Measuring permeation rate

Attempts to relate diffusion rates, measured permeation rates, and partition coefficients led Miller to consider the distinct regions of the membrane as independent compartments (polar headgroup, hydrocarbon interior) [20]. Although these adjustments were advances, they still were not universally applicable. A more detailed understanding of lipid bilayer structure, obtained by a consideration of data from both MD simulations and experiment, helps us to understand why this is so. 

Then, the solubility ratio Hcham Hchaw can be calculated from the experimantally measured permeation rates of CH4 arid the viscosities of the aqueous membrane solutions using Equation 29. It should be noted that Hch4,m is not the Henry constant of CH4 for the aqueous amine solution but that for the aqueous membrane solution which is in contact with CO2 and contains unreacted amine and reaction products, i.e., carbamate ion and protonated amine. 

Figure 5. An illustration of two types of permeation behaviors from two different chemicals tested against the same glove material. The dotted line indicates the level from which the standardized breakthrough time is measured. Permeation rate is expressed in ng/cm /min. The arrows show the normalized breakthrough times in minutes. 

A number of methods can be used to measure permeation rate through polymers including fluoropoly-mers. These methods are helpful for comparison of different materials. The extent of the information obtained is limited due to the inability of these techniques... 

Three general test procedures used to measure the permeability of plastic films are the absolute pressure method, the isostatic method, and the quasi-isostatic method. The absolute pressure method (ASTM D 1434, Gas Transmission Rate of Plastic Film and Sheeting) is used when no gas other than the permeant in question is present. Between the two chambers a pressure differential provides the driving force for permeation. Here the change in pressure on the volume of the low-pressure chamber measures the permeation rate. 

Water Permeation of Nitrile Copolymers. While the AN content bears a direct relationship to the gas barrier, the water permeability presents quite an anomaly. If the water permeation of the commercial SAN films (25% AN) is measured, the rate is higher than that of polystyrene. Thus it appears that films with greater AN content have even higher water permeation rates. It was discovered, however, (I, 7) that as the AN content increases there is a shift in permeation, and the higher AN/S materials show water barriers of excellent quality. Table VIII... 

Then Q = Ds (Section 23.4.2), with diffusion coefficient D also measurable by absorption testing, so that Q can be estimated. Permeation rate is then obtainable for a particular sample shape at specified conditions by using the appropriate permeation equation. 

When testing, the permeation rate is initially measured as change in pressure (E2) in the low-pressure reservoir versus time, to give a plot of the same form as shown in Figure 23.10, but with pressure rather than volume being plotted. To convert to volume, the following expression (referring strictly to an ideal gas) is applied to obtain rate Aq/At from the slope of the plots ... 

When testing, the permeation equation shown for conical samples in Section 23.4.2 is used to provide permeation coefficient Q. Measured values showed that the more normal methods employed for high-pressure permeation testing at MERE and elsewhere, using a sample support sinter, are likely to give underestimates of true permeation rates by a factor of about three. 

The advantage of using the time lag method is that the partition coefficient K can be determined simultaneously. However, the accuracy of this approach may be limited if the membrane swells. With D determined by Eq. (12) and the steady-state permeation rate measured experimentally, K can be calculated by Eq. (10). In the case of a variable D(c ), equations have been derived for the time lag [6,7], However, this requires that the functional dependence of D on Ci be known. Details of this approach have been discussed by Meares [7], The characteristics of systems in which permeation occurs only by diffusion can be summarized as follows ... 

We measure the permeation rate of liquids through bottles by filling them with the liquid of interest and placing them in a controlled atmosphere chamber. At intervals we remove the bottles, weigh them and return them to the chamber. We repeat this procedure over a period of days, or even weeks, until their rate of weight loss reaches a steady value. We calculate the permeability factor from Eq. 8.10. 

Increase in polarity, polarizability, and hydrogen-bonding ability leads to a decrease in BBB permeation rate. Compound size as measured by the McGowan volume leads to an increase in the permeation rate. 

The composite materials have been used to form selective membranes for the separation of liquid mixtures [181]. The membranes should consist of a polymer which is soluble in the liquid components) to be separated, as the dispersed phase-derived polymer, and a continuous phase-derived polymer which is insoluble in all components of the liquid mixture. Thus, membranes consisting of polystyrene in polyacrylamide will separate toluene from cyclohexane, and those comprising polyacrylamide in crosslinked polystyrene can be used for water removal from ethanol. Due to the very thin films of polymer which separate the polyhedral dispersed phase cells, the permeation rates, which are measured by pervaporation, are relatively high. 

The value of /, PR, and the pure water permeation rate (PWP) for a given area of film surface (13.2 cm2 in this work) were determined under the specified experimental conditions. The data on PR and PWP are corrected for 25 °C. Concentrations of NaCl were determined by conductance measurement, whereas concentrations of organic solutes were determined by the Beckman total carbon analyzer model 915-A. 

Table IV presents the comparative data on the permeation rates of the three types of membranes with two different porosities for various aqueous organic solutions and for pure water as measured over the duration of the study. The data shown here represent the relative chronological order in which the samples were tested. In the beginning, even though the percent rejection of NaCl is high for PA and CAc (indicating small-size pores), the rates of permeation of pure water are higher for denser membranes than for membranes having lower percent rejection of NaCl. In the case of the PBI membrane, the reverse of this phenomenon is observed.

Pervaporation separation index (PSI), which is a measure of the separation ability of a membrane was defined by Huang and Yeom [7] and expressed as the product of separation factor and permeation rate. 

Deposition of reduced sulfur compounds to enclosed surfaces could also be measured by adding low loss (5 ng/min, GC Industries) permeation tubes to the inlet of the chambers. This permeation rate adds an additional sulfur load which varied typically between 2 and S times that already within the enclosure. These studies were limited to surfaces where large temporal changes in natural emissions were not occurring, and ultimately became limited by the reliability of the permeation devices. 

The most common sources of error for permeation measurements are leaks in the film between the two test chambers. Measurements at two or more different pressures in the first chamber can help detect leaks. If for example the permeation rate... 

When measuring permeation into a gas stream the tests should be repeated at several different gas flow rates to detect possible influences of mass transport in the gas phase. Such influences are negligible if the values for P remain constant below a certain gas flow rate. 

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