(352s) Thermodynamic Analysis of NaCl-Water Hypersaline Solutions: An Ultrasonic Study for Water-Energy Nexus Applications

One of the major problems faced by urban and rural areas of both developing as well as developed countries is the availability of fresh water. The global climate crisis has disrupted the natural water cycle, limiting the availability of fresh water even in tropical countries. To address this pressing issue, the world economies have shifted their focus to desalination and have made remarkable progress in developing technologies which are energy efficient, affordable and available to the masses. While the main product of a desalination plant is pure water, the by product is a hypersaline (greater than 5000ppm) solution which is diluted and discharged into the oceans. This solution is a raw source of energy which can be tapped to supplement the energy requirements of the desalination plant. The recent advances in Salinity energy technologies have opened new avenues for research in the water-science sector. In this work, the authors have focused on understanding the molecular interactions of a hypersaline NaCl solution by simple Ultrasonic Interferometric techniques at three different temperatures (300.15K, 308.15K and 315.15K). The analysis of thermo-acoustical parameters such as isothermal compressibility, adiabatic expansivity, free volume, intermolecular free length, acoustic impedance and relaxation time, has painted a clear picture on the interactions occurring in the electrolyte solution. The free volume and intermolecular free length of the solution decreases with an increase in concentration of NaCl due to volume electrostriction and increases with temperature due to breakage of bonds and results in unbonded molecules. A linear increase in ultrasonic velocity with concentration and temperature gives a linear increase in acoustic impedance. This is due to the increase in interactions between salt ions and water molecules at higher concentrations and temperatures. The relaxation time studies showed a unique dip in values at around 25-40ppt which indicate that perhaps the most stable concentration of NaCl-water solution exists at this range and thus it explains the reason behind the ambient seawater concentrations (35-40ppt) existing across the globe. This work has also revealed several lacunae in the understanding of properties such as internal pressure which have been accounted for and addressed. Different correlations used to calculate Internal Pressure were analysed and results showed a gross deviation from reported values. Therefore, for the first time, a simple correlation between ultrasonic velocity and internal pressure has been proposed in this work. Osmotic Pressures and Vapour Pressures find importance in desalination processes such as Reverse Osmosis and Membrane Distillation respectively. Therefore, it is essential to determine these properties in a quick and efficient manner. To do so, correlations between ultrasonic velocity and these properties have been proposed in this work for a temperature range of 300.15K to 315.15K. The highly linear relation between these properties and ultrasonic velocity at different concentrations and temperatures, makes the derived equations highly reliable and reduces chances of error. Furthermore, the contribution of hydrogen bonding to the cohesive energy density of NaCl solutions upto 1 lakh ppm have also been understood. An in-depth analysis on the least work and heat of separation has also been done to analyse hypersaline solutions further and to judge whether extraction of power from the solution would be feasible. A framework for a facile determination of internal pressure, osmotic pressure, and vapour pressure has been proposed. By evaluating the ultrasonic velocity of the solution and using a simple TDS meter to check its salinity at a constant temperature, one may find these properties with ease. In addition to this, it gives an engineer a tool to make an unerring judgement between desalination (Reverse Osmosis, Membrane Distillation, Forward Osmosis) and salinity gradient energy generation (Pressure Retarded Osmosis, Thermo-Osmotic Energy Conversion, Reverse Electro-Dialysis). From this study, we can easily conclude that at concentrations of above 1 lakh ppm, it is more suitable to generate power than to produce pure water. This work deals with the application of thermodynamic properties for making engineering decisions for a Water-Energy Nexus with simple inputs such as salinity and ultrasonic velocity.