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Inorganic membrane

The membrane separation process can be packaged in one of four common integrated arrangements that called membrane modules: plate and frame, spiral wound, hollow fiber, capillary and tubular [1]. Both plate and frame and spiral wound modules, the flat sheet membrane is used. In the case of plate and frame modules, the membrane sheets are simply attached frames which are stacked together in such a fashion that a feed flow channel is formed between the frames. Plate and frame modules suffer from the fact that the packing density, or the amount of membrane area which can be packed into a given volume, is quite low and the manufacturing costs tend to be high. Spiral wound elements neatly address both these problems. In this modules, two membrane sheets are placed back to back separated by a permeate spacer and sealed with glue on three sides. Then, the remaining side is connected to a porous permeate tube which runs through the centre of the completed module. Finally, a feed spacer is placed adjacent to each active membrane surface and the membrane sheet is rolled around the permeate tube to create a cylindrical module. The feed spacers create feed channels by insuring that the rolled up membrane do not contact each other while the permeate spacers provide a spiral path for the permeate to reach the central tube [2].

The other common used membrane module is the tubular membrane. The membrane belonging to this group all have a tubular shape (high ratio of length to diameter). The length is ranging from one to three meter and the diameter of the membrane is ranging from half a millimeter to two centimeter. Tubular membranes with a diameter below than 0.5 mm are called hollow fiber membranes, ones with the diameter ranging from 0.5 to 5 mm are called capillary membranes, while membrane with a diameter larger than 5 mm a called tubular membrane. Tubular membranes are made by casting a membrane onto porous supporting tubes. These supporting tubes are manufactured from fiber glass, ceramics, carbon, porous plastics, stainless steel or paper and must be strong enough to withstand the feed pressures [3].

The advantages of inorganic membranes compares with organic membranes have been recognized i.e.: thermal and pH resistances, generally can withstand organic solvents, chlorine and other corrosive chemicals (see Figure 1.3). Most of inorganic membrane have multi-layered structure and consist of the separating layer and the underlying support layer(s). The available filtration area per unit per volume of support varies from 300 to 2,000 m2/m3. Each layer contains different pore size and porosity. The support, made of alumina, zirconia, titania, silica, spinel, aluminosilicate, cordierite or carbon, typically has a pore diameter of about 1 to 20 mm and a porosity of 30 to 60%. Any additional intermediate support layers have progressively smaller pore size than the underlayer of support. The intermediate support layers are typically 20-60 mm in thickness and 30-40% in porosity. The selective membrane material varies from alumina, zirconia, glass, titania, cordierite, mullite, carbon to such metals as stainless steel, palladium and silver. The overall membrane element shape comes in different types: sheet, single tube, hollow fiber and multi-channel monolith. A monolithic multi-channel honeycomb shape provides more filtration area per unit volume than either a sheet or a single tube [4].

References:

  1. R. Rautenbach and R. Albrecht, Membrane Processes, John Wiley and Sons, Ltd., Chichester, 1989, 459 pp.
  2. J. G. Pharoah, Fluid dynamics and mass transport in rotating channels with application to centrifugal membrane separation, PhD Dissertation, University of Victoria, 2002.
  3. J. Q. J. C. Verbeck, Application of air in membrane filtration, PhD Dissertation, Technische Universiteit Delft, 2005.
  4. H. P. Hsieh, Inorganic membranes for separation and reaction, Membrane Science and Technology Series 3, Elsevier Science B.V., Amsterdam, 1996, 591 pp.
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