Effect of channel height on the overall performance of direct contact membrane distillation

Abstract

• Temperature polarization was decreased significantly via decreasing channel height. • Feed temperature and flow rate are key parameters that affect the performance. • Permeate flux was increased by 21% due to addressing temperature polarization. • Specific energy consumption was improved by 10% due to enhanced polarization. Temperature polarization is a crucial issue in the application of the membrane distillation technique, as it can significantly affect its entire performance. Experimental and numerical studies were conducted to decrease the temperature polarization in the feed and permeate channels by reducing the channel height. This is the first time that a study addresses the temperature polarization in membrane distillation by decreasing channel height. Furthermore, the feed inlet temperature and flow rate were studied by changing the channels’ height to obtain a comprehensive conclusion. Results showed that decreasing temperature polarization had a significant effect on the membrane surfaces’ temperatures, which in turn, significantly improved system productivity and thermal performance. In this study, the channels’ heights decreased from 2.5 mm to 1.5 mm; whereas, the feed inlet temperature and flow rate ranged from 58 °C to 78 °C and from 100 ml/min to 400 ml/min, respectively, and at constant feed salinity of 10 g/l. A 21% increase in the permeate flux was achieved when the channel height decreased from 2.5 mm to 1.5 mm at feed inlet temperature and flow rate of 78 °C and 100 ml/min, respectively. Furthermore, at the maximum considered flow rate of 400 ml/min this improvement enhanced both the thermal efficiency and specific energy consumption by 12.7% and 10%, respectively. The results also showed that feed inlet temperature had a significant impact on the whole system performance: productivity was more than doubled when feed inlet temperature was increased from 58 °C to 78 °C (at 1.5 mm channel height). However, increasing flow rate negatively affected both thermal efficiency and specific energy consumption even though it was proven to positively affect productivity. Finally, increasing the flow rate beyond 200 ml/min proved to be less significant with respect to permeate flux, thermal efficiency, and specific energy consumption.