Experimental evaluation of two consecutive air-gap membrane distillation modules with heat recovery

Two rectangular modules with a total interior membrane surface area of 13.53 m(2) were consecutively combined to evaluate the use of heat recovery in an air-gap membrane distillation (AGMD) system. Several operating inlet parameters including feed water temperature, mass water flow rate and salinity were investigated. The experimental results revealed that the performance of the system was improved by virtue of efficient heat recovery resulting from combining two AGMD membrane modules in series. Under optimal inlet operating parameters of cooling water temperature of 20 degrees C, salinity of 0.05% and flow rate of 3 l/min, the system productivity (P-p) increased up to 192.9%, 179.3%, 176.5% and 179.2%, and the thermal efficiency (eta(th)) by 261.5%, 232.6%, 239.4% and 227.3% at feed water temperatures of 45 degrees C, 55 degrees C, 65 degrees C and 75 degrees C, respectively. Concurrently, the specific waste heat input (E (w.h.i)) decreased by 6.7%, 4.7%, 5.6% and 2.7% due to the efficient heat recovery. The results confirmed that heat recovery is an important factor affecting the AGMD system that could be improved by designing one of the two AGMD modules with polytetrafluoroethylene (PTFE) hollow fibers with a flow length shorter than the other one having a salt rejection rate of 99%.