A Two-Dimensional Transport Model of Bipolar Membrane Electrodialysis for the Electro-Regeneration of Carbon Capture Solvents

Abstract

Bipolar membrane electrodialysis (BPMED) is a promising technology that uses an electric field to split water into H+ and OH- ions. A repeating stack of cation selective, anion selective, and bipolar membranes are placed between two electrodes, creating a repeating pattern of acid, base, and diluting flow chambers. A major application of BPMED is the electrochemical solvent regeneration of capture solutions used in CO2 point source (PS) or direct air capture (DAC). Protons generated using BPMED can be used to shift the acid-base equilibria back to CO2, providing a more sustainable alternative to traditional thermal regeneration methods when integrated with renewable electricity sources. This method combines electrosynthesis and separation, producing a pure stream of CO2 for sequestration and a concentrated base stream.Despite its potential, current limitations to BPMED implementation for this application stem from a large power consumption and high membrane costs. In addition, gas bubbles in solution can cause high electrical resistances, and experimental studies of this phenomenon are challenging due to the small scale on which it occurs. To address these challenges, we present a two-dimensional model of BPMED for CO2 capture solvent recovery. This model depicts a single repeating cell triplet and was constructed using COMSOL Multiphysics version 6.1. The Navier-Stokes equations were implemented to compute a convection field, and the ionic species concentration was found by solving the Nernst-Planck equation. A source term in the concentration partial differential equations was implemented to account for ionic speciation and water splitting reactions, captured through kinetic rate expressions.Experimental validation was conducted on a PC Cell BED 1-4 recirculating batch system with 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 accuracy and robustness. The calculated pH and concentration profiles revealed that the majority of the speciation reaction takes place inside, or directly adjacent to, the bipolar membrane, including bubble nucleation. Bubbles form inside membrane pores and displace the electrolyte, greatly increasing the electrical resistance of the membrane, particularly when membrane porosity is low. However, the reaction plane location where bubbles are generated can be manipulated by increasing the channel width or reducing the applied voltage.Future research will focus on expanding to multiphase CFD simulations to investigate the effect on flow. Our two-dimensional model provides valuable insights into the complex and multifaceted nature of BPMED, offering a powerful tool for investigating small-scale processes and advancing the use of BPMED for CO2 capture solvent recovery. Figure 1