12
14
S
3
C bara
35.8 5.00 1.11 98.40 0.49
T (Overall) P (Overall) Water (vapour) Methane (vapour) Carbon dioxide (vapour)
13
23
mol % mol % mol %
CB
5
24
19
18
9
22
17
11
10
15
21
1
8
2
20
4
7
6
16
Figure 2 Schematic of biogas upgrading process
to upgrade the biogas to 0.5% ± 0.01% CO₂. Both absorber and regenerator contain Mellapak M250.X structured pack- ing with bed heights of 8 m and 5 m, respectively. In all simulations, the reboiler’s molar boil-up ratio was held constant at 0.1 (ratio of molar vapour flow produced by the reboiler to molar flow of liquid withdrawn from it). The solvent is 35 wt% MDEA with 5 wt% piperazine, and its flow rate is whatever the Solver calculates is necessary to produce biomethane with 0.5 mol% CO₂ (water-satu - rated basis). A CO₂ content of 0.5 mol% is not an especially stringent requirement for the performance of a piperazine-promoted solvent, so a very low lean solvent CO₂ loading is not needed to reach it. Case study In this discussion, we show the influence of pressure in the upgrading unit. With the fixed plant design as already described, the inlet gas and the flash pressures were varied, and the solvent flow rate was optimised to reach 0.5 vol% CO₂ in stream 3. All other parameters were kept constant.
The simulations were performed in ProTreat using the flowsheet shown in Figure 2. A total of 16 cases were run where the sulphur-free biogas pressure ranged from 1.05 to 6 bar(a). After the absorber, the solvent was fed to a flash unit so soluble CO₂/ CH₄ could be flashed out prior to the regeneration section. The pressure of the flash unit was also varied to see the effect on process performance. Table 1 shows the varied operating parameters (such as gas and flash pressures) and the main results of the simulations for each case. The solvent circulation rate, indicated by the liquid over gas ratio, decreases with increasing gas pressure indepen- dently of the flash pressure. This indicates that the size of the equipment could be reduced for plants operating pres - surised. As shown in Figure 3 Case 16 (about ambient pressure), the absorber is used at its full height, whereas in Case 1, about half of the absorber is not performing any separation (pinched in the upper half of the packing). The specific reboiler duty (SRD), the ratio between the reboiler duty (MW) and the total mass flow of CO₂ removed (flash and stripper, kg/s), also decreases with the
Case
Gas pressure, bara
Flash pressure, bara
L/G, kg/kg
SRD, MJ/kgCO2
CH₄ loss, %
CO2 flash out, %
1
6
1.5 1.5 1.5 1.5 1.5
7.4 7.7 8.1 8.6 9.4 7.4 7.7 8.1 8.6 9.4 7.4 7.7 8.1 8.6 9.4 11.2
1.9
0.08
9.2 5.9 1.8 0.2 0.0
2 3 4
5
2.0 2.1 2.2 2.3 2.0 2.0 2.1 2.2
0.07 0.06 0.05 0.04 0.08 0.07 0.06
4 3 2 6
5
6 7 8 9
2 2 2 2 2 3 3 3 3 2
3.7
5
1.1
4 3 2 6
0.2 0.0 0.0 0.3
0.05
10
2.
0.04 0.08 0.07 0.06 0.05 0.04 0.02
11
2.0 2.0 2.1 2.2 2.3 2.8
12 13 14 15 16
5
0.1
4 3 2
0.0 0.0 0.0 0.0
1.05
1.05
Table 1 Simulation results
24
PTQ Q4 2022
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