Differential permeation

Kameyama, T., M. Dokiya, K. Fukuda and Y. Kotera. 1979. Differential permeation of hydrogen sulfide through a microporous vycor-type glass membrane in the separation system of hydrogen and hydrogen sulfide. Separ. Sci. Technol. 14(10) 953-957. 

Pitkanen et al. [51] reported the isolated bovine RPE-choroid was up to 20 times more permeable to lipophilic than hydrophilic beta-blockers. Furthermore, the in vitro permeability of bovine RPE-choroid to hydrophilic compounds and macromolecules was 10 to 100 times less compared to sclera, whereas the permeability for lipophilic molecules was in the same range for both tissues. The isolated bovine RPE-choroid also exhibited differential permeation by molecular weight and Stokes radius. The permeation rate of 4, 10, and 20 kDa FITC dextrans was moderate compared to a good permeation rate for the 376 Da carboxyfluor-escein and a poor penetration rate for 40 and 80 kDa FITC-dextrans. The permeability to carboxyfluorescein was 35 times more than to 80 kDa FITC-dextran [51]. In a study on the permeability of the human ciliary epithelium to a horseradish peroxidase, Tonjum and Pedersen [52] reported that ciliary and iridial epithelium contained a system of paracellular zonulae occludentes. Peroxidase was applied on the stromal side of ciliary body and iris specimens obtained from freshly enucleated eyes. The 40 kDa peroxidase was blocked apically in the lateral intercellular spaces of the CNPE whereas in the iris the progression of peroxidase was blocked apically in the lateral intercellular spaces of the IPE. Freddo [53] studied the intercellular junctions in the posterior IPE cells of the rhesus monkey by electron microscopy, freeze-fracture, and horseradish peroxidase. Intravenously injected horseradish... 

A differential permeation method was used to determine diffusion kinetics of strongly adsorbing vapors through an Ajax activated carbon (type 976) (whose physical properties [7] particle density of 733 kg/m micropore porosity 0.40, macropore porosity 0.31 and mean macropore radius 0.8 pm). An activated carbon pellet was carefully mounted in a copper block, separating two reservoirs. One reservoir is much larger in volume than the... 

Bae J.-S. and Do D. D., Study on diffusion and flow of benzene, ra-hexane and CCI4 in activated carbon by a differential permeation method. Chem. Eng. Sci. in press (2002). 

Of further interest and concern is the operation of a membrane cell as a continuum, referred to as differential permeation. 

The most usual problem encountered is that of determining the degree of separation for a single-stage embodiment, which can certainly be complicated. The extension to multistage and differential permeation operations will only be alluded to, with referral made to Hoffman (2003). 

The use of differential permeation in countercurrent flow with recycle is developed in Chapter 7 and Appendix 7 of Hoffman (2003). 

MULTISTAGE AND DIFFERENTIAL PERMEATION DERIVATIONS AND CALCULATIONS FOR TWO KEY COMPONENTS... 

As can be expected, the derivations and calculations become increasingly complicated for multistage and differential permeation. The subject is detailed in the later pages of Hoffman (2003), with examples provided, including systematic spreadsheet calculations. 

E.J. Hoffman, Membrane Separations Technology Single-Stage, Multistage, and Differential Permeation, Gulf Professional Publishing, an Imprint of Elsevier Science, Amsterdam, 2003. 

Size-exclusion chromatography (SEC) is a method in which molecules are separated by size due to differential permeation into a porous support. It requires complete solubility of the analytes in the mobile phase and elimination of all interactions with the bonded phase. In these respects, SEC is not as useful for the separation of peptides as it is for proteins because peptides vary drastically in solubility, charge, and hydrophobicity. Peak capacity in SEC is fairly low compared to other HPLC methods because all separations must occur in the internal volume (Vi) of the support, which is generally less than half the volume of mobile phase in the column. Despite these deficiencies, SEC can be very effective for separating peptides from dimers, aggregates, small molecules, proteins, and other molecules which differ by size. 

Size-exclusion chromatography (SEC) is a method in which molecules are separated by size due to differential permeation into a porous support. This technique is especially useful for the separation of pro-... 

Gel permeation chromatography. A method of separating molecules by size, usually carried out on columns that are tightly packed with gel and completely filled with solvent. Differential permeation into the gel pores... 

Molar flow rates f are the variable rates inside the separator, with subscripts P andf designating, respectively, the permeate side and residue side of the membrane. A material balance around the differential volume is written for component i. The component differential permeate rate equals the differential change in the component residue rate ... 

The complexity and selectivity of the processes which may be determinants of permeation is well illustrated by the demonstration of differential permeation of isomeric drugs. The mechanisms and significance of this selectivity have been reviewed (Heard and Brain 1995). 

Permeation Properties. The data shown in Figure 2 are the toluene permeation rates of the fluorinated and untreated containers g. toluene/container per day are plotted vs. the time of toluene exposure on a logarithmic scale. These cumulative permeation rates were calculated based on the cumulative weight loss over the time of toluene exposure, as opposed to the differential permeation rates based on the differential weight loss over each time interval. The room temperature permeation rates for the in-situ fluorinated containers were less than 0.01 g./day and, hence, have been rounded up to 0.01 g./day for illustrative purposes. In Figure 2, the,flat portion of the curves for the untreated containers yielded the steady state permeation rates. From these values, the permeability coefficients (P) for the untreated containers were calculated using Equation 1. 

Tbe extraction of phenol has been used by Schlosser and Kossaczky11 and Halwachs et al.16 as a model for studying liquid-membrane transport. The former group compared the liquid-membrane method to a conventional liquid-liquid double extraction process. They concluded thet liquid membranes have disliact economic ndvsmages in cases where type I or type 2 facilitation methods can be used (e.g., phanel or metal ion extractions). However, where one must rely on simple differential permeation (bydrocarbon seperations) rhe ndvantage is less clear-cut. 

Suedee, R. Brain, K.R. Heard, C.M. Differential permeation of propranolol enantiomers across human skin in vitro from formulations containing an enantioselective excipient. Chirality 1999, 11 (9), 680-683. 

In the membrane separation processes, the importance of the effect of possible associations between the solute and solvent and the membrane material is quite clear. This behavior resulting in differential permeation rates of solutes due to these phenomena of interactions can be observed in the case of retention of Sudan IV, an organic compound of molecular weight equal to 384 in this molecnle are present four aromatic rings, and its rejection by a hydrophobic membrane was 25% and 10%, respectively, when dissolved in n-hexane, at a pressnre of 15 bar, and methanol, 20 bar. Under the same conditions, a crossing of two solutions in a hydrophilic membrane was performed, resulting in the values of the retention of 86% for the compound in the solution of methanol and 43% for the same compound in the solution of n-hexane [23]. 

Subramanian, R., Raghavarao, K. S. M. S., Nakajima, M., Nabetani, H., Yamagushi, T., and Kimura, T. (2003) Application of dense membrane theory for differential permeation of vegetable oil constituents. Journal of Food Engineering 60, 249-256. 

Size-exclusion chromatography is a method in which molecules are separated by size due to differential permeation into a porous support. This technique is especially useful for the separation of proteins because they are macromolecules frequently found in the presence of smaller and larger species. The peak capacity in SEC is fairly low compared to other HPLC methods because all separation must occur in the internal volume of the support,... 

Of further interest and concern is the operation of a membrane cell as a continuum. Such a view may be referred to as differential permeation. The permeate may be withdrawn at points along the membrane, as illustrated in Figure 1.8. Or the cell may be operated in concurrent flow, as shown in Figure 1.9, or countercurrent flow, as shown in Figure 1.10. There is the possibility, even, of producing two permeate products if two different membrane materials are employed separately in the same unit or module. This is indicated in Figure 1.11. 

Other procedures and calculation techniques have been developed for both stagewise and differential permeation, such as those presented S-T Hwang and K. Kammermeyer, but they are not pursued here, inasmuch as the analogy is to be made specific to vapor-liquid mass transfer unit operations. In this way, the conventions and techniques already developed for mass transfer operations can be more readily utilized. Also note that the symbols and terminology used for membrane permeation have evolved through the years and vary from one author to another. 

Of special consideration is the investigation of the cell as a continuum, first with point withdrawal of the permeate, then in both concurrent and countercurrent flow for the permeate and the reject phases. For this treatment, differential permeation is the mode of attack. The differential... 

The same is true in utilizing membrane calculations. As indicated by S. Y. Lee and B. S. Minhas, the observed permeabilities for the components of a mixture may be markedly less than that measured for the individual pure components. This indicates that there is a role for efficiency ratings, which may be stagewise or pointwise (as in the case of differential permeation) or an overall figure may be used. 

Differential Permeation with Point Permeate Withdrawal... 

In differential permeation, the point compositions of the phases are considered to vary linearly with position along the surface(s) of the membrane. A steady state is assumed, so that the compositions are independent of time. 

For membrane systems in concurrent or countercurrent flow of the reject and permeate, the concept of differential permeation applies. Here we consider the flow of the feed and reject stream as diagrammed in Figure 5.1. The end of the membrane cell where the feed stream is introduced... 

Figure 5.1 Differential permeation with point permeate withdrawal.

Differential Permeation ivith Point Permeate Withdrawal i 175... 

---