Vapor permeation

In CPA-rich blends, both the enthalpy and entropy of mixing are positive and, as a result, the Gibbs free energy of mixing (which is the difference between a relatively large enthalpy term and a smaller entropy term) would also become positive. The process may be similar to the isothermal fusion of CPA crystallites present in its structure under the influence of chitosan, which acts as a macromolecular solvent in the blend. The need for CPA fusion is an obstacle to its compatibility with chitosan, and the blends would tend to phase-separate, at least potentially. 

---

Gases and vapors permeate FEP resin at a rate that is considerably lower than that of most plastics. Because FEP resins are melt processed, they are void-free and permeation occurs only by molecular diffusion. Variation in crystallinity and density is limited, except in unusual melt-processing conditions. 

FIG. 25-19 Pervaporation of gas from liquid feed across membrane to vaporous permeate. (SOURCE Redrawn from Ref. 24.)... 

Vapor Permeation Vapor permeation is similar to vapor perva-poration except that the feed stream for permeation is a gas. The futnre commercial viability of this process is based npon energy and capital costs savings derived from the feed already being in the vapor-phase, as in fractional distillation, so no additional heat inpnt wonld be req iired. Its foreseen application areas wonld be the organics recov-eiy from solvent-laden vapors and pollntion treatment. One commercial nnit was installed in Germany in 1989 (Ref. 26). 

The most common membrane systems are driven by pressure. The essence of a pressure-driven membrane process is to selectively permeate one or more species through the membrane. The stream retained at the high pressure side is called the retentate while that transported to the low pressure side is denoted by the permeate (Fig. 11.1). Pressure-driven membrane systems include microfiltration, ultrafiltration, reverse osmosis, pervaporation and gas/vapor permeation. Table ll.l summarizes the main features and applications of these systems. 

Vapor permeation Vapor Vapor Less than saturation pressure of feed (permeate is under vacuum) Nonporous Volatile organic compounds... 

Three related methods based on the quasiisostatic method are used to measure permeability. The most commonly used technique allows the permeant gas or vapor to flow continuously through one chamber of the permeability cell. The gas or vapor permeates through the sample and is accumulated in the lower-concentration chamber. At predetermined time intervals, aliquots are withdrawn from the lower cell chamber for analysis. The total quantity of accumulated permeant is then determined and plotted as a function of time. The slope of the linear portion of the transmission-rate profile is related to the sample s permeability. 

The range of both gas and vapor permeation rates of polymers is well known (13). Table IV lists some typical commercial polymers, along... 

Zeolite A is a very successful membrane for separation of water from alcohols, but it suffers from stability issues under acid conditions [23]. Usually, a Hquid phase should be avoided and, for this reason, vapor permeation is preferred. Recent developments show that the hydrophilic MOR [23] and PHI [50] membranes are more stable under acidic conditions in combination with a good membrane performance. 

Figure 12.37 shows a flowsheet for the separation of an azeotropic mixture using a membrane, but this time using vapor permeation. The mixture is first distilled to approach the azeotrope using a distillation column with... 

Figure 12.37 Flowsheet for dehydration of ethanol using vapor permeation.

On-line pervaporation, 18 518 On-line probes, for fermentation, 11 39 On-line vapor permeation, 18 518 On/off feedback controllers, 20 691 Onstream analyzers, 20 682 On-tank leak detection systems, 24 311 Onyx thermal printing process, 19 321 Oocytes, retroviral infection of, 12 457 Oolitic limestone, 15 28 Opacification, colorants for ceramics, 7 344-345... 

K., Kondo, M., and NaA, Z. (2001) membrane preparation, single-gas permeation, and pervaporation and vapor permeation of water/organic liquid mixtures. Ind. Eng. Chem. Res., 40, 163-175. 

These materials (trade names Epichlomer, Herclor, Hydrin) have good resistance to fuel, oil, ozone, and heat combined with low-temperature flexibility and resistance to vapor permeation by air, hydrocarbons, and fluorocarbons. 

Stannett, V. Yasuda, H., "Liquid vs Vapor Permeation Through... 

This test is conducted on containers heat-sealed with foil laminate therefore, only the properties of the container are evaluated. The level of protection from water vapor permeation provided by a packaging system marketed with a heat-sealed foil laminate inner seal (up to the time the inner seal is removed) is expected to be approximately the same as that determined by this test. The acceptance criteria are those established in USP <671>. 

This test measures the water vapor permeation of a singleunit or unit-dose container closure system and establishes acceptance criteria for five standards (Class A-E containers). 

This test is intended for drugs being dispensed on prescription, but it has also been applied to the drug product manufacturer s container closure system. If the container closure system has an inner seal, it should be removed before testing. The results from this study reflect the contributions to water vapor permeation through the container and through the seal between the container and the closure. 

A clever means of dynamic generation of standards at the part-per-million level involves permeation through a polymer. In 1966 O Keeffe and Ortman (34) described this technique primarily for air pollution standards. A condensable gas or vapor is sealed as a liquid in a Teflon tube under its saturation vapor pressure as shown in Figure 4.14. After an initial equilibration period the vapor permeates through the tube wall at a constant rate. This rate is determined by weight loss over a period of time. Temperature must be controlled to within .0.1°C to maintain 1% accuracy. In use the tube is thermostatted in a chamber that permits a diluent gas to fully flush the chamber. The concentration is then determined by the same equation used for diffusion tubes. However, since the rate is generally much less in permeation tubes it is usually reported in ng/min. 

Gas Dehydration. It has been found that water vapor permeates cellulose acetate membranes at a rate approximately 500 times that of methane (Ref. 2). This exceptionally high selectivity for water vapor make cellulose acetate membrane systems attractive for dehydration of hydrocarbon gas streams to pipeline specifications on either a pure gas stream or while simultaneously removing contaminating acid gases. For these applications the small size, low weight and low maintenance of the SEPAREX system is particularly advantageous for offshore installations. 

B. Will, R.N. Lichtenthaler, Comparison of the separation of mixtures by vapor permeation and pervaporation using PVA composite membranes. Part I. Binary alcohol—water systems, J. Membr. Sci. 68 (1992) 119-125. 

T. Kataoka, T. Tsuro, S.-I. Nakao and S. Kimura, Membrane Transport Properties of Pervaporation and Vapor Permeation in an Ethanol-Water System Using Polyacrylonitrile and Cellulose Acetate Membranes, J. Chem. Eng. Jpn 24, 326 (1991). 

Spiral-wound when fluxes are higher, feed gases more contaminated and concentration polarization a problem (natural gas separations, vapor permeation)... 

Figure 4.19(b) shows an equivalent figure for a counter-flow module in which 5 % of the residue gas containing 100 ppm water vapor is expanded to 50 psia and introduced as a sweep gas. The water vapor concentration in the permeate gas at the end of the membrane then falls from 1900 ppm to 100 ppm, producing a dramatic increase in water vapor permeation through the membrane at the residue end of the module. The result is a two-thirds reduction in the size of the module. 

Both Mitsui [26] and Sulzer [27] have commercialized these membranes for dehydration of alcohols by pervaporation or vapor/vapor permeation. The membranes are made in tubular form. Extraordinarily high selectivities have been reported for these membranes, and their ceramic nature allows operation at high temperatures, so fluxes are high. These advantages are, however, offset by the costs of the membrane modules, currently in excess of US 3000/m2 of membrane. 

Figure 9.3 illustrates the concept of permeation from a saturated vapor phase in equilibrium with the feed liquid as a tool to obtain Equation (9.5). A number of workers have experimentally compared vapor permeation and pervaporation separations and have sometimes shown that permeation from the... 

An alternative carrier-gas system uses a condensable gas, such as steam, as the carrier sweep fluid. One variant of this system is illustrated in Figure 9.7(d). Low-grade steam is often available at low cost, and, if the permeate is immiscible with the condensed carrier, water, it can be recovered by decantation. The condensed water will contain some dissolved organic and can be recycled to the evaporator and then to the permeate side of the module. This operating mode is limited to water-immiscible permeates and to feed streams for which contamination of the feed liquid by water vapor permeating from the sweep gas is not a problem. This idea has been discovered, rediscovered, and patented a number of times, but never used commercially [37,38], If the permeate is soluble in the condensable... 

Figure 9.12 Isothermal vapor permeation and multistage pervaporation with intermediate heating. GFT poly (vinyl alcohol) membranes [45]...

---