Simulation and optimization study of the humidification–dehumidificatidesalination process

Abstract

The humidification–dehumidification method (HDH) is a thermal desalination process. The process is viewed as a promising technique for small-capacity production plants. The process has many features, which include operation at low temperatures and the ability to combine with a sustainable solar-energy source. This paper demonstrates the design, modeling, simulation, economy, and optimization of an HDH desalination unit simulated within one hour of operation. The designed unit has continued water circulation across the humidifier (HD), a cross-flow oil heater, and closed air circulation. The effects and economic impact of a circulating air-mass flow rate and HD-inlet water temperature on unit productivity are studied. Other parameters, such as energy consumption, heat-to-mass transfer ratio, humidifier enthalpy difference, humidifier thermal efficiency, gained-output ratio (GOR), and annual production costs, are evaluated. Experimentally obtained mass-transfer coefficients in the humidifier were used to simulate the desalination unit. The study reveals that increasing air-mass flow rate has a significant effect on improving unit productivity. Moreover, highest productivity was obtained at the maximum simulated humidifier-inlet water temperatures. Packing with a higher mass-transfer coefficient gives higher unit productivity. The unit was optimized to achieve the best operating conditions, with higher productivity, at lower operating costs, maintaining best energy use at the highest attainable GOR. The optimal operating air-mass flow rate for all conditions ranged between 0.131 and 0.1998 kg/s for both the experimental and the simulated conditions. The lowest production cost obtained was at the HD-inlet water temperature of 50°C. The best experimental production costs were US$/0.07/L compared to US$0.066/L for the simulation. Finally, the simulation process showed good agreement with the experimental results, which can be a useful tool to further evaluate the unit when coupled with a renewable-energy source.