Highly concentrated HCl and NaOH from brines using electrodialysis with bipolar membranes

Abstract

Electromembrane technologies, such as electrodialysis with bipolar membranes (EDBM) have been widely proposed for the treatment and valorization of seawater reverse osmosis (SWRO) concentrated streams. The EDBM technology applied to brines produces acids (HCl) and bases (NaOH) with just two inputs: electric energy and brine. Thus, the reagents self-supply to the SWRO plant could be achieved, which definitively fits the principles of the Circular Economy. However, previous works have shown low concentration values of the produced acids and bases by EDBM, which undoubtedly hampers the possibility of its use in the SWRO plant. Therefore, the aim of the present work is to demonstrate the viability of a new strategy to valorise SWRO brine producing commercial HCl at 35 wt.% by means of the integration of EDBM technology under constant (galvanostatic, equivalent to the use of electrical energy from the grid mix) and variable current intensity (equivalent to the use of PV solar energy) and azeotropic distillation. Concentrations of HCl and NaOH up to ~3.3 mol·L-1 and ~3.6 mol·L-1 are obtained respectively, which are almost 50% higher than any other reported in the literature so far using this technology. The specific energy consumption of the EDBM unit was in the range of 21.8 kWh·kg-1 of HCl and 43.5 kWh·kg-1 of HCl, being dependant on the average applied current density. These HCl concentrations, although being acceptable for internal use in the SWRO plant, did not reach commercial levels (35 wt.% of HCl), so a further concentration stage using distillation has been evaluated through simulation. Minimum values of the overall EDBM plus distillation process specific energy consumption (SECOV) was between ~40 kWh·kg-1 and ~60 kWh·kg-1 of HCl. Moreover, the environmental burdens associated with the energy consumption are quantified in terms of the carbon footprint (CF). Although the SECOV is slightly higher, the use of PV solar energy instead of the grid mix let the process to provide a better environmental performance. The renewable alternative provides values between 1.61 kg CO2-eq.·kg-1 of HCl (full PV solar energy) and 6.97 kg CO2-eq.·kg-1 of HCl (PV solar energy and steam).