(317b) Molecular Design Targets and Optimization of Low-Temperature Thermal Desalination Systems

Water consumption is critical to modern society; the average American family uses approximation 1140 l of municipal water per day [1], and 130 million Americans face severe water scarcity at least part of the year [2]. Globally, 3.8 billion people currently experience water scarcity [3], and it is estimated that 66% of the world's population could be living under water-stressed conditions by 2025 [4]. Although the costs of water obtained from desalination have fallen in the last decade, these costs are higher than obtaining freshwater from rivers, groundwater, or water recycling. In 2019, less than 1% of the water consumed globally was produced by desalination [5], [6], [7]. The expansion of oil and gas extraction in the US has created new water and environmental challenges [1]. Around 0.50 liters of produced water may be generated to supply energy for one hour in an average American household. While evaporative and reverse osmosis desalination technologies are commonly deployed, they remain energy-intensive and unable to treat high salinity water sources. There is no one-size-fits-all technology for water treatment, including desalination. Instead, there is a growing emphasis on fit-for-purpose treatment in decentralized networks [8]. In this paradigm, water is treated to only the specifications needed for specific end uses. In this talk, we demonstrate, through technoeconomic optimization, ionic liquids are extremely promising for a new desalination process.

We present a mathematical optimization framework to rapidly design desalination processes. Directional solvent extraction (DSE) uses a thermoresponsive solvent to facilitate treatment over a wide salinity range [9]. DSE does not require membranes, which often foul at high salinities, and can utilize low-grade heat, including waste or renewable (solar) sources. Prior work in DSE includes characterization of molecular phenomena, bench-scale demonstrations, and limited process analysis [9], [10]. The framework has two new capabilities [11]: first, we perform simultaneous process optimization and heat integration to rapidly screen directional solvent candidates in seconds. Second, we perform a sensitivity analysis to identify the necessary solvent properties to enable cost-effective DSE processes for treating high salinity water, which is difficult/impossible to desalinate with other technologies. We emphasize these advances in process-scale models can rapidly accelerate DSE development by reducing the need for expensive experiments and guiding (computational) molecular design.

Our framework allows for rapidly screening and identification of quantitative solvent thermophysical property targets. We find the Levelized Cost Of Water (LCOW) is most sensitive to three thermophysical properties: i) the change in water solubility for a fixed temperature change (thermoresponsiveness); ii) the solubility of water in the solvent; iii) the solubility of the directional solvent in freshwater. [11]. Technoeconomic optimization was performed for five candidate fatty directional solvents ranging, giving LCOW predictions between $1.3/m³ and $109/m³. Sensitivity analysis shows significant improvements in three solubility properties (thermoresponsiveness of solvent, the solubility of water in the solvent, and solubility of the solvent in freshwater) are needed for the hypothetical fatty acid-like DS to achieve less than $0.5/m³. In contrast, ILs show much greater promise as directional solvents. Using newly published data from [emim][Tf₂N] and assuming a moderate solvent price of $100/kg, we predict a modest $2.65/m³ LCOW. Sensitivity analysis shows the required combination of thermophysical properties necessary to achieve LCOW to below $0.5/m³. These results emphasize the potential of IL directional solvents to desalinate high salinity water, which is currently challenging with existing technologies. As ongoing work, we are considering extensions to hypersaline conditions.

References

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[2] M.M. Mekonnen, A.Y. Hoekstra, Sustainability: four billion people facing severe water scarcity, Sci. Adv., 2 (February) (2016), pp. 1-7, 10.1126/sciadv.1500323

[3] M. Kummu, J.H. Guillaume, H. De Moel, S. Eisner, M. Flörke, M. Porkka, S. Siebert, T.I. Veldkamp, P.J. Ward, The world’s road to water scarcity: shortage and stress in the 20th century and pathways towards sustainability, Sci. Rep., 6 (December) (2016), pp. 1-16, 10.1038/srep38495

[4] UNDESA, International decade for action "water for life" 2005–2015 (DoA: 2019), https://www.un.org/waterforlifedecade/scarcity.shtml

[5] N. Voutchkov, Energy use for membrane seawater desalination – current status and trends, Desalination, 431 (2018), pp. 2-14

[6] E. Jones, M. Qadir, M.T. van Vliet, V. Smakhtin, S. mu Kang, The state of desalination and brine production: a global outlook,, Science of The Total Environment, 657 (2019), pp. 1343-1356, 10.1016/j.scitotenv.2018.12.076

[7] H. Ritchie, Water use and stress, our world in data (DoA: 2020). URL https://ourworldindata.org/water-use-stress.

[8] K.R. Zodrow, Q. Li, R.M. Buono, W. Chen, G. Daigger, L. Dueñas-Osorio, M. Elimelech, X. Huang, G. Jiang, J.-H. Kim, B.E. Logan, D.L. Sedlak, P. Westerhoff, P.J.J. Alvarez, Advanced materials, technologies, and complex systems analyses: emerging opportunities to enhance urban water security, Environmental Science & Technology, 51 (18) (2017), pp. 10274-10281, 10.1021/acs.est.7b01679

[9] T. Luo, A. Bajpayee, G. Chen, Directional solvent for membrane-free water desalination—a molecular level study, Journal of Applied Physics, 110 (5) (2011), Article 054905, 10.1063/1.3627239

[10] S. Alotaibi, O. Ibrahim, S. Luo, T. Luo, Modeling of a continuous water desalination process using directional solvent extraction, Desalination, 420 (2017), pp. 114-124, 10.1016/j.desal.2017.07.004

[11] Alejandro Garciadiego, Tengfei Luo, Alexander W. Dowling, Molecular design targets and optimization of low-temperature thermal desalination systems, Desalination, 504 (2021), 10.1016/j.desal.2021.114941.