Aqueous piperazine derivatives for CO2 capture: Accurate screening by a wetted wall column

Concentrated aqueous piperazine (PZ) has been identified as a better solvent for CO2 capture than monoethanolamine (MEA), because it has a greater rate of CO2 absorption and greater CO2 capacity. This work evaluates the effect of substitute groups on PZ performance. Many previous screening studies measured absorption/desorption with CO2/N-2 sparging, which lacks accuracy and cannot be used to estimate actual absorber performance. In this work a wetted wall column was used to accurately measure absorption/desorption rate at typical rich and lean CO2 loading (alpha) and simulate performance of real packing. The method also provides accurate measurement of CO2 solubility at 40-100 degrees C. This study provided rate and solubility data at 40-100 degrees C and practical ranges of CO2 loading for 8m 1-methylpiperazine (1-MPZ), 8 m 2-methylpiperazine (2-MPZ), 4 m 2-MPZ/4 m PZ, 7.7 m N-(2-hydroxyethyl)piperazine (HEP), 6 m 1-(2-aminoethyl)piperazine (AEP), 8 m 2-piperidine ethanol (2-PE), and 2 m trans-2,5-dimethylpiperazine (2,5-DMPZ). With the measurements of CO2 flux (NCO2) and equilibrium driving force, liquid film mass transfer coefficients (k(g)') are calculated. The rate decreases as of 1-MPZ =PZ > 2-MPZ/PZ > 2-MPZ > HEP > MEA > AEP = 2-PE. This method also allows bracketing and determination of equilibrium CO2 partial pressure (PCO2.) at each condition. Semi-empirical solubility models of CO2 for each amine were regressed from experimental solubility data to find the lean and rich CO2 loading corresponding to 0.500 kPa and 5 kPa CO2 partial pressure respectively. Based on the solubility model, the actual operating capacity of the solvents without overstripping decreases in the sequence of 2-PE > 2-MPZ > 2-MPZ/PZ > 1-MPZ > PZ > HEP > AEP > MEA. The enthalpy of CO2 absorption (AI-labs) of all the piperazine derivatives is around 70 kj/mol CO2. (C) 2011 Published by Elsevier B.V. on behalf of The Institution of Chemical Engineers.