Types of Porous Membranes

2 GAS PERMEATION IN POROUS MEMBRANES 2.2.1 Types of Porous Membranes 

According to the lUPAC definition, porous membranes can be classified into macro-, meso-, and microporous membranes based on the pore size of the separation layer. The separation layer and the substrates can be made from different materials. Common inorganic porous membranes are listed in Table 2.1. 

Different types of pores may co-exist in the porous membranes isolated pore, dead-end pore, cylindrical pore, constricted pore, and conical 

The transport properties of porous membranes are highly related to the parameters of porosity, pore size distribution, pore shape, interconnectivity, orientation, and roughness of the surface. The separation activity of 

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Figure 1.2 Cross-sectional schematic diagrams of various types of porous membranes...

The difference in relative permeability of gases through membranes offers an attractive method of separation and enrichment of gases. The above types of porous membranes can be used for this purpose. For example, N2 shows much higher permeability than O2 through porous polystyrene membrane which therefore has potential for application in the production of oxygen-enriched air. Porous polymers... 

It should be noted that no other type of porous membranes has all of these unique properties. 

In both types of liquid-membrane ISEs, the membrane acts as an inunis-cible phase boundary between the aqueous and non-aqueous solutions inside the ISE (see the schematic diagram presented in Figure 3.13). In order to minimize mixing, the liquid membrane is held in place by an inert, porous material such as a rigid glass frit or a flexible synthetic polymer - the choice will depend on the manufacturer rather than on experimental considerations. 

The membranes used for pervaporation are similar to reverse osmosis membranes, i.e. both are composite membranes consisting of a very thin dense permselective film on top of a nonselective porous support. In pervaporation, however, the membrane is highly swollen at the feed side and relatively dry at the permeate side. Two different types of pervaporation membranes based on polymeric materials were developed at about the same time in the early 1980s [31] ... 

The search for models of biological membranes among porous membranes continued in the twenties and thirties. Here, Michaelis [67] and Sollner (for a summary of his work, see [90] for development in the field, [89]) should be mentioned. The existence and characteristics of Donnan membrane equilibria could be confirmed using this type of membrane [20]. The theory of porous membranes with fixed charges of a certain sign was developed by Teorell [93], and Meyer and Sievers [65]. 

What pressure must be applied to force water through an initially dry Teflon membrane which has a uniform pore size of 0.5 pm diameter What factors can reduce this pressure Give examples of the industrial and everyday use of this type of porous material. 

Studies with many types of porous media have shown that for the transport of a pure gas the Knudsen diffusion and viscous flow are additive (Present and DeBethune [52] and references therein). When more than one type of molecules is present at intermediate pressures there will also be momentum transfer from the light (fast) molecules to the heavy (slow) ones, which gives rise to non-selective mass transport. For the description of these combined mechanisms, sophisticated models have to be used for a proper description of mass transport, such as the model presented by Present and DeBethune or the Dusty Gas Model (DGM) [53], In the DGM the membrane is visualised as a collection of huge dust particles, held motionless in space. 

The carbon materials attract the increasing interest of membrane scientists because of their high selectivity and permeability, high hydrophobicity and stability in corrosive and high-temperature operations. Recently many papers were published considering last achievements in the field of carbon membranes for gas separation [2-5]. In particular, such membranes can be produced by pyrolyzing a polymeric precursor in a controlled condition. The one of most usable polymer for this goal is polyacrylonitrile (PAN) [6], Some types of carbon membranes were obtained as a thin film on a porous material by the carbonization of polymeric predecessors [7]. Publications about carbon membrane catalysts are not found up to now. 

In the characterization of porous membranes by liquid or gaseous permeation methods, the interpretation of data by the hyperbolic model can be of interest even if the parabolic model is accepted to yield excellent results for the estimation of the diffusion coefficients in most experiments. This type of model is currently applied for the time-lag method, which is mostly used to estimate the diffusion coefficients of dense polymer membranes in this case, the porosity definition can be compared to an equivalent free volume of the polymer [4.88, 4.89]. 

For convenience of discussion, modeling studies of packed-bed inert membrane tubular reactors will be divided into two categories depending on the type of inorganic membranes dense or porous. 

Besides the compact membrane catalysts described in Section II, there are two types of composite membrane catalyst porous and nonporous. Composite catalyst consists of at least two layers. The first bilayered catalyst was prepared by N. Zelinsky [112], who covered zinc granules with a porous layer of palladium sponge. The sponge became saturated with the hydrogen evolved during hydrochloric acid reaction with zinc and at room temperature actively converted hydrocarbon iodates into corresponding hydrocarbons. 

Two different approaches can be followed, the fabrication of porous membranes by constructing pillars or channels within the microchannel or by creating a shallow microchannel covered with a slide. Comb-shape, array of posts and weir-type filters are described by Wilding and co-workers [56-58],... 

To enhance the separation factor the average pore diameter should be decreased considerably. According to Eqs. (9.9a) and (9.15) the contribution to the total gas flux of the gas (Knudsen) diffusion decreases and at the same time that of surface flow (diffusion) increases with decreasing pore radius. In recent years modification of existing membranes for improving their separation efficiency has been actively pursued especially by attempts to decrease the pore size of membranes. This resulted in different types of microporous membranes. According to lUPAC convention these are porous systems with a pore diameter below 2 nm. In the literature the name microporous is frequently misused and this should be avoided. 

In the case of a composite membrane consisting of a skinless porous substrate and a dense film, permeability and permselectivity may be determined solely by the resistance of the denser film. Different membrane polymers may therefore be employed for the thin barrier layer and the thick support structure. This permits a combination of properties which are not available in a single material. Such membranes were initially developed for desalination by reverse osmosis where they are known as thin- or ultrathin-film composites or nonlntegrally-skinned membranes. A second type of composite membrane is utilized for gas separations. It is a composite consisting of an integrally-skinned or asymmetric membrane coated by a second, more permeable skin which is used to fill skin defects. The inventors of the latter have entitled their device a resfstanee model membrane, but the present author prefers the term coated integrally-skinned composites. 

In many cases it is possible to remove the membrane by chemical means, recondition the porous support, and reform either the same type or a different type of dynamic membrane at the application site. This feature gives each module a long operating life. 

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