Symmetric membrane

Because membranes appHcable to diverse separation problems are often made by the same general techniques, classification by end use appHcation or preparation method is difficult. The first part of this section is, therefore, organized by membrane stmcture preparation methods are described for symmetrical membranes, asymmetric membranes, ceramic and metal membranes, and Hquid membranes. The production of hollow-fine fiber membranes and membrane modules is then covered. Symmetrical membranes have a uniform stmcture throughout such membranes can be either dense films or microporous. 

Dense Symmetrical Membranes. These membranes are used on a large scale ia packagiag appHcations (see Eilms and sheeting Packaging materials). They are also used widely ia the laboratory to characterize membrane separation properties. However, it is difficult to make mechanically strong and defect-free symmetrical membranes thinner than 20 p.m, so the flux is low, and these membranes are rarely used in separation processes. Eor laboratory work, the membranes are prepared by solution casting or by melt pressing. 

Other resolutions of the Poisson Nernst Planck equations (i.e. using various simplifying assumptions) have been proposed that couple the adsorption, desorption and permeation of ions through a membrane (e.g. [273,274]) as might be observed for a carrier-mediated transport. For example, for a symmetrical membrane (identical electrolyte on both sides of the membrane) and variation in the electrical potential profile given by i//m, /int can be estimated from ... 

DNAPLs), in soil and ground water treatment, 25 834 Dense symmetrical membranes,... 

Symmetrical dialkyl peroxides, 75 436, 446 Symmetrical dyes, substituent effects in heterocyclic nuclei of, 20 509-510 Symmetrical membranes dense, 75 800-801 microporous, 75 801-804 Symmetrical polymethine dyes,... 

Terms such as symmetric and asymmetric, as well as microporous, meso-porous and macroporous materials will be introduced. Symmetric membranes are systems with a homogeneous structure throughout the membrane. Examples can be found in capillary glass membranes or anodized alumina membranes. Asymmetric membranes have a gradual change in structure throughout the membrane. In most cases these are composite membranes... 

Porous metals have long been commercially available for particulate filtration. They have been used in some cases as microfiltration membranes that can withstand harsh environments, or as porous supports for dynamic membranes. Stainless steel is by far the most widely used porous metal membrane. Other materials include silver, nickel. Monel, Hastelloy and Inconel. Their recommended maximum operating temperatures range from 200 to 650°C. Elepending on the pore diameter which varies from 0.2 to 5 microns, the water permeability of these symmetric membranes can exceed 3000 L/h-m -bar and is similar to that obtained with asymmetric ceramic microfiltration membranes. Due to the relatively high costs of these membranes, their use for microfiltration has not been widespread. 

This is called a symmetrical membrane arrangement. It is the original version found in all early ion-selective electrodes. The cell notation can be written as... 

In a rather unnecessary attempt to define the interface between the ion-selective membrane and the dielectric, an internal hydrogel layer saturated with, for example, a buffer solution, has been interposed between these two layers. It mirrors the miniaturization of conventional symmetrical membrane ion sensors. The volume of the hydrogel layer is very small. This approach creates more problems than it solves. What is the major problem ... 

An important point to consider about hollow fiber membranes is their morphology. Hollow fiber membranes can be either symmetric or asymmetric.16 Symmetric membranes have continuous pore structure throughout. Asymmetric membranes have a dense upper layer or skin layer that is then supported with a sublayer that is significantly more porous. Figure 6.2 shows SEM images of... 

Cross-section structure. An anisotropic membrane (also called asymmetric ) has a thin porous or nonporous selective barrier, supported mechanically by a much thicker porous substructure. This type of morphology reduces the effective thickness of the selective barrier, and the permeate flux can be enhanced without changes in selectivity. Isotropic ( symmetric ) membrane cross-sections can be found for self-supported nonporous membranes (mainly ion-exchange) and macroporous microfiltration (MF) membranes (also often used in membrane contactors [1]). The only example for an established isotropic porous membrane for molecular separations is the case of track-etched polymer films with pore diameters down to about 10 run. All the above-mentioned membranes can in principle be made from one material. In contrast to such an integrally anisotropic membrane (homogeneous with respect to composition), a thin-film composite (TFC) membrane consists of different materials for the thin selective barrier layer and the support structure. In composite membranes in general, a combination of two (or more) materials with different characteristics is used with the aim to achieve synergetic properties. Other examples besides thin-film are pore-filled or pore surface-coated composite membranes or mixed-matrix membranes [3]. 

Experimentally it is the transmembrane potential difference that is observed by the use of potential-sensitive fluorescent dyes. Two of the components of the transmembrane potential difference are the intra- and extracellular potential differences (Fig. 17.2). If (f)x is the potential in the interior of the cell and 0O the exterior potential, then the transmembrane potential, Em, is given, in the case of a symmetric membrane where D for the two sides cancels, by... 

Kg. 17.2 Schematic representation of transmembrane potential profile. Em, transmembrane potential difference Eo DL, exterior diffuse double layer potential difference Ei DL, interior diffuse double layer potential difference ED potential difference due to membrane molecular dipoles EDi = EDo, symmetric membrane potential EDiff diffusion potential difference. 

FIGURE 1.13. Diagram of a liposome showing external and internal aqueous regions and the symmetrical membrane. 

A novel, but not commercially practiced, method of making symmetric membranes is to press two aluminum sheets (or foils) together in a plastic holder and anodize from only one side. The nonporous barrier layer is displaced into the second aluminum sheet and the first alumina sheet becomes a membrane containing cylindrical pores extending across its entire thickness [Rai and Ruckenstein, 1975]. 

Opposiny-reactants mode. When immobilized with a catalyst or enzyme, the interconnected tortuous pores or the nearly straight pores of a symmetric inorganic membrane provides a relatively well controlled catalytic zone or path for the reactants in comparison with the pellets or beads in a fixed or fluidized bed of catalyst particles. This unique characteristic of a symmetric membrane, in principle, allows a novel reactor to be realized provided the reaction is sufficiently fast. The concept applies to both equilibrium and irreversible reactions and does not utilize the membrane as a separator. Consider a reaction involving two reactants, A and B ... 

Membrane and Membrane Design Most membranes are polymers in nature, but some inorganic membranes have become available. The most common membranes are based on polysulfone, cellulose acetate, polyamide, fluoropolymers, and other compounds. Formation of a symmetric membrane structure is an important element in the success of UF/NF membrane separation (16). The other considerations for membrane separation are as follows (1) separation capabilities (retention or selectivity), (2) separation rate (flux), (3) chemical and mechanical stabilities, and (4) membrane material cost. 

Preliminary experiments on such a strategy of coupled separation and photodegradation were carried out on a simplified experimental device schematically represented in Figure 25.15a. It consists of two tanks separated by a 1.8 pm pore size alumina microfiltration symmetric membrane with grains coated with an anatase layer. The feed tank contained methylene blue (MB) and the reception tank was initially filled with pure water. Under continuous UV irradiation, MB is completely destroyed as it arrives in the reception tank (Figure 25.15b). The quantity of destroyed MB per surface area unit is equal to 1.0 X 10 mol.s. m . 

Symmetric membranes were the first ones to be produced. Typical symmetric membranes are Vycor glass or solid-electrolyte ones, whereas, in general, an asymmetric structure is preferred for any other material so as to get a proper balance among membrane permselectivity, permeability (the lower the permeability, the lower is the transmembrane flux at a given pressure difference) and mechanical strength [8]. In fact, an inofganic... 

The theory for HLM was developed for flat thin uncharged symmetric membranes without variation in porosity and pore sizes across the membrane thickness. 

FIGURE 18.2 (a) Symmetric and (b) asymmetric membranes. (Micrograph of symmetric membrane. Courtesy of MiUipore Corporation. 

Symmetric membranes and asymmetric membranes are two basic types of membrane based on their structure. Symmetric membranes include non-porous (dense) symmetric membranes and porous symmetric membranes, while asymmetric membranes include integrally skinned asymmetric membranes, coated asymmetric membranes, and composite membranes. A number of different methods are used to prepare these membranes. The most important techniques are sintering, stretching, track-etching, template leaching, phase inversion, and coating (13,33). 

T. H. Young, Y. H. Huang, and L. Y. Chen, Effect of solvent evaporation on the formation of asymmetric and symmetric membranes with crystallizable EVAL polymer. Journal of... 

Thermoporometry is a method which measures cavity sizes and not inlet sizes. It has been mainly used for the characterisation of organic mesoporous membrane texture [70-73] but has been also applied to inorganic alumina symmetric membranes [73] with a good reliability. However the solidification of water in small pores may sometimes damage the membrane structure due to the expansion of the condensate and consequently different results can be obtained after several runs [74]. 

Data given in the form of permeability (mol m/ir s Pa) are usually meaningful only in symmetric membranes (single, homogeneous wall, non-supported). 

Because of the flat concentration profiles, the solution precipitates at virtually the same time over the entire film cross section, and no macroscopic gradients of activity or concentration of the polymer are obtained over the film cross-section. On a microscopic scale, however, because of thermal molecular motions, there are areas of higher and lower polymer concentration, which act as nucleation centers for polymer precipitation. These microscopic areas of higher polymer concentration are randomly distributed throughout the cast polymer film. Therefore, a randomly distributed polymer structure is obtained.during precipitation. This structure is also shown in Figure 13 in the form of a scanning electron microscope picture of the cross section of a symmetric membrane obtained with a vapor phase precipitant. 

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