Ind. Eng. Chem. Res. 2000, 39, 3397
3397
CORRESPONDENCE Comments on “A Study on Equilibrium Solubility for Carbon Dioxide in Methyldiethanolamine-Piperazine-Water Solution” Ralph H. Weiland† Koch-Glitsch, Inc., 4900 Singleton Boulevard, Dallas, Texas 77252
Sir: Liu et al.1 have collected valuable data on the effect of piperazine on the equilibrium partial pressure of CO2 over aqueous solutions of N-methyldiethanolamine (MDEA). They correlated some 80 data points using the Deshmukh-Mather model by fitting a twoparameter expression for the piperazine protonation reaction equilibrium constant in addition to 14 ion-ion and ion-molecule interaction parameters. Their model considers five equilibrium reactions. Unfortunately, they appear to have omitted one of the primary reactions, namely, the formation of piperazine carbamate from piperazine and CO2. Bishnoi and Rochelle2 have found that CO2 reacts nearly 10 times faster with piperazine than with monoethanolamine and they report a carbamate stability constant comparable to DEA and other secondary amines. For any but the smallest CO2 loading in piperazine-activated MDEA, a significant fraction of the piperazine is in the carbamate form (i.e., reacted) and the actual concentrations of free, unreacted piperazine will actually be quite a bit smaller than they † Tel.: (214)583-3160.
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calculated from their reaction equilibrium equation set during model regression. The net result is that the fitted parameters do not correspond to the chemistry. The CO2 concentrations all fall in the high-loading range where, for the small concentrations of piperazine that apply in activated MDEA systems, most of the piperazine is in the protonated and carbamate forms. Therefore, with the CO2-piperazine reaction having gone almost to completion for most of the data, the question arises as to whether there is enough variation in the free piperazine concentration to make data regression to piperazine protonation and carbamateformation constants valid at all. Literature Cited (1) Liu, H.-B.; Zhang, C.-F.; Xu, G.-W. A Study on Equilibrium Solubility for Carbon Dioxide in Methyldiethanolamine-Piperazine-Water Solution. Ind. Eng. Chem. Res. 1999, 38, 4032. (2) Bishnoi, S.; Rochelle, G. T. Absorption of Carbon Dioxide into Aqueous Piperazine: Reaction Kinetics, Mass Transfer and Solubility. Paper presented at 49th Annual CsChE Conference, Saskatoon, Saskatchewan, Oct 1999.
IE991061C
10.1021/ie991061c CCC: $19.00 © 2000 American Chemical Society Published on Web 07/22/2000
