Flow-Electrofiltration

General Reeerence Wankat, Rate-Controlled Separations, Elsevier, London, 1990. 

Mobility is affected by the dielectric constant and viscosity of the snspending flnid, as indicated in Eq. (20-25). The ionic strength of the flnid has a strong effect on the thickness of the double layer and hence on As a mle, mobility varies inversely as the square root of ionic strength [Overbeek, Adv. Colloid Set., 3,97 (1950)]. 

Modes of Operation There is a close analogy between sedimentation of particles or macromolecules in a gravitational field and their electrophoretic movement in an electric field. Both types of separation have proved valnable not only for analysis of colloids but also for preparative work, at least in the laboratory. Electrophoresis is applicable also for separating mixtures of simple cations or anions in certain cases in which other separating methods are ineffectual. 

Electrodecantation or electroconvection is one of several operations in which one mobile component (or several) is to be separated ont from less mobile or immobile ones. The mixture is introduced between two vertical semipermeable membranes for separating cations, anion membranes are used, and vice versa. When an electric field is apphed, the charged component migrates to one or another of the membranes bnt since it cannot penetrate the membrane, it accu-mnlates at the snrface to form a dense concentrated layer of particles which will sink toward the bottom of the apparatus. Near the top of the apparatus immobile components will be relatively pure. Murphy [/. Electrochem. Soc., 97(11), 405 (1950)] has used silver-silver chloride electrodes in place of membranes. Frilette [ J. Phys. Chem., 61, 168 (1957)], using anion membranes, partially separated and Na K and Li, and K and Na.  

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The resistances, when incorporated into equations descriptive of cross-flow filtration, yield the general expression for the permeate flux for particulate suspensions in cross-flow-electrofiltration systems. 

FIG. 22-28 Regimes of operation of cross-flow-electrofiltration (a) voltage less than critical, (h) voltage equal to the critical voltage, (c) voltage greater than critical. 

Cross-flow—electrofiltration (CF-EF) is the multifunctional separation process which combines the electrophoretic migration present in electrofiltration with the particle diffusion and radial-migration forces present in cross-flow filtration (CFF) (microfiltration includes cross-flow filtration as one mode of operation in Membrane Separation Processes which appears later in this section) in order to reduce further the formation of filter cake. Cross-flow—electrofiltration can even eliminate the formation of filter cake entirely. This process should find apphcation in the filtration of suspensions when there are charged particles as well as a relatively low conductivity in the continuous phase. Low conductivity in the continuous phase is necessary in order to minimize the amount of electrical power necessary to sustain the electric field. Low-ionic-strength aqueous media and nonaqueous suspending media fulfill this requirement. 

FIG. 2G26 Alternative electrode configurations for cross-flow-electrofiltration. 

Theory Cross-flow-electrofiltration can theoretically be treated as if it were cross-flow filtration with superimposed electrical effects. These electrical effects include electroosmosis in the filter medium and cake and electrophoresis of the particles in the slurry. The addition of the applied electric field can, nowever, result in some qualitative differences in permeate-flux-parameter dependences. 

In the intensification of chromate removal from water, a double-chain cationic surfactant, dioctadecyl-dimethylammonium chloride (DODDMAC), was used as a carrier and a cross-flow electrofiltration was used, in which both the transient and the steady-state fluxes and the rejection of metal ions and surfactant were measured.Dioctadecyldimethy-lammonium chloride in water forms multilamellar droplets, even at very low concentrations. This structure is shown in Fig. 10. Metal ions are entrapped within the water layers and organic toxins can be immobilized within the surfactant bilayers. Under an electric field. 

One of the more curious phenomena associated with cross-flow electrofiltration is that above some critical voltage (Ec), increases in the tangential velocity across the membrane may actually decrease the membrane flux as in Figures 3.54 and 3.55. This can be explained by referring to Figure 3.56. 

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