Capillary module

Pervaporation Most pervaporation systems are small so plate-and-frame systems were used in the first systems. Spiral-wound and capillary modules are being introduced. 

Air Bacfiflush A configuration unique to microfiltration feeds the process stream on the shell side of a capillary module with the permeate exiting the tube side. The device is run as an intermittent deadend filter. Every few minutes, the permeate side is pressurized with air. First displacing the liquid permeate, a blast of air pushed backward through the membrane pushes off the layer of accumulated solids. The membrane skin contacts the process stream, and while being backwashed, the air simultaneously expands the capillary and membrane pores slightly. This momentary expansion facilitates the removal of imbedded particles. 

In the case of purification of water the permeate is the cleaned water and the removed components are collected in the concentrate [111-113]. Various modules can be used, such as plate-modules, tubes, and capillary modules. For water purification and recycling processes the following aspects have to be considered ... 

Spiral-wound and capillary modules being introduced... 

Capillary membrane modules are not as inexpensive or compact as hollow fine fiber modules, but are still very economical. Their principal drawback is the limited pressure differential the fibers can support, typically not more than 10 to 15 bar. This limitation means capillary modules cannot be used at the high pressures needed for hydrogen or natural-gas processing applications. However, capillary modules are ideally suited to lower-pressure separations, such as nitrogen from air or air dehydration. In these applications, capillary modules have essentially the entire market. 

The largest current application of gas-separation membranes is separation of nitrogen (N2) from air. Capillary modules formed into bore-side feed modules are used almost exclusively in this application [10, 11]. The feed air is compressed to 6-10 bar and pumped through the membrane capillaries. Oxygen (02) permeates the membrane preferentially, leaving an oxygen-depleted, nitrogen-rich residue stream. The first membranes used for this application were based on poly(4-methyl-l-pentene) and ethyl cellulose, and had 02/N2 selectivities of about 4. Because of the modest... 

Counterflow modules are always more efficient than crossflow modules, but the advantage is most noticeable when the membrane selectivity is much higher than the pressure ratio across the membrane and a significant fraction of the most permeable component is being removed from the feed gas. This is the case for air-dehydration membrane modules, so counterflow capillary modules are almost always used. With most other gas-separation applications, the advantage offered by counterflow designs does not offset the extra cost of making the counterflow type of module, so they are not widely used. 

A hollow fiber module is conceptually similar to the capillary module, but differs in dimensions. In this case the diameter of the tubular membrane varies between 50 and 100 pm and several thousand of fibers can be placed in the vessel. The hollow fiber module is the configuration with the highest packing density (with values up to 30,000 m m ). 

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. 

Stoyanov, A. V. and Pawhszyn, J., CE in a nonuniform capillary modulated by a cylindrical insert, and zone-narrowing effects during sample injection. Analytical Chemistry, 75, 3324, 2003. 

Hollow fibers or capillary modules have not yet found an industrial application in pervaporation or vapor-permeation processes. A few data have been reported where organic capillary structures with an outside diameter of 0.5 to 1 mm have been coated with silicon and used in organophilic separation. With the flow on the shell side permeate pressure losses inside the bore of the fiber control the process. For specific organophilic applications, these pressure losses may be tolerable. For hydrophilic processes, however, the useful length of a module would be of the order of 20 to 30 cm only, even at an inner diameter of the capillary of 1 mm. Such a module, including housing and connection in any industrial application, is more costly than a plate module. So far no potting material is available that combines the necessary chemical and mechanical stability at the operation temperature and pressure of a dehydration plant. 

Another class of applications are aqueous solutions containing low concentrations of a volatile component such as occur in mixtures of ethanoi/waier or trichloroethylene/water. Here also a vacuum can be applied instead of water on the permeate side, resulting in a high driving force (p2 => 0). Becau.se the separation is based on a vapour-liquid equilibrium, the permeate is enriched in the volatile component. Although this process is sometimes referred to as pervaporation it is in fact a membrane distillation process. Membrane distillation can have a distinct advantage over distillation, especially for small-scale applications, because of the large surface area per volume as can be found in hoUow fiber and capillary modules. 

The capillary module consists of a large number of capillaries assembled together in a module, as shown schematically in figure Vni -11. The free ends of the fibers are potted with agents such as epoxy resins, polyurethanes, or silicone rubber. 

Figure VIII 11. Schemadc.drawing of a capillary module/hollow fiber module.(Ieft) inside-out or tube-side, feed outside-in or shell-side feed (right)...

Commercially available poly(propylene) membranes having inner and outer diameter of 1.8 and 2.6 mm, respectively, and 0.28 m in length were placed into a capillary module. The effective membrane area was 0.014 m. A solution of MB... 

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