Membranes permselective layer

HoUow fibers are usuaUy on the order of 25 p.m to 2 mm in diameter. They can be made with a homogeneous dense stmcture, or preferably with a microporous stmcture having a dense permselective layer on the outside or inside surface. The dense surface layer can be integral, or separately coated onto a support fiber. The fibers are packed into bundles and potted into tubes to form a membrane module. More than a kilometer of fibers may be requited to... 

The permeability of dense membranes is low because of the absence of pores, but the permeance of Component i in Equation 10.20 can be high if SM is very small, even though the permeability is low. Thickness of the permselective layer is typically in the range 0.1 to 10 tm for gas separations. The porous support is much thicker than this and typically more than 100 tm. When large differences in PM exist among species, both high permeance and high selectivity can be achieved in asymmetric membranes. 

There are essentially four different types of membranes, or semipermeable barriers, which have either been commercialized for hydrogen separations or are being proposed for development and commercialization. They are polymeric membranes, porous (ceramic, carbon, metal) membranes, dense metal membranes, and ion-conductive membranes (see Table 8.1). Of these, only the polymeric membranes have seen significant commercialization, although dense metal membranes have been used for commercial applications in selected niche markets. Commercial polymeric membranes may be further classified as either asymmetric (a single polymer composition in which the thin, dense permselective layer covers a porous, but thick, layer) or composite (a thick, porous layer covered by a thin, dense permselective layer composed of a different polymer composition).2... 

If a thinner membrane is required, then one must choose a supported membrane. The permselective metal layer may be palladium or, more commonly, palladium-silver alloy, palladium-copper alloy, or other alloy of palladium. The permselective layer ranges in thickness from about 2-25 /an thinner than 2/rm is very difficult to achieve without introducing pin holes and other adverse defects into the permselective layer. The support layer is porous and is composed of either metal (such as sintered stainless steel or tightly woven wire cloth) or an inert ceramic alumina is very common. Since all of the mechanical strength is derived from the support layer, consideration must be given to its shape and thickness. 

Interesting progress has been made recently in chemicaUy modifying the barrier-layer surface of asymmetric polymeric gas permeation membranes by reactive gaseous or liquid treatment (e.g., fluorination) to improve membrane permselectivity or stability [42]. Such surface treatments modify the ultrathin barrier layer almost exclusively and aUow conversion of that layer into a compositionaUy difierent structure. The result may be a more permselective membrane without significant permeabUity loss, a more fouling resistant membrane. 

Otherwise, the separation factor will be smaller. The narrow pore-size distributions and the small pores of ceramic and glass membranes allow separation due to Knudsen diffusion (for the appropriate pressure range) by preferential diffusion of the lighter component through the membrane. In composite membranes, the thin permselective layer can be in the Knudsen diffusion regime and thus be responsible for all the separation. The support layers, with their larger-diameter pores, are usually in the viscous-flow regime. 

The direct fixation of the biocatalyst to the sensitive surface of the transducer permits the omission of the inactive semipermeable membranes. However, the advantages of the membrane technology are also lost, such as the specificity of permselective layers and the possibility of affecting the dynamic range by variation of the diffusion resistance. Furthermore, the membrane technology has proved to be useful for reloading reusable sensors with enzyme. In contrast, direct enzyme fixation is mainly suited to disposable sensors. This is especially valid for carbon-based electrodes, metal thin layer electrodes printed on ceramic supports, and mass-produced optoelectronic sensors. Field effect transistors may also be envisaged as basic elements of disposable biosensors. 

Membranes are classified by whether the thin permselective layer is porous or dense, and by the type of material (organic, polymeric, inorganic, metal, etc.) this membrane film is made from. The choice of a porous vs. a dense film, and of the type of material used for manufacturing depends on the desired separation process, operating temperature and driving force used for the separation the choice of material depends on the desired permeance and selectivity, and on thermal and mechanical stability requirements. For membrane reactor applications, where the reaction is coupled with the separation process, the thin film has also to be stable under the reaction conditions. 

There are a number of membrane reactor systems, which have been studied experimentally, that fall outside the scope of this model, however, including reactors utilizing macroporous non-permselective membranes, multi-layer asymmetric membranes, etc. Models that have been developed to describe such reactors will be discussed throughout this chapter. In the membrane bioreactor literature, in particular, but also for some of the proposed large-scale catalytic membrane reactor systems (e.g., synthesis gas production) the experimental systems utilized are often very complex, in terms of their configuration, geometry, and, of course, reaction and transport characteristics. Completely effective models to describe these reactors have yet to be published, and the development of such models still remains an important technical challenge. 

Consider first a clean membrane processing solutions with significant osmotic pressure. The solvent is assumed to pass by laminar flow through the small pores of the permselective layer, and the driving force is the applied pressure differential minus the difference of osmotic pressure across the membrane. The volume flux, v, which is the superficial permeate velocity normal to the membrane surface, can be written for a clean membrane as... 

A timely review paper focusing on the hurdles to inorganic membrane use was presented by Saracco et They listed the main drawbacks as the high cost of membranes, low permeability, defects in permselective layers, instability of membranes and catalysts, and sealing. They were not optimistic for the future of the CNMR or CMR, and considered circumventing equilibrium in a PBMR to be most promising, if membranes with good characteristics become available. 

Strained in two dimensions (the % and y axes). If the membrane is a tube, fixed to a header at one end and free floating at the other, then it is also effectively constrained in two dimensions (the % and y axes). This is shown schematically in Fig. 5.3. Even worse would be a tubular membrane that is fixed at both ends to headers, resulting in a membrane that is constrained in three dimensions. Surprisingly, the latter is a design that is still often used in laboratory test-and-evalua-tion cells using tubular metal membranes (usually a thin permselective layer deposited onto a porous tube). 

For a well designed membrane module of reasonably large scale, the initial cost should be dominated by the cost of the palladium content of the thin metal membrane (assuming a palladium alloy comprises the permselective layer). The module itself will be made of steel, most likely a nickel alloy, with or without significant chromium addition. The cost of the steel and the assembly labor should not exceed the cost of the palladium alloy membrane. 

For laboratory research, various types of membranes mentioned earlier are used without any support in PV cells. For large-scale applications, where the membrane sizes are larger, the reinforcement of this top skin layer by an appropriate support is required to maintain dimensional stability. These types of membranes consisting of a skin layer (permselective layer) supported by a suitable support are called composite membranes. Ceramic-supported membranes already have a support, and hence, no additional support is required. 

Asynunetric membranes are much more common. These membranes are made of two layers, a thin (0.1 -1.0 ]un thick) permselective layer, supported by... 

Electrical resistance for layer membrane, electrolyte or layer j (i = m, e, j) Membrane permselectivity to ion i Membrane surface Thermodynamic temperature... 

Gas transport through SPPO-PPO composite membranes, depended on the nature of the solvents for the top layer coating solutions. Permeance was higher when DMA was used as the solvent. This effect was thought to be due to the formation of a thinner coated layer of SPPO when a DMA solution was used. For both kinds of membranes permselectivities of 6-8 for O2/N2 gas mixture and about 100 for CO2/N2 mixture were observed. The ionic character of SPPO was believed to have enhanced the CO2 permeance. 

Most solution-cast composite membranes are prepared by a technique pioneered at UOP (35). In this technique, a polymer solution is cast directly onto the microporous support film. The support film must be clean, defect-free, and very finely microporous, to prevent penetration of the coating solution into the pores. If these conditions are met, the support can be coated with a Hquid layer 50—100 p.m thick, which after evaporation leaves a thin permselective film, 0.5—2 pm thick. This technique was used to form the Monsanto Prism gas separation membranes (6) and at Membrane Technology and Research to form pervaporation and organic vapor—air separation membranes (36,37) (Fig. 16). 

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. 

---