General Characteristics of Inorganic Membranes

The separation efficiency (e.g. permselectivity and permeability) of inorganic membranes depends, to a large extent, on the microstructural features of the membrane/support composites such as pore size and its distribution, pore shape, porosity and tortuosity. The microstructures (as a result of the various preparation methods and the processing conditions discussed in Chapter 2) and the membrane/support geometry will be described in some detail, particularly for commercial inorganic membranes. Other material-related membrane properties will be taken into consideration for specific separation applications. For example, the issues of chemical resistance and surface interaction of the membrane material and the physical nature of the module packing materials in relation to the membranes will be addressed. 

Inorganic membranes commercially available today are dominated by porous membranes, particularly porous ceramic membranes which are essentially the side-products of the earlier technical developments in gaseous diffusion for separating uranium isotopes in the U.S. and France. Summarized in Table 3.1 are the porous inorganic membranes presently available in the market (Hsieh 1988). They vary greatly in pore size, support material and module geometry. 

To help understand the performance of membranes, brief explanations of a few terminologies are in order. Permeability of a membrane is determined by dividing permeate flux by the transmembrane pressure. It indicates the membrane s throughput per unit area (flux) per unit pressure difierence. An important factor afiecting flux and retention ability of the membrane is the direction of the feed flow relative to the membrane surface. In through-flow configuration, the feed flow is perpendicular to the membrane surface. In cross-flow configuration, the feed stream flows parallel to the membrane 

Manufacturer Trade Name Membrane Material Support Material Membrane Pore Diameter Geometry of Membrane Element Tube or Channel Inside Diameter (mm) 

Alcoa/SCT Membralox ZrOj AI2O, AI2O3 AI2O3 20-100 nm 0.2-5 pm Monolith/ Tube 4 and 6 

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The general characteristics of inorganic membranes that are not application specific but affect separation performance and their determination methods are reviewed in this chapter. Those application-specific characteristics will be treated in Chapters 6 through 11. 

H.P. Hsieh, General characteristics of inorganic membranes, in R.R Bhave (Ed.), Inorganic Membranes Characterisation and Applications, van Nostrand Reinhold, New York, 1991, pp. 64-94. 

Hsieh, H. P., General Characteristics of Inorganic Membranes, (R. R. Bhave, ed.). Inorganic Membranes Synthesis, Characteristics and Applications, Van Nostrand, Reinhold, New York (1991)... 

Another very important operating characteristics of inorganic membranes that is not shown in Table 1.4 has to do with the phenomena of fouling and concentration polarization. Concentration polarization is the accumulation of the solutes, molecules or particles retained or rejected by the membrane near its surface. It is deleterious to the purity of the product and the decline of the permeate flux. Fouling is generally believed to occur when the adsorption of the rejected componcni(s) on the membrane surface is strong enough to cause deposition. How to maintain a clean membrane surface so that... 

The preparation and fabrication methods and their conditions described in Chapter 3 dictate the general characteristics of the membranes produced which, in turn, affect their performance as separators or reactors. Physical, chemical and surface properties of inorganic membranes will be described in detail without going into discussions on specific applications which will be treated in later chapters. Therefore, much of this chapter is devoted to characterization techniques and the general characteristics data that they generate. 

Different membrane shapes are used, such as plates, foils, spirals, hollow fibers, tubes, and even monilithic multichannel elements have been mentioned in the context of membrane reactors. In the following section, a general survey will be given indicating the main characteristics of the different types of inorganic membranes used in CMRs. More details can be found elsewhere [13-15]. 

Some details of the microstructures and physical, chemical and surface properties of inorganic membranes (particularly the porous ones) have been described in Cluster 4. In this chapter, those properties related to membrane performance during and between applications and general features of commercial membrane elements, modules and systems will be discussed along with application characteristics and design and operating considerations. 

The discussion on the general characteristics of polymeric and inorganic membranes is treated separately partly due to their differences in production methods and also due to important differences in their operating characteristics. 

Quaternary Ammonium Ions. In a recent study (17), 1200 EW Nafion has been used to construct a membrane ion selective electrode. The electrode was placed in both the tetrabutylammonium ion and cesium ion forms, and the response characteristics of each form were measured. These electrodes show Nernstian responses, and the tetrabutylammonium ion electrode has no interference from inorganic cations such as Na" ", K" ", and Ca2" ". However, this electrode shows a marked interference with decyltri-methylammonium ion. In addition the cesium ion electrode response is sensitive to the presence of tetrabutylammonium ion and especially dodecyltrimethylammonium ion. Although membrane electrode sensitivities are not in general proportional to thermodynamic selectivity coefficients, the results do indicate that these large, hydrophobic cations are preferred over smaller inorganic cations by the polymer. The authors suggest that the surfactant character of the two asymmetric tetraalkylammonium ions may lead to non-electrostatic interactions with the fluorocarbon regions of the polymer, which would enhance their affinities (17). 

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