Permeation inorganic membranes

Some bead materials possess porous structure and, therefore, have very high surface to volume ratio. The examples include silica-gel, controlled pore glass, and zeolite beads. These inorganic materials are made use of to design gas sensors. Indicators are usually adsorbed on the surface and the beads are then dispersed in a permeation-selective membrane (usually silicone rubbers). Such sensors possess high sensitivity to oxygen and a fast response in the gas phase but can be rather slow in the aqueous phase since the gas contained in the pores needs to be exchanged. Porous polymeric materials are rarer and have not been used so far in optical nanosensors. 

The size of the crystallites forming the network of membrane pores and the porous nature of the network affect the permeation and separation properties of porous inorganic membranes. These features will be discussed below. 

Finally it should be noted that inorganic membranes suitable for separation by multilayer diffusion and capillary condensation are also appropriate for performing pervaporation (a technique in which the feed is liquid and the permeate is gas) and distillation at reduced pressure (where gases with overlapping condensation regions are separated). 

Another very important operating characteristics of inorganic membranes that is not shown in Table 1.4 has to do with the phenomena of fouling and concentration polarization. Concentration polarization is the accumulation of the solutes, molecules or particles retained or rejected by the membrane near its surface. It is deleterious to the purity of the product and the decline of the permeate flux. Fouling is generally believed to occur when the adsorption of the rejected componcni(s) on the membrane surface is strong enough to cause deposition. How to maintain a clean membrane surface so that... 

Many composite inorganic membranes have been prepared where a layer of dense separating membrane is deposited onto a porous support. The deposited layer is typically thin to avoid significant reduction in the permeation rate. 

The openness (e.g., volume fraction) and the nature of the pores affect the permeability and permselectivity of porous inorganic membranes. Porosity data can be derived from mercury porosimetry information. Membranes with higher porosities possess more open porous structure, thus generally leading to higher permeation rates for the same pore size. Porous inorganic membranes, particularly ceramic membranes, have a porosity... 

Elcctrokinctic phenomena such as the zeta potential and streaming potential can alter the surface properties of many porous inorganic membranes. The electrochemical properties of membrane surfaces can exert profound influence on the nature and magnitude of the interactions between the membrane and the liquid feed, thus affecting the permeating fluxes of the solvent and solute (or macromolecule/panicle) through the membrane pores. 

Four parameters related to the membrane, feed stream and operating conditions determine the technical as well as economic performance of an inorganic membrane system. They are the transmembrane flux, permselectivity, maintenance of the permeating flux and permselectivity over time and stability toward the applications environment These parameters are the primary considerations for all aspects of the membrane system design, ai lication, and operation. 

Separation of lactic and propionic acids. The lactose fraction in the sweet whey permeate from cheese whey ultrafiltration can be fermented to produce lactic acid. In conjunction with the fermentation step, inorganic membranes have been tested in a continuous process to separate the lactic acid. This approach improves the productivity and reduces energy consumption compared to a conventional fermentation process. In addition, it produces a cell-free product. In a conventional process, some cells, although immobilized, are often detached and released to the product Zirconia membranes with a MWCX) of 20,000 daltons were operated at 42 C and a crossflow velocity of 2-5 m/s for this purpose [Boyaval et al., 1987]. The resulting permeate flux is 12-16 L/hr-m. To... 

The above discussion applies to low level activity liquid wastes. Inorganic membranes have also been tested for treating medium level liquid wastes containing approximately 100 times the activity of the aforementioned low level wastes [Gutman et al., 1986]. The same type of zirconia membranes with a MWGO of 10,000 daltons used in treating low level wastes docs not perform as well (in terms of the permeate flux and decontamination factor), but can function acceptably when processing medium level wastes. The problem is particularly pronounced if there are some defects in the... 

Yet another unique class of inorganic membrane materials called pillared clay and carbon composite membranes has been studied for gas separation [Zhu et al., 1994]. The permeation rates of benzene, chlorobenzene and 1,3-dichlorobenzene vapors through these membranes can be different by orders of magnitude as indicated earlier. This may open the door for these types of membranes for separating organic mixtures. 

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