Experimental mass transfer comparison between vacuum and direct contact membrane distillation for the concentration of carbonate solutions

Abstract

• Membrane distillation was applied to the concentration of carbonate solutions. • The mass transfer coefficient revealed phenomena undetected by the flux. • Comparing configurations helps identifying the phenomena affecting mass transfer. • The mass transfer in DCMD is mostly affected by concentration and flow velocity. • The mass transfer in VMD is mostly affected by the vacuum level. Membrane distillation is a promising technique for numerous applications, namely in desalination, wastewater treatment, food, textile and chemical industry. Vacuum membrane distillation (VMD) offers numerous advantages in terms of flux, thermal efficiency and relevance for specific products. On the other hand, direct contact membrane distillation (DCMD) is the most studied configuration and it is preferred due to its conceptual simplicity. The literature is addressing their performance considering mainly the transmembrane flux, which is leading to a lack of full understanding of the phenomena behind each configuration. In this work, the concentration of carbonate solutions using vacuum and direct contact membrane distillation is performed, and results are analyzed by the flux and the overall mass transfer coefficient. This analysis has allowed the evaluation of the real performance of each configuration by keeping the effect of the driving force out of the discussion. The results reveal higher and more stable mass transfer coefficients in VMD, and a much more significant impact of concentration in DCMD. Thus, even though DCMD presents a very attractive configuration in terms of simplicity, its performance is poorer in comparison with VMD.