CO2 absorption rate in semi-aqueous monoethanolamine

Abstract

Post-combustion CO2 capture using amine scrubbing is the most promising technology to reduce CO2 emissions from coal- or gas-fired power plants. Increasing CO2 absorption rate (kg′) reduces the absorber capital cost, which is the cost center of the capture plant. By partially replacing water with N-methyl-2-pyrrolidone (NMP) in 7 m (30 wt%) aqueous monoethanolamine (MEA), the CO2 absorption rate (kg′) is significantly enhanced because of lower CO2 loading/higher free MEA at the same CO2 partial pressure (P∗CO2), greater CO2 physical solubility, and greater MEA activity. At 40 °C, in the operating range of 100–5000 Pa P∗CO2, the average kg′ of 7 m MEA in 3 water/1 NMP, 1 water/3 NMP, and 1 water/19 NMP is 1.1 times, 2 times, and 5 times that of 7 m aqueous MEA, respectively. CO2 physical solubility, solvent viscosity, and MEA activity were measured. A kinetic model was built in MATLAB® to better understand the mass transfer of CO2 into semi-aqueous MEA (MEA-NMP-water). The model suggests that the diffusion and reaction of CO2 into aqueous MEA can be approximated by pseudo-first-order (PFO) behavior and adding NMP causes deviation from PFO by the depletion of MEA at the surface. The semi-aqueous solvent provides an excellent rate of CO2 absorption, but the increased viscosity reduces normalized capacity and the volatility of the physical solvent must be addressed.