Sodium Hydroxide Production from Seawater Desalination Brine: Process Design and Energy Efficiency

Abstract

The ability to increase pH is a crucial need for desalination pretreatment (especially in reverse osmosis) and for other industries, but processes used to raise pH often incur significant emissions and nonrenewable resource use. Alternatively, waste brine from desalination can be used to create sodium hydroxide, via appropriate concentration and purification pretreatment steps, for input into the chlor-alkali process. In this work, an efficient process train (with variations) is developed and modeled for sodium hydroxide production from seawater desalination brine using membrane chlor-alkali electrolysis. The integrated system includes nanofiltration, concentration via evaporation or mechanical vapor compression, chemical softening, further ion-exchange softening, dechlorination, and membrane electrolysis. System productivity, component performance, and energy consumption of the NaOH production process are highlighted, and their dependencies on electrolyzer outlet conditions and brine recirculation are investigated. The analysis of the process also includes assessment of the energy efficiency of major components, estimation of system operating expense and comparison with similar processes. The brine-to-caustic process is shown to be technically feasible while offering several advantages, that is, the reduced environmental impact of desalination through lessened brine discharge, and the increase in the overall water recovery ratio of the reverse osmosis facility. Additionally, best-use conditions are given for producing caustic not only for use within the plant, but also in excess amounts for potential revenue.

Integrated Valorization of Desalination Brine through NaOH Recovery: Opportunities and Challenges

“…[18][19][20] In conjunction with different composite materials,o thers have improved the performance of nanofiltration by exploring different strategies to modify the surface of nanofiltration membranes, [18][19][20][21] which range from rapid co-deposition of polymers [21] to in situ surface reactions [18] to inkjet printing. Generally,t he mass transfer and process efficiencies of membrane-based processes can be mathematically modelled to provide approximations of the optimal operating conditions and configurations.S everal studies have developed models for NF by considering its properties such as recirculation, ion rejection, permeate production, and energy consumption; [9,27,28] as ar esult, trends between the different factors were identified. Like most membrane-based processes,NFcan be operated at different pressures,t emperatures,f lowrates, and concentrations,which can all affect the energy efficiency and consumption of the process.…”

“…[9] 3. NF and SED are currently more suited to pretreat brine with divalent ions and avoid membrane scaling.…”

“…Recent thermodynamic limits analyses [7,9] and the above discussion focus on the energy requirements for caustic production, which are promising compared to the incumbent chlor-alkali process. Recent thermodynamic limits analyses [7,9] and the above discussion focus on the energy requirements for caustic production, which are promising compared to the incumbent chlor-alkali process.…”

“…Thus,desalination plants can save money and reduce their environmental impact by recovering NaOH and HCl from brine,w hich contains sodium chloride. [7,9] 2. Membrane-Based Pretreatment Processes for Brine Valorization By integrating membrane-based processes,r esources in seawater reverse osmosis (SWRO) brine,s pecifically NaCl, can be recovered as valuable chemical commodities (i.e., NaOH and HCl).…”

“…[25] While the flat sheet membrane had ah igh transmembrane flux, the ion rejection and membrane permeance values were similar between the two configurations. Generally,t he mass transfer and process efficiencies of membrane-based processes can be mathematically modelled to provide approximations of the optimal operating conditions and configurations.S everal studies have developed models for NF by considering its properties such as recirculation, ion rejection, permeate production, and energy consumption; [9,27,28] as ar esult, trends between the different factors were identified. By identifying the optimal operating mode and conditions,N Fm ay be af avored pretreatment method for SWROb rine in large-scale use.…”