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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.

1. 4NH3 + 5O2 = 4NO + 6H2O ΔHR,298 = -226.60 kJ/mol

2. 4NH3 + 4O2 = 2N2O +6H2O ΔHR,298 = -275 kJ/mol

3. 4NH3 + 3O2 = 2N2+ 6H2O ΔHR,298 = -318 kJ/mol

Fig. 4.1 Process flow diagram of nitric acid production (T: steam turbine, C: compressor,

E: expander, HE-1-HE-7: heat exchanger, CC: cooler/condenser, AV: ammonia vaporizer,

AS: ammonia superheater, AB: absorber, AOR: reactor, M: static mixer).

At high pressure, 96% ammonia is converted into NO and reaming 4% is converted into

N2 and N2O (Kirova-Yordanova, 2011).The reaction temperature reaches up to 1189.15 K


Chapter-4 Page 64

due to highly exothermic nature. A train of heat exchangers is used to recover the heat.

Steam of 4.2 MPa is generated in waste heat boiler (WHB). Part of it is fed to steam

turbine after superheating. Large oxidation spools are included in the heat exchanger train

to promote reaction 4 and allow recovery of the reaction heat in the heat exchangers (HE-1

to HE-4).

4. 2NO + O2 = 2NO2 ΔHR,298 = -57.11 kJ/mol

After the energy recovery, nitrous gases are passed through a cooler condenser (CC) where

it is further cooled to 325.15 K at 1.2 MPa. In the nitric acid plant, the design of condenser

is crucial for obtaining the desired concentration of product acid from the absorption

section (Chatterjee and Joshi, 2008).Water is condensed due to lower temperature with the

simultaneous absorption of nitrogen oxides. The nitric oxide reacts with residual oxygen to

form nitrogen dioxide and its liquid dimer, nitrogen tetroxide.

5. 2NO2 = N2O4 ΔHR,298 = -57.32 kJ/mol

Weak nitric acid and nitrous gases in cooler condenser are separated and fed to the

absorber (AB). Water is fed from the top and air is from the bottom of the tower through

the bleaching section.

6. 3NO2 + H2O = 2HNO3 + NO ΔHR,298 = -20.53 kJ/mol

Reaction no. 6 is exothermic and continuous removal of heat is required. Upper trays of

the column are provided with chilled water coils and lower trays are provided with cooling

water coils. From reaction 5 & 6, every three moles of NO2 reacts with water to form two

moles of nitric acid and gives back one mole of NO. This NO should be reoxidized with

additional O2 from the air. Every time one-third of the oxidized nitrogen has to be re-

oxidized with oxygen during formation of nitric acid. Nitric acid (60%) and tail gas

(almost 97% N2) are going out form the column at 339.15 K and 287.15 K respectively.

Product acid is sent for storage. Tail gas is passed through various heat exchangers and


Chapter-4 Page 65

finally expanded to the gas turbine (E) which is part of turbo-compressor as shown in Fig.

4.1. Mass balance of overall process is shown in Table 4.1.

Table 4.1 Material balance of nitric acid plant

Input Flow (kg/t of 100% acid) Temperature (K) Pressure (MPa)

Air

5054.48 308.15 0.101 N2 (74.40%)

O2 (22.60%)

Water (3.01%)

Ammonia 281.11 240.15 1.376 Process water 300 313.15 1.96

Output Flow (kg/t of 100% acid) Temperature (K) Pressure (MPa)

Nitric acid 1634.84 319.15 1.213 Nitric acid (60%)

Water (40%)

Tail Gas

3885.3 320.15 0.101 N2 (97.01%)

O2 (2.86%)

H2O (0.1%)

NO (0.018%)

Water from compressor 115.45 310.15 Inter stage

pressure

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


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


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


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)


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


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.


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


Chapter-4 Page 72

Exergy of humid air is calculated by (Dincer and Rosen, 2007)

EAir = (CP,a + ωCP,V)T0 (T/T0 -1- ln T/T0) + (1+ ῶ)RaT0lnP/P0 +RaT0[((1+ ῶ)ln((1+ ῶ0)/

(1+ ῶ)+ ῶ ln(ῶ/ῶ0) (4.2)

Mechanical work and electricity both are having the same exergy value as that of energy.

Exergy destruction = [Physical exergy of air in + work + Physical exergy of cooling water

in] – [Physical exergy of compressed air + Physical exergy of interstage water discharge +

Physical exergy of cooling water out ]

Expander /Turbine

Turbocompressor is powered by expander and turbine. Hot tail gas is input to expander

and steam is for the turbine. Compressor, expander and turbine are mounted on the same

shaft. It is rotated using work produced by expander and turbine.

Fig. 4.9 Expander/Turbine

Exergy destruction = [Physical exergy in] – [Physical exergy out + Work]

Heat Exchanger

Exergy analysis of heat exchanger is discussed in chapter-2. Various heat exchangers in

the process consist of gas-gas, gas-liquid and liquid-liquid systems like expander gas

heater, turbine condenser, air cooler, ammonia heater, etc.


Chapter-4 Page 73

Fig. 4.10 Heat Exchanger

Exergy destruction = [Exergy given by hot fluid] – [Exergy taken by cold fluid]

Nature of exergy is either physical, chemical or both are depending upon the nature of the

stream.Exergy calculations for equiments are shown in Appendix-II.

4.4.2. Pervious Work

Nitric acid production involves mainly chemical exergy rather than physical exergy.

Szargut et al. (1988) found more irreversibility in a chemical reaction when ammonia is

converted into nitric oxide. The degree of the perfection of the process having capacity

100TPD is 30.5%. Kirova-Yordanova et al. (1994) carried out exergy analysis of two dual

pressure nitric acid plants and found efficiency in the range of 17-34 %. Both plants have

same operating conditions expects tail gas treatment method and heat recovery system.

Steam turbine provides half of the work required while expander provides remaining in the

first plant while in a second plant the expander supplies all work. Tail gas treatment is

done by extended absorption in the first plant. Natural gas is used for the treatment of tail

gas in the second plant and the gas is exhausted into the atmosphere at 453.15 K, which is

having considerable exergy. Total input exergy of the plant depends upon various utilities

used along with the raw material. The major exergy loss is in the reactor followed by a

turbo-compressor. Cooling tower water is used in both cooler condensers to extract heat. In

another study by Gaggioli et al. (1991) integration of nitric acid plant using exergy

analysis is done into an existing steam production site. Exergy input to nitric acid plant is

60 MW and maximum recoverable exergy is 30.6 MW. The existing site integration

proposal gives 3.965 MW generator output. Apart from reaction irreversibility major


Chapter-4 Page 74

losses are occurring in condensate and throttling. Some of the measures suggested by

Gaggioli et al. (1991) are

1. Matching heating streams with their specific heat values.

2. To raise steam pressure

3. To employ steam reheater after partial turbine expansion.

There are many possible solutions for the optimization of heat recovery in the process.

Plants operating all over the world are having different efficiencies though having same

process parameters. Climatic conditions and demand for steam or electricity are the

factors, which influence maximum heat recovery solutions.

4.4.3. Exergy Analysis of Nitric Acid Plant

In the plant under study, ammonia contributes almost 94.33 % of input exergy. Total input

exergy is 6091.95 MJ/t including chemical and physical exergy. Though air gives a

considerable amount of energy, its exergy input is negligible due to low temperature. Only

12% of input exergy is converted into nitric acid. MP steam available for export is having

8.81 % exergy. 73.81% of input exergy is destroyed in the plant. Exergy analysis of nitric

acid plant is shown in Table 4.3. The environmental parameters considered for exergy

analysis are P0 = 0.101 MPa and T0 = 298.15 K.

Overall plant exergy efficiency is

= (Exergy in Nitric acid + Exergy of steam exported)/ exergy input

= 20.83 %

Table 4.3 Exergy analysis of nitric acid plant

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 0.07 Nitric Acid(CH) 715.34 11.74 NH3(PH) 159.44 2.62 Tail Gas(PH) 2.67 0.04 NH3(CH) 5586.69 91.71 Tail Gas(CH) 112.95 1.85 Electricity 126.72 2.08 Steam Export 536.91 8.81 Makeup Water 192.12 3.15 Heat to Cooling Tower 211.24 3.47 NH3 Heating 20.45 0.34 Total 6091.95 100.00 Total 1595.60 26.19 Exergy Destruction 4496.36 73.81


Chapter-4 Page 75

Heat exchangers used for process gas heat recovery are considered as plug flow reactors

due to oxidation reaction taking place in it. Exergy given by hot fluid is not equal to

exergy taken by a cold fluid due to exergy destruction in the heat exchanger. Heat

generation in the pipeline is incorporated in the neighboring equipment. Exergy flow in

heat exchanger train including heat of reaction is shown in Fig. 4.2. Most of the energy in

the tail gas is recovered hence there is less loss of exergy. Heat available in cooling water

is also having very less exergy due to its low temperature. Table 4.4 shows exergy

efficiencies and exergy destruction in various equipments in the plant. Table 4.5 shows a

comparison of exergy analysis of different plants.

Table 4.4 Exergy efficiency and destruction in plant equipments

Equipment Code Description Exergy Efficiency of

equipment (%)

% of total exergy destruction of plant

in equipment TC Turbine Condenser 62.24 0.58

AV Ammonia Vaporiser 14.38 0.81

AS Ammonia Superheater 51.05 0.22

HE-4 Heat Exchanger-4 77.81 0.96

HE-1 Heat Exchanger- 1 81.69 3.89

WHB Waste Heat Boiler 69.44 9.21

HE-2 Steam Superheater 84.46 0.42


CC Cooler Condenser 24.44 12.48


HE-5 Economiser 87.03 0.55


AOR Ammonia Oxidation Reactor 73.91 40.84

C Air Compressor 50.08 12.21

E Gas Expander 90.68 4.07

T Steam Turbine 67.73 4.52

AB Absorber 85.98 5.76


Chapter-4 Page 76

Table 4.5 Comparison of exergy analysis of different nitric acid plants.

Reference Plant

Plant under

present study

Kirova et al. (1994)-1

Kirova et al. (1994)-2

Gaggioli (1991)

Szargut et al.

(1988)

Process Pressure Mono High Dual Dual Dual Dual

Oxidation Pressure (MPa) 1.3 0.33 0.39 0.35 0.115

Oxidation Temperature (K) 1189.15 1126.15 1123.15 1113.15 1206.15

Absorption Pressure (MPa) 1.3 1.24 1.07 1.025 0.52

HNO3 Concentration (%) 60 55 60 51.2 67

Power to Air Compressor by Steam Turbine (%)

70 50 0 38.15 0

Power to Air Compressor by Gas Turbine (%)

30 50 100 61.85 0

Input Exergy (MJ/t ) 6091.95 6000.1 9266.7 5833.3 6411.74 Useful Output Exergy (MJ/t) 1105.31 1047.8 2622.8 1162.3 1956.96

Exergy Loss (MJ/t) 4986.64 4952.3 6643.9 4671.01 4454.78 Exergy Efficiency (%) 20.83 17.46 28.30 19.93 30.5

4.4.4. Exergy Loss in Reactor

Ammonia is oxidized into nitrogen oxide i.e., conversion of high exergy raw material to

low exergy product. Heat produced in the process is not equal to the difference in exergy.

Exergy is lost in the process due to irreversibility in the oxidation reaction. It is the highest

loss in the plant. It is an unavoidable exergy loss until the process route is changed.

Another loss due to the reaction is in the conversion of nitric oxide to nitrogen dioxide.

Exergy loss can be reduced if simultaneous nitric oxide and electricity can be produced in

high temperature ammonia solid electrolyte fuel cell (Farr, 1979). If a cell has 100%

efficiency, the electrical energy produced is equal to exergy reduced by reactants.

Presently research work carried out about ammonia SOFC is to generate electricity

through maximum hydrogen production instead of NO.


Chapter-4 Page 77

4.4.5. Exergy Loss in Turbocompressor

Turbo compressor consists of the turbine, expander and compressor. Exergy loss in a

steam turbine is related to a reduction in steam temperature, pressure from its original state

and the functioning of mechanical parts. It can be improved by various measures such as

advanced shaft sealing, blade replacement, entire replacement of steam paths utilizing

state-of-the-art materials and condenser optimization (Pollak et al., 2004). Gas expander

performance can be improved by proper scheduled maintenance, improved blade designs

etc. CFD software can be used to design more efficient flow paths. Exergy loss in the

compressor is 510.84 MJ/t (Table 4.6). A large amount of heat is generated due to

compression that is removed by the cooling water through inter stage cooler.

4.4.6. Exergy Loss in Heat Exchangers

Efficient energy recovery from heat available in process gas using a train of the heat

exchanger is one the key factor in the process. Heat available in Nitrous gases is used in

waste heat boiler to produce steam. A Large amount of exergy destruction takes place in

WHB due to a large temperature difference in process gas and boiler feed water. Heat

available in cooling water is of very low grade. Only 6.5% exergy in cooling water is used

for ammonia vaporization (from CC to AV) and remaining is lost in the atmosphere (Fig.

4.3). Cooler condenser is the second largest candidate for exergy destruction after

ammonia converter. Total exergy loss in all heat exchangers is 1236.96 MJ/t, which

accounts 30% of the total. Simultaneous oxidation and absorption take place in the

absorber. Both are exothermic, hence cooling coils are provided on each tray. Exergy loss

in the absorber is through the transfer of reaction and absorption heat to the cooling water

in the lower part of the column and to the chilled water at the upper part.

4.5 Exergy Loss Minimization in Air Compressor

Compressed air is used to maintain high pressure in the process. The temperature of the

compressed air rises many times when it reaches the outlet of the multistage compressor.

Increased temperature at the inlet of each stage reduces compressor efficiency hence

intercoolers are provided at the end of each stage to improve efficiency. Total 1342.37


Chapter-4 Page 78

MJ/t of heat is taken out after compressing atmospheric air at to 1.3 MPa by providing

cooling tower water.

The compressor consumes about 23.7 % of the total input energy. Steam turbine and gas

expander power it. Small saving in compressor power will result in large saving in the

either gas or steam turbine. The power required for the compressor is 1885.82 MJ/t in the

present process. The work required can be reduced by reducing inlet air temperature.

Figure 4.12 shows the effect of inlet temperature on required power. In existing plant, the

compressor can handle minimum air temperature up to 290.15 K. At this temperature

power requirement of compressor goes down to 1800.28 MJ/t of 100% acid.

Table 4.6 Effect of the inlet temperature of air on compressor energy, cooling, and exergy values.

Sr No. 1 2 3 4 5 6 Inlet air temperature (K) 308.15 303.15 302.15 301.15 300.15 299.15 Power required for compressor (MJ/t) 1883.52 1875.00 1869.55 1864.20 1858.99 1853.89

Reduction in power required for compressor (MJ/t)

0.00 8.52 13.97 19.32 24.53 29.63

Heat removed by cooling tower water from interstage coolers of compressor (MJ/t)

1342.37 1274.26 1244.23 1215.36 1187.65 1161.00

Reduction in heat removal by cooling tower water (MJ/t)

0.00 68.10 98.14 127.01 154.72 181.37

Heat removed from air to bring down at required temperature from 308.15 K (MJ/t)

0.00 56.38 81.31 105.18 128.15 150.16

TR of refrigeration unit for cooling air 0.00 55.67 80.27 103.84 126.51 148.25

Exergy input to compressor (MJ/t) 1886.10 1876.92 1870.31 1864.65 1859.21 1853.96

Exergy output from compressor (MJ/t) 1375.26 1372.58 1371.39 1370.25 1369.16 1368.11

Exergy loss (MJ/t) 510.84 504.35 498.92 494.39 490.05 485.85 % reduction in exergy loss 0.00 1.27 2.33 3.22 4.07 4.89


Chapter-4 Page 79

Table 4.6 Effect of the inlet temperature of air on compressor energy, cooling and exergy values. (Continued)

7 8 9 10 11 12 13 14 15 298.15 297.15 296.15 295.15 294.15 293.15 292.15 291.15 290.15

1848.90 1844.03 1839.16 1833.64 1828.34 1822.45 1814.21 1806.37 1798.91

34.62 39.49 44.36 49.88 55.18 61.07 69.31 77.15 84.61

1135.38 1110.74 1086.91 1063.22 1040.49 1017.85 993.60 970.25 947.93

206.99 231.63 255.46 279.15 301.88 324.52 348.77 372.12 394.44

171.31 191.63 211.16 229.94 248.01 265.41 282.15 298.30 313.86

169.13 189.19 208.47 227.02 244.86 262.03 278.56 294.50 309.86

1848.91 1844.04 1839.24 1833.86 1828.75 1823.10 1815.16 1807.66 1800.57

1367.10 1366.13 1365.20 1364.26 1363.37 1362.47 1361.52 1360.60 1359.72

481.81 477.91 474.05 469.60 465.39 460.63 453.64 447.06 440.85 5.68 6.45 7.20 8.07 8.90 9.83 11.20 12.49 13.70

A new refrigeration unit is proposed to reduce inlet air temperature. The condenser is

maintained at 313.15 K by using cooling tower water. The inlet temperature of the

refrigerant in the evaporator is 284.85 K. A mixture of R290 and R600a in equal

proportion is used as refrigerant. It is having zero ozone depletion and very low global

warming potential. Though refrigerant is flammable, it can be handled safely in the plant

due to regular safety practices. Effect of reduced inlet air temperature on compressor

power and cooling duty is presented in Table 4.6. A net saving of 31.07 MJ/t can be

achieved by providing 53.53 MJ/t of compressor energy for refrigeration cycle to bring

down air temperature from 308.15 K to 290.15 K (Fig. 4.11). Almost 14% reduction in the

compressor exergy destruction takes place when inlet air temperature is reduced to 290.15

K (Fig. 4.12). Reduction in compressor power saves 51.2 kg/t of steam from a steam

turbine. Due to the availability of extra steam for export, the overall exergy efficiency of


Chapter-4 Page 80

the plant increases up to 21.33%. Effect of refrigeration unit on cooling tower and exergy

balance is discussed in section 4.7.

Fig. 4.11 Effect of inlet air temperature at different intercooler temperature

Fig. 4.12 Reduction in exergy destruction of compressor at different inlet air temperature

1750

1770

1790

1810

1830

1850

1870

1890

290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307

Pow

er (k

W)

Temperature K

310.37 K309.26 K308.15 K

Intercooler temperature

0

2

4

6

8

10

12

14

16

290 291 292 293 294 295 296 297 298 299 300 301 302 303

% r

educ

tion

in E

D

Temperature K


Chapter-4 Page 81

4.6. Heat Recovery in Cooler Condenser Cooler condenser is important equipment in nitric acid production placed before absorber.

It is not only a heat exchanger but also act as a reactor and absorber. Nitric oxide is

converted into nitrogen dioxide and nitrogen dioxide reacts with water to produce nitric

acid. Sensible heat, reaction heat, latent heat, the heat of absorption and heat of dilution

plays an important role. It gives output in two phases. Gas phase consists of Nitrous gases

while liquid phase consists of weak nitric acid. Weak nitric acid is produced in the form of

mist, which is separated at the end of the cooler condenser by using demister. Nitrous gas

enters cooler condenser at 470.15 K and leaves at 333.15 K. Heat available for utilization

is 2298.24 MJ/t. This heat is taken away by cooling water. Table 4.7 shows the percentage

of inlet and outlet composition of Nitrous gases in the cooler condenser. Nitric acid plant

under study is utilizing the exothermic heat of reaction and exporting excess heat in the

form of steam. Heat in the cooler condenser that is taken out by cooling water can be

converted into electricity.

Table 4.7 Inlet and outlet composition of cooler condenser

Component Mole % in Mole % out Nitrogen 71.17 73.87 Oxygen 2.15 1.18 Nitric Oxide 2.21 1.51 Nitrogen dioxide 7.66 3.26 Water 16.70 15.99 Dinitrogen tetra oxide 0.01 1.41 Nitric Acid 0.10 2.77 Total 100.00 100.00

The Organic Rankine Cycle (ORC) is used to produce electricity from low to medium

temperature heat sources. Steam is used as working fluid in Steam Rankine Cycle, which

is used in power plants while the organic liquid is used in ORC. Organic fluid is having

various advantages over steam. These fluids are characterized as dry fluids and having a

low boiling point. The heat of evaporation is very less compared to water (Li et al., 2012).

These fluids are having higher molecular weight than water hence useful to get more

power from the turbine.

ORC plants are employed for the production of energy from a heat source at different

temperatures. Quoilin et al. (2013) and Tchanche et al. (2011) carried out a review of


Chapter-4 Page 82

different ORC systems for the analysis of the application of various working fluids at

different heat sources. Most widely used fluids for low to medium temperature heat source

are isopentane, R-123, R-134a. ORC is commonly used for electricity production from

geothermal energy sources and also for heat recovery from flue gas in power plants.

Approximately 2.27% additional electricity above installed capacity in thermal power

plants can be generated by using R-123 as a working fluid to recover waste heat from flue

gas (Roy et al., 2010). Innovative approaches like solar desalination and biogas plants

using ORC are gaining importance nowadays (Schuster et al., 2009). Sun et al. (2011)

studied ORC optimization using R-134a to know the nonlinear functional relationship

between working fluid mass flow rate, condenser fan air flow rate and ambient dry bulb

temperature.

Exergy analysis can be used as a tool to assess the performance of ORC. Comparative

performance analysis of low-temperature Organic Rankine Cycle (ORC) using pure

working fluids and azeotropic working fluids shows that the exergy efficiency of the cycle

decreases when the expander inlet pressure increases (Aghahosseini and Dincer, 2013).

Using HCFC-123 as working fluid even at 353.15 K power generation is possible with

31% exergy efficiency (Li et al., 2012). Exergy analysis shows that maximum exergy loss

is taking place in evaporator followed by the turbine where the heat source is at more than

973.15 K (Mago et al., 2008). Comparison of exergy analysis between ORC and Kalina

cycle shows that higher ORC efficiency at higher temperature geothermal source

(Guzovicet al., 2010) and higher Kalina efficiency for lower temperature geothermal

source (Rodríguez et al., 2013). Comparative study of ORC of different temperature

geothermal sources using isobutene as the working fluid shows highest exergy efficiency

(El-Emama and Dincer, 2013; Tunc et al., 2013). Exergoeconomic model for the analysis

of geothermal power plants proposed by Coskun et al. ( 2011) shows that exergy

efficiency decreases with increasing outdoor temperature. The performance of the whole

cycle depends on the condenser performance that depends upon ambient temperature

(Sohel et al., 2011). R-123 can give maximum exergy efficiency for ORC used to recover

exhaust waste heat in the range of 500.15 K to 600.15 K from SOFC-GT cycle (Tuo,

2013). A series circuit of an ORC with isopentane as the working fluid and parallel circuit

ORC with R-227ea as working fluid are the most efficient for the same (Heberle and

Brüggemann, 2010). A trigeneration system of absorption chilling, water heating and


Chapter-4 Page 83

electricity generation with ORC gives almost 50% of exergy efficiency (Ahmadi et al.,

2012). SOFC trigeneration system is having a maximum cost per exergy unit than biomass

and solar trigeneration system (Al-Sulaiman et al., 2013). Energy and exergy efficiency of

different ORC is shown in Table 4.8.

Table 4.8 Energy and Exergy efficiency of different ORC

Fluid Exergy

Efficiency (%)

Energy Efficiency

(%)

Temp of heat

source (K)

Condenser Temp (K) Ref

R-12 30.01 12.09 413.15 300.15 Schuster et al. 2009 HCFC-123 31 4.6 347.15 275.95 Quoilin et al. 2013 R-113 31.4 19.72 1000.15 298.15 Mago et al. 2008 HCFC-123 37 8.2 379.15 277.25 Quoilin et al. 2013 R-134a 37.80 15.53 413.15 300.15 Schuster et al. 2009 HCFC-123 38 7.7 369.15 277.15 Quoilin et al. 2013 HCFC-123 40 7.4 358.15 275.95 Quoilin et al. 2013

R-227ea 40 10 448.15 288.15 Heberle & Brüggemann , 2010

iso-pentane 42 15 448.15 288.15 Heberle & Brüggemann , 2010

R-290 47.6 8.47 373.15 298.15 El-Emama et al. 2013

R-245fa 48 16 448.15 288.15 Desai & Bandopadhyay, 2009

iso-butane 48.8 16.37 438.15 288.15 Heberle & Brüggemann , 2010

iso-pentane 52 14.1 448.15 288.15 Rodríguez et al. 2013

iso-butane 54 14 448.15 288.15 Heberle & Brüggemann , 2010

R-123 64.40 25.30 413.15 300.97 Schuster et al. 2009

The ORC plant is proposed for 300 TPD of the nitric acid process is shown in Fig. 4.13.

Working parameters chosen are close to practical operating plant worldwide. The working

fluid is chosen as pentane to maintain cycle pressure above atmospheric pressure to avoid

leakage of air inside cycle (Desai and Bandyopadhyay, 2009). Leaked air will reduce the

thermodynamic performance of the cycle. Hydrocarbon fluids are flammable in nature, but

the chemical industry is well equipped to handle them. Flammability of pentane can be

reduced by adding carbon dioxide in it (Garget al., 2013). In this study, ORC is analyzed


Chapter-4 Page 84

using two fluids – n-pentane and iso-pentane. Cooling water is available at 306.65 K from

the existing cooling tower.

Fig. 4.13 a) Organic Rankine Cycle and b) TS diagram


Chapter-4 Page 85

Pentane is evaporated in the evaporator and vapors are sent to the turbine. The temperature

in the evaporator is maintained at 363.15 K to use the maximum heat available and avoid

condensation in the turbine. Vapors are expanded in dry turbine and exhaust vapors are

condensed in a condenser. The regenerator is used to recover heat available in turbine

outlet vapors that improve cycle efficiency. Liquid from the condenser is pressurized using

feed pump and send to preheater where the temperature is increased before sending to the

evaporator.

Simulation of ORC is done by Aspen Plus simulator using Peng-Robison equation of state.

The power of the turbine is given by

WT = mP (h1-h2) (4.3)

Heat rejected by pentane to water in the condenser is

QCON = mP (h3-h4) = mW (h12-h11) (4.4)

Heat available in turbine exhaust vapors is given to the fluid coming out of the pump in the

regenerator. Heat exchanged in the regenerator is

QR = mP (h2-h3) = mP (h6-h5) (4.5)

A booster pump is required to increase the pressure of pentane before sending to

evaporator. The power required to pump is given by

WP = mP (h5-h4) = mP (h5-h3)/ 0.8 (4.6)

In cooler condenser along with enthalpies of inlet vapor, heat of reaction is the source of

heat. Heat given by the evaporator and preheater to pentane is calculated as

QE = mP (h1-h7) = mNitrous (h8-h9) + HGE (4.7)

QPH = mP (h7-h6) = mNitrous (h9-h10) + HGP (4.8)

The thermal efficiency of ORC is

ORC = (WT - WP)/(QE + QPH) (4.9)

To get the best performance, maximum exergy of nitrous gases should be transferred to the

ORC working fluid. All equipments used in ORC have exergy destruction within them.

Exergy analysis will give a perfect picture about the quality use of energy in each of the

equipment. Exergy calculations are based on the equation shown in chapter 3.


Chapter-4 Page 86

4.7. Results and Discussion Energy and exergy profile of cooler condenser is shown in Fig.4.14. Heat is available

considerably when nitrous gases start condensing. Less heat is available from 470.15 K to

430.15 K compared to a temperature change from 403.15 K to 323.15 K. As gas progress

in the heat exchanger, vapor fraction reduces and more heat becomes available for

exchange due to the reaction and condensation (Fig. 4.15). Chemical exergy remains

almost constant in the condenser. The inlet temperature of the turbine is kept at 363.15 K

to recover heat available from 373.15 K to 333.15 K. If the temperature of pentane

increases above 363.15 K maximum heat recovery is not possible and hence reduces

turbine output.

Fig. 4.14 Energy and Exergy profile in cooler condenser

Heat energy used by ORC plant is 1961.56 MJ/t and 1921.24 MJ/t for iso-pentane and n-

pentane respectively. Nitrous gasses must be cooled further after recovering heat in ORC

to maintain absorber performance and to maintain nitrous concentration in the tail gas.

0

1000

2000

3000

4000

5000

6000

7000

323 333 343 353 363 373 383 393 403 413 423 433 443 453 463 473

MJ/

t

Temperature K

Energy

Exergy(PH)

Exergy(CH)


Chapter-4 Page 87

In the proposed ORC plant, turbine inlet pressure is maintained at 0.587 MPa for iso-

pentane and 0.46 MPa for n-pentane. Turbine efficiency is presumed to be 0.85 and pump

efficiency is 0.80. Turbine gives 213.58 MJ/t and 209 MJ/t output for iso-pentane and n-

pentane respectively. The work required for the pump is 8.35 MJ/t and 8 MJ/t respectively.

The results are given in Table 4.9 and 4.10.

Fig. 4.15 Molar vapor fraction and heat available in cooler condenser

Exergy values of nitrous gases are higher due to a considerable amount of physical and

chemical exergy present in them. Both ORC plants are giving 10.46 % thermal efficiency.

The power produced is limited by cooling water temperature. Power output can be

increased further by reducing turbine output pressure and using cooling water at lower

temperature. These conditions are not viable for the practical operation of the existing

plant. Exergy efficiency of proposed ORC is 33.57% for iso-pentane and 33.76% for n-

pentane with the mass flow rate of 17.4 and 16.2 Kg/s respectively. Exergy destruction of

ORC using iso-pentane as working fluid is 325.15 MJ/t. Exergy destruction in the

evaporator is highest followed by preheater in both plants as shown in Fig. 4.16. It is due

to a large temperature difference in hot and cold fluids. Condenser and pump are having

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

0

500

1000

1500

2000

2500

323 333 343 353 363 373 383 393 403 413 423 433 443 453 463

Vap

frac

tion

MJ/

t

Temperature K

Molar Vapor Fraction

Heat available for use after cooling NOx gases from 470 K


Chapter-4 Page 88

very less exergy destruction. Exergy destruction in cooler condenser is reduced by 42%

with ORC system. The proposed ORC plant gives 205.24 MJ/t of net output. Total exergy

output of nitric acid plant considering ORC is 1473.98 MJ/t.

Table 4.9 Stream parameters of ORC using iso-pentane

Stream Fluid m (kg/s) T (K) P (MPa) h (kJ/kg) s (kJ/kg K) e (kJ/kg) 1 iso-pentane 17.4 363.15 0.58 447 1.27 68.54 2 iso-pentane 17.4 334.01 0.15 404.37 1.29 19.95 3 iso-pentane 17.4 321.15 0.15 381.01 1.22 17.45 4 iso-pentane 17.4 311.65 0.15 30.49 0.1 0.69 5 iso-pentane 17.4 312.05 0.10 32.16 0.1 2.36 6 iso-pentane 17.4 322.15 0.10 55.52 0.17 4.86 7 iso-pentane 17.4 363.15 0.10 158.95 0.47 18.89 8 Nitrous Gas 15.39 469.89 1.22 427.14 0.51 526.28 9 Nitrous Gas 15.39 368.15 1.22 144.48 -0.18 441.27

10 Nitrous Gas 15.39 341.15 1.22 39.11 -0.47 414.05 11 Cooling Water 165.14 306.65 0.10 12 Cooling Water 165.14 315.48 0.10

Table 4.10 Stream parameters of ORC using n-pentane

Stream Fluid m (kg/s) T (K) P (MPa) h (kJ/kg) s (kJ/kg K) e (kJ/kg) 1 n-pentane 16.2 90 0.46 472.66 1.34 73.34 2 n-pentane 16.2 60.68 0.11 427.84 1.37 19.58 3 n-pentane 16.2 45 0.11 399.53 1.28 18.09 4 n-pentane 16.2 38.5 0.11 30.98 0.1 1.18 5 n-pentane 16.2 38.9 0.1 32.71 0.1 2.91 6 n-pentane 16.2 50.92 0.1 61.03 0.19 4.41 7 n-pentane 16.2 90 0.1 160.56 0.48 17.52 8 Nitrous Gas 15.39 196.74 1.22 427.14 0.51 526.28 9 Nitrous Gas 15.39 95 1.22 144.48 -0.18 441.27

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


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


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


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

plant is economically viable.

Table 4.13 Cost-benefit analysis of ORC

Unit Cost of Electricity (Rs/kWh)

Annual Income (Rs. Crores)

O&M Cost (Rs. Crores)

Net Annual Income (Rs. Crores)

Payback Period (Years)

3.00 1.71 0.70 1.01 8.22 3.50 2.00 0.70 1.29 6.40 4.00 2.28 0.70 1.58 5.25 4.50 2.57 0.70 1.86 4.44 5.00 2.85 0.70 2.15 3.85 5.50 3.14 0.70 2.43 3.40 6.00 3.42 0.70 2.72 3.04 6.50 3.71 0.70 3.00 2.76 7.00 3.99 0.70 3.29 2.52


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.


Chapter-4 Page 93

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