Removal of heat stable salts from N-methyldiethanolamine wastewater by anion exchange resin coupled three-compartment electrodialysis

Abstract

Heat stable salts (HSS) are well-known degradation products of gas desulfurization using N-methyldiethanolamine (MDEA) solvents. Excessive HSS content causes a decrease in desulfurization efficiency, a decrease in the active ingredients of MDEA, and the corrosion of pipelines. In this paper, a self-designed two-compartment electrodialysis (ED) stack, a three-compartment electrodialysis (TED) stack, and an anion exchange resin coupled three-compartment electrodialysis (RTED) stack were assembled to remove the HSS from spent MDEA wastewater. The HSS desalination mechanisms of the three electrodialyzers were investigated, and their HSS removal efficiencies and losses of MDEA were investigated and compared. According to the results, approximately 93.84% of HSS were removed from the spent MDEA wastewater in the RTED, 7.88% higher than those in the TED and 28.57% higher than those in the ED. The loss of MDEA for the RTED was 3.78%, which is 1.78% lower than that for the TED and 17.29% lower than that for the ED. A relatively high sodium hydroxide (NaOH) concentration in the NaOH compartment, but no more than 3%, is beneficial for both improving the HSS removal efficiency and decreasing the loss of MDEA in TED and RTED. Much less fouling of the anion exchange membranes occurs in the RTED process than in the ED process because of the effect of the filled resin and the NaOH compartment. The cost of RTED is estimated to be US $0.88/kg HSS, which is far lower than that of both ED and TED.