Membrane permeation definition

In order to link concentration polarization and membrane selectivity, Zhang et alP used the permeating fluxes in the definition of the coefficient. In the case where only one component is able to permeate through the membrane, this definition is very useful, because it does not depend on the particular permeation mechanism and, thus, is general. However, this approach is valid only for a binary mixture and cannot simply be generalized for a multi-component mixture. Additionally, the overall coefficient varies between zero and infinity, therefore not allowing an immediate perception of the polarization level. 

First we will illustrate the minimum energy required to separate a small amount of mixture for the following processes evaporation of water from a saline solution recovery of water by reverse osmosis separation of an ideal binary gas mixture by membrane permeation. Then we will consider the definition of net work consumption for thermally driven processes. Next we will consider a variety of separation processes vis-k-vis their minimum energy requirement for separation. 

For gas and vapor systems, by combining the laws of sorption and diffusion in the sequence (l)-(3), general permeation equations are obtained. For sheet membrane samples of polymers above Tg, if the definition is made that permeation coefficient Q = Ds,... 

Modules and Housings Modern gas membranes are packaged either as hollow-fiber bundles or as spiral-wound modules. The former uses extruded hollow fibers. Tube-side feed is preferable, but it is limited to about 1.5 MPa. Higher-pressure applications are usually fed on the shell side. A large industrial permeator contains fibers 400 pm by 200 pm i.d. in a 6-inch shell 10 feet long. Flat-sheet membrane is wound into spirals, with an 8- by 36-inch permeator containing 25 of membrane. Both types of module are similar to those illustrated in Background and Definitions. Spiral modules are useful when feed... 

The ion-selective membrane is the key component of all potentiometric ion sensors. It establishes the preference with which the sensor responds to the ion of interest in the presence of various other ionic components of the sample. By definition, the ion-selective membrane forms a nonpolarized interface with the solution. If the interface is permeable to only one ion, the potential difference at that interface is expressed by the Nernst equation (6.6). If more than one ion can permeate, the interface can be anything between the liquid junction and the mixed potential. The key distinguishing feature is the absolute magnitude of the total exchange current density. 

Many controlled release devices are not membranes by the conventional definition, since only transient release of an active agent, without permeation occurring between an upstream and a downstream, is typical. Nevertheless, some controlled release units do operate with a concentration driving force to achieve effectively steady state release from the internal reservoir of the device to the external surrounding. Such processes are included here for completeness. 

Monolith Ceramic membranes are usually monoliths of tubnlar c illaries (Fig. 22-52), although one supplier has square passages, channel sizes are in the millimeter range. By strict definition, a monolith becomes a module by attaching end fittings and a means of permeate collection. In practice, many monoliths are nsnally incorporated into one modnlar housing. 

In the characterization of porous membranes by liquid or gaseous permeation methods, the interpretation of data by the hyperbolic model can be of interest even if the parabolic model is accepted to yield excellent results for the estimation of the diffusion coefficients in most experiments. This type of model is currently applied for the time-lag method, which is mostly used to estimate the diffusion coefficients of dense polymer membranes in this case, the porosity definition can be compared to an equivalent free volume of the polymer [4.88, 4.89]. 

This abstract definition will be explained with the actual example of gaseous permeation through a zeolite/alumina composite membrane. Here, we must investigate the effect of the five following factors on the rate of permeation the temperature (T) when the domain is between 200 and 400 °C, the trans-membrane pressure (Ap) when the domain is between 40 and 80 bar, the membrane porosity (s) ranging from 0.08 to 0.18 m /m, the zeolite concentration within the porous structure (c ) from 0.01 to 0.08 kg/kg and the molecular weight of the permeated gas (M) which is between 16 and 48 kg/kmol. With respect to the first... 

Recent studies based on comparison between gel permeation chromatography and ultra/micro-filtration [119] have shown that whatever the chemical nature and shape of the model macromolecule used, it is possible to predict the cut-off value of a membrane by considering the hydrodynamic volume of the macromolecule. This parameter provides an appropriate definition of the effective solute size to be considered in hydrod)mamic models. 

A plot of Ft vs P for defect-free membranes which are definitely in the transition region )delds a curve with a certain slope, which intersects the permeation axis. This is shown in Fig. 9.5. 

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