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Journal of Natural Gas Science and Engineering 20 (2014) 132e146

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Journal of Natural Gas Science and Engineering

journal homepage: www.elsevier .com/locate/ jngse

Syngas production in a novel methane dry reformer by utilizing of tri-reforming process for energy supplying: Modeling and simulation

Mehdi Farniaei a, Mohsen Abbasi b, Hamid Rahnama c, Mohammad Reza Rahimpour c, *,Alireza Shariati c

a Department of Chemical Engineering, Shiraz University of Technology, Shiraz 71555-313, Iranb Department of Chemical Engineering, School of Chemical and Petroleum Engineering, Persian Gulf University, Bushehr 75169, Iranc Chemical Engineering Department, School of Chemical and Petroleum Engineering, Shiraz University, Shiraz 71345, Iran

a r t i c l e i n f o

Article history:Received 7 May 2014Received in revised form13 June 2014Accepted 15 June 2014Available online

Keywords:Syngas productionDry reforming of methaneTri-reforming of methaneSteam reforming of methane

* Corresponding author. Tel.: þ98 711 2303071; faxE-mail address: rahimpor@shirazu.ac.ir (M.R. Rahi

http://dx.doi.org/10.1016/j.jngse.2014.06.0101875-5100/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

In this study, tri-reforming process has been utilized as an energy source for driving highly endothermicprocess of methane dry reforming process in a multi-tubular recuperative thermally coupled reactor(TCTDR). 184 two-concentric-tubes have been proposed for this configuration. Outer tube sides of thetwo-concentric-tubes have been considered for the tri-reforming reactions while dry reforming processtakes place in inner tube sides. Simulation results of co-current mode have been compared with cor-responding predictions of thermally coupled tri- and steam reformer (TCTSR); in which the tri-reformingprocess has been coupled with steam reforming of methane in same conditions. A mathematical het-erogeneous model has been applied to simulate both dry and tri-reforming sides of the TCTDR. Resultsshowed that methane conversion at the output of dry and tri-reforming sides reached to 63% and 93%,respectively. Also, molar flow rate of syngas at the output of DR side of TCTDR reached to 7464 kmol h�1

in comparison to 3912 kmol h�1 for SR side of TCTSR.© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Synthesis gas (syngas), a fuel gas mixture consists of hydrogenand carbon monoxide is important intermediate for production ofseveral materials such as; methanol, hydrogen, ammonia andsynthetic petroleum for use as a fuel through FischereTropschprocess (Halmann and Steinfeld, 2004; Stelmachowski andNowicki, 2003; Van der Drift and Boerrigter, 2006; Zahedi nezhadet al., 2009).

One of the basic process to produce syngas with desired ratio ofH2/CO is the reforming of methane or natural gas. There are threemain reforming processes; steam reforming (SRM), carbon dioxidereforming (CDR) and partial oxidation of methane (POM). Also,recently a new process named as tri-reforming of methane (TR) hasbeen proposed by Song and Pan in 2001, for production of syngasfrom flue gases (Arena et al., 1996; Choi et al., 2004; Lee et al.,2003).

: þ98 711 6287294.mpour).

Dry reforming is the reaction of carbon dioxide with methanethat converting two major greenhouse gases with high globalwarming potential into valuable chemicals. Produced syngas fromdry reforming reaction has lowH2/CO ratio that is suitable for use inFischereTropsch synthesis (Kang et al., 2007; Lee et al., 2003;Nematollahi et al., 2011; Pino et al., 2011; Yin et al., 2007).

Dry reforming of methane reaction (DR):

CH4 þ CO242COþ 2H2; DH�298 ¼ 247:3 kJ mol�1 (1)

TR is a new process that synergistically combines three maincatalytic reforming reactions; CO2 reforming (Eq. (1)), steamreforming (Eqs. (2)e(4)) and oxidation of methane (Eqs. (5) and (6))in a single reactor (Song and Pan, 2004).

Steam reforming of methane reactions (SR):

SR 1 : CH4 þH2O4COþ 3H2; DH�298 ¼ 206:3 kJ mol�1 (2)

SR 2 : CH4 þ 2H2O4CO2 þ 4H2; DH�298 ¼ 164:9 kJ mol�1

(3)Wateregas shift reaction (WGSR):

M. Farniaei et al. / Journal of Natural Gas Science and Engineering 20 (2014) 132e146 133

COþ H2O4CO2 þ H2; DH�298 ¼ �41:1 kJ mol�1 (4)

Partial oxidation of methane (POM):

CH4 þ12O24COþ 2H2; DH

�298 ¼ �35:6 kJ mol�1 (5)

Complete oxidation of methane (COM):

CH4 þ 2O24CO2 þ 2H2O; DH�298 ¼ �880 kJ mol�1 (6)

WGSR is an important chemical reaction taking place during theTR process which has a significant impact on the whole process(Fiaschi and Baldini, 2009). Syngas from TR process has the H2/COratio about 1.5e2, suitable for production of other chemicals suchas methanol, dimethyl ether and liquid hydrocarbons (Jiang et al.,2007).

TR concept allows effective utilization and conversion of CO2 influe gases (that contain CO2 H2O, O2 and N2) from power plantswithout any pre-separation. On the other hand, because of exis-tence of methane oxidation reaction, heat generated in the TRreactor is as high as it can be used to drive another endothermicreaction like dry reforming reaction (Arab Aboosadi et al.,2011a,b).

By the concept of thermally coupled reactor the generated heatfrom TR process can be transferred to dry reforming reaction andthus both processes proceed simultaneously. Generally, in ther-mally coupled reactors, two reactions are coupled together in asingle reactor. One of the reactions is considered as heat source(exothermic reaction) for another reaction (endothermic reaction).Hence, endothermic reaction is driven by generated heat fromexothermic reaction in a reactor. Thermally coupled reactorsgenerally are classified into three main categorizes; direct coupling,regenerative coupling and recuperative coupling. Much attentionhas been paid to recuperative coupled reactors in much literature.Recuperative coupled reactors consist of two vertical concentric

Table 1Operating conditions of steam reforming side of TCTSR (same as the conventionalsteam reformer (CSR)).

Parameter Value

Total feed gas flow (kmol h�1) 9129.6

Feed composition (mol %)CO2 1.72CO 0.02H2 5.89CH4 32.59N2 1.52H2O 58.26

Steam to methane ratio 1.79

Inlet temperature (K) 793.15Design temperature (K) 1063.15Inlet pressure (kPa) 4000Design pressure (kPa) 4100Inside diameter (mm) 125Heated length (m) 12Number of tubes 184Catalyst name NiOeMg/CeeZrO2/Al2O3

Catalyst volume (m3) 27.8Catalyst shape 10-HOLE ringsParticle size (mm) 19 � 16Void fraction (e) 0.4Heat load on tube (100% of design case)

(MJ m�2 h�1) (based on tube ID)287.6

Reformer duty (100% of design case)[GJ h�1] net

248.2

tubes for each of reactions separated by awall. Generally, inner tubeside has been considered for exothermic reaction and outer tubeside is for exothermic one. Heat indirectly is transferred from outertube side to inner tube side through the wall (Rahimpour et al.,2012).

In the present work, a recuperative coupled reactor has beenproposed in which TR process is considered as heat source forhighly endothermic dry reforming reaction. This reactor is con-sisted of two fixed beds separated by a wall where heat is trans-ferred across the wall from outer tube side to inner tube side of thereactor. Thus, in this way both reactions are driven simultaneouslyin single reactor.

2. Processes description

2.1. Conventional steam reformer (CSR)

Conventional steam reformer has been used for syngas pro-duction in methanol synthesis unit of Zagros PetrochemicalCompany, Assaluyeh, Iran. CSR is consisted of a fixed bed reactorand a huge top fired-furnace for supplying needed energy for SRequal to 69 MWth in fixed bed reactors. A conventionalauto-thermal reformer (CAR) has been applied at the outlet ofCSR for complete conversion of methane and adjustment of H2/CO ratio that is suitable for methanol production (Rahnamaet al., 2014). Operational conditions of the CSR with catalystcharacteristics and feed compositions have been presented inTable 1.

2.2. Thermally coupled tri- and dry reformer (TCTDR)

Fig. 1 presents a schematic diagram of thermally coupled tri-and dry reformer reactor (TCTDR). 184 two concentric tubes hasbeen considered for the reactor configuration in which tri- and dryreforming processes occur in outer and inner tube sides of thereactor, respectively.

Fig. 1. A schematic diagram of TCTDR.

Table 2Optimized parameters for tri-reformer reactor (Arab Aboosadi et al.,2011a,b).

Inlet temperature (K) 1100

Composition (mol %)CO2 24.81CO 0.01H2 1.53CH4 18.7O2 8.78N2 0.01H2O 46.18

Or reactant ratiosSteam/methane ratio 2.46Oxygen/methane ratio 0.47Carbon dioxide/methane ratio 1.30

Table 3Simulation conditions of tri-reforming (TR) side of TCTDR reactor (ArabAboosadi et al., 2011a,b).

Total feed gas flow (kmol h�1) 28,115.4Methane feed rate (kmol h�1) 9264.4Inlet pressure (bar) 20Catalyst shape 10-HOLE ringsParticle size (mm) 19 � 16Shell inner diameter (m) 2

M. Farniaei et al. / Journal of Natural Gas Science and Engineering 20 (2014) 132e146134

2.2.1. Dry reforming side (inner tube side)Dry reforming side of the reactor has been loaded with Rh/Al2O3

catalysts and size of reactor is similar to CSR as presented in Table 1.Rh/Al2O3 is an effective catalyst for high dry reforming reaction

rate with CO2/CH4 ratio near 1 in feed. Also, in dry reforming ex-periments, no signs of coke formation on the catalyst surface havebeen found (Richardson and Paripatyadar, 1990).

2.2.2. Tri-reforming side (outer tube side)Pre-reformed gas is mixed with natural gas stream and proper

ratio of steam, CO2 and O2. Then pre-reformed gas is sent throughtri-reforming side that is loaded with NiOeMg/CeeZrO2/Al2O3catalysts. This type of catalyst reduces coke formation on thereactor wall and surface of the catalyst (Cho et al., 2009). Size of thecatalysts is same as the conventional steam reformer. The opti-mized inlet parameters of tri-reformer reactor have been consid-ered for inlet parameters of tri-reforming side of the reactor whichare listed in Table 2 (Arab Aboosadi et al., 2011a,b).

Table 3 shows other simulation conditions for tri-reforming sideof the reactor.

Table 4Arrhenius kinetic parameters, constants of reaction equilibrium and adsorption equilibri

DR

Rate constant of reaction (mol gcat�1 s�1)kDR ¼ 1290 ex

Equilibrium constants of reactionKDR ¼ exp

��ð4

Equilibrium constants of adsorption (Pa�1)KCO2

¼ 2:64�

KCH4¼ 2:63�

2.3. Thermally coupled tri- and steam reformer (TCTSR)

Schematic diagram of thermally coupled tri- and steamreformer (TCSTR) is similar to TCTDR.

This multi-tubular reactor has 184 two-concentric-tubes andendothermic steam reforming of methane occurs in the inner tubeside, while methane tri-reforming takes place in the outer tubeside. Tri-reforming reaction supply sufficient energy for drivingitself and also, steam reforming reaction in inner tube side. Speci-fications of steam reforming side have been considered same as theCSR reactor. Ni-based catalysts are loaded in vertical tubes of steamreforming side. Natural gas and steam mixed together and enteredto the inner tube side (steam reforming side). Generated heat fromtri-reforming side transfers to the inner tube side and reaction takeplaces.

Effective length of TCTSR is 12 m similar to TCTSR based onspecification of CSR in Zagros Petrochemical Company, Assaluyeh,Iran (see Table 1).

3. Kinetic of reactions

3.1. Dry reforming reactions

During production of synthesis gas by dry reforming, beside dryreforming reaction (Eq. (1)), reverse wateregas shift reaction(reverse of Eq. (4)) occurs, too. Kinetic of reaction for Eqs. (1) and (4)over Rh/Al2O3 catalyst is considered based on Richardson andParipatyadar model (Richardson and Paripatyadar, 1990), asfollows:

rDR ¼ kDR

" KCO2

KCH4PCO2

PCH4�1þ KCO2

PCO2þ KCH4

PCH4

�2!

� 1�

�PCOPH2

�2KDRPCO2

PCH4

!# (7)

rRWGS ¼ kRWGSKCO2KH2

PCO2PH2�

1þ KCO2PCO2

þ KH2PH2

�2"1�

�PCOPH2O

�2KRWGSPCO2

PH2

#(8)

The Arrhenius kinetic parameters, constants of reaction equi-librium and adsorption equilibrium constants are summarized inTable 4.

3.2. Tri-reforming side reactions

Eqs. (2)e(4) and also complete oxidation of methane (Eq. (6))have been considered to describe tri-reforming process. Among tri-reforming reactions, dry reforming reaction (Eq. (1)) is a dependentreaction as it can be written as SRM reaction minus WGS reaction.Hence, considering kinetic model for Eqs. (2) and (4) is enough and

um constants for DR and RWGS reactions.

RWGS

p��102;065

RT

�kRWGS ¼ 1:856� 10�5 exp

��73;105

RT

�2�0:3ðT�773ÞÞ

RT

�KRWGS ¼ exp

�4400T � 4:036

�

10�3 exp�37;641RT

�

10�3 exp�40;684RT

� KCO2¼ 5:6955� 10�6 exp

�9262RT

�

KH2¼ 1:4705� 10�5 exp

�6025RT

�

Table 6Van't Hoff parameters for species adsorption.

Components Koi (bar�1) DHi (J mol�1) KoiC (bar�1) DHi

C (J mol�1)

CH4 6.65 � 10�4 �38,280CO 8.23 � 10�5 �70,650H2 6.12 � 10�9 �82,900H2O 1.77 � 105 88,680CH4 (combustion) 1.26 � 10�1 �27,300O2 (combustion) 7.78 � 10�7 �92,800

Ki ¼ Koi � expð�DH=RTÞ.KCi ¼ KC

oi � expð�DHCi =RTÞ.

M. Farniaei et al. / Journal of Natural Gas Science and Engineering 20 (2014) 132e146 135

no need to consider kinetic model for CO2 reforming reaction (Eq.(1)). The kinetic model of Xu and Froment over Ni-based catalysts isused for Eqs. (2)e(4). This model has been extensively tested underlab scale and is more general for steam reforming reactions. Xu andFroment kinetic model for reactions (2)e(4) is as follows (Xiu et al.,2002; Xu and Froment, 1989):

R1 ¼ k1p2:5H2

pCH4

pH2O � p3H2pCOKI

!� 1f2 (9)

R2 ¼ k2p3:5H2

pCH4

p2H2O � p4H2pCO2

KII

!� 1f2 (10)

R3 ¼ k3pH2

�pCOpH2O � pH2

pCO2

KIII

�� 1f2 (11)

f ¼ 1þ KCOpCO þ KH2pH2

þ KCH4pCH4

þ KH2OpH2O

pH2

(12)

The kinetic model of Trimm and Lam (1980) is used for methanecombustion (Eq. (6)) as a rigorous study, but it was driven over sup-portedPt-basedcatalyst.Hence, themodel adsorptionparametersareadjusted for Ni-based catalysts as below (De Smet et al., 2001):

R4 ¼ k4apCH4pO2�

1þ KCCH4

pCH4þ KC

O2pO2

þ k4bpCH4pO2�

1þ KCCH4

pCH4þ KC

O2pO2

(13)

Table 5 shows reaction equilibrium constant and Arrheniusparameters. Van't Hoff parameters for species adsorption are listedin Table 6. The consumption or formation rate of species i, ri(mol kg�1 s�1), is determined by summing up the reaction rates ofspecies i (Rj (mol kg�1 s�1)). To account intra-particle transportlimitations, effectiveness factor, hi, are used (De Groote andFroment, 1996; Gosiewski et al., 1999). Therefore the reaction rateof each species is as below:

rCH4¼ �h1R1 � h2R2 � h4R4 (13-a)

rO2¼ �2h4R4 (13-b)

rCO2¼ h2R2 þ h3R3 þ h4R4 (13-c)

rH2O ¼ �h1R1 � 2h2R2 � h3R3 þ 2h4R4 (13-d)

rH2¼ 3h1R1 þ 4h2R2 þ h3R3 (13-e)

rCO ¼ h1R1 � h3R3 (13-f)

Table 5Reaction equilibrium constants and Arrhenius kinetic parameters for TRM reactions.

Reaction, j Equilibrium constant, Kj koj (mol kgcat�1 s�1) Ej (J mol�1)

1KI ¼ exp

��26;830

Tsþ 30:114

�ðbar2Þ 1.17 � 1015 bar0.5 240,100

2 KII ¼ KI$KIIIðbar2Þ 2.83 � 1014 bar0.5 243,900

3 KIII ¼ exp�4400Ts

� 4:036�

5.43 � 105 bar�1 67,130

4 8.11 � 105 bar�2 86,0006.82 � 105 bar�2 86,000

kj ¼ koj � expð�Ej=RTÞ.

where h1 ¼ 0.07, h2 ¼ 0.06, h3 ¼ 0.7, h4 ¼ 0.05 (De Groote andFroment, 1996).

4. Mathematical model

A one dimensional heterogeneous catalytic reaction model hasbeen developed in order to determine the concentration andtemperature distribution inside both sides of the reactor. Thismodel will accounts heat and mass transfer resistance along thereactor. Some assumptions are considered to model both sides ofthe reactor:

� The reactor operates at steady state conditions.� Gas phase is ideal.� Axial diffusions of mass and heat are negligible.� One dimensional plug flow pattern is considered in both sides ofthe reactor.

� Bed porosity in axial and radial directions is constant.� Heat loss to surrounding is neglected.

Mass and energy balance equations are obtained by consideringa differential element along the axial direction of the reactor.

4.1. Balance equation for solid phase

The mass and energy balance equations for solid phase havebeen developed for both sides of the reactor as follows:

kgiðyi � yisÞ þ hrirBa ¼ 0 (14)

avhf ðT � TsÞ þ rBaXNi¼1

hri��DHf i

¼ 0 (15)

where Ts and yis are temperature and the solid phase mole fractionof component i inside of reactor, respectively and h is effectivenessfactor (the ratio of the reaction rate observed to the real rate ofreaction), which is obtained from a dusty gas model calculations(Graaf et al., 1990). Surface areas per volume of catalyst pellet areshown by av (m2 m�3).

4.2. Balance equation for fluid phase

Mass and energy balance equations for both sides have beendeveloped as follows:

�FtAc

vyivz

þ avctkgiðyis � yiÞ ¼ 0 (16)

� 1Ac

CgpvðFtTÞvz

þ avhf ðTs � TÞ ¼ 0 (17)

Table 7Auxiliary correlations.

Parameter Equation Reference

Component heat capacity Cp ¼ aþ bT þ cT2 þ dT3 Barbieri and Di Maio (1997)Mixture heat capacity Based on local compositionsViscosity of reaction mixtures Based on local compositionsMixture thermal conductivity Based on local compositions Van Ness et al. (2001)Mass transfer coefficient between gas and solid phases kgi ¼ 1:17Re�0:45Sc�0:67

i ug � 103 Cussler (1997)

Re ¼ 2Rpugrm

Sci ¼m

rDim � 10�4

Dim ¼ 1�yiPisj

yiDij

Dij ¼ 10�7T3=2ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1=Miþ1=Mj

pPðv3=2ci þv

3=2cj Þ2

Reid et al. (1977)

Overall heat transfer coefficient 1U ¼ 1

hiþ Ai lnðDo=DiÞ

2pLKwþ Ai

Ao

1ho

Van Ness et al. (2001)

Heat transfer coefficient between gas phase and reactor wallh

Cprm

�Cpm

k

�2=3¼ 0:458

εB

�rudpm

��0:407 Smith (1980)

M. Farniaei et al. / Journal of Natural Gas Science and Engineering 20 (2014) 132e146136

where T and yi are temperature and the fluid phase mole fraction ofcomponent i inside of the reactor, respectively. kgi is the masstransfer coefficient between gas and solid phase. First and secondterms in Eq. (17) show heat transferred by convection and the heatexchanged between fluid phase and solid particle, respectively.

4.3. Pressure drop

The Ergunmomentum equation is used to consider the pressuredrop variations along the reactor axis:

dpdz

¼ 150ð1� εÞ2mug

ε3d2p

þ 1:75ð1� εÞu2gr

ε3dp

(18)

where the pressure drop is in Pa.

4.4. Boundary conditions

Temperature, pressure and gas compositions are known at theentrance of the reactor. Hence, the following boundary conditionsare applied at the inlet of reactor:

z ¼ 0; yi ¼ yi0; T ¼ T0; P ¼ P0 (19)

For analyzing performance of TCTDR and TCTSR, syngas quality(H2/CO) and CH4 conversion is calculated by following equations:

CH4 conversion ¼ FCH4;in � FCH4;out

FCH4 ;in(20)

Table 8Comparison between model prediction and plant data.

Parameter Plant data CSR

Methane conversion % 26.5 26

Composition (mol %)CO2 5.71 5.72CO 3.15 3.19H2 31.39 31.53CH4 20.41 20.33N2 1.29 1.30H2O 38.05 37.94

Fig. 2. Variations in mole fraction of all components along (a) tri-reforming (TR) sideand (b) dry reforming (DR) side of the TCTDR reactor.

M. Farniaei et al. / Journal of Natural Gas Science and Engineering 20 (2014) 132e146 137

Syngas quality ¼ FH2

F(21)

CO

4.5. Auxiliary correlations

To complete the simulation, some auxiliary correlations must beapplied into the developed model equations. The correlation esti-mations for heat and mass transfer between two phases and also,overall heat transfer coefficient between wall and gas phase in theinner tube side must be considered. Table 7 shows applied corre-lations for physical properties, mass and heat transfer coefficient.The heat transfer coefficient between gas phase and reactor wall isapplicable for heat transfer coefficient between gas bulk phase andsolid phase (hf).

5. Numerical solution

The developed model contains a set of differential equations byusing the backward finite difference approximation for solving themodel; the differential equations alter to a non-linear algebraic set

Fig. 3. Variations in reaction rates along (a) TR side, and (b) DR side of TCTDR reactor.

of equations. Then length of the reactor is divided into 100separate sections. Non-linear algebraic equations of each sectionare solved by GausseNewton method in MATLAB programmingenvironment. Validation of modeling results has been presented inTable 8. Results identify that simulation results at the output ofCSR are similar to plant data in Zagros Petrochemical Company,Assaluyeh, Iran.

6. Results and discussion

Fig. 2(a) and (b) represents variations of all components molefractions along tri- and dry reforming sides of the TCTDR, respec-tively. At the entrance of TR side, all oxygen is rapidly consumed byCOM reaction and CO2 is produced at first but then CO2 molefraction is decreased because of consuming by reforming reactions.Behavior of profile of H2O mole fraction is almost similar to CO2mole fraction to length of 2.8 m of the TR side of reactor but afterthat RWGS reaction controls process conditions and causesincreasing H2O mole fraction at the rest of the reactor. Profiles ofcomponent mole fractions along dry reforming (DR) side, Fig. 2(b),can be explained via stoichiometric coefficient components in dry-reforming process. Methane and carbon dioxide as reactants have

Fig. 4. (a) Changes in temperature of TCTDR along reactor axis and (b) variation of heatgeneration in TR side, heat consumption in DR side and heat transfer between sides ofTCTDR along reactor axis.

Fig. 5. Comparison between mole fraction profiles of (a) CO2 and H2, (b) H2O and (c)CH4 and CO along TR side of TCTDR and TSTSR reactors.

Fig. 6. Molar flow rate of syngas along TR side of TCTDR and TCTSR reactors.

Fig. 7. Variations of (a) CH4 conversion and (b) H2 yield along TR side of TCTDR andTCTSR reactors.

M. Farniaei et al. / Journal of Natural Gas Science and Engineering 20 (2014) 132e146138

Fig. 8. Changes of (a) temperature and (b) generated heat along TR side of TCTDR andTCTSR reactors.

Fig. 9. Comparison between mole fraction of (a) H2 and CH4, (b) H2O, and (c) CO andCO2 along DR side of TCTDR and SR side of TCTSR.

M. Farniaei et al. / Journal of Natural Gas Science and Engineering 20 (2014) 132e146 139

Fig. 10. Changes of (a) temperature and (b) consumed heat along DR side of TCTDR andSR side of TCTSR reactors.

Fig. 11. Molar flow rate of syngas along DR side of TCTDR and SR side TCTSR reactors.

Fig. 12. Variations in (a) syngas quality along DR and TR sides of TCTDR, (b) syngasquality along SR and TR sides of TCTSR and (c) CH4 conversion along DR side of TCTDRand SR side of TCTSR reactors.

M. Farniaei et al. / Journal of Natural Gas Science and Engineering 20 (2014) 132e146140

M. Farniaei et al. / Journal of Natural Gas Science and Engineering 20 (2014) 132e146 141

same stoichiometric coefficients and also both products of dryreforming reaction; carbon monoxide and hydrogen have stoi-chiometric coefficient of 2. Small amount of H2O is produced byRWGS.

Fig. 3(a) and (b) shows how reaction rates change along the tri-and dry reforming side of reactor, respectively. Fig. 3(a) revealsthat complete oxidation of methane controls conditions of TRprocess kinetically at the entrance of TR side. When most of ox-ygen is consumed by COM reaction (see Fig. 2(a)); rate of reactionreaches to its equilibrium value, so endothermic reactions of TRprocess control conditions (equilibrium controlling). Reaction rateof WGS is negative because of high reaction rate of RWGS reactionthat is due to high temperature and concentration of CO2 in feedgas.

Fig. 3(b) shows that rate of DR and RWGS reaction is high at theentrance of DR side because of high temperature and concentrationof CH4 and CO2 at first. Then reaction rates decreases because oftemperature reduction at the rest of DR side. Reaction rate of RWGSreaches near zero at the entrance of DR side of TCTDR reactor. Fig. 4demonstrates temperature profiles in TR and DR sides of thereactor. Also, generated heat from TR side, consumed heat by DRside and transferred heat between both sides are presented inFig. 4(b), for more explanation about thermal behavior of thereactor. Temperature increases along TR side at first, because a largeamount of heat is generated by methane combustion reaction (seeFig. 4(b)) at the entrance of TR side. This generated heat is spent forendothermic reactions in TR side and temperature along rest of TRside decreases gradually.

6.1. Comparison between tri-reforming side of two differentreactors

This section discus about comparison between two thermallycoupled tri- and dry reforming (TCTDR) reactor and second isthermally coupled tri- and steam reforming (TCTSR) reactor; inwhich tri-reforming process is coupled with steam reformingprocess instead of dry reforming process.

Fig. 5(a) and (b), represents comparison between profiles ofmole fraction of components along TR side of TCTDR and TSTSRreactors. As shown in these figures, there are no significant differ-ence between behavior of component mole fraction in TR side ofreactors; because of same feed and conditions of TR sides.

As shown in Fig. 6, molar flow rate of syngas along TR side ofTCTSR is higher than TCTDR one in the half part of TR side,although its final value is lower. Flow rate of syngas along TR sideof TCTSR increases to 56,860 kmol h�1 at length of 2.6 m then afterthat begin to decrease and reaches to 38,360 kmol h�1 at the endof the reactor (12 m length) while molar flow rate of syngas alongTR side of TCTDR increases to 45,440 kmol h�1 at the end of thereactor.

Fig. 7 shows variations of CH4 conversion along TR side of TCTDRand TCTSR reactors. CH4 conversion profiles along TR side of bothreactors are similar and final values at the end of the reactor arenear together; these are due to same feed conditions of TR side ofboth reactors.

Fig. 8(a) and (b) represents thermal behavior of TR side of bothreactors. Since all components and conditions in TR side of TCTDRand TCTSR reactors are considered same, the amount of oxygen that

is reacted with methane is same, too. Hence profile of heat gener-ation from TR side of TCTDR is overlapped to the TCTSR one.Temperature profile along TR side of TCTDR is lower than TCTSRone at the second part of reactor.

6.2. Comparison between DR side of TCTDR and SR side of TCTSRreactors

Fig. 9(aec) presents a comparison between profiles of compo-nent mole fractions along steam reforming (SR) side of TCTSR anddry reforming (DR) side of TCTDR reactors. High effect of WGS re-action on SR process in second part of SR side of TCTSR reactor(after reaching to equilibrium) led to a lower profile of H2 incomparisonwith H2 profile in DR side of TCTDR reactor, as shown inFig. 9(a).

Since H2O is one of the feed components of SR side ofTCTSR, profile of H2O mole fractions higher than H2O molefraction in DR side of TCTDR reactor (see Fig. 9(b)). Smallamount of H2O in DR side of TCTDR reactor is produced byreverse WGS.

Variations of CO and CO2 mole fraction along SR side ofTCTSR and DR side of TCTDR are illustrated in Fig. 9(c). CO2 inDR side of TCTDR is reactant while in SR side of TCTSR isproduct, so CO2 decreases along DR side and increases along SRside.

Thermal behavior of both DR side of TCTDR and SR side of TCTSRreactors are compared in Fig.10(a) and (b). Heat consumption alongDR side of TCTDR is higher than transferred heat so temperaturedecreases along DR side.

Fig. 11 shows syngas molar flow rate along DR and SR side ofTCTDR and TCTSR reactors, respectively. Molar flow rate of syngasreaches to 7464 kmol h�1 at the output of DR side of TCTDR whilethis value is 3912 kmol h�1 for SR side of TCTSR reactor.

Fig. 12(a) and (b) represents profiles of syngas quality (H2/CO) along sides of both reactors. Results identify that syngasquality is equal to 9.19 and 1.12 at the outlet of SR and TR sidesof TCTSR respectively. Also, syngas is produced with a quality of1.08 and 0.958 in TR and DR sides of TCTDR respectively.Therefore, by considering molar flow rate of syngas in each sidefor determination of reactors performance, TCTDR and TCTSRcan produce total syngas with a quality of 1.063 and 1.848respectively.

CH4 is more converted by DR side of TCTDR in comparison withSR side of TCTSR as shown in Fig. 12(c). CH4 conversion reached to63% and 26% at the end of endothermic side of TCTDR and TCTSRreactors, respectively.

Finally, the pros and cons of the TCTDR can be addressed asfollows to be overcome for this to become a more widely adoptedmethod.

Production of two types of syngas and reduction of energyconsumption is advantages of TCTSR and TCTDR. In CSR, hugeamount of energy is needed. For preparation of this energymethane should be consumed in fired-furnace for energy produc-tion with CO2 generation. But in TCTSR and TCTDR, energy is pre-pared from TR reactions and fired-furnace is removed. Eliminationof a low performance fired-furnace and replacing it with a highperformance reactor causes a reduction of full consumption withproduction of a new type of syngas.

M. Farniaei et al. / Journal of Natural Gas Science and Engineering 20 (2014) 132e146142

One of the problems with dry reforming reaction is formation ofcoke on the active surface and leading to deactivation of thecatalyst rapidly. Carbon deposition is dependent on temperature,type of catalyst and existence of oxygen, steam, etc. in process. Byadjusting one or all of these parameters and choosing an appro-priate catalyst carbon deposition can be significantly eliminatedduring reaction.

Molar flow rate of the feed gases to TR side should be sufficientfor preparation of enough heat for DR and SR endothermic re-actions. Coke formation on the catalyst and reactor wall is elimi-nated in TR side because of applying some operating conditions ofthe reactor such as high temperature, high steam to carbon ratio infeed composition, appropriate Ni-based catalysts and presence ofsteam and oxygen in the feed.

Of course, economic feasibility such as capital and operating costof the process is necessary in order to consider commercializationof the proposed configuration.

7. Conclusions

Tri-reforming process was supplied for providing necessary heatof endothermic dry reforming process plus production of two typesof syngas by different quality in a multi-tubular thermally coupledreactor (TCTDR). 184 two-concentric-tubes were simulatednumerically by a one dimensional heterogeneousmodel and resultsof each side compared with thermally coupled steam and tri-reformer (TCSTR) reactor. Results identified that CH4 conversion atthe output of dry reforming (DR) side of TCTDR reached to 63% thatis very higher than steam reforming (SR) side of TCTSR reactor with26%. Also, syngas quality of DR side in TCTDR is lower than SR sideof TCTSR (0.958 vs. 9.19). Molar flow rate of syngas at the output ofDR side in TCTDR reactor reached to 7464 kmol h�1 while this valuewas 3912 kmol h�1 for SR side in TCTSR. CH4 conversion at theoutput of TR side of TCTDR reached to 93%. Syngas molar flow rateat the output of TR side of TCTDR reached to 5.544 � 104 kmol h�1.Some advantages of TCTDR reactor are: both sides produced syngaswith different quality, needed energy for highly dry reformingprocess was supplied by another process (tri-reforming) instead ofhuge furnaces.

Nomenclature

av specific surface area of catalyst pellet, m2 m�3

Ac cross section area of each tube, m2

Cp specific heat at constant pressure, J mol�1

dp particle diameter, mDi tube inside diameter, mDij binary diffusion coefficient of component i in j, m2 s�1

Dim diffusion coefficient of component i in themixture, m2 s�1

Ft total molar flow rate, mol s�1

Fi flow rate of component i, mol s�1

hf gasesolid heat transfer coefficient, W m�2 K�1

hi heat transfer coefficient between fluid phase and reactorwall in exothermic side, W m�2 K�1

ho heat transfer coefficient between fluid phase and reactorwall in endothermic side, W m�2 K�1

ki rate constant of reaction i, mol kg�1 s�1

k4a first reaction rate constant for the fourth rate equation,mol kg�1 s�1

k4b second reaction rate constant for the fourth rate equation,mol kg�1 s�1

Kw thermal conductivity of reactorwall, W m�1 K�1

L reactor length, mMi molecular weight of component i, g mol�1

N number of components, eP total pressure, barpi partial pressure of component i, Pari reaction rate of component i, mol kg�1 s�1

R1 first rate of reaction for steam reforming of methane,mol kg�1 s�1

R2 second rate of reaction for steam reforming of methane,mol kg�1 s�1

R3 rate of reversed wateregas shift reaction, mol kg�1 s�1

R4 rate of complete oxidation of methane, mol kg�1 s�1

R universal gas constant, J mol�1 K�1

Rp particle radius, mRe Reynolds number, eSci Schmidt number of component, eT temperature, Ku superficial velocity of fluid phase, m s�1

ug linear velocity of fluid phase, m s�1

U overall heat transfer coefficient betweenexothermic and endothermic sides, W m2 K�1

vci critical volume of component i, cm3 mol�1

yi mole fraction of component i, mol mol�1

z axial reactor coordinate, m

Greek lettersa activity of catalyst (where a ¼ 1

for fresh catalyst)DHf,i enthalpy of formation of component i, J mol�1

εB void fraction of catalytic bed, eh effectiveness factor used for the intra-particle transport

limitation, em viscosity of fluid phase, kg m�1 s�1

r density of fluid phase, kg m�3

rB density of catalytic bed, kg m�3

Superscriptsg in bulk gas phases at surface catalyst

Subscripts0 inlet conditionsi chemical speciesj reactor side

Appendix A

The following source code has been applied for solving non-linear algebraic equations by GausseNewton method in MATLABversion:

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