Humidification dehumidification (HDH) is a thermal desalination technology that imitates the rain cycle in an engineered setting. It can be advantageous is small-scale, decentralized applications. In addition, the components used in HDH systems are fairly robust, and can treat highly saline water. The technology has recently been commercialized in order to treat highly saline produced water from hydraulically fractured oil and gas wells. That plant has proved HDH’s ability to treat water that most current seawater desalination technologies are unable to treat.
The major disadvantage of HDH is its low energy efficiency compared to other desalination technologies when treating seawater. Previous studies have shown that the system’s energy efficiency can be improved greatly by varying the water-to-air mass flow rate ratio within the system. This translates into operating two or more adjacent stages at different mass flow rate ratios, which is done by extracting an air stream from an intermediate location in the humidifier and injecting it at an intermediate location in the dehumidifier.
Previous models have used fixed effectiveness or fixed pinch approaches to evaluate the benefits of multi-staging, but these do not take account of the size of the system. In physical systems, what remains constant when going from a single-stage to a multi-stage system is the total size of the system and not the effectiveness or the pinch. Therefore, comparing systems with the same total heat exchanger area is the best way to understand the effect of extraction/injection and whether its implementation is beneficial or not.
In this paper, a numerical heat and mass transfer model is used to simulate the operation of HDH at various feed salinities. For each case, the performance of the single-stage system is compared to that of a two-stage system of the same size at different values of feed salinity. The ability of HDH to treat feeds with varying salinity is also studied.Center for Clean Water and Clean Energy at MIT and KFUPM (Project R4-CW-08
