Performance assessment of a modified air-gap and water-gap membrane distillation systems

This work experimentally evaluates and compares the performance of four membrane distillation (MD) units with and without an internal gap rotating impeller blade. The novelty of the current study lies in the installed impeller within the condensation cavity and the impeller design parameters. The investigated MD units include a novel modified water-gap MD (M-WGMD) system featuring an internal impeller blade; a novel modified air-gap MD (M-AGMD) system with an internally rotating impeller blade; a regular water-gap MD (WGMD) process; and a conventional air-gap MD (AGMD) system. The evaluated system performance index includes gained output ratio (GOR) and vapor flux. The performance of the four MD systems is assessed and compared against each other under the same test conditions for various system design and operating variables, including impeller blade material, impeller blade angle, saline feed temperature, coolant temperature, and saline feed flowrate. Findings indicate that M-WGMD and WGMD systems (modified and conventional) produce higher flux than M-AGMD and AGMD units. Meanwhile, air-gap MD units (AGMD and M-AGMD) exhibits better energy efficient (higher GOR) than WGMD and M-WGMD systems. Furthermore, modified MD systems recorded superior performance compared to conventional MD systems and the blade angle effect is more influential than the blade material. At 80 °C feed temperature and 3200 mg/L feed salinity, the M-WGMD reaches a maximum flux of 128.39 kg/m²h and a GOR of 0.7702. On the other hand, the WGMD attained a flux of 67.32 kg/m²h and a GOR of 0.6199. The M-AGMD recorded a flux of 30.20 kg/m²h and a GOR of 0.8696. For the AGMD system, the GOR and the flux values are 0.7357 and 15.58 kg/m²h, respectively.