Thermodynamics and Energy Efficiency of Zero Liquid Discharge
Ruoyu Wang, Shihong Lin
Abstract
Zero liquid discharge (ZLD) is an ambitious brine management approach that has received increasing attention in multiple sectors due to the growing water resource scarcity and tightening environmental regulations. Existing ZLD processes are known to be energy-intensive and of high capital cost, which motivates the development of new ZLD treatment trains. Most ZLD treatment trains comprise a brine concentrating stage and a brine crystallization stage, with the latter being the most energy-intensive. Compared to general desalination, our systematic understanding of the thermodynamics of ZLD processes is limited. Thermodynamics can help us to quantify and compare the energy efficiency of existing and emerging ZLD processes and guide the design and development of next-generation ZLD systems. Herein, we present a thermodynamic analysis of a generic reversible brine crystallization process and of a specific reversible evaporative crystallization process, demonstrating that in both cases, the work consumed by such a reversible crystallization process equals the Gibbs free energy change of crystallization. We also evaluated and compared the minimum work for crystalizing a series of common single salt solutions and discuss salt properties that influence the minimum work. Finally, the specific energy consumption and thermodynamic energy efficiency of various emerging ZLD processes were estimated. Compared with mechanical vapor compression, the state-of-the-art brine concentration/crystallization process which typically has an energy efficiency less than 15%, emerging ZLD processes integrating variants of reverse osmosis can potentially reach an energy efficiency above 20%.