(583h) Mathematical Optimization of Sustainable Water Desalination Processes Using Directional Solvent Extraction

Access to clean fresh water is an ever-growing concern for modern society as it is critical to ensure human health, to protect threatened ecosystems, and to promote economic growth and prosperity. Modern seawater desalination technologies remain energy intensive; they required three to four times the theoretical minimum energy for separation [1]. Further research is required to both reduce the energy intensity of seawater desalination and design new systems that are driven by renewable solar thermal and waste process heat.

Directional solvent extraction (DSE) is a promising new technology for solar desalination that can effectively utilize low temperature (40 °C to 80 °C) heat. DSE exploits special solubilities of certain solvents (typically saturated fatty acids) to extract fresh water from saline sources at elevated temperature and release the water when the solution is cooled. This approach has several unique features: (1) water can dissolve in the solvent and the solubility increases with temperature; (2) the solvent is virtually insoluble in water; (3) the solvent does not dissolve salts; and 4) no membrane is required. Previous work includes initial concept demonstration as a batch process [2,3], molecular simulation to understand solvent performance [4], and heat integration for a single-stage continuous process [5]. The predicted energy requirement for DSE, without process intensification or optimization, is >160 kWh/m³ of fresh water.

This presentation explores new process intensification opportunities for DSE. Most notably, previous work has focused on single-stage configurations [5]. Preliminary results suggest a counter-current configuration with a moderate number of equilibrium stages can reduce the energy requirement by up to 70%. Moreover, a mathematical optimization framework is proposed to optimize the DSE process and systematically explore trade-offs between product quality, energy requirements, and capital costs. This is done using an equation-oriented approach that simultaneously optimizes process operating conditions (such as temperatures, flow rates, compositions), design parameters (such as equipment sizes), and heat recovery opportunities [6,7]. Thus, the simultaneous approach can exploit tens to hundreds more degrees of freedom than conventional targeting methods (i.e., as employed in [5]), which consider optimal heat integration with fixed flowrates and temperatures. This capability is critically to fully optimize the DSE process and inform ratio molecular design of novel thermoresponsive solvents.

References:

[1] M. Elimelech, W. A. Philip (2011). The Future of Seawater Desalination: Energy, Technology, and the Environment. Science 333 (6043), pp. 712-717.

[2] A. Bajpayee, T. Luo, A. Muto, G. Chen (2011). Very low temperature membrane-free desalination by directional solvent extraction. Energy Environ. Sci. 4, pp. 1672–1675.

[3] D. Rish, S. Luo, B. Kurtz, T. Luo (2014). Exceptional ion rejection ability of directional solvent for non-membrane desalination. Appl. Phys. Lett. 104 (2), p. 024102.

[4] T. Luo, A. Bajpayee, G. Chen (2011). Directional solvent for membrane-free water desalination—A molecular level study. J Appl. Physics., 110 (5), p. 054905.

[5] S. Alotaibi, O. M. Ibrahim, S. Luo, T. Luo (2017). Modeling of a continuous water desalination process using directional solvent extraction. Desalination 420, pp. 114-124.

[6] A.W. Dowling, L. T. Biegler (2015). A Framework for Efficient Large Scale Equation-Oriented Flowsheet Optimization. Comp. Chem. Eng. 72, pp. 3-20.

[7] A. W. Dowling, L. T. Biegler (2013). Optimization-based process synthesis for sustainable power generation. Chem. Eng. Trans. 35, pp. 1-12.