Irreversibilities and nonidealities in desalination systems

Abstract

Energy requirements for desalination systems must be reduced to meet increasing global demand for fresh water. This thesis identifies thermodynamic limits for the energetic performance of desalination systems and establishes the importance of irreversibilities and solution composition to the actual performance obtained. Least work of separation for a desalination system is derived and generalized to apply to all chemical separation processes driven by some combination of work, heat, and chemical energy (fuel) input. At infinitesimal recovery, least work reduces to the minimum least work of separation: the true exergetic value of the product and a useful benchmark for evaluating energetic efficiency of separation processes. All separation processes are subject to these energy requirements; several cases relevant to established and emerging desalination technologies are considered. The effect of nonidealities in electrolyte solutions on least work is analyzed through comparing the ideal solution approximation, Debye-Hückel theory, Pitzer's ionic interaction model, and Pitzer-Kim's model for mixed electrolytes. Error introduced by using incorrect property models is quantified. Least work is a strong function of ionic composition; therefore, standard property databases should not be used for solutions of different or unknown composition. Second Law efficiency for chemical separation processes is defined using the minimum least work and characterizes energetic efficiency. A methodology is shown for evaluating Second Law efficiency based on primary energy inputs. Additionally, entropy generation mechanisms common in desalination processes are analyzed to illustrate the effect of irreversibility. Formulations for these mechanisms are applied to six desalination systems and primary sources of loss are identified. An economics-based Second Law efficiency is defined by analogy to the energetic parameter. Because real-world systems are constrained by economic factors, a performance parameter based on both energetics and economics is useful. By converting all thermodynamic quantities to economic quantities, the cost of irreversibilities can be compared to other economic factors including capital and operating expenses. By applying these methodologies and results, one can properly characterize the energetic performance and thermodynamic irreversibilities of chemical separation processes, make better decisions during technology selection and design of new systems, and critically evaluate claimed performance improvements of novel systems.