GREEN AMONIA PROCESS
OPTIMISATION
CHE4173 – SUSTAINABLE
ENGINEERING
NGOC GIA HAN NGUYEN
ATIQAH NOOR HISHAM
01/04/2022
1
Question 1 – Stream Table
The heat capacity (CP) can be calculated by:
𝐻𝑒𝑎𝑡 𝑑𝑢𝑡𝑦
𝑄
𝑄
𝐶𝑃 =
=
=
𝐶ℎ𝑎𝑛𝑔𝑒 𝑜𝑓 𝑇𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒
∆𝑇
𝑇𝑜𝑢𝑡 − 𝑇𝑖𝑛
The min temperature difference is ∆𝑇𝑚𝑖𝑛 = 20℃ so the
∆𝑇𝑚𝑖𝑛
2
= 10℃
The shifting rule is:
For hot stream, the shifted temperature would be 𝑇 ∗ = 𝑇ℎ𝑜𝑡 𝑠𝑡𝑟𝑒𝑎𝑚 −
∆𝑇𝑚𝑖𝑛
For cold stream, the shifted temperature would be 𝑇 ∗ = 𝑇𝑐𝑜𝑙𝑑 𝑠𝑡𝑟𝑒𝑎𝑚 +
2
∆𝑇𝑚𝑖𝑛
2
Based on above calculation, the stream table is shown in Table 1.
Table 1 shows the stream table in the base case scenario (dry recycle, no heating) with ∆𝑇𝑚𝑖𝑛 = 20℃
Stream Type
No.
Mass
P
Tin
Flowrate (bara) (oC)
(kg/hr)
1a
Cold 8858.1
40
25
1b
Cold 8858.1
40
250.3
1c
Cold 8858.1
40
251.3
2
Cold 2118.7
40
222.9
3
Cold 9545.7
40
379.9
4
Hot
8925.4
39.3
800
5
Hot
8925.4
39
195
6
Hot
2739.1
39.3
800
7
Hot
687.6
40
35
* T* was denoted as shifted temperature
Tout
(oC)
Q (kW)
CP
(kW/oC)
T*in
(oC)
T*out
(oC)
250.3
251.3
420
550
610
195
35
35
35
2453.15
4215.79
1133.54
207.20
1409.70
3854.80
5176.30
602.30
0
10.89
4215.79
6.72
0.63
6.13
6.37
32.35
0.79
0
35
260.3
261.3
232.9
389.9
790
185
790
25
260.3
261.3
430
560
620
185
25
25
25
Question 2
To be able to find the hot and cold utility targets, and process pinch temperature, the
problem table algorithms was carried out, as shown in Table 2:
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Table 2 shows the problem table cascade in the base case scenario (dry recycle, no heating) with ∆𝑇𝑚𝑖𝑛 = 20℃
Stream 1a
Ti (oC)
790
620
560
430
389.9
261.3
260.3
1b
1c
6.72
6.72
4215.7
9
232.9
10.89
185
10.89
35
10.89
25
*(i) was denoted as interval
2
0.63
0.63
0.63
0.63
0.63
3
6.13
6.13
6.13
6
CPc CPh
Ti
(oC)
Hi
(kW)
6.37
6.37
6.37
6.37
6.37
6.37
0.79
0.79
0.79
0.79
0.79
0.79
170.00
60.00
130.00
40.10
128.60
1.00
6.37
6.37
0.79
0.79
0.79
0.79
-7.16
-1.03
-0.40
6.32
0.19
4209.2
7
4.36
3.73
-22.25
-33.14
27.40
47.90
150.00
10.00
4
5
32.35
32.35
-1217.01
-61.95
-51.87
253.44
24.93
4209.27
Total
H (kW)
0.00
1217.01
1278.96
1330.82
1077.38
1052.45
-3156.81
Hadjust
(kW)
3455.00
4672.01
4733.96
4785.82
4532.38
4507.45
298.19
119.54
178.64
-3337.62
-331.39
-3276.36
-3455.00
-117.38
214.02
178.64
0.00
3337.62
3669.01
As we can see from the table, the shifted pinch temperature is 185 oC, where is equal to 0. Therefore,
∆𝑇
 The pinch temperature for hot stream would be 𝑇𝑝𝑖𝑛𝑐ℎ 𝑓𝑜𝑟 ℎ𝑜𝑡 𝑠𝑡𝑟𝑒𝑎𝑚 = 𝑇 ∗ 𝑝𝑖𝑛𝑐ℎ + 2𝑚𝑖𝑛 = 185℃ + 10℃ = 195℃.

The pinch temperature for cold stream would be 𝑇𝑝𝑖𝑛𝑐ℎ 𝑓𝑜𝑟 𝑐𝑜𝑙𝑑 𝑠𝑡𝑟𝑒𝑎𝑚 = 𝑇 ∗ 𝑝𝑖𝑛𝑐ℎ −
∆𝑇𝑚𝑖𝑛
2
= 185℃ − 10℃ = 175℃.
The hot utility target is 3455 kW, and the cold utility target is 3669.01 kW
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For the flue gas required, from the problem table cascade in Table 2, a grand composite
curve was plotted, as shown in Figure 1.
Figure 1: Grand Composite Curve of the process in the base case scenario (dry recycle, no heating) with ∆𝑇𝑚𝑖𝑛 = 20℃
The flue gas temperature will start at the theoretical flame temperature (1800 oC).
∆𝑇
This corresponds to a shifted temperature of 1800 ℃ − min −2𝑓𝑙𝑢𝑒 𝑔𝑎𝑠 = 1800 ℃ −
25 ℃ = 1775℃ on the grand composite curve. The heat duty of the flue gas is equal to
the utility target of 3455 kW.
The flue gas profile is not restricted above the pinch, so it can be cooled to the pinch
temperature of 185 oC before venting to the atmosphere.
∆𝑇
The actual stack temperature would be 185 ℃ + min −2𝑓𝑙𝑢𝑒 𝑔𝑎𝑠 = 185 ℃ + 25 ℃ = 210℃.
The actual stack temperature is above the acid dew point of 125 oC.
The Grand Composite Curve with the Hot Utility is shown in Figure 2.
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Figure 2: Grand Composite Curve of the process and flue gas as hot utility in the base case scenario (dry recycle, no heating)
with ∆𝑇𝑚𝑖𝑛 = 20℃
For the fuel consumption,
𝐶𝑃𝐹𝐿𝑈𝐸 𝐺𝐴𝑆
𝑄𝐻𝑚𝑖𝑛 = 3455 𝑘𝑊
𝑄𝐻𝑚𝑖𝑛
3455
=
=
= 2.17 𝑘𝑊/𝐾
∆𝑇
1775 − 185
The fuel consumption is calculated for flue gas from theoretical flame temperature to
ambient temperature
𝑄𝐹𝐿𝑈𝐸 𝐺𝐴𝑆 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 = 𝐶𝑃𝐹𝐿𝑈𝐸 𝐺𝐴𝑆 × ∆𝑇 = 2.17 × (1800 − 20) = 3867.86 𝑘𝑊
𝐹𝑢𝑟𝑛𝑎𝑐𝑒 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 =
𝑄𝐻𝑚𝑖𝑛
𝑄𝐹𝐿𝑈𝐸 𝐺𝐴𝑆 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑
× 100 =
3455
× 100 = 89.33%
3867.86
The flow rate of flue gas required is:
𝐹𝑙𝑢𝑒 𝑔𝑎𝑠 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒 = 𝑄𝐹𝐿𝑈𝐸 𝐺𝐴𝑆 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 × 𝐻𝑒𝑎𝑡𝑖𝑛𝑔 𝑣𝑎𝑙𝑢𝑒
1 𝑀𝑊
𝑀𝐽
= 3867.86 𝑘𝑊 ×
× 47.1
= 0.082 𝑘𝑔/𝑠
1000 𝑘𝑊
𝑘𝑔
For the CO2 stack emissions:
The stack loss is calculated for flue gas from stack temperature to ambient air temperature
𝑄𝑆𝑡𝑎𝑐𝑘 𝑙𝑜𝑠𝑠 = 𝐶𝑃𝐹𝐿𝑈𝐸 𝐺𝐴𝑆 × ∆𝑇 = 2.17 × (210 − 20) = 412.86 𝑘𝑊
The CO2 stack emissions are
𝐶𝑂2 𝑠𝑡𝑎𝑐𝑘 𝑒𝑚𝑖𝑠𝑠𝑖𝑜𝑛 = 𝑄𝑆𝑡𝑎𝑐𝑘 𝑙𝑜𝑠𝑠 × 𝐸𝑚𝑖𝑠𝑠𝑖𝑜𝑛 𝐹𝑎𝑐𝑡𝑜𝑟
1 𝐺𝑊
𝐶𝑂2
𝑘𝑔 𝐶𝑂2 3600𝑠
= 412.86 𝑘𝑊 × 6
× 58.3 𝑘𝑔
= 0.024
×
10 𝑘𝑊
𝐺𝐽
𝑠
1ℎ𝑟
= 86.65 𝑘𝑔 𝐶𝑂2 /ℎ𝑟
The design scenarios will produce 281 kg/hr H2. Therefore, CO2 footprint of SOEC process is
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𝐶𝑂2 𝑓𝑜𝑜𝑡𝑝𝑟𝑖𝑛𝑡 =
𝐶𝑂2 𝑠𝑡𝑎𝑐𝑘 𝑒𝑚𝑖𝑠𝑠𝑖𝑜𝑛
86.65 𝑘𝑔 𝐶𝑂2 /ℎ𝑟
=
= 0.31 𝑘𝑔 𝐶𝑂2 /𝑘𝑔 𝐻2
𝐻2 𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑖𝑜𝑛
281 𝑘𝑔 𝐻2 /ℎ𝑟
Question 3
Above Pinch
Figure 3: Heat exchange network design for the above pinch region
The heat exchange network is designed such that the hot utility target of 3455 kW is
supplied by the cooling of flue gas in the above pinch region, as represented by the red
“heat exchangers” in Figure 3. The boiler is divided into three parts namely the boiler
feedwater (BFW) pre-heater, the steam generator, and the superheater and are given by
streams 1a, 1b and 1c respectively. Figure 3 shows the heat exchange network design for
the above pinch region which treats flue gas as a hot stream in order to show how heat is
exchanged from the flue gas to the boiler. Heat exchange between the flue gas and the
steam generator (Stream 1b) principally relies on radiant heat as the steam generator will
typically operate at a very high pressure, whereby the CP of steam is high enough for
cooling the flue gas at its hottest end. The pre-heater (Stream 1a), as it operates at the
lowest pressure in the boiler, is placed at the colder end of the flue gas stream and the
superheater (Stream 1c) in between the two. Since the CPs of streams 1a and 1c are
significantly lower, both the pre-heater and superheater will rely on convective heat from
the flue gas.
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Below Pinch
Figure 4: Heat exchange network design for the below pinch region
Figure 4 shows the heat network design for the below pinch region, whereby cold utilities
are placed in streams 5 and 6, totalling up to 3669.01 kW which matches the cold utility
target that has been calculated in question 2.
Question 4
The heat exchanger which exchanges heat between Stream 1a and Stream 6 has the heat
duty of 66.9 kW. The heat duty is well below 100 kW, which is the standard of a normal
small heat exchanger. Therefore, if possible, this heat duty should be removed for the fact
that it is not efficient to invest capital in such a small duty heat exchanger.
The CP of flue gas in this scenario (dry recycle, no heating) is 2.17 kW/K. However, in other
cases with dry recycling and moderate heating, the CP of flue gas is slightly lower at 2.11
kW/K. It is expected that with more heating, the lower the CP of flue gas. With heating, the
feed temperature is higher, the temperature difference smaller, and thus the CP is lower
𝑄
since 𝐶𝑃 = ∆𝑇.
A higher CP means that more energy is required to raise or lower its temperature and there
is a higher number of heat transfer units. This is not cost-effective considering the capital
expenditure. Additionally, flue gas with a higher CP will take a longer time to be cooled
down, and hence the heat exchange with the boiler will also take a longer time. Therefore,
various improvements could be made to lower the CP of the flue gas as seen in the other
scenarios which include a recycling stream and heating.
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