High water recovery in direct contact membrane distillation using a series of cascades

Like all thermal processes, water recovery in membrane distillation (MD) is not limited by osmotic pressure, so that high recoveries are in principle achievable. The process can be carried out in batch recirculation mode or in continuous mode. In a number of earlier studies we have experimentally demonstrated very high water recovery in direct contact membrane distillation (DCMD) using cross flow hollow fiber membrane modules by operating in a batch recirculation mode without scaling seriously impairing membrane operation. However continuous operation is desirable for larger scale plants. Such operation may be carried out in a series of MD units each of which may consist of a countercurrent cascade of cross flow DCMD modules. A theoretical analysis of such a series of cascades has been carried out and expressions have been generated to show the relationship between recovery, temperature differences across and between the cascades, and the possible theoretical gained output ratio (GOR) and recoveries that are attainable. Depending on configuration (interstage heating or not) and temperature differences, recoveries of 60% and GOR values above 20 may be attainable. This could have important implications for the use of MD in the treating of desalination and other industrial brines.

► Direct contact membrane distillation (DCMD) has the potential for high water recovery in sea water desalination. ► A series of countercurrent DCMD cascades of two types has been theoretically investigated. ► It is preferable to use interstage heating of brine between each cascade. ► A GOR value above 20 may be attainable.