Tubular/hollow-fiber membrane

Figure 16.16 Examples of immersed tubular/hollow-fiber membrane systems.

Membrane module Tubular, hollow fiber Tubular, hollow fiber. Tubular, spiral wound, plate Tubular, spiral wound. 

Crossflow technology is increasing, as it proves practical. Micioliltration membranes are of an isotropic and homogeneous morphology, i.e., the pore structure is consistent throughout. There is some movement, however, toward ihe use of "skinned" anisotropic membranes. Microliltration membranes are available in a wide variety ol polymers, including some that arc quite chemically inert. They also tire available as tubular, hollow fiber, or capillary fiber elements. 

Hollow-fiber membranes form a tubular structure which is usually arrayed as a parallel fiber bundle within a cylindrical container. The cells are trapped on the shell side of the hollow fibers while aerated nutrient medium is rapidly recirculated through the fibers. This type of membrane support can provide extra protection against contamination. However, the major disadvantages of membrane... 

Salts rejected by the membrane stay in the concentrating stream but are continuously disposed from the membrane module by fresh feed to maintain the separation. Continuous removal of the permeate product enables the production of freshwater. RO membrane-building materials are usually polymers, such as cellulose acetates, polyamides or polyimides. The membranes are semipermeable, made of thin 30-200 nanometer thick layers adhering to a thicker porous support layer. Several types exist, such as symmetric, asymmetric, and thin-film composite membranes, depending on the membrane structure. They are usually built as envelopes made of pairs of long sheets separated by spacers, and are spirally wound around the product tube. In some cases, tubular, capillary, and even hollow-fiber membranes are used. 

Asymmetric Microporous Nonporous, skinned on microporous substrate Flat-sheet, tubular, hollow fiber Flat-sheet, tubular, hollow fiber Phase-inversion casting or spinning Phase-inversion casting or spinning Microfiltration, ultrafiltration, membrane reactors Reverse osmosis, gas separation, pervaporation, perstraction, membrane reactors... 

Microfiltration and UF membranes are available in tubular, spiral wound, and hollow fiber membrane module configurations. Tubular and spiral MF and UF modules are similar to RO tubular and spiral wound membrane modules described in Chapters 4.3.2 and 4.3.3. However, while the thickest feed spacer in a spiral RO module is 34-mil, UF and MF modules nominally have up to a 45-mil spacer due to the relatively high concentration of suspended solids these membranes are called upon to treat (TriSep Corporation offers a special 65-mil spacer for dairy applications). 

Hollow fiber membrane modules can be backwashed to remove foulants whereas tubular and most spiral configurations cannot be backwashed. Backwashing of traditional spiral-wound modules would break the glue lines holding the membrane leaves together or cause blistering and delamination of the membrane from the backing in both spiral and tubular modules (TriSep Corporation has recently developed a back-washable, spiral-wound module (SpiraSep—US patent 6,755,970), that is used in immersed systems see below). 

Membrane separation devices are assembled in a number of forms. In a flat sheet form the membrane is laid over a flat porous support. A unit would include a large number of the flat sheets separated by spacers and stacked together. In another configuration the fiat sheet may be spiral-wound with spacers around a perforated tube. Other arrangements involve tubular membranes or hollow fiber membranes assembled in bundles. In the tubular module the membrane is wrapped around a tubular... 

Membranes are manufactured in a diverse range of geometries they include flat, tubular, and multi-tubular, hollow-fiber, and spiral-wound membranes. The type of geometry the membrane is manufactured into depends on the material the membrane is made from. Ceramic membranes, generally, come in tubular, multi-tubular and flat geometries, whereas spiral-wound and hollow-fiber membranes seem, for the most part (with a few notable examples), to be made from polymers. 

Figure 1.33 Schematic diagram showing membrane modules presently used in industrial separation processes (a) pleated membrane filter cartridge (b) plate-and-frame membrane module (c) spiral wound membrane module (d) tubular membrane module (e) capillary membrane module (f) hollow fiber membrane module.

Based on the different modules used in technical scale membrane separation processes, there are four basic membrane configurations produced today on a large scale. These are flat sheet, tubular, capillary, and hollow fiber membranes. 

Detector UV 270 following post-column reaction. The column effluent mixed with 2 M NaOH and 0.05% sodium hypochlorite solution pumped at 0.1 mL/min in a 400 x 0.5 mm hollow fiber membrane reactor at 40° and this mixture flowed through a 1400 x 0.3 mm knitted open tubular reactor at 50° to the detector. 

Membrane materials are available in various shapes, such as flat sheets, tubular, hollow fiber, and monolithic. Flat sheets have typical dimensions of 1 m by 1 m by 200 pm thickness. Tubular membranes are typically 0.5 to 5.0 cm in diameter and up to 6 m in length. The thin, dense layer is on either the inside or the outside of the tube. Very small-diameter hollow fibers are typically 42 pm i.d. by 85 pm o.d. by 1.2 m long. They provide a very large surface area per unit volume. Honeycomb, monolithic elements of inorganic oxide membranes are available in hexagonal or circular cross section. The circular flow channels are typically 0.3 to 0.6 cm in diameter (Seader and Henley, 2006). 

For industrial applications, large surface areas are obtained using modules with many tubular or hollow-fiber membranes or by using large sheets in a spiral-wound arrangement. Tubular membranes are 5 to 25 mm inside diameter and up to 3 m long. Hollow-fiber UF membranes have diameters of 0.2 to 2 mm, and thousands of fibers are sealed in each cylindrical module. Spiral-wound modules of the type used for RO are widely used for UF. 

A useful variation of this design, shown in Figure 24.2b, consists of three concentric tubes (Oertel et al., 1987). The inner of the two annular spaces formed is filled with the catalyst, and selective permeation of products to the central (product) tube is achieved by placing a number of tubular membranes inside this packed volume. Therefore, it is called the packed-bed inert selective multimembrane reactor (IMMR-P). In yet another version of an IMR-P, the membrane is supported on the inner surface of a hollow fiber membrane tube and the catalyst is loaded around the hollow fiber (Figure 24.2c). 

A number of studies have been carried out to evaluate the effect of injection of air into the lumen of tubular and hollow fiber membranes on the performance of membrane filtration. Figure 10.38 shows the filtration results obtained by Cui et al. [85] using a vertically installed tubular membrane module (12.7 mm i.d., PVDF, MWCO, 100 kDa) with dextran solution... 

A separating layer on the outside of a tubular structure is useful on ceramic or hollow fiber membranes only, other structures would collapse. The pressure loss of the permeate in the bore of a hollow fiber is too large by far, it prevents the use of such structures is pervaporation and vapor permeation. Ceramic tubes need then to be fixed on one side only to a tube sheet, where the inner lumen of the tubes is connected to the permeate volume. Baffle plates are required over the outside of the tube bundle in order to achieve good flow distri-... 

Depending on their geometry, the membranes can be subdivided in tubular, hollow fiber, spiral wound and flat sheettubular membranes are the most common solution, even if they require relatively high volume per membrane area unit and present high costs. 

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