Membrane filtration polymer

Polymer Membranes These are used in filtration applications for fine-particle separations such as microfiltration and ultrafiltration (clarification involving the removal of l- Im and smaller particles). The membranes are made from a variety of materials, the commonest being cellulose acetates and polyamides. Membrane filtration, discussed in Sec. 22, has been well covered by Porter (in Schweitzer, op. cit., sec. 2.1). 

In addition to the insoluble polymers described above, soluble polymers, such as non-cross-linked PS and PEG have proven useful for synthetic applications. However, since synthesis on soluble supports is more difficult to automate, these polymers are not used as extensively as insoluble beads. Soluble polymers offer most of the advantages of both homogeneous-phase chemistry (lack of diffusion phenomena and easy monitoring) and solid-phase techniques (use of excess reagents and ease of isolation and purification of products). Separation of the functionalized matrix is achieved by either precipitation (solvent or heat), membrane filtration, or size-exclusion chromatography [98,99]. 

The separation of homogeneous catalysts by means of membrane filtration has been pioneered by Wandrey and Kragl. Based on the enzyme-membrane-reactor (EMR),[3,4] that Wandrey developed and Degussa nowadays applies for the production of amino acids, they started to use polymer-bound ligands for homogeneous catalysis in a chemical membrane reactor (CMR).[5] For large enzymes, concentration polarization is less of an issue, as the dimension of an enzyme is well above the pore-size of a nanofiltration membrane. 

In the early 1970 s, Bayer et al. reported the first use of soluble polymers as supports for the homogeneous catalysts. [52] They used non-crosslinked linear polystyrene (Mw ca. 100 000), which was chloromethylated and converted by treatment with potassium diphenylphosphide into soluble polydiphenyl(styrylmethyl)phosphines. Soluble macromolecular metal complexes were prepared by addition of various metal precursors e.g. [Rh(PPh3)Cl] and [RhH(CO)(PPh3)3]. The first complex was used in the hydrogenation reaction of 1-pentene at 22°C and 1 atm. H2. After 24 h (50% conversion in 3 h) the reaction solution was filtered through a polyamide membrane [53] and the catalysts could be retained quantitatively in the membrane filtration cell. [54] The catalyst was recycled 5 times. Using the second complex, a hydroformylation reaction of 1-pentene was carried out. After 72 h the reaction mixture was filtered through a polyamide membrane and recycled twice. 

New approaches to catalyst recovery and reuse have considered the use of membrane systems permeable to reactants and products but not to catalysts (370). In an attempt to overcome the problem of inaccessibility of certain catalytic sites in supported polymers, some soluble rho-dium(I), platinum(II), and palladium(II) complexes with noncross-linked phosphinated polystyrene have been used for olefin hydrogenation. The catalysts were quantitatively recovered by membrane filtration or by precipitation with hexane, but they were no more active than supported... 

To use membrane filtration for residence time decoupling the molecular weight of the chemical catalyst has to be increased, for example, by binding the catalyst to a homogeneously soluble polymer [4]. This allows for separation of reactants and catalysts by size. Due to their similarity to biological catalysts, the term chemzyme (chemical enzyme) [5, 6] has been coined for these polymer-enlarged but still homogeneously soluble chemical catalysts (Fig. 3.1.2) [7]. 

The step 4 product was placed in a reaction tube and heated to 200°C 0.5 mmHg for 24 hours. The cmde polymer was isolated as glassy orange solid and was purified using membrane filtration with a 2.4-nm cut-off and the product isolated in 40-70% yield. 

Continuous homogeneous catalysis is achieved by membrane filtration, which separates the polymeric catalyst from low molecular weight solvent and products. Hydrogenation of 1-pentene with the soluble pofymer-attached Wilkinson catalyst affords n-pentane in quantitative yield A variety of other catalysts have been attached to functionalized polystyrenes Besides linear polystyrenes, poly(ethylene glycol)s, polyvinylpyrrolidinones and poly(vinyl chloride)s have been used for the liquid-phase catalysis. Instead of membrane filtration for separating the polymer-bound catalyst, selective precipitation has been found to be very effective. In all... 

A major alternative to direct flow membrane filtration is depth filtration, in which particles are removed throughout the filtration matrix rather than just at the membrane surface, by various mechanisms such as size exclusion, electrostatic, and hydrophobic interactions. Depth filters are typically composed of a bed of cellulose or polypropylene fibers together with an inorganic filter aid such as diatomaceous earth and a binder to form a filter sheet. The filter aid imparts the matrix very high surface areas and plays an important role in increasing both retention and the capacity. Depth filters can also have an electrostatic charge usually associated with the binder polymer. 

Solomon, B. A. Colton, C. K. Friedman, L. 1. Castino, F. Wiltbank, T. B. Martin, D. M. "Microporous Membrane Filtration for Continuous-Flow Plasmapheresis" In Ultrafiltration Membranes and Applications Vol. 3 of Polymer Science and Technology Cooper, A. R., Ed. Henum Press New York, N.Y., 1980, pp 489-505. Zydney, A. L. "Cross-flow membrane plasmapheresis an analysis of flux and hemolysis PhD Thesis, Massachusetts Institute of Technology, 1985. 

Consequently, manufacturers of polymers show only a small interest in developing special polymers for membrane filtration. 

A careful choice of cleaning solutions and procedures will extend the service life of the membrane. In many polymer membrane filtration systems. 

For industrial biotransformations, catalyst recovery and reuse are major issues. This may be desirable either for reasons of downstream processing or for repeated use in order to reduce the specific catalyst costs per kg of product produced. A very simple method is the use of membrane filtration. Because of the increasing number of membranes from different materials (polymers, metal or ceramics) this is an attractive alternative. Whereas for whole cells microfiltration or centrifugation can be applied, for the recovery of soluble enzymes ultrafiltration membranes have to be used120-221. Often immobilization on a support is chosen to increase the catalyst s stability as well as to facilitate its recovery. The main advantages of immobilization are ... 

Alternatively, membrane filtration can be used to separate soluble metal complexes, containing polymer-attached ligands, from reactants and products [43]. A major advantage of using soluble polymer-enlarged catalysts in conjunction with membrane separation is that it is readily amenable to continuous operation. A novel example of this concept is the use of dendritic ligands [44] in combination with membrane filtration. 

Lactic acid is an important additive and preservative agent in the chemical, cosmetics, pharmaceutical, and food industries. It is also used as the base for the production of biodegradable polymers like polylactates [4.12]. Its current worldwide production is estimated to be 40,000 tons per year. The results reported by Olmos-Dichara and coworkers [4.13] are typical of the results reported in many of the prior studies of this reaction system. They carried out a study comparing the performance of a batch reactor and a MBR for the production of lactic acid using L. cassei sp. rhamnosus as a biocatalyst. The MBR consists of the batch bioreactor coupled with a cross-flow mineral membrane filtration unit. MBR productivity was eight times that of the batch reactor, while the biomass concentration (77g f ) in the MBR was 19 times that found in the batch culture. 

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