Gas permeation rate

Poly(vinyl chloride). To be converted into film, poly(viayl chloride) [9002-86-22] (PVC) must be modified with heat stabilizers and plasticizers, which increase costs. Plasticized PVC film is highly transparent and soft, with a very high gas-permeation rate. Water-vapor transmission rate is relatively low. At present, PVC film is produced by blown-film extmsion, although casting and calendering are employed for heavier gauges (see Vinyl POLYAffiRS). 

The question of relative rates of seal leakage and permeation (cf. points 1 and 3) has been considered by MERE. For the sealing of HPHT (5000 psi, ca. 345 bar, or 34.5 MPa 100°C) fluids with chevron seal-stack systems used at the bottom of oil wells in intermittently dynamic conditions, high-pressure permeation and seal-leakage tests using the equipment outlined in Section 23.3.1.2 have been conducted it has been shown that methane gas permeation rate was only ca. 1 % of the rate... 

To verify the membrane integrity prior to attempting separations, pure gas permeation rates for nitrogen and helium were determined and compared to the vendor s data supplied with the membrane. Figure 4 and Table V verify the vendor s data reasonably well for the only membrane which survived shipment and startup. The agreement of the nitrogen values is particularly indicative of the membrane s integrity. 

The separation efficiency for a given membrane with a particular binary gas mixture will be dependent mainly upon three factors gas composition, the pressure ratio between feed and permeate gas, and the sepration factor for the two components. A higher separation factor gives a more selective membrane, resulting in a greater separation efficiency. This parameter is a function of the membrane material and is determined by the individual gas permeation rates. 

On the contrary, the viscosity of gas at low density increases with increasing temperature. Therefore, as the temperature rises, the gas permeation rate becomes lower. 

Since the product Dk is known from the steady state rate of permeation, kp can also be obtained. This time lag method is the basis of most of the gas and some of the vapor transport studies made today. Little application of the time lag method was made until Barter introduced the use of vacuum on the downstream side of the membrane and measured the gas permeation rate by monitoring the increase in pres-arre in a fixed downstream receiving volume Recently the original isobaric method has been reintroduced in a number of commercial permeability instruments. 

Apparent porosity is very important in connection with flux corrosion and with thermal insulation which may be unfavourably affected by permeation of hot gases. The total volume of open pores can be determined, for example, by measuring the amount of absorbed water, but this value provides no information on pore size which is important with respect to these properties. The permeability for gases is therefore measured. The test specimen is fixed in a special vessel so that it allows a gas or air to be driven through. The gas permeation rate and the pressure difference are measured. The permeability coefficient K is then given by the equation... 

One approach that has been used is to compare the densities of two polymers and infer that the denser polymer has a lesser amount of free volume and thus lower gas permeation rates (9,10). This approach, however, has been abused in that it has been used to compare the free volumes of structurally dissimilar polymers. 

Threa types of laboratory apparatus are in common use for the measurement of gas permeation rates through polymers to obtain dela for anelyses such as those discussed previously S... 

Eor tubular membranes, the permeation flux measurements are generally carried out by a shell-and-tube arrangement as shown in Fig. 13.2 [7]. Since the membrane is plated on the outside of the porous substrate, the feed gas flows upward through the outside of the membrane (shell side), and the permeate gas is collected on the tube side. A purge gas can also be used on the tube side if desired. The upstream pressure is monitored by a capacitance pressure transducer and the permeate side pressure is kept atmospheric. The gas permeation rate (the volumetric flow rate) is measured in the permeate side at atmospheric pressure and room temperature. For mixture experiments, the permeate and retentate can be analyzed by an online gas chromatograph equipped with appropriate detectors, such as Thermal Conductivity Detector (TCD), Flame Ionization Detector (FID) or others, depending on the specific gas mixture... 

The knowledge of tortuosity factor of the mlcroporous hollow fibers used In this study Is Important for characterizing the gas permeation rate through the Immobilized liquid membrane. Since It Is not available, an Independent estimate was developed. We first consider these results. 

The LCPs retain 70% of their notched Izod impact values down to a temperature of about —270°C (—450°F). They are also resistant to bleaches, chlorinated organic solvents and alcohols. They have extremely low gas-permeation rates compared to commercial packaging films. Their ionic extractables are well below those needed for corrosion-free environments for integrated circuit chip apphcations. The... 

Approximately 200 ppm residual methanol leaves the scrubber and enters the separator train. As indicated in Fig. 20.2-3, the major feed components consist of Hj, CO2, CO, CH4, and Nj with a saturated vapor pressure of water pesent from the scmbbing operation. The stream is preheated prior to entering the separator train to prevent depression of the product gas permeation rate. This preheating prevents complications arising from water condensation and other more subtle permeation effiects discussed later in the fundamentals section. Then the feed is split and fed in parallel to the series of separators shown in Fig. 20.2-3. 

AF, a copolymer of TFA and 2,2-bis-trifluoromethyl-4,5-difluoro-l,3dioxole, display the same chemical resistance as PTFE but is completely amorphous and with the lowest dielectric constant of 1.89 to 1.9 reported so far and unusually high gas permeation rates [750,751]. 

PTFE has a gas permeation rate of 420 cB (centi-Barrers) for oxygen and 1200 cB for carbon dioxide [750]. 

As long as Equation 5.317 is valid, the permeation rate can be given by Equation 5.320, and all the theories developed for the electric circuit become valid. It should be noted that Equation 5.320 is not always valid. As will be shown later, the gas permeation rate is not always related to the pressure difference in a linear fashion. Similarly, the permeation rate of a component of a liquid mixture is not necessarily linear to the concentration difference. Therefore, the validity of the resistance model suffers severe restrictions under those circumstances. Nevertheless, the model offers a practical guideline to the design of the composite membrane. It should also be noted that the model is valid when... 

The total gas permeation rate. 0, is the sum of gas phase flow, and the surface flow, and therefore. 

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

Alternatively, a method entailing surface coating with an alcohol solution of 60% B rank phenol formaldehyde resin (PFR) and dispersant, followed by recarbonization (700°C, 60 min), was used to prepare carbon-carbon composite membranes with improved separation performance by Liang and co-workers [80], On the carbon-caibon composite membrane coated with PFR alcohol solution and added dispersant, the gas permeation rates decreased by a factor of 10, and the ideal separation factor increased, probably due to the PFR s dispersing effect. 

The single gas permeation system is illustrated in Fig. 5.5. Pure O2 and pure N2 are introduced into the system at feed pressures of 1-5 bar at ambient temperature. The permeate side of membrane is maintained at atmospheric pressure. A soap film flow meter is used to measure the gas permeation rate. 

During the development of PEM fuel cells, many techniques have been devised to measure the gas permeation rate and/or hydrogen crossover rate of membranes. These techniques include ex situ and in situ methods. Among them, the in situ electrochemical technique is a direct and effective method to measure the hydrogen crossover rate of membranes in PEM fuel cells. 

Ideally, it is necessary to use mass-transfer equations in the feed-gas phase and the product-gas phase along with the permeation equation (3.4.72). However, in general, the gas permeation rate through a nonporous membrane is so slow that mass-transfer equations in the feed-gas and product-gas phases are not needed. Note that, for species i transport through the membrane from the feed to the product gas, pif> pip, but fy need not be greater than Pp, although in practice it generally is. In such a case, flux expression (3.4.72) may also be expressed as... 

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