Analysis of water gap membrane distillation process with an internal gap circulation propeller

Abstract

A novel propeller-aided water gap membrane distillation (P-WGMD) system has been investigated through detailed mathematical modeling, and its performance was compared with that of a baseline/conventional WGMD system. The proposed system contains a rotating propeller blade, which improves mass and heat transfer within the gap space. The installed propeller and its modeling within the gap chamber represent the novelty of the present work. The influences of various design variables, including propeller diameter, propeller material, propeller thickness, position of the propeller, and the thickness of the gap on the distillate production and the specific thermal energy consumption (STEC) of the proposed propeller-aided P-WGMD system, are investigated. In addition, the effects of different operating parameters such as feed water temperature, coolant temperature, feed water flowrate, and coolant flowrate on the performance of the baseline WGMD and propeller-aided P-WGMD processes were examined. Results revealed that propeller material and propeller thickness showed marginal effects on the system's performance, especially when the thermal conductivity of the propeller material exceeded 100 W/m K. The propeller position has a strong influence on the system performance, and placing the propeller away from the membrane surface improves the system performance. Larger propeller diameters and higher propeller speeds exhibit better system performance. Propeller speed and propeller diameter are found to be the most dominant design factors affecting the performance of the proposed system. Furthermore, the P-WGMD system exhibits superior performance over the baseline WGMD system under the same operating conditions, where the flux of the P-WGMD system could reach a maximum value of 322 kg/m2h at a feed flowrate of 10 L/min, propeller speed of 1000 rev/min, feed temperature of 70 °C, coolant temperature of 20 °C, and coolant flowrate of 2.5 L/min. The presence of a propeller within the gap chamber reduces the resistance to heat transfer from about 0.002057 m2K/W to 0.0001144 m2K/W, reflecting the enhancement gained in the proposed system. Freshwater costs from the proposed system vary between 8.243 $/m3 and 11.57 $/m3 while those from the baseline system fall between 9.971 $/m3 and 21.11 $/m3.