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

Abstract

Bipolar membrane electrodialysis (BPMED) is a sustainable and highly tuneable electrocatalytic technology used to efficiently manipulate the pH of liquid streams through the electrosynthesis of H+ and OH- ions from water. This emerging reactor technology employs a repeating stack of cation selective, anion selective, and bipolar membranes placed between two electrodes to create a repeating pattern of acid, base, and diluting flow chambers. The applied electric field induces water splitting at the bipolar interface, due to the presence of an electrocatalyst, the products of which are transported to the adjacent channels. The salient benefits of BPMED arise from its fundamental design, as 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.A robust mathematical model of BPMED would be invaluable to identify situations where BPMED is commercially viable and the aspects in which technological advancements which would have the greatest effect. Due to the highly multifaceted and confounding nature of BPMED, the relationship between manipulated variables and process performance is unintuitive. The proposed model will thus have stark implications for process intensification.In this work, a novel circuit-based model of BPMED has been developed. 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 implemented is a novel model for the current efficiency, which is calculated as a function of the trans-membrane concentration difference. A single ‘innate current efficient’ experimental parameter is used, the value found when the trans-membrane concentrations are equal and is very often supplied by membrane manufactures.Experimental validation was conducted on a PC Cell BED 1-4 recirculating batch system. A stack of seven cell pairs of PC acid 60 (anion selective), PC SK (cation selective), and PC Bip (bipolar) membranes. Good agreement between the model and experimental results across a range of conditions and variables demonstrates the accuracy and robustness of the model.This model was subsequently used to investigate the power consumption of BPMED as a function of the desired outlet pH, and integrated into an Aspen Plus model. A process model was developed to simulate and optimise the electrosynthesis of concentrated sodium hydroxide and hydrochloric acid from seawater. The compact nature of BPMED and reliance on electricity only makes this a promising application for offshore drilling platforms. An economic analysis showed this to be a very preferable alternative to current technologies. Figure 1