Inorganic membranes types

Membranes. Membranes comprised of activated alumina films less than 20 )J.m thick have been reported (46). These films are initially deposited via sol—gel technology (qv) from pseudoboehmite sols and are subsequently calcined to produce controlled pore sizes in the 2 to 10-nm range. Inorganic membrane systems based on this type of film and supported on soHd porous substrates have been introduced commercially. They are said to have better mechanical and thermal stabiUty than organic membranes (47). The activated alumina film comprises only a miniscule part of the total system (see Mel rane technology). 

The ceramic membrane has a great potential and market. It represents a distinct class of inorganic membrane. In particular, metallic coated membranes have many industrial applications. The potential of ceramic membranes in separation, filtration and catalytic reactions has favoured research on synthesis, characterisation and property improvement of inorganic membranes because of their unique features compared with other types of membrane. Much attention has focused on inorganic membranes, which are superior to organic ones in thermal, chemical and mechanical stability and resistance to microbial degradation. 

The vast increase in the application of membranes has expanded our knowledge of fabrication of various types of membrane, such as organic and inorganic membranes. The inorganic membrane is frequently called a ceramic membrane. To fulfil the need of the market, ceramic membranes represent a distinct class of inorganic membrane. There are a few important parameters involved in ceramic membrane materials, in terms of porous structure, chemical composition and shape of the filter in use. In this research, zirconia-coated y-alumina membranes have been developed using the sol-gel technique. 

In this chapter, we Hmit ourselves to the topic of zeolite membranes in catalysis. Many types of membranes exist and each membrane has its specific field where it can be appHed best. Comparing polymeric and inorganic membranes reveals that for harsher conditions and high-temperature applications, inorganic membranes outperform polymeric membranes. In the field of heterogeneous catalYsis, elevated temperatures are quite common and therefore this is a field in which inorganic membranes could find excellent applications. 

Liquid membrane type ion-seleetive electrodes (ISEs) provide one of the most versatile sensing methods because it is possible to customize the sensory elements to suit the structure of the analyte. A wealth of different synthetic and natural ionophores has been developed, in the past 30 years, for use in liquid membrane type ISEs for various inorganic and organic ions [1], In extensive studies [2-4], the response mechanism of these ISEs has been interpreted in terms of thermodynamics and kinetics. However, there have been few achievements in the characterization of the processes occurring at the surface of ISEs at molecular level. 

In the recent years, many researchers have devoted attention to the development of membrane science and technology. Different important types of membranes, such as these for nanofiltration, ultrafiltration, microfiltration, separation of gases and inorganic membranes, facilitated or liquid membranes, catalytic and conducting membranes, and their applications and processes, such as wastewater purification and bio-processing have been developed [303], In fact, almost 40 % of the sales from membrane production market are for purifying wastewaters. 

A completely different type of inorganic membrane also has its origin in the nuclear industry the asymmetric alumina membranes obtained by the anodic oxidation of an aluminum sheet were first developed for uranium enrichment... 

There are limited data on the chemical resistance of various oxide materials in the literature (Samsonov 1982, Ryshkewitch and Richerson 1985). Furthermore, many of the studies are concerned with solid, nonporous materials. Nevertheless, these may provide an indication of the general trends. Until a definitive and quantitative database of chemical stability of various inorganic membranes becomes available, some simple dissolution-type test methods using membrane samples may be employed on a comparative basis to estimate the extent of attack by a chemical under the application conditions. An example of such a simple test is given below. 

Table 7.6. Summarized Results on Inorganic Membrane Reactors Used for Hydrogenation, Oxidation and Other Reaction Types... 

Currently, there is a great deal of activity in the development of various types of new membrane materials, both polymeric and inorganic. As many industrially relevant reactions require higher temperatures to achieve and maintain economic performance, temperature-resistant inorganic membranes are of especial interest. Indeed, many excellent reviews are currently available describing these materials and future trends in their development [e.g., Refs. 19-22]. 

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]. 

To facilitate discussions on the preparation methods, characteristics and applications of inorganic membranes in the following chapters, some terminologies related to the types of membranes according to the combined structures of the separating and support layers, if applicable, will be introduced. 

Historically there are two major types of dense inorganic membranes that have been studied and developed extensively. They are metal membranes and solid electrolyte membranes. 

In contrast to dense inorganic membranes, the rate of advances toward industrial-scale applications of porous inorganic membranes has been rapid in recent years. In the early periods of this century, microporous porcelain and sintered metals have been tested for microfiltration applications and, in the 1940s, microporous Vycor-type glass membranes became available. Then in the mid-1960s porous silver membranes were commercialized. These membranes, however, have not seen large scale applications in... 

The above early commercial developments of inorganic membranes, although slow at the beginning, have stimulated sufficient market interest to entice more companies to enter the field with new types of membranes. These activities and various features of currently available commercial inorganic membranes will be highlighted in Chapter 5. 

The processes discussed so far produce various types of inorganic membranes in one production process prior to applications and the membrane structures are fixed to the supports in the case of composite membranes. There are, however, other special types of inorganic membranes that are prepared either by a second process to modify the... 

The year 1980 marked the entry of a new type of commercial ceramic membrane into the separation market. SPEC in France introduced a zirconia membrane on a porous carbon support called Carbosep. This was followed in 1984 by the introduction of alumina membranes on alumina supports, Membralox by Ceraver in France and Ceraflo by Norton in the U.S. With the advent of commercialization of these ceramic membranes in the eighties, the general interest level in inorganic membranes has been aroused to a historical high. Several companies involved in the gas diffusion processes were responsible for this upsurge of interest and applications. 

There are a variety of porous inorganic membranes in the market today. Highlighted in Table 5.1 are selected major commercial inorganic membranes according to their material type. So far the most widely used inorganic membranes are alumina membranes, followed by zirconia membranes. Porous glass and metal (such as stainless steel and silver) membranes have also begun to attract attention. 

Successful performance of inorganic membranes depend on three types of variables and their interactions. The first type is related to the characteristics of the feed stream such as the molecular or particulate size and/or chemical nature of the species to be separated and concentration of the feed to be processed, etc. The second type is membrane dependent Those factors are the chemical nature and pore size of the membrane material and how the membrane and its accessory processing components are constructed and assembled. The third type is processing conditions such as pressure, transmembrane pressure difference, temperature, crossflow velocity and the way in which the membrane flux is maintained or restored as discussed earlier in this chapter. 

Fouling is one of the most prevalent operational problems associated with microfiluation and ultrafiltration applications. The high mechanical strength and chemical as well as structural stabilities of many inorganic membranes (especially the ceramic types) and... 

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