Commercially Available Inorganic Membranes

A variety of membrane materials has been investigated and reported [1,9] and an overview of commercially available systems has been given by Flsieh [9]. Alumina, zirconia and, more recently, titania membranes are used in large-scale applications. The more complex shapes, i.e. monolith and honeycomb, are almost exclusively based on a-alumina or cordierite. 

Some examples of different systems and new developments are given below with their trade name and producer. 

Carbosep membranes (Tech-Sep, France) are made of a zirconia layer attached to a porous carbon supporting tube assembled into modules containing up to 252 tubes. The same company produces Kerasep membranes of alumina or titania on a monolithic alumina-titania support containing 7-19 channels. 

Membralox membranes produced by US FUter/SCT (USA) is the name of a group of tubular and monolithic (multichannel) alumina membranes. The supporting system is formed by high-purity a-alumina multilayers with a final coating of alumina or zirconia. This system has now been developed further to obtain smaller pore diameters for nanofiltration and gas separation. 

Ceramem membranes (CeraMem, USA) produces honeycomb-shaped monolithic supports of cordierite, the channels of which are coated with zirconia, silica, y-alumina or a-alumina separation layers. 

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Among the commercially available inorganic membranes, zirconia- and alumina-based membranes are used in large-scale applications in a wide variety of areas. 

Shown in Table 5.3 is the packing density of the commercially available inorganic membrane modules. The single plate geometry has a very low packing density. Single tubes typically also show a low packing density unless the membrane tube and the module both have small diameters which are not practical other than for laboratory... 

A variety of inorganic membranes are summarized in Table 1.1. The metal membranes mainly include palladium-based membranes for hydrogen permeation and silver-based membranes for oxygen permeation. Currently, the commercially available inorganic membranes are porous membranes made from alumina, silica and titania, glass, and stainless... 

Microfiltration with inorganic membranes is a promising alternative. As early as 1964 porous silver membranes (composed of permanently molecular bonded pure silver particles) in the disk form were commercially available. Silver membranes with a maximum pore diameter of 1.2 pm were tested successfully on the pilot scale for cold sterilization of beer to remove any organisms that can cause spoilage in closed... 

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. 

Despite the fact that inorganic membranes are, in general, more stable mechanically than organic membranes, available mechanical properties data for commercial inorganic membranes are sketchy and these are not yet standardized for comparing various membranes. It appears that the methods used for obtaining various mechanical strength data are based on those for solid (nonporous) bodies and most of them arc listed as ASTM procedures. 

Nearly all of the commercially available membranes are based on Nafion. Nafion also has the largest body of literature devoted to its study because of its demonstrated industrial importance and availability. Nafion composite systems also have already become significant in both industrial and academic research. In composite structures, Nafion can be impregnated into an inert Teflon-like matrix (i.e. W. L. Gore membranes ), or inorganic additives can be added to a supporting Nafion matrix for improved physical or electrochemical properties (i.e. lon-omem °). Some critical aspects of Nation s molecular structure and physical properties will be briefly highlighted to provide a baseline for comparison with the other alternative materials discussed in this review. 

P-gp associated ATPase is vanadate sensitive. A membrane product prepared from baculovirus infected insect cells containing this activity is now commercially available from Gentest Corp. (Woburn, Massachusetts, U.S.). Substrates of P-gp, such as verapamil, have been demonstrated to stimulate this vanadate-sensitive membrane ATPase (123). By determination of inorganic phosphate liberated in the reaction containing a P-gp preparation and a test compound, in the presence and absence of vanadate, one can determine if the test compound is a substrate/inhibitor of P-gp (123,422). Any compound that binds to P-gp would stimulate the magnesium-dependent ATPase, and thus, this method cannot distinguish between a substrate and inhibitor of P-gp. 

This starting material is prepared in three steps from commercially available (from Research Organic/Inorganic Chemical Corp., Belleville, NJ) 3,4-methylenedioxyphenylacetic acid according to well-established procedures that have been applied to similar compounds. First, 16.0 g (88.8 mmol) of the acid, recrystallized from chloroform, is dissolved in 50 mL of tetra-hydrofuran, and the solution is added to a suspension of 5.98 g (158 mmol) of lithium aluminum hydride powder in 225 ml of distilled diethyl ether (Note 3) at 0°C. [Caution Lithium aluminum hydride ie very sensitive to mechanical shock, and very reactive towards moisture and other protic substances its dust is very irritating to skin and mucous membranes. It should not be allowed to come into contact with metallic species or apparatus, including metal... 

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. 

Although some inorganic membranes such as porous glass and dense palladium membranes have been commercially available for some time, the recent escalated commercial activities of inorganic membranes can be attributed to the availability of large-scale ceramic membranes of consistent quality. As indicated in Chapter 2, commercialization of alumina and zirconia membranes mostly has been the technical and marketing extensions of the development activities in gas diffusion membranes for the nuclear industry. 

Based on available product brochure information of commercial membranes and the literature data of developmental membranes. Table 5.5 provides some indication of the various mechanical properties of a few inorganic membrane elements. It should not be used for serious comparison as the testing conditions are usually not given or sketchy at best. In addition, the mechanical properties of a membrane element depend, to a great extent, on its shape, membrane thickness, porosity and pore size. While overall the majority of the data appears to be consistent, there is some strength data that seems to be off the trend line. 

Many catalytic processes of industrial importance, however, involve the combination of high temperature and chemically harsh environments, a factor that strongly favors inorganic membranes. So with the introduction of commercially available glass, ceramic and metal membranes, there has been a dramatic surge of interest in the field of membrane reactor or membrane catalysis. 

Thermal shock resistance. Temperature swing as part of the normal cycles of operation or regeneration of the membranes or membrane reactors can lead to deleterious thermal shock. The materials for the various components in a membrane reactor should be carefully selected to impart good thermal sh k resistance. This is particularly important for high temperature reactions. Also listed in Table 9.5 is a summary of various membrane materials along with qualitative description of their resistance to thermal shock. Again, the available data apply to dense materials. While various metal oxides have been made into commercial inorganic membranes, they tend to be affected by thermal shock much more than other ceramic materials. 

Microporous membranes. While dense metal or metal oxide membranes possess exceptionally good peimselectivities, their permeabilities are typically lower than those of porous inorganic membranes by an order of magnitude or more. Commercial availability of porous ceramic membranes of consistent quality has encouraged an ever... 

In many situations the conversion of a membrane reactor increases as the total permeate rate increases. This is to be expected if the membrane has a perfect or very high pcimselcctivity. In many commercially available porous inorganic membranes, however, the permselectivity is moderate and some reactants as well as products other than the most selective species "leak through the membrane. This leakage rate often increases with the total permeate rate, for example, as the feed side pressure increases. This has an important consequence on the reactor performance. 

Current commercial inorganic membranes come in a limited number of shapes disk, tube and monolithic honeycomb. Compared to other shapes such as spiral-wound and hollow-fiber that are available to commercial organic membranes, these types of membrane elements have lower packing densities and, therefore, lower throughput per unit volume of membrane element or system. 

The inorganic silica membranes, also commercial, have solved the problem of thermal and chemical stability however, these membranes are only used for dehydration purposes, leaving the problem of separation of organic mixtures unsolved. As we have seen previously, due to the versatility and special feamres of zeolites, new applications in pervaporation that are not possible with other membranes could be developed with zeolite membranes. GaUego-Lizon et al. [110] compared different types of commercial available membranes zeolite NaA from SMART Chemical Company Ltd., sUica (PERVAP SMS) and polymeric (PERVAP 2202 and PERVAP 2510) both from Sulzer Chemtech GmbH, for the pervaporation of water/f-butanol mixtures. The highest water flux was obtained with the silica membrane (3.5 kg/m h) while the zeolite membrane exhibited the highest selectivity (16,000). 

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