Permeability, Permeation composite

Equation (9.1) is the preferred method of describing membrane performance because it separates the two contributions to the membrane flux the membrane contribution, P /C and the driving force contribution, (pio — p,r). Normalizing membrane performance to a membrane permeability allows results obtained under different operating conditions to be compared with the effect of the operating condition removed. To calculate the membrane permeabilities using Equation (9.1), it is necessary to know the partial vapor pressure of the components on both sides of the membrane. The partial pressures on the permeate side of the membrane, p,e and pje, are easily obtained from the total permeate pressure and the permeate composition. However, the partial vapor pressures of components i and j in the feed liquid are less accessible. In the past, such data for common, simple mixtures would have to be found in published tables or calculated from an appropriate equation of state. Now, commercial computer process simulation programs calculate partial pressures automatically for even complex mixtures with reasonable reliability. This makes determination of the feed liquid partial pressures a trivial exercise. 

Figures 7 and 8 incorporate calculated tube- and shell-side concentration profiles for the unique CO2-N2 and O2-N2 (air) data, and also Illustrate local permeate composition variation. Each figure shows that the concentration of more permeable gas steadily decreases in the direction of flow along the high-pressure (tube)...

Example 26A An air-separation membrane has an O2/N2 selectivity of 5.0, and the O2 permeability is 0.2 scf/ft -h-atm. (a) For a counterflow separator operating with a residue containing 95 percent N2, what is the permeate composition and the fraction of the feed obtained as permeate if the feed pressure is 150 Ib /in. absolute and the permeate pressure 15 Ib /in. absolute (i) What membrane area is needed for a feed rate of 300 scfm ... 

Example 26.3. Laboratory tests of a pervaporation membrane exposed to liquid with 90 weight percent ethanol and 10 percent water at 60°C showed a flux of 0,20 kg/ra -h and a permeate compostion of 7.1 percent ethanol when the downstream pressure was 15 mmHg. (a) Calculate the permeability of the membrane to ethanol and to water at the test conditions and the selectivity for water. (6) Predict the local permeate composition for 90 percent ethanol and 60°C if the downstream pressure is kept at 30 mm Hg by a water-cooled condenser. What is the condensing temperature (c) Calculate the local permeate composition for 95 percent, 99 percent, and 99.9 percent ethanol at 60°C and 30 ram Hg assuming the permeabilities are the same as for part... 

If the other concentrations in the residue or the permeate composition do not match the data, the permeabilities are changed and the calculations repeated. 

It is agreed that the permeabilities of the components may not be constant, but rather depend on a number of factors, such as the retentate and/or permeate composition(s), and mass transfer through the membrane can be quite complex. However, for demonstration purposes, a simplified approach will be followed. For this reason, constant relative permeabilities have been assumed, resulting in what is known as a Knudsen membrane [12]. 

The important thing is that using recycle (or reflux) of the more-permeable product and recycle (or reflux) of the less-permeable product can induce sharper separations, just as in distillation. Furthermore, as far as the total number of stages is concerned, it makes very little difference as to whether the feedstream is introduced at the reject or permeate side of the cell at the feed location and whether or not it is partitioned. Not only this, it makes very little difference whether the feedstream composition coincides with the reject or permeate compositions. 

Figure 6.31 shows some experimental data for the pervaporation of water/ethanol mixtures by a silicone rubber membrane preferentially permeable to ethanol. The experiment was conducted at 23 C for downstream pressures of 667, 1200, and 2100 Pa (5,9, and 16 mmHg). As reported by Hoover and Hwang [250] and Tanigaki et al. [245], the silicone membrane showed preferential permeation to ethanol. Evidently, the downstream pressure has little effect on both permeate composition and permeation rate, supporting the calculated results shown in Figure 6.30. When the experimental data arc closely examined, however, the relative permeation rate decreases slightly with an increase in the downstream pressure. The calculated values in Figure 6.30 show exactly the same tendency, justifying the transport model on which the calculation is based. It has to be noted, however, that the saturation vapor pressure of water and ethanol at 60 C are 1.99 x l(f and 4.69 x lO Pa (149.4 and 351.9 mmHg), respectively. When the downstream pressure approaches the saturation vapor pressure, the assumption on which the theoretical calculation is based (i.e., (he vapor permeation prevails across the membrane cross-.section) becomes invalid, since liquid penetrates more deeply into the pore.

The permeate composition, j, can be determined depending on various parameters such as the feed composition (xin), the upstream (/ ) and downstream ip") pressure, the permeability to the fast (P/) and slow (Py) permeant. The mixture composition is usually expressed in mole fractions (which equal the volume fractions for a perfect gas mixture) with the fast compound as a reference. Figure 2.2 summarizes the main process variables which have to be taken into account for a single membrane module design study. 

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... 

The similar results were obtained in a series of experiments performed with the different polyphenylene oxides in a range of temperatures, pressures and gas mixture composition. A brief summary of the experimental results is given in Table 7. The steady-state values of permeability coefficients for Kr and Xe (Pkt and Pxe respectively) were obtained by monitoring the permeate composition vs. time on stream. The diffusion coefficients D were evaluated by the time-lag method. 

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). 

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). 

Such a composition reduces friction, permeates drilling mud wall cake, destroys binding wall cake, and reduces differential pressure. Unfortunately, many of such compositions are toxic to marine life. Synthetic PAOs are nontoxic and effective in marine environments when used as lubricants, retum-of-permeabiUty enhancers, or spotting fluid additives for water-based drilling muds. A continuing need exists for other nontoxic additives for water-based drilling muds, which serve as lubricants, retum-of-permeability enhancers, and spotting fluids. 

In situ perfusion studies assess absorption as lumenal clearance or membrane permeability and provide for isolation of solute transport at the level of the intestinal tissue. Controlled input of drug concentration, perfusion pH, osmolality, composition, and flow rate combined with intestinal region selection allow for separation of aqueous resistance and water transport effects on solute tissue permeation. This system provides for solute sampling from GI lumenal and plasma (mesenteric and systemic) compartments. A sensitive assay can separate metabolic from transport contributions. 

For this reason much work has been done at the ALZA Corporation and elsewhere to increase the water permeation rates by various technologies. For example, ALZA scientists utilized a composite membrane in the development of their first commercial product with this technology [21,22], In this system they first applied a CA membrane containing a high concentration of porosigens. A second dense membrane containing only CA was added. In this way the overall fluid permeability was increased, since the thickness of the dense portion of the film could be proportionately reduced. 

---