The effect of pH on CO2-separation from post combustion gas by polyvinylamine based composite membrane

Abstract

A high performance membrane based on polyvinylamine composite material has been reported with Fixed-Site- Carriers (FSC) for CO2 transport. This polyvinylamine based FSC membrane has a promising performance of high permeance as well as high selectivity and gives high potential to be applied for the post combustion CO2 capture where the driving force has been generally considered to be too low for membrane application. The membrane has been tested in both lab & pilot scale facilities and investigated further for evaluation by modelling and simulation, and its robustness against the toxic contaminants (SO2, CO and NOx) was reported in the previous GHGT-10.The chemical form of polyvinylamine changes with pH of the cast solution. The ratio of free amine group and the corresponding ammonium salt depends on pH of the cast solution. Furthermore the overall solubility of CO2 in the membrane increases as the pH increases. The membranes prepared from cast solutions of different pH have been characterized and tested for CO2 capture from post combustion gas.Permeation tests of membranes of different pH cast solutions have shown that the performance of the polyvinylamine composite membrane could be much affected and even enhanced remarkably by pH control. The performance increase was clearly pronounced when the post combustion gas contained more water molecules at higher level of humidity. According to the proposed mechanism of reaction for CO2 facilitated transport, water promotes the transport of CO2 by participating in the reaction as well as increasing the mobility of the transportation.The enhanced performance of the membrane at higher pH can be explained with regard to the increased number of free amino groups available on the polymer backbone chain for facilitated transport of CO2 or the degree of ionization of the functional groups. The increased CO2 solubility in the membrane at higher pH condition also assumed to contribute to the enhanced performance. At lower pH, the decreased number of free amines and the increased viscosity of casting solution have resulted in the decrease in membrane performance.