Polyaramide membrane

Membrane filtration using a polyaramide membrane [56] showed a retention of more than 99.8%. Application of this catalyst in a continuously operated membrane reactor showed conversion for more than 150 h. The ee dropped from 80% in the beginning (non-bonded analogue 97%) to about 20% after 150 h. The average ee for the first 80 h was 50%. 

New polyaramide membranes with outstanding properties (membrane UF-PA-5) met all requirements [27]. 

At the beginning the first requirement was not fulfilled, because experiments with commercially available membranes showed insufficient resistance to organic products, especially against oxo products. Later, a new polyaramide membrane... 

First results in membrane filtration with a polyaramide membrane and conventional Rh/TPP catalysts were unsatisfactory, even with the Rh complex itself (M < 1000). Only the use of the re-immobilized catalyst with a higher molecular mass (> 3000) showed an encouraging Rh retention of 96% as compared with 50% with TPP [32],... 

For ultrafiltration as a unit operation for the separation of polymer-bound soluble catalysts in particular, the recovery process for a rhodium catalyst from the hydroformylation of dicyclopentadiene is an illustrative example (for another detailed example, see Section 7.5) [26, 27]. Toluene can be used as a solvent with the polyaramide membrane employed. TPPTS or also a sulfonated bidentate phosphine with large ammonium counterions, are used as ligands. For efficient recovery, molecular weights of the catalyst of more than 3000 g mofi were required on the membrane used. Separation is performed in two steps [28]. A pilot plant was run successfully over an extended period of time of three months. 

In the past, the available membranes lost a significant fraction of their selectivity when operated at these high temperatures. They also became plasticized by absorbed heavy hydrocarbons in the feed gas. As a consequence, a number of early hydrogen-separation plants installed in refineries had reliability problems. The development of newer polyimide and polyaramide membranes that can safely operate at high temperatures has solved most of these problems and the market for membrane-based hydrogen-recovery processes in refineries is growing. 

The water uptake shows a nearly hnear dependence on the ion exchange capacity of the membranes. It is in the same range as observed for the Nafion membranes. Unexpectedly, no difference between the water uptake of random and multiblock copolymers was observed in contrast to sulfonated polyaramide membranes [309]. From studies following the polymerization process of a random sulfonated copoly (ether sulfone), it was deduced that during the preparation of random copolymers a block-like stmcture is obtained due to differences in the reactivity of the monomers. Therefore, it is likely that the morphology of the membranes prepared from random or block copolymers is very similar, resulting in a similar behavior of the membranes. 

Air Liquide, with the MEDAL polyimide and high selectivity polyaramide membranes. 

Membrane modules have found extensive commercial appHcation in areas where medium purity hydrogen is required, as in ammonia purge streams (191). The first polymer membrane system was developed by Du Pont in the early 1970s. The membranes are typically made of aromatic polyaramide, polyimide, polysulfone, and cellulose acetate supported as spiral-wound hoUow-ftber modules (see Hollow-FIBERMEMBRANEs). 

Kragl and Dreisbach (1996) have carried out the enantioselective addition of diethyl zinc to benzaldehyde in a continuous asymmetric membrane reactor using a homogeneous soluble catalyst, described in their paper. Here a,a-diphenyl-L-proline was used as a chiral ligand, coupled to a copolymer made from 2-hydroxy ethyl methacrylate and octadecyl methacrylate, which had a sufficiently high molecular weight to allow separation by ultra-filtration (U/F). The solvent-stable polyaramide U/F Hoechst Nadir UF PA20 retained more than 99.8% of the catalyst. The ee was 80 %, compared to 98 % for a noncoupled catalyst. 

Kragl 13) pioneered the use of membranes to recycle dendritic catalysts. Initially, he used soluble polymeric catalysts in a CFMR for the enantioselective addition of Et2Zn to benzaldehyde. The ligand a,a-diphenyl-(L)-prolinol was coupled to a copolymer prepared from 2-hydroxyethyl methyl acrylate and octadecyl methyl acrylate (molecular weight 96,000 Da). The polymer was retained with a retention factor > 0.998 when a polyaramide ultrafiltration membrane (Hoechst Nadir UF PA20) was used. The enantioselectivity obtained with the polymer-supported catalyst was lower than that obtained with the monomeric ligand (80% ee vs 97% ee), but the activity of the catalyst was similar to that of the monomeric catalyst. This result is in contrast to observations with catalysts in which the ligand was coupled to an insoluble support, which led to a 20% reduction of the catalytic activity. 

Removal of carbon dioxide is the only membrane-based natural gas separation process currently practiced on a large scale—more than 200 plants have been installed, some very large. Most were installed by Grace (now Kvaerner-GMS), Separex (UOP) and Cynara and all use cellulose acetate membranes in hollow fiber or spiral-wound module form. More recently, hollow fiber polyaramide (Medal) membranes have been introduced because of their higher selectivity. 

In 1996, a paper was published which was dedicated to selecting suitable membranes for separations in organic solvents [466]. Membranes tested in an asymmetrical channel included polysulfone MWCO 20,000 g/mol, regenerated cellulose MWCO 20,000 g/mol, PTFE pore size 0.02 mm, polyaramide MWCO 50,000 g/mol, poly(vinylidene fluoride) MWCO 50,000 g/mol, poly(phenylene oxide) MWCO 20,000 g/mol and a DDS fluoro polymer MWCO 30,000 g/mol. The first membrane was tested with water, the others with THF or a THF/ace-tonitrile mixture. Numerous problems occurred with the different membranes. The best membrane for THF was found to be the DDS fluoro polymer membrane. 

Moreover, ultrafiltration membranes derived from polyaramide were successfully modified by electron beam irradiation [63]. As displayed in Fig. 9, an increase in retention capability without any change in permeate flux was achieved, due to chemical reactions leading to crosslinks and to the introduction of hydrophilic groups into the polymer backbone. 

Fig. 9 Effect of electron beam irradiation on the performance of a polyaramide ultrafiltration membrane, (a) Permeate flux, (b) Retention (reproduced from [63] with permission from Wiley-VCH)...

Further developments are still being made in membrane materials and membrane modules. There are reports, in particular, of hydrophobic membranes made of polyolefins, crosslinked polyolefins, polyamides, polyaramids, poly(vinylidene fluoride), PTFE, polyimides, and suchlike. 

Superior mechanical, thermal, and fllm-forming ability of the soluble polyaramids made them attractive material for membrane-based applications [29]. Incorporation of specific groups into the PA backbone leads to high-performance polymeric materials that can be used as gas separation, PV, and PEM materials. 

The earliest report on CMS membranes obtained from hollow fiber polymeric membranes appears to be from Koresh and Soffer (1983) and Soffer et al. (1987). By comparing CMS membranes derived from different polymeric membranes, Jones and Koros (1994a) found that the ones from aromatic polyimides yielded the best separation and mechanical properties. The polymers tested by Jones and Koros were cellulose acetate, polyaramides, and polyimides. Polyfurfural alcohol was used by Foley and co-workers (Foley, 1995 Shiflett and Foley, 1999 Strano and Foley, 2002). A comparison of the O2 permeances and O2/N2 selectivities showed that the CMS membrane from polyimide was indeed much better than that from polyfurfural alcohol, as will be seen shortly. 

Separation of H2 from a variety of gas streams H2 from N2 in purge streams in aimnonia plants recovery of H2 from various streams in petroleum refineries and petrochemical plants H2/CO ratio adjustment in synthesis gas for oxo-alcohol plants. The membrane materials used are polysulfone, cellulose acetate, poly-imides and polyaramides. Selectivities of H2 over N2, CH4, CO in these applications vary between 30 and 200. 

Since the early 1980s, membrane technology has advanced rapidly and continues to advance. In addition to cellulose acetate and polysulfone, the polymers used in making gas separation membranes include polyimides, polyamides, polyaramid, polydimethylsiloxane, silicon polycarbonate, neoprene, silicone rubber, and others. Today membranes can be designed to withstand a 2,000 psi pressure differential. Membranes used in hydrogen or carbon dioxide applications operate at temperatures up to 200°F, while those used in solvent applications can operate at temperatures up to about 400°F (Baker, 1985). 

Segregation of catalyst (Co-factor in a reactor) Production of amino acid (ultra filtration) Enantio selective addition of zinc to benzalde-hyde (homogenously soluble catalyst retained by a solvent stable polyaramide ultraiiltration membrane)... 

In addition to the more conventional membranes, such as those of poly-sulfone and silicone rubber, there has been an exponential growth in the R feD of other membrane materials, such as polyimides and polyaramides. Inorganic or carbon sieving materials can also separate many gases, although their fragility and cost have made them less attractive. Robeson has noted a trade-off between the permeability (productivity) and selectivity (efficiency) of typical polymeric membranes. To transcend this well-known upper-bound trade-off curve and maximize both membrane permeability and... 

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