4 Minimization of Exergy losses in Mono High Pressure Nitric Acid Process 4.1. Introduction Energy is required for every process to get desired products. There are some processes in inorganic chemical industries where raw material itself acts as a source of energy. Nitric acid production is that kind of process due to highly exothermic reactions involved in it. Irreversibility in the reaction is the major cause for the exergy loss. Using exergy analysis, we can pinpoint true losses of available energy in this process. Chemical industries can increase their profit margins with the help of exergy analysis combined with other techniques (Nimkar and Mewada, 2014). Various studies have been carried out for the exergy analysis of inorganic chemical process industries (Kirova-Yordanova, 2004; Radgen, 1996; Rasheva and Atanasova, 2002; Atanasova, 2002; Atanasova, 2010). Commercially nitric acid plants are operated by two methods - mono pressure and dual pressure. In the present work, an attempt has been made to carry out exergy analysis of mono high-pressure nitric acid process and suggestions have been proposed to reduce exergy losses. First time attempt has been made for the recovery of heat from cooler condenser in the nitric acid plant. 4.2. Process Description Ammonia is oxidized to produce nitric oxide. The heat generated by this reaction is recovered in heat exchangers and used to produce work. Oxidation of nitric oxide and subsequent absorption is carried out in an absorber where heat is removed by cooling tower water and chilled water. A PFD of the mono high-pressure process of nitric acid manufacturing is shown in Fig.4.1. Air at 308.15 K with a flow rate of 5054.48 kg/t of 100% acid is compressed up to 1.3 MPa (470.15 K) and send to the mixer (M). A Part of compressed air is sent to the absorber. Ammonia at 240.15 K with a flow rate of 281.11 kg/t of 100% acid is vaporized (AV), superheated (AS) and send to the mixer. A mixture
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4
Minimization of Exergy losses in Mono High Pressure Nitric Acid Process
4.1. Introduction Energy is required for every process to get desired products. There are some processes in
inorganic chemical industries where raw material itself acts as a source of energy. Nitric
acid production is that kind of process due to highly exothermic reactions involved in it.
Irreversibility in the reaction is the major cause for the exergy loss. Using exergy analysis,
we can pinpoint true losses of available energy in this process. Chemical industries can
increase their profit margins with the help of exergy analysis combined with other
techniques (Nimkar and Mewada, 2014). Various studies have been carried out for the
exergy analysis of inorganic chemical process industries (Kirova-Yordanova, 2004;
Radgen, 1996; Rasheva and Atanasova, 2002; Atanasova, 2002; Atanasova, 2010).
Commercially nitric acid plants are operated by two methods - mono pressure and dual
pressure. In the present work, an attempt has been made to carry out exergy analysis of
mono high-pressure nitric acid process and suggestions have been proposed to reduce
exergy losses. First time attempt has been made for the recovery of heat from cooler
condenser in the nitric acid plant.
4.2. Process Description Ammonia is oxidized to produce nitric oxide. The heat generated by this reaction is
recovered in heat exchangers and used to produce work. Oxidation of nitric oxide and
subsequent absorption is carried out in an absorber where heat is removed by cooling
tower water and chilled water. A PFD of the mono high-pressure process of nitric acid
manufacturing is shown in Fig.4.1. Air at 308.15 K with a flow rate of 5054.48 kg/t of
100% acid is compressed up to 1.3 MPa (470.15 K) and send to the mixer (M). A Part of
compressed air is sent to the absorber. Ammonia at 240.15 K with a flow rate of 281.11
kg/t of 100% acid is vaporized (AV), superheated (AS) and send to the mixer. A mixture
Exergy Analysis of Nitric Acid, Ethylene Oxide/Ethylene Glycol Processes and Methanol Reactor
Chapter-4 Page 63
of ammonia and air (1: 9.7) is fed to the ammonia oxidation reactor (AOR) where
reactions no. 1 to 3 take place in the presence of a platinum catalyst.
Due to stringent environmental regulations, nitrous gas concentration in tail gas must be
within the limit before going to the atmosphere. Nitric acid (15-35 wt %) may be
effectively used to scrub tail gas for reduction of nitrous gases (Carta, 1986). Mowla and
Razavi (2004) studied the use of activated carbon in a fluidized bed for adsorption of
nitrous gases. Traces of nitrogen oxides are present in the tail gas due to reaction cycle
(Reaction no. 5 & 6). The cycle can be broken if NO formed in reaction 6 is reacted with
nitric acid to form N2O3. It will be ultimately converted to nitric acid after reacting with O2
and water. There will not be any nitrogen oxide coming out of absorber (Drinkard, 2001).
But practically in the plants all over the world tail gas is treated by either use of a catalyst
(Joshi et al., 1985; Qajar and Mowla, 2009) (selective catalytic reduction and non-selective
Exergy Analysis of Nitric Acid, Ethylene Oxide/Ethylene Glycol Processes and Methanol Reactor
Chapter-4 Page 66
catalytic reduction) or limiting its content in absorber itself (Extended absorption) (Kirova-
Yordanova, 2011). In this process, extended absorption method is used for reduction of
Nitrous gases in tail gas.
4.3. Energy Balance Three forms of energy are used in the process 1) Mechanical (Shaft work) 2) Thermal and
3) Electrical. Ammonia itself acts as a source of energy. The energy balance of the process
is shown in Table 4.2. Enthalpy values are calculated at 298.15 K reference temperature.
Half of input energy is provided by oxidation of ammonia. Electricity is required to
operate various pumps in the plant. Though nitric acid formation from NO2 and its dilution
gives 21% heat, it does not contribute for any useful work production. The heat of outlet
gas from the reactor is initially recovered in expander gas heater (HE-1) followed by waste
heat boiler (WHB) and steam superheater (HE-2). Hot tail gas enters gas expander at
894.15 K and 1.1 MPa while steam enters a turbine at 589.15 K and 4.22 MPa. Excess
steam is exported after providing sufficient quantity to steam turbine. The energy input to
the compressor is 1857 MJ/t through the turbine and expander out of which only 30 % is
converted into work. Turbine contributes 70% and expander contributes 30% power to run
the compressor.
Table 4.2 Energy balance of nitric acid process
Input MJ/t of 100% acid % Output MJ/t of
100% acid %
Air 562.39 6.99 Nitric Acid 180.75 2.25 Water 22.18 0.28 Tail Gas 90.06 1.12
Electricity 126.72 1.58 Heat to Cooling Tower 5457.21 67.86
Makeup Water 690.22 8.58 Steam Export 2314.10 28.77 NH3 Heating 89.72 1.12 Heat of NH3 Oxidation 3806.70 47.33 Heat of NO Oxidation 906.59 11.27 Heat of HNO3 Formation 1035.43 12.52 Heat of HNO3 Dilution 224.85 2.80 Heat of Reoxidation of NO 454.69 5.65 Heat of NO2 to N2O4 Formation 122.63 1.88 Total 8042.13 100.00 Total 8042.13 100.00
Exergy Analysis of Nitric Acid, Ethylene Oxide/Ethylene Glycol Processes and Methanol Reactor
Chapter-4 Page 67
Energy flow including sensible heat and heat of reaction in heat exchanger train is shown
in Fig.4.2. Heat generated in absorber due to oxidation, absorption, acid formation and
dilution is taken away by cooling coils provided on the trays. Top trays are chilled with
chilled water of 274.85 K and bottom trays (except bleacher) are cooled with cooling water
of 307.15 K.
Fig. 4.2 Energy and Exergy flow in heat exchanger train including heat of reaction.
About 68 % of input energy is lost in the cooling tower. Cooling tower circuit is shown in
Fig.4.3. Compressor and cooler condenser are the major candidates contributing large
amount of heat. Outlet cooling water from the cooler condenser (CC) is used to heat
ammonia in ammonia vaporizer (AV). Total 5457 MJ/t heat is discarded into the
atmosphere from the cooling tower. Energy balance shows a large quantity of heat is being
wasted but does not give any idea about its usefulness. It is possible by carrying out exergy
analysis.
0
1000
2000
3000
4000
5000
6000
HE-1 WHB HE-2 HE-3 HE-4 CC
MJ/
t
EnergyTotal ExergyPhysical ExergyChemical Exergy
Exergy Analysis of Nitric Acid, Ethylene Oxide/Ethylene Glycol Processes and Methanol Reactor
Chapter-4 Page 68
Figure 4.3 Cooling tower circuit in nitric acid plant
4.4. Exergy Analysis
4.4.1 Exergy Balance
The energy balance of the system shows input energy is the sum of output energy and
accumulation. The case is not true for exergy balance because some part of exergy is lost
in the system due to irreversibilities present in the process. The Exergy balance of the
system can be written as
Exergy in = useful process work (product) + external exergy loss with waste stream +
exergy destruction (4.1)
Calculations for exergy analysis are carried according to equations explained in chapter-2.
Values of chemical exergy are shown in Appendix-I. Exergy balances of individual
equipments are as follows (Fig. 4.4 to 4.10)
Exergy Analysis of Nitric Acid, Ethylene Oxide/Ethylene Glycol Processes and Methanol Reactor
Chapter-4 Page 69
Ammonia Oxidation Reactor
Ammonia is a source of chemical exergy that will be utilized in the entire plant. Physical
and chemical exergy of ammonia and air is input to the reactor and hot nitrous gases are
coming out of the reactor
Fig. 4.4 Ammonia oxidation reactor
.
Exergy destruction in reactor = (Physical exergy of air in + Chemical exergy of air in +
Physical exergy of ammonia in + Chemical exergy of ammonia in) – (Physical exergy of
nitrous gas out + Chemical exergy of nitrous gas out)
Waste Heat Boiler
Heat available in hot nitrous gas is used to produce steam in waste heat boiler. Nitrous
gases are reacting while traveling through pipes and equiments hence change in chemical
exergy will take place.
Fig. 4.5 Waste heat boiler
Exergy Analysis of Nitric Acid, Ethylene Oxide/Ethylene Glycol Processes and Methanol Reactor
Chapter-4 Page 70
Exergy destruction in waste heat boiler = (Exergy given by hot gas) – (Exergy taken by
feed water) = {[(Physical exergy of nitrous gas in + Chemical exergy of nitrous gas out)] –
[(Physical exergy of nitrous gas out + Chemical exergy of nitrous gas out)]} – {[Physical
exergy of feed water] – [Physical exergy of steam]}
Cooler Condenser
It is one of the important equipment in the process. Before going to the absorber, product
gas is cooled and water is condensed with the formation of weak acid. Cooling tower water
is used to take out heat. It is discussed in section 4.6.
Fig. 4.6 Cooler condenser
Exergy destruction in cooler condenser = (Exergy given by hot gas) – (Exergy taken by
cooling water) = {[(Physical exergy of nitrous gas in + Chemical exergy of nitrous gas
out)] – [(Physical exergy of nitrous gas out + Chemical exergy of nitrous gas out +
Physical exergy of weak acid + Chemical exergy of weak acid)]} – {[Physical exergy of
cooling water in] – [Physical exergy of cooling water out]}
Absorber
Simultaneous absorption and oxidation take place in the absorber. Heat evolved is taken
out by cooling medium provided through cooling coils placed on trays.
Exergy Analysis of Nitric Acid, Ethylene Oxide/Ethylene Glycol Processes and Methanol Reactor
Chapter-4 Page 71
Fig. 4.7 Absorber
Exergy destruction in absorber = [(Physical exergy of nitrous gas + Chemical exergy of
nitrous gas + Physical exergy of weak acid + Chemical exergy of weak acid + Physical
exergy of air + Chemical exergy of air) + (Physical exergy of cooling water in)] –
[(Physical exergy of weak nitric acid + Chemical exergy of weak nitric acid + Physical
exergy of tail gas + Chemical exergy of tail gas) + (Physical exergy of cooling water out)]
Compressor
Air compressor in the nitric acid is powered by turbine and expander. Multistage
compressor with cooling water at each stage to remove heat is used. Water is removed at
each stage.
Fig. 4.8 Compressor
Exergy Analysis of Nitric Acid, Ethylene Oxide/Ethylene Glycol Processes and Methanol Reactor
Chapter-4 Page 72
Exergy of humid air is calculated by (Dincer and Rosen, 2007)
10 Nitrous Gas 15.39 71 1.21 45.21 -0.45 414.47 11 Cooling Water 165.14 33.5 0.10 12 Cooling Water 165.14 42.33 0.10
In refrigeration unit and ORC, cooling water at 310.15 K is used in the condenser. Fig.
4.17 shows cooling tower circuit after adding heat load of both condensers. The cooling
duty of compressor is reduced from 1342.37 MJ/t to 948.10 MJ/t in the new system. Outlet
nitrous gas temperature from the cooler condenser is 341.15 K. It is cooled up to 325.15 K
Exergy Analysis of Nitric Acid, Ethylene Oxide/Ethylene Glycol Processes and Methanol Reactor
Chapter-4 Page 89
by using cooling tower water in a new heat exchanger to maintain inlet temperature at
absorber. This cooling water cannot provide required heat to ammonia vaporizer (AV)
hence directly sent to the cooling tower.
Fig. 4.16 Exergy destruction in ORC
Fig. 4.17 Cooling tower circuit after installation of new systems
Saving of steam in turbine reduces its condenser (TC) duty by 111 MJ/t. If all exported
steam is used to generate work or electricity additional condenser will be required which
0
10
20
30
40
50
60
70
Exer
gy D
estr
uctio
n (%
)
isopentane
npentane
Exergy Analysis of Nitric Acid, Ethylene Oxide/Ethylene Glycol Processes and Methanol Reactor
Chapter-4 Page 90
can increase the load on cooling tower. In present scenario (after installation new units),
cooling tower load is reduced by only 0.2%. The existing cooling tower can handle this
load easily. Additional low pressure steam is required to vaporize ammonia vapors (AV)
after installation of ORC. Energy and exergy balance of the revised system is shown in
Table 4.11 & 4.12.
Table 4.11 Energy balance after installation of new systems
Input MJ/t of 100% acid
% Output MJ/t of 100% acid
%
Air 562.39 6.67 Nitric Acid 180.75 2.14 Water 22.18 0.26 Tail Gas 90.06 1.07 Electricity 126.72 1.50 Heat to Cooling Tower 5446.75 64.62 Makeup water (Boiler) 690.22 8.19 Steam Export 2505.82 29.73 NH3 heating 476.22 5.65 Electricity (ORC) 205.24 2.44 Heat of NH3 Oxidation 3806.70 45.16 Heat of NO Oxidation 906.59 10.76 Heat of HNO3 Formation 1035.43 12.28 Heat of HNO3 Dilution 224.85 2.67 Heat of Reoxidation of NO 454.69 5.39 Heat of NO2 to N2O4 Formation 122.63 1.45 Total 8428.63 100.00 Total 8428.63 100.00
Table 4.12 Exergy balance after installation of new systems
Input MJ/t of 100% acid
% Output MJ/t of 100% acid
%
Air 2.58 0.04 Nitric Acid(PH) 16.49 0.27 Process Water 3.96 0.06 Nitric Acid(CH) 715.34 11.58 NH3 (PH) 159.44 2.58 Tail Gas(PH) 2.67 0.04 NH3 (CH) 5586.69 90.45 Tail Gas(CH) 112.95 1.83 Electricity 126.72 2.05 Steam Export 585.92 9.49 Make Up Water 212.56 3.44 Electricity (ORC) 205.24 3.32 NH3 Heating 84.71 1.37 Heat to Cooling Tower 210.84 3.41 Total 6176.66 100.00 Total 1849.46 29.94 Exergy Destruction 4324.34 70.06
Exergy Analysis of Nitric Acid, Ethylene Oxide/Ethylene Glycol Processes and Methanol Reactor
Chapter-4 Page 91
4.8 Cost Benefit Analysis
Financial analysis is important for implementation of any energy conservation project.
Waste heat recovery by using ORC is producing electricity with a net output of 712.64
kW. Installation of ORC required large capital cost that is higher than Steam Rankine
Cycle. Electricity produced can be used in the plant itself or can be given to state
electricity board grid if excess. Cost per unit of electricity is depending upon various
factors that include production cost and taxes. Central, state electricity boards are making a
contract with power plants for the purchase of electricity for longer periods and selling it to
their customers. If the quantity of power purchased from electricity board is reduced due
to proposed ORC, the industry can get a higher benefit than selling power to the grid.
Electricity cost for industry varies all over the country and it is in the range of 3.5 to 7
Rs/kWh.
In the present financial analysis, the payback period is calculated for the different unit cost
of electricity. Power available from ORC plants is 5.70 million units per year considering
8000 operating hours. Installation cost of thermal power plant in India is approximately
Rs. 6.45 Crores/MW (IFFCO Chhattisgarh Power Ltd, 2014). ORC cost is 1.8 times
thermal power plant that are equal to Rs. 8.24 Crores/MW. Operation and maintenance
cost is around 8.5% of capital cost. Table 4.13 shows cost-benefit analysis of proposed
ORC plant. Acceptable payback period in most of the energy conservation projects is
between 4 to 5 years. If unit cost of electricity is above 4 Rs./kWh, installation of ORC
Exergy Analysis of Nitric Acid, Ethylene Oxide/Ethylene Glycol Processes and Methanol Reactor
Chapter-4 Page 92
4.9 CO2 Reduction Climate change due to global warming is a serious concern for the whole world.
Industrialization and generation of electricity are mainly based on fossil fuels. It leads to
the emission of thousand tons of carbon dioxide in the atmosphere. In the year 2012
carbon dioxide emission from consumption of energy was 32310.287 million metric tons
(International energy statistics, 2014). Carbon di oxide emission from coal-based power
plants in India is 0.87 kg/kWhel (Raghuvanshi et al., 2006). ORC installed in the nitric acid
plant and reduction in compressor power can generate 254.26 MJ/t of additional power. It
is having a potential of reduction of 6302.73 metric tons of carbon dioxide per annum in
300 TPD nitric acid plant.
4.10 Conclusion The nitric acid plant is a net exporter of energy. Ammonia oxidation is a major source of
energy. Plants energy efficiency is 31% and exergy efficiency is 20.83 %. Heat energy
discarded through cooling tower water is 67.86 % of the total energy. Reaction
irreversibility is a major cause of exergy loss. The Exergy efficiency of the plant can be
increased by reducing exergy losses. It is concluded that
By reducing the inlet air temperature in the compressor, exergy efficiency of the
plant can be increased up to 21.33%.
By using heat given by cooler condenser in Organic Rankine Cycle power plant,
exergy efficiency can be increased up to 24.15%. ORC plant is economically viable
if unit cost of electricity above 4 Rs./kWh.
The additional available energy of 254.26 MJ/t from both measures will increase
exergy efficiency of the nitric acid plant from 20.83 % to 24.65%.
Apart from above benefits, reduction of CO2 emission is an added advantage. Above
savings can reduce 57.56 kg of CO2 per ton of acid (100%) Carbon credits will help to
reduce expenditure required for above improvements.
Exergy Analysis of Nitric Acid, Ethylene Oxide/Ethylene Glycol Processes and Methanol Reactor
Chapter-4 Page 93
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