Cellulose acetate membrane discussion

For purposes of illustration, the following discussion, unless otherwise specified, is limited to single-solute aqueous feed solutions, cellulose acetate membranes, and reverse osmosis systems for which osmotic pressure effects are essentially negligible. 

This presentation will discuss the membrane performance and its physical and chemical changes under unfavourable conditions. This kind of studies will give us information on trouble-shooting counter-measures for unexpected membrane deteriorations, and on the durability of a cellulose acetate membrane under adverse conditions. 

The development of asymmetric membrane technology in the 1960 s was a critical point in the history of gas separations. These asymmetric structures consist of a thin (0.1 utol n) dense skin supported on a coarse open-cell foam stmcture. A mmetric membranes composed of the polyimides discussed above can provide extremely high fluxes throuj the thin dense skin, and still possess the inherently hij separation factors of the basic glassy polymers from which they are made. In the early 1960 s, Loeb and Sourirajan described techniques for producing asymmetric cellulose acetate membranes suitable for separation operations. The processes involved in membrane formation are complex. It is believed that the thin dense skin forms at the... 

It is to this topic of solute preferential sorption in reverse osmosis that this paper is dedicated. Specifically, this discussion will involve a description of solute preferential sorption, an overview of the literature in the area, and finally a presentation of some recent work on the removal of aromatic hydrocarbons from water. The significance of this work is at least two-fold. From a practical point of view the classes of solutes which demonstrate preferential attraction to the membrane material tend to be organic compounds and the removal and recovery of these solutes from water is environmentally and economically important. From a theoretical point of view an understanding of the phenomena involved is essential to the achievement of a fundamental description of the RO process. Although this paper deals solely with aqueous solutions and cellulose acetate membranes, it Is important to recognize that the concepts discussed can be extended to Include other membrane materials and non-aqueous systems. 

Figure 6.3927 shows a two-stage cascade for the purification and dehydration of sour gases, mainly removing C02 and H2S. Again, spiral wound modules with asymmetric cellulose acetate membranes are employed. It should be noted, that in this case as in all other cases discussed here, no compressors had to be installed. This is the main reason why these applications show excellent payback times. 

It is the objective of this article to present recent developments and applications of low-pressure membranes which have been developed in the past few years. Improvements and optimization of casting conditions of asymmetric membranes as well as composite membranes are discussed. Comparisons are made where applicable. Some data obtained in our laboratory for the effect of casting conditions on the performance of asymmetric cellulose acetate membranes are also presented and compared with the data from literature. 

Interconversions based on a subunit model of the A, B, and S forms of isoenzymes of the j8-o-2-acetamido-2-deoxyhexosidase in human tissues have been accomplished by means of preparative polyacrylamide gel electrophoresis." It was concluded that the A, B, and S forms are composed of a heteropolymer containing a- and j8-chains, a jS-chain homopolymer, and an a-chain homopolymer, respectively. Separation of the isoenzymes of jS-D-2-acetamido-2-deoxyhexosidase in human tissues by electrophoresis on cellulose acetate membranes has been used in the diagnosis of GM -gangliosidosis." The relative abundances and properties of the three isoenzymes were determined, and the relevance of the results to the disease was discussed. 

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). 

As previously discussed, solvents that dissolve cellulose by derivatization may be employed for further functionahzation, e.g., esterification. Thus, cellulose has been dissolved in paraformaldehyde/DMSO and esterified, e.g., by acetic, butyric, and phthalic anhydride, as well as by unsaturated methacrylic and maleic anhydride, in the presence of pyridine, or an acetate catalyst. DS values from 0.2 to 2.0 were obtained, being higher, 2.5 for cellulose acetate. H and NMR spectroscopy have indicated that the hydroxyl group of the methy-lol chains are preferably esterified with the anhydrides. Treatment of celliflose with this solvent system, at 90 °C, with methylene diacetate or ethylene diacetate, in the presence of potassium acetate, led to cellulose acetate with a DS of 1.5. Interestingly, the reaction with acetyl chloride or activated acid is less convenient DMAc or DMF can be substituted for DMSO [215-219]. In another set of experiments, polymer with high o -celliflose content was esterified with trimethylacetic anhydride, 1,2,4-benzenetricarboylic anhydride, trimellitic anhydride, phthalic anhydride, and a pyridine catalyst. The esters were isolated after 8h of reaction at 80-100°C, or Ih at room temperature (trimellitic anhydride). These are versatile compounds with interesting elastomeric and thermoplastic properties, and can be cast as films and membranes [220]. 

The importance of proper RO membrane selection has already been discussed. A review of commercially available RO membranes revealed five different basic membranes that could provide organic recovery. Cellulose acetate and cellulose acetate blends, aromatic polyamide, polyamide thin-film composite, cross-linked polyimine thin-film composite (FT-30), and polybenzimidazole were available when this work was performed. Only the first four types were commercially available. All membranes were available with excellent salt rejection (>97 sodium chloride). Two types of membranes, cellulose acetate and FT-30, have shown short-term (<2-months intermittent use) resistance... 

The discussion directly following Eq (6) provides a simple, physically reasonable explanation for the preceding observations of marked concentration dependence of Deff(C) at relatively low concentrations. Clearly, at some point, the assumption of concentration independence of Dp and in Eq (6) will fail however, for our work with "conditioned" polymers at CO2 pressures below 300 psi, such effects appear to be negligible. Due to the concave shape of the sorption isotherm, even at a CO2 pressure of 10 atm, there will still be less than one CO2 molecule per twenty PET repeat units at 35°C. Stern (26) has described a generalized form of the dual mode transport model that permits handling situations in which non-constancy of Dp and Dh manifest themselves. It is reasonable to assume that the next generation of gas separation membrane polymers will be even more resistant to plasticization than polysulfone, and cellulose acetate, so the assumption of constancy of these transport parameters will be even more firmly justified. 

Two most common families of RO membranes, based on the type of polymer backbone, are cellulose acetate and polyamide.12 Membranes made from these polymers differ in many respects, including performance, physical properties, structure and the manner in which they are created. These aspects are discussed below. 

Immobilization of proteins from solutions has been discussed in Section 13.3. Several matrices can be used for the transfer of proteins from gels, the most widely used being nitrocellulose. Other alternatives are diazobenzyloxymethyl-cellulose (DBM filters Alwine et al., 1979), diazophenylthioether-cellulose (DPT filters Reiser and War-dale, 1981), cellulose-acetate or paper activated with CNBr (Clarke et al., 1979), commercially available nylon-based membranes such as Gene screen (New England Nuclear), Zetabind (available from Bio-Rad as Zeta Probe). Nitrocellulose is better for EIA than Zeta Probe (considerably less background). 

Meiny different supports have been used to prepare ILMs Including cellulose acetate reverse osmosis membranes (1 6, 25, 29, ), micro-porous polypropylene ultrafiltration membranes (31-3 T7 polyvinyl chloride filters (35), and hollow fiber cellulose acetate reverse osmosis membranes T36). Way et al. ( ) discuss the chemical and physical properties that must be considered when an ILM support Is selected. 

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