Permeation rates, various gases

Beside the partial pressure differences between the various gas components, the total pressure on both sides of the membrane is also important. Though mean total pressure does not directly affect the permeation rate in the case of a Knudsen diffusion mechanism, it governs the gas flow through... 

Although these two expressions have the same form, the coefficients are different. In particular, the solubility constant varies much more between various gas solutes than does the diffusion coefficient D. This implies that polymer membranes tend to be much more selective in separation various gas species than porous membranes. Unfortunately, diffusion coefficients in solids and liquids are much smaller than for gases so this increase in selectivity is often traded off with lower permeation rates. 

In a conventional isothermal plug-flow reactor (PFR), two important rates govern its performance - the rate of reaction and the rate of reactant feed per catalyst volume to the reactor. The ratio of these gives the Damkohler number, = (reactor volume)(maximum reaction rate per volume)/(inlet flow rate), which also involves reactor tube dimensions. The membrane reactor brings in at least one additional rate, the permeation rate of the fastest gas. The ratio of these has been labelled differently by various authors we will follow Bernstein and Lund and term it the Damkohler-Peclet product, DaPe = (maximum reaction rate per volume)/(maximum permeation rate per volume). For proper perfor-... 

The estimation of the transmission rates for gases other than helium can also be obtained, provided a preliminary calibration is carried out. This is particularly important for water, which is the main gas permeating through the barrier skin. To run the calibration procedure, the helium transmission rate is measured for various samples having different and known permeation rates for water. The linear correlation between helium and water transmission rates is then established, as shown in Figure 4.8. 

Both CA and polyimides are susceptible to plasticization by CO2 [25-27]. When CO2 solnbihzes in the polymer matrix the polymer chains become more flexible and allow faster transport of CO2 molecules. If other gas molecules such as methane or nitrogen are present they also increase their transport rate often even more so than CO2. This means that mixed gas measurements often generate lower a than pure gas. Also, if pure gas measurements are done then methane is usually measured before CO2 since the polymer remembers exposure to CO2 and needs time to relax the extra free volume that has been created. Various studies have documented the response of CA permeation rates to tem-peratnre, pressure and CO2 concentration [28-31]. When pressure is increased more CO2 solubilizes in the polymer matrix increasing this plasticization. Going up in temperature also softens the polymer and causes loss of a. In addition, common heavy hydrocarbons in gas fields such as hexane or toluene can negatively impact performance of polyimides in treating natural gas [32-34]. 

A linear pressure dependence of the permeation rate is indicated by the results of various observers (24,30,28,31,32). It was immaterial whether the gas was monatomic (He) or diatomic (Hg, Og, Ng), the same linear relation was obtained. There is one contrary result (33), data on the velocity of permeation of helium through pyrex, soda, lead, and Jena 16 ii glasses being reported as conforming to an equation dpjdt = ccp. The values of n were given as follows ... 

Figure 6.20. Contribution of various flow mechanisms to the total gas permeation rate. (Reproduced from [236] with permission.)...

There are substantial differences in the rates at which water vapor and other gases can permeate different plastics. For instance, PE is a good barrier for moisture or water vapor, but other gases can permeate it rather readily. Nylon, on the other hand, is a poor barrier to water vapor but a good one to other vapors. The permeability of plastic films is reported in various units, often in grams or cubic centimeters of gas per 100 in.2 per mil of thickness (0.001 in.) of film per twenty-four hours. The transmission rates are influenced by such different factors, as pressure and temperature differentials on opposite sides of the film. 

Test leaks (also known as standard leaks or reference teaks) normally comprise a gas supply, a choke with a defined conductance value, and a valve. The configuration will be in accordance with the test teak rate required. Figure 5.9 shows various test leaks. Permeation teaks are usually used for leak rates of 10 ° < < 10, capillaries, between 10 and 10 ... 

Our main concern here is to present the mass transfer enhancement in several rate-controlled separation processes and how they are affected by the flow instabilities. These processes include membrane processes of reverse osmosis, ultra/microfiltration, gas permeation, and chromatography. In the following section, the different types of flow instabilities are classified and discussed. The axial dispersion in curved tubes is also discussed to understand the dispersion in the biological systems and radial mass transport in the chromatographic columns. Several experimental and theoretical studies have been reported on dispersion of solute in curved and coiled tubes under various laminar Newtonian and non-Newtonian flow conditions. The prior literature on dispersion in the laminar flow of Newtonian and non-Newtonian fluids through... 

A gas identification set includes containers of small quantities of various gases of known concentration that are used as standards against which to calibrate analytical equipment. Permeation devices contain small containers of compressed or liquefied gases that, on opening, pass through a gas permeable membrane at a known rate and constant temperature. They are also used for calibration. 

Integration of Eq. (61.1) for the desired geometry and boundary conditions yields the total rate of permeation of the penetrant gas through the polymer membrane. Integration of Eq. (61.2) yields information on the temporal evolution of the penetrant concentration profile in the polymer. Equation (61.2) requires the specification of the initial and boundary conditions of interest. The above relations apply to homogeneous and isotropic polymers. Crank [3] has described various techniques of solving Pick s equations for different membrane geometries and botmdary conditions, for constant and variable diffusion coefficients, and for both transient and steady-state transport. 

Swelling or sorption test of liquids and vapours through the rubber is obtained by the standard method of ASTM D 814 and ISO 6179 2010 as a general method for various types of rubber. Other standard methods to test liquid and water vapour transmission rate are displayed in Table 27.1. Gas permeation test is obtained by the standard method of ASTM D 1434-82 and ISO 2782 as a general method, especially for oxygen gas mostly using ASTM D 3985 as a protocol for measurement. Table 27.2 exhibits the standard methods of gas permeability test. 

Since different compounds permeate the membranes with different rates, spectral interference can be reduced [60], The membrane materials, used for that purpose, include polypropylene, poly(tetrafluoroethylene), cellulose, silicone rubber, dimethylvinyl silicone, polyethylene, and zeolite. The membrane can be interfaced to the mass spectrometer in different configurations - directly or with the aid of a sweep gas. Using MIMS, the sampling probe can be taken away from the mass spectrometer to which it is connected by a tube. Therefore, sampling of various environmental matrices can be conducted in situ. In fact, portable MIMS systems can be used as monitors, providing vital information which is not offered by other technologies (e.g., fluorescence, infrared spectroscopy) [64]. 

Other studies of the diffusion of nascent hydrogen have given empirical relations between the rate of passage through the metal and the concentration of the acid generating the gas at the surface of the metal (55) and for the influence of various capillary active substances (i05) upon the velocity of permeation. For example, the addition of mercuric chloride caused an acceleration in the rate of permeation. This accelera-... 

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