Ceramic membranes, synthetic

Surface media Captures particles on the upstream surface with efficiencies in excess of depth media, sometimes close to 100% with minimal or no off-loading. Commonly rated according to the smallest particle the media can repeatedly capture. Examples of surface media include ceramic media, microporous membranes, synthetic woven screening media and in certain cases, wire cloth. The media characteristically has a narrow pore size distribution. 

Microfiltration units can be configured as plate and frame flat sheet equipment, hollow fiber bundles, or spiral wound modules. The membranes are typically made of synthetic polymers such as Polyethersulfone (PES), Polyamide, Polypropylene, or cellulosic mats. Alternate materials include ceramics, stainless steel, and carbon. Each of these come with its own set of advantages and disadvantages. For instance, ceramic membranes are often recommended for the filtration of larger particles such as cells because of the wider lumen of the channels. However, it has been shown that spiral wound units can also be used for this purpose, provided appropriate spacers are used. 

The MF membranes are usually made from natural or synthetic polymers such as cellulose acetate (CA), polyvinylidene difiuoride, polyamides, polysulfone, polycarbonate, polypropylene, and polytetrafiuoroethylene (FIFE) (13). Some of the newer MF membranes are ceramic membranes based on alumina, membranes formed during the anodizing of aluminium, and carbon membrane. Glass is being used as a membrane material. Zirconium oxide can also be deposited onto a porous carbon tube. Sintered metal membranes are fabricated from stainless steel, silver, gold, platinum, and nickel, in disks and tubes. The properties of membrane materials are directly reflected in their end applications. Some criteria for their selection are mechanical strength, temperature resistance, chemical compatibility, hydrophobility, hydrophilicity, permeability, permselectivity and the cost of membrane material as well as manufacturing process. 

C and 80 bars. For that, four ionic liquids, namely [bmim INTf "], [bmim ] [PFg"]) [bdimim ][PFg ] and [omim+][PF ], were used. Figure 8.4 shows the synthetic activity and selectivity of immobilized Candida antarctica lipase B, CaLB, on ceramic membranes in scCO medium as well as in four different IL/scCO biphasic systems. 

Membranes can be prepared from both ceramic and polymeric materials. Ceramic materials have several advantages over polymeric materials, such as higher chemical and thermal stability. However, the market share of polymeric membranes is far greater than ceramic membranes as the polymeric materials are easier to process and less expensive. A handful of technical polymers are currently used as membrane materials for 95% of all practical applications [2]. Polymeric materials that are used to prepare separation membranes are mostly organic compounds. A number of different techniques are available to prepare synthetic membranes. 

These arious techniques allow to prepare microfiltration membranes from Trtually all kinds of materials of which polymers and ceramics are the most important. Synthetic polymeric membranes can be divided in two classes, i.e. hydrophobic and hydrophilic. Various polymers which yield hydrophobic and hydrophilic membranes are listed below. Ceramic membranes are based mainly on two materials, alumina (A1203) and ziiconia (Zr02). However, other materials such as titania (TiOj) can also be used in principle. A number of organic and inorganic materials are listed below ... 

LaCoOs ceramic exhibits interesting electrical, magnetic, and catalytic properties, being used as cathode material for solid oxide fuel cells (SOFCs), catalysts for light hydrocarbon oxidation, and gas detection sensors. For other applications such as oxygen separation from air and for synthetic gas production, dense ceramic membranes with well-defined microstructure are required. 

Depending on the particular filtration devices and applications, various filter media have been used, such as sand [2], charcoal [3], cotton [4], wool [5], zeolite [6], earthenware [7], ceramic [8], synthetic nanofibrous membranes [9], and more sophisticated laboratory biological membranes [10]. In this chapter, we focus on filters based on electrospun nanofibers. 

Figure 16.22 shows SEM micrographs for the porous media of ceramic support at different magnifications. The non-uniformity resulted from the synthetic foam used as a base to absorb the ceramic solution before vaporising any water from the inorganic mixture. Uniform porous media as a solid support for the membrane was obtained. 

The third main class of separation methods, the use of micro-porous and non-porous membranes as semi-permeable barriers (see Figure 2c) is rapidly gaining popularity in industrial separation processes for application to difficult and highly selective separations. Membranes are usually fabricated from natural fibres, synthetic polymers, ceramics or metals, but they may also consist of liquid films. Solid membranes are fabricated into flat sheets, tubes, hollow fibres or spiral-wound sheets. For the micro-porous membranes, separation is effected by differing rates of diffusion through the pores, while for non-porous membranes, separation occurs because of differences in both the solubility in the membrane and the rate of diffusion through the membrane. Table 2 is a compilation of the more common industrial separation operations based on the use of a barrier. A more comprehensive table is given by Seader and Henley.1... 

Two main criteria for the membrane selection are pore size and material. As peroxidases usually have sizes in the range of 10-80 kDa, ultrafiltration membranes with a molecular cutoff between 1 and 50 kDa are the most adequate to prevent enzyme leakage [99]. The materials commonly applied to ultrafiltration membranes are synthetic polymers (nylon, polypropylene, polyamide, polysulfone, cellulose and ceramic materials [101]. The adequate material depends on a great number of variables. When enzyme is immobilized into the matrix, this must be prepared at mild conditions to preserve the enzymatic activity. In the case of enzyme immobilization onto the membrane, this should be activated with the reactive groups necessary to interact with the functional groups of the enzyme. If an extractive system is considered, the selection of the hydrophilicity or hydro-phobicity of the membrane should be performed according to the features of reactants, products, and solvents. In any case, the membrane should not interfere with the catalytic integrity of the enzyme. 

However, in view of the high-temperature capabilities of thermoplastic materials based on aromatic rings linked by thermally and oxidatively stable units such as ether, ketone, sulfone, or direct bonds,3 we have attempted to develop linear, carborane-copolymers of this same general type. We here describe synthetic and crystallographic studies of such materials, and report on their potential applications in membrane separation and in ceramic-precursor chemistry. 

Membranes are used for a wide variety of separations. A membrane serves as a barrier to some particles while allowing others to selectively pass through. The pore size, shape, and electrostatic surface charge are fundamental to particle removal. Synthetic polymers (cellulose acetate, polyamides, etc.) and inorganic materials (ceramics, metals) are generally the principal materials of construction. Membranes may be formed with symmetric or asymmetric pores, or formed as composites of ultra thin layers attached to coarser support material. Reverse osmosis, nanofiltration, ultrafiltration, and microfiltration relate to separation of ions, macromolecules, and particles in the 0.001 to 10 pm range (Rushton et al. 1996). 

A number of novel applications of zeolites depend on the ability to create thin, adhesive films on various substrates. While zeolite films or layers are commonly prepared on dense substrates such as silicon wafers, zeolite membranes are made on porous supports in order to permit permeation through the zeolite layer. Numerous synthetic studies have addressed the goal of obtaining adhesive layers of zeolites on various substrates such as noble and nonnoble metals, glass, ceramics, silicon, and even biological substrates such as cellulose fibers. For a more detailed discussion of zeolite membranes the reader is referred to the article by Julbe in this book. Pertinent reviews to this subject are given in the following references.[57,58]... 

Tubular (membrane, sponge) Composite CoUagen/synthetic polymer CoUagen/biological polymer CoUagen/ceramic... 

Thus, the hydrophilic head group and hydrophobic tail of lipids ensure assembly into the oriented bilayer array of cell membranes. The amphiphilic sheet, bilayer, and vesicle are now familiar mofits in biomimetic materials and structures. Synthetic liposomes are employed as biocompatible, biodegradable drug-delivery vehicles. Amphiphile assemblies may serve as templates mono-disperse nanoparticles are synthesized inside reverse micelles, and inorganic structures and materials such as ceramic tubules or mesoporous silica are formed around tubular micelles, rather as inorganics are patterned by vesicles in the formation of the exoskeletons of radiolarians and diatoms. 

Membranes can be natural or synthetic. Regarding the type of material, synthetic membranes can be divided into organic, made of various polymers (Figure 23.4), aud iuor-ganic, composed of ceramic or metal (Figure 23.5). 

The use of membranes specifically for desalination dates to at least the late 1940 s. At The University of California at Los Angeles, Hassler proposed using synthetic membranes as biomimetic surrogates for cellular membranes [8]. His design utilized two membranes (cellophane sheets supported by a porous ceramic support) separated by an air gap that would permit the evaporation of water across the gap and subsequent condensation. The air gap itself was... 

---