enhancement of methanol, dme and hydrogen production via employing hydrogen permselective membranes...

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Journal of Natural Gas Science and Engineering 14 (2013) 158e173

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Journal of Natural Gas Science and Engineering

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Enhancement of methanol, DME and hydrogen production viaemploying hydrogen permselective membranes in a novel integratedthermally double-coupled two-membrane reactor

Mahdi Farniaei a, Mohsen Abbasi b, Ali Rasoolzadeh b, Mohammad Reza Rahimpour b,*aDepartment of Chemical Engineering, Shiraz University of Technology, Shiraz 71555-313, IranbDepartment of Chemical Engineering, 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 2 June 2013Received in revised form22 June 2013Accepted 24 June 2013Available online 15 July 2013

Keywords:Cyclohexane dehydrogenationDMEHydrogenMethanolThermally double-coupled two-membranereactor

* Corresponding author. Tel.: þ98 711 2303071; faxE-mail address: [email protected] (M.R. Rahi

1875-5100/$ e see front matter � 2013 Elsevier B.V.http://dx.doi.org/10.1016/j.jngse.2013.06.010

a b s t r a c t

In this paper, a thermally double-coupled two-membrane reactor for simultaneous production ofmethanol, hydrogen and dimethylether (DME) by employing Pd/Ag membranes in co-current mode hasbeen investigated. In this novel multi-tubular reactor configuration, cyclohexane dehydrogenation as anendothermic reaction has coupled with two exothermic reactions of methanol production and directDME synthesis from syngas. Two Pd/Ag membranes are used for improving the efficiency of hydrogenproduction and other products in exothermic sides by separation of hydrogen from unconverted outputsof exothermic sides that are recycled to the reactor. A steady state heterogeneous catalytic reactionmodel is applied to analyze the performance of thermally double-coupled two-membrane reactor andcomparison of the result with corresponding predictions for a conventional methanol reactor, thermallycoupled reactor (coupling of methanol synthesis with cyclohexane dehydrogenation only) and thermallydouble coupled reactor.

Modeling results show that by employing this novel configuration, methanol yield reaches to 0.4017 incomparison with 0.3885, 0.3735 and 0.362 for conventional methanol reactor, thermally double coupledreactor and thermally coupled reactor respectively.

Also methanol production increases 3.39%, 7.03% and 10.94% compared with conventional methanolreactor, thermally double coupled reactor and thermally coupled reactor respectively. Simulation resultsillustrate that by using Pd/Ag membranes, DME production raises from 277.24 kmol h�1 in thermallydouble coupled reactor to 325.3 kmol h�1 in thermally double-coupled two-membrane reactor. Addi-tionally, hydrogen production in endothermic side of thermally double coupled reactor is enhanced from1076 to 1219 kmol h�1 in thermally double-coupled two-membrane reactor.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

1.1. Non-renewable energy sources

Fossil fuels as their names exhibit are consisted of high per-centages of carbon and include coal, petroleum, and natural gas(Eisenberg and Nocera, 2007; Kamat, 2007). Fossil fuels are pro-ducing significant amounts of energy per unit weight and becauseof this reason, they are very important. They are named as non-renewable energy resources because it will take millions of yearsfor them to form, and the depletion and extraction of accumulation

: þ98 711 6287294.mpour).

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is faster than making of them. The production and use of fossil fuelsincreased environmental concerns.

1.2. Hydrogen

Recently interest in alternative fuels like hydrogen, which is oneof the great alternative, renewable energy sources, has beenestablished (Goltsov et al., 2006; Ni et al., 2006a, 2006b). There aremany different methods and various kinds of sources that canproduce hydrogen (Marshall et al., 2007; Navarro et al., 2007). In-dustrial production of hydrogen is mainly from the steam reform-ing of natural gas, and less often from more energy-intensivehydrogen production methods like the electrolysis of water (Wanget al., 2008; Vizcaino et al., 2007; Haryanto et al., 2005; Breen et al.,2002). By using the steam reforming method, the toxic and

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M. Farniaei et al. / Journal of Natural Gas Science and Engineering 14 (2013) 158e173 159

greenhouse gases like carbon monoxide, hydrogen sulfide andcarbon dioxide produces which are corrosive and a large amount ofenergy is needed (Itoh, 2003; Kariya et al., 2003; Yolcular andOlgun, 2008). Because of these bad effects of steam reformingmethod, the alternative method is the dehydrogenation of organichydrocarbons with high amount of hydrogen content. This methodhas many advantages like: The only products of this method arehydrogen and dehydrogenated hydrocarbons, no toxic, corrosiveand greenhouse gas is formed, because this method is reversible,the products can be hydrogenated and regenerated again andfinally, because no by-products such as carbon monoxide hasformed, the purification of hydrogen is not needed (Khademi et al.,2009a,b; Rahimpour and Asgari, 2009).

1.3. Coupling reactors

Chemical engineering focused on making the processes moreefficient and reducing the cost and bad environmental effects of theprocesses. One of the processes for improving the efficiency andreducing the costs is coupling of endothermic and exothermic re-actions through them; the heat that released from the exothermicreaction is the driving force for proceeding of the endothermic re-action (Zanfir and Gavriilidis, 2001; Dautzenberg and Mukherjee,2001). There are many publications on coupling of exothermic andendothermic reactions in a reactor (Kolios et al., 2002;Moustafa andElnashaie, 2000; Ramaswamy et al., 2008). Abo-Ghander et al.(2008) coupled the endothermic reaction with the exothermic re-action by usingno direct heat transfer. Itoh andWu (1997) suggestedan adiabatic type of palladium membrane reactor for couplingendothermic and exothermic reactions. Elnashaie et al. (2000)studied a heterogeneous model to study the performance of thecatalytic membrane reactor for the dehydrogenation of ethyl ben-zene to styrene. Altimari and Bildea (2009) designed plant systems,which are including coupling of exothermic and endothermic re-actions. Ramaswamyet al. (2006) have analyzed the steady state andthe dynamic behavior of coupling exothermic and endothermic re-actions in directly coupled adiabatic packed-bed reactors for thefirstorder reactions using one-dimensional pseudo-homogeneous plugflowmodel. Khademi et al. (2009a,b) optimizedmethanol synthesisand cyclohexane dehydrogenation in a thermally coupled reactor byusing differential evolution method. Additionally Khademi et al.(2010) presented a mathematical model for thermally coupledhydrogen-perm selective membrane reactor that is composed ofthree sides for synthesis of methanol and benzene. Glockler et al.(2009) optimized the reactor design for coupling the exothermicand endothermic reactions in a fixed bed reactor. Rahimpour et al.(2011) have studied theoretically hydrogen production fromcoupling of methanol synthesis and cyclohexane dehydrogenationin hydrogen perm-selective membrane and dual membrane ther-mally coupled reactor. Rahimpour and Pourazadi (2010) comparedco-current and counter-current patterns in a thermally coupledmembrane reactor in which, the heat that is required for the endo-thermic reactionofbenzeneproduction is provided fromexothermicreaction of methanol synthesis. Vakili et al. (2011) investigateddimethylether synthesis directly in a thermally coupled heatexchanger reactor. Farsi et al. (2011) and Farsi and Jahanmiri (2011),investigated simulation and optimization of DME production andcyclohexane dehydrogenation in a thermally coupled dual mem-brane reactor and suggested the optimum conditions for hydrogensynthesis from coupling of DME and benzene production processes.

1.4. Cyclohexane and dehydrogenation of cyclohexane

Cyclohexane is used as a nonpolar solvent in the chemical in-dustry. In the industrial scale, reaction of aromatic compounds like

benzenewith hydrogen produces cyclohexane. Dehydrogenation ofcyclohexane is the reverse reaction of benzene production, which isan endothermic reaction, which its products are hydrogen andbenzene. The heat that is required for this endothermic reaction isprovided from the exothermic reaction (Biniwale et al., 2005,2008). There exists many ways for benzene production that dehy-drogenation of cyclohexane is one of them. The dehydrogenation ofcyclohexane for producing hydrogen has many advantages incomparison with the other methods such as: the dehydrogenatedproducts like benzene could be generated and recycled, highhydrogen content of cyclohexane that is 7.1 wt.%. This amount ofhydrogen content is much higher than metal hydrides, because ofhigh boiling point; it is safer for transportation and storage. There isno carbon dioxide emission in hydrogen production by cyclohexanedehydrogenation that is environmental friendly (Ali et al., 1994;Garcia Villaluenga and Tabe-Mohammadi, 2000). As previouslymentioned one or two exothermic reactions can provide the heatthat is necessary for the endothermic reaction, which coupled inone reactor.

1.5. Exothermic reactions

1.5.1. DME synthesisDME is a promising fuel in diesel engines, petroleum engines

(30% DME-70% LPG), and gas turbines due to its high octanenumber, which is 55, in comparison with diesels octane number,which is 40e53. Just finite modification is needed to convert adiesel engine to burn DME (Semelsberger et al., 2006; Galvita et al.,2001; Fleisch et al., 1995).

Nowadays, by gasification of coal or steam reforming ofmethane, synthesis gas is produced and then it is converted intomethanol in the presence of catalyst that is usually belonging tothe copper group. DME is produced with dehydration of meth-anol in the presence of another different catalyst such as silica-alumina. This process includes two steps that firstly, starts withmethanol synthesis and ends with DME synthesis or methanoldehydration (Guo et al., 2000; Arcoumanis et al., 2008; Makarandet al., 1992; Troy et al., 2006; Ng et al., 1999). Alternatively, DMEcan be produced through direct synthesis. In this process, we canuse a dual catalyst system, which allows both methanol synthesis,and methanol dehydration in the same process unit, with nomethanol isolation and purification, a procedure that by omittingthe intermediate methanol synthesis stage, yields more efficiencyand low cost. The two-steps process is widely used and moreapplicable than other method because it is relatively simple andstart-up costs are relatively low (Lu et al., 2004; Rahimpour et al.,2008).

1.5.2. MethanolMethanol has many applications like using as a solvent, using as

a hydrate inhibitor in pipelines, using as antifreeze and using as afuel. Methanol is clean burning fuel with many applications(Rahimpour et al., 2005; Lovik et al., 1998; Semelsberger et al.,2006; Rahimpour et al., 1998). Firstly, methanol was produced asa byproduct of themortal distillation of wood and because of that, itwas named as wood alcohol. In advanced case, methanol is pro-duced directly in a catalytic industrial process from synthesis gaseslike carbonmonoxide, carbon dioxide, and hydrogen. The synthesisgases react over a catalyst to produce methanol. Nowadays, themost applicable catalyst is a mixture of copper (Cu), zinc (Zn) andalumina.

An alternative process instead of production of methanol fromsynthesis gas is the direct catalytic conversion of methane tomethanol by using Cu-Zeolites or other catalysts (Rezaie et al.,2005; Rahimpour and Ghader, 2003; Amandusson et al., 2001).

M. Farniaei et al. / Journal of Natural Gas Science and Engineering 14 (2013) 158e173160

1.6. Membrane

Recently, membranes are used to improve the yield and selec-tivity of reactions. Membranes have some advantages like:

� Membranes are less energy-intensive, since they do not requiremajor phase changes.

� Do not require adsorbents or solvents, whichmay be expensiveor difficult to handle.

� Equipment simplicity and modularity, which facilitates theincorporation of more efficient membranes.

� They provide low maintenance operation.

Table 1The catalyst characteristics and the specifications of conventionalmethanol reactor.

Parameter Value

Methanol synthesis process (exothermic side)Feed composition (mole fraction)CH3OH 0.0050CO2 0.0940CO 0.0460H2O 0.0004H2 0.6590N2 0.0930CH4 0.1026Total molar flow rate (mol s�1) 0.64Inlet pressure (bar) 76.98Catalyst particleDensity (kg m�3) 1770Particle diameter (m) 5.47 � 10�3

Heat 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 0.123Bed void fraction 0.39Density of catalyst bed (kg m�3) 1140Length of reactor (m) 7.022Number of tubes 2962Tube inner diameter (m) 3.8 � 10�2

Tube outer diameter (m) 4.2 � 10�2

1.6.1. Pd/Ag membraneThe most widely used application of membrane in separation of

gases is the separation of hydrogen from nitrogen, argon andmethane in the ammonia purge gas stream. The Palladium mem-brane is commonly a metallic tube, which is consisted of palladiumand silver alloy material. This characteristic gives the membranethe unique property that only allows the monatomic hydrogen gasto pass through the membrane crystal lattice when it is heatedabove 300 �C. Because of this membrane property, this type ofmembrane is mostly used in dehydrogenation of organic com-pounds and in hydrogen purifying (Shu et al., 1991). By eliminatingthe hydrogen by means of Pd/Ag membrane through the reversiblereaction, the reaction is shifted toward the product side. Shu andcoworkers have investigated the hydrogenation and dehydroge-nation reactions on Pd/Agmembrane (Uemiya et al., 1991). In Pd/Agmembrane, the hydrogen permeability in the membrane increasesby increasing silver content until it reaches its maximum perme-ability that is about 23 wt.% Ag (Graaf et al., 1988).

1.7. Objective

The aim is study is theoretical investigation of employing Pd/Agmembranes for increasing performance of a thermally doublecoupled reactor. In fact, the new idea of this work is coupling twoexothermic reactions with one endothermic reaction and using twoPd/Ag membranes for improving methanol, DME and hydrogenproduction.

Two Pd/Ag membranes are used for improving the efficiency ofhydrogen production and other products in exothermic sides byseparation of hydrogen from unconverted outputs of exothermicsides that are recycled to the reactor. In fact by addition of hydrogento methanol and DME synthesis sides, reversible reactions areshifted to front along to reactor axes. Therefore unconvertedhydrogen at the output of exothermic sides of thermally doublecoupled reactor can be recycled to the reactor for increasingmethanol, DME and hydrogen production by employing Pd/Agmembranes in thermally double-coupled two-membrane reactor. Asteady state heterogeneous catalytic reaction model is applied toanalyze the performance of thermally double-coupled two-mem-brane reactor and comparison of the result with correspondingpredictions for an industrial methanol reactor, thermally coupledreactor (coupling of methanol synthesis with cyclohexane dehy-drogenation only) and thermally double coupled reactor.

2. Process description

2.1. Conventional methanol synthesis reactor

Conventional methanol synthesis reactor is consisted of a ver-tical shell and tube heat exchanger. The vertical tubes are packedwith CuO/ZnO/Al2O3 catalyst and in their surroundings is boiling

water. The synthesis gas is entering the tubes from top of thereactor and methanol synthesis reactions occur. The heat of thisexothermic reaction that produced is transferred to the boilingwater and steam is produced. The catalyst characteristics and theconventional methanol reactor specifications are exhibited inTable 1.

2.2. Thermally coupled reactor

In the thermally coupled reactor dehydrogenation of cyclo-hexane to benzene is used in the shell side instead of using coolingwater. The heat is transferred from exothermic reaction (methanolproduction) to the endothermic reaction (cyclohexane dehydro-genation) continuously, and the total feed flow rate of endothermicside is equal to 0.1 mol s�1 (Khademi et al., 2009a,b).

2.3. Thermally double coupled reactor

Fig. 1 shows a schematic diagram of the thermally doublecoupled reactor in co-current mode. Thermally double coupledreactor is a multi tubular reactor and consisted of 2962 threeconcentric tubes. In the central tube, the endothermic reaction,which is dehydrogenation of cyclohexane, received the heat fromtwo exothermic reactions of methanol and DME synthesis, whichhave taken place in the inner and outer tubes respectively. DMEsynthesis reactions occur over catalyst of conventional methanolreactor (Cu/Zn/Al2O3) and dehydration of methanol process (g-Al2O3). In addition, Pt/Al2O3 is used as a catalyst for dehydrogena-tion of cyclohexane reaction. The properties and input data ofthermally double coupled reactor are showed in Tables 2 and 3. Theother properties of thermally double coupled reactor related tomethanol synthesis process are the same as conventional methanolreactor. In this configuration, three reactions have done in a propertemperature range to have proper thermal couple.

2.4. Thermally double coupled two-membrane reactor

Thermally double-coupled two-membrane reactor is a multitubular reactor and consisted of 2962 three concentric tubes with

Fig. 1. Thermally double coupled reactor configuration in co-current mode.


two membrane that covered inside and outsides of each threeconcentric tubes. A schematic diagram of one three concentrictubes with coveredmembrane inside and outside of it in co-currentmode is showed in Fig. 2. For better explanation of this configura-tion, Fig. 3 represents a differential element along the axial

Table 2The operating conditions for DME synthesis (one of the exothermicsides) and dehydrogenation of cyclohexan (endothermic side).

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 0.044Inlet temperature (K) 493Inlet pressure (bar) 50Typical properties of catalystParticle diameter (mm) 5 � 5fDensity of catalyst bed (kg m�3) 1200Porosity 0.455Dehydrogenation of C6H12 (endothermic side)Feed compositionC6H12 0.1C6H6 0.0H2 0.0Ar 0.9Inlet temperature (K) 503Inlet pressure (Pa) 1.013 � 105

Particle diameter (m) 3.55 � 10�3

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

direction of thermally double-coupled two-membrane reactor withdirection of heat and mass transfer.

Two Pd/Ag membranes separate hydrogen from recycled gasesand hydrogen are added in exothermic sides for shifting reversiblereactions to front. Pressure of recycle streams is increased to 90 barby using compressors. The pressure of recycle stream should besufficient because the driving force for hydrogen separation in Pd/Ag membranes is temperature difference. The properties and inputdata of thermally double-coupled two-membrane reactor aresimilar to thermally double coupled reactor.

3. Reaction scheme and kinetics

3.1. Direct DME synthesis (exothermic reaction)

The reactions in DME production from synthesis gas are: pro-duction of methanol from CO and CO2 and dehydration of methanolas follows (Vakili et al., 2011):

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

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

2CH3OH4CH3OCH3 þ H2O DH298 K ¼ �21:003kJ=mol (3)

Table 3The characteristics of thermally double coupled reactor.

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) 7.022

Fig. 2. A schematic diagram of one three concentric tubes with covered membrane inside and outside of it in thermally double-coupled two-membrane reactor.


These reactions are taken happened over a catalyst that containstwo active sites; one is applied for methanol synthesis and theother one for DME formation.

The reaction rates of direct DME synthesis are as follow:

rCO ¼ k1fCOf 2H2ð1� b1Þ�

1þ KCOfCO þ KCO2fCO2

þ KH2fH2

�3 (4)

rCO2¼ k2fCO2

f 3H2ð1� b2Þ�

1þ KCOfCO þ KCO2fCO2

þ KH2fH2

�4 (5)

Fig. 3. The differential element along the axial direction insid

rDME ¼ k3fCH3OHð1� b3Þ� ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffip �2 (6)
1þ KCH3OHfCH3OH
b1 ¼ fCH3OH

Kf1 fCOf2H2

(7)

b2 ¼ fCH3OHfH2O

Kf2 fCO2f 3H2

(8)

e the thermally double-coupled two-membrane reactor.

Table 4Reaction rate constants for DME synthesis reactions.

k 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) 30,275KH2

2.1 � 10�3 (bar�1) 31,846KH2

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

Table 6The reaction rate constant, the adsorption equilibrium constant and the reactionequilibrium constant for cyclohexane 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


b3 ¼ fDMEfH2O

Kf3 f2CH3OH

(9)

In the above equations, fi and Kfi represent the fugacity ofcomponent i and equilibrium constant of reaction i respectively.The kinetic parameters are exhibited in Table 4.

3.2. Methanol production reaction (exothermic reaction)

In methanol synthesis reaction, three equilibrium reactions takeplace simultaneously:

COþ 2H24CH3OH DH298 K ¼ 90:55 kJ=mol (10)

CO2 þ 3H24CH3OHþ H2O DH298 K ¼ 49:43kJ=mol (11)

CO2 þ H24COþH2O DH298 K ¼ 41:12kJ=mol (12)

In this work, methanol synthesis reaction kinetic parameters aretaken from Graaf et al. (1999). This kinetic model can be used in thetemperature range of 495e533 K and pressure range of 5e8 MPa.The rates of reactions for methanol synthesis are:

r1 ¼k1KCO

hfCOf

3=2H2

� fCH3OH=f1=2H2

KP1

i�1þ KCOfCO þ KCO2

fCO2

�hf 1=2H2

þ�KH2O=K

1=2H2

�fH2O

i (13)

r2 ¼k2KCO2

hfCOf

3=2H2

� fCH3OHfH2O=f3=2H2

KP2

i�1þ KCOfCO þ KCO2

fCO2

�hf 1=22 þ

�KH2O=K

1=2H2

�fH2O

i (14)

r3 ¼ k3KCO2

�fCO2

fH2� fH2OfCO=KP3

��1þ KCOfCO þ KCO2

fCO2

�hf 1=2H2

þ�KH2O=K

1=2H2

�fH2O

i (15)

Table 5The 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)

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=2H2

Þ (6.37 � 2.88) � 10�9 84,000 � 1400

A (K) B (K)

KP15139 12.621

KP23066 10.592

KP3�2073 �2.029

Table 5 exhibits the reaction rate constants, the adsorptionequilibrium constants and the reaction equilibrium constants formethanol synthesis.

3.3. Dehydrogenation of cyclohexane (endothermic side)

The reaction dehydrogenation of cyclohexane reaction is asfollows (Farsi et al., 2011):

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

In this case, the following rate equation is used for the aboveequation:

rC ¼�k

�KPPC=P3H2

� PB�

1þ�KBKPPC=P3H2

� (17)

Where k, KB, KP are the reaction rate constant, the adsorption equi-librium constant and the reaction equilibrium constant respectivelywhich are showed inTable 6. Pi is the partial pressure of component iin Pa. The temperature range of the reaction is 423e523 K.

3.4. Pd/Ag membrane

For estimating the hydrogen flux, Equation (18) can be used,which in this equation; the hydrogen flux is a function of diffusivity,membrane thickness and hydrogen partial pressure (Barbieri andMaio, 1997; Lindsay and Bromley, 1950).

JH2¼

Q0exp��EH2

RT

�dH2

ffiffiffiffiffiffiffiffiffiffiffiffiPTubeH2

q�

ffiffiffiffiffiffiffiffiffiffiffiffiPShellH2

q

Q0 ¼ 1:65e�5molm�1 s�1kPa�12 ; EH2

¼ 15:7kJmol�1

(18)

4. Mathematical model

The one-dimensional heterogeneous catalytic reaction modelfor mathematical modeling of thermally double-coupled two-membrane reactor is based on the following assumptions:

� It is assumed that this model is to be at steady-state conditions.� Ideal gas phase.� Plug flow mode is applied in each side of reaction.� Constant bed porosity in axial and radial directions.

The differential equations which are describingmole and energybalances in the axial direction of thermally double-coupled two-membrane reactor for all sides are exhibited in Table 7.

4.1. Auxiliary correlations

For solving of mentioned set of differential equations, auxiliarycorrelations including physical properties of components, mass andheat transfer coefficients, and the Ergun equation should have beenlisted in Table 8.

Table 7Mole and energy balances in the axial direction of thermally double-coupled two-membrane reactor.

Mass and energy balances equations Number

Endothermic sideMass balanceSolid phase ancjkgi;jðygi;j � ysi;jÞ þ hri;jrb ¼ 0 (19)

Gas phase 1AC

dFt;jdz þ ancjkgi;jðysi;j � ygi;jÞ ¼ 0 (20)

Energy balanceSolid phase anhf ðTgj � Ts

j Þ þ rbPN

i¼1hri;jðDHf ;iÞ ¼ 0 (21)

Gas phase �FtAcCgpdTtdz þ anhf ðTsj � Tgj Þ þ

pDj

AcU1�2ðTg1 � Tg2 Þ þ

pDj

AcU3�2ðTg3 � Tg2 Þ ¼ 0 (22)

Exothermic sidesMass balanceSolid phase ancjkgi;jðygi;j � ysi;jÞ þ hri;jrb ¼ 0 (23)

Gas phase � 1Ac

dFt;jdz þ ancjkgi;jðysi;j � ygi;jÞ þ

JH2m1;H2m2

Ac¼ 0 (24)

Energy balanceSolid phase anhf ðTgj � Ts

j Þ þ rbPN

i¼1hri;jðDHf ;iÞ ¼ 0 (25)

Gas phase �FtAcCgpdTtdz þ anhf ðTsj � Tg

j Þ �pDj

AcU1;3 2ðTg1;3 � Tg

2 Þ þpDj

AcU1;3 m1;m2ðTg

1;3 � Tgm1;m2Þ þJH2m1;H2m2

Ac

R Tm1;m2T1;3 Cg

pdT ¼ 0 (26)

Membranes sidesMass balance � 1

Ac

dFm;j

dz � jH2m1;H2m2

Ac¼ 0 (27)

Energy balance �FmAcCgpdTtdz �

pDj

AcU1;3 m1;m2ðTg1;3 � Tgm1;m2Þ �

JH2m1;H2m2

Ac

R Tm1;m2T1;3 Cg

pdT ¼ 0 (28)


4.2. Numerical solution

The proposed model is consisted of the set of ordinary differ-ential equations (ODEs) resulted from mass and energy conserva-tion rules as well as non-linear algebraic equations of the kineticmodel, auxiliary and hydrodynamic correlations. Backward finitedifference calculation is applied for solving set of ODEs. Hence, theODEs are changed into a set of nonlinear algebraic equations. Thelength of each reactor is divided to 100 distinct segments and theGausseNewton method is used to solve the obtained set of non-linear algebraic equations in each segment for both sides, simul-taneously. This procedure is repeated for all nodes in the reactorthat the results of each node are used as the inlet conditions for thefollowing node.

4.3. Model validation

Model validation has been carried out by comparison betweenthe simulation results and conventional methanol synthesis reactor

Table 8Auxiliary correlations (Lindsay and Bromley, 1950; Cussler, 1984; Wilke, 1949; Reidet al., 1997; Smith, 1980).

Parameter Equation

Component heat capacity Cp ¼ aþ bT þ cT2 þ dT3

Mixture heat capacity Based on local compositionsViscosity of reaction mixtures Based on local compositionsMixture thermal conductivity Based on local compositionsMass transfer coefficient between

gas and solid phaseskgi ¼ 1:17Re0:42Sc0:67i ug � 103

Re ¼ rugdpm

Sci ¼ mrDim�104

Dim1�yiP

isj

yiDij

Dij ¼ 107T3=2ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1=Miþ1=Mj

pPðv3=2ci þv3=2cj Þ

Overall heat transfer coefficient 1U þ 1

hiþ Ai lnðDo=DiÞ

2pLKwþ Ai

Ao

1ho

Heat transfer coefficient betweengas phase and reactor wall

hCprm

�CpmK

�2=3 ¼ 0:4583B

�m

rudp

�0:407Ergun equation dP

dz ¼ 150 ð1� 3Þ232d2

pþ 1:75

ð1� 3Þu2g

33d2p

in industrial scale by Rahimpour et al. (2011) fortunately; very goodagreement was achieved between proposed model and experi-mental plant data showing feasibility of this novel process.

5. Result and discussions

In this part of paper, the results of thermally double-coupledtwo-membrane reactor simulation with the validated conven-tional methanol reactor, thermally coupled reactor in whichmethanol synthesis is coupled with cyclohexane dehydrogenationonly and thermally double coupled reactor have been compared.The following results will exhibit the various observed behaviors ofthermally double-coupled two-membrane reactor by some figures.In order to calculate methanol and hydrogen yields as well ascyclohexane and carbon monoxide conversions in DME productionside, following definitions have been used:

Hydrogen recovery yield ¼ FH2 ; out

FC6H12; in(29)

Methanol yield ¼ FCH3OH; out

FCO þ FCO2; in(30)

Cyclohexane conversion ¼ FC6H12; in � FC6H12; out

FC6H12; in(31)

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

(32)

5.1. Molar behavior

Fig. 4(a) shows the products mole fractions of methanol syn-thesis side, which are hydrogen, carbon monoxide, carbon dioxide,water and methanol that pass through inner side of thermallydouble-coupled two-membrane reactor. The results show thathydrogen has the highest mole fraction profile. The mole fractionsof components in DME synthesis versus reactor length are plottedin Fig. 4(b). As this plot shows, hydrogen has the highest profile like

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Dimensionless length

Mol

e fr

acti

on

MethanolWaterCarbon dioxideHydrogenCarbon monoxide

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5


Mol

e fr

acti

on

DMEMethanolWaterHydrogenCarbon monoxideCarbon dioxide

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2


Mol

e fr

acti

on

CyclohexaneBenzeneHydrogen

a

b

c

Fig. 4. Variations of components mole fraction (a) methanol synthesis (innerexothermic side), (b) DME synthesis (outer exothermic side) and (c) dehydrogenationof cyclohexane (middle endothermic side) with reactor dimensionless length in ther-mally double-coupled two-membrane reactor.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1500

505

510

515

520

525

530

535

540

545


Tem

pera

ture

(K

)

Methanol productionCyclohexane dehydrogenationDME productionMembrane insideMembrane outside

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1500

505

510

515

520

525

530

535


Tem

pera

ture

(K

)

Thermally double coupled reactorConventional methanol reactorThermally coupled reactorThermally double-coupled two-membrane reactor

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1500

505

510

515

520

525

530


Tem

pera

ture

(K

)

Thermally double coupled reactorThermally double-coupled two-membrane reactor

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1490

495

500

505

510

515

520

525

530

535

540


Tem

pera

ture

(K

)

Thermally double coupled reactorThermally coupled reactorThermally double-coupled two-membrane reactor

a

b

c

d

Fig. 5. (a) Temperature pattern of thermally double-coupled two-membrane reactor inthe three sides (b) Thermal behavior of methanol synthesis process in thermallydouble-coupled two-membrane reactor in comparison with the ones in conventionalmethanol reactor, thermally coupled reactor and thermally double coupled reactor (c)Thermal behavior of DME reactor in thermally double-coupled two-membrane reactorin comparison with thermally coupled reactor (d) Thermal behavior of cyclohexanedehydrogenation process in thermally double-coupled two-membrane reactor incomparison with thermally coupled reactor and thermally double coupled reactor.


Fig. 4(a) because hydrogen is added along reactor axes inexothermic sides with Pd/Ag membranes. Fig. 4(c) is the plot ofcomponents mole fractions in endothermic side; cyclohexane,hydrogen and benzene. The hydrogen mole fraction profile is abovethe benzene mole fraction profile because of its higher stoichio-metric coefficient in cyclohexane dehydrogenation reaction.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.5

1

1.5

2

2.5

3

3.5

4

4.5


Hea

t (k

W)


Fig. 6. Heat generation in methanol synthesis reactor in thermally double-coupledtwo-membrane reactor, thermally coupled reactor and thermally double coupledreactor.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 11

1.5

2

2.5

3

3.5

4

4.5


Hea

t (k

W)


Fig. 8. Heat of cyclohexane dehydrogenation process in thermally double-coupledtwo-membrane reactor, thermally double coupled reactor and thermally coupledreactor.


5.2. Thermal behavior

Fig. 5(a) exhibits the temperature profile of thermally double-coupled two-membrane reactor in the three sides. Generated heatof both exothermic sides (methanol and DME syntheses) is used forproceeding endothermic reaction as well as heating the mixtures ofall three sides. As shown in thisfigure, both exothermic profiles haveapproximately the same manner, above the endothermic one. Alsotemperature variationofmembrane inside andoutsideflowarenearmethanol and DMEproduction respectively. Temperature reductionin the secondhalf of exothermic sides is decreasing rate of reversiblereactions due to equilibrium conditions, Therefore temperature ofendothermic sidedecreases slightlyalong reactor axes. Temperatureof DME side is lower thanmethanol side due to greater area contactof DME side with endothermic side.

At the entrance of the thermally double-coupled two-mem-brane reactor, generated heat is less than consumed heat leadingto temperature of endothermic side to fall and a cold spotdeveloped.

Fig. 5(bed) illustrates the temperature profiles of methanolsynthesis, DME synthesis and cyclohexane dehydrogenation for

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 12

3

4

5

6

7

8

9


Hea

t (k

W)

Thermally double-coupled two-membrane reactorThermally double coupled reactor

Fig. 7. Heat of DME reactor in thermally double-coupled two-membrane reactor andthermally double coupled reactor.

various types of coupling reactors and conventional one. Resultsshow that temperature of methanol production in conventionalmethanol reactor is higher than coupled reactor due to moretransferring heat to endothermic side in comparison with boilingwater. It must be noted that in thermally coupled reactors, anotherproducts such as benzene, hydrogen and DME are produce but inconventional methanol reactor only steam is produced. For DMEside, temperature of thermally double-coupled two-membranereactor is more than thermally double coupled reactor because ofaddition of hydrogen with membranes increases heat generationby far out of chemical equilibrium in reversible reactions. There-fore endothermic side in thermally double-coupled two-mem-brane reactor is received more heat compared with thermallycoupled reactor and thermally double coupled reactor and tem-perature of it is highest (see Fig. 5(c)). For better explanation ofthis subject, Figs. 6e9 present the heat generation and con-sumption for exothermic and endothermic sides of thermallydouble-coupled two-membrane reactor, thermally double coupledreactor and thermally coupled reactor. As demonstrated in Fig. 6for methanol production, heat generation in thermally double-

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

1

2

3

4

5

6

7

8


Hea

t (k

W)

Methanol productionCyclohexane dehydrogenationDME production

Fig. 9. Heat of three sides of the thermally double-coupled two-membrane reactor.


coupled two-membrane reactor is bigger than thermally doublecoupled reactor and thermally coupled reactor due to highertemperature and higher rate of methanol production in thermallydouble-coupled two-membrane reactor compared to thermallydouble coupled reactor and thermally coupled reactor (ratebehavior has been illustrated in next part of paper). Fig. 7 exhibitsthe heat of DME reactor in thermally double-coupled two-mem-brane reactor and thermally double coupled reactor. As this figurepresents the heat generation of DME production in thermallydouble-coupled two-membrane reactor is much larger than ther-mally double coupled reactor because temperature and rate ofreactions in thermally double-coupled two-membrane reactor ishigher. Fig. 8 displays the heat consumption of cyclohexanedehydrogenation process for thermally double-coupled two-membrane reactor, thermally double coupled reactor and ther-mally coupled reactor. As the results explain the heat consumptionof dehydrogenation process for thermally double-coupled two-membrane reactor is larger than thermally double coupled reactorand thermally coupled reactor. This subject is due to higher tem-perature and rate of cyclohexane dehydrogenation reaction inthermally double-coupled two-membrane reactor. Fig. 9 illustratesthe heat of three sides of the reactors. As the results display thegenerated heat of DME side is larger than methanol side andsummation of generated heat is more than consumed heat incyclohexane dehydrogenation side.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-2

0

2

4

6

8

10


Rat

e (m

ol k

g-1 s

-1)

Hydrogenation of CO in Thermally coupled reactorHydrogenation of CO inThermally coupled reactor

Water gas shift in Thermally coupled reactorHydrogenation of CO in Thermally double coupled reactorHydrogenation of CO in Thermally double coupled reactor

Water gas shift in Thermally double coupled reactorHydrogenation of CO in Thermally double-coupled two-membrane reactorHydrogenation of CO in Thermally double-coupled two-membrane reactor

Water gas shift in Thermally double-coupled two-membrane reactor

Water gas shift

Hydrogenation of CO2

Hydrogenation of CO

0 0.1 0.2 0.3 0.4 0

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

Dimension

Rat

e (m

ol k

g-1s-1

)

Thermally double coThermally coupled rThermally double-co

a

c

Fig. 10. (a) Rate profile in (a) methanol exothermic side (b) DME exothermic side (c) cyclohecoupled reactor and thermally double coupled reactor.

5.3. Rate behavior

Fig. 10(a) demonstrates variations in rates of CO hydrogenation,CO2 hydrogenation and water gas shift in methanol synthesis sideof thermally coupled reactor, thermally double coupled reactorand thermally double-coupled two-membrane reactor. Resultsshow that rate of reactions in thermally double-coupled two-membrane reactor is more than thermally double coupled reactorand thermally coupled reactor due to higher temperature ofthermally double-coupled two-membrane reactor (see Fig. 5(b)).For exothermic reactions, the temperature is closed to the indus-trial temperature which is optimum temperature. In thermallydouble-coupled two-membrane reactor, temperature of methanolside becomes near temperature of conventional methanol reactor.Of course, by moving along the reactor length, rate of reactionsdecreases because of equilibrium conditions at the end of thereactor.

Rate behavior of DME side has been shown in Fig. 10(b). Withemploying membranes in thermally double-coupled two-mem-brane reactor increases rate of DME production. This subjectmanner is based on far away of chemical equilibrium in reversiblereactions cause more heat generation and higher temperate.Fig. 10(c) shows the rate profiles of cyclohexane dehydrogenationreactor for thermally double-coupled two-membrane reactor,thermally double coupled reactor and thermally coupled reactor. As

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-2

0

2

4

6

8

10

12

14


Rat

e (m

ol k

g-1 s

-1)

Hydrogenation of CO in Thermally double-coupled two-membrane reactorHydrogenation of CO2 in Thermally double-coupled two-membrane reactor

DME production in Thermally double-coupled two-membrane reactorHydrogenation of CO in Thermally double coupled reactorHydrogenation of CO2 in Thermally double coupled reactor

DME production in Thermally double coupled reactor

DME production

Hydrogenation of CO

Hydrogenation of CO2

.5 0.6 0.7 0.8 0.9 1less length

upled reactoreactorupled two-membrane reactor

b

xane endothermic side in thermally double-coupled two-membrane reactor, thermally


the results show, the rate in thermally double-coupled two-mem-brane reactor is more than thermally double coupled reactor andthermally coupled reactor because of receiving more heat fromexothermic sides and highest temperature of cyclohexane dehy-drogenation can be achieved in this configuration.

5.4. Molar flow rate behavior

Fig. 11 displays methanol production molar flow rate profiles ofall components in each three concentric tubes of multi tubularreactors.

As shows in Fig.11(d) thefinal hydrogenmolarflowrate at the endof the reactor for thermallydouble-coupled two-membrane reactor ismore than 18% higher than the other reactors approximately due tomembranes are added hydrogen to methanol production side.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10.042

0.044

0.046

0.048

0.05

0.052

0.054

0.056

0.058

0.06

0.062


CO

2 flo

w r

ate

(mol

s-1

)

Thermally double coupled reactorConventional methanol reactorThermallycoupled reactorThermally double-coupled two-membrane reactor

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

0.02


Wat

er f

low

rat

e (m

ol s

-1)


0 0.1 0.2 0.3 0.40

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

Dimensio

Met

hano

l flo

w r

ate

(mol

s-1

)

ThermaConvenThermaTherma

a b

c d

e

Fig. 11. (a) CO2 (b) CO (c) H2O (d) H2 (e) CH3OH molar flow rate in methanol synthesis prreactor, thermally coupled reactor and conventional methanol reactor.

Fig. 11(e) presents methanol molar flow rate profile for ther-mally double-coupled two-membrane reactor, thermally doublecoupled reactor, thermally coupled reactor and conventionalmethanol reactor. The results display that methanol molar flow rateat the end of the reactor is 0.036 mol s�1 that is higher than ther-mally double coupled reactor, thermally coupled reactor and con-ventional methanol reactor. Methanol molar flow rate at the outputof the reactors is equal to 0.03244, 0.03346 and 0.03481 mol s�1 forthermally coupled reactor, thermally double coupled reactor andconventional methanol reactor respectively. This subject is theadvantages of thermally double-coupled two-membrane reactor incomparison with other reactors.

Fig. 12 illustrates the DME side molar flow rate profiles forthermally double-coupled two-membrane reactor and thermallydouble coupled reactor. The results exhibit that the DME

0 0.2 0.4 0.6 0.8 10.01

0.015

0.02

0.025

0.03

0.035

0.04


CO

flo

w r

ate

(mol

s-1

)


0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0.32

0.34

0.36

0.38

0.4

0.42

0.44

0.46

0.48

0.5


Hyd

roge

n fl

ow r

ate

(mol

s-1

)


0.5 0.6 0.7 0.8 0.9 1nless length

lly double coupled reactortional methanol reactorllycoupled reactorlly double-coupled two-membrane reactor

ocess for thermally double-coupled two-membrane reactor, thermally double coupled

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04


DM

E f

low

rat

e (m

ol s

-1)


0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0.16

0.18

0.2

0.22

0.24

0.26

0.28

0.3

0.32


Hyd

roge

n fl

ow r

ate

(mol

s-1

)


0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

0.11


CO

flo

w r

ate

(mol

s-1

)


0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.002

0.004

0.006

0.008

0.01

0.012

0.014


Met

hano

l flo

w r

ate

(mol

s-1

)


a

b

c

d

Fig. 12. (a) DME (b) H2 (c) CO (d) CH3OH molar flow rate profile in DME reactor forthermally double-coupled two-membrane reactor and thermally double coupledreactor.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16


Hyd

roge

n fl

ow r

ate

(mol

s-1

)


0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045


Ben

zene

flo

w r

ate

(mol

s-1

)


0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.01

0.02

0.03

0.04

0.05

0.06

0.07


Cyc

lohe

xane

flo

w r

ate

(mol

s-1

)


0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8


Hyd

roge

n fl

ow r

ate

(mol

s-1

)

Methanol side in Thermally double coupled reactor

Cyclohexane side inThermally double coupled reactor

DME side in Thermally double coupled reactor

Methanol side in Thermally double-coupled two-membrane reactor

Cyclohexane side in Thermally double-coupled two-membrane reactor

DME side in Thermally double-coupled two-membrane reactor

a

b

c

d

Fig. 13. (a) H2 (b) Benzene (c) cyclohexane (d) H2 molar flow rate profiles in cyclo-hexane dehydrogenation reactor for thermally double-coupled two-membrane reactor,thermally double coupled reactor and thermally coupled reactor.



production in each three concentric tubes of multi tubular reactorincreases from 0.0262 to 0.0305 mol s�1 by employing mem-branes in thermally double-coupled two-membrane reactor. Inaddition, the results demonstrate that hydrogen molar flow rate atthe end of thermally double-coupled two-membrane reactor isincrease 33.31% is comparison with thermally double coupledreactor.

Fig. 13 shows hydrogen, benzene and cyclohexane molar flowrate profile of cyclohexane dehydrogenation side in coupled re-actors. The results show that hydrogenmolar flow rate at the end ofthermally double-coupled two-membrane reactor, thermally dou-ble coupled reactor and thermally coupled reactor is 0.1143, 0.1009and 0.0235mol s�1 that indicate 13.28% enhancement for hydrogenproduction in thermally double-coupled two-membrane reactor incomparison with thermally double coupled reactor.

Based on importance of hydrogen, hydrogen molar flow rateprofile for methanol side, DME side and cyclohexane dehydro-genation side for thermally double-coupled two-membranereactor and thermally double coupled reactor has been presentedin Fig. 13(d). The results exhibit that hydrogen flow rate inthermally double-coupled two-membrane reactor is higher thanthermally double coupled reactor and enhancement of 13.28%,33.31% and 18.07% can be achieved for cyclohexane dehydroge-nation, DME production and methanol synthesis sides respec-tively. This results show that one the major advantage ofthermally double-coupled two-membrane reactor is productionof hydrogen.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45


Met

hano

l yie

ld


0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9


Cyc

lohe

xane

con

vers

ion


a b

c d

Fig. 14. (a) Methanol yield for thermally double-coupled two-membrane reactor, thermally(b) CO conversion in DME reactor for thermally double-coupled two-membrane reactor angenation reactor for thermally double-coupled two-membrane reactor and thermally doublfor thermally double-coupled two-membrane reactor and thermally double coupled reacto

5.5. Yield and conversion changes

Fig. 14(a) shows methanol yield for all types of reactors. As theresults illustrate, yield of methanol at the end of the thermallydouble-coupled two-membrane reactor is 0.4017 which is higherthan the other three reactors (0.3885, 0.3735 and 0.362 for con-ventional methanol reactor, thermally double coupled reactor andthermally coupled reactor respectively).

Fig. 14(b) shows the conversion of carbon monoxide in DMEreactor for thermally double-coupled two-membrane reactor andthermally double coupled reactor. The results demonstrate thatconversion of carbon monoxide in thermally double-coupled two-membrane reactor is 0.78 in comparison with thermally doublecoupled reactor that is 0.64. As shown in Fig. 14(c) the conversion ofcyclohexane increases from 0.68 in thermally double coupledreactor to 0.76 in thermally double-coupled two-membranereactor. Fig. 14(d) demonstrates the recovery yield of hydrogen incyclohexane dehydrogenation side for thermally double-coupledtwo-membrane reactor and thermally double coupled reactor.The results show that hydrogen recovery yield in thermally double-coupled two-membrane reactor is 12.98% higher than thermallydouble coupled reactor.

5.6. Pressure variation

Fig. 15(aec) presents pressure variations along thermallydouble-coupled two-membrane reactor axis in methanol synthesis,

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8


CO

con

vert

ion


0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.5

1

1.5

2

2.5


Hyd

roge

n re

cove

ry y

ield


double coupled reactor, thermally coupled reactor and conventional methanol reactord thermally double coupled reactor (c) hydrogen conversion in cyclohexane dehydro-e coupled reactor (d) hydrogen recovery yield in cyclohexane dehydrogenation reactorr.

Fig. 15. Variations of pressure along reactor axes for (a) methanol synthesis (b) DME synthesis (c) cyclohexane dehydrogenation.


DME synthesis and cyclohexane dehydrogenation sides respec-tively. The pressures drop is 2.68, 1.12 and 5.17 bar for methanolproduction, DME production and cyclohexane dehydrogenationside respectively.

6. Conclusion

In this study thermally coupling of two exothermic reactionswith one endothermic reaction using two Pd/Ag membranes forimproving methanol, DME and hydrogen production has beeninvestigated theoretically in a thermally double-coupled two-membrane reactor. Thermally double-coupled two-membranereactor is a multi tubular reactor and consisted of 2962 threeconcentric tubes with two membrane that covered inside andoutsides of each three concentric tubes. A steady state heteroge-neous catalytic reaction model is applied to analyze the perfor-mance of thermally double-coupled two-membrane reactor andcomparison of the result with other conventional and coupledreactors.

The simulation results display that methanol production ofthermally double-coupled two-membrane reactor is 383.87 kmolh�1 that is higher than thermally double coupled reactor(356.79 kmol h�1), thermally coupled reactor (345.91 kmol h�1)and conventional methanol reactor (371.19 kmol h�1). Also DMEproduction increases from 279.38 in thermally double coupledreactor to 325.23 kmol h�1 by employing membranes in thermallydouble-coupled two-membrane reactor. Yield of methanol at the

end of thermally double-coupled two-membrane reactor is 0.4017which is higher than the other three reactors (0.3885, 0.3735 and0.362 for conventional methanol reactor, thermally double coupledreactor and thermally coupled reactor respectively).

By employing Pd/Ag membranes in thermally double-coupledtwo-membrane reactor, carbon monoxide conversion in DMEside, cyclohexane conversion and hydrogen recovery yield inendothermic side increase 14%, 8% and 12.98%. Finally, based onsimulation result it can be said that thermally double-coupled two-membrane reactor increases methanol and DME production fromsyngas and production of hydrogen from cyclohexanedehydrogenation.

Nomenclature

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

Ac Cross section area of each tube, m2

Ao Inside area of inner tube, m2

Ai 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

Do Tube outside diameter, m


fi Partial fugacity of component i, barFt Total molar flow rate, mol s�1

hf Gas-solid 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, gmol�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 water-gas 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 betweenexothermic and endothermic sides, W m�2 K�1

vci Critical volume of component i, cm3 mol�1

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

Abo-Ghander, N.S., Grace, J.R., Elnashaie, S., Lim, C.J., 2008. Modeling of a novelmembrane reactor to integrate dehydrogenation of ethyl benzene to styrenewith hydrogenation of nitrobenzene to aniline. Chem. Eng. Sci. 63, 1817e1826.

Ali, J.K., Newson, E.J., Rippin, D.W.T., 1994. Exceeding equilibrium conversion with acatalytic membrane reactor for the dehydrogenation of methylcyclohexane.Chem. Eng. Sci. 13, 2129e2134.

Altimari, P., Bildea, C.S., 2009. Integrated design and control of plant wide systemscoupling exothermic and endothermic reactions. Comp. Chem. Eng. 33, 911e923.

Amandusson, H., Ekedahl, L.G., Dannetun, H., 2001. Hydrogen permeation throughsurface modified Pd and Pd/Ag membranes. J. Membr. Sci. 193, 35e47.

Arcoumanis, C., Bae, C., Crookes, R., Kinoshita, E., 2008. The potential of dimethylether (DME) as an alternative fuel for compression-ignition engines: a review.Fuel 87, 1014e1030.

Barbieri, G., Maio, F.P.D., 1997. Simulation of the methane steam reforming processin a catalytic Pd-membrane reactor. Ind. Eng. Chem. Res. 36, 2121e2127.

Biniwale, R.B., Kariya, N., Ichikawa, M., 2005. Dehydrogenation of cyclohexane overNi based catalysts supported on activated carbon using spray-pulsed reactorand enhancement in activity by addition of a small amount of Pt. Catal. Lett.105, 83e88.

Biniwale, R.B., Pande, J., Dhakad, M., Labhsetwar, N., Ichikawa, M., 2008. Nitric oxidereduction using hydrogen over perovskite catalysts with promotional effect ofplatinum on catalytic activity. Catal. Lett. 123, 164e171.

Breen, J.P., Burch, R., Coleman, H.M., 2002. Metal-catalyzed steam reforming ofethanol in the production of hydrogen for fuel cell applications. Appl. Catal. 39,65e74.

Cussler, E.L., 1984. Diffusion, Mass Transfer in Fluid Systems. University Press,Cambridge.

Dautzenberg, F.M., Mukherjee, M., 2001. Process intensification using multifunc-tional reactors. Chem. Eng. Sci. 56, 251e267.

Eisenberg, R., Nocera, D., 2007. Overview of the forum on solar and renewableenergy. Inorg. Chem. 44, 6799e6801.

Elnashaie, S.S.E.H., Moustafa, T., Alsoudani, T., Elshishini, S.S., 2000. Modeling andbasic characteristics of novel integrated dehydrogenationehydrogenationmembrane catalytic reactors. Comp. Chem. Eng. 24, 1293e1300.

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, 14416e14472.

Farsi, M., Khademi, M.H., Jahanmiri, A., Rahimpour, M.R., 2011. Optimal conditionsfor hydrogen production from coupling of dimethyl ether and benzene syn-thesis. Int. J. Hydrogen Energy 36, 299e310.

Fleisch, T.H., Basu, A., Gradassi, M.J., Masin, J.G., 1995. Dimethyl ether: a fuel for the21st century. Nat. Gas Convers. 107, 17e25.

Galvita, V.V., Semin, G.L., Belyaev, V.D., Yurieva, T.M., Sobyanin, V.A., 2001. Pro-duction of hydrogen from dimethyl ether. Appl. Catal. 216, 85e90.

Garcia Villaluenga, J.P., Tabe-Mohammadi, A., 2000. A review on the separation ofbenzene/cyclohexane mixtures by pre-evaporation processes. J. Membr. Sci.169, 159e174.

Glockler, B., Kolios, G., Tellaeche, C., Nieken, U., 2009. A heat-integrated reverse-flow reactor concept for endothermic high temperature syntheses, part i:fundamentals short-cut theory and experimental verification of a travelingendothermic reaction zone. Chem. Eng. Technol. 32, 1339e1347.

Goltsov, V.A., Veziroglu, T.N., Goltsova, L.F., 2006. Hydrogen civilization of the futurea new conception of the IAHE. Int. J. Hydrogen Energy 31, 153e159.

Graaf, G.H., Stamhuis, E.J., Beenackers, A.A.C.M., 1988. Kinetics of low-pressuremethanol synthesis. Chem. Eng. Sci. 43, 3185e3195.

Guo, J.W., Niu, Y.Q., Zhang, B.J., 2000. Macro Kinetics Study on DME Synthesis FromSyngas in Slurry Reactor, vol. 25. Natural Gas Chemical Industry, pp. 4e10.

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

Itoh, N., 2003. Hydrogen recovery from cyclohexane as a chemical hydrogen carrierusing a palladium membrane reactor. Catal. Today 82, 119e125.

Itoh, N., Wu, T.H., 1997. An adiabatic type of palladium membrane reactor forcoupling endothermic and exothermic reactions. J. Membr. Sci. 124, 213e222.

Kamat, P.V., 2007. Meeting the clean energy demand: nanostructure architecturesfor solar energy conversion. J. Phys. Chem. 111, 2834e2860.

Kariya, N., Fukuoka, A., Utagawa, T., Sakuramoto, M., Goto, Y., Ichikawa, M., 2003.Efficient hydrogen production using cyclohexane and declain by pulse-spraymode reactor with Pt catalysts. Appl. Catal. 247, 247e259.

Khademi, M.H., Jahanmiri, A., Rahimpour, M.R., 2009a. A novel configuration forhydrogen production from coupling of methanol and benzene synthesis in ahydrogen perm selective membrane reactor. Int. J. Hydrogen Energy 34, 5091e5107.

Khademi, M.H., Setoodeh, P., Jahanmiri, A., Rahimpour, M.R., 2009b. Optimization ofmethanol synthesis and cyclohexane dehydrogenation in a thermally coupledreactor using differential evolution (DE) method. Int. J. Hydrogen Energy 34,6930e6944.

Khademi, M.H., Rahimpour, M.R., Jahanmiri, A., 2010. Differential evolution (DE)strategy for optimization of hydrogen production, cyclohexane dehydrogena-tion and methanol synthesis in a hydrogen-perm selective membrane ther-mally coupled reactor. Int. J. Hydrogen Energy 35, 1936e1950.

http://refhub.elsevier.com/S1875-5100(13)00054-1/sref1















































































































Kolios, G., Frauhammer, J., Eigenberger, G., 2002. Efficient reactor concepts for couplingof endothermic and exothermic reactions. Chem. Eng. Sci. 57, 1505e1510.

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

Lovik, I., Hillestad, M., Hertzberg, T., 1998. Long term dynamic optimization of acatalytic reactor system. Comp. Chem. Eng. 22, 707e710.

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

Makarand, L., Gogate, R., Kulika, C.J., 1992. A novel single-step dimethyl ether (DME)synthesis in a three-phase slurry reactor from co-rich syngas Sunggyu. Chem.Eng. Sci. 47, 3769e3776.

Marshall, A., Sunde, S., Tsypkin, M., Tunold, R., 2007. Performance of a PEM waterelectrolysis cell using IrxRuyTazO2 electro catalysts for the oxygen evolutionelectrode. Int. J. Hydrogen Energy 32, 2320e2324.

Moustafa, T.M., Elnashaie, S., 2000. Simulation production of styrene and cyclo-hexane in an integrated membrane reactor. J. Membr. Sci. 178, 171e184.

Navarro, R.M., Pena, M.A., Fierro, J.L.G., 2007. Hydrogen production reactions fromcarbon feed stocks: fossil fuels and biomass. Chem. Rev. 107, 3952e3991.

Ng, K.L., Chadwick, D., Toseland, B.A., 1999. Kinetics and modeling of dimethyl ethersynthesis from synthesis gas. Chem. Eng. Sci. 54, 3587e3592.

Ni, M., Leung, D.Y.C., Leung, M.K.H., Sumathy, K., 2006a. An overview of hydrogenproduction from biomass. Fuel Process. Technol. 87, 461e472.

Ni, M., Leung, M.K.H., Sumathy, K., Leung, D.Y.C., 2006b. Potential of renewablehydrogen production for energy supply in Hong Kong. Int. J. Hydrogen Energy31, 1401e1412.

Rahimpour, M.R., Asgari, A., 2009. Production of hydrogen from purge gases ofammonia plants in a catalytic hydrogen-perm selective membrane reactor. Int.J. Hydrogen Energy 34, 5795e5802.

Rahimpour, M.R., Ghader, S., 2003. Theoretical investigation of a Pd-membranereactor for methanol synthesis. Chem. Eng. Technol. 26, 902e907.

Rahimpour, M.R., Pourazadi, E., 2010. A comparison of hydrogen and methanolproduction in a thermally coupled membrane reactor for co-current andcounter-current flows. Int. J. Energy Res. 10, 863e882.

Rahimpour, M.R., Fathikalajahi, J., Jahanmiri, A., 1998. Selective kinetic deactivationmodel for methanol synthesis from simultaneous reaction of CO2 and CO withH2 on a commercial copper/zinc oxide catalyst. Can. J. Chem. Eng. 76, 753e761.

Rahimpour, M.R., Moghtaderi, B., Jahanmiri, A., Rezaie, N., 2005. Operability of anindustrial methanol synthesis reactor with mixtures of fresh and partiallydeactivated catalyst. Chem. Eng. Technol. 28, 226e234.

Rahimpour, M.R., Khosravanipour Mostafazadeh, A., Barmaki, M.M., 2008. Appli-cation of hydrogen-perm selective Pd-based membrane in an industrial single-type methanol reactor in the presence of catalyst deactivation. Fuel. Process.Technol. 89, 1396e1408.

Rahimpour, M.R., Rahmani, F., Bayat, M., Pourazadi, E., 2011. Enhancement ofsimultaneous hydrogen production and methanol synthesis in thermallycoupled double-membrane reactor. Int. J. Hydrogen Energy 36, 284e298.

Ramaswamy, R.C., Ramachandran, P.A., Dudukovic, M.P., 2006. Recuperativecoupling of exothermic and endothermic reactions. Chem. Eng. Sci. 61, 459e472.

Ramaswamy, R.C., Ramachandran, P.A., Dudukovi, M.P., 2008. Coupling exothermicand endothermic reactions in adiabatic reactors. Chem. Eng. Sci. 63, 1654e1667.

Reid, R.C., Sherwood, T.K., Prausnitz, J., 1997. The Properties of Gases and Liquids,third ed. McGraw-Hill, New York.

Rezaie, N., Jahanmiri, A., Moghtaderi, B., Rahimpour, M.R., 2005. A comparison ofhomogeneous and heterogeneous dynamic models for industrial methanol re-actors in the presence of catalyst deactivation. Chem. Eng. Process. 44, 911e921.

Semelsberger, T.A., Borup, R.L., Greene, H.L., 2006. Dimethyl ether (DME) as an alter-native fuel. J. Power Sources 156, 497e511.

Shu, J., Grandjean, B.P.A., Neste, A.V., Kaliaguine, S., 1991. Catalytic palladium-basedmembrane reactors: a review. Can. J. Chem. Eng. 69, 1036e1060.

Smith, J.M., 1980. Chemical Engineering Kinetics. McGraw-Hill, New York.Troy, A.S., Rodney, L.B., Howard, L.G., 2006. Dimethyl ether (DME) as an alternative

fuel. J. Power Sources 156, 497e511.Uemiya, S., Matsuda, T., Kikuchi, E., 1991. Hydrogen permeable palladium silver

alloy membrane supported on porous ceramics. J. Membr. Sci. 56, 315e325.Vakili, R., Pourazadi, E., Setoodeh, P., Eslamloueyan, R., Rahimpour, M.R., 2011. Direct

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

Vizcaino, A.J., Carrero, A., Calles, J.A., 2007. Hydrogen production by ethanol steamreforming over CueNi supported catalysts. Int. J. Hydrogen Energy 32, 1450e1461.

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

Wilke, C.R., 1949. Estimation of liquid diffusion coefficients. Chem. Eng. Prog. 5,218e224.

Yolcular, S., Olgun, O., 2008. Ni/Al2O3 catalysts and their activity in dehydrogenationof methyl cyclohexane for hydrogen production. Catal. Today 138, 198e202.

Zanfir, M., Gavriilidis, A., 2001. Modeling of a catalytic plate reactor for dehydro-genationecombustion coupling. Chem. Eng. Sci. 56, 2671e2683.





















































































































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