Polymeric/membrane

The use of synthetic polymeric membranes in separation processes [19,20] took off with the breakthrough of asymmetric membrane formation first developed by Loeb and Sourirajan [21]. These membranes have a thin dense polymer layer that governs the separation on top of a much thicker porous layer that provides mechanical support. They were first produced in the laboratory by spreading 

FIGURE 13.12 Machine used to prepare polymeric membranes by the Loeb-Sourirajan wet casting process. (Data from Baker, R. W. Membrane Technology and Applications, 2nd edn., WUey, Hoboken, NJ, 2004. Copyright Wiley-VCH Verlag GmbH Co. KGaA. Reproduced with permission.) 

Thermoporometry has become a popular tool for measuring pore size distribution in polymer membranes, as illustrated in reviews by Nakao [39] and Kim et al. [40]. 

In 2000, Hay et al. [41] reported a study of cellulose membranes with water TPM. The authors found a pore size distribution with radii between 5 and 50 nm but noted an important migration of water during DSC measurements. 

In 2003, Ksiqzczak et al. [42] used water TPM for the characterization of nitrocellulose prepared by nitration of natural cellulose. The hydrophobic nature of the membrane made the measurements difficult and only partial conclusions were drawn. Despite this, pore size distributions were measured which showed good consistency and confirmed the value of TPM for such studies. Even more recently, Rohman et al. used water TPM to measure pore size distributions in porous polymers networks [43]. 

FIGURE 4.4 A proposed mechanism of facilitated transport of CO2 in an FSC membrane. (From Kim T.J., Hagg M.B., J. Pol. Set Part B Polym. Phys., 42, 4326, 2004. With permission.) 

The solubility in glassy polymers is usually described by the so-called dual-mode model, which implies that there is a need for a more detailed definition of the sorption, c, in the flux Equation 4.1. Equations 4.20 and 4.21 illustrate this and can relate to 

FIGURE 4.5 SEM-picture of a typical composite membrane comprising of support structure of PP and a selective skin layer of PDMS. 

The dual-mode model has been extensively covered by several authors [25,47 9]. 

FIGURE 4.7 Polymeric specific volume as a function of temperature. (From Chem R.T., Koros W.J., Sanders E.S., Chen S.H., Hopfenberg H.B. In Whyte T., Yon C.M., Wagener E.H., eds. ACS Symposium Series 223 on Industrial Gas Separations. American Chemical Society, Washington DC, 47, 1983. With permission.) 

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More recendy, two different types of nonglass pH electrodes have been described which have shown excellent pH-response behavior. In the neutral-carrier, ion-selective electrode type of potentiometric sensor, synthetic organic ionophores, selective for hydrogen ions, are immobilized in polymeric membranes (see Membrane technology) (9). These membranes are then used in more-or-less classical glass pH electrode configurations. 

Interfacial polymerization membranes are less appHcable to gas separation because of the water swollen hydrogel that fills the pores of the support membrane. In reverse osmosis, this layer is highly water swollen and offers Httle resistance to water flow, but when the membrane is dried and used in gas separations the gel becomes a rigid glass with very low gas permeabiUty. This glassy polymer fills the membrane pores and, as a result, defect-free interfacial composite membranes usually have low gas fluxes, although their selectivities can be good. 

R. E. Kestiug, Synthetic Polymeric Membranes, 2nd ed., John Wiley Sons, Inc., New York, 1985. 

This system utilizes specific membranes, between which the dmg reservoir is enclosed (Fig. 4). A tiny ehiptical disk, inserted into the cul-de-sac of the eye, releases pilocarpiae steadily. The dmg is deUvered through selected polymeric membranes. The dmg reservoir maintains a saturated solution between the membranes which acts osmoticaHy as the driving force for the dmg to diffuse through the rate-limiting membranes. 

Fleece-Back Sheet. A fleece-back sheet is a nonreinforced polymeric membrane that has had a nonwoven mat made of polyester, weighing 101.7—203.4 g/m, laminated to the back of the sheet. The prime use of the fleece-back sheet is in the fully adhered roofing systems. The fleece provides the chemical separator, which eliminates the need for an adhesive that is compatible with the specific membrane or a compatible substrate. 

S. Hwang and K. Kammermeyer, Membranes in Separations,]ohn Wdey Sons, Inc., New York, 1975 good study of membrane transport phenomenon. R. E. Kesting, Synthetic Polymeric Membranes, McGraw-HiU, New York, 1971 good bibhographies. 

Membra.ne Diffusiona.1 Systems. Membrane diffusional systems are not as simple to formulate as matrix systems, but they offer much more precisely controlled and uniform dmg release. In membrane-controlled dmg deUvery, the dmg reservoir is intimately surrounded by a polymeric membrane that controls the dmg release rate. Dmg release is governed by the thermodynamic energy derived from the concentration gradient between the saturated dmg solution in the system s reservoir and the lower concentration in the receptor. The dmg moves toward the lower concentration at a nearly constant rate determined by the concentration gradient and diffusivity in the membrane (33). 

Polymeric Membranes Economically important applications required membranes that could operate at higher pH than could CA, for which the optimum is around pH = 5. Many polymeric membranes are now available, most of which have excellent hydrolytic stabihty. Particularly prominent are polysulfone, polyvinyhdene fluoride, poly-ethersulfone, polyvinyl alcohol-polyethylene copolymers, and aciylic copolymers. 

UF Membranes as a Substrate for RO An important use of UF membranes is as a substrate for composite reverse-osmosis membranes. After the UF membrane (usually polysulfone) is prepared, it is coated with an aqueous solution of an amine, then dipped in an organic solution of an acid chloride to produce an interfacially polymerized membrane coating. 

Selective gas permeation has been known for generations, and the early use of p adium silver-alloy membranes achieved sporadic industrial use. Gas separation on a massive scale was used to separate from using porous (Knudsen flow) membranes. An upgrade of the membranes at Oak Ridge cost 1.5 billion. Polymeric membranes became economically viable about 1980, introducing the modern era of gas-separation membranes. Hg recoveiy was the first major apphcation, followed quickly by acid gas separation (CO9/CH4) and the production of No from air. 

Leading Examples These apphcations are commercial, some on a very large scale. They illustrate the range of application for gas-separation membranes. Unless otherwise specified, all use polymeric membranes. 

Despite the fact that a great lot of ion-selective electrodes (ISEs) with liquid and film polymeric membranes for the determination of physiologically active amines (PhAA) has been described, the factors responsible for their selectivity have not yet been studied sufficiently. In this work, the influence of plasticizer and ion-exchanger nature on the selectivity of ISEs reversible to PhAA cations of various stmctures has been discussed. 

By the time the next overview of electrical properties of polymers was published (Blythe 1979), besides a detailed treatment of dielectric properties it included a chapter on conduction, both ionic and electronic. To take ionic conduction first, ion-exchange membranes as separation tools for electrolytes go back a long way historically, to the beginning of the twentieth century a polymeric membrane semipermeable to ions was first used in 1950 for the desalination of water (Jusa and McRae 1950). This kind of membrane is surveyed in detail by Strathmann (1994). Much more recently, highly developed polymeric membranes began to be used as electrolytes for experimental rechargeable batteries and, with particular success, for fuel cells. This important use is further discussed in Chapter 11. 

Polymeric ionic conductors. One of the most unexpected developments in recent decades in the whole domain of electrochemistry has been the invention of and gradual improvements in ionically conducting polymeric membranes, to the... 

The achievements of a small Canadian startup company, Ballard Power Systems, in Vancouver, are the main reason for my view that polymeric-membrane cells have the automotive market at their feet. The stages of the company s achievements. 

Through these processes dissolved substances and/or finely dispersed particles can be separated from liquids. All five technologies rely on membrane transport, the passage of solutes or solvents through thin, porous polymeric membranes. 

Polymeric membranes are most commonly produced in the form of flat sheets, but they are also widely produced as tubes of diameter 10-25 mm and in the form of hollow fibres of diameter 0.1-2 mm. 

The solid-liquid separation of shinies containing particles below 10 pm is difficult by conventional filtration techniques. A conventional approach would be to use a slurry thickener in which the formation of a filter cake is restricted and the product is discharged continuously as concentrated slurry. Such filters use filter cloths as the filtration medium and are limited to concentrating particles above 5 xm in size. Dead end membrane microfiltration, in which the particle-containing fluid is pumped directly through a polymeric membrane, is used for the industrial clarification and sterilisation of liquids. Such process allows the removal of particles down to 0.1 xm or less, but is only suitable for feeds containing very low concentrations of particles as otherwise the membrane becomes too rapidly clogged.2,4,8... 

It is possible to separate a soap-LSDA dispersion by ultrafiltration through a polymeric membrane [33]. The filtrate contained sodium and some magnesium ions but no calcium soaps or LSDA. The separated substances on the membrane could be readily dispersed in water in which they retained a high degree of surface activity. 

Today, the term solid electrolyte or fast ionic conductor or, sometimes, superionic conductor is used to describe solid materials whose conductivity is wholly due to ionic displacement. Mixed conductors exhibit both ionic and electronic conductivity. Solid electrolytes range from hard, refractory materials, such as 8 mol% Y2C>3-stabilized Zr02(YSZ) or sodium fT-AbCb (NaAluOn), to soft proton-exchange polymeric membranes such as Du Pont s Nafion and include compounds that are stoichiometric (Agl), non-stoichiometric (sodium J3"-A12C>3) or doped (YSZ). The preparation, properties, and some applications of solid electrolytes have been discussed in a number of books2 5 and reviews.6,7 The main commercial application of solid electrolytes is in gas sensors.8,9 Another emerging application is in solid oxide fuel cells.4,5,1, n... 

In this chapter, we Hmit ourselves to the topic of zeolite membranes in catalysis. Many types of membranes exist and each membrane has its specific field where it can be appHed best. Comparing polymeric and inorganic membranes reveals that for harsher conditions and high-temperature applications, inorganic membranes outperform polymeric membranes. In the field of heterogeneous catalYsis, elevated temperatures are quite common and therefore this is a field in which inorganic membranes could find excellent applications. 

Consider an equilibrium-limited esterification reaction. One way to drive the reaction to completion is to remove the water formed by the reaction selectively through a membrane. This can be an attractive strategy when higher temperatures are undesirable due to factors like colouration of the materials and formation of undesirable products even though these may be present at a low level. As another example, consider the air oxidation of cyclohexane or cyclododecane to cyclohexanone/-ol or cyclododecanone/-ol, where the product can undergo more facile oxidation to unwanted or much lower value products. Consequently, industrial processes operate at a level of less than 5% conversion. If a membrane can selectively remove cyclohexanone as it is formed, the problems mentioned above can be thwarted. However, selective polymeric membranes, which can work at oxidation temperature, have not yet been proved. 

Novel chiral. separations using enzymes and chiral surfactants as carriers have been realized using facilitated transport membranes. Japanese workers have reported the synthesis of a novel norbornadiene polymeric membrane with optically active pendent groups that show enantio.selectivity, which has shown promi.se in the. separation of propronalol. 

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