6 thermally coupled reactors for methanol synthesis - an...
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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.
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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)
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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.
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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
Exergy Analysis of Nitric Acid, Ethylene Oxide/Ethylene Glycol Processes and Methanol Reactor
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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.
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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.
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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
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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
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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.
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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
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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
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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
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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
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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
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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
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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.
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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
Exergy Analysis of Nitric Acid, Ethylene Oxide/Ethylene Glycol Processes and Methanol Reactor
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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.
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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
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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
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.
Exergy Analysis of Nitric Acid, Ethylene Oxide/Ethylene Glycol Processes and Methanol Reactor
Chapter-6 Page 163
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