Mass transfer electrodialysis

Electrodialysis. In reverse osmosis pressure achieves the mass transfer. In electro dialysis (qv), dc is appHed to a series of alternating cationic and anionic membranes. Anions pass through the anion-permeable membranes but are prevented from migrating by the cationic permeable membranes. Only ionic species are separated by this method, whereas reverse osmosis can deal with nonionic species. The advantages and disadvantages of reverse osmosis are shared by electro dialysis. 

Membrane operation is a specific, but not exotic, operation. In fact it is a hybrid of classical heat and mass transfer processes (Figure 4.1). Direct contact mass transfer operations tend to reach equilibrium due to a difference of chemical potential between two phases that are put into contact. In the same way, temperature equilibrium is aimed at during heat transfer operations, for which driving force is a temperature gradient. In contrast, for membrane operations, by using the specific properties of separation of the thin layer material that constitutes the membrane, under the particular driving force that is applied, it is possible to deviate from the equilibrium that prevails at fluid-to-fluid interphase with classical direct contact mass exchange systems and to reorientate the mass transfer properties. In particular, this is the case with classical operations such as microfiltration (MF), ultrafiltration (UF), reverse osmosis (RO), gas separation (GS), pervaporation (PV), dialysis (DI) or electrodialysis (ED), for which a few characteristics are recalled in Table 4.1. 

Chloride Industrial effluents Electrodialysis UV-Vis 200-3200 mg L-1 Flow injection system enhanced mass transference through passive neutral membranes [288]... 

Electrodialysis. The driving force is an external electric field that accelerates mass transference across the membrane. Charged compounds are transferred, whereas uncharged compounds are retained. As the dialysis efficiency depends on the external applied electric field, a potentiostat should be linked to both sides of the membrane. Details are given elsewhere [277,278]. 

O. Kurokawa, H. Matsusaki and T. Takahashi, Characteristics of net-type spacer as mass transfer promoter in electrodialysis, Kagaku Kogaku Ronbunshu, 1983, 9, 142. 

Included also in this chapter is a qualitative description of separations based on intraphase mass transfer (dialysis, permeation, electrodialysis, etc.) and discussions of the physical property criteria on which the choice of separation operations rests, the economic factors pertinent to equipment design, and an introduction to the synthesis of process flowsheets. 

Because EY5 is equal to k the mass transfer coefficient, is strongly dete. There are two other effects that influences the performance of the process as well, i) osmotic flow and ii) less effective Donnan exclusion. Osmotic flow is inherently part of the process and can not be avoided. Since ions are transferred from one compartment to another an osmotic pressure difference is generated and this drives the osmotic transport of water from the diluate to the concentrated side. Secondly, in case of high ionic concentrations the Donnan exclusion becomes less effective. This effect and the less favoured energy consumption at high concentrations makes the process of electrodialysis mote competitive at relatively low concentrations. 

Profiles in which this latter profile can be found are electrodialysis, per/aporation, gas separation, dialysis, diffusion dialysis, facilitated transport or carrier mediated transport and membrane contactors. The extent of the boundary layer resistance varies from process to process and even for a specific process it is quite a lot dependent on application. Table Vn.2 summarises the causes and consequences of concentration polarisation in various membrane processes. The effect of concentration polarisation is very severe in microfiltration and ultrafiltration both because the fluxes (J) are high and the mass transfer coefficients k (= EV8) are low as a result of the low diffusion coefficients of macromolecuiar solutes and of small particles, colloids and emulsions. Thus, the diffusion coefficients of macromolecules are of the order of lO ° to 10 m /s or less. The effect is less severe in reverse osmosis both because the flux is lower and the mass transfer coefficient is higher. The diffusion coefficients of low molecular weight solutes are roughly of the order of 10 m /s. In gas separation and pervaporation the effect of concentration polarisation is low or can be neglected. The flux is low and the mass transfer coefficient high in gas separation (the diffusion coefficients of gas molecules are of the... 

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. 

In plate-and-frame systems, such as in electrodialysis, turbulence promoters are introduced to increase the mass transfer. Figure VII - 8 is a schematic drawing of a flow channel in which corrugations have been introduced and where A1 is the distance between successive corrugations. 

This relation illustrates how the salt mass-transfer coefflcient in the region adjacent to the membrane on the left-hand side controls the current through the membrane. Since the solution in the left-hand side is being depleted of salt and is becoming more dilute, it is calied the dlluate (note, fca). The solution on the right-hand side of the membrane is called the concentrate in this electrodialysis (ED) process. 

Advantages to Membrane Separation This subsertion covers the commercially important membrane applications. AU except electrodialysis are pressure driven. All except pervaporation involve no phase change. All tend to be inherently low-energy consumers in the-oiy if not in practice. They operate by a different mechanism than do other separation methods, so they have a unique profile of strengths and weaknesses. In some cases they provide unusual sharpness of separation, but in most cases they perform a separation at lower cost, provide more valuable products, and do so with fewer undesirable side effects than older separations methods. The membrane interposes a new phase between feed and product. It controls the transfer of mass between feed and product. It is a kinetic, not an equihbrium process. In a separation, a membrane will be selective because it passes some components much more rapidly than others. Many membranes are veiy selective. Membrane separations are often simpler than the alternatives. 

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