Absorption of carbon dioxide into aqueous piperazine: reaction kinetics, mass transfer and solubility

This work studies the absorption of carbon dioxide into aqueous solutions o f piperazine in a wetted wall contactor. Absorption was studied from 298 to 333 K in solutions of 0.6 and 0.2 M aqueous piperazine. The apparent react ion rate is first order in both carbon dioxide and piperazine with a value of 53,700 m(3)/kmols at 25 degreesC. The apparent second-order rate constan t follows an Arrhenius temperature dependence over the range studied with a n activation energy of 3.36 x 10(4) k(3)/kmol. Solubility in 0.6 M PZ was m easured by bracketing absorption and desorption in the wetted wall contacto r at 313 and 343 K. Chemical and phase equilibrium was modeled by consideri ng the following piperazine species: piperazine carbamate, dicarbamate, pro tonated carbamate, and protonated piperazine. Henry's law and the dissociat ion of carbon dioxide to form bicarbonate and carbonate were also considere d. The carbamate stability constant and pK(a) for piperazine carbamate were regressed from the VLE data. Although shown to be present by NMR, the dica rbamate is not the dominant reaction product at any loading. The carbamate stability constant is comparable to other secondary amines such as diethano lamine (DEA) but the apparent second-order rate constant is an order of mag nitude higher than primary amines such as monoethanolamine (MEA) or diglyco lamine (DGA(R)). The second-order rate constant obtained in this work is mu ch higher than previously published values for the piperazine/carbon dioxid e reaction. These previous studies were limited by mass transfer limitation of products and reactants and were not a true measurement of the kinetics of carbon dioxide/piperazine. There is some evidence that the reactivity of piperazine is due to its cyclic and diamine characteristics. (C) 2000 Else vier Science Ltd. All rights reserved.