Types of Inorganic Membranes

Inorganic membranes are made of inorganic materials such as metals, ceramics, zeolites, glasses, carbon, and so on. Actually, inorganic membranes usually consist of several layers from one or more different inorganic materials. Details of inorganic membranes with respect to their syntheses, characterizations, transport theories, and scaling-up problems have been well reviewed and summarized by several authors [5,6]. 

The separation layer may be dense (non-porous), such as Pd or Pd-alloy membranes for hydrogen separation and mixed (electronic, ionic) conducting oxide membranes for oxygen separation, or porous, such as metal oxides, silicalite, or zeolite membranes. Inorganic membranes are generally named for this separation layer, since it determines the properties and application of the membrane. The flux and selectivity of inorganic membranes are mainly determined by the quality of the separation layer, which is required to be defect-free and as thin as possible. 

Metal or alloys Symmetric/ Dense Tube plate hollow 

Stainless steel Symmetric Porous lube hollow fiber 

Metal oxides (AI2O3, Symmetric/ Porous/ Tube hollow fiber  

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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... 

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]. 

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... 

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. 

Zeolite membranes are not the only kind of membranes that have been used in pervaporation, organic and other types of inorganic membranes, different from zeolites, have been employed. Polymeric membranes of PVA (polyvinyMcohol) have been widely employed for dehydratation and separation of organic mixmres however, their main limitations are related to their low thermal and chemical stability. When the water content in the feed mixmre is high, polymeric membranes suffer from swelling moreover, in the separation of organic mixtures they usually present a low selectivity. 

A different type of inorganic membrane made from zeolites with a pore size of 4.2 A can allow water through and produce a full range of dry solvents including aprotic ones. It even excludes methanol. The zeolite membrane allows molecular sieves to be operated continuously unlike the molecular sieve beds that have been used in the past. A positive method has been developed to locate holes in the membrane and seal them. 

Another type of inorganic membranes used to the PV separation is a zeolite membrane. Na-type zeolite membranes have been applied for dehydration of aqueous alcohol. Kita et al. [9] reported that a permeation flux of 3kgm h and separation factor (a) over 10 000 isopropyl alcohol aqueous solution (90wt% isopropyl alcohol), which corresponds to much larger flux and selectivity compared with polymeric membranes (normally a 1000 flux <0.1 kgm h h) On the other hand, a silicalite membrane, which is hydrophobic, preferentially permeates alcohol over water, showing a selectivity of 60 and flux of 0.8kgm h - at 5wt% of ethanol at60°C [8). 

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. 

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]. 

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. 

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. 

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. 

The above discussion applies to low level activity liquid wastes. Inorganic membranes have also been tested for treating medium level liquid wastes containing approximately 100 times the activity of the aforementioned low level wastes [Gutman et al., 1986]. The same type of zirconia membranes with a MWGO of 10,000 daltons used in treating low level wastes docs not perform as well (in terms of the permeate flux and decontamination factor), but can function acceptably when processing medium level wastes. The problem is particularly pronounced if there are some defects in the... 

Yet another unique class of inorganic membrane materials called pillared clay and carbon composite membranes has been studied for gas separation [Zhu et al., 1994]. The permeation rates of benzene, chlorobenzene and 1,3-dichlorobenzene vapors through these membranes can be different by orders of magnitude as indicated earlier. This may open the door for these types of membranes for separating organic mixtures. 

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