Perm-selectivity, membrane separation

The membrane perm-selectivity (y.m) is defined as the ratio between the actual and theoretical transfer of counterions through any IEM. It can be simply determined as the percentage ratio between the experimental and theoretical Donnan potential differences as measured using a test system consisting of two cells provided with calomel electrodes and filled with well-mixed standardized aqueous solutions of KC1 (at 0.1 and 0.5 kmol/m3), kept at 25 °C, and separated by the IEM sample under testing. 

Collings, C.W., Huff, G.A. and Bartels, J.V. (January 2004) Separation Processes Using Solid Perm-Selective Membranes in Multiple Groups for Simultaneous Recovery of Specified Products from a Fluid Mixture. US Patent Appl. Publ. 20040004040 Al. 

Regenesys uses DuPont s Nafion (Section 6.1.7) as the perm-selective sodium ion transfer membrane, separating the two half cells. Figure 2.1. Diffusion of sodium ions in the concentration difference across the Nafion membrane is one of the irreversibilities of the system. The low-cost plastic (e.g. polyethylene) tanks and pipework are treated with fluorine to provide bromine resistance, and are able to operate with, and contain, both electrolytes at ambient temperature. 

The effects of operating conditions on separation [119] performance were studied. POMS membrane was more perm-selective to the aroma compound than PDMS. /3 higher for ETH than for ETB... 

The experimental procedures are quite similar to and often confused with pervaporation. The main difference between VMD and pervaporation is the nature of the membrane used, which plays an important role in the separations. While VMD uses a porous hydrophobic membrane and the degree of separation is determined by vapor-liquid equilibrium conditions at the membrane-solution interface, pervaporation uses a dense membrane and the separation is based on the relative perm-selectivity and the diffusivity of each component in the membrane material. 

A number of methods of producing chlorine dioxide electrochemically using perm selective membranes are found in the patent literature. Harke et. aL produced chlorine dioxide in a three compartment electrolytic cell [56]. A buffer compartment located between the anode and cathode compartments was separated from the cathode side by a cation exchange membrane and from the anode side by an am on exchange membrane. Hydrogen chloride was fed to the anode compartment, aqueous alkali metal chlorate and chloride to the buffer compartment, and water to the cathode. Chlorine and chlorine dioxide are taken off from the anode compartment while hydrogen and alkali metal hydroxide are removed from the cathode compartment. 

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). 

Instead, the enzyme is separated from the sample by a perm-selective membrane that allows the analyte of interest to enter an internal solution. This internal solution contains all the reagents required for the analytical reaction, including the enzyme. Figure 9 is a schematic diagram of an internal enzyme biosensor with a fiber-optic detection scheme. As the sample enters the enzyme-containing internal solution, the analytical reaction takes place. The rate of this reaction is monitored and related to the concentration of the analyte in the sample solution. 

Matrix Functionality Template Prepa- ration Membrane thickness (pm) Separation by Source cone. (mmol/L) Solvent Flux (nmol/ cm h) Perm- selectivity Adsorption selectivity qualitatively Reference... 

Among all types of membranes, the inorganic dense membranes have attracted the interest of many researchers due to their capacity to separate completely a product from gaseous mixtures [19]. In particular, the dense palladium membranes are used because of their complete hydrogen perm-selectivity. In the last years, the increasing interest towards this type of membranes is, also, due to hydrogen application as energy carrier. 

Two-bed membrane reactor for integrated process involving aqueous-phase glycerol reforming to synthesis gas coupled with DME synthesis process combines two physically separated enclosures (Figure 5.13). The first unit is a tube-in-tube fixed-bed water perm-selective membrane reactor for DME synthesis process. The reaction-side compartment (outer tube) is packed with bifunctional catalytic particles for DME synthesis, whereas the permeate-side compartment (inner tube) is an empty... 

The need for the membrane reactors also primarily stems from the equilibrium conversion limitation of a reversible reaction. These reactors are used when the boiling point differences are not sufficient to use distillation column (combo) reactors. The thermal sensitivity of the reactive domain may inhibit the use of boiling point differences for the product separation, hence the use of distillation column reactors. A perm selective membrane can be used within the reactor providing product separation. The other advantage of the membrane reactors is the possibility they offer to run the reaction either in the gas or in the liquid phase, thus... 

The perm-selectivity of a membrane toward a mixture is generally expressed by one of two parameters the separation factor and retention. The separation factor is defined by... 

It is known that zeolite membranes essentially contain intercrystalline non-zeolitic pores (defects). This irregular nature of zeolite membranes with intercrystalline pores adds to the complexity of the transport process in addition to the contribution of a support layer to the permeation resistance. For zeolite membranes, selectivity similar to that expected for Knudsen flow generally indicates the presence of intercrystalline pores. Separation based primarily on adsorption differences, which is generally true in the separation of liquid mixtures by pervaporation, may have tolerance to the intercrystalline pores. However, in order to obtain high perm-selectivity, the zeolite membranes must have negligible amounts of intercrystalline pores and pinholes of larger than 2nm so as to reduce the gas flux from these defects [3]. 

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