Simultaneous production of methanol, DME and hydrogen in a thermally double coupled reactor with different endothermic reactions: Application of cyclohexane, methylcyclohexane and decalin dehydrogenation reactions

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

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Simultaneous production of methanol, DME and hydrogen in a thermally double coupled reactor with different endothermic reactions: Application of cyclohexane, methylcyclohexane and decalin 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, Iran Department of Chemical Engineering, School of Chemical and Petroleum Engineering, Persian Gulf University, Bushehr, Iran c Chemical Engineering Department, School of Chemical and Petroleum Engineering, Shiraz University, Shiraz 71345, Iran b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 March 2014 Received in revised form 21 May 2014 Accepted 22 May 2014 Available online

Three different units of a multi-tubular reactor with 2962 three concentric tubes have been investigated as a thermally double coupled reactor (TDCR) in co-current mode. Exothermic reactions are same for all three units and are occurred in the inner (methanol synthesis) and outer tubes (direct DME synthesis from syngas) while endothermic reaction is different for each unit. Three endothermic dehydrogenation reactions of cyclohexane (CH), methylcyclohexane (MCH) and decalin (DC) have been considered for middle tube side of each unit. A steady-state heterogeneous catalytic reaction model is applied to evaluate the performance of TDCR for simultaneous production of methanol, hydrogen and dimethylether (DME) in one reactor. The simulation results of each unit are compared with others that operated at the same feed conditions. Results show that conversion of CH, MCH and DC reaches to 67%, 56% and 77% at the output of each unit respectively. Output methanol yield with application of CH and MCH dehydrogenation as endothermic reactions are same equal to 0.37. Also, methanol yield for DC unit is 0.33. In addition, conversion of monoxide at the output of DME synthesis side reaches to 64%, 65% and 62% for CH, MCH and DC units, respectively. © 2014 Elsevier B.V. All rights reserved.

Keywords: Methanol Hydrogen DME Cyclohexane Methylcyclohexane Decalin

1. Introduction Non-renewability and environmental problems of fossil fuels caused a global movement toward clean, renewable and alternative energy carriers such as hydrogen (Brown, 2001; Jain et al., 2010; Song, 2002). Advantageous properties of hydrogen are: sustainability, high energy content, high efficiency, ease of storage and distribution, cost attractive and finally environmentally friendly characteristics (Badmaev and Snytnikov, 2008; Cai and Wang, 2012; Edwards et al., 2007; Haryanto et al., 2005; Hoffman, 2002; Schrope, 2001; Turner, 2004). Among different developed methods for production of hydrogen, more attraction is placed on process of catalytic steam reforming of natural gas and other hydrocarbons. This method emits CO and CO2 to atmosphere that are corrosive and greenhouse

* Corresponding author. Tel.: þ98 711 2303071; fax: þ98 711 6287294. E-mail address: [email protected] (M.R. Rahimpour). http://dx.doi.org/10.1016/j.jngse.2014.05.019 1875-5100/© 2014 Elsevier B.V. All rights reserved.

gases (Kariya et al., 2003; Wang et al., 2008) and works at high reaction temperatures (more than 700
C) (Rahimpour et al., 2011b). An alternative way to produce, store and transport of hydrogen is the dehydrogenation of cyclic hydrocarbons with high hydrogen content (such as cyclohexane, methyl-cyclohexane, decalin, etc.) without any containments (Rahimpour et al., 2011b). Cyclohexane (C6H12) with 7.1 wt.% hydrogen content can be dehydrogenated to gaseous hydrogen and condensable benzene (Jain et al., 2010). Benzene as a byproduct material can be used in production of phenol, styrene, aniline, drugs, dyes, insecticides, plastics, etc. (Othmer, 1978). Cyclohexane dehydrogenation reaction is predominantly carried out over Pt/Al2O3 catalyst at a temperature range and total pressure of 423e523 K and 101.3 kPa, respectively (Rahimpour et al., 2011a). Dehydrogenation of decalin (DC) as another hydrogen storage material is performed at 210
C over carbon supported Pt based catalyst in a batch reactor and a condenser removes hydrogen from the reactor (Hodoshima et al., 2003). Wang et al. (2008) employed

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

dehydrogenated decalin to produce pure hydrogen for fuel cell applications. Byproduct of decalin dehydrogenation reaction is tetralin which can be further dehydrogenated into naphthalene and more hydrogen. Dehydrogenation of methyl-cyclohexane (MCH) is one of the most attractive hydrogen energy storage materials. Besides hydrogen, toluene is produced during this reaction which is used in the petrochemical complexes for various end products. Also, toluene can play the role of carrier because it is easily hydrogenated and then dehydrogenated again (Vakili et al., 2011). Each of the three mentioned endothermic dehydrogenation reactions needs an energy source for proceeding process. In this study, concept of thermally coupled reactors is employed to produce the necessary heat for each of cyclohexane, MCH and decalin dehydrogenation reactions. In this type of reactors, released heat from one or two exothermic reactions is used as energy source for proceeding an exothermic reaction without any mixing of reactors. For the first time, Hunter and McGuire (1980) coupled endothermic and exothermic reactions without direct heat transfer. Bhat and Sadhukhan (2009) presented a comprehensive overview of the process integration aspects for methane steam reforming in a thermally coupled membrane separation technology. Khademi et al. (2010) investigated coupling of methanol synthesis and cyclohexane dehydrogenation reaction. Methanol synthesis and cyclohexane dehydrogenation reactions in a single and dual hydrogen perm-selective membrane thermally coupled reactor have been investigated by several researchers (Aboosadi et al., 2011; Rahimpour, 2007). Synthesis of dimethylether (DME) in a thermally coupled heat exchanger reactor was investigated by Vakili et al. (2011). Farsi and Jahanmiri (2011) simulated and optimized DME production and cyclohexane dehydrogenation in a thermally coupled heat exchanger reactor. Methanol dehydration and cyclohexane dehydrogenation reactions were investigated in a thermally coupled reactor by Khademi et al. (2011). Rahimpour et al. (2013) studied coupling of methanol and DME synthesis with the endothermic reaction of cyclohexane dehydrogenation in a thermally double coupled reactor. Methanol and DME as important chemical materials have wide ranges of applicability in industry and daily lives. DME can be used as an alternative fuel and has applications in heating and cooking instead of liquefied petroleum gas (LPG) (Ji et al., 2011; Naik et al., 2011). Also, it can be used as a raw material for some chemical productions such as: olefins, gasoline, jet fuel, spray, and hydrogen carrier in fuel cells (Naik et al., 2011). Methanol is utilized in synthesis of biodiesel, DME, methyl tbutyl-ether, etc. as well as in novel processes such as direct methanol fuel cell, micro channel methanol steam reforming for production of hydrogen. Also, it can be used as a near-zero emissions 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 dimethylether (DME) synthesis are considered as energy source for proceeding each of endothermic reactions of cyclohexane (CH), methylcyclohexane (MCH) and decalin (DC) dehydrogenation. Therefore, three individual units of thermally double coupled reactor (TDCR) are investigated by employing exothermic reactions of DME and methanol synthesis that take place in outer and inner tubes, respectively. Also, each of endothermic reaction (CH, MCH and DC dehydrogenation) is replaceable and occurs in middle tube. Result of each unit is compared together and effect of two exothermic reactions on performance of each endothermic reaction is investigated. It must be noted that, results of this paper are novel and there are not similar works in literature. In fact, thermally double coupled reactors with application of MCH and DC dehydrogenation

325

endothermic reactions have not been presented yet in literature. On the other hand, performance of each case of TDCR with different endothermic reaction for simultaneous production of methanol, hydrogen and dimethylether (DME) in one reactor is evaluated in this novel paper. The simulation results of each unit are compared with other units that operated at the same feed conditions. 2. Process description Thermally double coupled reactor (TDCR) is composed of three concentric tubes that each endothermic reaction (dehydrogenation of CH, MCH and DC) is occurred in middle tube and exothermic reactions of methanol and DME synthesis take place in the inner and outer tubes, respectively. Generated heat from endothermic reactions is continuously transferred to endothermic reaction side. This multi-tubular configuration in cocurrent mode is illustrated in Fig. 1. Inner and outer tubes are loaded with CuO/ZnO/Al2O3 (same as the conventional methanol reactor). Middle tube is loaded with Pt/Al2O3 catalysts for units in which CH and MCH dehydrogenation reactions are considered for endothermic reaction and with PteSn/g-Al2O3 for DC dehydrogenation unit. Conventional methanol reactor in Zagros Petrochemical Company in Assaloyeh, Iran; has 2962 packed tubes with length equal to 7.022 m. Based on this subject, number of tubes and length of them; feed composition and flow rate of methanol side in TDCR have been selected. Also, feed compositions in endodermic and DME sides were selected based on results of Wang et al. (2008), Itoh (1987), Maria et al. (1996) and Hu et al. (2008). After that, for achieving good thermally coupled reactors from thermal and molar flow rate view, temperature and feed flow rate of endothermic reaction were selected. Finally, Tables 1e6 represent characteristics, the properties and input data of endothermic and exothermic reactions in each unit of TDCR. 3. Reaction scheme and kinetics 3.1. Methanol synthesis (exothermic inner tube side) Three main reactions that occur in methanol synthesis are as follows: CO hydrogenation:

CO þ 2H2 4CH3 OH;

DH298 K ¼ 90:55 kJ=mol

(1)

CO2 hydrogenation:

CO2 þ 3H2 4CH3 OH þ H2 O;

DH298 K ¼ 49:43 kJ=mol

(2)

Water gas shift reaction:

CO2 þ H2 4CO þ H2 O;

DH298 K ¼ 41:12 kJ=mol

(3)

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

h . i 3=2 1=2 k1 KCO fCO fH2  fCH3 OH fH2 KP1 .  i r1 ¼  h 1=2  1=2 1 þ KCO fCO þ KCO2 fCO2 fH2 þ KH2 O KH2 fH2 O

(4)

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

h . i 3=2 3=2 k2 KCO2 fCO fH2  fCH3 OH fH2 O fH2 KP2 .  i r2 ¼  h 1=2  1=2 1 þ KCO fCO þ KCO2 fCO2 fH2 þ KH2 O KH2 fH2 O

(5)

   k3 KCO2 fCO2 fH2  fH2 O fCO KP3 h .  i  r3 ¼   1=2 1=2 1 þ KCO fCO þ KCO2 fCO2 fH2 þ KH2 O KH2 fH2 O

(6)

Table 7 gives the reaction rate constants, the adsorption equilibrium constants and the reaction equilibrium constants for methanol synthesis. 3.2. Direct DME synthesis

DH298 K ¼ 90:55 kJ=mol

k2 fCO2 fH32 ð1  b2 Þ rCO2 ¼  4 1 þ KCO fCO þ KCO2 fCO2 þ KH2 fH2

(11)

k3 fCH3 OH ð1  b3 Þ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2 1 þ KCH3 OH fCH3 OH

(12)

b1 ¼

b2 ¼

fCH3 OH

(13)

Kf 1 fCO fH22 fCH3 OH fH2 O

(14)

Kf 2 fCO2 fH32

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

DH298 K ¼ 49:43 kJ=mol

(8)

Dehydration of methanol:

2CH3 OH4CH3 OCH3 þ H2 O;

(10)

(7)

Water gas shift reaction:

CO2 þ 3H2 4CH3 OH þ H2 O;

k1 fCO fH22 ð1  b1 Þ rCO ¼  3 1 þ KCO fCO þ KCO2 fCO2 þ KH2 fH2

rDME ¼ 

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

CO þ 2H2 4CH3 OH;

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

DH298 K ¼ 21:003 kJ=mol (9)

Parameter

Value

Inner tube or methanol synthesis side diameter (m) Middle tube or endothermic side diameter (m) Outer tube or DME synthesis side diameter (m) Length of the reactor (m) Number of tubes

3.8  102 8.52  102 9.75  102 7.022 2962

M. Farniaei et al. / Journal of Natural Gas Science and Engineering 19 (2014) 324e336 Table 2 The operating conditions for DME synthesis (outer exothermic side) in TDCR. Parameter

Value

DME synthesis (exothermic reaction) Feed composition (mole fraction) CO CO2 DME CH3OH H2O H2 N2 CH4

0.1716 0.0409 0.0018 0.003 0.0002 0.4325 0.316 0.044

Inlet temperature (K) Inlet pressure (bar) Inlet flow rate in each tube (mol s1)

493 50 0.6

Typical properties of catalyst Number of three concentric tubes Size of column grain catalysts (mm) Density of catalyst bed (kg m3) Porosity

2962 f5  5 1200 0.455

b3 ¼

fDME fH2 O

(15)

2 kf 3 fCH 3 OH

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

Table 4 The operating conditions for dehydrogenation of CH (endothermic side of CH unit). Parameter

Value

Dehydrogenation of C6H12 (endothermic side) Gas phase Feed composition C6H12 C6H6 H2 Ar

0.1 0.0 0.0 0.9

Inlet temperature (K) Inlet pressure (bar) Particle diameter (m) Bed void fraction Total flow rate (mol s1)

503 20 3.55  103 0.39 0.5

where k, KB and KP are the reaction rate constant, the adsorption equilibrium constant and the reaction equilibrium constant, respectively which are listed in Table 9. Pi is the partial pressure of the component i in Pa. 3.3.2. Dehydrogenation of methylcyclohexane (MCH) The reaction scheme for MCH dehydrogenation is expressed as follows:

C7 H14 4C7 H8 þ 3H2 ;

DH298 K ¼ þ205 kJ=mol

(18)

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

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):

C6 H12 4C6 H6 þ 3H2 ;

327

DH298 K ¼ 206:2 kJ=mol

(16)

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

.   3 P k KP PC PH B 2 .   rC ¼ 3 1 þ KB KP PC PH 2

(17)

rMCH ¼ kpMCH 1 

ptol p3H2

# (19)

Keq pMCH

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



E 1 1  k ¼ A exp  R T 650

(20)

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

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

Value

Gas phase Feed composition (mole fraction) CH3OH CO2 CO H2O H2 N2 CH4

0.0050 0.0940 0.0460 0.0004 0.6590 0.0930 0.1026

Total molar flow rate (mol s1)

0.64

Catalyst particle Particle diameter (m) Density (kg m3) Heat capacity (kJ kg1 K1) Thermal conductivity (W m1 K1) Specific surface area (m2 m3) Ratio of void fraction to tortuosity of catalyst particle

5.47  103 1770 5.0 0.004 626.98 0.123



217; 650 1 1  Keq ¼ 3600 exp R T 650

(21)

Keq in bar3, R in J mol1, and T in K. Table 5 Operating conditions and the properties of the catalyst for dehydrogenation of MCH (endothermic side of MCH unit). Parameter

Value

Feed composition (mole fraction) C7H14 C7H8 H2 Ar

0.12 0.0 0.0 0.88

Total molar flow rate (mol s1) Inlet temperature (K) Inlet pressure (Pa) Shell inner diameter (m) Bed void fraction (e) Particle diameter (m)

0.1 503 8  105 0.07 0.39 3.55  103

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

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 Cisdecalin (CDC) and Trans-decalin (TDC) can be dehydrogenated into tetralin and hydrogen. Furthermore, produced tetralin is dehydrogenated to form naphthalene and more hydrogen. All of these endothermic reactions are reversible and are presented as follows (Wang et al., 2008):

D ¼ 1 þ KCDC PCDC þ KTDC PTDC þ KTT PTT þ KH2 PH2 þ KNP PNP (31) . 0 PCDC U4 r4 ¼ kr4 KCDC

(32)

. 0 r40 ¼ k0r4 KTDC PTDC U4

(33)

(22)

0 0 PTDC þ KCDC PCDC U ¼ 1 þ KTDC

(34)

(23)

RTDC ¼ r1  r10  r4 þ r40

(35)

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

RCDC ¼ r2  r20 þ r4  r40

(36)

RTT ¼ r1  r10 þ r2  r20 þ r30  r3

(37)

RNP ¼ r3  r30

(38)

r40 ;r4

Cis  Decaline ðCDCÞ ! Trans  Decaline ðTDCÞ

Decalin4Naphthalene þ 5Hydrogen

(24)

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

. r1 ¼ kr1 KTDC PTDC D4

(25)

For the rate coefficient:

. r2 ¼ kr2 KCDC PCDC D4

(26)

ki ¼ Ai expð  Ei =RTÞ

(39)

For the adsorption constant:

. r3 ¼ kr3 KTT PTT D4

(27)

. 3 r10 ¼ k0r1 KTT KH2 PTT PH D4 2

(28)

. 3 D4 r20 ¼ k0r2 KTT KH2 PTT PH 2

(29)

. 2 D3 r30 ¼ k0r3 KNP KH2 PNP PH 2

(30)

Ki ¼ Ai expð  DHi =RTÞ

(40)

The parameters for calculation of the reaction rate constant and adsorption equilibrium constant for dehydrogenation of DC reaction are tabulated in Table 10.

Table 7 The reaction rate constants, the adsorption equilibrium constants and the reaction equilibrium constants for methanol synthesis. A (mol kg1 s1 bar1/2)

Table 6 The operating conditions for dehydrogenation of DC to naphthalene (endothermic side of DC unit). Parameter Feed composition (mole fraction) Trans-Decalin (TDC) Cis-Decalin (CDC) Total molar flow rate (mol s1) Reactor pressure (bar) Feed temperature (K) Catalyst equivalent diameter (m) Bed void fraction (e)

k1 k2 k3

Value 0764 0.236 0.1 1 550 3.55  10 0.39

KCO KCO2 1=2 ðKH2 O =KH2 Þ

KP1 KP2 KP3

7

B (J mol1)

(4.89 ± 0.29)  10 (1.09 ± 0.07)  105 (9.64 ± 7.30)  106

63,000 ± 300 87,500 ± 300 152,900 ± 6800

A (bar1)

B (J mol1) 5

(2.16 ± 0.44)  10 (7.05 ± 1.39)  107 (6.37 ± 2.88)  109

46,800 ± 800 61,700 ± 800 84,000 ± 1400

A (K)

B (K)

5139 3066 2073

12.621 10.592 2.029

M. Farniaei et al. / Journal of Natural Gas Science and Engineering 19 (2014) 324e336 Table 8 Reaction rate constants for DME synthesis reactions.

k1 k2 k3 KCO KCO2 KH2 KCH3 OH

A

B (J mol1)

1.828  103 (mol g1 h1 bar3) 0.4195  102 (mol g1 h1 bar3) 1.939  102 (mol g1 h1 bar1) 8.252  104 (bar1) 2.1  103 (bar1) 0.1035 (bar1) 1.726  104 (bar1)

43,723 30,253 24,984 30,275 31,846 11,139 60,126

4. Mathematical modeling Following assumption are modeling of each unit of TDCR:

considered

for

mathematical

 One-dimensional heterogeneous model for each side of the reactor 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 comparison with radial diffusions.  Bed porosity in axial and radial directions is constant. A differential element along the axial direction of the reactor is presented in Fig. 2. The necessary heat for endothermic dehydrogenation reaction (middle tube) is provided by exothermic reactions of methanol and DME synthesizes (inner and outer tube, respectively). Mole and energy balance differential equations of TDCR are presented in Table 11. h is the effectiveness factor and ysi;j and Tjs are the mole fraction of the component i in the solid phases and solid temperature of j side of the TDCR, respectively. Fi,j is the molar flow rate of component i in the fluid phase and j side of the reactor. Tjg is the fluid temperature of j side of the reactor. 4.1. Auxiliary correlations

Methanol yield ¼

Table 10 The parameters for calculation of the reaction rate constant and adsorption equilibrium constant for dehydrogenation of DC reaction. Parameter

Value

ATDC ACDC ATT ANP A H2 ATDC0 ACDC0 ATDC-TT ACDC-TDC ATT-NPATTNP ACDC-TT DHTDC DHCDC DHTT DHNP DHH DHTDC0 DHCDC0 ETDC-TT ECDC-TT ETT-NP ECDC-TDC

1.54906 7.89457 9.33959Eþ1 2.26323 3.31108 2.21726Eþ2 4.0854Eþ1 3.63431Eþ5 1.24695Eþ1 3.35724Eþ3

FCH3 OH;out FCO þ FCO2 ;in

1.28343Eþ4 4.41280Eþ3 3.88825Eþ3 1.07247Eþ4 6.54869Eþ3 1.42062Eþ4 6.89845Eþ3 2.34547Eþ4 9.05243Eþ3 7.06251Eþ3 2.84393Eþ4

CH conversion ¼

FC6 H12 ;in  FC6 H12 ;out FC6 H12 ;in

(49)

CO conversion ¼

FCO;in  FCO;out FCO;in

(50)

MCH conversion ¼

Since heat and mass transfer between solid and fluid phases has been considered for mathematical modeling of TDCR, physical properties of components are estimated for the calculations. Table 12 collected auxiliary correlations including physical properties of components, mass and heat transfer coefficient and Ergun equation. Ergun equation was used for calculating pressure drop through the catalytic bed. Endothermic reaction for each unit is different to others while exothermic reactions were similar for all units. Hence, all units are named due to their own endothermic reactions; CH, MCH and DC units, respectively. Following definitions have been used to calculate methanol and hydrogen yields as well as CH and carbon monoxide conversions:

329

DC conversion ¼

FMCH;in  FMCH;out FMCH;in

FDC;in  FDC;out FDC;in

(51)

(52)

(48)

Table 9 The reaction rate constant, the adsorption equilibrium constant and the reaction equilibrium constant for CH dehydrogenation. A k KB KP

B (K) 3

1

0.221 mol m Pa 2.03  1010 Pa1 4.89  1035 Pa3

1

s

4270 6270 3190

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

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

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

Solid phase (both exothermic side and endothermic side)

Fluid phase (both exothermic sides)

Mass and energy balances equations

#

av cj kgi;j ðygi;j  ysi;j Þ þ hri;j rb ¼ 0 P av hf ðTjg  Tjs Þ þ rb N i¼1 hri;j ðDHf ;i Þ g 1 dFt;j s Ac dz þ av cj kgi;j ðyi;j  yi;j Þ ¼ 0

(41)

C g dðFj Tjg Þ pD þ ai;j hf ðTjs  Tjg Þ þ Ac j dz g 1 dFt;j s Ac dz þ av cj kgi;j ðyi;j  yi;j Þ ¼ 0

 Ap;jc

Fluid phase (endothermic side)

C g dðFj T g Þ ± Ap;jc dz j þ ai;j hf ðTjs z ¼ 0; ygi;j ¼ ygi0;j ; Tj

Boundary conditions

Table 12 Auxiliary correlations. Parameter

Equation

Component heat capacity Mixture heat capacity Component viscosity

Cp ¼ a þ bT þ cT2 þ dT3 P Cpm ¼ yi Cpi

Mixture thermal conductivity Mass transfer coefficient between gas and solid phases

m¼ km

Ref.

P T ¼ ðyi ki Þ

kgi ¼

Re ¼

rug dp m m 4 im 10 1yi P



103

Lindsay and Bromley (1950) Cussler (1997)

Sci ¼ rD Dim ¼

isj

107 T 3=2

Dij ¼ Overall heat transfer coefficient Heat transfer coefficient between gas phase and reactor wall Ergun equation

1 U

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

2=3

Cp m K

¼ 0:458 3B 2

3Þ ¼ 150 ð1 2 þ 1:75 33d p

m rudp

Smith (1980)

ð13 Þu2g 2 3 2 dp

Developed model is formed by a set of ordinary differential equations (ODEs) which are consisted of mass and energy balance equations. ODEs are coupled with non-linear algebraic equations of kinetic models and auxiliary and hydrodynamic correlations. Set of ODEs are solved by backward finite difference approximation to change them into a set of non-linear algebraic equations. The length of each reactor is divided into 100 separate segments and GausseNewton method is applied to solve the obtained set of nonlinear algebraic equations in each segment for each tube side,

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

CH3OH CO2 CO H2O H2 CH4 N2 Feed flow rate (mol s1) Temperature (K)

g U23 ðTj



g T3 Þ

(46)

¼0

(47)

simultaneously. This procedure is repeated for all segments of the reactor as the results of each segment are used as the inlet conditions for the next segment.

Simulation results were compared with an industrial scale conventional methanol synthesis reactor in Zagros Petrochemical Company, Assaloyeh, Iran that has been presented in Table 13. Fortunately, a good agreement was achieved between proposed model and experimental plant data. In addition, for validating the proposed model in DME side, a good agreement was observed between simulation results and experiments of Hu et al. (2008) in Table 14. 5. Results and discussion

0:407

4.2. Numerical solution

Composition (mol%)

(45)

pD pD g g g  Tj Þ  Ac j U12 ðTj  T1 Þ  Ac j g g g ¼ Tjo ; Pj ¼ Pj0 ; j ¼ 1; 2; 3

Reid et al. (1977)

1=Mi þ1=Mj

Ai lnðDo =Di Þ þ AAoi h1 2pLKw o

i



dP dz

(44)

UðT2g  Tjg Þ ¼ 0

Pðv3=2 þv3=2 Þ ci cj

¼ h1 þ

h Cp rm

yi Dij

(43)

4.3. Model validation

C1 T C2 C C 1þ T2 þ 42

1:17Re0:42 Sc0:67 ug i

(42)

¼0

Reactor inlet

0.032 3.45 4.66 0.08 79.55 11.72 0.032 0.565 527

Reactor output Plant data

Simulation results

5.49 2.18 1.44 1.74 75.71 12.98 0.16 0.51 528

5.5 2.43 1.52 1.47 76.54 12.96 0.035 0.511 524.1

Variations of component mole fractions along reactor axes in methanol synthesis side (inner tube side) for all three units are illustrated in Fig. 3. As seen, trend of each component mole fraction in the inner tube side is similar for all units. Hydrogen has the highest profile and CO2 mole fraction decreases slightly in all units of TDCR while other components have a curved profile lower than it. Comparisons between behaviors of component mole fractions in outer tube side (DME synthesis) of all units are shown in Fig. 4. All components in DME synthesis side have a same behavior in all three units and hydrogen has the highest profile (like inner tube side). Fig. 5 represents the changes of component mole fractions in each endothermic reaction in the middle tube side of all units. Since reaction scheme of CH dehydrogenation is almost same as the MCH dehydrogenation reaction, it is expected the same behavior for component mole fractions of both reactions is observed (see Fig. 5(a) and (b)). The hydrogen trajectory is upper than benzene/

Table 14 Comparison between steady state simulation results and experiments of Hu et al. (2008). Components Output composition (%) DME CH3OH H2O H2 CO CO2 N2 Temperature

Simulation results

Experiments of Hu et al. (2008)

4.95 1.03 3.51 33.3 8.9 6.4 41.91 511

4.91 1.06 3.38 33 8.77 6.71 42.17 517

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.

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

toluene owing its higher stoichiometric coefficient in CH and MCH dehydrogenation reaction. Fig. 5(c) illustrates variations of component mole fractions in DC unit. The highest product is hydrogen. DC as one of the best hydrogen carriers has a high hydrogen production rate. Since

stoichiometric ratio of naphthalene to hydrogen is 1/5; hydrogen production is five times naphthalene production as illustrated in Fig. 5(c). Tetralin as an intermediate product turns into naphthalene and hydrogen immediately. The rate of tetralin dehydrogenation reaction is very high thus all tetralin is converted into

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

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

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333

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 along endothermic sides of CH, MCH and DC units.

products and its mole fraction at the end of the reactor almost reaches to zero. Fig. 6 shows a comparison between conversion of CH, MCH and DC along the endothermic side of CH, MCH and DC units, respectively. 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 that other units. Therefore, conversion in DC unit is higher than other units. Variation of CO conversion along outer exothermic tube side (DME synthesis) for all units is compared in Fig. 7. Monoxide conversion at the output of CH, MCH and DC units reached to 64%, 65% and 62%, respectively. This subject is due to better thermal conditions for production of DME in MCH unit. On the other hand, reaction rate in DME side of MCH unit is higher than other units. As shown in Fig. 8(aec), trend of changing in H2 molar flow rate along the exothermic sides (methanol and DME synthesis sides) is similar for all three units. Hydrogen molar flow rate at the output of endothermic side of CH, MCH and DC units arrives to 0.10, 0.08 and 0.05 mol s1, respectively. Also, simultaneous plots of H2 molar flow rate along the endothermic sides of all units are illustrated in Fig. 8(d). Higher H2 molar flow rate in MCH unit in comparison to other units is due to receiving more heat from exothermic side. Therefore, endothermic reaction rate in MCH unit is higher than other cases and more hydrogen is produced. Variations of methanol yield along inner tube side for all three units are demonstrated in Fig. 9. As illustrated in this figure, yield of methanol in CH and MCH units is equal to 0.37 that is higher than DC unit with a value of 0.33. These results indicate that by employing of DC dehydrogenation reaction, reaction rate of methanol production decreases due to unsuitable thermal

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

conditions. In fact, in DC unit, temperature of methanol side decreases and is get away from optimum temperature. Temperature profiles of all three sides in each unit are demonstrated in Fig. 10(aec). To create thermal driving force, temperature of exothermic side should be higher than endothermic side. In all units temperature profile of endothermic side is lower than temperature profiles of both methanol and DME synthesis (see Fig. 10). In fact, exothermic sides generate the necessary heat for driving the each endothermic reaction (CH, MCH and DC dehydrogenation) as well as heating the mixtures of all three sides. In all units, the generated heat at the entrance of the reactor is less than consumed heat. Therefore, the temperature of endothermic side begins to fall and a cold spot developed. After a certain length, conditions are changed and temperature increases along the rest of the reactor. The generated and consumed heat along exothermic and endothermic sides of all units have been presented in Fig. 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 high temperature and reaction rate. But at the second half, due to conversion of feed and reduction of reaction rate, heat generation decreases. Similar phenomenon is observed for endothermic sides. For TDCR with DC dehydrogenation (Fig. 11(c)), heat consumption at the entrance of reactor is very high due to high reaction rate but it decreases sharp at the rest of the reactor. Production of hydrogen, methanol and DME depends on exchange of heat between reactor sides. Of course, it is difficult to

quantitative correlation between products during reaction process from exchange of heat due to carrying out a lot of reactions in three sides of TDCR. From the results in Fig. 11(aec), it can be said that for all units, sum of heat generation in methanol and DME sides is higher than heat consumption in the endothermic sides. 6. Conclusions In this study, methanol DME synthesis processes are coupled with three different dehydrogenation reactions (Cyclohexane (CH), Methylcyclohexane (MCH) and Decalin (DC) dehydrogenations) in three individual units of thermally double coupled reactor (TDCR). Simultaneous production of hydrogen, methanol and DME is investigated via a one-dimensional heterogeneous catalytic reaction model. Achieving high degree of in situ energy integration by coupling two exothermic reactions with an endothermic reaction can be expressed as advantages of these configurations. The simulation results of each TDCR unit are compared with other units. Results indicated that hydrogen molar flow rate at the output of endothermic side of CH unit reached to higher value in comparison with MCH and DC units (0.101, 0.084 and 0.046 mol s1 in each tube, respectively). Yield of methanol in CH and MCH unit obtained to a same value of 0.373 while it is 0.335 for DC unit. Monoxide is more converted along DME synthesis side of MCH unit with the value of 65% in comparison with CH and DC units with the values of 64% and 62%, respectively. Output H2 molar flow rate in methanol and DME synthesis sides in each tube

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335

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 dehydrogenation along reactor axes in MCH unit and c) DC dehydrogenation along reactor axes in DC unit.

was 0.35 and 0.15 mol s1, respectively and was same for all three units.

Kp Kpi

Nomenclature

Kw L Mi N P Pi r1

av Ac Ai Ao C Cp dp Di Do Dij Dim fi Ft hf hi ho

DHf,i k ki kg,i K Ki

specific surface area of catalyst pellet, m2 m3 cross section area of each tube, m2 inside area of inner tube, m2 outside area of inner tube, m2 total concentration, mol m3 specific heat of the gas at constant pressure, J mol1 particle diameter, m tube inside diameter, m tube outside diameter, m binary diffusion coefficient of component i in j, m2 s1 diffusion coefficient of component i in the mixture, m2 s1 partial fugacity of component i, bar total molar flow rate, mol s1 gasesolid heat transfer coefficient, W m2 K1 heat transfer coefficient between fluid phase and reactor wall in exothermic side, W m2 K1 heat transfer coefficient between fluid phase and reactor wall in endothermic side, W m2 K1 enthalpy of formation of component i, J mol1 rate constant of dehydrogenation reaction, mol m3 Pa1 s1 rate constant of reaction i, mol kg1 s1 bar1/2 mass transfer coefficient for component i, m s1 conductivity of fluid phase, W m1 K1 adsorption equilibrium constant for component i, bar1

r2 r3 rCO rCO2 rDME rC R Rp Re Sci T u ug U vci

equilibrium constant for dehydrogenation reaction, Pa3 equilibrium constant based on partial pressure for component i in methanol synthesis reaction thermal conductivity of reactor wall, W m1 K1 reactor length, m molecular weight of component i, g mol1 number of components total pressure, bar partial pressure of component i, Pa rate of reaction for hydrogenation of CO in methanol synthesis, mol kg1 s1 rate of reaction for hydrogenation of CO2 in methanol synthesis, mol kg1 s1 rate of reversed wateregas shift reaction in methanol synthesis, mol kg1 s1 rate of reaction for hydrogenation of CO, mol kg1 s1 rate of reaction for hydrogenation of CO2, mol kg1 s1 rate of reaction for dehydration of methanol, mol kg1 s1 rate of reaction for dehydrogenation of cyclohexane, mol m3 s1 universal gas constant, J mol1 K1 particle radius, m Reynolds number Schmidt number of component temperature, K superficial velocity of fluid phase, m s1 linear velocity of fluid phase, m s1 overall heat transfer coefficient between exothermic and endothermic sides, W m2 K1 critical volume of component i, cm3 mol1

336

yi Z

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mole fraction of component i axial reactor coordinate, m

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