Thermodynamic and mass transfer modeling of carbon dioxide absorption into aqueous 2-piperidineethanol

by Sherman, Brent J.; Ciftja, Arlinda F.; Rochelle, Gary T.

CHEMICAL ENGINEERING SCIENCE, 153, pg 295-307, DOI: 10.1016/j.cec.2016.07.019

Amine scrubbing is a necessary technology to offset CO2 emissions from fossil-fuel power plants. Of the many solvents studied, hindered amines are of particular interest for their marriage of the capacity of tertiary amines with rates a hundredfold greater than tertiary amines. The relatively rapid rates of hindered amines have not been adequately explained, despite their extensive use in commercial solvents. This work seeks to explain the rapid rate of mass transfer of 2-piperidineethanol (2PE) and uses this rationale to draw general conclusions on hindered amines.

Quantitative C-13 NMR data were collected to determine the equilibrium of carbamate in 30 wt% 2PE. Using these data along with VLE and pK(a) data, a rigorous thermodynamic model of 8 molal 2PE was built with electrolyte-NRTL and activity-based kinetics. Wetted-wall column flux data were fit to create the activity-based mass transfer model. Using this comprehensive model, the mass transfer rate was examined through sensitivity studies and Bronsted correlations.

This work shows that 2PE forms a more stable carbamate than 2-amino-2-methyl-1-propanol. The carbamate reaction is the most significant component of mass transfer at 40 degrees C. The Bronsted correlation for carbamate reactions of unhindered amines predicts the rate of carbamate reaction of 2PE, but the Bronsted correlation for bicarbonate underpredicts the regressed rate. The CO2 solubility is fit with five parameters with an ARD of 0.84%. The kinetics are fit with a carbamate- and a bicarbonate-forming reaction with an ARD of 7.03%.

The chief conclusions are (1) that the rapid mass transfer of hindered amines is due to the formation of carbamate and the high pKa of the amine, (2) the carbamate formation rate appears unimpeded by steric hindrance and is predicted by a Bronsted correlation, suggesting that hindered amines react in the same manner as unhindered amines. (C) 2016 Elsevier Ltd. All rights reserved.

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