Multi-Objective Optimisation of Hybrid CO2 Capture Processes Using Exergy Analysis

Carbon dioxide (CO2) purification is an essential step in the carbon capture and storage (CCS) process. The leading technology consists of a solvent absorption carbon capture process followed by a multi-stage CO2 gas compression into supercritical state for sequestration. This study considers a hybrid system of vacuum swing adsorption (VSA), membranes and cryogenic separation. Replacing the multi-stage gas compression with the cryogenic separation has two main advantages: firstly, it further purifies the CO2 stream, which is valuable for both VSA and membrane processes since both processes struggle to achieve high purity product. Secondly, it produces liquid CO2 that can be pumped to the supercritical state, which is required for transport and sequestration. Due to the higher degree of freedom available in hybrid processes, a new methodology using multi-objective optimisation combined with exergy analysis was used to optimise the process. This allowed different decision variables to be considered to find the range of optimum operating conditions for each of the processes. It was determined that the refrigerant flow rate, multi-stage compression and cryogenic minimum temperature had the biggest impact on the recovery rate. Furthermore, it was observed that the total specific shaft work had a linear relationship with the specific exergy loss rate.