A Feasibility Study of Vortex Tube-Powered Membrane Distillation (MD) for Desalination

Abstract

This work theoretically studies the capability of using vortex tubes to provide the necessary heating and cooling energies required by a typical direct-contact membrane distillation (MD) process. The vortex tube generates a temperature separation that can supply the membrane distillation process with sufficiently hot feed and cold permeate with a temperature difference as large as 70 °C. Several structures integrating vortex tubes and MD with and without heat recovery and cascading are proposed and their respective performances are assessed and compared. A maximum distillate production of 38.5 kg/h was obtained at an inlet air pressure of 9 bar, cold air mass ratio of 0.7, and air-to-water mass ratio of 9. The corresponding energy consumption was found to be 25.9 kWh/m3. The production rate can be increased up to 75.2 kg/h and the specific energy consumption can be reduced to 13.3 kWh/m3 when three MD stages were connected in series using the same single vortex tube at the same operating conditions. It is found that the cold fraction plays an important role in the balance between heating and cooling operations. In addition, cold fraction values smaller than 0.7 should be avoided to prevent water from freezing inside the membrane.