Asymmetric membranes processes

Phase Inversion (Solution Precipitation). Phase inversion, also known as solution precipitation or polymer precipitation, is the most important asymmetric membrane preparation method. In this process, a clear polymer solution is precipitated into two phases a soHd polymer-rich phase that forms the matrix of the membrane, and a Hquid polymer-poor phase that forms the membrane pores. If precipitation is rapid, the pore-forming Hquid droplets tend to be small and the membranes formed are markedly asymmetric. If precipitation is slow, the pore-forming Hquid droplets tend to agglomerate while the casting solution is stiU fluid, so that the final pores are relatively large and the membrane stmcture is more symmetrical. Polymer precipitation from a solution can be achieved in several ways, such as cooling, solvent evaporation, precipitation by immersion in water, or imbibition of... 

Interfdci l Composite Membra.nes, A method of making asymmetric membranes involving interfacial polymerization was developed in the 1960s. This technique was used to produce reverse osmosis membranes with dramatically improved salt rejections and water fluxes compared to those prepared by the Loeb-Sourirajan process (28). In the interfacial polymerization method, an aqueous solution of a reactive prepolymer, such as polyamine, is first deposited in the pores of a microporous support membrane, typically a polysulfone ultrafUtration membrane. The amine-loaded support is then immersed in a water-immiscible solvent solution containing a reactant, for example, a diacid chloride in hexane. The amine and acid chloride then react at the interface of the two solutions to form a densely cross-linked, extremely thin membrane layer. This preparation method is shown schematically in Figure 15. The first membrane made was based on polyethylenimine cross-linked with toluene-2,4-diisocyanate (28). The process was later refined at FilmTec Corporation (29,30) and at UOP (31) in the United States, and at Nitto (32) in Japan. 

These solvents include tetrahydrofuran (THF), 1,4-dioxane, chloroform, dichioromethane, and chloroben2ene. The relatively broad solubiHty characteristics of PSF have been key in the development of solution-based hoUow-fiber spinning processes in the manufacture of polysulfone asymmetric membranes (see Hollow-fibermembranes). The solvent Hst for PES and PPSF is short because of the propensity of these polymers to undergo solvent-induced crysta11i2ation in many solvents. When the PES stmcture contains a small proportion of a second bisphenol comonomer, as in the case of RADEL A (Amoco Corp.) polyethersulfone, solution stabiHtyis much improved over that of PES homopolymer. 

Most commercially available RO membranes fall into one of two categories asymmetric membranes containing one polymer, or thin-fHm composite membranes consisting of two or more polymer layers. Asymmetric RO membranes have a thin ( 100 nm) permselective skin layer supported on a more porous sublayer of the same polymer. The dense skin layer determines the fluxes and selectivities of these membranes whereas the porous sublayer serves only as a mechanical support for the skin layer and has Httle effect on the membrane separation properties. Asymmetric membranes are most commonly formed by a phase inversion (polymer precipitation) process (16). In this process, a polymer solution is precipitated into a polymer-rich soHd phase that forms the membrane and a polymer-poor Hquid phase that forms the membrane pores or void spaces. 

The second major membrane type is a composite. Starting with a loose asymmetric membrane, usually a UF membrane, a coating is applied which is polymerized in situ to become the salt rejecflng membrane. This process is used for most high-performance flat-sheet RO membranes, as well as for many commercial nanofiltration membranes. The chemistry of the leading RO membranes is known, but... 

Asymmetrical membrane capsules were prepared, for example, by dip coating mandrels with solutions containing 15 wt% CA and 33 wt% ethanol in acetone. After the mandrels were withdrawn from the coating solution they were immersed in water to precipitate the polymer and create the asymmetrical membrane capsule shell. This process was used to create both the body and the cap for the capsules. The capsules were sealed at the juncture between the cap and body by banding with a solution of CA in acetone. 

RW Korsmeyer, SM Herbig, KL Smith, JR Cardinal, AC Curtiss, MB Fergione, RA Wilson. Single-step process for coating multiparticulates with asymmetric membranes. Proceedings of the International Symposium on the Controlled Release of Bioactive Materials, Stockholm, 1997, pp 531-532. 

AG Thombre, JR Cardinal, AR DeNoto, SM Herbig, KL Smith. Asymmetric membrane capsules for osmotic drug delivery. I. Development of manufacturing process. J Controlled Release 57 55-64, 1999. 

RO process for the desalination of seawater was proposed for the first time by Reid in 1953, but no significant advancement was observed until the invention of asymmetric membrane with high water flux by Loeb and Sourirajan in I960. Since that time, RO process showed remarkable progresses in practical applications in the field of desalination of brackish water for potable and pure water. 

It has been shown (, , 2.) that a membrane casting dope is a strongly structurlzed polymer solution, and that the morphology of the membrane surface layer can be correlated to the structure of the casting solution. The latter parameter affects the nature and details of the phase inversion process occuring in the upper part of the cast solution, in an incipient skin. Thus the solution structure is one of the factors responsible for the skin properties, and consequently for the performance of the ultimately formed asymmetric membrane. 

These observations have several practical consequences for membrane processes where the selective layers are as thin as or even thinner than the low end of the range studied here. First, it is clear that use of thick film data to design or select membrane materials only gives a rough approximation of the performance that might be realized in practice. Second, because the absolute permeability of a thin film may be severalfold different than the bulk permeability, use of the latter type of data to estimate skin thickness from flux observations on asymmetric or composite membranes structures is also a very approximate method. Finally, these data indicate that one could expect... 

Figoli et al. [96,97] reported the preparation of polymeric capsules combining the phase-inversion technique with the membrane process. Polyetheretherketone (PEEKWC) capsules of different size (300-800 micrometer) and morphology (asymmetric with a porous or dense layer) have been prepared. The SEM pictures of the prepared PEEKWC capsules are shown in Figure 21.16. The capsules can find application both in chemical and in food packaging fields [98],... 

Thombre, A. G., Cardinal, J. R., DeNoto, A. R., Herbig, S. M., and Smith, K. L. (1999), Asymmetric-membrane capsules for osmotic drug delivery. Part I Development of a manufacturing process,/. Controlled Release, 57,55-64. 

If a membrane has a graded pore structure but is made in one processing step, frequently from the same material across its thickness, it is called an asymmetric membrane. If, on the other hand, the membrane has two or more distinctively different layers made at different steps, the resulting structure is called a composite membrane. Almost invariably in the case of a composite membrane, a predominantly thick layer provides the necessary mechanical strength to other layers and the flow paths for the permeate and is called the support layer or bulk support. Composite membranes have the advantage that the separating layer and the support layer(s) can be tailored made with different materials. Permselective and permeation properties of the membrane material are critically important while the material for the support layer(s) is chosen for mechanical strength and other consideration such as chemical inertness. The composite membranes can have... 

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... 

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