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: π»πππ‘ ππ’π‘π¦ π π πΆπ = = = πΆβππππ ππ ππππππππ‘π’ππ βπ πππ’π‘ − πππ The min temperature difference is βππππ = 20β so the βππππ 2 = 10β The shifting rule is: For hot stream, the shifted temperature would be π ∗ = πβππ‘ π π‘ππππ − βππππ For cold stream, the shifted temperature would be π ∗ = πππππ π π‘ππππ + 2 βππππ 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 βππππ = 20β 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: 2 Table 2 shows the problem table cascade in the base case scenario (dry recycle, no heating) with βππππ = 20β 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 οCPc οCPh οTi (oC) οHi (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 οH (kW) 0.00 1217.01 1278.96 1330.82 1077.38 1052.45 -3156.81 οHadjust (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, βπ ο· The pinch temperature for hot stream would be πππππβ πππ βππ‘ π π‘ππππ = π ∗ ππππβ + 2πππ = 185β + 10β = 195β. ο· The pinch temperature for cold stream would be πππππβ πππ ππππ π π‘ππππ = π ∗ ππππβ − βππππ 2 = 185β − 10β = 175β. The hot utility target is 3455 kW, and the cold utility target is 3669.01 kW 3 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 βππππ = 20β The flue gas temperature will start at the theoretical flame temperature (1800 oC). βπ This corresponds to a shifted temperature of 1800 β − min −2πππ’π πππ = 1800 β − 25 β = 1775β 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. βπ The actual stack temperature would be 185 β + min −2πππ’π πππ = 185 β + 25 β = 210β. 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. 4 Figure 2: Grand Composite Curve of the process and flue gas as hot utility in the base case scenario (dry recycle, no heating) with βππππ = 20β For the fuel consumption, πΆππΉπΏππΈ πΊπ΄π ππ»πππ = 3455 ππ ππ»πππ 3455 = = = 2.17 ππ/πΎ βπ 1775 − 185 The fuel consumption is calculated for flue gas from theoretical flame temperature to ambient temperature ππΉπΏππΈ πΊπ΄π ππππ’ππππ = πΆππΉπΏππΈ πΊπ΄π × βπ = 2.17 × (1800 − 20) = 3867.86 ππ πΉπ’πππππ ππππππππππ¦ = ππ»πππ ππΉπΏππΈ πΊπ΄π ππππ’ππππ × 100 = 3455 × 100 = 89.33% 3867.86 The flow rate of flue gas required is: πΉππ’π πππ ππππ€ πππ‘π = ππΉπΏππΈ πΊπ΄π ππππ’ππππ × π»πππ‘πππ π£πππ’π 1 ππ ππ½ = 3867.86 ππ × × 47.1 = 0.082 ππ/π 1000 ππ ππ For the CO2 stack emissions: The stack loss is calculated for flue gas from stack temperature to ambient air temperature πππ‘πππ πππ π = πΆππΉπΏππΈ πΊπ΄π × βπ = 2.17 × (210 − 20) = 412.86 ππ The CO2 stack emissions are πΆπ2 π π‘πππ ππππ π πππ = πππ‘πππ πππ π × πΈπππ π πππ πΉπππ‘ππ 1 πΊπ πΆπ2 ππ πΆπ2 3600π = 412.86 ππ × 6 × 58.3 ππ = 0.024 × 10 ππ πΊπ½ π 1βπ = 86.65 ππ πΆπ2 /βπ The design scenarios will produce 281 kg/hr H2. Therefore, CO2 footprint of SOEC process is 5 πΆπ2 ππππ‘πππππ‘ = πΆπ2 π π‘πππ ππππ π πππ 86.65 ππ πΆπ2 /βπ = = 0.31 ππ πΆπ2 /ππ π»2 π»2 πππππ’ππ‘πππ 281 ππ π»2 /βπ 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. 6 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 π since πΆπ = βπ. 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. 7