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Water gap membrane distillation is an effective configuration to improve the productivity of the membrane distillation process. This study provides a comprehensive theoretical analysis of a novel water gap membrane distillation (WGMD) design where the gap water is circulated. Circulating the gap water improves the output flux due to the forced convection characteristics inside the water gap and hence improves mass and heat transfer coefficients. The model outcomes are compared with the experimental findings for validation, and it shows a very good agreement with a maximum deviation of less than 10%. The influences of operational variables on the energy efficiency, output flux, and freshwater cost are presented and compared to the standard WGMD process, without circulated gap, under the same operation conditions. Results reveal that the circulated water gap enhances the output flux by a maximum of 350% and reduces the production cost by a maximum of 32%. Regarding energy efficiency, a slight improvement is obtained when the gap water is circulated at a rate of up to 4 L/min; however, using a higher circulation rate reduces the energy efficiency. Using a multistage system to increase productivity can minimize production costs by roughly 82%. [ABSTRACT FROM AUTHOR]

Summary: A novel configuration of humidification dehumidification (HDH) desalination system is presented using direct contact bubble column (BC) humidifier and dehumidifier. Experiments are carried out for performance analysis and comparison between a single‐stage, HDH air heated system (one humidifier and a dehumidifier) and a double‐stage HDH system (two BC humidifiers with air reheating). Effect of various operating parameters such as airflow rate, air temperature, and humidifier water levels is studied on performance metrics such as productivity, gain output ratio (GOR), input energy, specific energy consumption, and effectiveness. Results show that higher airflow rate, air temperature, and humidifier water level favor the system performance. The single‐stage system has a maximum productivity rate of 0.19 L/h and GOR of 0.21 obtained at 140°C and 1.5 standard cubic feet per minute (SCFM). However, the productivity of the single‐stage HDH system is limited due to low air outlet temperature at the exit of the BC humidifier. The double‐stage humidification system performs better in terms of productivity and GOR. The two‐stage humidification HDH system produced a distillate rate of 0.46 L/h and GOR of 0.4 with thermal energy increment of only 0.27 kW compared to the single‐stage system. The obtained results are promising in comparison with previous HDH studies, especially of air heated cycle. The estimated cost of production is 0.090 USD/L for double‐stage HDH BC. [ABSTRACT FROM AUTHOR]