International Journal of Heat and Mass Transfer 78 (2014) 515–521

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International Journal of Heat and Mass Transfer

journal homepage: www.elsevier .com/locate / i jhmt

Experimental and modeling study of catalytic steam reformingof methane mixture with propylene in a packed bed reactor

http://dx.doi.org/10.1016/j.ijheatmasstransfer.2014.06.0840017-9310/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +43 1 58801 0; fax: +43 1 58801 41099.E-mail address: parham.sadooghi@tuwien.ac.at (P. Sadooghi).

1 Tel.: +43 3322 42606 152; fax: +43 3322 42606 199.

Parham Sadooghi a,⇑, Reinhard Rauch b,1

a Vienna University of Technology, Vienna, Austriab Bioenergy 2020+ GmbH, Güssing, Austria

a r t i c l e i n f o

Article history:Received 25 September 2013Received in revised form 28 June 2014Accepted 29 June 2014Available online 28 July 2014

Keywords:ReactorSteam reformingMethanePropyleneHeatModeling

a b s t r a c t

Producer gas from biomass gasification contains mainly hydrogen, carbon dioxide, carbon monoxide,methane and some other low molecular hydrocarbons like propylene. This paper reports mathematicalsimulation and experimental study of steam reforming of methane mixture with propylene in a packedbed reactor filled with nickel based catalysts. Due to the high heat input through the reformer tube walland the endothermic reforming reactions, a two-dimensional pseudo-heterogeneous model that takesinto account the diffusion reaction phenomena in gas phase as well as inside the catalyst particles hasbeen used to represent temperature distribution and species concentration within the reactor. Steamreforming of propylene is faster and more selective than methane and it is shown that addition of pro-pylene to the methane steam mixture reduces the conversion of methane. The obtained results play akey role in optimization and design of a commercial reactor.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Climate change and global warming are the most importantchallenges facing the world today; therefore research and develop-ments have focused on the use of renewable energy as an alterna-tive to fossil fuels, [1]. Amongst the renewable energies, one of themost important energy sources in near future is biomass. Becauseof its wide spread availability, renewable in nature and potential inneutral in relation to global warming, biomass has proved to helpto meet the world energy demands and supply much largeramounts of useful energy with fewer environmental impacts thanfossil fuels, [2]. Biomass can be converted into commercial prod-ucts via either biological or thermochemical processes, [3,4]. Gasi-fication is a well established technique to convert biomass fuelsinto liquid and gaseous products, [5,6]. The gaseous products aremainly hydrogen, carbon monoxide, carbon dioxide, methane,and other low-molecular hydrocarbons like propylene. Steamreforming of hydrocarbons produced by biomass gasification isregarded as one of the most important and economic processesfor the industrial production of hydrogen and synthesis gas, whichcan subsequently be converted to numerous valuable basic chem-icals for refinery industry, ammonia production or as feedstock to

the Fischer–Tropsch process for liquid hydrocarbons production,[7–11].

Although steam methane reforming process is widely investi-gated, [12–17], no data is available in the literature on steamreforming of methane mixture with propylene. The present workinvestigates a heterogeneous model to present steam reformingof methane mixture with propylene in a fixed bed reactor filledwith catalyst particles. The reaction system was mathematicallymodeled in steady-state and model solutions by Finite ElementMethod software, COMSOL Multiphysics were carried out. Due tothe strongly endothermic nature of the reforming process, a largeamount of heat is supplied by means of electrical heating whichkeep the outer surface of the reactor at certain temperature. There-fore reformer tube wall and the catalyst tubes are exposed to sig-nificant axial and radial temperature gradients, [12]. In developingof these kinds of reactors the knowledge of the temperature pro-files and gas compositions within the reactor play an importantrole and are important for designing and optimizing the catalystsstructure and the reactor geometry to achieve the bestperformance.

2. Mathematical model

Several chemical reactions can take place during steam meth-ane reforming process, [12,18–21]. The most important and ther-modynamically probable reaction of steam reforming of methane

Nomenclature

Cpi heat capacity of component i (J/kg � K)dp catalyst pellet diameter (m)do outside diameter of the reactor tube (m)di inside diameter of the reactor tube (m)Der effective radial diffusivity (m2/s)Dei effective diffusivity of component i (m2/s)E activation energy (kJ/kmol)f friction factorFi molar flow rate of component i (mol/s)Ft total molar flow rate (mol/s)G mass flow velocity (kg/m2s)DHi enthalpy of the reaction i (kJ/kmol)Mi molar weight of the component i (kg/kmol)Pi partial pressure of the component i (bar)ps,i partial pressure inside catalyst (bar)p total pressure (bar)ri inside radius (m)rCH4 methane reaction rate (kmol/kgcats)rCO2 carbon dioxide reaction rate (kmol/kgcats)Ri reaction rate of the reaction i (kmol/kgcats)R universal gas constant (kJ/kg K)T gas temperature (K)Tw outside reactor surface temperature (K)us gas velocity (m/s)U overall heat transfer coefficient (Kw/m2 K)

V volume of the catalyst particle (m3)yi molar fraction of component iz axial coordinate (m)

Greek lettersaw convective heat transfer coefficient (Kw/m2 K)gi effectiveness factor for reaction iker effective radial thermal conductivity (Kw/m K)kw thermal conductivity of the tube (kw/m K)n radial coordinates of the catalyst particle (m)qb bulk density of the catalyst (kgcat/m3)qg gas density (kg/m3)t void fraction of the packed bed

Superscripts0 inlet conditionss catalyst surface conditionsStag stagnant

Subscriptscat catalysti initialp particlew wall

Table 1Kinetics parameters for the reactions involved in methane steam reforming.

Rate coefficientsk1 ¼ 9:49:1016 � e�28879=T kmol � kPa0:5

=ðkg � hrÞk2 ¼ 4:39:104 � e�8074:3=T kmol � kPa�1

=ðkg � hrÞk3 ¼ 2:29:1016 � e�29336=T kmol � kPa0:5

=ðkg � hrÞ

Adsorption coefficient constants

KCH4 ¼ 6:65:10�6 � e4604:28=T kPa�1

KH2 O ¼ 1:77:103 � e�10666:35=T kPa�1

KH2 ¼ 6:12:10�11 � e9971:13=T kPa�1

KCO ¼ 8:23:10�7 � e8497:71=T kPa�1

Equilibrium constants K1 ¼ 10266:76 � eð�2630=Tþ30:11Þ kPa2

K2 ¼ eð4400=T�4:063Þ

K3 ¼ K1 � K2 kPa2

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mixture with propylene consists of four reversible reactions: thestrongly endothermic reforming reactions (1), (3), (4) and the mod-erately exothermic water–gas shift reaction (2), [22–24]:

CH4 þH2O $ COþ 3H2 DH0298 ¼ þ206 kJ=mol ð1Þ

COþH2O $ CO2 þH2 DH0298 ¼ �41 kJ=mol ð2Þ

CH4 þ 2H2O $ CO2 þ 4H2 DH0298 ¼ þ165 kJ=mol ð3Þ

C3H6 þ 6H2O $ 3CO2 þ 9H2 DH0298 ¼ þ110 kJ=mol ð4Þ

The mathematical model is based on the Langmuir–Hinshelwoodmechanism were the reaction rates are investigated and given byXu and Froment [23]:

R1 ¼k1

P2:5H2

PCH4 PH2O �P3

H2PCO

K1

� �DEN2 ð4aÞ

R2 ¼k2

PH2PCOPH2O �

PH2PCO2

K2

h iDEN2 ð4bÞ

R3 ¼k3

P3:5H2

PCH4 P2H2O �

P4H2

PCO2K3

� �DEN2 ð4cÞ

where

DEN ¼ 1þ KCH4 PCH4 þ KCOPCO þ KH2 PH2 þKH2OPH2O

PH2

ð4dÞ

where R1 and R3 are the reaction rates of reactions (1) and (3) pro-ducing carbon monoxide and carbon dioxide, and the water gasshift reaction (WGS) with reaction rate R2 as independent reaction.The rate constants and adsorption constants are Arrhenius functiontype and are function of temperature, [23]:

kj ¼ Aj � e�Ej=RT j ¼ 1; 2; 3 ð5aÞKi ¼ Bi � e�DHi=RT i ¼ CH4; H2; H2O; CO; CO2 ð5bÞ

Regarding propylene reforming reaction, (4), intrinsic kineticexpressions reported by Figueiredo and Trimm are adopted, [24]:

R4 ¼ 2:81 � 10�2 PC3H6

1þ 0:09PC3H6

� expð�8000=TÞ mol=ðs � gÞ ð5cÞ

The formation rate of each component was then calculated byusing Eqs. (1)–(4) and (4a)–(5c); for example for methane propyl-ene and carbon dioxide components:

RCH4 ¼ �R1 � R3 ð5dÞ

RC3H6 ¼ �R4 ð6eÞ

RCO2 ¼ R2 � R3 þ 3R4 ð6fÞ

Partial pressures of gases were correlated to their own concen-trations by using the ideal gas law. The values of pre-exponentialfactors and activation energies, with the correspondent dimen-sions, used in the Arrhenius expression, are listed in Table 1, [23].

Considering Reynolds number in steam reformers, it is assumedthat axial diffusion effects are negligible compared to the axial con-vection. There is no radial convection compare to axial convectionand external heat and mass transfer resistance are negligible;

P. Sadooghi, R. Rauch / International Journal of Heat and Mass Transfer 78 (2014) 515–521 517

therefore, the mass balance equations for two major components,i.e. methane and carbon-dioxide under steady-state conditionsare written as, [20]:

@xCH4

@z¼

Derqg

G1r@xCH4

@rþ @

2xCH4

@r2

" #þ qbM

Gy0CH4

ðR1g1 þ R3g3Þ ð7aÞ

@xCO2

@z¼

Derqg

G1r@xCO2

@rþ @

2xCO2

@r2

" #þ qbM

Gy0CH4

ðR2g2 þ R3g3Þ ð7bÞ

where g1; g2 and g3 are the effectiveness factors defined as theratio of the observed reaction rate to the reaction rate calculatedat the external catalytic surface conditions, or at bulk fluid condi-tions in the absence of external mass transport resistance. Analyti-cal expressions of effectiveness factors are very useful to evaluatethe effective reaction rate to be used in mass balance and are calcu-lated as follows, [20]:

gi ¼R V

0 riðps;jÞ dVV

ri pss;j

� � i ¼ 1; 2; 3 ð7cÞ

xCH4 is the total conversion of methane and xCO2 is the conversionof methane and propylene into carbon dioxide and are defined as,[20]:

xCH4 ¼ F0CH4� FCH4

� �F0

CH4

.ð7dÞ

xCO2 ¼ FCO2 � F0CO2

� �F0

CH4þ F0

C3H6

� �.ð7eÞ

Carbon monoxide selectivity, propylene conversion and hydro-gen yield are defined as:

CO Selectivity ¼ FCO= FCO þ FCO2

� �ð7fÞ

xC3H6 ¼ ðF0C3H6� FC3H6 Þ

.F0

C3H6ð7gÞ

H2 Yield ¼ 0:25 � FH2out

�F0

CH4þ F0

C3H6

� �ð7hÞ

The energy transport in axial direction is dominated by thetransport from axial convection, and thus axial conduction isneglected. With no radial convection, the only energy transportmechanism in radial direction is the effective conduction. Theenergy balance equation for the reactor can be written as, [20]:

GCp@T@z¼ ker

1r@T@rþ @

2T@r2

" #þ qb

X3

i¼1

ð�DHiÞrigi

!ð8aÞ

Momentum equation which shows the pressure distribution inthe packed-bed reactor was described by the Tallmadge who pro-posed an extension of Ergun’s equation under higher Reynoldsnumbers, [25,26]:

dpdz¼ �

fqgu2s

dpð8bÞ

f ¼ ð1� tÞt3 1:75þ 4:2ð1� tÞ

Re1=6

� �ð8cÞ

3. Boundary conditions

The boundary conditions are written as, [20]:

xCH4 ¼ xCO2 ¼ xC3H6 ¼ 0

T ¼ T0

p ¼ p0 at z ¼ 0 ð9aÞ

@xCH4

@r¼ @xCO2

@r¼ @xC3H6

@r¼ 0

@T@r¼ 0 at r ¼ 0 ð9bÞ

@xCH4

@r¼ @xCO2

@r¼ @xC3H6

@r¼ 0

ker@T@r¼ UðTw � TiÞ at r ¼ R ð9cÞ

U ¼ di

2 � kwln

do

di

þ 1

awat r ¼ R ð9dÞ

Wall heat transfer coefficient is calculated using the correlationreported by Peters et al. [27]:

Nuw ¼ 5:1do

di

0:26

Re0:45p Pr0:33

p ð10Þ

Effective thermal conductivity is dependent on the geometryof the particles, and is a function of heat transfer and flowconditions, (Reynolds and Peclet number). This effective thermalconductivity includes conduction in both the solid pellets andthe gas phase and also some contribution from turbulence andradiation between pellets. There is a wide variety in how ker iscorrelated. For catalyst particles in cylindrical shape, it is writtenas follow, [28,29]:

ker ¼ kstag þ 0:0105

3600 1þ 0:46 dp

dR

� �2� � � Rep ð11aÞ

kstag is the condition at stagnation point:

kstag ¼ kgep þ kg

�0:895 1� ep

� �ln kp

kg� 0:5439 kp

kg� 1

� �h i� 0:4561 kp�kg

kp

� �0:3521 kp�kg

kp

� �2

ð11bÞ

where ep is the porosity of the catalyst.The reactions are strongly endothermic therefore, a suitable cat-

alyst structure design plays an important role in obtaining highactivity and stability. The selected catalyst particles are nickelbased catalyst supported on alumina, cylindrical shaped pelletswith an axial hole. Catalyst properties and operating conditionsfor the steam reformer are shown in Table 2. Since the partial pres-sure gradients are limited to a very thin layer near the surface, pla-nar geometry was used. The continuity equations for methane andcarbon dioxide then become, [20]:

De;CH4

1n

ddn

ndps;CH4

dn

¼ RT r1ðps;jÞ þ r3ðps;jÞ

� �qp ð12aÞ

De;CO2

1n

ddn

ndps;CO2

dn

¼ RT r2ðps;jÞ þ r3ðps;jÞ

� �qp ð12bÞ

De;C3H6

1n

ddn

ndps;C3H6

dn

¼ RT r1ðps;jÞ þ r3ðps;jÞ

� �qp ð12cÞ

With boundary conditions:

dps;CH4

dn¼

dps;CO2

dn¼

dps;C3H6

dn¼ 0 at n ¼ 0

ps;CH4¼ pCH4

ð12dÞ

ps;CO2¼ pCO2

at n ¼ 1

Table 2Catalyst parameters and operating conditions.

Parameter Value

no Outside diameter of the catalyst 0:005 ðmÞni Inside diameter of the catalyst 0:002 ðmÞh Height of the catalyst particle 0:005 ðmÞT0 Inlet temperature 973 ðKÞp0 Inlet pressure 1:163 ðbarÞy0

CH4Inlet methane dry molar fraction 10 ð%Þ

y0C2H4

Inlet propylene dry molar fraction 5 ð%Þ

y0H2O Inlet steam molar fraction 40 ð%Þ

y0H2

Inlet hydrogen dry molar fraction 40 ð%Þy0

CO2Inlet carbon dioxide dry molar fraction 25 ð%Þ

y0CO Inlet carbon monoxide dry molar fraction 20 ð%Þ

qb Bulk density 760 ðkg=m3ÞF0

t Total molar flow rate 100 ðmol=hrÞOutside diameter of the reactor 0:008 ðmÞInside diameter of the reactor 0:007 ðmÞHeight of the catalyst in the reactor 0:1 ðmÞ

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ps;C3H6¼ p C3H6

Detailed simulations of the set of equations described abovewere implemented using COMSOL Multiphysics programmingsoftware 4.2. Gas velocity through the reactor was described usingthe Navier–Stokes equations assuming incompressible fluids intandem with the continuity equation while mass transfer of indi-vidual species is described by Maxwell–Stefan multicomponentdiffusion, [30]. According to Rostrup-Nielsen et al. [31], the axialdiffusion is negligible compared to the axial convection in steamreformers. Therefore, only the radial diffusion coefficient will beconsidered in this paper. The only energy transport mechanism isassumed to be axial convection and radial conduction. Fluid den-sity was calculated assuming ideal gas law and dynamic viscosityof the gas mixture is evaluated as a function of composition andtemperature. Reaction, mass diffusion and heat conduction withinthe catalyst film was modeled assuming a combination of Darcy’sLaw (to describe convective transport in porous media) andMaxwell–Stefan diffusion and reaction. By imposing a constant

Fig. 1. (a) Methane and propylene conversion (b) comparison of methane conversion witlength.

temperature boundary condition at the reactor wall interface,numerical solutions to the above system of partial differentialequations predict extent of catalytic reactions, concentration ofspecies and temperature distribution within the reactor.

4. Experimental procedure

Reactor tube is filled with 11 g of catalyst and the height of thecatalyst is 0.1 (m), and total flow rate is 100 ðmol=hrÞ. Due to endo-thermic nature of the process, heat should be supplied into thereactor; therefore, outer surface temperature of the reformer iskept constant by a series of electric heaters and the reactor is insu-lated to keep the temperature constant. Thermocouples are placedin the centerline at the outlet of the reactor and are connected to atemperature indicator, computer monitoring system and tempera-ture controllers. Steam reforming catalysts in this study are nickelbased catalysts supported in alumina, (NiAl2O3). They are in cylin-drical shape with a hole inside. Catalyst prosperities and operatingconditions for the steam reactor are shown in Table 1.

5. Result and discussion

The aim of this work is to study the behavior of the reaction ofsteam methane reforming mixture with propylene in a fixed bedreactor at high temperatures and atmospheric pressure. The reac-tor performance was evaluated through the following quantities:conversion of methane and propylene, hydrogen yields, and carbonmonoxide selectivity. The two-dimensional model also providesthe temperature and concentration profiles along the reactor radialand axial coordinates. The effect of different reaction temperatureswithin the range of 973–1123 (K) on the performance of the reac-tor is investigated carefully. Using the same inlet and process con-dition for the experiments, makes it possible to compare themodeling and experimental results and see how well the modelingpredicts the experimental results.

Fig. 1a and b indicate the effect of the temperature variation onmethane conversion, propylene conversion and hydrogen yieldalong the reactor length, respectively. As soon as the gas enters

h and without propylene in the gas at different wall temperatures along the reactor

Fig. 2. (a) CO selectivity, and (b) CH4 conversion to CO2 at different wall temperature along the reactor length.

P. Sadooghi, R. Rauch / International Journal of Heat and Mass Transfer 78 (2014) 515–521 519

the reactor conversion of methane and propylene begins. As it isshown, propylene conversion is much faster than methaneconversion and at very short distance from the inlet it is almostcomplete, (100%). Heat is conducted into the reactor via the wallbut due to the highly endothermic nature of the process, itdecreases the temperature within the reactor, and therefore meth-ane conversion decreases. Fig. 1b, shows the decreasing effect ofpropylene on methane conversion in a mixture of methane andpropylene and compares it to the case when only methane isincluded in the gas. When propylene conversion is complete, theprocess is less endothermic and the conducted heat into the reac-tor increases the temperature distribution within the reactor,therefore methane conversion increases. Hydrogen yield shows

Fig. 3. Temperature distribution in the reactor, (a) centerline temperature profiles of the r1023 (K).

the ratio of hydrogen at the outlet to the methane and propyleneat the inlet and is presented by Eq. (6f). Increasing the walltemperature leads to increasing the reaction temperature withinthe packed bed of the reactor and increases the conversion ofmethane, propylene and therefore hydrogen yield increases. Attemperature of 1123 K the conversion of methane is also almostcomplete and closes to 100% and corresponds to the highest levelof hydrogen yield, Fig. 2a. Dry mole concentration of each speciesalong the reactor bed is shown in Fig. 2b at 1023 (K) and helps toexplain carbon monoxide selectivity in Fig. 3a.

It is shown that, the carbon monoxide selectivity dropped atvery short distance from the inlet of the reactor, and then increasesalong the axial bed of the catalyst. At higher temperature the same

eactor for different wall temperatures. (b) Temperature distribution in the reactor at

Fig. 4. (a) Radial temperature of the reactor at different positions along the reactor, wall temperature 1023 (K). (b) Radial and axial temperature distributions in the reactor atoutlet, wall temperature 1023 (K).

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behavior is shown but the decreasing rates are slower compare tolower temperatures. Decreasing in short distance from the inlet isdue to the quick conversion of methane and propylene to carbondioxide and water gas shift reaction behavior which is faster thanreforming reactions and decreases carbon monoxide concentra-tion, Fig. 2b. Increasing hydrogen due to methane and propyleneconversion causes water gas shift reaction to proceed in otherdirection. Completion of propylene conversion moderates theincreasing rate. Higher temperatures increases methane conver-sion and therefore carbon monoxide selectivity increases.

Axial and radial temperature profiles are shown in Figs. 3b and4a, b. Fig. 3b shows the profile of the predicted reactor temperaturealong the bed length at various reaction temperatures and Fig. 4ashows radial temperature gradients at four different axial positionsin the catalytic tube. As soon as gas enters the reactor its temper-ature is drastically decreases. The gas contacts with the catalystand loses its temperature due to the occurrence of highly endo-thermic reforming reactions. Then, the heat is conducted into thereactor via reactor wall and the temperature of the process gasincreases from center of the reactor toward the wall and there isa temperature gradient between the centerline and wall of thereactor along the reactor length. It is shown that radial tempera-ture gradients tend to decrease towards the reactor outlet becauseof the lower local heat fluxes and reforming reaction rates. The rateof the reforming reactions is more rapid near the wall because thegas temperature is higher. It is shown that increasing reaction tem-peratures lead to the more uniform temperature profiles along theaxial bed that finally results in a more efficient usage of the reactor.

6. Conclusions

An experimental and mathematical modeling study was devel-oped to investigate catalytic steam reforming of methane mixturewith propylene to synthesis gas at high temperatures and atmo-spheric pressure. A two dimensional heterogeneous model is used

to simulate reforming reactions in a packed bed reactor filled withcatalyst particles. Effect of wide range of temperatures on methaneand propylene conversion as well as hydrogen yield and carbonselectivity are investigated. Result shows that high temperaturegradients exist near the wall toward the center of the reactor ateach axial position and are more significant close to the reactorentrance. It is shown that addition of propylene to the methanesteam mixture reduces the conversion of methane. Using the sameinlet and process condition for the experiments, makes it possibleto compare the modeling and experimental results and see howwell the modeling predicts the experimental results.

Conflict of interest

There is no conflict of interest.

Acknowledgments

The authors wish to acknowledge the financial support of theAustrian Climate and Energy Fund since this work was carriedout under the Projects ‘‘distributed SNG’’ and ‘‘BioH2-4industries’’.Moreover the authors acknowledge the financial support of theAustrian COMET program, where within the Bioenergy2020+ pro-jects ‘‘BioH2’’ and "Mixed Alcohols" a part of the work was carriedout.

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