Thermodynamic analysis of CO2 capture solvents

There is enormous and growing world-wide activity in developing new and improved processes for post-combustion capture of CO2, frequently using chemical absorption. Developers of new processes and solvents make positive claims for their technology in terms of low energy consumption, but these are usually difficult to validate. A practical issue that hampers clear analysis of CO2 capture with chemical solvents is that superior process performance is a complex combination of solvent characteristics (e.g., absorption capacity, enthalpy of absorption, mass transfer and kinetics, hydraulic properties) and process innovations (e.g., absorber intercooling, lean-vapor compression, interheated strippers). This work breaks up the problem by focusing on the solution thermodynamics of the CO2-solvent system (a subset of solvent characteristics), which may well be the most important contributor to superior CO2-capture technology. What are the characteristics of the “best” solvent based upon solution thermodynamics? This question is tackled by creating artificial solvents for which the only restriction is thermodynamic consistency, and then performing a relative evaluation using process simulation with a fixed, standard flowsheet. The study provides useful insight into the interacting roles of absorption capacity and enthalpy of solution in understanding and identifying the “optimum thermodynamic solvent,” and may serve as a guide for research into new solvents. The key insight from the study is that solvents similar to the common solvents in use today and with low- to mid-range enthalpies of solution are optimum based upon thermodynamics alone. Further, for a common solvent it is usually detrimental to increase the absorption capacity while keeping the enthalpy of solution constant.