Permeation factor

Table III. Required Permeation-factors of Polymer to Meet Package Criteria...

Since many IV packs are overwrapped (some prior to autoclaving), usually to reduce gaseous or moisture permeation factors, this may also add to the overall pack integrity and needs evaluation before and after autoclaving. 

Table 33.1 Permeation Factor of Common Packaging Materials...

F is the permeation factor of fhe material the notation sc refers to semicrystalline. P is the permeation factor of the amorphous phase and is the crystalline content fraction [14]. Experimental data is presented in Table 33.4 for both thermal and strain-induced crystallinity in terms of oxygen ingress to the package. 

Salt Effects. The definition of a capacity factor k in hydrophobic interaction chromatography is analogous to the distribution coefficient, in gel permeation chromatography ... 

A third factor is the ease with which various membrane materials can be fabricated into a particular module design. Almost ah membranes can be formed into plate-and-frame, spiral, and tubular modules, but many membrane materials caimot be fabricated into hollow-fine fibers or capihary fibers. Finahy, the suitabiHty of the module design for high pressure operation and the relative magnitude of pressure drops on the feed and permeate sides of the membrane can sometimes be important considerations. 

A key factor determining the performance of ultrafiltration membranes is concentration polarization due to macromolecules retained at the membrane surface. In ultrafiltration, both solvent and macromolecules are carried to the membrane surface by the solution permeating the membrane. Because only the solvent and small solutes permeate the membrane, macromolecular solutes accumulate at the membrane surface. The rate at which the rejected macromolecules can diffuse away from the membrane surface into the bulk solution is relatively low. This means that the concentration of macromolecules at the surface can increase to the point that a gel layer of rejected macromolecules forms on the membrane surface, becoming a secondary barrier to flow through the membrane. In most ultrafiltration appHcations this secondary barrier is the principal resistance to flow through the membrane and dominates the membrane performance. 

The dimensions of permeabiUty become clear after rearranging equation 1 to solve for P. The permeabiUty must have dimensions of quantity of permeant (either mass or molar) times thickness ia the numerator with area times a time iaterval times pressure ia the denomiaator. Table 1 contains conversion factors for several common unit sets with the permeant quantity ia molar units. The unit nmol/(m-s-GPa) is used hereia for the permeabiUty of small molecules because this unit is SI, which is preferred ia current technical encyclopedias, and it is only a factor of 2, different from the commercial permeabihty unit, (cc(STP)-mil)/(100 in. datm). The molar character is useful for oxygen permeation, which could ultimately involve a chemical reaction, or carbon dioxide permeation, which is often related to the pressure in a beverage botde. 

Starches. Starch (qv) granules must be cooked before they wiU release their water-soluble molecules. It is common to speak of solutions of polysaccharides, but in general, they do not form tme solutions because of their molecular sizes and intermolecular interactions rather they form molecular dispersions. The general rheological properties of polysaccharides like the starch polysaccharides are described below under the discussion of polysaccharides as water-soluble gums. Starch use permeates the entire economy because it (com starch in particular) is abundantly available and inexpensive. Another key factor to its widespread use is the fact that it occurs in the form of granules. 

Salt flux across a membrane is due to effects coupled to water transport, usually negligible, and diffusion across the membrane. Eq. (22-60) describes the basic diffusion equation for solute passage. It is independent of pressure, so as AP — AH 0, rejection 0. This important factor is due to the kinetic nature of the separation. Salt passage through the membrane is concentration dependent. Water passage is dependent on P — H. Therefore, when the membrane is operating near the osmotic pressure of the feed, the salt passage is not diluted by much permeate water. 

Chlorine Tolerance Most of the best RO membranes are attacked by oxidants, and they are particularly susceptible to chlorine. A particularly sensitive locus for attack is the amidic hydrogen. Cellu-losic membranes are generally less sensitive, and pass the chlorine into the permeate giving downstream biocidal activity, veiy useful for under-the-sink RO. These factors are largely responsible for CA s survival in RO membranes. Chlorine, whatever its vices, has the virtue of being a known, effective, residual bactericide and a good inhibitor of... 

Osmotic Pinch Ejfect Feed is pumped into the membrane train, and as it flows through the membrane array, sensible pressure is lost due to fric tion effects. Simultaneously, as water permeates, leaving salts behind, osmotic pressure increases. There is no known practical alternative to having the lowest pressure and the highest salt concentration occur simultaneously at the exit of the train, the point where AP — AH is minimized. This point is known as the osmotic pinch, and it is the point backward from which hydrauhe design takes place. A corollary factor is that the permeate produced at the pinch is of the lowest quality anywhere in the array. Commonly, this permeate is below the required quahty, so the usual prac tice is to design around average-permeate quality, not incremental quahty. A I MPa overpressure at the pinch is preferred, but the minimum brine pressure tolerable is 1.1 times H. 

Membrane thickness is a factor in microbial retention, Tortiioiis-pore membranes rated at 0,22 jlrn tvpicallv have surface openings as large as 1 jlrn (Fig, 22-71), Narrovv er restrictions are found beneath the surface. In challenge tests, P. diminnta organisms are found vv ell beneath the surface of an 0,2 jlrn membrane, but not in the permeate. 

Plasticization Gas solubility in the membrane is one of the factors governing its permeation, but the other factor, diffusivity, is not always independent of solubility. If the solubility of a gas in a polymer is too high, plasticization and swelhng result, and the critical structure that controls diffusion selectivity is disrupted. These effects are particularly troublesome with condensable gases, and are most often noticed when the partial pressure of CO9 or H9S is high. H9 and He do not show this effect This problem is well known, but its manifestation is not always immediate. 

Modules Eveiy module design used in other membrane operations has been tried in peivaporation. One unique requirement is for low hydraulic resistance on the permeate side, since permeate pressure is veiy low (O.I-I Pa). The rule for near-vacuum operation is the bigger the channel, the better the transport. Another unique need is for neat input. The heat of evaporation comes from the liquid, and intermediate heating is usually necessary. Of course economy is always a factor. Plate-and-frame construc tion was the first to be used in large installations, and it continues to be quite important. Some smaller plants use spiral-wound modules, and some membranes can be made as capiUaiy bundles. The capillaiy device with the feed on... 

All organic coatings show varying degrees of solubility and permeability for components of the corrosive medium, which can be described as permeation and ionic conductivity (see Sections 5.2.1 and 5.2.2). An absolute separation of protected object and medium is not possible because of these properties. Certain requirements have to be met for corrosion protection, which must also take account of electrochemical factors [1] (see Section 5.2). 

Permeation efficiency Reduction factor for latent heat exchange through clothing. 

A number of studies have recently been devoted to membrane applications [8, 100-102], Yoshikawa and co-workers developed an imprinting technique by casting membranes from a mixture of a Merrifield resin containing a grafted tetrapeptide and of linear co-polymers of acrylonitrile and styrene in the presence of amino acid derivatives as templates [103], The membranes were cast from a tetrahydrofuran (THF) solution and the template, usually N-protected d- or 1-tryptophan, removed by washing in more polar nonsolvents for the polymer (Fig. 6-17). Membrane applications using free amino acids revealed that only the imprinted membranes showed detectable permeation. Enantioselective electrodialysis with a maximum selectivity factor of ca. 7 could be reached, although this factor depended inversely on the flux rate [7]. Also, the transport mechanism in imprinted membranes is still poorly understood. 

The anode compartment contains a reference electrode and counterelectrode and by means of a potentiostat the anode side is maintained at a constant potential. The coverage of adsorbed hydrogen on the cathode side will depend on the current density i and the nature of the electrolyte solution, and the cell can be used to study the effect of a variety of factors (composition and structure of alloys, pH of solution, effect of promoters and inhibitors) on hydrogen permeation. 

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. 

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