Numerical fixed-effectiveness and fixed-area models of the humidification dehumidification desalination system with air extractions and injections/

Abstract

The humidification dehumidification (HDH) desalination system can be advantageous in small-scale, off-grid applications. This system is very robust and can tolerate a wide range of feed salinities, making it a good candidate for treating produced water from hydraulically fractured natural gas wells. The main drawback of this technology has been its low energy efficiency, which results in high water production costs. This work focuses on the thermodynamic balancing of HDH. The first part uses a fixed-effectiveness approach to model 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. Further, 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. Fixing the effectiveness of the heat and mass exchangers allows them to be modeled without explicitly sizing the components and gives insight on how the cycle design can be improved. However, linking the findings of fixed-effectiveness models to actual systems can be challenging, as the performance of the components depends mainly on the available surface areas and the flow rates of the air and water streams. In the second part of this study, we present a robust numerical solution algorithm for a heat and mass transfer model of a complete humidification-dehumidification system consisting of a packed-bed humidifier and a multi-tray bubble column dehumidifier. We look at the effect of varying the water-to-air mass flow rate ratio on the energy efficiency of the system. In addition, we study the effect of the top and bottom temperatures on the performance of the system. We recommended the implementation a control system that varies the mass flow rate ratio in order to keep the system balanced in off-design conditions, especially with varying top temperature. Finally we consider a single air extraction, and look at the effect of the location of extraction, and its direction. We define the criteria for achieving a completely balanced system.