PTQ Q4 2022 Issue

(A)

(B)

Eqm partial pressure Actual partial pressure

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

Partial pressure (bar)

Figure 3 Actual and equilibrium partial pressures calculated with ProTreat for: (A) case 16; and (B) case 1

increase in the gas pressure, and it is almost independent of the flash pressure. However, the CH₄ loss increases almost linearly with the increase in pressure, as shown in Figure 4 . Nevertheless, methane loss is still negligible compared to other processes such as water scrubbing and membranes, which can show methane losses up to 20%.² If the upgrad - ing plant is operated under atmospheric conditions, CH₄ loss is lower than 0.02%. This benefit is also enhanced by the fact that gas compression is avoided in the upgrad - ing phase, and the final biomethane volume to be further compressed for transportation is reduced by about 40%, significantly saving compression costs. Chemical absorption with amine-based solvents can be used for biogas upgrading. The process can take advantage of working at nearly ambient pressure and avoid compres - sion costs. However, operations at elevated pressures can also be performed. The operating pressure depends on the chosen pretreatment (such as H₂S removal technology). When operating under pressure, the size of the upgrading unit can be reduced. In our example, for a process oper - ating at 6 bar, the absorber and stripper columns could be reduced by almost half compared to operations under atmospheric conditions. The design of biogas upgrading plants should be per - formed on a case-by-case basis. Plants where biogas pre - treatment is done under atmospheric conditions require

larger equipment while equipment size reduces as the operating pressure increases. Larger equipment, most likely, increases the Capex, while operating at elevated pressures increases the Opex. This trade-off should be evaluated, and the optimal plant should be designed to avoid oversized or undersized equipment. Targeted CO₂ residual vs targeted CO₂ rejection The challenge in conventional CO₂ removal operations is to reach sufficiently low CO₂ remaining in the treated gas to meet fairly stringent specifications. This requires sufficient solvent regeneration, so that absorber performance is con - trolled by events at the absorber’s lean end. In other words, the absorber should be operated lean-end pinched. The solvent flow rate needs to be high enough to elimi - nate the solvent capacity for CO₂ as a limiting factor. Exceptions tend to be piperazine-promoted MDEA-based solvents because piperazine is so highly reactive that even modest regeneration yields a solvent lean enough for the treated gas to meet (CO₂-residual) specifications even with a modest reboiler steam flow. Figure 5 Case 16 shows the loading profile with high reboiler energy flow. Figure 5 Case 1 shows what happens when reboiler energy flow is low – the upper half of the regenerator enters into an idling state, and the CO₂ loading

Case 1 Case 16

0.10

0.08

0.06

0.04

0.02

0.1

0.2

0.3

Loading (mol/mol)

0.00

Figure 5 Lean solvent loading at stripper height for: (orange line) Case 16; and (blue line) Case 1. Calculations from ProTreat

Absorber pressure (bar)

Figure 4 CH₄ loss variation with inlet gas pressure

PTQ Q4 2022