Use of multiple extractions and injections to thermodynamically balance the humidification dehumidification desalination system

Humidification dehumidification (HDH) desalination systems are well suited for small scale, off-grid desalination. These systems are very robust and can tolerate a wide range of feed salinities, making them a good candidate for treating produced water from hydraulically fractured natural gas wells. A primary engineering challenge for these systems is their high thermal energy consumption. In this study, we examine the use of multiple air extractions and injections to thermodynamically balance the HDH system, so as to make it more energy efficient. The effect of the number of extractions on several performance parameters is studied. In addition, we study the effect of the enthalpy pinch, which is a measure of performance for a heat and mass exchanger, on these performance parameters. Finally, we present results that can be used as guidelines in designing HDH systems. These results include the identification of appropriate temperatures for the extracted/injected air streams, the division of the heat duty between stages, and the value of the mass flow rate ratio in each stage at various values of enthalpy pinch.