Superstructure optimization of hybrid thermal desalination configurations

Abstract

As the global demand for freshwater continues to increase, a larger number of resources are dedicated to seawater desalination technologies. In areas with high temperature and salinity water, thermal desalination technologies are often employed. In other areas, reverse osmosis technologies are more popular. While both these technologies have witnessed improvements in recent years, economic and performance issues still pose significant barriers to their universal implementation, which has left many countries, including ones bordering oceans and seas, suffering from dire water scarcity issues. This thesis proposes a methodology which enables the identification of improved thermal-based desalination structures. It is based on the notion of superstructure, which allows for the representation of numerous feed, brine and vapor routing schemes. A superstructure is developed. By adjusting the flow routings, the superstructure is capable of representing the common thermal desalination structures, as well as an extremely large number of alternate structures, some of which might exhibit advantageous behavior. The superstructure is built around a repeating unit which is a generalization of an effect in a multi-effect distillation system (MED) and a stage in a multi-stage flash system (MSF). Allowing for just 12 repeating units, more than 1040 different structures can be represented. The superstructure is thus proposed as an ideal tool for the structural optimization of thermal desalination systems, whereby the optimal selection of components making up the final system, the optimal routing of the vapors as well as the optimal operating conditions are all variables simultaneously determined during the optimization problem. The proposed methodology is applicable to both stand-alone desalination plants and dual purpose (water and power) plants wherein the heat source to the desalination plant is fixed. It can be extended to also consider hybrid thermal-mechanical desalination structures, as well as dual purpose plants where the interface of power cycle and desalination is also optimized for. A multi-objective structural optimization of stand-alone thermal desalination structures is performed in Chapter 2, whereby the performance ratio of the structures is maximized while the specific area requirements are minimized. It is found that for any particular distillate production requirement, alternate structures with non-conventional flow patterns require lower heat transfer areas compared to commonly implemented configurations. Examples of these non-conventional configurations are identified, which include a forward feed - forward feed MED structure, involving the integration of two forward feed MED plants. Chapter 3 highlights how the superstructure can be adapted to optimize integrated thermal desalination and thermal compression systems. Specifically, the conducted study investigates whether there is any merit to the thermal compression of vapor streams produced in intermediate MED effects as opposed to the common practice of compressing vapors produced in the last effect. The study concludes that intermediate vapor compression results in significant reductions in area requirements, as well as significant increases in maximum distillate production capacities. Moreover, the study confirms that the optimal location of vapor extraction is heavily dependent on the exact distillate production requirement in question. Two novel configuration forms are informed by the optimization. The first is an integrated MED-TVC + MED + MSF system, while the second is an integrated MED-TVC + MSF system.