Thermodynamic analysis of an efficient pressure-swing CO₂ capture system based on ionic liquid with residual pressure energy recovery

In this work, an efficient pressure-swing ionic liquid-based CO₂ capture system with residual pressure energy recovery is proposed. In this system, the N₂–O₂ mixture from flue gas which is not absorbed by the ionic liquid in the absorber flows into the turbine to recover the energy for the purpose of reducing energy consumption. 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF₄]) and 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF₆]) are selected as the capture solvent and a comprehensive thermodynamic analysis is performed. The total energy consumption of the new system is 1.770 GJ/tCO₂ under design conditions which is 28.45% lower than the conventional system for [Bmim][BF₄] and 1.737GJ/tCO₂ which is 28.84% lower than the conventional system for [Bmim][PF₆]. The effect of absorption pressure, desorption temperature, heat storage fluid flow rate, distribution ratio of heat storage fluid in the intercooler, flue gas composition and type of heat storage fluid on system thermodynamic performance were analyzed. The result shows that relatively lower capture pressure, the lower desorption temperature, the lower heat storage fluid flow rate, the more heat storage fluid distributed in the first intercooler and the higher CO₂ fraction in flue gas can improve the thermodynamic performance of the system. Also, the exergy analysis represents that cold tank and the capture module are the two components with maximum exergy destruction. And the comparison results with the conventional system show that the new system has significantly lower energy consumption and capture cost.