Coupling mass transport and chemical equilibrium models for improving the prediction of SWRO permeate boron concentrations

A new simulation approach is presented for predicting boron concentrations in the product water of seawater reverse osmosis operations. The new (numerical) approach links traditional mass-transfer models (the solution-diffusion transport approach and the concentration polarization film-layer model) with full aqueous-phase thermodynamic species characterization, performed by chemical equilibrium software (PHREEQC), based on the Pitzer approach. The new approach results in a more accurate calculation of the boric acid (B(OH)₃) molar fraction which develops close to the membrane wall, on the feed side, thereby improving the prediction accuracy of B(OH)₃ permeation. Specifically, acknowledging that the pH value of the feed invariably changes as seawater brine progresses through the membranes' train, calculation of this pH change, as performed in the new approach, enables a more physically-accurate and better simulation of the boric acid fraction. The new approach is shown in the paper to result in a prediction that matches better empirical results obtained from the operation of a pilot-scale SWRO plant, as compared to the traditional approach.