Permeance

Thickness. The traditional definition of thermal conductivity as an intrinsic property of a material where conduction is the only mode of heat transmission is not appHcable to low density materials. Although radiation between parallel surfaces is independent of distance, the measurement of X where radiation is significant requires the introduction of an additional variable, thickness. The thickness effect is observed in materials of low density at ambient temperatures and in materials of higher density at elevated temperatures. It depends on the radiation permeance of the materials, which in turn is influenced by the absorption coefficient and the density. For a cellular plastic material having a density on the order of 10 kg/m, the difference between a 25 and 100 mm thick specimen ranges from 12—15%. This reduces to less than 4% for a density of 48 kg/m. References 23—27 discuss the issue of thickness in more detail. 

Moisture. Absorbed and retained moisture, especially as ice, has a significant effect on the stmctural and thermal properties of insulation materials. Most closed-ceU plastic foams have low permeance properties most notably where natural or bonded low permeance surface skins exist (29,30). Design, building, and constmction practices requite adequate vapor retarders, skins, coatings, sealants, etc, in order to prevent the presence of moisture. However, moisture vapor cannot be completely excluded, thus the possibiUty of moisture absorption and retention is always present. The freezing of moisture and mpturing of cells result in permanent reduction of thermal and stmctural performance. 

Costs of ceUular plastic insulations are stiU higher than those of fibrous and other mass insulation types, but these can often be justified based on overall advantages of combined stmctural, thermal, and permeance properties. It is difficult to provide a single cost for each material type since there are many different forms of a material-based product avaUable and differing forms of manufacture and appHcation, often in combination with other materials. In the United States, EPS board costs on the order of 0.12 to 0.18 XEPS, 0.25 to 0.30 and PU, 0.30 to 0.35, per board foot ( 0.30/board ft fx 127/m ). 

Layered Structures. Whenever a barrier polymer lacks the necessary mechanical properties for an appHcation or the barrier would be adequate with only a small amount of the more expensive barrier polymer, a multilayer stmcture via coextmsion or lamination is appropriate. Whenever the barrier polymer is difficult to melt process or a particular traditional substrate such as paper or cellophane [9005-81-6] is necessary, a coating either from latex or a solvent is appropriate. A layered stmcture uses the barrier polymer most efficiently since permeation must occur through the barrier polymer and not around the barrier polymer. No short cuts are allowed for a permeant. The barrier properties of these stmctures are described by the permeance which is described in equation 16 where and L are the permeabiUties and thicknesses of the layers. 

The permeance can be used in equation 17 (which is a modification of eq. 1) to estimate package performance. 

Fig. 8. Average permeance of a two-layer stmcture. See Table 1 for unit conversions.

Equation (22-106) gives a permeate concentration as a function of the feed concentration at a stage cut, 0 = 0, To calculate permeate composition as a function of 0, the equation may be used iteratively if the permeate is unmixed, such as would apply in a test cell. The calculation for real devices must take into account the fact that the driving force is variable due to changes on both sides of the membrane, as partial pressure is a point function, nowhere constant. Using the same caveat, permeation rates may be calciilated component by component using Eq. (22-98) and permeance values. For any real device, both concentration and permeation require iterative calculations dependent on module geometiy. 

FIG. 22-75 Air fractionation by membrane. O2 in retentate as a function of feed fraction passed tbrougb tbe membrane (stage cut) showing tbe different result with changing process paths. Process has shell-side feed at 690 kPa (abs) and 298 K. Module comprised of hollow fibers, diameter 370 im od X 145 im id X 1500 mm long. Membrane properties (X = 5.7 (O2/N2), permeance for O2 = 3.75 X 10 Barrer/cm. Coutiesy Innovative Membrane Systems/ Fraxair)... 

We have been studying the novel process for CO2 separation named membrane/absorption hybrid method. The advantages of this process are that high gas permeance and selectivity were obtained. The concept of this process is shown in Fig. 1. Both feed gas and absorbent solution are supplied to the inside of hollow fibers. While Ae liquid flows upward inside the hollow fibers, absorbent solution absorbs CO2 selectively and it becomes a rich solution. Most of rich solution permeates the membrane to the permeate side maintained at reduced pressure, where it liberated CO2 to become a lean solution. Compared to a conventional gas absorption... 

The experimental results are summarized in Table 2. CO2 permeance (Rco2), selectivity (oicx)2/N2) and CO2 recovery (Y) increased with decreasing CO2 mole fraction in feed gas. CO2 in the feed gas was successfully concentrated to 97-99 % by the single-stage operation. CO2... 

For similar temperature conditions the permeance for nitrogen varies with the applied pressure in a different way according to nature of the sample (i.e. the two starting supports or the zeolite-alumina composite. Figure 7). 

Figure 7. N2 permeances at 30 C in the a-Al203 suppon alone (I, ), in the a-Al203 support plus the y-Al203 toplayer (2, ) and in the zeolite-support composite (3, A).

In the zeolite-alumina composite the behaviour of different gases (permeance of pure gases as a function of the temperature. Figure 8) behave very differently from those predicted by ideal Knudsen diffusion processes. 

Figure 7 shows that N2 permeability strongly depends on the pore size. For the macroporous support (curve 1) Poiseuille flow occurs, leading to an increase of the permeance... 

When using the microporous zeolite membrane (curve 3) the N2 permeance decreases when the pressure increases such a behaviour can be accounted for by activated diffusion mechanisms [21], which are typical of zeolite microporous systems. In such systems the difflisivity depends on the nature and on the concentration of the diffusing molecule which interacts with the surface of the pore. For gases with low activation energies of diffusion, a decrease of the permeability can be observed [22]. 

The flux, and hence the permeance and permeability, can be defined on the basis of volume, mass or molar flowrates. The accurate prediction of permeabilities is generally not possible and experimental values must be used. Permeability generally increases with increasing temperature. Taking a ratio of two permeabilities defines an ideal separation factor or selectivity awhich is defined as ... 

The permeability of dense membranes is low because of the absence of pores, but the permeance of Component i in Equation 10.20 can be high if SM is very small, even though the permeability is low. Thickness of the permselective layer is typically in the range 0.1 to 10 tm for gas separations. The porous support is much thicker than this and typically more than 100 tm. When large differences in PM exist among species, both high permeance and high selectivity can be achieved in asymmetric membranes. 

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