(670a) A Novel Circuit-Based Model of a Bipolar Membrane Electrodialysis Reactor for Offshore Chemical Synthesis

Bipolar membrane electrodialysis (BPMED) is a sustainable and highly tuneable electrocatalytic technology used to efficiently manipulate the pH of liquid streams through electrosynthesis of H+ and OH- ions from water. The salient benefits of BPMED arise from its fundamental design; it consumes electricity alone to correct stream pH, resulting in greater sustainability and tunability. However, current barriers to implementation of BPMED stem from a high energy consumption resulting from a high membrane electrical resistance or a low selectivity.

In this work, a novel circuit-based model of a BPMED reactor has been developed to evaluate trouble areas and optimise design. The repeating unit cell is represented as a system of resistors in series, with membranes and electrolytes comprising the resistive elements. The water splitting potential is computed using Wein’s second law, Ohm’s law is used to relate the cell voltage to a current density on a differential volume, and Faraday’s law translates the current density to an ion flux. Ion speciation is computed in these differential volumes as well. Delayed differential material balances are then used to calculate the distribution of ionic species in time and space. A critical feature is a novel model for the current efficiency, calculated as a function of the trans-membrane concentration difference rather than assumed constant. Experimental validation was conducted on a PC Cell BED 1-4 recirculating batch system and showed good agreement across a range of conditions and variables.

This model was integrated into an Aspen Plus model to simulate and optimise the electrosynthesis of concentrated NaOH and HCl from seawater. The compact nature of BPMED and reliance on electricity only makes this a promising application for offshore platforms. An economic analysis showed this to be a very preferable alternative to current technologies.