THERMOCHEMICAL MODELING, TECHNOECONOMIC OPTIMIZATION, AND CONTROL OF FREEZE DESALINATION PROCESS

Abstract

Freeze desalination (FD) is a method in which saline water is cooled below its freezing point and freshwater is separated from the brine in the form of ice crystals. FD is relatively insensitive to the salinity of the feed solution, making it suitable for desalination of high concentration brines such as the brine rejected from the seawater desalination plants. The design of the FD system and the thermochemical behavior of the brine upon freezing are critical factors in the energy performance of this method. To date, thermochemical properties of the concentrated seawater during cooling, such as the threshold of formation of ice and salt-hydrates and their corresponding cooling load of formation, are not well known. Likewise, the optimal configuration of the FD system to achieve the maximum energy efficiency has not been investigated. This work provides comprehensive data about the cooling load of freezing of concentrated brine rejected from seawater desalination plants along with the threshold of formation of ice and salt-hydrates backed-up by validation. Furthermore, the optimal configuration of the FD system is identified and the effects of the compressor isentropic efficiency and effectiveness of the system’s heat exchangers on the work consumption of the FD system were investigated. In addition, the lack of knowledge of FD’s economic performances represents a critical research gap that impedes the technology’s progression toward commercialization. To address the issue, this work presents a cost optimization framework for tuning the equipment size and identifying the optimal control parameters at the design stage. Utilizing the developed framework, the outcomes provide benchmark economic metrics for the FD technology considering practical ranges of electricity prices, brine disposal cost, and feed total dissolved solids (TDS). For a feed brine TDS of 75,000 ppm and at a freshwater selling price of 1.5 $/ton, electricity price of 8 ¢/kWh, and brine disposal cost of 0.02 $/bbl, the FD costs $0.33 to treat 1 barrel (bbl) of feed. For feed brine TDS of 100,000 ppm and 200,000 ppm at the same electricity, disposal, and freshwater selling prices, FD costs 0.38 $/bbl and 0.62 $/bbl respectively. The generated benchmark cost metrics can inform the future market analysis and commercialization of the FD technologies. In addition, this work investigates the potential for enhancing the power flexibility of the FD technology being a thermal desalination method known for its high energy consumption. The study introduces a design modification to the FD configuration, aiming to improve its adaptability to fluctuating power availability. By employing a causal modeling approach, the modified design is analyzed with a focus on minimizing brine treatment costs over a 24-hour period. The effectiveness of this design is evaluated through scenario-based testing, with a particular emphasis on its potential for brine treatment cost savings by participating in day-ahead electricity markets.