Thermodynamics of electrosorption-based separation processes and cycles

Abstract

Understanding the thermodynamics associated with ion mixing and separation processes is important in order to meet the rising demands for clean energy and water production. State-of-the-art separations technologies such as reverse osmosis and thermal-based distillation methods desalinate through processes that occur at elevated pressure and/or temperature. This can result in an increase in the energy requirements or system footprint. Several electrochemical-based technologies such as capacitive deionization (CDI) and capacitive mixing (CapMix) are also capable of achieving desalination and energy production through ion separation and mixing processes, yet experimental investigations suggest energy conversion is inefficient. Here, a theoretical investigation of reversible and irreversible CapMix and CDI cycles aims to explore the irreversibilities that prevent CDI from operating efficiently. Analogies drawn between heat engines and blue engines aim to provide insight into designing optimal separation and mixing cycles and processes. In addition to examining efficiencies for CapMix and CDI, an analysis of the theoretical and experimental heat of electroadsorption provides knowledge regarding how field-assisted sorption technologies compare to passive-sorption technologies.