rate of hydrogen permeation ðQH2Þ is given by [13]:

QH2¼ 7:21� 10�5

�pdH2

NH2L

dH2

�exp

��15700:0

RT

�� ffiffiffiffiffiffiffiffiPrH2

q�

ffiffiffiffiffiffiffiffiPpH2

q (17)

where dH2, NH2

, dH2, R, T, PH2

are hydrogen membrane tube

diameter (m), number of hydrogen membrane tubes, thick-

ness of membrane (mm), gas constant (kJ/mol K), temperature

(K) and hydrogen partial pressure (kPa), respectively.

The total number of moles of component i that have been

produced by a unit mole of heptane in the feed is expressed as

dimensionless yield:



Table 2 e Simulation data.


Yield of component i ¼ Fi � Foi

Fo (18)

Reformer length (m) 2.00

Reformer inside diameter (m) 0.10

Reformer outside diameter (m) 0.12

Catalyst particle diameter (mm) 186

Catalyst density (kg/m3) 2835

Tube outside diameter (m) 0.01

Membrane side pressure (kPa) 100.0

Initial conditions (S/C¼1.0)

FoC7H161.43

FoH2O10.0

FoCO 0.1

FoH20.1

FoCH40.0

FoCO20.0

FPoH20.0

C7H16

The CFFBR and CFFBMR operate within the fast fluidization

region. In this region, the gas velocity is very high and fine

solids dispersed in the gas with following hydrodynamics

characteristics [20]:

1. Extensive back mixing of solids.

2. Solid concentration somewhere between dense-phase beds

and pneumatic transport conditions.

3. Clusters and strands of particles that break apart and

reform in quick succession.

4. Slip velocity of particles one order of magnitude larger than

the particle terminal velocity.

The physical properties of the phases in the reactor and the

superficial gas velocity affect the fast fluidization hydrody-

namics. The following dimensionless quantities:

ReP ¼dcuorg

mg

; Ar ¼ d3c

"rg

�rc � rg

g

m2g

#(19)

u� ¼ ReP

Ar1=3(20)

d�c ¼ Ar1=3 (21)

are used to locate the fast fluidization regime as shown by

Kunii and Levenspiel [20]. Where dc, uo, rg, Ar, u*, d�c , Rep, mg are

catalyst pellet diameter (m), superficial gas velocity (m/s), gas

density (kg/m3), dimensionless Archimedes number, dimen-

sionless gas velocity, dimensionless catalyst particle diam-

eter, dimensionless particle Reynolds number and gas

viscosity (kg/m s), respectively. The dimensionless particle

size (dc) and dimensionless gas velocity (u*) are used as axes

for the general flow diagram of fluidization regimes because

these dimensionless quantities are manipulated variables

which can be adjusted freely.

The reformer model equations (10)e(16) are initial

value differential equations and their initial conditions are

the feed molar flow rates of the components ðFoC7H16; FoH2O

;

FoCO; FoH2; FoCH4

; FoCO2; FPoH2

Þ and given in Table 2. The equations

were solved by a FORTRAN subroutine. Fibonacci method has

been used to find the optimum values. The simulation data is

presented in Table 2.

Fig. 2 e Comparison of the CFFBR and CFFBMR: Effect of

steam to carbon feed ratio (S/C) on heptane conversion.

3. Results and discussion

Only simulated results are presented here because the

experimental data of steam reforming of heptane in the CFFBR

and CFFBMR is scarce.

3.1. Effect of steam to carbon feed ratio (S/C)

The feed ratio of steam to heptane flow in the feed is usually

expressed by steam to carbon feed ratio (S/C). Excess steam is

usually desirable to favor the reforming reactions such as

reactions (2)e(4) for more production of hydrogen and syngas

and to prevent carbon deposition over the catalyst. Fig. 2

shows the effect of different S/C feed ratios on the heptane

conversion profiles in the CFFBR and CFFBMR at 400 �C. Theresults show that adding more steam coupled with the

hydrogen membrane substantially enhances the heptane

conversion achieved by the CFFBMR. In the case of S/C feed

ratio of 3.0 complete conversion of heptane (100% conver-

sion) is attained by both reactors, but the CFFBMR achieves

this high heptane conversion at a shorter reactor length.

However, at an S/C ¼ 5.0 the membrane has a limited effect

and the significant effect of increasing the S/C feed ratio to

high values on reducing the effective dimensionless reactor

length is obvious. We mean by the effective dimensionless

reactor length is the length at which almost complete

conversion of heptane is achieved. The effect of the

membrane is limited because the complete conversion of

reaction (1) occurs at the beginning of the reactor at a limited

membrane permeation area. Fig. 3 shows the corresponding

hydrogen yield profiles. Significant improvement of

hydrogen yield is gained by implementing the hydrogen

membrane in the CFFBMR. Inflection points of maximum



0.0 0.2 0.4 0.6 0.8 1.0

Dimensionless reactor length

0.00

4.00

8.00

12.00

16.00

Hy

dro

ge

ny

ie

ld

S/C=1.0 CFFBR

S/C=1.0 CFFBMR

S/C=3.0 CFFBR

S/C=3.0 CFFBMR

S/C=5.0 CFFBR

S/C=5.0 CFFBMR

Tf = 400.0 oC

P = 20.0 (bar)

NH2 = 30 tubes

= 3.0 m

Fig. 3 e Comparison of the CFFBR and CFFBMR: Effect of

steam to carbon feed ratio (S/C) on hydrogen yield.


nature are developed for the cases of S/C ¼ 3.0 and S/C ¼ 5.0.

These inflection points occur at locations corresponding to

100% conversion of heptane shown in Fig. 2, i.e. reaction (1)

completely stopped. In case of the CFFBR, the hydrogen

yield has been dropped drastically after the maxima to low

equilibrium values almost the same as the exit hydrogen

yield at S/C ¼ 1.0. This substantial decrease in the hydrogen

yield could be due to 100% conversion of heptane and no

further supply of hydrogen by reaction (1) and a part of the

hydrogen available in the reactions media is consumed by

one or more of reactions (2)e(4) according to their thermo-

dynamic equilibrium directions. For the CFFBMR, the figure

shows that a weak drop in the hydrogen yield at S/C ¼ 5.0

occurs after the maximum point. Mild steady increases of

hydrogen yield after the maximum point for the case of S/

C ¼ 3.0 is observed. An interesting observation that the exit

hydrogen yield obtained by both reactors at S/C¼ 3.0 is higher

than that at S/C ¼ 5.0.

1.0 2.0 3.0 4.0 5.0

Steam to carbon feed ratio (S/C)

0.00

4.00

8.00

12.00

16.00

Ex

it

hy

dro

ge

ny

ie

ld

CFFBR

CFFBMR

Tf = 400.0 oC

P = 20.0 (bar)

NH2 = 30 tubes

= 3.0 m

Fig. 4 e Comparison of the CFFBR and CFFBMR: exit

hydrogen yield as a function of steam to carbon feed ratio

(S/C).

3.2. The optimal conditions

Fig. 4 depicts the exit hydrogen yield vs S/C feed ratio. The

profileof theexithydrogenyieldobtainedby theCFFBMRhasan

inflection point of a maximum value (13.87) at which the S/C is

optimal at a value of 3.23, while the profile of the CFFBR shows

a weak maximum point. The reason for occurrence of this

phenomenon is thedevelopment of themaximumpoint shown

in Fig. 3. At the optimal value of S/C ¼ 3.23 the ratio of exit

hydrogen yield obtainedby theCFFBMRandCFFBR is 13.87: 2.42

i.e. 473.14% increase in the exit hydrogen yield is achieved by

the CFFBMR. Fig. 5 shows the hydrogen to carbon monoxide

ratio (H2/CO) profiles for the reactors at the optimal conditions.

The hydrogen to carbonmonoxide ratio (H2/CO) in this study is

for the reaction side only. This ratio is very important for the

quality of the syngas used for the GTL. The recommended

optimumvalueof this ratio liesbetween0.7up to 3.0 [30]. Ascan

be seen in Fig. 5, the H2/CO ratio profiles assume inflection

points of minimum nature. The minimum value of H2/CO ratio

obtained by the CFFBR (8.55) is too high than the recommended

industrial limits. The CFFBMR achieves the minimum value of

H2/CO ratio of 0.74597 at a dimensionless reactor length of 0.57

as shown in the enlargement part of Fig. 5. It seems that the

controlling of the length of the CFFBMR is essential to achieve

the practical range of the H2/CO ratio. The corresponding

methane and carbon dioxide yield profiles are shown in Fig. 6a

and b, respectively. Fig. 6a shows that the hydrogenmembrane

is suppressing the formation ofmethanewhich is desirable, i.e.

shifting the thermodynamic equilibrium of reaction (2) to the

left and of reaction (4) to right to favor more hydrogen forma-

tion. In addition, the thermodynamic equilibriumof reaction (3)

is shifted to the right by the membrane to enhance further the

hydrogen formation. This positive shift of the thermodynamic

equilibrium of reactions (3) and (4) is accompanied by more

production of carbon dioxide as shown in the corresponding

carbon dioxide profile in [Fig. 6b]. The above discussion shows

clearly the superiority of theCFFBMRover theCFFBR.Therefore,

the next parametric sensitivity analysis is conducted for the

CFFBMR at the optimal conditions of S/C ¼ 3.23 that gives

maximum hydrogen yield of 13.87 as shown in Fig. 4.

0.0 0.2 0.4 0.6 0.8 1.0


0.00

40.00

80.00

120.00

H2/C

O ratio

CFFBR

CFFBMR

Tf = 400.0 oC

P = 20.0 (bar)

S/C = 3.23

NH2 = 30 tubes

= 3.0 m

Fig. 5 e Comparison of the CFFBR and CFFBMR at the

optimal conditions: H2/CO ratio profiles.



0.0 0.2 0.4 0.6 0.8 1.0


0.00

2.00

4.00

6.00M

eth

an

ey

ie

ld

without H2 membrane

with H2 membrane

Tf = 400.0 oC

P = 20.0 (bar)

S/C = 3.23

NH2 = 30 tubes

= 3.0 m

0.0 0.2 0.4 0.6 0.8 1.0


0.00

2.00

4.00

6.00

Ca

rb

on

dio

xid

ey

ie

ld

without H2 membrane

with H2 membrane

Tf = 400.0 oC

P = 20.0 (bar)

S/C = 3.23

NH2 = 30 tubes

= 3.0 m

a

b

Fig. 6 e Comparison of the CFFBR and CFFBMR at the

optimal conditions: (a) methane yield profiles, (b) carbon

dioxide yield profiles.

0.0 0.2 0.4 0.6 0.8 1.0


0.00

0.20

0.40

0.60

0.80

1.00

He

pta

ne

co

nv

ers

io

n

P = 20.0 (bar)

S/C = 3.23

NH2 = 30 tubes

= 3.0 m

Tf

=400

o

C

Tf=

300

o C

Tf = 500o

C

Fig. 7 e Heptane conversion at different feed temperatures.

0.0 0.2 0.4 0.6 0.8 1.0


0.00

4.00

8.00

12.00

16.00

20.00

Hy

dro

ge

ny

ie

ld

P = 20.0 (bar)

S/C = 3.23

NH2 = 30 tubes

= 3.0 m

Tf =400

oC

Tf =300

o C

Tf=

500

o C

Fig. 8 e Heptane yield at different feed temperatures.


3.3. Sensitivity analysis for the CFFBMR at the optimalconditions

Le Chatelier’s principles show that the direction of displace-

ment of the thermodynamic equilibrium depends on many

factors such as: addition or removal of reactants and products,

addition or removal of heat and increasing or decreasing of

the pressure. The addition of a reactant and removal of

a product has been demonstrated above by the influence of

the S/C feed ratio and the effect of the hydrogen removal from

the reactions media by the membrane. In the following anal-

ysis we investigated further the effect of other remaining

factors of the feed temperature and reaction side pressure.

3.3.1. Influence of feed temperatureThe feed temperature has profound effect on the reactions

rate constants and participates in supplying the necessary

heat required to shift the thermodynamic equilibriums of the

endothermic reactions. Fig. 7 shows the heptane conversion

profiles for various feed temperatures at the optimal condition

of S/C ¼ 3.23. The figure shows that the feed temperature has

a significant effect in increasing the heptane conversion, for

example the exit heptane conversion increased from 38.05%

to 100% due to the rise of the feed temperature by 100 �C from

300 �C to 400 �C, respectively. At the same time, the dimen-

sionless reactor length is reduced from 100% to 43%, respec-

tively. This reduction in the effective reactor length by

increasing the feed temperature has an important impact in

reducing the reactor cost. However, the feed temperature

effect is more pronounced at low feed temperatures e.g. from

300 �C to 400 �C than at high feed temperatures e.g. from

400 �C to 500 �C. The corresponding hydrogen yield profiles at

different feed temperatures are shown in Fig. 8. The figure

shows that considerable improvement in exit hydrogen yield

has been achieved by the rise of the feed temperature. It is also

shown that weak inflection points appear at 400 �C and 500 �C,whichmight have a negative effect and cause a drop along the

hydrogen yield profile. This drop is caused by the complete

conversion of heptane and no further contribution of reaction

(1) to supply hydrogen. Fig. 9 shows the corresponding H2/CO

ratio profiles at different feed temperatures. As it can be seen

that at a feed temperature of 500 �C the profile tends to reflect



0.0 0.2 0.4 0.6 0.8 1.0


0.00

4.00

8.00

12.00

16.00

20.00

24.00

28.00

H2

/C

Ora

tio

P = 20.0 (bar)

S/C = 3.23

NH2 = 30 tubes

= 3.0 m

Tf

=400

C

Tf = 300 C

Tf=

500

C

Fig. 9 e H2/CO ratio at different feed temperatures.0.0 0.2 0.4 0.6 0.8 1.0


0.00

4.00

8.00

12.00

16.00

20.00

Hy

dro

ge

ny

ie

ld

Tf = 400o

C

S/C = 3.23

NH2 = 30 tubes

= 3.0 m

P= 10 (bar)

P = 30 (bar)

P = 20 (bar)

Fig. 11 e Hydrogen yield at different reaction side

pressures.

Tf = 400o

C


high values of H2/CO ratio that out of the industrial recom-

mended range. An important and interesting result is that the

profile at 400 �C shows aminimum value of H2/CO ratio within

the recommended range and less than the values obtained at

300 �C. This implies that further rigorous optimization is

needed for the entire parameter space.

3.3.2. Influence of reaction side pressureThe effect of the reaction side pressure is very complex

because it affects simultaneously the reversible reactions and

hydrogen permeation, which are strongly interrelated. The

increase of the reaction side pressure favors reactions (2) and

(4) in the direction of decreasing number of moles i.e. to right

and left for more production of methane and steam, respec-

tively. Also, the reaction side pressure enhances the perme-

ation rate of hydrogen to favor reactions (2)e(4) for more

hydrogen production i.e. thermodynamic equilibrium

displacement to left and right, respectively. This implies that

the thermodynamic equilibriums of reactions (2) and (4) are

under opposite driving forces of decreasing number of moles

and reduction of hydrogen concentration. It seems that the

dominant effect controls the direction of the thermodynamic

equilibrium according to the conditions in the reactions

0.0 0.2 0.4 0.6 0.8 1.0


0.00

0.20

0.40

0.60

0.80

1.00

He

pta

ne

co

nv

ers

io

n

Tf = 400o

C

S/C = 3.23

NH2 = 30 tubes

= 3.0 m

P=

10

(bar)

P=

30

(b

ar)

P = 20 (bar)

Fig. 10 e Heptane conversion at different reaction side

pressures.

media. Fig. 10 shows the heptane conversion profiles for

various side reaction pressures (10 bar, 20 bar, 30 bar) at the

optimal condition of S/C ¼ 3.23. The increase of the reaction

side pressure and its complex consequences affects the partial

pressures of steam, heptane and hydrogen and affects

implicitly the rate of heptane steam reforming kinetics shown

in equation (5), which has direct tangible effect on the

conversion of heptane as shown in Fig. 10. As shown in Fig. 10

that the effect of increasing the reaction side pressure ismuch

less pronounced than the effect of the feed temperature in

decreasing the effective dimensionless reactor length. The

influence of the reaction side pressure on corresponding

hydrogen yield profiles is shown in Fig. 11. The hydrogen yield

is boosted by the increase of the reaction side pressure. It

seems that the hydrogen permeation rate is the dominant

effect in shifting the thermodynamic equilibriums of reac-

tions (2)e(4) towards hydrogen production. Also, an increment

of 10 bar increase in the reaction side pressure has different

0.0 0.2 0.4 0.6 0.8 1.0


0.00

5.00

10.00

15.00

20.00

25.00

H2

/C

Ora

tio

S/C = 3.23

NH2 = 30 tubes

= 3.0 m

P=

10

(b

ar)

P=

30

(b

ar)

P = 20 (bar)

Fig. 12 e H2/CO ratio at different reaction side pressures.




effects on hydrogen yield. The effect is stronger from 10 bar to

20 bar than from 20 bar to 30 bar. The influence of side reac-

tion pressure on the H2/CO ratio at the optimal condition of S/

C ¼ 3.23 is shown in Fig. 12. The profiles show the minimum

phenomena at all pressures and theminimum is shifted to the

right and becomes more flat by increasing the reaction side

pressure to produce low values of H2/CO ratio within the

recommended range. It is interesting that the increase of the

reaction side pressure givesmore flexibility for the selection of

the appropriate reactor length for the practical H2/CO ratio.

4. Conclusions

We have been able to build a preliminary picture of the steam

reforming of heptane in the CFFBR and CFFBMR for simulta-

neous production of pure hydrogen and syngas using mathe-

matical modeling and numerical simulation. The ultraclean

hydrogen produced can be used as fuel for fuel cells. The

simulation results show that the CFFBMR is superior to the

CFFBR and of high potential to be in the near future leading

reactor for efficientproductionofhydrogenand syngas that can

beusedbyseveralpetrochemicalprocessessuchtheFTprocess.

It seems that, the CFFBMR performance is affected by the

complex interaction of many variables, parameters and the

position of the thermodynamic equilibrium. Phenomena of

inflection points of different nature for exit hydrogen yield and

H2/CO ratio are observed and explanations offered. Sensitivity

analysis for the CFFBMR at the optimal conditions shows that

the feed temperature is more effective than the reaction side

pressure inboosting theheptaneconversionandalmosthas the

same impact as the reaction side pressure on the exit hydrogen

yield. To control the optimumH2/CO ratio on the recommended

range for the GTL processes the reaction side pressure has an

important positive role, whereas in the case of the temperature

careful measures must be taken because the increase of the

temperature may have adverse effects on the optimal H2/CO

ratio.This implies that further rigorousoptimizationstudiesare

needed for the entire parameter space. It seems the picture is

very complex and very promising for further investigations.

Acknowledgments

This project was supported by King Saud University, Deanship

of Scientific Research, College of Engineering Research Center.

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Steam reforming of n-heptane for production of hydrogen and syngas