Capacity and absorption rate of tertiary and hindered amines blended with piperazine for CO2 capture

Aqueous piperazine (PZ) blended with a tertiary or hindered amine combines the high CO2 capacity of the tertiary or hindered amine with the fast rate of PZ. For flue gas CO2 capture, the optimum pKa (that which offers the highest CO2 cyclic capacity) of a mono-tertiary amine blended with PZ is 9.1. A generic Aspen Plus® model for PZ/tertiary amine was developed to predict the CO2 vapor-liquid-equilibrium (VLE) from the pKa of the tertiary amine. The polarity of the tertiary amine also affects the CO2 solubility of the PZ/tertiary amine. Hindered amines that form little carbamate show similar CO2 VLE to tertiary amines with the same pKa, when blended with PZ. The CO2 absorption rate of most 2.5 m PZ/2.5 m tertiary amines was slightly slower than 2.5 m PZ, probably due to the higher viscosity of the blends.