Permeation Through a membrane

When two species are permeating through a membrane, the ratio of their fluxes can be written following Eqs. (22-97) and (22-98) as ... 

A further application of this technology will certainly be the fabrication of membranes of these materials. Membrane reactors have shown great utility in many systems, where one component of a reaction mixture can be separated by permeation through a membrane, thus driving a reaction forwards, by continuous separation. Such continuous processes can themselves save a great deal of waste. 

One of the newest commercial methods of sweetening gas is the use of membranes. This separation works on the principle that there are different rates of permeation through a membrane for different gases. 

The measurement of sorption, diffusion and permeability coefficients takes place as a rule using one of three methods sorption of the gases in the polymer, permeation through a membrane (film or sheet) into a sealed container or permeation through a membrane into a gas stream. As far as possible sorption methods should be used together with permeation methods that are specific for the measured gas/polymer system in order to uncover any possible anomalies or errors in the measurements by comparison of results. 

Flux through membrane as a function of pressure drop. The flow of permeate through a membrane may be considered as a microscopic form of cake filtration, where the solute that polarizes at the feed side of the membrane may be considered as the cake. In cake filtration (see Chapter 7), the volume of filtrate V that passes through the cake in time t can be solved from... 

Globally two methods to measure permeation through a membrane exist. One is a static method the other a continuous method Figure 12). 

More recently, Teramoto et al. [24-25] referred to the use of a novel facihtated transport membrane for gas separation in which a carrier was supphed to the feed side (high-pressure side) and it was forced to permeate through a membrane to the permeate side (low-pressure side), and then the permeated carrier solution was recirculated to the feed side. Since the membrane was always wet with the carrier solution, the membrane became very stable with no open or unfilled pores present which usually caused membrane unselectively in traditional SLM. This new type of membrane has been named a bulk flow liquid membrane (BFLM). The membrane resulted to be stable over a discontinuous one-month testing period. 

FIGURE 20,1-1 Terminology and coordinate system convention for one-dimensional permeation through a membrane of thickness f. 

Electrodialysis is a process by which ionic substances with molecular weight below several hundreds permeate through a membrane in the presence of an... 

A diffusive/passive sampler is a device which is capable of taking samples of gas or vapour pollutants from the atmosphere at a rate controlled by a physical process such as diffusion through a static air layer or permeation through a membrane, but which does not involve the active movement of air through the sampler (Brown, 1993 Moore, 1987). Compared with methods requiring pumps for active air movement, diffusive samplers are more convenient and have lower associated costs. 

The process of permeation through a membrane gives rise to a flux, or permeation. Commonly used units are mol s i and mL min = mb min (STP). In order to obtain (area specific) flux densities we divide by area and get, for eiample, units of mol s i cm 2 or mLn mim cm . ... 

PV combines the evaporation of volatile components with their permeation through a membrane. It involves the use of a Hquid feed to produce a vapour permeate and a hquid reject. The feed is usually suppHed at above 100°C and at a pressure sHghdy above 1 bar. The permeate pressure is maintained at vacuum well below atmospheric. Vapourisation occurs as the permeating species pass through the membranes. Mosdy asymmetric composite hydrophihc membranes such as composite poly vinyl alcohol (PVA)/PS (or polyacrylonitrile) are used [43,44]. Membranes made from a thin layer of PDMS cast on PAN have been found to be usefiil in separating polar compounds from non-polar ones in deal-coholisation of Hquors. Flat sheet and spiral wound modules are commonly used. 

Ultrafiltration Water permeation through a membrane— reverse osmosis. 

Now, we proceed to discuss a simple model for the kinetics of ion permeation through a membrane that may be used to deduct ion permeability from the experimentally accessible electric conductivity. 

Source Adapted from Kesting based on data from R. C. Binning, R. Lee, J. E Jennings, and E. C. Martin. Separation of Liquids by Permeation through a Membrane. Ind. Eng. Chem. 53 (1961), p. 45. 

For a pure component permeating through a membrane, it is possible to employ linear relations to describe transport However, when two or more components permeate simultaneously, such relations cannot be generally employed since coupling phenomena may occur in the fluxes and forces. These coupling phenomena can be described in tenns of e formalism of non-equilibrium theimodynamics. 

Probably the most used experimental technique is the one that involves solvent permeation measurements through brush-coated membranes [7,27,35]. The rate of solvent permeation through a membrane of cylindrical pores depends on the pore size and may be related to their radius by the Hagen-Poiseuille equahon ... 

Fig. 15.6 Schematic of methanol permeation through a membrane from reservoir compartment (A) to receiving compartment (B)...

The intercept of this line on the time axis is S lGDim, called the time lag in simple diffusive permeation through a membrane for constant diffusion coefficient in linear diffusion. The value of the time lag provides a good order of magnitude estimate of the time from f = 0 after which steady state permeation through the membrane may be assumed. Correspondingly, equations (4.3.24) and (4.3.25) for gas permeation in a device (see also Chapters 6, 7 and 8) are valid only after this initial unsteady state. Experimental measurement of the time lag allows one to measure the value of Knowing Qj from steady state experiments, one can determine Sjm-... 

In Sections 6.3.3.5 and 6.4.2.2, we described gas separation by permeation through a membrane under the conditions of bulk flow parallel to the force and having a well-mixed flow on both sides of the membrane. We learned that the configuration of bulk gas flow parallel to the force direction was not very useful it could not be used in practice with large membrane surface area, and it led to unsteady state... 

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