6 thermally coupled reactors for methanol synthesis - an...

23
6 Thermally coupled reactors for methanol synthesis - An exergetic approach 6.1 Introduction An alternative to the petroleum fuels is today's need due to their impact on global economy and depletion of sources. Crude oil and natural gas reserves are located in politically unstable regions hence becomes a threat to nation’s energy security. There are various alternate fuels like ethanol, methanol, hydrogen, coal gas etc. emerged as promising one. Due to high octane number i.e. 108.7 methanol can be mixed in gasoline. Dimethyl ether is produced by dehydration of methanol which can be used as diesel fuel substitute due to high cetane number i.e. 55. Though today methanol is produced by using natural gas, renewable sources are also available which can be transformed into synthesis gas. Biomass, municipal waste, industrial waste and carbon dioxide are the renewable sources for the production of methanol. Apart from fuel, methanol is also used as hydrogen carrier in fuel cell, in production of biodiesel, feedstock for formaldehyde, acetic acid, olefin etc. In the present study exergy analysis of various thermally coupled reactors are carried out. Methanol synthesis is exothermic reaction and dehydrogenation of cyclohexane or methyl cyclohexane is endothermic reaction. Organic chemical hydrides are prominent source of hydrogen because they consisting of 6-8 % (wt) hydrogen. It can also act as hydrogen storage to produce hydrogen without emitting pollutants (Kumar et. al., 2009). 6.2 Production of Methanol Feed for methanol is synthesis gas, which contains carbon monoxide, carbon dioxide and hydrogen. Natural gas is used worldwide for the production of synthesis gas. It is carried out in two steps – production of synthesis gas and synthesis of methanol. Natural gas is desulfurized to avoid catalyst poisoning and then fed to the catalytic reformer with steam. Conventional steam reforming is a widely practiced method for synthesis gas production.

Upload: lamduong

Post on 05-Mar-2018

232 views

Category:

Documents


4 download

TRANSCRIPT

Page 1: 6 Thermally coupled reactors for methanol synthesis - An ...shodhganga.inflibnet.ac.in/bitstream/10603/90267/9/17_exergy... · Thermally coupled reactors for methanol synthesis -

6

Thermally coupled reactors for methanol synthesis

- An exergetic approach 6.1 Introduction

An alternative to the petroleum fuels is today's need due to their impact on global economy

and depletion of sources. Crude oil and natural gas reserves are located in politically

unstable regions hence becomes a threat to nation’s energy security. There are various

alternate fuels like ethanol, methanol, hydrogen, coal gas etc. emerged as promising one.

Due to high octane number i.e. 108.7 methanol can be mixed in gasoline. Dimethyl ether is

produced by dehydration of methanol which can be used as diesel fuel substitute due to

high cetane number i.e. 55. Though today methanol is produced by using natural gas,

renewable sources are also available which can be transformed into synthesis gas. Biomass,

municipal waste, industrial waste and carbon dioxide are the renewable sources for the

production of methanol. Apart from fuel, methanol is also used as hydrogen carrier in fuel

cell, in production of biodiesel, feedstock for formaldehyde, acetic acid, olefin etc. In the

present study exergy analysis of various thermally coupled reactors are carried out.

Methanol synthesis is exothermic reaction and dehydrogenation of cyclohexane or methyl

cyclohexane is endothermic reaction. Organic chemical hydrides are prominent source of

hydrogen because they consisting of 6-8 % (wt) hydrogen. It can also act as hydrogen

storage to produce hydrogen without emitting pollutants (Kumar et. al., 2009).

6.2 Production of Methanol

Feed for methanol is synthesis gas, which contains carbon monoxide, carbon dioxide and

hydrogen. Natural gas is used worldwide for the production of synthesis gas. It is carried

out in two steps – production of synthesis gas and synthesis of methanol. Natural gas is

desulfurized to avoid catalyst poisoning and then fed to the catalytic reformer with steam.

Conventional steam reforming is a widely practiced method for synthesis gas production.

Page 2: 6 Thermally coupled reactors for methanol synthesis - An ...shodhganga.inflibnet.ac.in/bitstream/10603/90267/9/17_exergy... · Thermally coupled reactors for methanol synthesis -

Exergy Analysis of Nitric Acid, Ethylene Oxide/Ethylene Glycol Processes and Methanol Reactor

Chapter-6 Page 143

Synthesis gas is cooled and compressed before sending to methanol synthesis reactor.

Following reactions takes place in reformer

1. CH4 + H2O ⇌ CO + 3 H2 ΔHR,298 = +206 kJ/mol

2. CO + H2O ⇌ CO2 + H2 ΔHR,298 = - 41 kJ/mol

Methanol synthesis is exothermic and accomplished by following reactions

Hydrogenation of carbon monoxide

3. CO + 2H2 ↔ CH3OH ΔHR,298 = -90.55 kJ/mol

Hydrogenation of carbon dioxide

4. CO2 + 3H2↔CH3OH + H2O Δ HR,298 = -49.43 kJ/mol

Reverse water gas shift reaction

5. CO2 + H2 ↔ CO + H2O Δ HR,298 = +41.12 kJ/mol

According to Le Chatelier's principle, higher methanol yield is obtained at higher pressure

and lower temperature. A commercial CuO/ZnO/Al2O3 catalyst is used for synthesis

reaction. The chemical equilibrium limits the conversions. Methanol synthesis reactor is

multi-tubular reactor working like shell and tube heat exchanger. The catalyst is placed in

tubes and water is placed in shell. Heat generated in the reaction is taken out by boiling

water to produce steam. The temperature in the reactor is controlled by steam pressure

(Fundamentals of methanol synthesis, 2015).

A temperature rise must be controlled in methanol synthesis reactor to get good

equilibrium value as well as to control catalyst activity. Maximum conversion of CO and

CO2 can give maximum methanol yield. Product gases from reactor come out at 523.15-

543.15 K which exchange heat with incoming synthesis gas. Further cooling is required

before sending product gas into the separator. Crude methanol is separated from the

unreacted gas. This gas is compressed and recycled back to the reactor. A small amount of

gas is purged to maintain the concentration of inert components in the reactor. The crude

methanol is distilled to get pure methanol (Fig.6.1)

Page 3: 6 Thermally coupled reactors for methanol synthesis - An ...shodhganga.inflibnet.ac.in/bitstream/10603/90267/9/17_exergy... · Thermally coupled reactors for methanol synthesis -

Exergy Analysis of Nitric Acid, Ethylene Oxide/Ethylene Glycol Processes and Methanol Reactor

Chapter-6 Page 144

Fig. 6.1 Production process of methanol

6.3 Production of Hydrogen

Steam methane reforming (SMR) process is most commercially used process for the

hydrogen production. A reformer is used in for synthesis reaction. Methane and steam are

used as feed to the reformer and heat is provided to the endothermic reaction by burning

extra methane along with recycle gas from the separator (Fig.6.2).

Fig. 6.2 Production process of hydrogen

Water gas shift reaction takes place in gas shift reactor for further hydrogen yield. Reaction

1 take place in the reformer and reaction 2 takes place in water gas shift reactor.

Page 4: 6 Thermally coupled reactors for methanol synthesis - An ...shodhganga.inflibnet.ac.in/bitstream/10603/90267/9/17_exergy... · Thermally coupled reactors for methanol synthesis -

Exergy Analysis of Nitric Acid, Ethylene Oxide/Ethylene Glycol Processes and Methanol Reactor

Chapter-6 Page 145

Energy produced by combustion of methane is the source of heat for the reformer. Though

water gas shift reaction is exothermic, heat produced cannot be utilized in the process due

to low temperature.

6.4 Methanol Synthesis Reactor

Synthesis reactor is a core of methanol production process. Conversion of synthesis gas per

pass is low due to equilibrium nature of the reaction. The Higher temperature is required at

the initial part of the reactor for higher kinetic constant and lower temperature is required

at end part to increase thermodynamic equilibrium conversion value. (Fig. 6.3) (Khademi

et al., 2009a)

Fig. 6.3 Temperature profile in methanol reactor (Kordabadi and Jahanmiri, 2005)

Methanol reactor is a multi-tubular reactor having exothermic reaction inside tubes and

water heating in the shell as shown in Fig. 6.4. For the better performance of reactor, the

entropy generation should be minimized. Various reactor designs have been proposed

during last decade. Thermally coupled reactor is used to utilize heat generated by the

exothermic reaction by an endothermic reaction. Points to be considered while designing

of the reactor is – temperature and pressure difference between endothermic and

exothermic reaction, phase of both reactions and rate of reaction. When exothermic

Page 5: 6 Thermally coupled reactors for methanol synthesis - An ...shodhganga.inflibnet.ac.in/bitstream/10603/90267/9/17_exergy... · Thermally coupled reactors for methanol synthesis -

Exergy Analysis of Nitric Acid, Ethylene Oxide/Ethylene Glycol Processes and Methanol Reactor

Chapter-6 Page 146

reactions are coupled with endothermic reaction hot spots may be produced due to

complete conversion in exothermic reaction which can lead to catalyst deactivation

Fig. 6.4 Conventional methanol reactor

Following types of reactors are classified by Rahimpour et al. for coupling exothermic and

endothermic reactions (Rahimpour et al., 2012).

1. Direct coupled adiabatic reactor: Exothermic and endothermic reactions are

taking place in the same reaction zone. Mass and energy is directly exchanged in

the reactor. This reactor can be used to couple exothermic reactions with an

endothermic reaction like oxidation and reduction, hydrogenation and

dehydrogenation, hydration and dehydration, etc.

2. Regenerative coupling: Thermal energy produced by the exothermic reaction is

stored in regenerative bed which is utilized by an endothermic reaction. It will lead

to efficient heat recovery. This type of reactor is used for producer gas. An

exothermic reaction is carried out in blow period while the endothermic reaction is

carried out in run period. Direct interchange of energy and mass is possible in this

reactor also.

Page 6: 6 Thermally coupled reactors for methanol synthesis - An ...shodhganga.inflibnet.ac.in/bitstream/10603/90267/9/17_exergy... · Thermally coupled reactors for methanol synthesis -

Exergy Analysis of Nitric Acid, Ethylene Oxide/Ethylene Glycol Processes and Methanol Reactor

Chapter-6 Page 147

3. Recuperative coupling: Recuperative reactors are used to conduct exothermic and

endothermic reactions simultaneously. Both reaction compartments are separated

by a metal wall. Only energy interchange is possible in this scheme. The material

can be exchanged by applying membrane system. Recuperative reactors are

subdivided into two types – Without membrane reactor and a membrane reactor.

Without membrane reactors are shell and tube, channels in monolith or micro

reactors. Each reactor has its own advantage. In membrane reactor energy is

transferred indirectly but mass can be transferred directly. Conversion in membrane

reactor can be increased for reversible reaction by removing one on the product.

The membrane is popularly used in dehydrogenation reactions for separation of

hydrogen from production mass. It is more useful if one side produces hydrogen

and another side consumes it.

6.5 Recuperative Reactor for Methanol Synthesis

Conventional methanol reactor (CMR) converts the heat of exothermic reaction into steam.

Steam is utilized either in a plant or for the production of electricity. Exported power is

only 2% of input energy (Rosen and Dincer, 1988). Almost 46% energy is lost through

cooling water. Production of methanol can be increased if heat energy available in the

reactor is utilized judiciously instead of making steam. CMR product stream contains only

5% methanol due to lower conversion of synthesis gas (Fig.6.5).

Energy integration in the reactor can increase production of either methanol or product

from the endothermic reaction. Dehydrogenation reaction is chosen as an endothermic

reaction. Various reactor schemes are proposed by Rahimpour and his group for methanol

synthesis.

Page 7: 6 Thermally coupled reactors for methanol synthesis - An ...shodhganga.inflibnet.ac.in/bitstream/10603/90267/9/17_exergy... · Thermally coupled reactors for methanol synthesis -

Exergy Analysis of Nitric Acid, Ethylene Oxide/Ethylene Glycol Processes and Methanol Reactor

Chapter-6 Page 148

Fig. 6.5 Methanol concentration in the reactor (Kordabadi and Jahanmiri, 2005)

6.5.1 Thermally Coupled Reactor (TCR)

TCR operates on the same principle as that of a conventional reactor, but instead of boiling

water in shell side dehydrogenation of the aromatic compound is carried out as shown in

Fig 6.6.

Fig. 6.6 Thermally coupled reactor

Page 8: 6 Thermally coupled reactors for methanol synthesis - An ...shodhganga.inflibnet.ac.in/bitstream/10603/90267/9/17_exergy... · Thermally coupled reactors for methanol synthesis -

Exergy Analysis of Nitric Acid, Ethylene Oxide/Ethylene Glycol Processes and Methanol Reactor

Chapter-6 Page 149

Both exothermic and endothermic reactions are carried out in the catalyst bed. Differential

Evolution method is used for optimization of the problem and calculates a best fit score for

the reactor. Methanol concentration at reactor outlet is objective function and other

parameters can be varied within the available limit. Khademi et al. (2009a) optimized

methanol and benzene production from cyclohexane in synthesis reactor. Cyclohexane

dehydrogenation consumes heat at a higher temperature in the first part and then reducing

the temperature at end part favoring thermodynamic equilibrium. It will give similar

temperature profile as that of CMR. Dehydrogenation of methyl cyclohexane is another

endothermic reaction coupled with methanol synthesis. (Rahimpour et al.,2011a)

6.5.2 Thermally Double Coupled Reactor (TDCR)

TDCR consist of three concentric tubes wherein the endothermic reaction is carried out in

the middle tube and exothermic reaction is in outer and inner tube. A multi-tubular

assembly of TDCR is shown in the Fig. 6.7.

Fig. 6.7 Thermally double coupled reactor

Page 9: 6 Thermally coupled reactors for methanol synthesis - An ...shodhganga.inflibnet.ac.in/bitstream/10603/90267/9/17_exergy... · Thermally coupled reactors for methanol synthesis -

Exergy Analysis of Nitric Acid, Ethylene Oxide/Ethylene Glycol Processes and Methanol Reactor

Chapter-6 Page 150

Endothermic reaction receives heat from both exothermic reactions. Three different

reactions are carried out in three tubes, out of which one reaction is endothermic. Catalyst

required for these reactions are packed in tubes. Inlet composition of synthesis gas is same

as CMR and composition of other reactants are selected according to the kinetics of the

reaction. In TDCR heat received by the endothermic reaction is more compared to other

thermally coupled reactors. Dehydrogenation reaction is coupled to methanol synthesis to

produce hydrogen as one of the product. Dimethyl ether (DME) synthesis is preferred as a

second exothermic reaction. In TDCR hydrogen production can be more than TCR due to

extra heat available from DMR synthesis (Farniaei et al., 2014).

6.5.3 Membrane Coupled Reactor (MCR)

Use of membrane for permeation of hydrogen helps to increase the yield of

dehydrogenation traction. It will shift the equilibrium of reversible reaction due to the

removal of the product during the reaction. A schematic arrangement of MCR is shown in

Fig. 6.8. Methanol synthesis reaction is a source of heat in the MCR as like CMR.

Dehydrogenation of cyclohexane is the endothermic reaction in the second side. Argon is

used as sweep gas in the third side that is separated by a semipermeable membrane to

remove hydrogen. Heat is transferred from exothermic side to dehydrogenation reaction

and hydrogen is transferred from endothermic side to permeate side. Pure hydrogen is

produced from dehydrogenation reaction using a membrane. Two hydrogen perm-selective

Pd-Ag membranes are used in thermally coupled double membrane reactor (TCDMR) on

the exothermic and endothermic side each (Rahimpour et al., 2011b). Membrane at the

exothermic side is used to remove hydrogen from methanol product gas and recycle it to

synthesis gas feed increasing the concentration of hydrogen in it. It will shift reversible

reaction in the forward direction and enhances the yield of methanol compared to TCR.

Page 10: 6 Thermally coupled reactors for methanol synthesis - An ...shodhganga.inflibnet.ac.in/bitstream/10603/90267/9/17_exergy... · Thermally coupled reactors for methanol synthesis -

Exergy Analysis of Nitric Acid, Ethylene Oxide/Ethylene Glycol Processes and Methanol Reactor

Chapter-6 Page 151

Table.6.1 Various schemes of thermally coupled reactors for methanol synthesis

Reactor type Exothermic

reaction

Endothermic

reaction

Methanol

yield (%)

Cyclohexane /

Methyl

cyclohexane

conversion

(%)

Reference

Conventional

Methanol reactor

Methanol synthesis

(tube side) Steam production 38.83 NA

Khademi et al.,

2009a

Thermally

coupled reactor

Methanol synthesis

(tube side)

Dehydrogenation of

cyclohexane (shell

side)

40.02 100 Khademi et al.,

2009a

Thermally

coupled reactor

Methanol synthesis

(tube side)

Dehydrogenation of

methylcyclohexane

(shell side)

35.40 67.5 Rahimpour et

al., 2011a

Thermally

coupled

membrane

reactor

Methanol synthesis

(tube side)

Dehydrogenation of

cyclohexane (shell

side)

38.08 85.13

Khademi et al.,

2009b;

Khademi et al.,

2010;

Rahimpour

and Pourazadi,

2011

Thermally

coupled double

membrane

reactor

Methanol synthesis

(tube side)

Dehydrogenation of

cyclohexane (shell

side)

42.73 92.29 Rahimpour et

al., 2011b

Thermally

double coupled

two membrane

reactor

Methanol synthesis

(inner tube side)

DME synthesis

(Outer tube)

Dehydrogenation of

cyclohexane (middle

tube)

42.36 82.66 Farniaei et

al.,2014

Thermally

double coupled

reactor

Methanol synthesis

(inner tube side)

DME synthesis

(Outer tube)

Dehydrogenation of

cyclohexane (middle

tube)

37.00 67.00 Farniaei et al.,

2014

Thermally

double coupled

reactor

Methanol synthesis

(inner tube side)

DME synthesis

(Outer tube)

Dehydrogenation of

methyl cyclohexane

(middle tube)

37.00 56.00 Samimi et al.,

2014

Page 11: 6 Thermally coupled reactors for methanol synthesis - An ...shodhganga.inflibnet.ac.in/bitstream/10603/90267/9/17_exergy... · Thermally coupled reactors for methanol synthesis -

Exergy Analysis of Nitric Acid, Ethylene Oxide/Ethylene Glycol Processes and Methanol Reactor

Chapter-6 Page 152

Fig. 6.8 Thermally coupled membrane reactor

In thermally double coupled two membrane reactor (TDCTMR), double coupled reactor

with two membranes is used to separate water from methanol. One membrane placed near

the center tube and another hydrogen perm selective membrane is used to remove

hydrogen from exothermic reaction (Farniaei et al., 2014). Through this reactor we can

achieve production of multiple reactant-product configurations, production of pure

hydrogen and energy integration between exothermic and endothermic reaction. (Khademi

et al., 2009b)

These reactor configurations are yet to be commercialized fully but research is going on to

find out the possibilities regarding the commercial implementation of these schemes in

methanol process. In many exothermic processes heat is ultimately used to produce steam.

If this steam is not required in the plant then it is converted into electricity. Losses in the

steam system and turbines reduce useful output and most of the heat is wasted through

cooling tower due to its low grade. Thermally coupled reactors can exchange heat instantly

at the place by utilizing of heat in a better way than producing steam. Hydrogen is

important industrial gas required in many processes and it is also acquiring place as a clean

Page 12: 6 Thermally coupled reactors for methanol synthesis - An ...shodhganga.inflibnet.ac.in/bitstream/10603/90267/9/17_exergy... · Thermally coupled reactors for methanol synthesis -

Exergy Analysis of Nitric Acid, Ethylene Oxide/Ethylene Glycol Processes and Methanol Reactor

Chapter-6 Page 153

fuel. Due to storage issue its use as fuel is limited. Industrial hydrogen requirement is

fulfilled by producing synthesis gas form methane. Thermally coupled reactors can

produce almost 40-45% hydrogen consumed in methanol synthesis reaction. Cyclohexane

and methyl cyclohexane can be used as a hydrogen carrier. Hydrogen produced in the

endothermic side can be purified using membrane and used for the methanol production.

Make up hydrogen and carbon dioxide are supplied to synthesis reactor along with

hydrogen from the endothermic side. As there is market limitation for benzene and toluene

production, this hydrogen source cannot fulfill entire requirement for methanol synthesis

reaction but will be helpful to reduce consumption of natural gas. Table 6.1 shows various

schemes of thermally coupled reactors used for methanol synthesis.

6.6 Exergy Analysis

Irreversibility in the chemical reaction is major cause of exergy destruction. In exothermic

reaction chemical exergy of reactant is converted into physical exergy in the form of heat.

Part of it is lost due to unavoidable irreversibility in the reaction. Most of the exothermic

processes utilize this heat in the plant itself in a usable form. As seen in the previous study

of mono high pressure nitric acid process, heat is utilized to rise the temperature of

expander gas and then to produce high pressure steam which is used in the turbine. Total

heat available in the reactor cannot be used at one step hence it is exchanged at later stages

in various heat exchangers. Exergy will go on reducing as temperatures reduce though

energy is in considerable amount. At each stage of energy conversion process some amount

of exergy is lost. Exergy loss in ammonia oxidation reactor is 40.84 % of total exergy

destruction of the plant (Mewada and Nimkar, 2015). In another exothermic process of

ethylene oxide production, exergy destruction in reactor is 47% of total exergy destruction

(Nimkar and Mewada, 2014)

Heat released during methanol synthesis reaction is used for the production of steam. The

temperature in the reactor is 806.15 K and pressure is 7.7 MPa. Synthesis reaction takes

place in tubes filled with CuO/ZnO/Al2O3 catalyst and boiling water is circulated in shell

side through steam drum as shown in Fig 6.4. If hydrogen from purge gas is separated and

recycled back to the reactor, methanol yield can be increased. In TCDMR and TDCTMR

hydrogen from the product gas is separated and recycled back to the reactor. In present

Page 13: 6 Thermally coupled reactors for methanol synthesis - An ...shodhganga.inflibnet.ac.in/bitstream/10603/90267/9/17_exergy... · Thermally coupled reactors for methanol synthesis -

Exergy Analysis of Nitric Acid, Ethylene Oxide/Ethylene Glycol Processes and Methanol Reactor

Chapter-6 Page 154

study different types of thermally coupled reactors are analyzed based on exergy. Energy is

directly transferred to the endothermic reaction that results in better utilization of exergy.

Dehydrogenation reaction is carried at another side of synthesis reactor. The overall

efficiency of SMR for hydrogen production is 67.35% (Boyano et al, 2011). Exergy

analysis of methanol production process and hydrogen production process is shown in

Table 6.2 and 6.3 respectively. Major exergy destruction takes place in the reformer due to

combustion of methane in the combustor. Product hydrogen consists of 67% of input

exergy mainly in the form of chemical exergy. About 6% exergy is lost in the cooling

water and flue gas.

Hydrogen produced in the thermally coupled reactor can be mixed with carbon dioxide

from the reformer and send to second thermally coupled reactor. Another source of

hydrogen is purge gas stream that is also sent to the second reactor. This scheme will

increase overall methanol capacity of the plant. New plant layout is shown in Fig. 6.9.

Fig.6.9 Proposed plant layout for methanol production

Page 14: 6 Thermally coupled reactors for methanol synthesis - An ...shodhganga.inflibnet.ac.in/bitstream/10603/90267/9/17_exergy... · Thermally coupled reactors for methanol synthesis -

Exergy Analysis of Nitric Acid, Ethylene Oxide/Ethylene Glycol Processes and Methanol Reactor

Chapter-6 Page 155

Table 6.2 Exergy analysis of 100 TPD methanol plant

Component Exergy PH (kW)

Exergy CH (kW)

Total Exergy (kW)

Component Exergy PH (kW)

Exergy CH

(kW)

Total Exergy (kW)

Natural gas 435.11 37422.8 37857.87 Methanol 11.82 25729.78 25741.61 Steam 832.51 2099.2 2931.71 Hydrogen 329.58 7897.26 8226.85 Compressor 1910.20 0 1910.20 Water 9.44 90.79 100.24 Reformer

Fuel 163.12 23869.8 24032.92 Steam 4764.21 0 4764.21

Air 1.64 241.10 242.75 Flue Gas 1052.32 130.56 1182.89

Electricity 94.25 0 94.25

Cooling

Water 198.65 0 198.65

Total 3342.61 63632.9 66975.47 Total 6460.30 33848.42 40308.72

Exergy

Destruction 26666.75

Table 6.3 Exergy analysis of 100 TPD hydrogen plant

Component Exergy PH (kW)

Exergy CH

(kW)

Total Exergy (kW)

Component Exergy PH (kW)

Exergy CH

(kW)

Total Exergy (kW)

Methane 0.00 178.83 178.83 Hydrogen 1.71 135.21 136.92 Air 0.00 0.02 0.02 Flue gas 6.39 4.61 11.00 Reformer fuel 0.00 21.46 21.46 Cooling water 0.51 0.00 0.51 Water 0.01 0.03 0.04 0.00 Electricity 2.94 0.00 2.94 0.00 Total 2.95 200.34 203.29 Total 8.61 139.82 148.43

Exergy Destruction 54.86

6.6.1 Conventional Methanol Reactor

Exergy analysis of CMR is carried out for 100TPD methanol production. Heat given to

boiling water during reaction through reactor length is 2420 kW. Steam of 2.9 MPa at

505.15 K is produced from steam drum. Inlet composition of synthesis gas is kept same for

all reactors shown in Table 6.4. Process parameters for reactor operation are shown in

Table 6.5. The temperature at the endothermic side is lower to enable transfer of heat from

Page 15: 6 Thermally coupled reactors for methanol synthesis - An ...shodhganga.inflibnet.ac.in/bitstream/10603/90267/9/17_exergy... · Thermally coupled reactors for methanol synthesis -

Exergy Analysis of Nitric Acid, Ethylene Oxide/Ethylene Glycol Processes and Methanol Reactor

Chapter-6 Page 156

exothermic side to endothermic side. Input exergy in the reactor is mainly chemical exergy

of reactant. Chemical exergy of the product is always lower in an exothermic reaction.

Data required for analysis is extracted from the respective work of the reactors cited in

Table 6.1. Exergy given by exothermic reaction is 1779.21 kW and exergy of steam

produced is 977.6 kW. Exergy taken by steam is lower due to the temperature difference

between boiling water and the temperature inside the tubes. Exergy destruction is 45% of

the exergy given for steam generation.

Table 6.4 Feed compositions for exothermic and endothermic reactions (Khademi et al., 2009a, Rahimpour et al., 2011a, Farniaei et al., 2014)

Component

(Mole Fraction)

Methanol

synthesis

gas

Cyclohexane

feed

Methylcyclohexane

feed

DME

synthesis

gas

Methanol 0.005 0 0 0.0030

Carbon dioxide 0.094 0 0 0.0409

Carbon

monoxide 0.046 0 0 0.1716

Water 0.000 0 0 0.0002

Hydrogen 0.659 0 0 0.4325

Nitrogen 0.093 0 0 0.3060

Methane 0.103 0 0 0.0440

Dimethyl ether 0 0 0 0.0018

Cyclohexane 0 0.1 0 0

Methyl cyclohexane

0 0 0.12 0

Argon 0 0.9 0.88 0

Total 1 1 1 1

Page 16: 6 Thermally coupled reactors for methanol synthesis - An ...shodhganga.inflibnet.ac.in/bitstream/10603/90267/9/17_exergy... · Thermally coupled reactors for methanol synthesis -

Exergy Analysis of Nitric Acid, Ethylene Oxide/Ethylene Glycol Processes and Methanol Reactor

Chapter-6 Page 157

Table 6.5 Inlet and outlet parameters in reactors (Khademi et al., 2009a, Rahimpour et al., 2011a, Farniaei et al., 2014)

Reactor type

Exothermic side Endothermic side Inlet

temp (K) Outlet

temp (K) Pressure (MPa)

Inlet temp (K)

Outlet temp (K)

Pressure (Mpa)

CMR 503.15 525.15 7.7 459.15 505.15 6.20 TCR-CH 527.15 505.15 7.7 423.15 495.15 0.11 TCR-MCH 505.15 522.15 7.7 503.15 519.15 0.80 MCR 503.15 515.15 7.7 503.15 501.15 0.10

TDCR 504.15 503.15

518.15 511.15

7.7 5.0 503.15 506.15 2.0

6.6.2 Thermally Coupled Reactor

Energy integration between exothermic reaction (methanol synthesis) and endothermic

reaction (dehydrogenation of cyclohexane) are helpful to reduce exergy loss. The short

distance between heat source and sink will result in efficient heat transfer. Inlet

composition of synthesis gas is same as that of CMR. For comparison only, 100 TPD

methanol production is taken as basis like in CMR. Benzene and hydrogen are produced in

the dehydrogenation of cyclohexane.

7. C6H12 ↔ C6H6 + 3H2 ΔHR,298 = +206.2 kJ/mol

Cyclohexane in the feed at the endothermic side is diluted with argon shown in Table 6.4.

The catalyst used for dehydrogenation is Pt/Al2O3. Heat input in the reactor on the both

side is available from feed gas heating. Total heat transferred in each section is a

combination of heat in feed gas and heat of reaction. Cyclohexane conversion is 100 % in

TCR and hydrogen production is 6.30 TPD, which is 41% of hydrogen required for

methanol synthesis. Exergy destruction is 173.77 kW compared to 801.6 kW in CMR.

When dehydrogenation of methyl cyclohexane is used as endothermic reaction synthesis

gas feed must be increased by 27% to get 100 TPD of methanol. It increases chemical and

physical exergy input in the reactor. Exergy destruction is 68.42% of exergy received from

the exothermic reactor. Hydrogen production has come down to 4.87 TPD for this

combination.

Page 17: 6 Thermally coupled reactors for methanol synthesis - An ...shodhganga.inflibnet.ac.in/bitstream/10603/90267/9/17_exergy... · Thermally coupled reactors for methanol synthesis -

Exergy Analysis of Nitric Acid, Ethylene Oxide/Ethylene Glycol Processes and Methanol Reactor

Chapter-6 Page 158

6.6.3 Thermally Double Coupled Reactor

TDCR provide more heat compared to TCR due to two exothermic reactions taking place

in it. The endothermic side will receive heat from inside as well as from outside also

because it is placed in between two concentric tubes (Fig.6.7). Heat given by DME

reaction is more than methanol synthesis. DME production involves methanol synthesis

reactions and dehydration of methanol. Inlet composition in DME synthesis side is shown

in Table 6.4.

8. 2CH3OH ↔ CH3OCH3 + H2O ΔHR,298 = -21.003 kJ/mol

Physical exergy inlet into the reactor has heat and pressure component. Exergy destruction

is 68% of the exergy provided by both exothermic reactions. Higher exergy destruction is

due to the exchange of heat at different temperature regimes in the reactor at various

sections. Inlet molar flow rate for both exothermic reactions are almost same. Hydrogen

production is 20 TPD, which is almost 3.2 times more than TCR due to the availability of

more heat.

6.6.4 Membrane Coupled Reactor

Hydrogen gas is separated using perm-selective membrane in MCR. The reaction is

favored by removing one the product i.e. hydrogen in dehydrogenation section. Total

hydrogen production is 5.06 TPD, and almost 95% is recovered by using a membrane.

Physical and chemical exergy is transferred to permeation side while transferring

hydrogen. Exergy given by exothermic reaction is 1086.5 kW and exergy taken by the

endothermic reaction is 938.46 kW.

6.7 Conclusion

Irreversibility in exothermic reactions is the major sources of exergy loss in the process.

Irreversibility in the combustion of methane to provide heat for reforming is unavoidable in

the present combustion system. Almost 18% of input exergy is lost in the reformer in SMR

process. Exergy efficiency of the reformer is 87.3% and exergy destruction in reformer per

Page 18: 6 Thermally coupled reactors for methanol synthesis - An ...shodhganga.inflibnet.ac.in/bitstream/10603/90267/9/17_exergy... · Thermally coupled reactors for methanol synthesis -

Exergy Analysis of Nitric Acid, Ethylene Oxide/Ethylene Glycol Processes and Methanol Reactor

Chapter-6 Page 159

ton of hydrogen is 392.47 kW as shown in Table 6.6. This value will further increase to

557 kW when pure hydrogen is coming out as a product. Efficiency can be increased if

losses in the reformer are reduced. Due to high temperature in the reformer it is difficult to

integrate directly it with another reactor. Change in the hydrogen production route will give

better results. Coupling of hydrogen production with exothermic methanol synthesis

reaction reduces exergy losses. These two products have practical aspects for

implementation in the existing plant or future plants.

Table 6.6 Exergy analysis of various reactor of 100 TPD methanol production

Reactor Type

Reactor

Exergy

Efficiency

(%)

H2

Production

in Reactor

(TPD)

Exergy

Destruction

(kW)

Exergy

Destruction

(kW) Per ton

of CH3OH

Exergy

Destruction

(kW) Per

ton of H2

Conventional Methanol

Reactor 54.94 NA 801.59 8.01 NA

Thermally coupled reactor

(Cyclohexane

dehydrogenation)

91.11 6.3 173.77 1.73 27.58

Thermally coupled reactor

(methylcyclohexane

dehydrogenation)

31.57 4.87 5449.72 54.49 1119.03

Thermally Double

coupled Reactor 32.04 20.39 2900.81 29.00 142.26

Membrane Coupled

Reactor 86.37 5.06 148.03 1.48 29.25

Steam Methane Reformer 87.3 431.42 169320 NA 392.47

The heat required for the production of hydrogen is 41.83 GJ/t in the reformer. It is far

more than heat produced in methanol synthesis 2.21 GJ/t. Heat requirement for hydrogen

production can be brought down up to 34.36 GJ/t if dehydrogenation reaction is used. The

temperature required for dehydrogenation is less than methanol synthesis that enables

coupling of both reactions. As heat requirement for hydrogen production is 16 times higher

than heat produced by synthesis reaction, dedicated hydrogen production facility is

economically not feasible.

Page 19: 6 Thermally coupled reactors for methanol synthesis - An ...shodhganga.inflibnet.ac.in/bitstream/10603/90267/9/17_exergy... · Thermally coupled reactors for methanol synthesis -

Exergy Analysis of Nitric Acid, Ethylene Oxide/Ethylene Glycol Processes and Methanol Reactor

Chapter-6 Page 160

Fig. 6.10 Hydrogen production in 100 TPD methanol thermally coupled reactors

Fig. 6.11 Exergy efficiency of various reactors

0

5

10

15

20

25

TCR-CH TCR-MCH TDCR MCR

Hyd

roge

n pr

oduc

tion

(TPD

)

Reactor type

0

10

20

30

40

50

60

70

80

90

100

CMR TCR-CH TCR-MCH TDCR MCR SMR

Exer

gy e

ffici

ency

(%)

Reactor type

Page 20: 6 Thermally coupled reactors for methanol synthesis - An ...shodhganga.inflibnet.ac.in/bitstream/10603/90267/9/17_exergy... · Thermally coupled reactors for methanol synthesis -

Exergy Analysis of Nitric Acid, Ethylene Oxide/Ethylene Glycol Processes and Methanol Reactor

Chapter-6 Page 161

Fig. 6.12 Exergy destruction per ton of methanol in various reactors

Fig. 6.13 Exergy destruction per ton of hydrogen in various reactors

0

10

20

30

40

50

60

CMR TCR-CH TCR-MCH TDCR MCR

Exer

gy d

estr

uctio

n (k

W/t

of C

H3O

H)

Reactor type

0

200

400

600

800

1000

1200

TCR-CH TCR-MCH TDCR MCR SMR

Exer

gy d

estr

uctio

n (k

W/t

of H

2)

Reactor type

Page 21: 6 Thermally coupled reactors for methanol synthesis - An ...shodhganga.inflibnet.ac.in/bitstream/10603/90267/9/17_exergy... · Thermally coupled reactors for methanol synthesis -

Exergy Analysis of Nitric Acid, Ethylene Oxide/Ethylene Glycol Processes and Methanol Reactor

Chapter-6 Page 162

Though methanol itself is emerging as a fuel for future its scope is totally dependent upon

the availability of feedstock. Large capacity plants are using natural gas as feed stock;

hence hydrogen production in thermally coupled reactor is limited by methanol production

capacity of the plant. MCR with 100 TPD methanol production can produce 5 TPD of

hydrogen. If more heat is added by another exothermic reaction keeping same methanol

production capacity, hydrogen production can be increased by 4 times. TDCR can produce

20 TPD of hydrogen by using DME synthesis as another source of heat as shown in Fig

6.10. Exergy destruction is more in TDCR due to the coupling of extra exothermic

reaction. Exergy efficiency will reduce due to this loss as shown in Fig 6.11.

TCR-C6H12 is having highest exergy efficiency among all reactors followed by MCR.

TDCR and TCR-MCH are having less efficiency because chemical exergy values are

playing an important role in the reaction. In exothermic process chemical exergy of the

product is always lower than reactants. In the case of MCH, reaction kinetics limits the

conversion of methyl cyclohexane compared to cyclohexane. Exergy destruction per ton of

methanol is 1.48 kW in MCR and 1.73 kW in TCR-CH (Fig.6.12) but for hydrogen

production TCR-CH is a better candidate than MCR (Fig.6.13).

Finally, it is concluded that TCR-CH and MCR are the best thermally coupled reactors on

the basis of exergy analysis. These reactors can get the advantage of the heat generated by

exothermic reaction in exergy efficient way than other reactors. Use of this reactor will

reduce the number of equipments for the production of methanol and hydrogen, results

saving in capital cost. Equilibrium conversion can be enhanced by keeping lower output

temperature. Along with methanol it produces hydrogen that is valuable industrial gas and

future fuel. Though hydrogen production capacity cannot be matched as per SMR, it can be

advantages to use produced hydrogen as make up quantity.

Page 22: 6 Thermally coupled reactors for methanol synthesis - An ...shodhganga.inflibnet.ac.in/bitstream/10603/90267/9/17_exergy... · Thermally coupled reactors for methanol synthesis -

Exergy Analysis of Nitric Acid, Ethylene Oxide/Ethylene Glycol Processes and Methanol Reactor

Chapter-6 Page 163

References Boyano, A., Blanco-Marigorta, A.M., Morosuk, T. and Tsatsaronis, G (2011).

‘Exergoenvironmental analysis of a steam methane reforming process for hydrogen production’, Energy, Vol. 36, pp. 2202-2214.

Farniaei, M., Abbasi, M., Rasoolzadeh, A. and Rahimpour, M.R. (2014) ‘Performance

enhancement of thermally coupling of methanol synthesis, DME synthesis and cyclohexane dehydrogenation processes: Employment of water and hydrogen perm-selective membranes via different recycle streams’, Chemical Engineering and Processing, Vol, 85, pp. 24–37.

Farniaei, M., Abbasi, M., Rahnama, H. and Rahimpour, M.R. (2014) ‘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, Vol 19, pp. 324-336

Fundamentals of methanol synthesis available at

http://www.supermethanol.eu/index.php?id=21&rid=12&r=methanol_synthesis&PHPSESSID=c9ngo2ijq0pbon5n1jgd2d2tr2 (accessed on 12/03/2015)

Khademi, M. H., Setoodeh, P., Rahimpour, M.R. and Jahanmiri, A. (2009a) ‘Optimization

of methanol synthesis and cyclohexane dehydrogenation in a thermally coupled reactor using differential evolution (DE) method’, International Journal of Hydrogen Energy, Vol. 34, No. 16, pp. 6930-6944.

Khademi, M.H., Jahanmiri, A. and Rahimpour, M.R. (2009b) ‘A novel configuration for

hydrogen production from coupling of methanol and benzene synthesis in a hydrogen-permselective membrane reactor’. International Journal of Hydrogen Energy, Vol. 34, pp. 5091-5107.

Khademi, M.H., Rahimpour, M.R. and Jahanmiri, A. (2010) ‘Differential evolution (DE)

strategy for optimization of hydrogen production, cyclohexane dehydrogenation and methanol synthesis in a hydrogen-permselective membrane thermally coupled reactor’, International Journal of Hydrogen Energy , Vol. 35, pp. 1936-1950.

Kordabadi, K., and Jahanmiri A.(2005) ‘Optimization of methanol synthesis reactor using

genetic algorithms’, Chemical Engineering Journal, Vol. 108, No. 3, pp. 249-255. Kumar, S., Gaba, T. and Kumar, S. (2009) ‘Simulation of Catalytic Dehydrogenation of

Cyclohexane in Zeolite Membrane Reactor’, International Journal of Chemical Reactor Engineering, 7(Article A13): DOI: 10.2202/1542-6580.1797.

Mewada, R.K. and Nimkar, S.C. (2015). ‘Minimization of exergy losses in mono high

pressure nitric acid process’, International Journal of Exergy, Vol. 17, No.2, pp. 192-218.

Page 23: 6 Thermally coupled reactors for methanol synthesis - An ...shodhganga.inflibnet.ac.in/bitstream/10603/90267/9/17_exergy... · Thermally coupled reactors for methanol synthesis -

Exergy Analysis of Nitric Acid, Ethylene Oxide/Ethylene Glycol Processes and Methanol Reactor

Chapter-6 Page 164

Nian, W.C. and You, F. (2013). ‘Design of methanol plant’, available at http://bari.upc.es/eurecha/docs/ESC_2013_winner.pdf (accessed on 01/04/2015)

Nimkar, S.C. and Mewada, R.K.(2014) ‘An overview of exergy analysis for chemical

process Industries’, .International Journal of Exergy, Vol. 15, No.4, pp. 468-507. Rahimpour, M.R., Dehnavi, M.R., Allahgholipour, F., Iranshahi, D. and Joker, S.M. (2012)

‘Assessment and comparison of different catalytic coupling exothermic and endothermic reactions: A review’, Applied energy, Vol. 99, pp. 496-512.

Rahimpour, M.R. and Pourazadi, E. (2011) ‘A comparison of hydrogen and methanol

production in a thermally coupled membrane reactor for co-current and counter-current lows’, International Journal of Energy Research, Vol. 35, No. 10, pp. 863-882.

Rahimpour, M.R., Rahmani, F., Bayat, M. and Pourazadi, E. (2011b) ‘Enhancement of

simultaneous hydrogen production and methanol synthesis in thermally coupled double-membrane reactor’, International Journal of Hydrogen Energy, Vol. 36, pp. 284-298.

Rahimpour, M.R., Vakili, R., Pourazadi, E., Bahmanpour, A.M. and Iranshahi, D. (2011a)

‘Enhancement of hydrogen production via coupling of MCH dehydrogenation reaction and methanol synthesis process by using thermally coupled heat exchanger reactor’, International Journal of Hydrogen Energy, Vol. 36, pp. 3371-3383.

Rosen, M.A. and Scott, D.S. (1988). ‘Energy and exergy analyses of a production process

for methanol from natural gas’, International Journal of Hydrogen Energy, Vol. 13, No. 10, pp. 617-623.