Direct contact membrane distillation mechanism for high concentration NaCl solutions

Abstract

Using high concentration NaCl aqueous solutions, direct contact membrane distillation (DCMD) experiments are implemented in this work where water fluxes are measured at different feed/permeation temperatures, feed concentrations and flow rates. The results under the experimental conditions show that (1) water fluxes decrease as concentrations of NaCl solutions increase; (2) when concentrations of NaCl solutions are above 25%, water fluxes decrease sharply; (3) when concentrations of NaCl solutions are up to saturation, water fluxes gradually approach steady levels. Variations of membrane-fouling resistance are found to result in the above observations. Analysis also shows that concentrations of NaCl solutions, operation temperatures and flow rates can affect the speed of membrane fouling. Among them, the concentration is the most important factor. In this work, a new model is also proposed which takes membrane resistance, concentration polarization resistance and membrane fouling resistance all into consideration. Model predictions show good agreement with the experimental results.