Analysis of Water Flow Inside the Compartment of Continuous Electro-Deionization (CEDI) Unit Using Computational Fluid Dynamics

Abstract

Continuous Electrode Deionization (CEDI) is based on a combination of electro dialysisand conventional ion exchange. Electro dialysis is today by far the most importantindustrial membrane separation process using ion-exchange membranes and the drivingforce of an electrical potential gradient. However, electro dialysis cannot be appliedeconomically for the treatment of dilute solution because of their high electricalresistance and the development of polarization phenomena. This limits the electricalcurrent that can be applied and thus increase the necessary membrane area. In orderto overcome this problem, ion conducting materials such as ion-exchange resin andtextile which are three to four orders magnitude more conductive than the solution tobe treated, are introduced into the ion-depleting compartments of an electro dialysisstack. This measure signicantly reduces stack resistance (power consumption) andincreases the available surface area for ion exchange. The ion-conducting materials arecontinuously regenerated electrochemically by hydrogen (H+) and hydroxide (OH-)ions that are produced by water splitting in an applied direct current (DC) electriceld. The major use for this technology is the production of high-purity water. Otherelds of application are the removal and recovery of heavy metal and precious metalions from industrial euents. [1].The method of producing ultra pure water is a hybrid process combining threetechnologies- Reverse Osmosis (RO), Continuous Electro Deionization (CEDI) andDeionization. The RO product water (25-40 M:cm2) is supplied to CEDI unit intwo inlet streams; one for treated compartment (product water) and another for concentratedcompartments (rejected water). Finally the treated water of CEDI unit ispassed through the polishing unit (Deionization process) and ultra pure water of 18.2M:cm2 is collected for further use. The ultra pure water has number of applicationin industries as well as in academic institutes and research laboratories. Looking tothe interest of multinational companies towards Research and Development, most ofvithe industries in India require ultra pure water system for research work. Thus fromthe market demand, it appears that ultra pure water system has a lot of scope inthe Indian market as well as abroad. The most important part of the ultra purewater system is CEDI unit, in which the RO product water is passed through treatedand rejected compartment (separated by ion exchange membrane). The ions will gettransferred from the treated compartment to the rejected one due to applied voltagewhich enhance the transfer of ions in the compartments lled with ion exchange resin.The CFD is the most eective tool to analyze the ow behavior and its pattern insidethe compartment. For the design optimization of the compartment, the parametersare namely, the inlet and outlet openings and the contact area of water ow withthe membrane. The water inside the compartment ows in narrow streams due tothe path of least resistance which also reduces the contact time and the membranecontact area inside the compartment. Due to this, ions will not have sucient time toget transported from treated compartment to rejected one. Thus the improvement inthe design of the inlet and outlet section of each compartment is mainly responsiblefor ow distribution vis-a-vis to maximize the eective area of membrane which signicantly improves transportation of ions. The theoretically calculated ow velocity(0.04-0.08 m/s) for the porous media is found to conform to the value predicted byusing FLUENT software. The ow pattern shows the maximum utilization of membranearea in the active area which enhances the ions transfer during the process.