Inorganic membranes refer to membranes made of materials such as ceramic, carbon, silica, zeolite, various oxides (alumina, titania, zirconia) and metals such as palladium, silver and their alloys. Inorganic membranes can be classified into 2 major categories based on its structure: porous inorganic membranes and dense (non-porous) inorganic membranes. Microporous inorganic membranes have 2 different structures: symmetric and asymmetric; and include both amorphous and crystalline membranes.
Microporous inorganic membranes can be obtained by coating of a porous support with a colloidal solution, called sol. The sol can consist of either dense spherical particles (colloids of oxides such as Al2O3, SiO2 or ZrO2) or polymeric macromolecules.
The Table showed the advantages and disadvantages of inorganic membranes in comparison with polymeric membranes. It can be seen that although inorganic membranes are more expensive than organic polymeric membranes, they possess advantage of: temperature stability, resistance towards solvents, well-defined stable pore structure, and the possibility for sterilization.
Application of dense inorganic membranes is primarily for highly selective separation of gases such as hydrogen and oxygen. However, dense membranes have limited industrial application due to their low permeability compared to porous inorganic membranes. Therefore, today's commercial inorganic membrane market is dominated by porous membranes.
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Dense, metal membranes are being considered for the separation of hydrogen from gas mixtures. Palladium (Pd) and its alloys are the dominant material used, due to its high solubility and permeability for hydrogen. Palladium, however, is expensive. One alternative is to coat a thin layer of palladium on a tantalum or vanadium support film. Alternative to palladium and less expensive are tantalum and vanadium, which are also quite permeable to hydrogen.
Recent focus is on supported thin metallic membranes with thickness ranging from submicron to a few ten microns. The advantages include reduced material costs, improved mechanical strength and possibly higher flux. The main developments have been the production of composite palladium membranes for use in catalytic membrane reactors (CMRs). This development is based on the concept of process intensification, one important aspect if which is the potential for combining the reaction and separation stages of a process in one unit. One such application is the CMR. Apart from the benefit inherent in cost reduction of plant and maintenance, there is also the potential attainment of higher conversions and product yields.
The composite palladium membrane used in the CMR is composed of a thin layer of Pd, or Pd alloy, deposited onto a porous substrate, such as a ceramic or stainless steel.
The composite palladium membrane is placed adjacent to a catalyst bed and effects the selective removal of hydrogen from the catalytic reaction source. Another application is the use of these membranes to control the feed rate during partial oxidation reactions (e.g. addition of hydrogen).
A major problem associated with metal membranes is the surface poisoning effects (e.g. by a carbon-containing source) which can be more significant for thin metal membranes.
These membranes are made from aluminum, titanium or silica oxides. They have the advantages of being chemically inert and stable at high temperatures. This stability makes ceramic microfiltration and ultrafiltration membranes particularly suitable for food, biotechnology and pharmaceutical applications in which membranes require repeated steam sterilization and chemical cleaning. Ceramic membranes have also been proposed for gas separations.
An example application of recent development is in the production and processing of syngas (synthetic gas - a mixture of hydrogen and carbon monoxide). The key part of the process involves the separation of oxygen from air in the form of ions to oxidize the methane.
A schematic representation of the process is given in the Figure.
Oxygen feeding from air is split at the perovskite-type membrane surface and is transported as O2-. The advantage of the membrane-based process is that the production of syngas takes place in a single-step operation occurring on one of the membrane sides, This process eliminates the need for a separate oxygen production plant, and might lead to significantly lower energy and capital costs.
Some remaining problems include:
Difficulties in proper sealing of the membranes in modules operating at high temperature
Extremely high sensitivity of membranes to temperature gradient, leading to membrane cracking
Chemical instability of some perovskite-type materials in the high temperature reduction conditions
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Carbon molecular sieve (CMS) membranes have been identified as very promising candidates for gas separation, both in terms of separation properties and stability. Carbon molecular sieves are porous solids that contain constricted apertures that approach the molecular dimensions of diffusing gas molecules. As such, molecules with only slight differences in size can be effectively separated through molecular sieving.
Carbon membranes can be divided into 2 categories: supported and unsupported. Unsupported membranes have 3 different configurations: flat (film), hollow fiber and capillary while supported membranes consisted of 2 configurations: flat and tube.
The Figure showed a comparison between carbon asymmetric membrane and polymeric asymmetric membrane where the main difference is in the skin layer.
CMS membranes can be obtained by pyrolysis of many thermosetting polymers such as poly(vinylidene chloride) or PVDC, poly(furfural alcohol) or PFA, cellulose triacetate, polyacrylonitrile or PAN, and phenol formaldehyde.
Zeolites are microporous crystalline alumina-silicate with a uniform pore size. Zeolites are used as catalysts or adsorbents in a form of micron or submicron-sized crystallites embedded in millimeter-sized granules. The zeolite type prepared most often as a membrane is MFI.
Main problem - relatively low gas fluxes compared to other inorganic membranes . Due to the fact that relatively thick zeolite layers are necessary to get a pinhole-free and crack-free zeolite layer. Overcome: use thin layer supported on others.
Other problem: thermal effect of zeolites. The zeolite layer can exhibit negative thermal expansion, i.e. in the high temperature region the zeolite layer shrinks . But the support continuously expands, resulting in thermal stress problems for the attachment of the zeolite layer to the support, as well as for the connection of the individual micro-crystals within the zeolite layer.
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