Technical specifications of ceramic membranes

Structure

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.
The ceramic membranes are often formed into an asymmetric, multi-channel element. These elements are grouped together in housings, and these membrane modules can withstand high temperatures extreme acidity or alkalinity and high operating pressures, making them suitable for many applications where polymeric and other inorganic membranes cannot be used. Several membrane pore sizes are available to suit specific filtration needs covering the microfiltration, the ultrafilatration, and nanofiltration ranges (from 5 mm down to 1000 Daltons).

Materials

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.

Flow

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.
A small fraction of feed, called permeate or filtrate, separates out as purified liquid passing through the membrane. The remaining fraction of feed, called retentate or concentrate, contains
particles rejected by the membrane.
The separation is driven by the pressure difference across the membrane, or the trans-membrane pressure. The parallel flow of the feed stream, combined with the boundary layer turbulence created by the cross flow velocity, continually sweeps away particles and other material that would otherwise build up on the membrane surface.

Element shapes

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.

Applications

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.
The membrane modules can withstand elevated temperatures, extremes of pH (0 to 14), and high operating pressures up to 10 bar (145 psi) without concern for membrane compaction, delamination or swelling. This makes these membranes suitable for many applications where polymeric and other inorganic membranes cannot be used.
Additionally, ceramic membranes are ideal for in-place chemical cleaning at high temperatures, while using caustic, chlorine, hydrogen peroxide, ozone and strong inorganic acids, and/or by using steam sterilization.

Industrial applications

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.
In the filtrations of sugar cane juice ceramic membranes can be used in several different stages in the raw and refine sugar production. the need to purchase, use and dispose of filter aids is then eliminated.
Ceramic membranes are used for fermentation broth clarification at numerous installations worldwide, successfully competing with other technologies such as polymeric membranes, vacuum filtration and centrifugation.

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.

Future development

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.
The sometimes harsh conditions encountered in these processes favour the use of ceramic membranes. In order to steer the future developments in membrane synthesis for both solvent resistant and aqueous NR/RO a good fundamental insight in the transport mechanisms is essential. Whereas the polymer-water interactions are well documented and understood, much more work still needs to be done for the solvent resistance applications.

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Source: Rishi Sondhi, Ramesh Bhave and Gary Jung, 'Applications and benefits of ceramic membranes', Membrane Technology November 2003