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Comparative Study on Simultaneous Production of Methanol, Hydrogen, and DME Using a Novel Integrated Thermally Double-Coupled Reactor Mohammad Reza Rahimpour,*,†,‡ Mahdi Farniaei,§ Mohsen Abbasi,† Jafar Javanmardi,§ and Sedigheh Kabiri† †

Department of Chemical Department of Chemical California 95616, United § Department of Chemical ‡

Engineering, School of Chemical and Petroleum Engineering, Shiraz University, Shiraz 71345, Iran Engineering and Materials Science, University of California, Davis, 1 Shields Avenue, Davis, States Engineering, Shiraz University of Technology, Shiraz 71555-313, Iran

ABSTRACT: The present paper focuses on simulation of a catalytic thermally double-coupled reactor (TDCR) in cocurrent mode. In this novel configuration, the endothermic reaction of cyclohexane dehydrogenation has coupled with two exothermic reactions: methanol production and direct DME synthesis from syngas to improve the heat transfer between the endothermic and the exothermic sides. A multitubular reactor with 2962 three concentric tubes has been considered for TDCR. A steady state heterogeneous catalytic reaction model is applied to analyze the performance of TDCR for simultaneous production of methanol, hydrogen, and dimethyl ether (DME). Simulation results of TDCR have been compared with corresponding predictions for an industrial methanol reactor (CMR) and thermally coupled reactor (coupling of methanol synthesis with cyclohexane dehydrogenation), operated at the same feed conditions. Results showed that by this novel configuration production of methanol and hydrogen increases from 345.48 to 373.21 kmol h−1 and 250.6 to 1066.3 kmol h−1 in comparison with TCR, respectively. In addition, production of DME with a rate of 277.24 kmol h−1 is another superiority of TDCR. In addition, hydrogen production in the endothermic side of TDCR in each of the three concentric tubes (0.1 mol s−1) is higher than hydrogen consumption in methanol synthesis (0.076 mol s−1).

1. INTRODUCTION 1.1. Hydrogen. Recently, considerable attention has been paid to environmentally friendly and efficient alternative energy sources.1−3 Among various fuels, hydrogen is the leading fuel which can satisfy the prerequisites well because of its especial properties.4 First, hydrogen has the highest calorific value between other common fuels except the nuclear fuels.5 Secondary, during hydrogen combustion no pollutant emissions emitted and its products are only water and a tiny amount of NOx, which can be removed by appropriate methods.6 Hydrogen is used in large quantities as a raw material in petroleum and chemical industry processes such as electricity, ammonia, methanol, oil refining, fuel cell, vehicle engines, power plants, etc.7,8 At present, demand for hydrogen is increasing fast and is higher than annual world production.9 Hydrogen is produced efficiently in large scale by a traditional method like steam reforming, coal gasification, electrolyses, biomass gasification, etc. Currently, efforts are being made to produce hydrogen from innovative approaches including biological and photobiological water splitting, fermentative methods, etc.5 Most of the mentioned methods have some disadvantages, such as requiring a large amount of energy and producing pollutant components, such as CO, CO2, H2S, etc.10 One of the high-quality candidates for hydrogen production in a less polluting way is dehydrogenation of cyclic hydrocarbons (e.g., cyclohexane). 1.2. Dehydrogenation of Cyclohexane. It has been reported that cycloalkanes like cyclohexane, methylcyclohexane, © 2013 American Chemical Society

decaline, etc., are promising hydrogen storage materials due to their specific characteristics including high hydrogen content, high boiling point, etc.11−13 Dehydrogenation is a reversible endothermic reaction limited by chemical equilibrium; its products are hydrogen and aromatics (benzene, toluene, etc) and free from any containments.14 Cyclohexane with chemical formula C6H12 has 7.1 wt % hydrogen capacities, which can be dehydrogenated to produce gaseous hydrogen and condensable benzene.15 Several studies on the cyclohexane reaction are reported which occurred over various catalysts such as nickel, platinum, and palladium, although palladium-based catalyst is more popular than others.16 Benzene is a vital multifunctional raw chemical material that has been used worldwide for production of phenol, styrene, aniline, drugs, dyes, insecticides, plastics, etc.17 Among the many technological methods for benzene production, dehydrogenation of cyclohexane has been studied in this work.18 The necessary heat for this endothermic reaction can be provided by means of one or two exothermic reactions by coupling them in one reactor. 1.3. Coupling Reactors. As environmental controls are growing fast, chemical industries are facing big challenges of developing innovative products and processes for endurance.19 In this field, the chemical engineering community has given special attention to process intensification (PI) known as a Received: October 16, 2012 Revised: February 26, 2013 Published: February 26, 2013 1982

dx.doi.org/10.1021/ef301683j | Energy Fuels 2013, 27, 1982−1993

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used as a raw material for chemical productions including olefins, gasoline, jet fuel, and spray and as a hydrogen carrier in fuel cells.37 There are various sources for DME production such as natural gas, coal, biomass, agricultural residues, crude oil, waste products, residual oil, methanol, etc.38 Dehydration of methanol is the indirect traditional DME production, while a combined synthesis and dehydration of methanol in one reactor at the presence of synthesis gas as feed is a direct method for this purpose. The direct method is more economical compared to the indirect one because of eliminating the cost of methanol purification and higher conversion of methanol.39,40 Reactions in DME production from synthesis gas are methanol formation, dehydration of methanol, and water−gas shift reaction as follows

strategy of improving the energy efficiency at minimum cost and reducing harmful emissions. A novel concept in process intensification is multifunctional reactors used for different purposes of which coupling exothermic and endothermic reactions are more interested.20,21 This type of reactor aims to use energy released by exothermic reaction for proceeding endothermic reaction without any mixing of reactants. Hunter and McGuire were pioneers in coupling endothermic and exothermic reactions without direct heat transfer.22 A review on the PI for methane steam reforming in a thermally coupled membrane separation technology was investigated by Bhat and Sadhukhan.23 Altimari and Bildea studied the design and control of plant systems including coupling of exothermic and endothermic reactions.24 Analysis and optimization of reactor design for coupling the exothermic and endothermic reactions in a fixed bed reactor was presented by Golckler.25 Chen et al. investigated an integrated process for hydrogen production by coupling of gasoline prereforming with autothermal reforming (ATR) in fixed-bed reactors.26 Khademi et al. proposed and optimized methanol synthesis and cyclohexane dehydrogenation in a thermally coupled reactor.27 Rahimpour et al. studied theoretical hydrogen production from coupling of methanol synthesis and cyclohexane dehydrogenation in a hydrogen permselective membrane and dual-membrane thermally coupled reactor.28,29 Ramaswamy et al. used a pseudohomogeneous plug flow model for coupling exothermic and endothermic reactions in directly coupled adiabatic packed bed reactors (DCAR).30 Vakili et al. studied dimethyl ether (DME) synthesis directly in a thermally coupled heat exchanger reactor.31 Farsi et al. investigated simulation and optimization of DME production and cyclohexane dehydrogenation in a thermally coupled dualmembrane reactor.32 Patel et al. developed a distributed mathematical model for methane steam reforming using a thermally coupled membrane reactor containing three channels.33 Simultaneous dehydration of methanol and dehydrogenation of cyclohexane reactions in an optimized thermally coupled reactor was considered by Khademi et al.34 On the basis of research made in the literature, most of the studies in this field were on the coupling of one exothermic and one endothermic reaction. It seems that coupling of two exothermic reactions with one endothermic reaction provides higher energy efficiency than conventional coupling. It made us compare the performance of two reactors. In this study, methanol and DME production are chosen as exothermic reactions for coupling with the endothermic reaction of hydrogenation of cyclohexane. 1.4. Exothermic Reactions. 1.4.1. DME Synthesis. As transportation energy consumption and environmental problems increase around the world, people face an energy shortage and a need to find substitute fuels.35 In this way, many investigations have been performed to verify the suitability of DME as an alternative fuel because of following reasons.36 • DME is a superior candidate for use in diesel engines for its high octane number of about 55−60 and instantaneous vaporization as injected in the cylinder. • DME combustion is free from any particulate matter and greenhouse gases such as NOx, CO2, and SOx due to its low carbon to hydrogen ratio and high oxygen content of about 34.5%. • The low boiling point of DME is a good condition for its storage and transportation. Besides, DME has a wide range of applications in heating and home cooking instead of liquefied petroleum gas (LPG) and is

CO + 2H 2 ↔ CH3OH

ΔH298K = − 90.55 kJ/mol

(1)

CO2 + 3H 2 ↔ CH3OH + H 2O

ΔH298K = − 49.43 kJ/mol (2)

2CH3OH ↔ CH3OCH3 + H 2O

ΔH298K = − 21.003 kJ/mol (3)

These reactions occur over a bifunctional catalyst which contains two active sites: one is applied for methanol synthesis and the another for DME formation.41 1.4.2. Methanol Production. Methanol is a chemical commodity produced, traded, and transported on a large industrial scale in the world.42 Demand for methanol is growing rapidly due to its applications in synthesis of biodiesel, DME, and methyl tert-butyl ether and also as a near-zero emission alternative fuel, hydrogen carrier, solvent, etc.43 Another important use of methanol is in novel processes like direct methanol fuel cell, microchannel methanol steam reforming, etc., for production of hydrogen.44,45 Methanol is mainly recovered from natural gas, although it can be produced from pyrolysis of organic materials or by Fisher−Troupsch from syngas.46 Three main reactions occur in methanol synthesis: hydrogenation of CO, carbon dioxide hydrogenation and reverse water gas shift reaction. These reactions were proceed over various Cu/Zn-based catalyst including Cu/Zn/Al, Cu/Zn/Cu, Cu/Zn/Zr, Cu/Zn/ Cr, Cu/Zn/Ga, and Cu/Zn/Ge, in which Cu/Zn/Al has been more popular.47−49 1.5. Objective. In the present work, simultaneous production of hydrogen, methanol, and DME has been investigated theoretically in a thermally double-coupled reactor (TDCR). On the endothermic side of the TDCR dehydrogenation of cyclohexane takes place, and methanol dehydration and indirect synthesis of DME are chosen for the exothermic sides. Its novelty is coupling two exothermic with one endothermic reaction and production of three important products simultaneously (methanol, DME, and hydrogen). Also, in order to prove the suitability of this suggestion, a one-dimensional heterogeneous catalytic reaction mathematical model is applied for steady state simulation. Results of TDCR are compared with a conventional methanol synthesis reactor (CMR) and thermally coupled reactor (TCR) with similar diameter and length to show the possibility of employing one multitubular reactor instead of two or three reactors for production of methanol, DME, and benzene.

2. PROCESS DESCRIPTION 2.1. Conventional Methanol Synthesis Reactor (CMR). A conventional methanol synthesis reactor (CMR) is a vertical shell and tube heat exchanger. Vertical tubes are packed with CuO/ZnO/Al2O3 catalyst and surrounded by boiling water. 1983


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Table 2. Characteristics of TDCR 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)

3.8 × 10−2 8.52 × 10−2 9.75 × 10−2 7.022

Table 3. Reaction Rate Constants for DME Synthesis Reactions A k1 k2 k3 KCO KH2 KH2 KCH3OH

Figure 1. TDCR configuration in cocurrent mode.

DME synthesis (exothermic reaction) feed composition (mole fraction) CO CO2 DME CH3OH H2O H2 N2 CH4 inlet temperature (K) inlet pressure (bar) inlet flow rate in each tube (mol s−1) typical properties of catalyst number of three concentric tubes particle diameter (mm) density of catalyst bed (Kg m−3 porosity dehydrogenation of C6H12 (endothermic side) gas phase feed composition C6H12 C6H6 H2 Ar inlet temperature (K) inlet pressure (bar) particle diameter (m) bed void fraction total flow rate (mol s−1)

−1

−3

1.828 × 10 mol g h bar 0.4195 × 102 mol g−1 h−1 bar−3 1.939 × 102 mol g−1 h−1 bar−1 8.252 × 10−4 bar −1 2.1 × 10−3 bar −1 0.1035 bar −1 1.726 × 10−4 bar −1

−1

)

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

Table 4. Reaction Rate Constants, Adsorption Equilibrium Constants, and Reaction Equilibrium Constants for Methanol Synthesis

Table 1. Operating Conditions for DME Synthesis (one of the exothermic sides) and Dehydrogenation of Cyclohexan (endothermic side) in TDCR parameter

B ( J mol −1

3

A (mol kg−1 s−1 bar

0.1716 0.0409 0.0018 0.003 0.0002 0.4325 0.316 0.044 493 50 0.6

)

(4.89 ± 0.29) × 10 (1.09 ± 0.07) × 105 (9.64 ± 7.30) × 106 A (bar −1) 7

k1 k2 k3

value

−1/2

B ( J mol−1) −63 000 ± 300 −87 500 ± 300 −152 900 ± 6800 B ( J mol−1)

KCO KCO2

(2.16 ± 0.44) × 10−5 (7.05 ± 1.39) × 10−7

46 800 ± 800 61 700 ± 800

(KH2O/K1/2 H2 )

(6.37 ± 2.88) × 10−9

84 000 ± 1400

KP1 KP2 KP3

A (K)

B (K)

5139 3066 −2073

12.621 10.592 −2.029

Table 5. Reaction Rate Constant, Adsorption Equilibrium Constant, and Reaction Equilibrium Constant for Cyclohexane Dehydrogenation

2962 ϕ5 × 5 1200 0.455

k KB KP

A

B (K)

0.221 mol m−3 Pa−1 s−1 2.03 × 10−10 Pa−1 4.89 × 1035 Pa3

−4270 6270 3190

2.2. Thermally Double Coupled Reactor (TDCR). Figure 1 illustrates the TDCR in multitubular configuration in cocurrent mode. This system consists of 2962 three concentric tubes reactor. In the middle tube, endothermic reaction of cyclohexane dehydrogenation occurs, and in the inner and outer tubes exothermic reactions of methanol and DME synthesis take place, respectively. Generated heat from the inner and outer tubes transfers to the middle tube. DME synthesis reactions occur over the same catalyst as a conventional methanol reactor (Cu/Zn/Al2O3) and dehydration of methanol process (γ-Al2O3). Pt/Al2O3 is used as a catalyst for dehydrogenation of the cyclohexane reaction. The properties and input data of TDCR are listed in Tables 1 and 2. The other properties of the methanol synthesis process in TDCR are the same as the CMR. 2.3. Thermally Coupled Reactor (TCR). In the thermally coupled reactor (TCR), a catalytic dehydrogenation of cyclohexane

0.1 0.0 0.0 0.9 503 20 3.55 × 10−3 0.39 0.5

Syngas is fed to the tubes from the top of the reactor, and methanol synthesis reactions take place. The generated heat of exothermic reactions is transferred to the boiling water, and steam is produced. The schematic and specifications of CMR and catalyst characteristics are described extensively in previous publications.27−29 1984


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Figure 2. Differential element along the axial direction inside the reactors of TDCR.

to benzene reaction is used instead of the cooler water in the shell side of the methanol synthesis reactor. Catalytic dehydrogenation of cyclohexane to benzene is assumed to take place in the shell, whereas methanol synthesis occurs inside the tube, and a fixed bed of different catalysts is used for both sides. Heat is transferred continuously from the exothermic reaction to the endothermic reaction. The feed composition of the endothermic side for TCR is similar to TDCR, but the total feed flow rate of the endothermic side of TDCR and TCR is different (0.5 vs 0.1 mol s−1, respectively).26

3. REACTION SCHEME AND KINETICS 3.1. Direct DME Synthesis (Exothermic Reaction). The rate equations of direct DME synthesis reactions are as follows50 k1 fCO fH2 (1 − β1) 2

rCO =

(1 + K CO fCO + K CO2 fCO + K H2 fH )3 2

2

2

(1 + K CO fCO + K CO2 fCO + K H2 fH )4 2

rDME =

β2 =

β3 =

(4)

k 2 fCO fH3 (1 − β2)

rCO2 =

β1 =

2

2

(5)

k 3 fCH OH (1 − β3) 3

K CH3OH fCH OH )2

(6)

2

(7)

(1 +

3

Figure 3. (a) Variation of components mole fraction in (a) methanol synthesis (inner exothermic side) with reactor dimensionless length. (b) Variations of components mole fraction in DME synthesis (outer exothermic side) with reactor dimensionless length. (c) Variations of components mole fraction in dehydrogenation of cyclohexane (middle endothermic side) with reactor dimensionless length.

fCH OH 3

K f 1 fCO fH2

fCH OH fH O 3

CH4, and CH3OH. The following equations have been considered as the methanol production reactions

2

K f 2 fCO fH3 2 2

(8)

CO + 2H 2 ↔ CH3OH

fDME fH O

CO2 + 3H 2 ↔ CH3OH + H 2O

2

2 K f 3 fCH OH 3

ΔH298K = − 90.55 kJ/mol

(9) CO2 + H 2 ↔ CO + H 2O

where f i and Kf j are the fugacity of component i and equilibrium constant of reaction j, respectively. Kinetic parameters are tabulated in Table 3. 3.2. Methanol Synthesis Reaction (Exothermic Reaction). Methanol synthesis components include CO, H2, CO2, H2O,

(10)

ΔH298K = − 49.43 kJ/mol (11)

ΔH298K = 41.12 kJ/mol

(12)

The kinetic model is valid in the temperature range of 495−533 K and pressure range of 5−8 MPa. In this study, the rate expressions have been chosen from Graaf et al.51 Information about the kinetics of methanol synthesis will be provided with 1985


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Figure 4. Variations of velocity of gases along reactor axis for methanol synthesis, DME synthesis, and cyclohexane dehydrogenation.

the rate equations and equilibrium rate constants. The following expressions are the rate equations for hydrogenation of CO and CO2 and reverse water−gas shift reaction. Table 4 represents the reaction rate constants, adsorption equilibrium constants, and reaction equilibrium constants for methanol synthesis. r1 =

1/2 k1K CO[ fCO fH3/2 − fCH OH / fH2 KP1] 2

3

(1 + K CO fCO + K CO2 fCO )[ fH1/2 + (K H2O/K H1/2 ) fH O ] 2 2

2

2

(13)

r2 =

k 2K CO2[ fCO fH3/2 2

3/2 fCH OH fH O / fH2 KP 2] 3 2

−

(1 + K CO fCO + K CO2 fCO )[ fH1/2 + (K H2O/K H1/2 ) fH O ] 2 2

2

2

(14)

r3 =

k 3K CO2[ fCO fH − fH O fCO /KP 3] 2

(1 + K COfCO +

2

2

K CO2fCO )[fH1/2 2 2

) fH O ] + (K H2O/K H1/2 2 2

(15)

3.3. Dehydrogenation of Cyclohexane (Endothermic Side). The reaction scheme for dehydrogenation of cyclohexane is as follows27 C6H12 ↔ C6H6 + 3H 2

ΔH298K = 206.2 kJ/mol (16)

In the current study, the following rate equation is used for the above reaction rC =

Figure 5. (a) Variations of pressure along reactor axes for methanol synthesis. (b) Variations of pressure along reactor axis for DME synthesis. (c) Variations of pressure along reactor axis for cyclohexane dehydrogenation.

k(KPPC/PH32 − PB) 1 + (KBKPPC /PH32)

(17)

where k, KB, and KP are the reaction rate constant, adsorption equilibrium constant, and reaction equilibrium constant, respectively, that are listed in Table 5. Pi is the partial pressure of component i in Pa.

(4) Axial diffusion of heat and mass is negligible in comparison with radial diffusion. (5) Bed porosity in axial and radial directions is considered to be constant. Figure 2 shows the differential element along the axial direction inside the reactors. The required heat for cyclohexane dehydrogenation is provided by a couple of exothermic reactions, methanol and DME syntheses. The differential equations describing mole and energy balances in the axial direction of a thermally double-coupled reactor are shown in Table 6.

4. MATHEMATICAL MODEL The one-dimensional heterogeneous catalytic reaction for the mathematical model of TDCR is based on the following assumptions. (1) The model is investigated at steady-state conditions. (2) Gas phases are ideal. (3) Plug flow mode is employed in each side of the reaction. 1986


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4.1. Auxiliary Correlations. In order to solve the mentioned set of differential equations, auxiliary correlations including physical properties of components, mass and heat transfer coefficients, and the Ergun equation should be added, see Table 7. In this study, the Ergun equation is used for Table 7. Auxiliary Correlations parameter

equation

Cp = a + bT + cT + dT

mixture heat capacity viscosity of reaction mixtures mixture thermal conductivity mass transfer coefficient between gas and solid phases

based on local compositions based on local compositions based on local compositions

2

Sci = Di m =

52 53

ρugd p μ μ ρDi m × 104 1 − yi ∑i ≠ j

Dij

10 T Dij =

54

yi

7 3/2

In eqs 18 and 19, η is the effectiveness factor obtained from a dusty gas model. Also, ysi,j and Tsj are the mole fraction of component i in the solid phase and solid temperature of j side of the reactor, respectively. In eqs 20−23, Fi,j and Tgj are the molar flow rate of component i in the fluid phase and j side of the reactor and fluid temperature of j side of the reactor, respectively.

3

kgi = 1.17Re0.42Sci0.67ug × 103

Re =

Figure 6. (a) Temperature pattern of TDCR in the three sides. (b) Thermal behavior of the methanol synthesis process in TDCR in comparison with the ones in CMR and TCR. (c) Temperature profile of methanol synthesis and cyclohexane dehydrogenation in TDCR and TCR.

ref

component heat capacity

55

1/Mi + 1/Mj

3/2 P(vc3/2 i + vcj )

overall heat transfer coefficient

A ln(Do /Di) A 1 1 1 = + i + i U hi 2πLK w Ao ho

heat transfer coefficient between gas phase and reactor wall

h ⎛ Cpμ ⎞ ⎟ ⎜ Cpρμ ⎝ K ⎠

Ergun equation

(1 − ε)ug2 (1 − ε)2 dP = 150 3 2 + 1.75 dz ε dp ε 3d p2

2/3

=

0.407 0.458 ⎛⎜ μ ⎞⎟ ⎟ ⎜ εB ⎝ ρud p ⎠

56

calculating pressure drop through the catalytic bed. In this equation, dP is the pressure gradient, Q is volumetric flow rate and dp is the diameter of a particle.

Table 6. Mole and Energy Balances in the Axial Direction of Thermally Double Coupled Reactor mass and energy balances equations

avcjkgi , j(yig, j − yis, j ) + ηri , jρb = 0

solid phase (both exothermic sides and endothermic side)

(18)

N

avhf (T jg − T js) + ρb ∑ ηri , j(ΔHf , i) = 0 i=1

fluid phase (both exothermic sides)

1 dFt , j + avcjkgi , j(yis, j − yig, j ) = 0 A c dz −

fluid phase (endothermic side)

(20)

⎞ ⎞ ⎛ π Dj ⎛ g ) 1 g d(FT Cp , i t Uw⎜T2 − T1g ⎟ = 0 + avhf ⎜T js − T jg ⎟ + Ac dz Ac ⎠ ⎝ ⎠ ⎝

1 dFt , j + avcjkgi , j(yis, j − yig, j ) = 0 A c dz

−

(19)

(22)

⎛ ⎞ ⎛ ⎞ π Dj ) 1 g d(FT Cp , i t U1 − 2⎜T jg − T1g ⎟ + avhf ⎜T js − T jg ⎟ − Ac dz Ac ⎝ ⎠ ⎝ ⎠ −

π Dj Ac

⎞ ⎛ U2 − 3⎜T jg − T3g ⎟ = 0 ⎠ ⎝

z = 0, yig, j = yig0, j , Tj = Tjg0 , P jg = P jg0 , j = 1, 2, 3

boundary conditions 1987

(21)

(23) (24)


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Figure 7. (a) Heat generation in methanol exothermic side in TCR and TDCR. (b) Consumed heat in the endothermic side of TCR and TDCR.

Figure 8. (a) Rate of reactions in the methanol exothermic side of TCR and TDCR. (b) Rate of reaction in the endothermic side of TCR and TDCR.

4.2. Numerical Solution. The developed model is composed of the set of ordinary differential equations (ODEs) related to mass and energy conservation rules as well as nonlinear algebraic equations of the kinetic model, auxiliary and hydrodynamic correlations. A backward finite difference approximation is applied to solve the set of ODEs. Therefore, the ODEs are changed into a set of nonlinear algebraic equations. The length of each reactor is divided into 100 separate segments, and the Gauss−Newton method is used to solve the obtained set of nonlinear algebraic equations in each segment for both sides, simultaneously. This procedure is repeated for all nodes in the reactor; the results of each node are used as the inlet conditions for the following node.

yields as well as cyclohexane and carbon monoxide conversions, the following definitions have been used hydrogen recovery yield =

methanol yield =

CO conversion =

FC6H12,in

(26)

FCH3OH,out FCO + FCO2,in

cyclohexane conversion =

6. RESULT AND DISCUSSIONS 6.1. Model Validation. Model validation has been carried out by comparison between simulation results and conventional methanol synthesis reactor in industrial scale by Rahimpour et al.28 Fortunately, good agreement was achieved between the proposed model and experimental plant data, showing the feasibility of this novel process. In this paper, we compared the results of thermally double-coupled reactor (TDCR) simulation with the validated conventional methanol reactor (CMR) and thermally coupled reactor (TCR) in which methanol synthesis is coupled with cyclohexane dehydrogenation. The following sections will present the various observed behaviors of TDCR via some figures. In order to calculate methanol and hydrogen

FH2,out

(27)

FC6H12,in − FC6H12,out FC6H12,in

(28)

FCO,in − FCO,out FCO,in

(29)

6.2. Molar Behavior. Figure 3a depicts the simultaneous mole fraction plot for products of methanol synthesis including H2, CO, CO2, H2O, and CH3OH flowing through the inner side of TDCR. As seen, hydrogen has the highest mole fraction profile. The CO2 mole fraction decreases slightly along the reactor, while each of the other components has a curved profile lower than it. The mole fraction of components in DME synthesis versus reactor length is presented in Figure 3b. According to this graph, hydrogen has the highest profile (like the inner exothermic side). Figure 3c gives plots for 1988


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Figure 9. (a) Total molar flow rate of exothermic and endothermic sides of TDCR. (b) Hydrogen flow rate of exothermic and endothermic sides of TDCR.

Figure 10. (a) Hydrogen molar flow rate of the endothermic side in TDCR and TCR. (b) Benzene and cyclohexane molar flow rates of the endothermic side in TDCR and TCR.

component mole fractions in the endothermic side: cyclohexane, hydrogen, and benzene. Due to the equation for the cyclohexane dehydrogenation reaction, it is clear that with increasing mole fraction of benzene and hydrogen along the reactor the mole fraction of cyclohexane decreases. The hydrogen trajectory is higher than benzene owing to its higher stoichiometric coefficient in the cyclohexane dehydrogenation reaction. It is well known that hydrogen demand is increasing fast, so planning an industrial process with higher hydrogen production can be an important victory. In addition, Figures 5 and 6 present velocity and pressure along the TDCR reactor axis in methanol synthesis, DME synthesis, and cyclohexane dehydrogenation sides. As shown in Figures 4 and 5, the velocity of gases in three side of TDCR is near 1 m s−1 and the pressure drop in the methanol synthesis, cyclohexane dehydrogenation, and DME synthesis sides is 2.49, 3.56, and 1.06 bar, respectively. 6.3. Thermal Behavior. Figure 6a demonstrates the temperature pattern of TDCR in the three sides. Thermal driving force is created by a temperature difference between two energy sources. Thus, if an exothermic reaction is coupled with an endothermic one, the temperature of the exothermic reaction must be higher than the endothermic one; this fact is clearly shown in Figure 6a in which both exothermic profiles have almost the same trend, higher than the endothermic one. Generated heat of both exothermic sides (methanol and DME syntheses) is used for the proceeding endothermic reaction as well as heating the mixtures of all three sides. At the reactor entrance, the generated heat is less than the consumed heat, causing the temperature of the endothermic side to fall and a

cold spot develop after that condition is changed so that an increase along the rest of the reactor length occurs. Figure 6b illustrates the thermal behavior of the methanol synthesis process in TDCR in comparison with the ones in CMR and TCR. As shown in this figure, the temperature of the methanol reactor in TDCR is higher than the methanol reactor in TCR and is near to CMR. This is due to generation of heat in the DME side of TDCR. Figure 6c depicts the temperature profiles of methanol synthesis and cyclohexane dehydrogenation in TDCR and TCR. As can be seen, TDCR works in the higher temperature range compared to TCR because of receiving heat from two exothermic sides. Figure 7a and 7b illustrates heat generation of the methanol exothermic side in TCR and TDCR and consumed heat in the endothermic side of TCR and TDCR. As shown in this figure, heat generation and heat consumed in TDCR is higher than TCR due to the higher temperature (see Figure 6b and 6c) and rate of TDCR in comparison with TCR. Finally, the heat transfer coefficient in the methanol synthesis, cyclohexane dehydrogenation, and DME synthesis sides is 1070, 220, and 2630 W m−2 K−1 in TDCR, respectively. 6.4. Rate Behavior. Figure 8a presents variations in rates of CO hydrogenation, CO2 hydrogenation, and water−gas shift in methanol synthesis in the exothermic side of TCR and TDCR. Results indicate that the rate of reactions in TDCR is higher than TCR because of the higher temperature of methanol reaction in TDCR in comparison with TCR. In addition, with going ahead along the reactor length, the rate 1989


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of reactions decreases due to equilibrium conditions at the end of the reactor. Also, Figure 8b gives changes in the rate of the endothermic side of TCR and TDCR. As shown in this figure, replacing TCR with TDCR increases the rate of cyclohexane dehydrogenation. 6.5. Molar Flow Rate Behavior. The total molar flow rate of the exothermic and endothermic sides of TDCR is shown in Figure 9a. It is observed that the total molar flow rates of both exothermic sides decrease and the endothermic one increases along the reactor due to reaction stoichiometries. Variations of flow rate affect the velocity and pressure drop in three sides. Figure 9b shows the hydrogen flow rate along all sides of TDCR, which has the same trend as total molar flow rate. As shown in Figure 9b, for each of the three concentric tubes, hydrogen molar flow rate decreases from 0.4217 to 0.3457 mol s−1 and 0.262 to 0.148 mol s−1 in the methanol and DME synthesis sides due to consumption in reactions 1, 2, 10, and 11, respectively. In the hydrogenation of cyclohexane, hydrogen molar flow rate increases along the TDCR because of its production in reaction 11 and the flow rate of hydrogen at the end of the endothermic side is equal to 0.1 mol s−1. Hydrogen production in the endothermic side of TDCR is higher than hydrogen consumption in methanol synthesis due to the high feed flow rate of cyclohexane dehydrogenation. Therefore, it can be said that the produced hydrogen in the endothermic side can be utilized as feed in methanol synthesis. Figure 10a presents the hydrogen molar flow rate of the endothermic side in TDCR and TCR. It can be understood that using double exothermic reactions in a multitubular reactor instead of a single one in a shell and tube reactor makes better conditions for hydrogen production rate in the endothermic side by increasing the hydrogen production rate from 0.0235 to 0.1 mol s−1. Figure 10b shows similar results for benzene and

Figure 11. (a) Methanol flow rate in TDCR, TCR, and CMR. (b) Methanol yield in TDCR, TCR, and CMR.

Figure 12. (a) Variations of cyclohexane conversion with total flow rate of feed in the endothermic side. (b) Variations of hydrogen flow rate with total flow rate of feed in the endothermic side. (c) Variations of CO conversion with total flow rate of feed in the endothermic side. (d) Variations of methanol yield with total flow rate of feed in the endothermic side. 1990


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cyclohexane flow rates. This figure shows that the benzene production rate increased in TDCR in comparison with TCR (increasing from 0.0078 to 0.0472 mol s−1) in each of the three concentric tubes. Of course, it must be noted that the feed flow rate of cyclohexane dehydrogenation in TDCR is higher than TCR for consumption of the generated heat in the two exothermic sides of TDCR. 6.6. Yield and Conversion Changes. Profiles of methanol flow rate in TDCR, TCR, and CMR are shown in Figure 11a. There is no considerable difference between methanol flow rate at the end of the reactor for TDCR and CMR. The methanol production profile in TDCR is higher than that of TCR due to its higher temperature as a consequence of using the doublecoupling concept, and therefore, the rate of methanol synthesis reaction in TDCR is higher that of than TCR. Figure 11b presents the methanol yield in TDCR, TCR, and CMR. Results of this figure are similar to Figure 11a. Methanol reaches a yield of 0.3735 and 0.362 in TDCR and TCR, respectively. However, in an investigation on the experimental design, commercial viability and economic feasibility of the proposed configuration are necessary to commercialize the process. Of course, it must be noted that TDCR is the coupling of two exothermic reactions with an endothermic reaction in one multitubular reactor instead of two or three shell and tube reactors for production of methanol, DME, and benzene. 6.7. Effect of Endothermic Side Feed Flow Rate on TDCR Performance. The TDCR performance has been studied for different molar flow rates of the endothermic stream. The variation range for the molar flow rate of the endothermic side stream was 0.35−0.6 mol s−1. All other parameters are kept at their base case values. Figure 12a illustrates variations of cyclohexane conversion with the endothermic side feed flow rate in each of the three concentric tubes. Cyclohexane conversion significantly decreases from 93.8% to 55.5% by increasing the endothermic side feed flow rate from 0.35 to 0.6 mol s−1. Decreasing cyclohexane conversion is an obvious consequence of the fact that the amount of catalyst on the endothermic side is not enough for higher flow rates. As can be seen in Figure 12b, by increasing the molar flow rate of the endothermic stream, the hydrogen flow rate increases from 0.0997 to 0.101 mol s−1. Figure 12c and 12d illustrate how CO conversion and methanol yield change with increasing flow rate of the endothermic stream from 0.35 to 0.6 mol s−1. CO conversion and methanol yield slightly decrease from 63.81% to 63.4% and 0.3718 to 0.3685, respectively. This is due to the lower axial temperature profile and consequently lower rate of reactions in the exothermic sides.

thermally coupled reactor (TCR) in which methanol synthesis is coupled with cyclohexane dehydrogenation. Results indicated that methanol reaches a yield of 0.3735 and 0.362 in TDCR and TCR, respectively. Using double exothermic reactions instead of a single one in a multitubular reactor makes better conditions for methanol and hydrogen production rate in the endothermic side with 8.02% and 325.5%, respectively. In addition, hydrogen production in the endothermic side of TDCR is higher than hydrogen consumption in methanol synthesis. Therefore, the produced hydrogen in the endothermic side can be utilized as feed in methanol synthesis. Simulation results showed that by increasing the molar flow rate of the endothermic stream cyclohexane conversion decreases significantly but methanol yield and CO conversion decrease slightly, whereas hydrogen production in the endothermic side increases slightly.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +98 711 2303071. Fax: +98 711 6287294. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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7. CONCLUSIONS In this paper, methanol synthesis and DME production coupled with dehydrogenation of cyclohexane to benzene in a thermally double-coupled reactor (TDCR) by means of indirect heat transfer and simultaneous production of pure hydrogen, methanol, DME, and benzene was studied by a one-dimensional heterogeneous catalytic reaction model. The advantages of this integrated catalytic reactor include achieving multiple products (methanol and DME) from syngas, production of hydrogen, and the possibility of achieving a higher degree of in situ energy integration between the coupled endothermic dehydrogenation reaction and the exothermic reactions. Simulation results are compared with corresponding predictions for an industrial methanol reactor (CMR) and 1991

NOMENCLATURE av = specific surface area of catalyst pellet, m2 m−3 Ac = cross-section area of each tube, m2 Ai = inside area of inner tube, m2 Ao = outside area of inner tube, m2 C = total concentration, mol m−3 Cp = specific heat of the gas at constant pressure, J mol−1 dp = particle diameter, m Di = tube inside diameter, m Do = tube outside diameter, m Dij = binary diffusion coefficient of component i in j, m2s−1 Dim = diffusion coefficient of component i in the mixture, m2s−1 Do = tube outside diameter, m f i = partial fugacity of component i, bar Ft = 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 reactor wall in the exothermic side, W m−2 K−1 ho = heat transfer coefficient between fluid phase and reactor wall in the endothermic side, W m−2 K−1 ΔHf,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 the fluid phase, W m−1 K−1 Ki = adsorption equilibrium constant for component i, bar −1 Kp = equilibrium constant for the dehydrogenation reaction, Pa3 Kpi = equilibrium constant based on the partial pressure for component i in the methanol synthesis reaction Kw = thermal conductivity of the reactor wall, W m−1 K−1 L = reactor length, m Mi = molecular weight of component i, g mol−1 N = number of components P = total pressure, bar Pi = partial pressure of component i, Pa dx.doi.org/10.1021/ef301683j | Energy Fuels 2013, 27, 1982−1993

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r1 = rate of reaction for hydrogenation of CO in methanol synthesis, mol kg−1s−1 r2 = rate of reaction for hydrogenation of CO2 in methanol synthesis, mol kg−1s−1 r3 = rate of reversed water-gas shift reaction in methanol synthesis, mol kg−1s−1 rCO = rate of reaction for hydrogenation of CO, mol kg−1 s−1 rCO2 = rate 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, m Re = Reynolds number Sci = Schmidt number of component T = temperature, K u = superficial velocity of the fluid phase, m s−1 ug = linear velocity of the fluid phase, m s−1 U = overall heat transfer coefficient between exothermic and endothermic sides, W m−2 K−1 vci = critical volume of component i, cm3 mol−1 yi = mole fraction of component i Z = axial reactor coordinate, m Greek Letters

μ = viscosity of the fluid phase, kg m−1 s−1 P = density of the fluid phase, kg m−3 ρb = density of catalytic bed, kg m−3 T = tortuosity of catalyst

Superscripts

g = in bulk gas phase s = at surface catalyst Subscripts

0 = inlet conditions i = chemical species j = reactor side

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