Composite permeability

Mineral composition, permeability, and porosity, grain-size distribution, in-situ density, moisture content... 

Permeability of doxorubicin in model membrane bilayers. From [7]. Permeability was measured in artificial unilamellar vesicles of known composition. Permeability varies with the addition of cholesterol (squares). Permeability also varies with change in phospholipid composition. 

Mica flakes embedded in a polymer and properly oriented in a plane can provide a tortuous path to vapors and liquids, similarly to the natural composites shown in Figure 1.1. Barrier properties can be imparted in blow-molded containers, packaging films, and corrosion-resistant coatings not only by mica but also by other impermeable lamellar fillers, including glass flakes, talc, and nanoclays. In blown LDPE film, the addition of 10% mica was found to reduce the oxygen permeabihty from 4.16 to 3.03 Barrer [37]. Assuming an impermeable, fully oriented lamellar filler, Eq. (8.1), [38] maybe used to predict the composite permeability. Pc, perpendicularly to the filler plane as a function of the matrix permeability, P , filler volume fraction, V(, matrix volume fraction, V , and filler aspect ratio a... 

Theoretical approaches on the barrier properties of nanocomposites beat fillers as impermeable nonoverlapping particles and assume no permeability changes in the polymer matrix. Effectively, this means that the permeability of the composite will be smaller than the permeability of the matrix (unfilled polymer) by a factor equal to path tortuosity in the composite (simply assuming that the penetrant path cannot cross any filler particles). This path tortuosity was calculated by Nielsen for completely aligned filler particles (aU fillers have then-larger surface parallel to the film surfaces, but there is no order in the filler center of mass), and its contribution to the composite permeability was derived to be... 

We were concerned that this apparent shear-thinning behavior in our short cores might be an experimental artifact (associated with microgels or very-high-MW polymer species) that could not be expected to propagate far into a real reservoir. To test this idea, we performed an experiment in a 57-md Berea sandstone core that was 122 cm long, with four equally spaced internal pressure taps, which created five 24.4-cm sections within the core. The core porosity was 17.3%, and the core cross section was 15.24 cm. The permeabilities of the five core sections were 53, 64, 65, 74, and 41 md, respectively, giving a composite permeability of 57 md. The core was saturated with filtered 2.52%-TDS brine (2.3% NaCl -H 0.22% NaHCOj), and our polymer solution contained 600-ppm CP Kelco xanthan in this same brine. 

Various models for composite permeability as they relate to nanocomposites have been reviewed and different models have been proposed [41—44]. The simplest way to model any composite property is to use a rule of mixtures approach. Polymer nanocomposite properties, however, do not generally follow this rule. Instead, fillers with high aspect ratio particles will influence the permeability of gases through the matrix more than filler particles with lower aspect ratios. Alignment/orientation of the filler particles (with respect to the axis of gas permeation) also plays a significant role in bulk permeability. Five models are briefly described in Sections 8.5.1-8.5.5. Predictions from these models are later compared to experimental mass loss rates. 

Environmental regulations are the driving force behind the need for and selection of FGD systems and dictate many design criteria. For example, they limit the amounts of the pollutants which can be discharged to the atmosphere and to any waterway. They also place limits on the concentration of toxic metals and other chemicals in landfilled byproduct, which can significantly affect the FGD process selection. Landfill material characteristics, such as leachate composition, permeability, and con >ressive strength and the availability of a suitable landfill site can also be important. Expected future regulations on traces of toxic substances and fine particulate may also affect the selection of a sulfur dioxide removal process. 

B.4 Permeability of Composite Laminates. Consider a laminar composite structure, in which the laminae, L are normal to the direction of permeation. These laminae can be either slabs, or hollow cylinders, or spherical shells (Fig. 4.23). Prove that the composite permeability, P, is given by the relationship... 

Let us first consider experiments without composition gradients. These are permeability measurements, in which flow is induced by a pressure gradient Consider first the flow of pure substance 1, setting x = 1, = 0 and... 

Potentiometric electrodes also can be designed to respond to molecules by incorporating a reaction producing an ion whose concentration can be determined using a traditional ion-selective electrode. Gas-sensing electrodes, for example, include a gas-permeable membrane that isolates the ion-selective electrode from the solution containing the analyte. Diffusion of a dissolved gas across the membrane alters the composition of the inner solution in a manner that can be followed with an ion-selective electrode. Enzyme electrodes operate in the same way. 

As an excellent barrier resin, PTEE is widely used in the chemical industry. However, it is a poor barrier for fluorocarbon oils because similarity in the chemical composition of a barrier and a permeant increases permeation. Most Hquids and gases (other than fluorocarbons) do not permeate highly crystalline PTFE. Permeabilities at 30deg C (in mol/(m-s-Pa) X 10 ) are as follows CO2, 0.93 N2, 0.18 He, 2.47 anhydrous HCl, <0.01 (89). 

Optimum permeability is achieved by choosing the composition where the anisotropy constants X are near zero, by using high purity raw... 

The effect of copolymer composition on gas permeability is shown in Table 9. The inherent barrier in VDC copolymers can best be exploited by using films containing Htde or no plasticizers and as much VDC as possible. However, the permeabiUty of even completely amorphous copolymers, for example, 60% VDC—40% AN or 50% VDC—50% VC, is low compared to that of other polymers. The primary reason is that diffusion coefficients of molecules in VDC copolymers are very low. This factor, together with the low solubiUty of many gases in VDC copolymers and the high crystallinity, results in very low permeabiUty. PermeabiUty is affected by the kind and amounts of comonomer as well as crystallinity. A change from PVDC to 50 wt °/ VC or 40 wt % AN increases permeabiUty 10-fold, but has Httle effect on the solubiUty coefficient. 

Table 9. Effect of Composition on the Permeability of Various Gases Through VDC Copolymers ...

Barrier Layers. Depending on composition, barrier layers can function simply as spatial separators or they can provide specified time delays by swelling at controlled rates or undergoing reactions such as hydrolysis or dissolution. Suitable barrier materials include cellulose esters and water-permeable polymers such as gelatin and poly(vinyl alcohol) (see Barrier polymers). 

Membrane Pervaporation Since 1987, membrane pei vapora-tion has become widely accepted in the CPI as an effective means of separation and recovery of liquid-phase process streams. It is most commonly used to dehydrate hquid hydrocarbons to yield a high-purity ethanol, isopropanol, and ethylene glycol product. The method basically consists of a selec tively-permeable membrane layer separating a liquid feed stream and a gas phase permeate stream as shown in Fig. 25-19. The permeation rate and selectivity is governed bv the physicochemical composition of the membrane. Pei vaporation differs From reverse osmosis systems in that the permeate rate is not a function of osmotic pressure, since the permeate is maintained at saturation pressure (Ref. 24). 

Here the permeability of the membrane to the solute is defined in terms of reflection coefficients aQ and for osmosis and filtration respectively. When (To = 1, then perfect semi-permeabihty results. in Eq. (4) is the diffusive permeabihty of the membrane, while (Cj) is the average composition of the solute in the membrane. 

Figure 4 Permeability of porous PVA-PVAc composite film to organic solvents, (a) n-Hexane at 0.5 kg/cm (b) cyclohexane at 0.5 kg/cm (c) benzene at 0.5 kg/cm (d) benzene at 60 kg/cm. ...

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