Electrodialysis stack design

A portion of the electric current can be carried by the stack manifold, bypassing the membrane cell. Modem electrodialysis stack designs generally make losses due to this effect negligible. 

A typical electrodialysis stack design is shown in Figure 8. An electrodialysis stack is essentially a device to hold an array of membranes between electrodes in such a way that the streams being processed are kept separated. 

The membranes in an electrodialysis cell are separated by spacer gaskets as indicated in Figure 5.3, which shows schematically the design of a so-called sheet flow electrodialysis stack. The spacer gasket consists of a screen that supports the membranes and controls the flow distribution in the cell and a gasket that seals the cell to the outside and also contains the manifolds to distribute the process fluids in... 

Electrodialysis equipment and process design The performance of electrodialysis in practical applications is not only a function of membrane properties but is also determined by the equipment and overall process design. As far as the stack design is concerned there are two major concepts used on a large scale. One is the sheet-flow concept, which is illustrated in Figure 5.3 and the other is the so-called tortuous path concept, which is illustrated in Figure 5.5. 

Stack design in bipolar membrane electrodialysis The key component is the stack which in general has a sheet-flow spacer arrangement. The main difference between an electrodialysis desalination stack and a stack with bipolar membranes used for the production of acids and bases is the manifold for the distribution of the different flow streams. As indicated in the schematic diagram in Figure 5.10 a repeating cell unit in a stack with bipolar membranes is composed of a bipolar membrane and a cation- and an anion-exchange membrane and three flow streams in between, that is, a salt... 

Because of these detrimental conseqoences of excessive concentration polarization, the minimization of polarization is an important factor in the design of electrodialysis stacks. 

Electrodialysis as a unit operation is determined by several process and equipment design parameters, such as feed flow velocities, cell and spacer construction, stack design etc. These parameters effect the costs of the process directly and also indirectly by means of the limiting current density and the current utilization [33]. 

The gaskets not only separate the membranes but also contain manifolds to distribute the process fluids in the different compartments. The supply ducts for the diluate and the brine are formed by matching holes in the gaskets, the membranes, and the electrode cells. The distance between the membrane sheets, i.e. the cell thickness, should be as small as possible to minimize the electrical resistance. In industrial size electrodialysis stacks membrane distances are typically between 0.5 to 2 mm. A spacer is introduced between the individual membrane sheets both to support the membrane and to help control the feed solution flow distribution. The most serious design problem for an electrodialysis stack is that of assuring uniform flow distribution in the various compartments. In a practical electrodialysis system, 200 to 1000 cation- and anion-exchange membranes are installed in parallel to form an electrodialysis stack with 100 to 500 cell pairs. 

In designing an electrodialysis stack several general criteria concerning mechanical, hydrodynamic, and electrical properties have to be considered. Since some of the criteria are counter effective, the final stack construction is generally a compromise between several conflicting parameters [38,39]. 

In stack designs employing the sheet-flow principle, a peripheral gasket provides the outer seal and the solution flow is approximately in a straight path from the entrance to the exit ports which are located on opposite sides in the gasket. This is illustrated in Figure 7 a) which shows the schematic diagram of a sheet-flow spacer of an electrodialysis stack. 

The AMJ Electro Cell System is an electrodialysis unit with a tubular design claimed to be free of leaks and more efficient than the conventional rectangular stack design. This is a two-compartment system which according to AMJ can be adapted to make the three-compartment cell required by our sodium silicate electrodialysis process to make colloidal silica. 

There are several common elements in the design of any electrodialysis stack end blocks, end frames with electrodes, membranes, spacers between membranes, and manifolds for inlet and outlet of fluids. The electrodes used in an electrodialysis stack must withstand the electrochemical reactions and the solutions that circulate within the electrode chambers, as well as to the electrolytes, which are carried there due to electrodialysis and form as a result of electrolysis. Residue must not be formed inside the electrode chambers, while the chamber itself must be thin enough to not create too much resistance to the electric current... 

The value of /jim is determined by the discontinuity in the dependence of cell current on applied cell voltage which occurs when the interfacial concentration approaches zero. The polarisation parameter is convenient in the design and scale-up of electrodialysis equipment. It can be easily measured in small-scale stacks at a given value of bulk concentration and then used to predict limiting current densities in larger stacks at other concentrations. Most stacks use operating values of the polarisation parameter that are 50-70 per cent of the limiting values. 

Current utilization In practical application electrodialysis is affected by incomplete current utilization. The reasons for the incomplete current utilization are poor membrane permselectivity, parallel current through the stack manifold, and water transport by convection and due to osmosis and electro-osmosis. In a well-designed stack with no pressure difference between diluate and the concentrate convective water transport is negligibly low and also the current through the manifold can be neglected. Under these conditions the overall current utilization is given by ... 

The use of electrodialytic water demineralizers with daily capacity of several hundred thousand to several million gallons has been stimulated by the availability of a field-tested basic membrane stack of suitable size to form a logical building block for such plants. This stack, the Mack III stack, was first described two years ago (5). Since that time, field tests at Oxnard, Calif., have been completed, improvements have been made in stack production and characteristics, and commercial plants are now being designed. In this paper, we describe briefly the Oxnard field tests, present and discuss the revised characteristics of the Mark III stack, and describe certain aspects of the design and economics of the Buckeye unit as a typical example of a municipal electrodialysis plant. 

The stack type Ionics electrodialysis cell was put together according to the manufacturer s instructions. Preliminary tests with hot water developed external leaks which were corrected. Further tests with hot sodium sihcate and in actual electrodialysis operation developed both internal leaks and new external leaks. The nature of some of the leaks was inherent to the design of the ceU and could not be corrected without changing the design of the cell. 

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