carbon dioxide (co2) reforming of methane to syngas over ni/sba-15 ...
Steam reforming of n-heptane for production of hydrogen and syngas
Transcript of Steam reforming of n-heptane for production of hydrogen and syngas
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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
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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.
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.
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 9864
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.
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 9 865
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
Dimensionless reactor length
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
Dimensionless reactor length
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
Dimensionless reactor length
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
Dimensionless reactor length
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
Dimensionless reactor length
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.
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 9866
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
Dimensionless reactor length
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
Dimensionless reactor length
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
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 9 867
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
Dimensionless reactor length
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
Dimensionless reactor length
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.
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 9868
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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