Membrane Material

Membranes have been made from over 150 different polymers or inorganic materials, but only about a dozen have achieved widespread commercial use for UF. The most common are polymers such as polysulfone, polyethersulfone, polyvinylidene fluoride, polyacrylonitrile, cellulose acetate and regenerated cellulose as well as inorganic materials such as alumina, zirconia and titania. Most polymeric UF membranes are asymmetric in structure, i.e. they have a thin 'skin' 0.1-0.2 |im thick on the surface of the membrane. This skin contains the pores of the required size and determines the separation characteristics of the UF membrane. Polymer layers under the skin usually consist of voids which support the skin layer. The skin and void layer are one structure and one polymer when made by a phase-inversion process, but they could be two or more different polymers in composite membranes. The membrane is then laid on a backing such as polyester or polypropylene and then formed into the module. In some cases, such as hollow fibres, a concentrated solution of the polymer is spun or extruded to form self-supporting single polymer hollow tubes with the pores on the inside surface of the tube.

Inorganic membranes have considerably widened the range of membrane applications, particularly in food processing, waste treatment, recovery of chemicals and biotechnology applications, where high temperature, acid and/or alkali stability, steam steril-izability and cleanability are important. A macropor-ous substrate of a fine dispersion of the powder is first formed, e.g. by thermal sintering of an extruded paste of the powder. If a tubular geometry is used, pastes from two powders of different grain sizes may be co-extruded, with the finer grain being closer to the axis. After baking at high temperatures (>1000°C), the inside may be coated by slip casting with the final fine grain powder. A series of such layers may be necessary to obtain the asymmetric-type ultrastructure. The membrane is finally set by a series of pressurizing, drying and baking steps. The most common ceramic materials are a-alumina, zirconia and titania. Composites of zirconia or titania membranes on alumina, carbon or stainless-steel supports are available.

Most inorganic membranes are available in tubular form, either as a single channel tube or multi-channel element, the latter containing up to 60 individual circular channels, depending on the relative diameters of the channel and the element. The inner diameter of individual channels vary from 2 to 6 mm and lengths from 0.8 to 1.2 m. As many as 99 of these elements may be put together in a single housing, resulting in

8-12 m2 per module. Normal process ratings are 15 bar and 150°C.

Inorganic membranes have several desirable properties. They are inert to common chemicals and solvents and have wide temperature limits. Depending on the seals and type of housing, some inorganic membranes can be operated as high as 350°C and within wide limits of pH from 0.5 to 13.5. The biggest advantage is their extended operating lifetimes. Operating life of membranes is most affected by the frequency and nature of the cleaning regime. In contrast to polymeric membranes which typically have 9-18-month lifetimes with normal daily cleaning cycles, inorganic membranes are able to tolerate frequent aggressive cleaning regimes. Many are still operating 10-14 years after installation with the first set of membranes. One major limitation is that they are 10-30 times more expensive than polymeric membranes.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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