Ceramic Membrane Nanotechnology Filters
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A brief review of the manufacture and structure of ceramic membrane filters used used for micro/ultra-filtration and an overview of their applications. By Dr. Leslie Henshall. Ceramic Technology International 1994, Sterling plc, London, (1993) p. 74-76 Contents |
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An integral and important aspect of many processing industries is the step involving the separation of the desired product and/or impurities from the reaction/carrier medium. The current total market for separation devices in the European Community area is in excess of three billion Euros a year. The conventional separation techniques such as distillation, crystallisation, filtration and solvent extraction were challenged in the 1960s by the use of membrane separation techniques. A membrane can be described as a semi-permeable barrier that allows preferential transport of certain matter through its structure. Pressure driven membrane processes can be divided according to the mean pore diameter or the size of particles they will retain: ultra-filtration (UF) describes the separation of macromolecules or particles in the approximate size range 0.001 to 0. 1 m m and micro-filtration (MF) is used to separate particles in the size range 0.1 to 20 m m, while reverse osmosis (R0) will remove sub-nanometre constituents from a liquid. Membrane separation processes are in many cases faster, more efficient and more economical than conventional processes since the fractionation takes place without a phase change. By the year 2000 the world market for membrane equipment is expected to be 10-15 per cent. Polymeric membranes will continue to fulfil the majority of the membrane separations. However, they suffer from significant limitations with respect to the environmental situations in which they can operate, due to swelling/degradation and the operating pressures, which can induce compaction of the polymer. Another factor is that as the membranes become "fouled" in service, regeneration by back-flushing or other means is problematical with polymers. The advantages and disadvantages of ceramic membranes are listed in Table 1. There are several different varieties of ceramic membranes, and only the main categories are considered here. The processing details are often only available through the patent literature. The principle shapes of the filters are flat plate, hollow fibre, and tubular or multi-channel monolithic elements. The different processing routes and materials are described below. Asymmetric membranes consist of up to four different layers and comprise the most common form of ceramic membrane filter, these are:
The requirements of the support are that it should provide reasonable strength and chemical resistance, and provide a suitable surface for depositing another layer onto it, while increasing the hydraulic resistance as little as possible, The main manufacturing routes for ceramic supports are extrusion, calendaring/tape casting and slip casting. The substrate materials that have been used include alumina, zirconia, titania, silloa, carbon, silicon carbide, silicon nitride and steel and nickel. The second layer is commonly formed by slip casting using very fine ceramic particles. The liquid from the slip should move into the substrate as a result of the capillary pressure drive forces. If an ultra-filtration layer is required the particle size of the ceramic needs to be of order 10nm to obtain the requisite pore size. Such fine particle sizes cannot be obtained by a comminution process, in the case of oxide based layers it is possible to deposit the fine particles from a sol-gel or polymer-gel formed from a metal salt or alioxide precursor. Non-oxide based ceramic membranes often have greater chemical stability and, probably, enhanced permeate flux rates. The obvious process for producing these is from a colloidal dispersion of nano sized particles. At present for most materials such fine particles can only be produced in laboratory quantities using for example, laser or thermal evaporation methods. However, given the superior properties that fully dense bodies manufactured from nano size particles exhibit, it is likely that commercial quantities of these powders will be available in the foreseeable future. The materials that have been fused for the second and/or third layers include: alumina, zirconia, titania, silica, carbon, silicon carbide and sialon in all cases, the top layer must be defect free (no cracks or pinholes) and preferably have a narrow pore size distribution. This provides the main technological challenge and difficulty in the fabrication process. Also, it would appear at the present time that there is no satisfactory theoretical model to predict the sintering time/temperature combination to produce a specified mean flow pore size. Surface modifications to the top membrane layer have been effected by several researchers to reduce pore sizes and/or improve selectivity. Glass membranes can be produced by acid etching of the Na2 O-B2 O3 rich phase obtained by thermal remixing of a homogeneous Na 2 O-B2 O3 -SIO2 glass. The remaining silica rich glass structure has suitable micro-porous interconnected porosity, but is not very chemically resistant. Anodic oxidation of aluminium generally results in the formation of a porous oxide layer. This has been adapted to produce homogeneous membranes with pores of 200nm and asymmetric membranes with pores of 25nm in the top layer of alumina. The remaining aluminium is removed by acid etching. Pyrolysis of carbon-based polymers for example polyacrylonitrile, or silicon rubbers can be used to produce sieve carbon or silica membranes with pore diameters about 1nm. In this case a thin layer of material is bombarded with energetic particles from a radioactive source. This sensitises the material to an etchant, which is then used to form straight pores of uniform shape and size with controlled diameters in the range 6 - 1200nm. This is applied commercially to polymeric membranes, for example Nucleopore, and experimentally to some inorganics, such as mica. These are typically thin plates of ceramic oxide, such as stabilised zirconia, or metals, such as Pd. and its alloys. These are fully dense and rely on the permeability of ionic or atomic oxygen or hydrogen through them. Their main disadvantage is low permeability. The main potential application for the zirconia membranes is in fuel cells. The main transport and separation mechanisms in porous membranes are Knudsen diffusion (gas phase transport), surface diffusion, multilayer diffusion and capillary condensation. When the pore size is of molecular dimensions molecular sieving or micropore diffusion are the determining mechanisms. The first major application of inorganic membrane filters started in the 1940s for the separation of uranium isotopes by the selective gaseous diffusion of UF2 . Over 1000 stages are required to achieve sufficient enrichment using this chemically very aggressive material. The commercial applications of gas separation membranes are limited at present, although there are several active areas of study. It has been shown that alumina membranes can be used to separate water from alcohols at temperatures up to 90° C. Therefore it is possible to go beyond the ageotropic point and produce high purity alcohols. Is has also been shown for example that a 1: 1: 1 CH 4 :C2 H6 :C3 H8 can be separated at room temperature and 15 bar pressure to give a 63.8 wt% CH4, 26 wt% C2 H4 and 0.5 wt% C 3 H8 mixture in one-step. It has been estimated that the market for ceramic membranes for gas separations will be $80 million in 1999, principally in refineries.
Food
and biotechnology applications Membranes for micro and ultrafiltration are generally used in a cross flow configuration. The main purpose of the filtration process is either to clarify liquids (for example, milk, wine) by removing suspended solids or to concentrate soluble molecules and/or suspended solids. The principal reasons for using micro-porous membrane filters in wine making are that the number of processing stages can be decreased from typically 10 to between 5 and 7 and the ability to remove yeast and bacterial cells. The main improvement required in this area is an increase in flux of the filtrate. The use of ceramic microfilters in the production of beer has been shown to be beneficial and has also been reported for the commercial production of vinegar. Other major application areas are in the dairy industry for the removal of bacteria from milk, concentration of pasteurised, skimmed and whole milks, production of fresh cream cheeses and the processing of whey to produce whey protein concentrate and in clarification of fruit juices and purees. Other emerging biotechnology applications which depend upon chemical stability and biological resistance of ceramic membranes include recovery of proteins and antibiotics, use as supports in bioreactors for enzymes and micro-organisms, cell debris filtration and plasma separation. Water and wastewater treatment Inorganic membrane based filtration systems for the production of drinking water have been used commercially since 1984. The increasing demand on environmental grounds for the removal of chemical and biological contaminants from effluent and other processing plants is likely to be only economically feasible using ceramic micro/ultra-filtration. Inorganic membrane reactors for example for dehydrogenations, oxidations, catalytic decompositions offer the possibility of combing reaction and separation in a single stage. However, at the present time further material and membrane module construction developments are required prior to commercial application. This article has outlined several of the manufacturing and application areas of ceramic membranes. The principal production method is the asymmetric membrane construction manufactured from oxide-passed ceramics, with the main usages in the food and water treatment industries. Forthcoming improvements are likely to be required in the chemical/thermal
stability of the membranes and their support modules and increased permeate
flux rates through improved micro-structural control.
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