Hollow fibre membranes materials

Because of the very fine nature of membrane media, it is normal practice to employ a filter, ahead of the membrane unit, which is intended to ranove any particulate material that might interfere with the membrane process. This is especially necessary where the flow passages are very narrow, such as in hollow fibre membranes. In fact, some membranes themselves are used as prefilters to membranes operating at a finer... 

Another factor is the ease with which various membrane materials can be fabricated into a particular module design.1618 Almost all membranes can be formed into plate-and-frame, spiral-wound and tubular modules, but many membrane materials cannot be fabricated into hollow fine fibres or capillary fibres. Finally, the suitability of the module design for high-pressure operation and the relative magnitude of pressure drops on the feed and permeate sides of the membrane can be important factors.4-11 The types of module generally used in some of the major membrane processes are listed in Table 16.2. 

Membrane-reservoir systems based on solution-diffusion mechanism have been utilized in different forms for the controlled delivery of therapeutic agents. These systems including membrane devices, microcapsules, liposomes, and hollow fibres have been applied to a number of areas ranging from birth control, transdermal delivery, to cancer therapy. Various polymeric materials including silicone rubber, ethylene vinylacetate copolymers, polyurethanes, and hydrogels have been employed in the fabrication of such membrane-reservoir systems (13). 

One can also envisage structures containing packages of active materials (capsules, hollow fibres) wrapped in an ICP membrane with the capacity determined by the internal volume rather than the dopant capacity of the ICP itself. 

The hollow fibre is the most crucial part of the microdialysis probe. It acts as a membrane, and its characteristics affect performance in the sampling step as well as the probe s suitability for the selected application. Hollow fibres are commercially available in different materials, the most common being polycarbonate (PC), regenerated cellulose (Cuprophan, CU), cellulose acetate (CA), polyacrylonitrile (PAN), polyethersulphone (PES), polysulphone (PE), and polyamide (PA). Generally, the fibres have an outer diameter between 200... 

The most simple form is a single, uniformly structured wall of a certain material, the so-called symmetric, stand-alone membranes. Examples are dense metal or oxide tubes and porous hollow fibres. To obtain sufficient mechanical strength, single-walled symmetric systems usually have a considerable thickness. 

S.P.J. Smith, V.M. Linkov, R.D. Sanderson, L.F. Petrik, C.T. O Connor and K. Keizer, Preparation of hollow fibre composite carbon zeolite membranes. Microporous Materials, 4 (1995) 385-390. 

Various modules or contactors have been used ranging from flat sheets to hollow fibres and with various acceptors volumes. Examples of such modules and different types of membranes have been reviewed recently (34). Flat modules are made of two blocks of inert materials and grooves with depth 0.1-... 

The need for large volumes of material often leads to a scale-up of systems. With UF, for example, hollow fibre systems and stirred cells have been compared for the concentration and fractionation of HS. Kwak and Nelson (1977) showed that retention increased with pressure, decreased with ionic strength and was not concentration dependent. HoUow fibres proved ineffective for UF fractionation compared to flat sheet membranes due to clogging and indiscriminate retention effects (Kiichler et al (1994)). 

Not only does the bulk chemical industry employ all of the membrane separation processes, but partly because of that, the sector gives home also to all of the various physical embodiments of membranes flat sheets, plate and frame, pleated cartridges, tubular, hollow fibre, capillary module, and spiral w ound. More particularly, this end Use sector has considerable demaitd for membranes able to resist high temperature or highly corrosive fluids, such that metallic membranes and ceramic materials, especially of the monolith type w ith parallel cylindrical chambers, are w idely used. 

Although there are a number of materials with the desired pore structure, for instance silicone rubbers, hydrocarbon rubbers, polyesters, polycarbonates and others, their use for industrial applications is limited to polysulfones and cellulose acetates. While the latt have been used with good success for dehydration, technical gas separation relies exclusively on polysulfones which can be used up to approximately 70 °C (their melting point is around 200 °C) and at pressures between IS and 140 bar. The lowest pressure differential between the feed gas side and the permeate gas side is 3 1 and this differential pressure determines the wall thickness of the membranes. Figure 2.8 shows the design of a membrane element developed by Monsanto Company, USA and marketed by the name of Prism separator. Each of these elements or modules contains thousands of hollow fibres packed to a density of approximately 1(X) per cm. ... 

The membranes used in GS can be distinguished in two main categories, polymeric and inorganic. Polymeric membranes, specifically used for GS, are generally asymmetric or composite and based on a solution-diffusion transport mechanism. These membranes, made as flat sheet or hollow fibres, have a thin, dense skin layer on a micro-porous support that provides mechanical strength [7]. Typically, polymeric membranes show high, but finite, selectivities with respect to porous inorganic materials due to their low free-volume... 

The actual basis for separation can be the differential solubility in the membrane material or the membrane hole size. When separating gases hollow membrane fibres are popular, as the small diameter of the fibres allows than to withstand the substantial driving pressures involved. Separation of the components in liquid systems usually relies on some form of spirally wound membrane sheet and spacer. Once again, substantial differential pressures are involved across the membrane surface and this conventionally requires the use of some form of spacer to prevent closure of the channels. Not only do these spacers (e.g. coarse woven fabric)... 

---