Polymeric membranes pervaporation

Membrane Porosity Separation membranes run a gamut of porosity (see Fig. 22-48). Polymeric and metallic gas separation membranes, electrodialysis membranes, pervaporation membranes, and reverse osmosis membranes are nonporous, although there is hnger-ing controversy over the nonporosity of the latter. Porous membranes are used for microfiltration and ultrafiltratiou. Nanofiltration membranes are probably charged porous structures. 

G. Ellinghorst, H. Steinhauser and A. Hubner, Improvement of Pervaporation Plant by Choice of PVA or Plasma Polymerized Membranes, in Proceedings of Sixth International Conference on Pervaporation Processes in the Chemical Industry, R. Bakish (ed.), Bakish Materials Corp., Englewood, NJ, pp. 484-493 (1992). 

The use of silicone membranes as an interface in MIMS for direct extraction and analysis by MS has fostered their implementation for extraction purposes that can be combined off-line or on-line with other analytical instrumentation, such as GC. The technique of membrane extraction with sorbent interface (MESI) (Figure 4.2) employs the pervaporation principle in a nonporous polymeric membrane unit, where the membrane is used as a selective barrier for the extraction of VOCs and SVOCs in gaseous or liquid samples. 

Polymeric membranes with a less porous structure, pervaporation membranes as well as nanofiltration membranes, can be described by a solution-diffusion mecha-... 

Problems to be solved are related to membrane stability (of polymeric membranes, but also the development of hydrophobic ceramic nanofiltration membranes and pervaporation membranes resistant to extreme conditions), to a lack of fundamental knowledge on transport mechanisms and models, and to the need for simulation tools to be able to predict the performance of solvent-resistant nanofiltration and pervaporation in a process environment. This will require an investment in basic and applied research, but will generate a breakthrough in important societal issues such as energy consumption, global warming and the development of a sustainable chemical industry. 

Yoshikawa M, Shimada H, Tsubouchi K, and Kondo Y. Specialty polymeric membranes. 12. Pervaporation of benzene/cyclohexane mixtures through carbon graphite-nylone 6 composite membranes. J Membr Sci 2000 177 49-53. 

Zeolite membranes are not the only kind of membranes that have been used in pervaporation, organic and other types of inorganic membranes, different from zeolites, have been employed. Polymeric membranes of PVA (polyvinyMcohol) have been widely employed for dehydratation and separation of organic mixmres however, their main limitations are related to their low thermal and chemical stability. When the water content in the feed mixmre is high, polymeric membranes suffer from swelling moreover, in the separation of organic mixtures they usually present a low selectivity. 

The different mechanisms that operate in the separation of gases have been previously described in Section 10.4.1. In pervaporation, the transport mechanism can be described by an adsorption-diffusion mechanism [74,114] similar to one for polymeric membranes [115]. However, it is necessary to consider that the specific interactions between the permeating component and the zeolitic material are different in zeolites. Moreover, the diffusion through the ordered zeolite nanopores is different than in the dense organic matrix. 

General-purpose organic solvents used in the chemical industry that are difficult to separate with conventional methods represent a potential area where pervaporation finds applications. The use of polymeric membranes in this case was not very successful due to the low chemical stability of the polymers in the organic solvent. The separations that have been accomplished up-to-date with zeolite membranes include tetrahydrofuran, dimethylformamide, and dioxane. 

T. S. Chung, J. J. Shieh, J. Qin, W. H. Lin, and R. Wang, Polymeric membranes for reverse osmosis, ultrafiltration, microfdtration, gas separation, pervaporation, and reactor applications. In Advanced Functional Molecules and Polymers, H. S. Nalwa (ed.). Chapter 7, Gordon Breach, pp. 219-264 (2001). 

These data are important in understanding vapor permeation through polymeric membranes, which occurs in the pervaporation process. 

Hirotsu, T. Graft polymerized membranes of methacrylic acid by plasma for water-ethanol pervaporation. Ind. Eng. Chem. Res. 1987, 26, 1287-1290. 

One of the first zeolite based membranes were composite membranes, obtained by dispersion of zeolite crystals in dense polymeric films in order to make zeolite filled polymeric membranes [59,60,61], These membranes have been developed at the end of the 80 s for both gas separation and pervaporation. The clogging of zeolite pores by the matrix and the quality of the interface between the zeolite crystals and the polymer matrix (non-selective diffusion pathways) were key points. 

T.C. Bowen, L. M. Vane, Ethanol recovery by pervaporation through zeolite, zeolite filled polymeric membranes, US Envir. Protection Agency-National risk Management Research Lab-EPA research bull.. 

Pervaporation membrane reactors are not a recent discovery. The use of a PVMR was proposed in a U.S. patent dating back to 1960 [3.6]. Though the technical details on membrane preparation and experimental apparatus were rather sketchy, the basic idea was described there, namely, the use of a water permeable polymeric membrane to drive an esterification reaction to completion. A more detailed description of a PVMR can be found in a later European patent [3.7], which described the use of a flat membrane (commercial PVA or Nafion ) placed in the middle of a reactor consisting of two half-cells. The reaction studied was the acetic acid esterification reaction with ethanol. For an ethanol to acetic acid ratio of 2, liquid hourly space velocities (LHSV) in the range of 2-5, and a temperature of 90 °C complete conversion of the acetic acid was reported. The use of PVMR for this reaction shows promise for process simplification, as indicated schematically in Figure 3.2, which shows a side-by-side comparison of a conventional and a proposed PVMR plant for ethyl acetate production. 

Further details about the use of this equation can be found elsewhere [5.90, 5.93]. The pervaporative transport of a component i through a dense polymeric membrane is generally described in the literature [5.85, 5.86] by the following equation... 

Chapter 3 is devoted to the topic of pervaporation membrane reactors. These are unique systems in that they use a liquid feed and a vacuum on the permeate side they also mostly utilize polymeric membranes. Chapter 4 presents a survey of membrane bioreactor processes these couple a biological reactor with a membrane process. Reactions studied in such systems include the broad class of fermentation-type or enzymatic processes, widely used in the biotechnology industry for the production of amino acids, antibiotics, and other fine chemicals. Similar membrane bioreactor systems are also fin-... 

All the above mentioned high perm-selectivity of zeolite membranes can be attributed to the selective sorption into the membranes. Satisfactory performance can be obtained by defective zeolite membranes. Xylene isomers separation by zeolite membranes compared with polymeric membranes are summarized in Table 15.4. As shown, zeolite membranes showed much higher isomer separation performances than that of polymeric membranes. Specially, Lai et al. [41] prepared b-oriented silicalite-1 zeolite membrane by a secondary growth method with a b-oriented seed layer and use of trimer-TPA as a template in the secondary growth step. The membrane offers p-xylene permeance of 34.3 x 10 kg/m. h with p- to o-xylene separation factor of up to 500. Recently, Yuan et al. [42] prepared siUcalite-1 zeolite membrane by a template-free secondary growth method. The synthesized membrane showed excellent performance for pervaporation separation of xylene isomers at low temperature (50°C). 

For illustration, rubbery polymeric membranes, whose polymeric network is sufficiently elastic and mobile to allow comparatively large organic compounds to diffuse through it (Table 3.6-2), are in general used for the recovery of organic compounds from aqueous solutions. Because of its small size, the bulk solvent, water, unfortunately diffuses through the membrane even better. This is why in organo-philic pervaporation the selectivity is mainly achieved and determined by the ratio of the solubility coefficients (sorption selectivity. Table 3.6-2). Membrane selectivity, as defined in Eq. (7), is an intrinsic parameter and can differ from the overall process selectivity, as wiU be shown later. 

A similar diffusion mechanism applies for the other membrane processes mencioiied above - of a vapour out of a liquid mixture for pervaporation, or of one gas out of a mixture for gas or vapour permeation. These diffusion processes all require the continuity of material represented by a polymeric membrane, in sheet or tubular form, and cannot be undertaken in ceramic materials. 

Poly(tetrafluoroethylene)/polyamide thin-film composite membranes via interfacial polymerization for pervaporation dehydration on an isopropanol aqueous solution. Journal of Membrane Science 315 106-115. 

Ray S, Singha NR, Ray SK. 2009. Removal of tetrahydrofuran (THF) from water by pervaporation using homo and blend polymeric membranes. Chem. Eng. J. 149 153-161. 

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