Derivation of the Theoretical Minimum Energy of Separation of Desalination Processes

Minimizing the energy consumption of desalination processes is an important goal for augmenting freshwater production and mitigating water scarcity. Chemical, civil, mechanical, and environmental engineering students can derive and analyze the energy consumption of desalination processes by applying engineering fundamentals such as thermodynamics, transport phenomena, and process design. We explore the fundamental thermodynamic limits of the most prominent desalination technologies in a format designed for engineering students and instructors. Two thermodynamically reversible processes for reverse osmosis (RO) and electrodialysis (ED) are developed to demonstrate that reversible processes consume the theoretical minimum energy, which is the Gibbs free energy of separation. We then quantify the practical minimum energy consumption for RO and ED, showing that the energy consumption of these processes approaches the minimum thermodynamic limit with increased process staging.