Investigations on Flow Behavior Inside the Compartments of Electrodialysis(ED) Stack

Abstract

Electro dialysis (ED) is one of the electrically driven process using ion-exchange mem- branes which has been developed for several years, mainly for desalting of brackish water and sea water. ED process is now days used for separation of chemicals from a mixture of ionic and non ionic substances. The ED stack is generally used in var- ious industrial processes namely, water puri cation, e uent treatment, desalination of chemicals, recovery of valuable chemicals, etc. In ED process, the ow of chemical over the surface of the membrane plays an important role in overall e ciency, the performance and the energy consumption etc. That determines the product cost of chemicals and ultimately the capital cost. In a ED unit, the compartments are formed by alternate arrangement of cation and anion exchange membranes. The arrangement of compartments play a vital role for equal distribution of liquid in the compartments. There are mainly three di erent types of arrangements adopted in a ED stack: (1) Parallel ow pattern, (2) Series ow pattern, and (3) Parallel-cum- series ow pattern. The arrangement of compartments in parallel and group of parallel compartments in series (parallel- cum-series) is preferred for handling huge amount of liquid in the ED stacks. The present investigation has been conducted by taking a parallel-cum-series pattern in the existing ED unit. For the design optimization of the stack and compartment, the parameters are namely, the inlet and outlet openings and the e ective area (contact area of water

ow with membrane) of membrane. The water inside the compartment ows in narrow streams due to the path of least resistance which leads to high velocity of the water jet. Due to this, its covers the less area of membrane and ions will not have su cient time to get transported from treated compartment to rejected one. Thus for the improvement in the design of the ED stack and its compartments, the inlet and outlet openings are mainly responsible for the e ective ow distribution in stack and compartments which signi cantly improves the transportation of ions and hence the performance of the ED unit. From the literature review pertaining to parallel- cum-series arrangement, it was noted that there was a possibility of an uneven uid

ow through the compartments and hence a lesser e cient system. The present work is an attempt to ascertain the ow behaviour inside an existing ED stack and the compartments in each stage of the unit. Further, investigations have been made with new proposed design aspects adopted in an Electro dialysis unit viz. the inlet header design, and, design of compartment inlet and outlet openings in order to improve its performance and e ciency. As the ED stack is a closed system hence it is very di cult to observe the ow inside the compartments and in the stack itself. Hence CFD (Computational Fluid Dynamics) is the most e ective tool to analyze the ow behaviour and its pattern inside the compartment. The theoretically calculated ow velocity and pressure and are found to conform to the value predicted by using FLUENT software. The ow patterns inside the ED stack and the compartments (with existing and new proposed design) observed experimentally are found to conform to the ow patterns observed in FLUENT.