Porous membranes classification

Many classification schemes for hemodialysis membranes exist. Water permeability through the porous membranes is frequently used.14 Water permeability for a dialyzer is defined by the ultrafiltration coefficient for the particular device (KUF, mL/ h/mmHg). The KUF of any individual fiber will be related to the pore size and has an... 

Transport models fall into three basic classifications models based on solution/diffusion of solvents (nonporous transport models), models based on irreversible thermodynamics, and models based on porous membranes. Highlights of some of these models are discussed below. 

For porous membranes the molecular size of the species to be separated plays also an important role in determining the pore size of the membrane to be utilized, and the related membrane process. According to the lUPAC classification, porous membranes with aver-... 

A classification will be made between the open porous membranes, which are applied in microfiltration and ultrafiitration and the dense nonporous membranes, applied in gas separation and pervaponition. The reason for this classification is the different... 

A verj large number of combinations of solvent and nonsolvent are possible all with their own specific thermodynamic behaviour. Table III.8 shows a very general classification of various solvent/nonsolvent pairs. Where a high mutual affinity exists a porous membrane is obtained, whereas in the case of low mutual affinity a nonporous membrane (or better an asymmetric membrane with a dense nonporous top layer) is obtained. It should be noticed that this holds for ternary systems. In case of multicomponent systems with additives the thermodynaics and kinetics change, as do the membrane properties. 

This chapter reviews the possibilities that the application of a membrane in a catalytic reactor can improve the selectivity of a catalytic oxidation process to achieve a more compact system or to otherwise increase competitiveness. Classification differentiates between those reactors using dense membranes and those using porous membranes. Dense membranes provide high selectivity towards oxygen or hydrogen and the selective separation of one of these compounds under the reaction conditions is the key element in membrane reactors using such membranes. Porous membranes may have many different operation strategies and the contribution to the reaction can be based on a variety of approaches reactant distribution, controlled contact of reactants or improved flow. Difficulties for the application of membrane reactors in industrial operation are also discussed. 

While the role of the membrane is straightforward in membrane reactors using dense membranes (simply to selectively permeate a reactant or product), the use of porous membranes has created a large variety of configurations, depending on the characteristics of the reaction. Therefore, for this kind of membrane, a classification based on the reactor structure rather than on the membrane material is more appropriate, since the same membrane can be used in different ways to improve the performance of different reactions. The following four operating modes, illustrated in Fig. 27.3, will be considered ... 

The texture properties of the ultrathin porous glass membranes prepared in our laboratory were initially characterized by the equilibrium based methods nitrogen gas adsorption and mercury porosimetry. The nitrogen sorption isotherms of two membranes are shown in Fig. 1. The fully reversible isotherm of the membrane in Fig. 1 (A) can be classified as a type I isotherm according to the lUPAC nomenclature which is characteristic for microporous materials. The membrane in Fig. 1 (B) shows a typical type IV isotherm shape with hysteresis of type FIl (lUPAC classification). This indicates the presence of fairly uniform mesopores. The texture characteristics of selected porous glass membranes are summarized in Tab. 1. The variable texture demanded the application of various characterization techniques and methods of evaluation. 

The porous structure of ceramic supports and membranes can be first described using the lUPAC classification on porous materials. Thus, macroporous ceramic membranes (pore diameter >50 nm) deposited on ceramic, carbon, or metallic porous supports are used for cross-flow microfiltration. These membranes are obtained by two successive ceramic processing techniques extrusion of ceramic pastes to produce cylindrical-shaped macroporous supports and slip-casting of ceramic powder slurries to obtain the supported microfiltration layer [2]. For ultrafiltration membranes, an additional mesoporous ceramic layer (2 nm<pore diameter <50 nm) is deposited, most often by the solgel process [11]. Ceramic nanofilters are produced in the same way by depositing a very thin microporous membrane (pore diameter <2 nm) on the ultrafiltration layer [4]. Two categories of micropores are distinguished the supermicropores >0.7 nm and the ultramicropores <0.7 nm. 

The following brief classification of membrane electrodes can be used [42] inert membranes (cellulose, some sorts of porous glass) ion exchange membranes. 

Nowadays, a wider variety of inorganic membranes are commercialized. A rough classification can be based on the type of the inorganic materials (e.g., carbon, metal, ceranfic, glass, zeolite) and on the structure (e.g., porous or dense). The final catalytic membrane can be a composite of different inorganic or organic-inorganic materials. 

Not all membranes and membrane structures are covered by the classification given in figure III - 1. This approach is used for the sake of simplicity so chat the basic principles can be understood more readily. There is distinct transition from one type to the other. Reverse osmosis membranes, for example, can be considered as being intermediate between porous and nonporous membranes. 

Different extraction techniques have been developed. These techniques have been classified as porous and nonporous, based on their structure, as a flat (like a paper sheet with less than 1 pm of thickness) or hollow fiber (200-500 pm i.d.) configuration. Other classification refers to the number of phases involved in the extraction (one-, two-, or three-phase extraction techniques) [186]. A distinction can be based on the nature of the acceptor phase liquid membrane extractions, where the acceptor phase is a liquid, such as supported liquid membrane (SLM) extraction, microporous membrane liquid-liquid... 

On the other hand, membranes can be symmetric or asymmetric. Asymmetric membranes have a porous support and a thin skin layer which gives selectivity. If the two layers are made of different materials, the membrane is called composite. A more or less complete classification of membranes, along with the main features of each kind of membrane, is shown in Fig. 2. 

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