Strong Bicarbonate Forming Solvents for Absorption of CO2 - Experimental Determination and Modelling of Vapor-Liquid Equilibria Data

Abstract

Absorption by amines provides a technology for CO2 capture. New solvents are of interest to ameliorate the process, and the two alkanoamines 2-piperidine-ethanol (2-PPE) and 1-(2-hydroxyethyl)-pyrrolidine (1-2HE-PRLD) were investigated in this work.

Ebulliometric vapor-liquid equilibrium (VLE) data of pure 2-PPE and 1-2HEPRLD were provided by this work. 1-2HE-PRLD was found to be more volatile than the 2-PPE. VLE data of the binary system 2-PPE/H2O were obtained by ebulliometric measurements. Quantification of the vapor and liquid phase concentration were done by amine titration and Ion Chromatography. The data were used to calculate the experimental activity coefficient. Due to the low volatility of 2-PPE, the uncertainty of the calculated activity coefficient were significant at low temperatures. At 50C, the uncertainty was about 40%. Such deviations are also found in other works with amines of similar volatility (50% for MDEA). The relative uncertainty decreased substantially when the temperature increased, it was below 8% at 100C.

The experimental data were used to develop a Non-Random-Two-Liquid (NRTL) model. This was also done for the system 1-2HE-PRLD/H2O, with the experimental data taken from literature. Two optimization routines were applied to find the model parameters. The first routine simultaneously fitted parameters for Antoine equation of pure amine and binary interaction parameters, yielding model predictions of the amine activity coefficients with AARD less than 14% for both the 2-PPE and the 1-2HE-PRLD system. The second routine fitted the two parameter sets separately, and gave better fit with respect to the total pressure of the solution (AARD <4%).

Furthermore, rigorous electrolyte-NRTL models were developed for the loaded systems 2-PPE/H2O/CO2 and 1-2HE-PRLD/H2O/CO2. The model parameters from the binary systems were fixed during the optimization. The AARD between the experimental data and the model predictions were for both systems found to be within 12% (PCO2 ) and 9% (Ptotal). These values indicate very good accordance between the models and the experimental data.

The critical properties of 2-PPE and 1-2HE-PRLD were not found in literature, and thus estimated. They were required for regression of the NRTL and eNRTL model parameters, but are however seen to have small influence on the final result due to low pressures.