Vapour permeation measurements

More careful analysis of the permeation curves show that the two vapours actually display completely different behaviour. The tangent method can be applied without problems to the DCM permeation curve, resulting in a time lag of ca. 400 s. hi contrast, this method shows that methanol has an unusually wide transient period. While the extrapo- 

In this light it is too simplistic to assume that the three time lags are related to the presence of single, dimeric and trimeric species, each with its own characteristic diffusion 

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In this work preliminary vapour permeation measurements were carried out with two different species, the rather bulky dichloromethane (DCM) molecules and the much smaller methanol molecules. Two typical permeation curves are displayed in Figure 4.8. The transport parameters, determined on the basis of the tangent method and Equations (4.9)-(4.11), are listed in Table 4.3. It contains the parameters dehned above as well as solubility C in the membrane in equilibrium with the feed pressure of penetrants. 

Permeability and solubility measurements Permeability of the free films for water vapour was measured by means of the wet cup method (15). Oxygen permeability was measured using the Polymer Permeation Analyser of Dohrmann Envlrotech (16,17). Results are summarized In Table I. 

Permeability is so low that, in sorption experiments, large pressure differences and membranes of small thickness have to be employed. These restraints do not apply to condensable vapours or highly permeant gas. In the last case, relaxation processes in the polymer matrix cause changes in the transport behaviour and make the permeability time-dependent. In contrast, only vapours of sufficiently high solubiUty are suitable for the gravimetric measurements used in the sorption experiments. Consequently, there are only few systems for which both sorption and permeation results have been reported. Low et al. conducted the two types of moisture transport experiments on a polyamide 6/elay nanocomposite. The authors found that the activation energy of moisture permeation obtained from the sorption experiment is lower than that derived from the permeation measurement. They concluded that the interaction and contribution of the diffusion and solubility parameters show complex transport behaviour in these nanocomposite films. 

In order to ensure accurate and reproducible results, a Vapour Permeation System (VPS) was constructed for the measurement of permeation rates of methane and water vapor in a mixture through PPO membranes and determination of water/methane selectivity. The schematic diagram and the components of the equipment are illustrated in Figure 1. The system is a sweep gas system that is used to measure the permeability of water and methane in a mixture. Using films of proper thickness, sweep gas as well as purging the sample line and sample loop prior to sample collection ensures accurate and reproducible data. 

Vapour-pressure-dependent up to svp, supplying the same data as in (1) per test tests likely to be medium-term to give sufficient permeant for reliable measurement. Can reduce vapor pressure (e.g., by using a carrier gas) on low-pressure side to maximize permeation by keeping P2 near-zero. 

Further important information about these products comes from measurements of their molar masses. Of the available methods, vapour pressure osmometry (VPO) and gel permeation chromatography (GPC) are the most widely used, but, because of its sensitivity to low-molar-mass components, VPO tends to yield results which are too low. In contrast, GPC provides a detailed picture of the molar mass distribution. 

No polymeric material forms an impervious barrier to gas or vapor molecules. The transmission of such molecules through a membrane is a result of intermolccular spaces, pinhole defects, or porosity, or some combination of these three structural features of the material. In this chapter we are restricting the term permeation to the movement of gas or vapour molecules through molecular scale voids. The quantity of permeant is usually measured as the volume at STP for a gas, or the mass in the case of a vapor. 

Measurements were carried out in the time lag mode, starting to record the permeate pressure as a function of time as soon as the membrane was exposed to the vapour. 

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. 

Following the same procedure, these authors incorporated a-TOC, BHT and fert-bufylhydroquinone (TBHQ) into PLA films at concentrations ranging from 0 to 10%. DSC results showed that the polymer glass transition temperature and the crystallinity slightly decreased with the increase of the antioxidant content. Permeability towards water vapour showed a decrease in the wettability of the prepared materials with the increase of the antioxidants content. Similar results were obtained for gas permeation. Barrier improvements could not be measured at high concentrations due to phase separation. 

The number average molecular weight (MJ was measured by vapour pressure osmometer weight average molecular weight (M ) was measured by gel permeation chromatography ... 

An important effect making the MD process different from traditional heat exchanges, the temperature polarisation and concentration polarisation occur in the membrane wall due to the transfer of both water vapour and latent heat. As previously stated, the heat and mass transfer across the membrane move from the hot feed stream to the cold permeate one. The temperature gradients cause a difference in temperature between the Uquid-vapour interfaces and the bulk temperatures on both sides of the membrane. This effect, in membrane science called temperature polarisation, reduces the water vapour flux and in literature it is measured by the so-called temperature polarisation coefficient (t), given by ... 

Table 7.5b. Permeability coefficient P for the permeation of water vapour (quantity of diffused water vapour measured in mass units) in HDPE pipes...

Since dimethylmercury is a supertoxic chemical that can quickly permeate common latex gloves and form a toxic vapour after a spill, its synthesis, transportation, and use by scientists should be kept to a minimum, and it should be handled only with extreme caution and with the use of rigorous protective measures . 

Measurement of water-vapour diffusion, solubility and permeability requires special techniques and care because of the unique nature of water. A number of techniques have been developed for the routine measurement of the water-vapour permeability of polymer membranes. " More accurate measurements may be obtained by using the techniques described for gases however, care must be taken to avoid the possibility of water sorbing or desorbing on to or from glass surfaces. With due caution, steady-state permeation rates may be measured in this fashion, but permeation lag-times are unreliable. 

A polymeric system is, thus, characterized by three transport coefficients, which are the permeability, the solubility, and the diffusion coefficients. The permeability coefficient, P, indicates the rate at which a permeant traverses polymer film. The solubility coefficient, S, is a measure of the amount of permeant sorbed by the polymer when equilibrated with a given pressure of gas or vapour at a particular temperature. Finally, the diffusion coefficient, D, indicates how fast a penetrant is transported through the polymer system. For steady state permeation of simple gases into a homogeneous film, the permeability coefficient, P, can be written as the product of diffusion coefficient D and solubility S (Equation 11.2) ... 

One very common method of measuring the rate of water permeation through a film is cup method which is the gravimetric method described by Fu et al. [22]. The membrane to be tested is placed and fixed over a standard cup, at least half filled with water. The cup is placed in a chamber in which air at constant relative humidity is circulated at a constant temperature. This method measures water permeation rate at a high water activity on the feed side of the membrane since the atmosphere is saturated with water vapour. The VPS described above provides more flexibility in the feed water vapour composition and allows for the measurement of water transport in a mixture. 

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