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Journal of Natural Gas Science and Engineering 19 (2014) 324e336

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Journal of Natural Gas Science and Engineering

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

Simultaneous production of methanol, DME and hydrogen in athermally double coupled reactor with different endothermicreactions: Application of cyclohexane, methylcyclohexane anddecalin dehydrogenation reactions

Mehdi Farniaei a, Mohsen Abbasi b, Hamid Rahnama c, Mohammad Reza Rahimpour 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, 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 19 March 2014Received in revised form21 May 2014Accepted 22 May 2014Available online

Keywords:MethanolHydrogenDMECyclohexaneMethylcyclohexaneDecalin

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

http://dx.doi.org/10.1016/j.jngse.2014.05.0191875-5100/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

Three different units of a multi-tubular reactor with 2962 three concentric tubes have been investigatedas a thermally double coupled reactor (TDCR) in co-current mode. Exothermic reactions are same for allthree units and are occurred in the inner (methanol synthesis) and outer tubes (direct DME synthesisfrom syngas) while endothermic reaction is different for each unit. Three endothermic dehydrogenationreactions of cyclohexane (CH), methylcyclohexane (MCH) and decalin (DC) have been considered formiddle tube side of each unit. A steady-state heterogeneous catalytic reaction model is applied toevaluate the performance of TDCR for simultaneous production of methanol, hydrogen and dimethy-lether (DME) in one reactor. The simulation results of each unit are compared with others that operatedat the same feed conditions. Results show that conversion of CH, MCH and DC reaches to 67%, 56% and77% at the output of each unit respectively. Output methanol yield with application of CH and MCHdehydrogenation as endothermic reactions are same equal to 0.37. Also, methanol yield for DC unit is0.33. In addition, conversion of monoxide at the output of DME synthesis side reaches to 64%, 65% and62% for CH, MCH and DC units, respectively.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Non-renewability and environmental problems of fossil fuelscaused a global movement toward clean, renewable and alternativeenergy carriers such as hydrogen (Brown, 2001; Jain et al., 2010;Song, 2002). Advantageous properties of hydrogen are: sustain-ability, high energy content, high efficiency, ease of storage anddistribution, cost attractive and finally environmentally friendlycharacteristics (Badmaev and Snytnikov, 2008; Cai andWang, 2012;Edwards et al., 2007; Haryanto et al., 2005; Hoffman, 2002;Schrope, 2001; Turner, 2004).

Among different developed methods for production ofhydrogen, more attraction is placed on process of catalytic steamreforming of natural gas and other hydrocarbons. This methodemits CO and CO2 to atmosphere that are corrosive and greenhouse

: þ98 711 6287294.mpour).

gases (Kariya et al., 2003; Wang et al., 2008) and works at highreaction temperatures (more than 700 �C) (Rahimpour et al.,2011b).

An alternative way to produce, store and transport of hydrogenis the dehydrogenation of cyclic hydrocarbons with high hydrogencontent (such as cyclohexane, methyl-cyclohexane, decalin, etc.)without any containments (Rahimpour et al., 2011b).

Cyclohexane (C6H12) with 7.1 wt.% hydrogen content can bedehydrogenated to gaseous hydrogen and condensable benzene(Jain et al., 2010). Benzene as a byproduct material can be used inproduction of phenol, styrene, aniline, drugs, dyes, insecticides,plastics, etc. (Othmer, 1978). Cyclohexane dehydrogenation reac-tion is predominantly carried out over Pt/Al2O3 catalyst at a tem-perature range and total pressure of 423e523 K and 101.3 kPa,respectively (Rahimpour et al., 2011a).

Dehydrogenation of decalin (DC) as another hydrogen storagematerial is performed at 210 �C over carbon supported Pt basedcatalyst in a batch reactor and a condenser removes hydrogen fromthe reactor (Hodoshima et al., 2003). Wang et al. (2008) employed

M. Farniaei et al. / Journal of Natural Gas Science and Engineering 19 (2014) 324e336 325

dehydrogenated decalin to produce pure hydrogen for fuel cellapplications. Byproduct of decalin dehydrogenation reaction istetralinwhich can be further dehydrogenated into naphthalene andmore hydrogen.

Dehydrogenation of methyl-cyclohexane (MCH) is one of themost attractive hydrogen energy storage materials. Besideshydrogen, toluene is produced during this reactionwhich is used inthe petrochemical complexes for various end products. Also,toluene can play the role of carrier because it is easily hydrogenatedand then dehydrogenated again (Vakili et al., 2011).

Each of the three mentioned endothermic dehydrogenationreactions needs an energy source for proceeding process. In thisstudy, concept of thermally coupled reactors is employed to pro-duce the necessary heat for each of cyclohexane, MCH and decalindehydrogenation reactions. In this type of reactors, released heatfrom one or two exothermic reactions is used as energy source forproceeding an exothermic reaction without any mixing of reactors.

For the first time, Hunter and McGuire (1980) coupled endo-thermic and exothermic reactions without direct heat transfer. Bhatand Sadhukhan (2009) presented a comprehensive overview of theprocess integration aspects for methane steam reforming in athermally coupled membrane separation technology. Khademiet al. (2010) investigated coupling of methanol synthesis andcyclohexane dehydrogenation reaction. Methanol synthesis andcyclohexane dehydrogenation reactions in a single and dualhydrogen perm-selective membrane thermally coupled reactorhave been investigated by several researchers (Aboosadi et al.,2011; Rahimpour, 2007). Synthesis of dimethylether (DME) in athermally coupled heat exchanger reactor was investigated byVakili et al. (2011). Farsi and Jahanmiri (2011) simulated and opti-mized DME production and cyclohexane dehydrogenation in athermally coupled heat exchanger reactor. Methanol dehydrationand cyclohexane dehydrogenation reactions were investigated in athermally coupled reactor by Khademi et al. (2011).

Rahimpour et al. (2013) studied coupling of methanol and DMEsynthesis with the endothermic reaction of cyclohexane dehydro-genation in a thermally double coupled reactor.

Methanol and DME as important chemical materials have wideranges of applicability in industry and daily lives. DME can be usedas an alternative fuel and has applications in heating and cookinginstead of liquefied petroleum gas (LPG) (Ji et al., 2011; Naik et al.,2011). Also, it can be used as a raw material for some chemicalproductions such as: olefins, gasoline, jet fuel, spray, and hydrogencarrier in fuel cells (Naik et al., 2011).

Methanol is utilized in synthesis of biodiesel, DME, methyl t-butyl-ether, etc. as well as in novel processes such as directmethanol fuel cell, micro channel methanol steam reforming forproduction of hydrogen. Also, it can be used as a near-zero emis-sions alternative fuel, hydrogen carrier, solvent, etc. (Hao et al.,2011; Park et al., 2012; Semelsberger et al., 2006).

In this work, two exothermic reactions of methanol and dime-thylether (DME) synthesis are considered as energy source forproceeding each of endothermic reactions of cyclohexane (CH),methylcyclohexane (MCH) and decalin (DC) dehydrogenation.Therefore, three individual units of thermally double coupledreactor (TDCR) are investigated by employing exothermic reactionsof DME and methanol synthesis that take place in outer and innertubes, respectively. Also, each of endothermic reaction (CH, MCHand DC dehydrogenation) is replaceable and occurs in middle tube.Result of each unit is compared together and effect of twoexothermic reactions on performance of each endothermic reactionis investigated.

It must be noted that, results of this paper are novel and thereare not similar works in literature. In fact, thermally doublecoupled reactors with application of MCH and DC dehydrogenation

endothermic reactions have not been presented yet in literature.On the other hand, performance of each case of TDCR withdifferent endothermic reaction for simultaneous production ofmethanol, hydrogen and dimethylether (DME) in one reactor isevaluated in this novel paper. The simulation results of each unitare compared with other units that operated at the same feedconditions.

2. Process description

Thermally double coupled reactor (TDCR) is composed ofthree concentric tubes that each endothermic reaction (dehy-drogenation of CH, MCH and DC) is occurred in middle tube andexothermic reactions of methanol and DME synthesis take placein the inner and outer tubes, respectively. Generated heat fromendothermic reactions is continuously transferred to endo-thermic reaction side. This multi-tubular configuration in co-current mode is illustrated in Fig. 1. Inner and outer tubes areloaded with CuO/ZnO/Al2O3 (same as the conventional methanolreactor). Middle tube is loaded with Pt/Al2O3 catalysts for units inwhich CH and MCH dehydrogenation reactions are considered forendothermic reaction and with PteSn/g-Al2O3 for DC dehydro-genation unit.

Conventional methanol reactor in Zagros Petrochemical Com-pany in Assaloyeh, Iran; has 2962 packed tubes with length equal to7.022m. Based on this subject, number of tubes and length of them;feed composition and flow rate of methanol side in TDCR have beenselected. Also, feed compositions in endodermic and DME sideswere selected based on results of Wang et al. (2008), Itoh (1987),Maria et al. (1996) and Hu et al. (2008). After that, for achievinggood thermally coupled reactors from thermal and molar flow rateview, temperature and feed flow rate of endothermic reactionwereselected.

Finally, Tables 1e6 represent characteristics, the properties andinput data of endothermic and exothermic reactions in each unit ofTDCR.

3. Reaction scheme and kinetics

3.1. Methanol synthesis (exothermic inner tube side)

Three main reactions that occur in methanol synthesis are asfollows:

CO hydrogenation:

COþ 2H24CH3OH; DH298 K ¼ �90:55 kJ=mol (1)

CO2 hydrogenation:

CO2 þ 3H24CH3OHþ H2O; DH298 K ¼ �49:43 kJ=mol (2)

Water gas shift reaction:

CO2 þ H24COþ H2O; DH298 K ¼ 41:12 kJ=mol (3)

These reactions are carried out over the CuO/ZnO/Al2O3 cata-lysts in the inner tube of TDCR. The following reaction rate equa-tions for hydrogenation of CO and CO2 and reverse water gas shiftreaction are chosen from Graaf et al. (1988):

r1 ¼k1KCO

hfCOf

3=2H2� fCH3OH

.f 1=2H2

KP1

i�1þ KCOfCO þ KCO2

fCO2

�hf 1=2H2þ�KH2O

.K1=2H2

�fH2O

i (4)

Fig. 1. Schematic diagram of thermally double coupled reactor (TDCR).

M. Farniaei et al. / Journal of Natural Gas Science and Engineering 19 (2014) 324e336326

r2 ¼k2KCO2

hfCOf

3=2H2� fCH3OHfH2O

.f 3=2H2

KP2

i�1þ KCOfCO þ KCO2

fCO2

�hf 1=2H2þ�KH2O

.K1=2H2

�fH2O

i (5)

r3 ¼k3KCO2

�fCO2

fH2� fH2OfCO

�KP3

��1þ KCOfCO þ KCO2

fCO2

�hf 1=2H2þ�KH2O

.K1=2H2

�fH2O

i (6)

Table 7 gives the reaction rate constants, the adsorption equi-librium constants and the reaction equilibrium constants formethanol synthesis.

Table 1The characteristics of thermally double coupled reactor (TDCR) units.

Parameter Value

Inner tube or methanol synthesis side diameter (m) 3.8 � 10�2

Middle tube or endothermic side diameter (m) 8.52 � 10�2

Outer tube or DME synthesis side diameter (m) 9.75 � 10�2

Length of the reactor (m) 7.022Number of tubes 2962

3.2. Direct DME synthesis

Traditional or indirect DME production is methanol dehydrationwhile direct production of DME is combining methanol synthesisand dehydration in one reactor at the presence of synthesis gas(CO þ H2) in the feed. The direct method is more economicalbecause in this way cost of methanol purification is eliminated andmethanol conversion will reach to a higher value in comparisonwith traditional method (Bercic and Levec, 1993; Lu et al., 2004).Reactions that occur in DME production are as follows:

Methanol formation:

COþ 2H24CH3OH; DH298 K ¼ �90:55 kJ=mol (7)

Water gas shift reaction:

CO2 þ 3H24CH3OHþ H2O; DH298 K ¼ �49:43 kJ=mol (8)

Dehydration of methanol:

2CH3OH4CH3OCH3 þ H2O; DH298 K ¼ �21:003 kJ=mol(9)

These reactions are carried out over a bi-functional catalyst (Cu/Zn/Al2O3) with two active sites; one is applied for methanol syn-thesis and another for DME formation (Flores et al., 2011).Following reaction rate equations are considered for direct DMEsynthesis reactions (Rahimpour et al., 2013):

rCO ¼k1fCOf 2H2

ð1� b1Þ�1þ KCOfCO þ KCO2

fCO2þ KH2

fH2

�3 (10)

rCO2¼ k2fCO2

f 3H2ð1� b2Þ�

1þ KCOfCO þ KCO2fCO2þ KH2

fH2

�4 (11)

rDME ¼k3fCH3OHð1� b3Þ�

1þ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiKCH3OHfCH3OH

p �2 (12)

b1 ¼fCH3OH

Kf1fCOf 2H2

(13)

b2 ¼fCH3OHfH2O

Kf2fCO2f 3H2

(14)

Table 2The operating conditions for DME synthesis (outer exothermic side) in TDCR.

Parameter Value

DME synthesis (exothermic reaction)Feed composition (mole fraction)CO 0.1716CO2 0.0409DME 0.0018CH3OH 0.003H2O 0.0002H2 0.4325N2 0.316CH4 0.044

Inlet temperature (K) 493Inlet pressure (bar) 50Inlet flow rate in each tube (mol s�1) 0.6

Typical properties of catalystNumber of three concentric tubes 2962Size of column grain catalysts (mm) f5 � 5Density of catalyst bed (kg m�3) 1200Porosity 0.455

Table 4The operating conditions for dehydrogenation of CH (endothermicside of CH unit).

Parameter Value

Dehydrogenation of C6H12 (endothermic side)Gas phaseFeed compositionC6H12 0.1C6H6 0.0H2 0.0Ar 0.9

Inlet temperature (K) 503Inlet pressure (bar) 20Particle diameter (m) 3.55 � 10�3

Bed void fraction 0.39Total flow rate (mol s�1) 0.5

M. Farniaei et al. / Journal of Natural Gas Science and Engineering 19 (2014) 324e336 327

b3 ¼fDMEfH2O

kf3f 2CH3OH

(15)

where fi and Kfj are the fugacity of component i and equilibriumconstant of reaction j, respectively. The kinetic parameters aretabulated in Table 8.

3.3. Endothermic middle tube side reactions

3.3.1. Dehydrogenation of cyclohexane (CH)The reaction scheme for CH dehydrogenation is as follows

(Khademi et al., 2010):

C6H124C6H6 þ 3H2; DH298 K ¼ 206:2 kJ=mol (16)

In the current study, the reaction rate is considered as follows:

rC ¼k�KPPC

.P3H2� PB

�1þ

�KBKPPC

.P3H2

� (17)

Table 3The operating conditions for methanol synthesis (inner exothermic side) in TDCR.

Parameter Value

Gas phaseFeed composition (mole fraction)CH3OH 0.0050CO2 0.0940CO 0.0460H2O 0.0004H2 0.6590N2 0.0930CH4 0.1026

Total molar flow rate (mol s�1) 0.64

Catalyst particleParticle diameter (m) 5.47 � 10�3

Density (kg m�3) 1770Heat capacity (kJ kg�1 K�1) 5.0Thermal conductivity (W m�1 K�1) 0.004Specific surface area (m2 m�3) 626.98Ratio of void fraction to tortuosity of catalyst particle 0.123

where k, KB and KP are the reaction rate constant, the adsorptionequilibrium constant and the reaction equilibrium constant,respectively which are listed in Table 9. Pi is the partial pressure ofthe component i in Pa.

3.3.2. Dehydrogenation of methylcyclohexane (MCH)The reaction scheme for MCH dehydrogenation is expressed as

follows:

C7H144C7H8 þ 3H2; DH298 K ¼ þ205 kJ=mol (18)

The following rate expression over commercial Pt/Al2O3 catalystis considered for MCH dehydrogenation reaction (Maria et al.,1996):

rMCH ¼ kpMCH

"1� ptolp

3H2

KeqpMCH

#(19)

where Pi and Keq are partial pressure (atm) and equilibrium con-stant (atm3) respectively (Akyurtlu and Stewart, 1978). k is reactionconstant and is obtained by following equation:

k ¼ A exp� ER

1T� 1650

��(20)

where A ¼ 20:46 mol=ðgcat h atmÞ and E/R ¼ 26,540 K, where Keq isequilibrium constant for MCH dehydrogenation and is given by

Keq ¼ 3600 exp�217;650

R

1T� 1650

��(21)

Keq in bar3, R in J mol�1, and T in K.

Table 5Operating conditions and the properties of the catalyst for dehydrogenationof MCH (endothermic side of MCH unit).

Parameter Value

Feed composition (mole fraction)C7H14 0.12C7H8 0.0H2 0.0Ar 0.88

Total molar flow rate (mol s�1) 0.1Inlet temperature (K) 503Inlet pressure (Pa) 8 � 105

Shell inner diameter (m) 0.07Bed void fraction (e) 0.39Particle diameter (m) 3.55 � 10�3

M. Farniaei et al. / Journal of Natural Gas Science and Engineering 19 (2014) 324e336328

3.3.3. Dehydrogenation of decalin (DC)DC dehydrogenation over PteSn/g-Al2O3 catalyst includes three

reactions. DC isomers; Cis and Trans, turn into each other. Both Cis-decalin (CDC) and Trans-decalin (TDC) can be dehydrogenated intotetralin and hydrogen. Furthermore, produced tetralin is dehydro-genated to form naphthalene and more hydrogen. All of theseendothermic reactions are reversible and are presented as follows(Wang et al., 2008):

(22)

Table 7The reaction rate constants, the adsorption equilibrium constants and the reactionequilibrium constants for methanol synthesis.

A (mol kg�1 s�1 bar�1/2) B (J mol�1)

Cis� Decaline ðCDCÞ������������������! ������������������r04;r4 Trans� Decaline ðTDCÞ(23)

The overall reaction of these two series reactions can be written asfollows:

Decalin4Naphthaleneþ 5Hydrogen (24)

The rate equations for DC dehydrogenation on Pt-active site can bepresented as follows (Wang et al., 2008):

r1 ¼ kr1KTDCPTDC.D4 (25)

r2 ¼ kr2KCDCPCDC.D4 (26)

r3 ¼ kr3KTTPTT.D4 (27)

r01 ¼ k0r1KTTKH2PTTP

3H2

.D4 (28)

r02 ¼ k0r2KTTKH2PTTP

3H2

.D4 (29)

r03 ¼ k0r3KNPKH2PNPP

2H2

.D3 (30)

Table 6The operating conditions for dehydrogenation of DC to naphthalene(endothermic side of DC unit).

Parameter Value

Feed composition (mole fraction)Trans-Decalin (TDC) 0764Cis-Decalin (CDC) 0.236Total molar flow rate (mol s�1) 0.1Reactor pressure (bar) 1Feed temperature (K) 550Catalyst equivalent diameter (m) 3.55 � 10Bed void fraction (e) 0.39

D ¼ 1þ KCDCPCDC þ KTDCPTDC þ KTTPTT þ KH2PH2þ KNPPNP

(31)

r4 ¼ kr4K0CDCPCDC

.U4 (32)

r04 ¼ k0r4K0TDCPTDC

.U4 (33)

U ¼ 1þ K 0TDCPTDC þ K 0CDCPCDC (34)

RTDC ¼ r1 � r01 � r4 þ r04 (35)

RCDC ¼ r2 � r02 þ r4 � r04 (36)

RTT ¼ r1 � r01 þ r2 � r02 þ r03 � r3 (37)

RNP ¼ r3 � r03 (38)

For the rate coefficient:

ki ¼ Ai expð � Ei=RTÞ (39)

For the adsorption constant:

Ki ¼ Ai expð � DHi=RTÞ (40)

The parameters for calculation of the reaction rate constant andadsorption equilibrium constant for dehydrogenation of DC reac-tion are tabulated in Table 10.

k1 (4.89 ± 0.29) � 107 �63,000 ± 300k2 (1.09 ± 0.07) � 105 �87,500 ± 300k3 (9.64 ± 7.30) � 106 �152,900 ± 6800

A (bar�1) B (J mol�1)

KCO (2.16 ± 0.44) � 10�5 46,800 ± 800KCO2

(7.05 ± 1.39) � 10�7 61,700 ± 800ðKH2O=K

1=2H2Þ (6.37 ± 2.88) � 10�9 84,000 ± 1400

A (K) B (K)

KP1 5139 12.621KP2 3066 10.592KP3 �2073 �2.029

Table 10The parameters for calculation of the reaction rate con-stant and adsorption equilibrium constant for dehydro-genation of DC reaction.

Parameter Value

ATDC 1.54906ACDC 7.89457ATT 9.33959Eþ1ANP 2.26323AH2

3.31108ATDC0 2.21726Eþ2ACDC0 4.0854Eþ1ATDC-TT 3.63431Eþ5ACDC-TDC 1.24695Eþ1ATT-NPATT�NP 3.35724Eþ3ACDC-TT

DHTDC 1.28343Eþ4DHCDC 4.41280Eþ3DHTT 3.88825Eþ3DHNP 1.07247Eþ4DHH 6.54869Eþ3DHTDC0 1.42062Eþ4DHCDC0 6.89845Eþ3ETDC-TT 2.34547Eþ4ECDC-TT 9.05243Eþ3ETT-NP 7.06251Eþ3ECDC-TDC 2.84393Eþ4

Table 8Reaction rate constants for DME synthesis reactions.

A B (J mol�1)

k1 1.828 � 103 (mol g�1 h�1 bar�3) �43,723k2 0.4195 � 102 (mol g�1 h�1 bar�3) �30,253k3 1.939 � 102 (mol g�1 h�1 bar�1) �24,984KCO 8.252 � 10�4 (bar�1) 30,275KCO2

2.1 � 10�3 (bar�1) 31,846KH2

0.1035 (bar�1) �11,139KCH3OH 1.726 � 10�4 (bar�1) 60,126

M. Farniaei et al. / Journal of Natural Gas Science and Engineering 19 (2014) 324e336 329

4. Mathematical modeling

Following assumption are considered for mathematicalmodeling of each unit of TDCR:

� One-dimensional heterogeneous model for each side of thereactor is applied.� The model is investigated at steady state conditions.� Gas phases are ideal.� Plug flow pattern is dominant in each side of the TDCR.� Axial diffusions of heat and mass are negligible in comparisonwith radial diffusions.� Bed porosity in axial and radial directions is constant.

A differential element along the axial direction of the reactor ispresented in Fig. 2. The necessary heat for endothermic dehydro-genation reaction (middle tube) is provided by exothermic re-actions of methanol and DME synthesizes (inner and outer tube,respectively). Mole and energy balance differential equations ofTDCR are presented in Table 11. h is the effectiveness factor and ysi;jand Ts

j are the mole fraction of the component i in the solid phasesand solid temperature of j side of the TDCR, respectively. Fi,j is themolar flow rate of component i in the fluid phase and j side of thereactor. Tgj is the fluid temperature of j side of the reactor.

4.1. Auxiliary correlations

Since heat and mass transfer between solid and fluid phases hasbeen considered for mathematical modeling of TDCR, physicalproperties of components are estimated for the calculations.

Table 12 collected auxiliary correlations including physicalproperties of components, mass and heat transfer coefficient andErgun equation. Ergun equation was used for calculating pressuredrop through the catalytic bed.

Endothermic reaction for each unit is different to others whileexothermic reactions were similar for all units. Hence, all units arenamed due to their own endothermic reactions; CH, MCH and DCunits, respectively. Following definitions have been used to calcu-late methanol and hydrogen yields as well as CH and carbonmonoxide conversions:

Methanol yield ¼ FCH3OH;out

FCO þ FCO2;in(48)

Table 9The reaction rate constant, the adsorption equilibrium constant and the reactionequilibrium constant for CH dehydrogenation.

A B (K)

k 0.221 mol m�3 Pa�1 s�1 �4270KB 2.03 � 10�10 Pa�1 6270KP 4.89 � 1035 Pa3 3190

CH conversion ¼ FC6H12;in � FC6H12;out

FC6H12;in(49)

CO conversion ¼ FCO;in � FCO;outFCO;in

(50)

MCH conversion ¼ FMCH;in � FMCH;out

FMCH;in(51)

DC conversion ¼ FDC;in � FDC;outFDC;in

(52)

Fig. 2. The differential element along the axial direction inside the sides of TDCR.

Table 12Auxiliary correlations.

Parameter Equation Ref.

Component heat capacity Cp ¼ a þ bT þ cT2 þ dT3

Mixture heat capacity Cpm ¼P

yiCpiComponent viscosity m ¼ C1TC2

1þC2T þ

C4T2

Mixture thermalconductivity

km ¼PðyikiÞ Lindsay and

Bromley (1950)Mass transfer coefficient

between gas and solidphases

kgi ¼ 1:17Re0:42Sc0:67i ug � 103 Cussler (1997)

Re ¼ rugdpm

Sci ¼ mrDim�104

Dim ¼ 1�yiPisj

yiDij

Dij ¼ 107T3=2ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1=Miþ1=Mj

pPðv3=2ci þv

3=2cj Þ

Reid et al. (1977)

Overall heat transfercoefficient

1U ¼ 1

hiþ Ai lnðDo=DiÞ

2pLKwþ Ai

Ao

1ho

Heat transfer coefficientbetween gas phase andreactor wall

hCprm

Cpm

K

�2=3¼ 0:458

3B

m

rudp

�0:407 Smith (1980)

Ergun equation dPdz ¼ 150 ð1� 3Þ2

33d2pþ 1:75

ð1� 3Þu2g

32d2p

Table 11Mole and energy balances in the axial direction of TDCR.

Mass and energy balances equations #

Solid phase (both exothermic side and endothermic side) avcjkgi;jðygi;j � ysi;jÞ þ hri;jrb ¼ 0 (41)

avhf ðTgj � Tsj Þ þ rbPN

i¼1hri;jðDHf ;iÞ ¼ 0 (42)

Fluid phase (both exothermic sides) 1Ac

dFt;jdz þ avcjkgi;jðysi;j � ygi;jÞ ¼ 0 (43)

�Cgp;j

Ac

dðFjTgj Þ

dz þ ai;jhf ðTsj � Tgj Þ þ

pDj

AcUðTg2 � Tgj Þ ¼ 0 (44)

Fluid phase (endothermic side) 1Ac

dFt;jdz þ avcjkgi;jðysi;j � ygi;jÞ ¼ 0 (45)

±Cgp;j

Ac

dðFjTgj Þ

dz þ ai;jhf ðTsj � Tgj Þ �pDj

AcU1�2ðTgj � Tg1 Þ �

pDj

AcU2�3ðTg

j � Tg3 Þ ¼ 0 (46)

Boundary conditions z ¼ 0; ygi;j ¼ ygi0;j; Tj ¼ Tgjo; Pgj ¼ Pgj0; j ¼ 1;2;3 (47)

M. Farniaei et al. / Journal of Natural Gas Science and Engineering 19 (2014) 324e336330

4.2. Numerical solution

Developed model is formed by a set of ordinary differentialequations (ODEs) which are consisted of mass and energy balanceequations. ODEs are coupled with non-linear algebraic equations ofkinetic models and auxiliary and hydrodynamic correlations. Set ofODEs are solved by backward finite difference approximation tochange them into a set of non-linear algebraic equations. Thelength of each reactor is divided into 100 separate segments andGausseNewton method is applied to solve the obtained set of non-linear algebraic equations in each segment for each tube side,

Table 13Comparison between simulation and plant data for conventional methanol reactor.

Composition (mol%) Reactor inlet Reactor output

Plant data Simulation results

CH3OH 0.032 5.49 5.5CO2 3.45 2.18 2.43CO 4.66 1.44 1.52H2O 0.08 1.74 1.47H2 79.55 75.71 76.54CH4 11.72 12.98 12.96N2 0.032 0.16 0.035Feed flow rate (mol s�1) 0.565 0.51 0.511Temperature (K) 527 528 524.1

simultaneously. This procedure is repeated for all segments of thereactor as the results of each segment are used as the inlet condi-tions for the next segment.

4.3. Model validation

Simulation results were compared with an industrial scaleconventional methanol synthesis reactor in Zagros PetrochemicalCompany, Assaloyeh, Iran that has been presented in Table 13.Fortunately, a good agreement was achieved between proposedmodel and experimental plant data. In addition, for validating theproposed model in DME side, a good agreement was observedbetween simulation results and experiments of Hu et al. (2008) inTable 14.

5. Results and discussion

Variations of component mole fractions along reactor axes inmethanol synthesis side (inner tube side) for all three units areillustrated in Fig. 3. As seen, trend of each component mole fractionin the inner tube side is similar for all units. Hydrogen has thehighest profile and CO2 mole fraction decreases slightly in all unitsof TDCR while other components have a curved profile lower thanit.

Comparisons between behaviors of component mole fractionsin outer tube side (DME synthesis) of all units are shown in Fig. 4.All components in DME synthesis side have a same behavior in allthree units and hydrogen has the highest profile (like inner tubeside).

Fig. 5 represents the changes of component mole fractions ineach endothermic reaction in themiddle tube side of all units. Sincereaction scheme of CH dehydrogenation is almost same as the MCHdehydrogenation reaction, it is expected the same behavior forcomponent mole fractions of both reactions is observed (seeFig. 5(a) and (b)). The hydrogen trajectory is upper than benzene/

Table 14Comparison between steady state simulation results and experiments of Hu et al.(2008).

Components Simulationresults

Experiments ofHu et al. (2008)

Output composition (%)DME 4.95 4.91CH3OH 1.03 1.06H2O 3.51 3.38H2 33.3 33CO 8.9 8.77CO2 6.4 6.71N2 41.91 42.17Temperature 511 517

Fig. 3. Variation of component mole fractions along reactor axes in methanol synthesis side for a) CH unit, b) MCH unit and c) DC unit.

Fig. 4. Variation of component mole fractions along reactor axes in DME synthesis side for a) CH unit, b) MCH unit and c) DC unit.

Fig. 5. Variation of component mole fractions along reactor axes in a) CH dehydrogenation side, b) MCH dehydrogenation side and c) DC dehydrogenation side.

M. Farniaei et al. / Journal of Natural Gas Science and Engineering 19 (2014) 324e336332

toluene owing its higher stoichiometric coefficient in CH and MCHdehydrogenation reaction.

Fig. 5(c) illustrates variations of component mole fractions inDC unit. The highest product is hydrogen. DC as one of the besthydrogen carriers has a high hydrogen production rate. Since

Fig. 6. Conversion changes of CH, MCH and DC along reactor axes in CH, MCH and DCunits, respectively.

stoichiometric ratio of naphthalene to hydrogen is 1/5; hydrogenproduction is five times naphthalene production as illustrated inFig. 5(c). Tetralin as an intermediate product turns into naphtha-lene and hydrogen immediately. The rate of tetralin dehydroge-nation reaction is very high thus all tetralin is converted into

Fig. 7. Variations of CO conversion in DME synthesis side along reactor axes for CH,MCH and DC units.

Fig. 8. Changes of hydrogen molar flow rate along all three sides of a) CH unit, b) MCH unit, and c) DC unit. d) Simultaneous plots for changes of hydrogen molar flow rate alongendothermic sides of CH, MCH and DC units.

M. Farniaei et al. / Journal of Natural Gas Science and Engineering 19 (2014) 324e336 333

products and its mole fraction at the end of the reactor almostreaches to zero.

Fig. 6 shows a comparison between conversion of CH, MCH andDC along the endothermic side of CH, MCH and DC units, respec-tively. CH, MCH and DC conversions at the output of each unit

Fig. 9. Changes of methanol yield in methanol synthesis side along reactor axes for CH,MCH and DC units.

reached to 67%, 56% and 77%, respectively. It must be noted that,inlet feed flow rate of endothermic side for DC unit is lower thatother units. Therefore, conversion in DC unit is higher than otherunits.

Variation of CO conversion along outer exothermic tube side(DME synthesis) for all units is compared in Fig. 7. Monoxideconversion at the output of CH, MCH and DC units reached to64%, 65% and 62%, respectively. This subject is due to betterthermal conditions for production of DME in MCH unit. On theother hand, reaction rate in DME side of MCH unit is higher thanother units.

As shown in Fig. 8(aec), trend of changing in H2 molar flowrate along the exothermic sides (methanol and DME synthesissides) is similar for all three units. Hydrogen molar flow rate atthe output of endothermic side of CH, MCH and DC units arrivesto 0.10, 0.08 and 0.05 mol s�1, respectively. Also, simultaneousplots of H2 molar flow rate along the endothermic sides of allunits are illustrated in Fig. 8(d). Higher H2 molar flow rate in MCHunit in comparison to other units is due to receiving more heatfrom exothermic side. Therefore, endothermic reaction rate inMCH unit is higher than other cases and more hydrogen isproduced.

Variations of methanol yield along inner tube side for all threeunits are demonstrated in Fig. 9. As illustrated in this figure, yield ofmethanol in CH and MCH units is equal to 0.37 that is higher thanDC unit with a value of 0.33. These results indicate that byemploying of DC dehydrogenation reaction, reaction rate ofmethanol production decreases due to unsuitable thermal

Fig. 10. Temperature profiles along all three sides of a) CH unit, b) MCH unit and c) DC unit.

M. Farniaei et al. / Journal of Natural Gas Science and Engineering 19 (2014) 324e336334

conditions. In fact, in DC unit, temperature of methanol side de-creases and is get away from optimum temperature.

Temperature profiles of all three sides in each unit aredemonstrated in Fig. 10(aec). To create thermal driving force,temperature of exothermic side should be higher than endo-thermic side. In all units temperature profile of endothermic side islower than temperature profiles of both methanol and DME syn-thesis (see Fig. 10). In fact, exothermic sides generate the necessaryheat for driving the each endothermic reaction (CH, MCH and DCdehydrogenation) as well as heating the mixtures of all three sides.In all units, the generated heat at the entrance of the reactor is lessthan consumed heat. Therefore, the temperature of endothermicside begins to fall and a cold spot developed. After a certain length,conditions are changed and temperature increases along the restof the reactor.

The generated and consumed heat along exothermic andendothermic sides of all units have been presented inFig. 11(aec).

As shown in Fig. 11(a) and (b), at the first half of the TDCR,heat generation in exothermic sides increases sharp due to hightemperature and reaction rate. But at the second half, due toconversion of feed and reduction of reaction rate, heat generationdecreases. Similar phenomenon is observed for endothermicsides. For TDCR with DC dehydrogenation (Fig. 11(c)), heat con-sumption at the entrance of reactor is very high due to high re-action rate but it decreases sharp at the rest of the reactor.Production of hydrogen, methanol and DME depends on ex-change of heat between reactor sides. Of course, it is difficult to

quantitative correlation between products during reaction pro-cess from exchange of heat due to carrying out a lot of reactionsin three sides of TDCR. From the results in Fig. 11(aec), it can besaid that for all units, sum of heat generation in methanol andDME sides is higher than heat consumption in the endothermicsides.

6. Conclusions

In this study, methanol DME synthesis processes are coupledwith three different dehydrogenation reactions (Cyclohexane (CH),Methylcyclohexane (MCH) and Decalin (DC) dehydrogenations) inthree individual units of thermally double coupled reactor (TDCR).Simultaneous production of hydrogen, methanol and DME isinvestigated via a one-dimensional heterogeneous catalytic reac-tion model. Achieving high degree of in situ energy integration bycoupling two exothermic reactions with an endothermic reactioncan be expressed as advantages of these configurations. Thesimulation results of each TDCR unit are compared with otherunits. Results indicated that hydrogen molar flow rate at theoutput of endothermic side of CH unit reached to higher value incomparison with MCH and DC units (0.101, 0.084 and0.046 mol s�1 in each tube, respectively). Yield of methanol in CHand MCH unit obtained to a same value of 0.373 while it is 0.335for DC unit. Monoxide is more converted along DME synthesis sideof MCH unit with the value of 65% in comparison with CH and DCunits with the values of 64% and 62%, respectively. Output H2molar flow rate in methanol and DME synthesis sides in each tube

Fig. 11. Variation of generated heat along methanol and DME synthesis sides and consumed heat in a) CH dehydrogenation along reactor axes in CH unit, b) MCH dehydrogenationalong reactor axes in MCH unit and c) DC dehydrogenation along reactor axes in DC unit.

M. Farniaei et al. / Journal of Natural Gas Science and Engineering 19 (2014) 324e336 335

was 0.35 and 0.15 mol s�1, respectively and was same for all threeunits.

Nomenclature

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

Ac cross section area of each tube, m2

Ai inside area of inner tube, m2

Ao outside area of inner tube, m2

C total concentration, mol m�3

Cp specific heat of the gas at constant pressure, J mol�1

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

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

fi partial fugacity of component i, barFt total molar flow rate, 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

DHf,i enthalpy of formation of component i, J mol�1

k rate constant of dehydrogenation reaction,mol m�3 Pa�1 s�1

ki rate constant of reaction i, mol kg�1 s�1 bar�1/2

kg,i mass transfer coefficient for component i, m s�1

K conductivity of fluid phase, W m�1 K�1

Ki adsorption equilibrium constant for component i, bar�1

Kp equilibrium constant for dehydrogenation reaction, Pa3

Kpi equilibrium constant based on partial pressure forcomponent i in methanol synthesis reaction

Kw thermal conductivity of reactor wall, W m�1 K�1

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

N number of componentsP total pressure, barPi partial pressure of component i, Par1 rate of reaction for hydrogenation of CO in methanol

synthesis, mol kg�1 s�1

r2 rate of reaction for hydrogenation of CO2 in methanolsynthesis, mol kg�1 s�1

r3 rate of reversed wateregas shift reaction in methanolsynthesis, mol kg�1 s�1

rCO rate of reaction for hydrogenation of CO, mol kg�1 s�1

rCO2rate of reaction for hydrogenation of CO2, mol kg�1 s�1

rDME rate of reaction for dehydration of methanol, mol kg�1 s�1

rC rate of reaction for dehydrogenation of cyclohexane,mol m�3 s�1

R universal gas constant, J mol�1 K�1

Rp particle radius, mRe Reynolds numberSci Schmidt number of componentT temperature, Ku superficial velocity of fluid phase, m s�1

ug linear velocity of fluid phase, m s�1

U overall heat transfer coefficient between exothermic andendothermic sides, W m�2 K�1

vci critical volume of component i, cm3 mol�1

M. Farniaei et al. / Journal of Natural Gas Science and Engineering 19 (2014) 324e336336

yi mole fraction of component iZ axial reactor coordinate, m

Greek lettersm viscosity of fluid phase, kg m�1 s�1

P density of fluid phase, kg m�3

rb density of catalytic bed, kg m�3

T tortuosity of catalyst

Superscriptsg in bulk gas phases at surface catalyst

Subscripts0 inlet conditionsi chemical speciesj reactor side

References

Aboosadi, Z., Rahimpour, M., Jahanmiri, A., 2011. A novel integrated thermallycoupled configuration for methane-steam reforming and hydrogenation ofnitrobenzene to aniline. Int. J. Hydrogen Energy 36, 2960e2968.

Akyurtlu, J.F., Stewart, W.E., 1978. Competitive hydrogenation of benzene andtoluene and dehydrogenation of the corresponding naphthenes over platinumwire. J. Catal. 51, 101e107.

Badmaev, S.D., Snytnikov, P.V., 2008. Hydrogen production from dimethyl ether andbioethanol for fuel cell applications. Int. J. Hydrogen Energy 33, 3026e3030.

Bercic, G., Levec, J., 1993. Catalytic dehydration of methanol to dimethyl ether. Ki-netic investigation and reactor simulation. Ind. Eng. Chem. Res. 32, 2478e2484.

Bhat, S.A., Sadhukhan, J., 2009. Process intensification aspects for steam methanereforming: an overview. AIChE J. 55, 408e422.

Brown, L.F., 2001. A comparative study of fuels for on-board hydrogen productionfor fuel-cell-powered automobiles. Int. J. Hydrogen Energy 26, 381e397.

Cai, J., Wang, G., 2012. Hydrogen production by a marine photosynthetic bacterium,Rhodovulum sulfidophilum P5, isolated from a shrimp pond. Int. J. HydrogenEnergy 37, 15070e15080.

Cussler, E.L., 1997. Diffusion: Mass Transfer in Fluid Systems. Cambridge UniversityPress.

Edwards, P., Kuznetsov, V., David, W., 2007. Hydrogen energy. Philos. Trans. R. Soc. A365, 1043e1056.

Farsi, M., Jahanmiri, A., 2011. Mathematical simulation and optimization of meth-anol dehydration and cyclohexane dehydrogenation in a thermally coupleddual-membrane reactor. Int. J. Hydrogen Energy 36, 14416e14427.

Flores, J., Peixoto, D., Appel, L., De Avillez, R., Silva, M., 2011. The influence ofdifferent methanol synthesis catalysts on direct synthesis of DME from syngas.Catal. Today 172, 218e225.

Graaf, G., Stamhuis, E., Beenackers, A., 1988. Kinetics of low-pressure methanolsynthesis. Chem. Eng. Sci. 43, 3185e3195.

Hao, Y., Du, X., Yang, L., Shen, Y., Yang, Y., 2011. Numerical simulation of configu-ration and catalyst-layer effects on micro-channel steam reforming of meth-anol. Int. J. Hydrogen Energy 36, 15611e15621.

Haryanto, A., Fernando, S., Murali, N., Adhikari, S., 2005. Current status of hydrogenproduction techniques by steam reforming of ethanol: a review. Energy Fuels19, 2098e2106.

Hodoshima, S., Arai, H., Saito, Y., 2003. Liquid-film-type catalytic decalindehydrogeno-aromatization for long-term storage and long-distance trans-portation of hydrogen. Int. J. Hydrogen Energy 28, 197e204.

Hoffman, P., 2002. Tomorrow's Energy. MIT Press, 55 Hayward St. Cambridge MA02142 USA, p. 301.

Hu, Y., Nie, Z., Fang, D., 2008. Simulation and model design of pipe-shell reactor forthe direct synthesis of dimethyl ether from syngas. J. Nat. Gas Chem. 17,195e200.

Hunter, J.B., McGuire, G., 1980. Method and Apparatus for Catalytic Heat Exchange,pp. 214e867. Google Patents, G. US Patent; 4.

Itoh, N., 1987. A membrane reactor using palladium. AIChE J. 33, 1576e7158.Jain, I., Lal, C., Jain, A., 2010. Hydrogen storage in Mg: a most promising material. Int.

J. Hydrogen Energy 35, 5133e5144.Ji, C., Liang, C., Wang, S., 2011. Investigation on combustion and emissions of DME/

gasoline mixtures in a spark-ignition engine. Fuel 90, 1133e1138.Kariya, N., Fukuoka, A., Utagawa, T., Sakuramoto, M., Goto, Y., Ichikawa, M., 2003.

Efficient hydrogen production using cyclohexane and decalin by pulse-spraymode reactor with Pt catalysts. Appl. Catal. A 247, 247e259.

Khademi, M., Farsi, M., Rahimpour, M., Jahanmiri, A., 2011. DME synthesis andcyclohexane dehydrogenation reaction in an optimized thermally coupledreactor. Chem. Eng. Process. 50, 113e123.

Khademi, M., Rahimpour, M., Jahanmiri, A., 2010. Differential evolution (DE) strat-egy for optimization of hydrogen production, cyclohexane dehydrogenationand methanol synthesis in a hydrogen-permselective membrane thermallycoupled reactor. Int. J. Hydrogen Energy 35, 1936e1950.

Lindsay, A.L., Bromley, L.A., 1950. Thermal conductivity of gas mixtures. Ind. Eng.Chem. 42, 1508e1511.

Lu, W.-Z., Teng, L.-H., Xiao, W.-D., 2004. Simulation and experiment study ofdimethyl ether synthesis from syngas in a fluidized-bed reactor. Chem. Eng. Sci.59, 5455e5464.

Maria, G., Marin, A., Wyss, C., Muller, S., Newson, E., 1996. Modelling and scaleup ofthe kinetics with deactivation of methylcyclohexane dehydrogenation forhydrogen energy storage. Chem. Eng. Sci. 51, 2891e2896.

Naik, S.P., Ryu, T., Bui, V., Miller, J.D., Drinnan, N.B., Zmierczak, W., 2011. Synthesis ofDME from CO2/H2 gas mixture. Chem. Eng. J. 167, 362e368.

Othmer, K., 1978. Encyclopedia of Chemical Technology, third ed. Wiley, New York.Park, Y.-C., Kim, D.-H., Lim, S., Kim, S.-K., Peck, D.-H., Jung, D.-H., 2012. Design of a

MEA with multi-layer electrodes for high concentration methanol DMFCs. Int. J.Hydrogen Energy 37, 4717e4727.

Rahimpour, M.R., 2007. A dual catalyst bed concept for industrial methanol syn-thesis. Chem. Eng. Commun. 12, 1638e1653.

Rahimpour, M., Mirvakili, A., Paymooni, K., 2011a. Hydrogen as an energy carrier: acomparative study between decalin and cyclohexane in thermally coupledmembrane reactors in gas-to-liquid technology. Int. J. Hydrogen Energy 36,6970e6984.

Rahimpour, M., Mirvakili, A., Paymooni, K., 2011b. Simultaneous hydrogen pro-duction and utilization via coupling of FischereTropsch synthesis and decalindehydrogenation reactions in GTL technology. Int. J. Hydrogen Energy 36,2992e3006.

Rahimpour, M.R., Farniaei, M., Abbasi, M., Javanmardi, J., Kabiri, S., 2013. Comparativestudyon simultaneous productionofmethanol, hydrogen, andDMEusing anovelintegrated thermally double-coupled reactor. Energy Fuels 27, 1982e1993.

Reid, R.C., Sherwood, T.K., Prausnitz, J., 1977. The Properties of Gases and Liquids,third ed. McGraw-Hill, New York.

Schrope, M., 2001. Which way to energy utopia? Nature 414, 682e684.Semelsberger, T.A., Borup, R.L., Greene, H.L., 2006. Dimethyl ether (DME) as an

alternative fuel. J. Power Sources 156, 497e511.Smith, J.M., 1980. Chemical Engineering Kinetics. McGraw-Hill, New York.Song, C., 2002. Fuel processing for low-temperature and high-temperature fuel

cells: challenges, and opportunities for sustainable development in the 21stcentury. Catal. Today 77, 17e49.

Turner, J.A., 2004. Sustainable hydrogen production. Science 305, 972e974.Vakili, R., Pourazadi, E., Setoodeh, P., Eslamloueyan, R., Rahimpour, M., 2011. Direct

dimethyl ether (DME) synthesis through a thermally coupled heat exchangerreactor. Appl. Energy 88, 1211e1223.

Wang, B., Goodman, D.W., Froment, G.F., 2008. Kinetic modeling of pure hydrogenproduction from decalin. J. Catal. 253, 229e238.