18th European Symposium on Computer Aided Process Engineering – ESCAPE 18
Bertrand Braunschweig and Xavier Joulia (Editors)
© 2008 Elsevier B.V./Ltd. All rights reserved.
Multi-scale
framework
for
Steam-Methane
Reforming reaction with in-situ adsorption
Ankur Kapil, Shrikant Bhat, Jhuma Sadhukhan
Centre for Process Integration, University of Manchester, Manchester, M60 1QD, UK
Abstract
In future hydrogen economy with hydrogen as a carrier of energy is proposed to replace
the current energy scenario. One of the main reaction for production of hydrogen is
steam methane reforming. However the reaction is equilibrium and diffusion limited
and hence most process have an efficiency of 65-75%. Particles with in-situ catalytic
and sorption functionalities have obvious advantages in achieving high purity and
productivity. A multi-scale simulation and characterisation framework has been
developed for sorption enhanced Steam Methane Reforming (SMR) reaction processes
with heterogeneous multifunctional catalysts with sorption properties. In order to tackle
the problem of particle diffusion a more detailed characterization at particle level is
essential, which is the main objective of this work. In this paper, a unified framework
has been developed that integrates continuum model at bulk scale with the diffusionreaction-sorption model at particle porous scale in a fixed bed reactor. It has been
demonstrated that considering detailed diffusivity model at porous level offers better
insights into catalyst design and process intensification.
Keywords: sorption enhanced reaction, multi-scale design, multifunctional catalyst,
porous diffusion, natural gas reforming, hydrogen
1. Introduction
Hydrogen is one of the important carriers of energy as its only byproduct is water and
has an excellent energy density by weight. Hydrogen has specific advantages for
transportation in terms of versatility, utilization efficiency and safety[1]. It can solve the
ill effects of current hydrocarbon fuels where carbon is directly released to
atmosphere.[2, 3]. A hydrogen economy is proposed where energy is stored as hydrogen
(H2). Steam methane reforming is one of the important reactions for production of
hydrogen. It accounts for roughly about 48 % of hydrogen produced globally[4].
However, it is an interesting case of equilibrium driven reactions with integrated
separation of one or more components based on the application of the Le Chatelier’s
principle[5]. Also, SMR processes has been determined to be strongly intraparticular
diffusion controlled[6].
The reactions are as:
CH 4  H 2 O  CO  3H 2 , H R , I , 298  206.2 KJ / mol
CH 4  2 H 2 O  CO2  4 H 2 , H R , II , 298  164.9 KJ / mol
(1)
CO  H 2 O  CO2  H 2 , H R , III , 298  41.1KJ / mol
Sorption enhanced reaction process offers some distinct advantages in terms of
tolerance of materials to high temperature and pressure, wider range of availability of
sorbents for a desired separation, and productivity of pure product[7, 8]. This concept
gives an additional degree of freedom in the design of multifunctional catalysts for
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Ankur Kapil et al.
intensifying processes. Ding and Alpay[9] developed a laboratory scale homogeneous
experimental system with catalyst and sorbent crushed and sieved to uniform fine
particles that were packed into the reactor. They also developed pseudo-homogeneous
model for representing the system. However, homogeneous model is a crude
assumption and such processes are not feasible for industrial scale reactor due to large
pressure drop for such an arrangement. Xiu et al.[10-12] developed a theoretical model
to describe fixed bed sorption enhanced reaction process. However, all these studies are
restricted to cases with catalyst and sorbent present as different particles. In such cases
the inter and intra mass transfer poses serious constraints in terms of intensifying
sorptive reactors[13]. If catalyst and sorbent are integrated within single particle, the
mass transfer limitations can be reduced and process intensification effects can be
realised. Dietrich et al.[14] introduced the concept of spatial integration of catalytic and
adsorptive functionality at the particle level. However, their particle scale model
assumes fixed effective porous diffusivity and no variation in diffusion through pores
due to molecular and molecule to pores interactions. Additionally, their isothermal
process scale model will not be valid for many systems such as SMR, where
temperature variation is an important variable to control, due to coking and metal
dusting problems. Rusten et al.[6] have introduced a more comprehensive
heterogeneous model for SMR with multifunctional catalysts. The main thrust of this
model is bulk scale characterization. However, similar to Dietrich et al.[14] their
particle scale model does not consider detailed molecular and porous hindrance effects
on particle diffusivity. It does not either involve particle scale characterisation which is
important for the design and performance of multifunctional particles.
The development of heterogeneous multi-scale simulation framework is imperative for
decision making in sorption enhanced reaction processes with multi-functional catalysts
that are proved to be more efficient than the cases with catalyst and sorbent in separate
particles[14]. In this research, we have applied a more generic particle scale porous
diffusion model accounting for the effects of porous hindrance as well as molecular
interactions to SMR. The diffusivity inside particle is an important characterisation
variable for a better performance. In addition to allowing temperature variations and
integration between bulk and more detailed particle scales in our model. Here we have
presented a novel methodology for multi-scale simulation and characterisation of
heterogeneous catalytic sorption enhanced SMR with sorbent and catalyst integrated in
single particle.
2. Methodology
2.1. Algorithm[15]
The algorithm for homogeneous model can be described as (Figure 1)
1. Given concentration, pressure and temperature at initial time, evaluate the
parameters such as particle and axial diffusivities, heat transfer coefficients,
ergun equation coefficients respectively.
2. Discretize bulk concentration along the length
3. Discretize temperature along the length domain
4. Solve the resulting system of ordinary differential equations
5. Bulk scale sensitivity analysis is carried out with respect to various decision
variables such as bed voidage, steam to Methane ratio, bed temperatures.
Multi-scale framework for heterogeneous catalytic sorption enhanced reaction process
3
Concentration (bulk, particle), Temperature, Pressure
Concentration (bulk), Temperature, Pressure
1.Homogeneous simulation
framework
Solution strategy
2-3. Discretize using
finite difference
6. Heterogeneous multi-scale
simulation framework
Solution strategy
7. Particle – discretize
using orthogonal
collocation
8-9. Bulk discretize using
finite difference
4.Solve using ode solver
10.Solve using ode solver
5. Characterization with
Homogeneous Model
Bulk scale decision variables
12. Productivity and purity
tradeoffs and identification of
operating zones
11. Characterization with
Heterogeneous Multi-scale Model
12. Porous particle
scale
characterization
12. Particle coreshell analysis
Figure 1: Methodology for characterization and simulation of sorption enhanced reaction
The algorithm for heterogeneous model can be described as:
6. Given concentration (bulk and particle), pressure and temperature at initial
time, evaluate the parameters such as porous and axial diffusivity, heat transfer
coefficients, ergun equation coefficients etc.
7. Discretize particle concentration along the radius
8. Discretize bulk concentration along the length
9. Discretize temperature along the length
10. Solve the resulting system of ordinary differential equation
11. Using the optimal values of bulk scale decision variables from step 5 of
homogeneous simulation framework, the operating ranges of particle scale
design variables such as sorbent inside particles, particle voidage and diameter
are established.
12. An analysis of the effect of particle level characterization parameters such as
the ratio of pore radius to tortuosity and Thiele modulus on the process
objectives such as productivity and purity is presented.
3. Results and discussions
The homogeneous model was used to validate the results against the experimental data
on steam methane reforming by Ding and Alpay[9]. The results are in excellent
aggreement with the experimental data. The hydrogen produced for fuel cell
applications should have a very low concentration of carbon oxides specially carbon
monoxide which acts as a catalyst poison. For other applications such as internal
combustion engines the quantity of CO2 should be controlled to reduce the greenhouse
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Ankur Kapil et al.
gas emission in environment. However, for combined heat and power generating
systems utilising hydrogen, yield is more important than purity. Based on the above
discussion, we can define two objective functions considering the tradeoffs between
purity and productivity.
t
Objective function =
C
H2
dt
(2)
0
Yield, where t = time to run a reactor. The time to run the reactor is decided based on a
desired purity of product or to exhaust the capacity of sorbent
Yield1, where t is the time for which concentration of CO2 is lower than a certain
threshold.
Figure 2: Effect of fraction of sorbent in a multifunctional particle on the yield and the time for
production of pure hydrogen
5000
9000
4500
8000
4000
3500
7000
Yield (mol-s/m3)
Yield (mol-s/m3)
If we increase the fraction of sorbent inside particles the sorption of the product (CO 2)
increases and so does the rate of forward reaction according to the Le Chatelier’s
principle. However, this is at the cost of the concentration of catalyst inside particles
that reduces the overall methane conversion. This offsets any increase in forward
reaction, decreasing the overall hydrogen yield as shown in Figure 2. Therefore we use
Yield1 in Eq 2 as a measure of the quantity of pure hydrogen, devoid of oxides of
carbon. Alongside, the time in which pure hydrogen can be produced and the quantity of
pure hydrogen have been plotted against the fraction of sorbent in Figure 2. Both time
and quantity of pure hydrogen increase with the increasing fraction of sorbent. Figure 2
also presents an important trade off between hydrogen productivity and purity presented
by Yield and Yield1 respectively in Eq 2. As discussed before, the objective function is
decided based on the end use of a product. If the hydrogen produced is to be used for
power generation we should work in zone I in Figure 2, however, for fuel cell
applications, zone II should be the preferred region for deciding upon catalyst to sorbent
ratio inside particles.
3000
2500
2000
1500
1000
6000
5000
4000
3000
2000
1000
500
0
0
0
0.2
0.4
0.6
0.8
1
1.2
0
200
400
Bed Voidage
(a)
Figure 3: Effect of a) bed voidage, b) feed temperature on the H2 yield
600
800
Temperature (K)
(b)
1000
1200
1400
Multi-scale framework for heterogeneous catalytic sorption enhanced reaction process
5
2.85
3000
2.845
2500
Yield (mol-s/m3)
Yield (Kmol-s/m3)
The sensitivity analysis is carried out with respect to bulk scale decision variables. The
overall concentration of sorbent and catalyst decreases with increasing voidage. As a
result the overall rate of reaction and sorption decreases (Figure 4a). By increasing the
temperature the rate of forward reaction can be increased for the first two reactions in
Eq 1. However, this decreases the rate of water gas shift reaction (Eq 1). Consequently,
by increasing the temperature, yield of H2 is increased (Figure 4b). The maximum
temperature permissible is limited by material of construction of the reactor. Decrease
in the steam to methane ratio increases the reaction till a stoichiometric amount of steam
to methane is reached between 2 and 1. However lower steam to methane ratios favour
soot and coke formation, which is not desirable for steam methane reforming
process[16]. Steam to methane ratio of 4 to 6 is generally accepted in the industry.
Hence the steam to methane ratio is kept at a constant value of 6 for all further study.
Additionally, multi-functional particles have other important design variables such as
particle voidage, diameter of particle, pore radius and tortuosity. Increase in the
diameter of particle leads to decrease in yield due to increase in mass transfer
limitations as depicted in Figure 4. Yield of the process also decreases with the increase
in particle voidage due to the reduction in number of active sites. Pore radius and
tortuosity affect the diffusivity inside particle significantly and hence the overall rate of
reaction and sorption. With catalyst in shell and adsorbent in core, initially the feed
comes in contact with the catalyst shell and the product is adsorbed at the adsorbent
core. In the case of adsorbent shell and catalyst core feed has to penetrate the depth of
the particle to come in contact with the catalyst core and the resultant product is then
adsorbed in the shell. The mass transfer inside the particle poses a significant resistance
to the reaction and separation process in the latter case. Hence, yield is higher if the
catalyst is present in the outer shell compared to the other two cases where the catalyst
and sorbent fraction are uniform throughout or the catalyst is present in the shell.
2.84
2.835
2.83
2.825
2000
1500
1000
500
2.82
0
0
0.1
0.2
0.3
0.4
0.5
Diameter of the particle (mm)
(a)
0.6
0.7
0
0.2
0.4
0.6
0.8
1
Particle Voidage
(b)
Figure 4: Effect of a) diameter of particle and b) particle voidage on the yield for a
multifunctional particle
4. Conclusions
A heterogeneous multi-scale framework has been developed that integrates transfer
models at bulk and porous scales for in-situ sorption enhanced SMR. The homogeneous
models are first validated against experimental results and extended to heterogeneous
framework. The productivity and purity tradeoffs are dependent on both reactor as well
as particle scale designs, such as bed voidage, temperature, steam to methane ratio,
fraction of sorbent, particle voidage and ratio between porous radius and toruosity etc.
The catalyst and sorbent distribution inside particles strongly affects the purity and
productivity tradeoffs. The respective zones dominated by productivity or purity were
determined in terms of sorbent ratio inside particles for sorption enhanced natural gas
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Ankur Kapil et al.
reforming reaction producing pure hydrogen for combined heat and power and fuel cell
applications respectively. The yield of pure product can be increased by increasing the
length of a reactor; however such a design would lead to high pressure drop and
problems associated with regeneration of the sorbent. The particle scale diffusion model
considers the effects of porous diffusivity, which is a function of Knudsen diffusivity
due to hindrance and molecular interactions, on particle level transport. . It was derived
that the shell core model with sorbent at the centre of particles and catalyst at the outer
surface is better than the reverse design and the uniform catalyst and sorbent
concentrations inside particles for the given reforming reaction.
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