Technical specifications of ceramic membranes
Ceramic membranes normally have an asymmetrical structure composed of at least two, mostly three, different porosity levels. Indeed, before applying the active, microporous top layer, a mesoporous intermediate layer is often applied in order to reduce the surface roughness. The macroporous support ensures the mechanical resistance of the nanofilter.
Ceramic membranes today run the gamut from A to Z in terms of materials (from alpha alumina to zircon). The most common membranes are made of Al, Si, Ti or Zr oxides, with Ti and Si being more stable than Al or Si oxides. In some less frequent cases, Sn or Hf are used as base elements. Each oxide has a different surface charge in solution. Other membranes can be composed of mixed oxides of two of the previous elements, or are established by some additional compounds present in minor concentration.
Ceramic membranes are operated in the cross flow filtration mode. This mode has the benefit of maintaining a high filtration rate for membrane filters compared with the direct flow filtration mode of conventional filters. Cross flow filtration is a continuous process in which the feed stream flows parallel (tangential) to the membrane filtration surface and generates two outgoing streams.
Ceramic membranes are available form several manufacturers in different shapes, mainly round and hexagonal, and with various channel diameters. A multi-channel construction provides a higher membrane packing density than a tubular element of the same length. The ceramic membrane elements have sealing gaskets attached at each end and are then assembled within housings, available in 316L SS, polyvinylidene and other alloys. A typical industrial installation will have several of these modules arranged in series and/or parallel configuration.
Ceramic membranes are increasingly being used in a broad range of industries such as biotechnology and pharmaceutical, dairy, food and beverage, as well as chemical and petrochemical, microelectronics, metal finishing, and power generation. Each industry presents specific needs and opportunities.
Clarification of natural fruit juices such as apple, cranberry and grape is one of the most successful and widely practiced industrial applications of ceramic membranes.
Finally, in many chemical process application there is the need to treat not only the waste streams, but also to recover and reuse chemicals. Ceramic membranes can be applied for this purpose, i.e. filtration of chemical solvents, dye and pigment wastewater from dye processing and colouring plants and highly variable wastewater containing detergents, polymers and organic solvents.
Polymeric and ceramic membranes clearly form two separate kinds of modern nanofiltration/reverse osmosis membranes, each with their peculiarity and possibilities. With the aqueous applications momentarily still heavily dominating the NF/RO market, the advent of new solvent resistant NF membranes reveals ample new and exciting opportunities in industrial chemical processes.
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Source: Rishi Sondhi, Ramesh Bhave and Gary Jung, 'Applications and benefits of ceramic membranes', Membrane Technology November 2003