Ultrafiltration membranes transport

Electroultrafiltration (EUF) combines forced-flow electrophoresis (see Electroseparations,electrophoresis) with ultrafiltration to control or eliminate the gel-polarization layer (45—47). Suspended colloidal particles have electrophoretic mobilities measured by a zeta potential (see Colloids Elotation). Most naturally occurring suspensoids (eg, clay, PVC latex, and biological systems), emulsions, and protein solutes are negatively charged. Placing an electric field across an ultrafiltration membrane faciUtates transport of retained species away from the membrane surface. Thus, the retention of partially rejected solutes can be dramatically improved (see Electrodialysis). 

A different approach is the use of an ultrafiltration membrane with an immobilized chiral component [31]. The transport mechanism for the separation of d,l-phenylalanine by an enantioselective ultrafiltration membrane is shown schematically in Fig. 5-4a. Depending on the trans-membrane pressure, selectivities were found to be between 1.25 and 4.1, at permeabilities between 10 and 10 m s respectively (Fig. 5-4b). 

For relatively porous nanofiltration membranes, simple pore flow models based on convective flow will be adapted to incorporate the influence of the parameters mentioned above. The Hagen-Poiseuille model and the Jonsson and Boesen model, which are commonly used for aqueous systems permeating through porous media, such as microfiltration and ultrafiltration membranes, take no interaction parameters into account, and the viscosity as the only solvent parameter. It is expected that these equations will be insufficient to describe the performance of solvent resistant nanofiltration membranes. Machado et al. [62] developed a resistance-in-series model based on convective transport of the solvent for the permeation of pure solvents and solvent mixtures ... 

In the absence of suspended solutes or colloids, the pure solvent flux through an ultrafiltration membrane is directly proportional to the applied pressure difference and inversely proportional to the viscosity of the solvent and the membrane thickness. Transport within the pores occurs in the creeping flow regime, since kinematic viscosities of liquids are sufficient to make Re < C 1 for practical pore sizes. In the simplest case, the membrane can be considered to be a packed array of straight, equal diameter nonintersecting capillary tubes. The observed volumetric flux, nAvA (cc/sec cm2), equals the product of the mass flux of solvent based on the total membrane area, nA... 

In this review dealing with recent advances in membrane science, the term membrane" will be used to indicate any medium which acts as a barrier to transport into or out of a region, provides selective transfer of one species over another or regelates the transport of a material to its environment at a controlled rate. In addition to the common usage of the word membrane" to indicate a dense polymer film, the above definition includes a variety of interesting cases such as highly porous ultrafiltration membranes and hydrophobic liquid membranes with selectivity properties which can be tailored by incorporation of materials which selectively complex with one of the species to be processed. The important topics of controlled release of chemicals from polymeric devices and removal of volatile monomers from addition polymers such as poly (vinyl chloride and poly (acrylonitrile are also treated here. 

Chmielewski, A.G. and Harasimowicz, M., Influence of gamma and electron irradiation on transport properties of ultrafiltration membranes, Nukleonika, 37, 4, 61, 1992. 

An adjunct to the assessment of adequacy in PD patients is the peritoneal equilibration test (PET). The PET assesses peritoneal membrane transport characteristics in terms of solute clearances and ultrafiltration. The results are used to select a dialysis regimen appropriate to the transport characteristics of the patient (e.g., high transporters may do better on short dwell APD regimens). The PET is typically undertaken at the same time as adequacy assessments in PD patients. 

The magnitude of the Peclet number indicates the importance of the convective relative to the diffuse process for solute transport. The solute concentration profiles for representative values of Pe are illustrated in Fig. 12.2 according to Bungay [7]. When diffusion is dominant (Pe 0) the concentration varies nearly linearly in z. For large absolute values of the Peclet number, diffusion is significant only in a thin zone adjacent to the low pressure face of the membrane in which the concentration profile is very steep. For micro- and ultrafiltration membranes, the solute concentration varies little from the value at high pressure face. For nanofiltration the Peclet number can vary considerably depending on membrane characteristic almost dense or porous membranes. 

In the presence of solutes with small molecular weights, concentration polarization is likely to occur but with much less effect than in the case of ultrafiltration as explained in Section 12.2.1. A theoretical model concerning separation of sucrose and raffinose by ultrafiltration membranes has been proposed by Baker et al. [53] which assumes transport of solvent and solute exclusively through pores. This model can apply to ceramic nanofilters as they exhibit a porous structure with a pore size distribution. The retention characteristics of a given membrane for a given solute is basically determined by its pore-size distribution. The partial volume flux jy through the pores which show no rejection to the solute can be expressed as a fraction of the total volume flux fy. 

Information about the porous support layer rather than the skin layer. The techniques used by these authors, as well as those reviewed by Pusch and Welch (21), provide valuable Insight Into the mechanism of membrane formation and thus may assist membrane scientists In developing better membranes. However, many of these techniques do not characterize the membrane under the conditions of application for example, the ultrafiltration membranes (23,24) are dried prior to gas sorption studies and microscopy. Therefore, caution must be exercised In Interpreting the results of these characterization methods and relating them to membrane performance and transport mechanisms. 

The ability to determine the pore size and pore size distribution for porous membranes has existed for a number of years (11,40-42). Recent advances have permitted the determination of pore size and distribution for finely porous ultrafiltration membranes (, 1 ). Determination of i j, (the average cross-sectional area of the transport corridor) for the skin layer of reverse osmosis and tight ultrafiltration membranes as well as pervaporatlon membranes at present remains a challenge, although advances are being made in this direction (9,10,48-50). 

Brun, J.-P. "Ultrafiltration et Microfiltration In Precedes de separation par membranes Transport, Techniques membranaires. Applications Masson Paris, France, 1989 pp 137-158. 

Szymezyk, A. et al.. Characterisation of surface properties of ceramic ultrafiltration membranes by studying diffusion-driven transport and streaming potential. Desalination, 119, 303, 1998. 

Pressure Control of the Ultrafiltration Rate During Hemodialysis with High-Flux Dialyzers and the Time Dependence of Membrane Transport Parameters... 

Rejection characteristics of ultrafiltration membrane were analysed and method to determine the transport coefficients is developed. Also the rejection characteristics of membrane with gel layer were analysed and it is found that the transport coefficients of gel layer have a definite relation with the gel layer resistance, and perhaps with gel layer thickness. 

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