Perstraction of Heat-Stable Salts from Aqueous Alkanolamine Solutions

Amine absorption processes designed to remove acid gases from gas streams generally face a major challenge of solvent degradation. This degradation leads to the formation of heat-stable salts (HSS), corrosive agents that irreversibly bind free alkanolamine. The present study proposes, for the first time, a method for HSS perstraction using a liquid-liquid membrane contactor that allows HSS to transfer through porous membranes from the solvent into a hydrophobic extractant represented by a methyltrioctylammonium solution in 1-octanol. The perstraction provides selective extraction of HSS anions without direct mixing of liquid phases or the formation of stable emulsions of the solvent and the extractant. For this purpose, a number of industrial and laboratory porous membrane samples fabricated from polyvinylidene fluoride, polypropylene, and polysulfone were investigated. Their chemical and morphological stability, surface properties, and transport properties were tested under prolonged (> 600 h) contact with a model solvent (an aqueous monoethanolamine solution) and with the components of the selective extractant. The feasibility of HSS perstraction was demonstrated using the formic acid (as an HSS model) extraction from the model solvent. The most promising results were obtained for a system with a polyvinylidene fluoride membrane: up to 50% of formic acid was extracted over 18 h.