Lost work: A comparison of water-lean solvent to a second generation aqueous amine process for CO2 capture

Energy use is a critical parameter for CO2 capture from flue gas. Proponents of water-lean solvents claim that they will provide better energy performance than aqueous amine solvents, but papers on water-lean solvents frequently fail to compare their energy performance to second-generation (2 G) aqueous amine systems. These papers claim that the weakest point of aqueous amine systems is the energy loss associated with boiling and condensing water. However, unlike first generation processes, 2 G scrubbing processes use innovative stripper configurations to minimize the loss of heat as excess water vapor from the top of the stripper. Proponents of water-lean solvents also claim that these solvents reduce sensible heat losses because of the lower specific heat of the organics that replace the water. In 2 G aqueous systems, the sensible energy of heating the solvent is minimized by high efficiency plate-and-frame exchangers where the optimized energy and capital cost is proportional not only to the specific heat of the solvent, but also to the viscosity and the thermal conductivity: CAPEX, OPEX, TC proportional to (m) over dot k(n-1/2)C(p)(1-n/2)mu(m-n/2) Delta T-crx(0.5) proportional to (m) over dot *k(-0.325)C(p)(0.825)mu(0.175) * Delta T-crx(0.5) Because water-lean solvents typically have higher viscosity and lower thermal conductivity, they do not provide significantly better exchanger performance than 2 G aqueous solvents. This paper develops a method to estimate the energy use of 2 G aqueous solvents by summing the minimum work of separation and lost work (irreversibility) of each process unit. This method allows the comparison of energy use for different solvents (aqueous or water-lean) on the same basis without rigorous process simulation. The results demonstrate that a representative water-lean solvent does not provide significantly better energy performance than a representative 2 G aqueous solvent with an advanced stripper configuration.