The operation pressure of a distillation column is one of the key variables for optimizing the required energy in a carbon capture and storage (CCS) chain. It affects the steam drag point in power plants, the regeneration energy in capture process and the compression energy in the liquefaction process. A new semi-analytical method for determining optimal stripper pressure for the CCS process using MEA as an absorbent is proposed based on the integrated simulation model. Total required energy is represented as a function of the pressure based on the equivalent work.
Pro/II with Soave-Redlich-Kwong (SRK), Non-Random Two Liquid (NRTL) and Benedict-Webb-Rubin-Starling (BWRS) models were employed for simulation. The SRK equation is used for gaseous components while the NRTL and BWRS models were chosen for the CO2 capture process and steam cycle in power plants, respectively. The SRK equation is commonly used for predicting the behavior of a CO2 mixture at high pressure. The simulated power plant was a conventional coal power plant with a 550-MW power generation capacity using Illinois No.6 bituminous coal. 30wt% of monoethanolamine (MEA) was used as an absorbent in the CO2 capture process to remove 90% of CO2 in the flue gas emitted from the power plant. The compression and liquefaction process were composed of a series of compressors to achieve the given terminal pressure. During the liquefaction process, the water content was maintained below 50 vppm, which is lower than 500 vppm for avoiding hydrate formation. For the transmission process, the pipeline was simulated using the specifications reported in the literature. The location of reservoir for CO2 storage was assumed to be at 2000m below sea level.
The results show that the compression work can be reduced at high pressures and that the total energy can be represented as a decreasing function of the stripper pressure. The evaluated optimal pressure decreases as the terminal pressure increases, indicating that the crucial condition for determining stripper operation pressure depends on the terminal pressure of the liquefaction process. Additionally, a general analytical solution for optimal pressure including both the capture and the liquefaction process, could be evaluated by differentiation based on Abel-Ruffini theorem. The total required energy in the possible pressure range could be estimated directly using approximation with the given input variables without using the commercial simulator.