Thermodynamics and kinetics of aqueous piperazine with potassium carbonate for carbon dioxide absorption
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This work proposes an innovative blend of potassium carbonate (K2CO3) and piperazine (PZ) as a solvent for CO2 removal from combustion flue gas in an absorber/stripper. The equilibrium partial pressure and the rate of absorption of CO2 were measured in a wetted-wall column in 0.0 to 6.2 m K+ and 0.6 to 3.6 m PZ at 25 to 110o C. The equilibrium speciation of the solution was determined by 1 H NMR under similar conditions. A rigorous thermodynamic model, based on electrolyte non-random two-liquid (ENRTL) theory, was developed to represent equilibrium behavior. A rate model was developed to describe the absorption rate by integration of eddy diffusivity theory with complex kinetics. Both models were used to explain behavior in terms of equilibrium constants, activity coefficients, and rate constants. The addition of potassium to the amine increases the concentration of CO3 2- /HCO3 - in solution. The buffer reduces protonation of the amine, but increases the amount of carbamate species, yielding a maximum reactive species concentration at a K+ :PZ ratio of 2:1. The carbamate stability of piperazine carbamate and dicarbamate resembles that of primary amines and has approximately equal values for the heats of reaction, ∆Hrxn (18.3 and 16.5 kJ/mol). The heat of CO2 absorption is lowered by K+ from -75 to -40 kJ/mol. The capacity increases as total solute concentration increases, comparing favorably with 5 M monoethanolamine (MEA). The rate approaches second-order behavior with PZ and is highly dependent on other strong bases. In 1 M PZ, the overall rate constant is 102,000 s-1, 20 times higher than in MEA. The activation energy is 35 kJ/kmol. In K+ /PZ, the most significant reactions are PZ and piperazine carbamate with CO2 catalyzed by carbonate. Neutral salts in aqueous PZ increase the apparent rate constant, by a factor of 8 at 3 M ionic strength. The absorption rate in 5 m K+ /2.5 m PZ is 3 times faster than 30 wt% MEA. A pseudo-first order approximation represents the absorption rate under limited conditions. At high loadings, the reaction approaches instantaneous behavior. Under industrial conditions, gas film resistance may account for >80% of the total mass transfer resistance at low loadings.