Electrodialysis modules

Plate-and-Frame (Conceptually the simplest, it is veiv much like a filter press. Once found in RO, UF, and IVIF, it is still the only module commonly used in electrodialysis (ED). A fevy applications in pressure-driven membrane separation remain (see Sec. 18 for a description of a plate-and-frarne filter press). 

This paper reviews the most recent innovations in electrodialysis (ED) modules and/or processes that appear to affect the food and drinks industries in the short-medium term, together with their basic mass transport equations that might help ED unit design or optimization. Future perspectives for ED processing in the food sector are also outlined. 

Plate-and-frame units have been developed for some small-scale applications, but these units are expensive compared to the alternatives, and leaks through the gaskets required for each plate are a serious problem. Plate-and-frame modules are now only used in electrodialysis and pervaporation systems and in a limited number of reverse osmosis and ultrafiltration applications with highly fouling feeds. An example of one of these reverse osmosis units is shown in Figure 3.39 [111],... 

Mainly four types of membrane modules are used plate-and-frame, spiral-wound, tube-in-shell, and hollow fiber. The plate-and-frame module consists of a series of membranes (10-500 pm thick) sandwiched between spacers that act as flow channels (Figure 5.69). (The membranes are often laminated on a porous support that offers no flow resistance.) The feed flows in one set of channels and the permeate, with or without carrier fluid, flows in alternate channels. Plate-and-frame modules find use in ultrafiltration and dialysis applications which include hemodialysis and electrodialysis. 

A new application area for membranes is energy production. Reverse electrodialysis and pressure retarded osmosis could provide significant quantities of energy from the mixing of fresh water with seawater or mixing the concentrated brine effluent of desalination plants with seawater. However, these applications will require significant reduction in membrane and module costs. 

Commercial membrane separation processes include reverse osmosis, gas permeation, dialysis, electrodialysis, pervaporation, ultrafiltration, and microfiltration. Membranes are mainly synthetic or natural polymers in the form of sheets that are spiral wound or hollow fibers that are bundled together. Reverse osmosis, operating at a feed pressure of 1,000 psia, produces water of 99.95% purity from seawater (3.5 wt% dissolved salts) at a 45% recovery, or with a feed pressure of 250 psia from brackish water (less than 0.5 wt% dissolved salts). Bare-module costs of reverse osmosis plants based on purified water rate in gallons per day are included in Table 16.32. Other membrane separation costs in Table 16.32 are f.o.b. purchase costs. 

Industrial cells consist of one or two hundred modules with membrane surfaces that can reach up to one m. These facilities are able to soften brackish water with flow rates spanning from a few hundred up to one thousand m per day, and all with an energy cost of about 1 kWh per m . The precise nature of the electrode reactions taking place in the compartments at both ends of the cell plays no direct role in the electrodialysis process. The intermembrane space has a thickness lower than 1 mm, in order to decrease the ohmic drop. However if the solution rec uires a stronger demineralising effect, then the ohmic drop can be very large because this solution will become poorly conducting. 

Concentration polarisation is not generally severe in dialysis and diffusion dialysis because of the low fluxes involved (lower than in reverse osmosis) and also because the mass transfer coefficient of the low molecular solutes encountered is of the same order of magnitude as in reverse osmosis. In carrier mediated processes and in membrane contactors the effect of concentration polarization may become moderate mainly due to the flux through the membrane. Finally, the effect of concentration polarisation may become ver severe in electrodialysis. In the following sections concentration polarization will be described more in detail. In some module configurations such as plate-and-frame and spiral wound spacer materials are used in the feed compartment (see chapter VIII). These spacers effect the mass transfer coefficient and can be considered as turbulence promoters. 

Specified for application in commercial module F = fiat plate S = spiral wound T = tubular H = hollow fiber. MF = microfiltration UF = ultrafiltration NF = nanofiltration RO = reverse osmosis ED = electrodialysis. 

J.H. Balster, Membrane Module and Process Development for Monopolar and Bipolar Membrane Electrodialysis. University of Twente, 2006. 

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