Membrane Crystallization of Sodium Carbonate for Carbon Dioxide Recovery: Effect of Impurities on the Crystal Morphology

Membrane contactors have been proposed as an advanced tool for CO2 capture from flue gases by absorption in alkaline solutions. However, regeneration of the alkaline reagent and further CO2 sequestration are pending issues. In this paper, membrane-assisted crystallization is proposed for crystallizing Na2CO3, which allows its reuse, after CO2 absorption from flue gases. Due to the presence of compounds other than CO2 in flue gases (i.e., SO2, NOx), other compounds (Na2SO4 and NaNO3) may interfere with Na2CO3 crystallization. This was evaluated by measuring the flux through the membrane and the morphology, crystallography, and purity of the crystals. Furthermore, the presence of NaCl possibly transferred from the osmotic solution to the feed solution was evaluated. The experimental results indicate that the presence of impurities decreases the flux through the membrane due to the decrease of water activity, although there is no influence on the overall mass transfer coefficient. The presence of Na2SO4 affected the morphology of the Na2CO3 crystals while NaNO3 and NaCl had no apparent effect on the crystalline products. It was confirmed that Na2CO3•10H2O was formed during the crystallization. Moreover, the purity of Na2CO3 crystals reaches up to ca. 99.5%. Membrane-assisted crystallization was concluded to be feasible in recovering CO2 as a carbonate salt, which can possibly be reused in the industry.