Enhanced ultraclean hydrogen production by multi-stage reformers

Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx

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JIEC-1930; No. of Pages 5

Short communication

Enhanced ultraclean hydrogen production by multi-stage reformers

M.E.E. Abashar *

Department of Chemical Engineering, 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 12 November 2013

Accepted 22 February 2014

Available online xxx

Keywords:

Multi-stage reformers

Methane

Ultraclean hydrogen

A B S T R A C T

The performance of multi-stage circulating fast fluidized bed membrane reformers (CFFBMRs) for

production of ultraclean hydrogen is investigated in comparison to a single circulating fast fluidized bed

membrane reformer (CFFBMR). The two-stage reformer configuration gives significant increase in the

methane conversion of 27.46% and ultraclean hydrogen yield of 29.61% compared to the single CFFBMR.

The concept of the multi-stage short reformers policy is introduced. Impressively, substantial increase of

ultraclean hydrogen yield of 83.91% is achieved.

� 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights

reserved.

Contents lists available at ScienceDirect

Journal of Industrial and Engineering Chemistry

jou r n al h o mep ag e: w ww .e lsev ier . co m / loc ate / j iec

1. Introduction

Ultraclean hydrogen is currently being used as a fuel forPolymer Electrolyte Membrane Fuel Cells (PEMFCs). The PEMFCsconstitute essential future devices for power generations. ThePEMFCs have several advantages such as [1]: very efficientcompared to combustion processes because they are electrochem-ical energy conversion devices, very clean power generationdevices, pollution free without greenhouse gas emissions and freefrom noise pollution because they have no moving parts. Despitethe benefits of the PEMFCs, these devices are very sensitive totraces of carbon monoxide (5–10 ppm), because the carbonmonoxide poisons the active sites of the platinum catalyst inthe anode affecting the current density and causing severe anodedamage [2]. Also, CO poisoning can develop transient oscillationsin cell potential [3]. Other challenges facing this technology are thecost, delivery and storage of hydrogen. Realization of importance ofproduction of ultraclean hydrogen necessitates improvement inthe current hydrogen production technologies.

The high demand for ultraclean hydrogen and drawbacks of theconventional fixed bed steam reformers are great challenges facingthe hydrogen industry and academia [4]. The recent research ofElnashaie and coworkers indicates that the novel circulating fastfluidized bed membrane reformers (CFFBMRs) have the potentialto be the next best generations of steam reformers for productionof ultraclean hydrogen [5,6].

* Tel.: +966 1 4675843; fax: +966 1 4678770.

E-mail address: [email protected]

Please cite this article in press as: M.E.E. Abashar, J. Ind. Eng. Chem

http://dx.doi.org/10.1016/j.jiec.2014.02.040

1226-086X/� 2014 The Korean Society of Industrial and Engineering Chemistry. Publis

Surprisingly, there is a lack of investigations of multi-stageCFFBMRs configurations in chemical engineering literature. Thus,the objective of this study is to gain a better insight intoimplementation of the multi-stage CFFBMRs for efficient produc-tion of ultraclean hydrogen. The investigation also explores a lowfeed temperature operation and heat integration for energysavings. The concept of the short multi-stage reformers policy istested. The benefits of introducing inter-stage fresh sweep gas onthe hydrogen permeation driving force are also investigated. Also,this preliminary investigation evaluates the effect of inter-stageinjections of oxygen on the performance of the multi-stageCFFBMRs configurations.

2. Reactions kinetics

The kinetics of the catalytic steam reforming of methane(CSRM) is developed by Xu and Froment over a commercial nickelcatalyst as follows [7]:

CH4 þ H2O Ð 3H2 þ CO;

r1 ¼k1

P2:5H2

ðPCH4PH2O � ðP3

H2PCO=K1ÞÞ

DEN2(1)

CO þ H2O Ð H2 þ CO2;

r2 ¼k2

PH2

ðPCOPH2O � ðPH2PCO2

=K2ÞÞDEN2

(2)

CH4 þ 2H2O Ð 4H2 þ CO2;

r3 ¼k3

P3:5H2

ðPCH4P2

H2O � ðP4H2

PCO2=K3ÞÞ

DEN2

(3)

. (2014), http://dx.doi.org/10.1016/j.jiec.2014.02.040

hed by Elsevier B.V. All rights reserved.


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Nomenclature

Ac free cross-sectional area of the reactor for catalyst

circulation (m2)

Cpj specific heat of component j (kJ/kmol K)

dH2diameter of hydrogen membrane tube (m)

Fj molar flow rate of component j (kmol/h)

DHi heat of reaction i (kJ/mol)

ki rate coefficient of reaction i

Ki equilibrium constant of reaction i

l reactor length (m)

NH2number of hydrogen membrane tubes

Pj partial pressure of component j (kPa)

ri rate of reaction i (kmol/kgcat h)

R gas constant (kJ/mol K)

T temperature (K)

Greek letters

d thickness of hydrogen membrane (mm)

e void fraction

rc catalyst density (kg/m3)

sji stoichiometric coefficient of component j in

reaction i

M.E.E. Abashar / Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx2

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Jin et al. [8] have developed the reaction kinetics for thecatalytic partial oxidation reactions of methane (CPOX) over anickel based catalyst as follows:

CH4 þ 2O2! CO2 þ 2H2O; r4 ¼ k4PCH4PO2

(4)

CH4 þ H2O Ð 3H2 þ CO;

r5 ¼ k5PCH4PH2O 1 �

P3H2

PCO

K5PCH4PH2O

!(5)

Fig. 1. Schematic representation


CH4 þ CO2Ð 2H2 þ 2CO;

r6 ¼ k6PCH4PCO2

1 �P2

H2P2

CO

K6PCH4PCO2

!(6)

where

DEN ¼ 1 þ KCH4PCH4

þ KH2PH2þ KCOPCO þ

KH2OPH2O

PH2

(7)

3. CFFBMR model

Fig. 1 shows a schematic diagram of the multi-stage circulatingfast fluidized bed membrane reformers (CFFBMRs) with freshsweep gas and inter-stage oxygen injections. The reformer isassumed to operate under steady state isobaric operation withnegligible diffusion limitations due to the fine catalyst particlesused. In the reaction side, the molar balance equations for allcomponents and hydrogen are given by:

dF j

dl¼ rcAcð1 � eÞ

X5

i¼1

s jiri (8)

dFH2


X5

i¼1

sH2 iri

� 7:21 � 10�5pdH2NH2

dexp �15700:0

RT

� � ffiffiffiffiffiffiffiffiPr

H2

q�

ffiffiffiffiffiffiffiffiP p

H2

q� �(9)

The steady state energy balance in the reaction side is given by:

dT


P6i¼1ð�DHiÞriP6

j¼1 F jC p j

(10)

In the permeation side, the hydrogen molar balance gives [9]:

dF pH2

dl¼ 7:21 � 10�5pdH2

NH2

dexp �15700:0

RT

� � ffiffiffiffiffiffiffiffiPr

H2

q�

ffiffiffiffiffiffiffiffiP p

H2

q� �(11)

The main model parameters are given elsewhere [6].

of the multi-stage CFFBMRs.



0.0 0.2 0.4 0.6 0.8 1.0Reactor length (m)

0.00

0.50

1.00

1.50

2.00

2.50

H2yield

Total

Reaction side

Permeati

on side

(ultracle

an H2)

Fig. 3. Profiles of H2 yield in the reaction side, permeation side (ultraclean H2) and

the total H2 yield along the length of the single CFFBMR.

M.E.E. Abashar / Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx 3

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4. Results and discussion

This study is solely a numerical simulation due to the lack ofexperimental data of the multi-stage CFFBMRs configuration in theliterature.

4.1. Single CFFBMR

This part of study is focused on a single CFFBMR in order toobtain the key features of the reformer. Fig. 2 compares theinfluence of oxygen to methane feed ratio (O/M) on the exit totalhydrogen yield obtained by the single CFFBMR at two feedtemperatures (Tf = 400 C, Tf = 800 C) for various steam to methanefeed ratios (S/M). As it can be seen that for both feed temperaturesinflection points of maximum nature are developed at different S/M feed ratios. The optimal values probably due to the excessiveoxygen in the reaction media that favors the CPOX for moreconsumption of methane at the expense of methane used toproduce hydrogen leading to decrease of the exit total H2 yield. Theincrease of the feed temperature increases the maximum point andshifts its position to left toward low O/M feed ratio. This could bedue to the fact that the high feed temperature (800 C) has asignificant contribution to supply part of the necessary energyrequired for the endothermic reforming reactions. At the high feedtemperature of 800 C the effect of the increase of the S/M feed ratiois pronounced on the optimum exit total hydrogen yield. However,at the low feed temperature of 400 C the improvement in theoptimum exit total H2 yield is marginal due to the highendothermicity of the reforming reactions. It appears that the O/M feed ratio is an important key factor and should be consideredwith care for optimal production of hydrogen.

We are interested to operate the reformer at the low feedtemperature of Tf = 400 C for energy saving and to minimize theeffect of elevated temperatures on the catalyst and the metallur-gical properties of the hydrogen membrane tubes as well as thereformer wall. Fig. 3 shows the profiles of hydrogen yield in thereaction side, permeation side and the total hydrogen yield alongthe length of the single CFFBMR. As it can be seen that most of thehydrogen produced is separated in the permeation side asultraclean hydrogen. The hydrogen yield profile in the reactionside shows an optimal value and this could due to the balancebetween hydrogen permeation rate and production rate.

0.1 0. 2 0.3 0.4 0. 5 0. 6O/M

2.80

3.00

3.20

3.40

3.60

ExittotalH2yield

S/M=5.0

Tf = 800.0 oC

S/M=4.0

S/M=3.0

Tf = 400.0 oC

S/M = 4.0, 5.0

S/M = 3.0

Fig. 2. Single CFFBMR. Exit total H2 yield as a function of oxygen to methane ratio (O/

M) for various steam to methane ratios (S/M) and feed temperatures (Tf).


4.2. Multi-stage CFFBMRs

Two-stage configurations are considered (CFFBMR1 andCFFBMR2). In this configuration direct oxygen feed (co-feed) is usedfor the first CFFBMR1 and inter-stage oxygen injection is utilizedin the second CFFBMR2. Each reformer has a length of 0.5 m.Fresh sweep gas is introduced for each stage. Fig. 4 compares thetemperature profiles of the single reformer (CFFBMR) and two-stagereformers (CFFBMR1 and CFFBMR2) configuration. A narroweffective preheating zone prevails at the beginning of the reformers.Considerable amount of methane is converted (about 24.52%) bythe partial oxidation reaction in the preheating zone and releasinglarge amount of heat to give a maximum temperature of 838.2090 C.As it can be seen in Fig. 4, a good temperature profile is maintainedby the multi-stage configuration to supply the heat for the reformingreactions. The role of the inter-stage oxygen injection is obviousand reflected as a temperature jump at the entrance of the CFFBMR2.

0.0 0.2 0.4 0.6 0.8 1. 0Reactor length (m)

400.00

500.00

600.00

700.00

800.00

900.00

Temperature(oC)

Singe stageEffective preheating zoneTwo stages

CFFBMR2CFFBMR

1

CFFBMR

838.1090 oC

Fig. 4. Comparison of different reformer configurations. Temperature along the

length of the single stage reformer (CFFBMR) and two-stage reformers (CFFBMR1,

CFFBMR2).



0.0 0.2 0.4 0.6 0.8 1.0Reactor length (m)

0.00

1.00

2.00

3.00

UltracleanH2yield

Singe stageTwo stages

CFFBMR2

CFFBMR

CFFBMR

1

0.0019 (Effective preheating zone)

Fig. 5. Comparison of different reformer configurations. Ultraclean H2 yield along

the length of the single stage reformer (CFFBMR) and two-stages reformers

(CFFBMR1, CFFBMR2).

Fig. 7. Comparison of the different reformer configurations. Hydrogen permeation

driving force along the length of the single stage reformer (CFFBMR) and three-stage

short reformers (CFFBMR1, CFFBMR2, CFFBMR3).

M.E.E. Abashar / Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx4

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The corresponding ultraclean H2 yield profiles are shown in Fig. 5.Despite the large amount of methane consumed in the effectivepreheating zone, unsatisfactory low ultraclean H2 yield of 0.0019 isproduced in this zone. It can be seen also, that considerableimprovement in the exit ultraclean H2 yield of 29.61% is obtained byimplementing the multi-stage configuration compared to the singleCFFBMR. This is corresponding to an increase of 27.46% in the exitmethane conversion. Almost complete conversion of methane isachieved by the multi-stage configuration.

0.0 0.1 0.2 0.3Reactor length (m)

0.00

0.50

1.00

1.50

2.00

2.50

UltracleanH2yield

Singe stageThree stages

CFFBMR

2

CFFBMR

CFFBMR1

CFFBMR

3

(Effective preheating zone)0.0019

Fig. 6. Comparison of different reformer configurations. Ultraclean H2 yield along

the length of the single stage short reformer (CFFBMR) and three-stage short

reformers (CFFBMR1, CFFBMR2, CFFBMR3).


4.3. Multi-stage short reformers policy

The results from Fig. 4 indicate that the beginning of theCFFBMR1 and CFFBMR2 is the most effective part in theperformance of the reformers. This implies that a benefit can beobtained by using reformers of short lengths. In this part weexplore the potential for further improvement of the multi-stageconfiguration performance by implementing multi-stage shortreformers policy. Three short reformers are considered (CFFBMR1,CFFBMR2, CFFBMR3) each has a length of 0.1 m. The total length ofthe reformers is reduced by 70% compared to the previous cases.Fig. 6 compares the ultraclean H2 yield profiles of the single stageshort reformer and three multi-stage short reformers configura-tions. The exit ultraclean H2 yield obtained by the three shortreformers is 2.4701 compared to 1.3431 obtained by the singleCFFBMR at the same total length of 0.3 m. This represents asignificant increase in the ultraclean hydrogen yield of 83.91%.Fig. 7 shows the corresponding profiles of the hydrogenpermeation driving force for the two configurations. Thisillustrates impressively the benefits gained by introducing thefresh sweep gas and oxygen at the beginning of each stage.

5. Conclusions

The potential application of multi-stage reformers for produc-tion of ultraclean hydrogen via steam reforming of methane hasbeen explored. Modeling and simulation as powerful tools havebeen utilized to predict the performance of the single CFFBMR andmulti-stage CFFBMRs configurations. An effective preheating zoneis developed at the beginning of the reformers mainly by the partialoxidation reaction. This zone can be utilized rationally for in situgeneration of heat that necessary for heat integration and energysavings. However, considerable amount of methane could beconsumed within the preheating zone for this purpose. The multi-stage CFFBMRs configuration in general and the multi-stage shortreformers policies in particular demonstrate significant improve-ment in the reformers performance. Controlled O/M feed ratio canprovide a platform beneficial for energy integration betweenendothermic and exothermic reaction schemes. The advantages ofthe hydrogen membrane coupled with oxygen addition, fresh



M.E.E. Abashar / Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx 5

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sweep gas, high temperature profiles and the circulating fastfluidization hydrodynamics show that the multi-stage shortreformers policy is capable to produce high yield of ultracleanhydrogen at a low feed temperature and achieving almost completeconversion of methane. Future experimental results are required forthe proper design of the multi-stage CFFBMRs configuration.

Acknowledgements

This project was supported by King Saud University, Deanshipof Scientific Research, College of Engineering Research Center.


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[5] Z. Chen, Y. Yan, S.S.E.H. Elnashaie, Chem. Eng. Sci. 58 (2003) 4335–4349.[6] M.E.E. Abashar, F.M. Alhabdan, S.S.E.H. Elnashaie, Int. J. Hydrogen Energy 33 (2008)

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Enhanced ultraclean hydrogen production by multi-stage reformers