Separation factors in permeation stages

A comparison is made of separation factors commonly used to characterize separations in single-entry barrier-permeation stages. These include the overall, heads, tails and Sandell's separation factors together with Rony's extent of separation. Equations are developed which show how the various separation indexes are related to stage cut, recovery and loss terms, and to each other. Calculations were made to compare the various separation indexes to determine which are suitable to characterize the “goodness” of separation in ideal countercurrent, cross-flow, cocurrent, and perfect mixed permeation stages. When compared at the same stage cut all of the separation indexes can be used as a qualitative measure of separation. However, only Rony's extent of separation appears to be generally applicable when comparing separations that are based on different stage cuts, or product stream rates. In all cases the extent of separation index gives a qualitative, and presumably a quantitative, measure of the goodness of separation.     

On the separation mechanism of gel permeation chromatography

The separation mechanism of gel permeation chromatography was investigated by static experiments. It was found that the solute molecule is excluded from part of the inner space of the gel particle, which is entirely available to the solvent molecule. The excluded volume, ΔV, increases with increase of molecular size of the solute. A linear relationship was observed between the ΔV and the logarithm of molecular size. Excluded volume was found to be independent of solute concentration as was expected. Absorption effect was negligible with polystyrene gel. However, a strong effect was observed between acids and polydextran gel. The possibility of using absorption effects to increase the separability of GPC is suggested.     

Stochastic model for gel permeation separation of polymers

The stochastic theory of chromatography developed by Giddings and Eyring and by McQuarrie is applied to gel permeation chromatography (GPC) by first recasting their assumptions to fit the GPC process and secondly by making specific assumptions about the molecular size dependence of the rate constants λ₁ and λ₂ for entrapment and elution of a polymer molecule, respectively. The model assumes that: (1) a monodisperse sample is injected; (2) molecules behave independently within the column; (3) no molecular diffusion occurs; (4) the polymer molecules are unperturbed random coils; (5) entrapment sites in the bed are identical; (6) λ₁ is proportional to the probability P_e that the square of the polymer end-to-end distance is less than the square of the average pore radius in the bed; (7) λ₂ is constant. The calculated difference in retention times, t_R-t_o (where t_R is the retention time for a molecule of arbitrary molecular size and t_o is the retention time for a molecule whose size totally precludes entrance into the pores of the bed), is shown to be proportional to P_e. The model as thus applied is based on only one parameter. The theory is tested by examining the ratio P_e/(t_R ? t_o)exp, predicted to be constant, for narrow polystyrene fractions in the molecular weight range 1.1 × 10⁴ – 8.9 × 10⁵. Chromatographs were obtained by Moore and Arrington by using a θ solvent and a single column packed with a porous glass bed having a sharp distribution of pore sizes. Ratio values ranged from 0.046 to 0.073 with an average value of 0.058 ± 0.009. This relative constancy demonstrates semiquantitative utility of the model for gaining insight into the GPC process.     

Stage extent of separation in ideal countercurrent recycle membrane cascades

Rony's extent of separation ξ is useful to characterize the quality of separation of a binary feed that occurs in membrane stages, membrane modules and cascades. The stage extent of separation, ξ_i, has a maximum value for a specified stage separation factor, α_i. The ideal cascade design requires a minimum total interstage flow compressor duty, and membrane area to make a specified overall separation. It is shown that ξ_i has the maximum value at each stage in an ideal membrane cascade composed of ideal perfect-mix stages. This is also true in a “properly designed” no-mix cascade composed of crossflow stages. This suggests that ξ is a basic separation index.     

Effect of permeation temperature on permeation and separation of a benzene/cyclohexane mixture through liquid-crystalline polymer membranes

When a benzene/cyclohexane mixture of 10 wt % benzene was permeated through side-chain liquid-crystalline polymer (LCP) membranes by pervaporation at various temperatures, the permeation rate increased with increasing permeation temperature. The LCP membranes also exhibited a benzene permselectivity. The permselectivity for the benzene/cyclohexane mixture through the LCP membrane was different in the glassy, liquid-crystalline, and isotropic states. The LCP membrane had different apparent activation energies for permeation at each state. LCP membrane in the liquid-crystalline state had the highest apparent activation energy of the three states. Results suggest that the benzene permselectivity was influenced by changes in the LCP membrane structure, i.e., a state-transformation. It was found that a balance of the orientation of mesogenic groups and the flexibility of the siloxane chains was very important for benzene permselectivity. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36 : 281–288, 1998     

Analysis of phase change during pervaporation with single component permeation

Individual (single) component pervaporation study helped to address some of the basic curiosities for the process of pervaporation. Investigations were carried out to focus on the location of vaporization during single component pervaporation. A mathematical model was developed for single component permeation during pervaporation, assuming two zones inside the membrane; namely, liquid permeation and vapour permeation zones. Considering a pressure distribution across the thickness of the membrane, Kelvin equation (saturation vapour pressure gets modified inside the membrane due to permeant membrane interactions) proved to be useful in developing the model. According to the model assumptions, the sorbed liquid first transports as liquid; and as soon as it finds the region, where pressure is Kelvin pressure, it evaporates and continues to transport as vapor. Further, the developed model was found to be useful in describing the flux in terms of downstream pressure variations. Accordingly, location of vaporization was determined. It was observed that vapor phase transport dominates in the membrane at low downstream pressures. Importance of consideration for both the phases, during modeling, is discussed. Activity profile, determined across the membrane, was observed to be in agreement with the experimental observations (as per literature). The study may help to establish a fundamental framework in turn to model for binary and/or multi-component mixtures.     

Gel permeation chromatography—I. The effect of flow rate and molecular weight on separation efficiency

Gel Permeation Chromatography experiments have been performed to determine the effect of flow rate on polymer elution volumes and the widths of the chromatographic peaks. Using a single polystyrene gel column, nominal porosity 1 x 10⁵ Å, a series of polystyrenes, having a narrow molecular weight distribution, has been chromatographed at a series of flow rates. Significantly lower flow rates have been used than have been previously reported. Corrections were made to account for the finite molecular weight distribution of the samples and instrumental broadening. Comparisons with the theoretical predictions for the effect of flow rate on the chromatogram have been made. The efficiency of the chromatography process is discussed in terms of the theoretical plate concept and the peak widths measured at the base. Under the conditions used, the elution volume was found to be independent of flow rate. The efficiency of the separation process was found to be highly dependent on flow rate in the range 0·05–1·5 ml/min.     

Comparison of gel permeation and ion-exchange chromatographic procedures for the separation of hyaluronate oligosaccharides

For the separation of hyaluronate oligosaccharides, gel permeation chromatography on Sephadex G-25 and ion-exchange chromatography on Dowex 1-X8 (formate form), DEAE Sephacel (chloride, acetate and formate forms) and Trisacryl M (acetate and formate forms) were compared. Best results were obtained from DEAE Sephacel (formate form) and Dowex 1-X8 (formate form). Even- and odd-numbered hyaluronic acid oligosaccharides up to decasaccharide were well separated. Contaminations were detected by high-performance liquid chromatography.     

The effects of water vapor on the separation of methane and carbon dioxide by gas permeation through polymeric membranes

The effects of water vapor on the separation of CO₂ and CH₄ using cellulose acetate, polyethersulfone, polysulfone and sulfolene modified poly(vinylidene fluoride) membranes were determined. Response surfaces of flux on a dry basis and separation factor clearly show the effects of feed gas water vapor content and temperature on the permeation characteristics of the film. Strong trends of a plasticizing effect due to the concentration of water in the membranes were apparent. In general, in the commercial films, the flux went through a maximum with increasing feed water content while the separation factor decreased. These variables changed little in the sulfolene modified film. At 90°C, water permeation accounted for up to 35 percent of the total permeate.     

Age determination of single plutonium particles after chemical separation

Age determination of single plutonium particles was demonstrated using five particles of the standard reference material, NBS 947 (Plutonium Isotopic Standard. National Bureau of Standards, Washington, D.C. 20234, August 19, 1982, currently distributed as NBL CRM-137) and the radioactive decay of ²⁴¹Pu into ²⁴¹Am. The elemental ratio of Am/Pu in Pu particles found on a carbon planchet was measured by wavelength dispersive X-ray spectrometry (WDX) coupled to a scanning electron microscope (SEM). After the WDX measurement, each plutonium particle, with an average size of a few μm, was picked up and relocated to a silicon wafer inside the SEM chamber using a micromanipulator. The silicon wafer was then transferred to a quartz tube for dissolution in an acid solution prior to chemical separation. After the Pu was chemically separated from Am and U, the isotopic ratios of Pu (²⁴⁰Pu/²³⁹Pu, ²⁴¹Pu/²³⁹Pu and ²⁴²Pu/²³⁹Pu) were measured with a thermal ionization mass spectrometer (TIMS) for the calculation of Pu age. The age of particles determined in this study was in good agreement with the expected age (35.9 a) of NBS 947 within the measurement uncertainty.     

Improved separation of palladium species in biological matrices by using a combination of gel permeation chromatography and isotachophoresis

The binding of palladium to high-molecular-mass compounds in palladium-treated lettuce is investigated as an example for a biological matrix. The total palladium concentration in lettuce leaves is 10.3 ng/g wet weight. After homogenization, high-molecular-mass compounds (> 10 kDa) are isolated by ultrafiltration. For separation of these palladium species a combination of preparative gel permeation chromatography (GPC) and preparative isotachophoresis (ITP) is used. Palladium is determined in separated fractions by using a highly sensitive total reflection X-ray fluorescence (TXRF) method after preconcentration. After GPC separation, four main fractions of palladium species are collected, each containing palladium in ng quantities (3-10 ng). Two of these fractions are further separated by ITP, yielding at least three main peaks per GPC fraction, each containing palladium in the range of 0.3-3 ng. These palladium containing peaks are characterized by high-performance size exclusion chromatography (HPSEC) and capillary isotachophoresis (cITP) in parallel. HPSEC enables the estimation of the molecular mass of six main palladium peaks, covering a molecular mass range of 69-200 kDa. It is also shown that the estimation of molecular mass after separation is more reliable than the respective estimation directly in the first GPC run. However, cITP reveals that each of the separated peaks is still a mixture of at least five different compounds.     

The permeation of organophosphorus compounds in silicone rubber membranes

The permeabilities, solubilities, and diffusivities of eight organophosphorus chemicals in silicone rubber were measured at saturation concentration using two different experimental methods: permeation experiments and absorption experiments. All tests were carried out at 25°C (±3°C). The eight organophosphorus chemicals investigated are dimethyl methylphosphonate, diethyl methylphosphonate, dimethyl hydrogenphosphonate, diethyl hydrogenphosphonate, trimethylphosphate, triethylphosphate, trimethylphosphite, and triethylphosphite. These eight chemicals were selected based on their similarities to organophosphorus chemicals used as pesticides and chemical warfare agents. The experimental data were analyzed using solutions of Fick's second law of diffusion and boundary conditions representative of the experimental settings. An unsteady-state diffusion model using boundary conditions that represent uniform initial concentration in the polymer and constant but different surface concentrations was used to interpret the permeation experimental data. In this model, the effective diffusivity calculated from the steady-state permeability and equilibrium solubility of each chemical was used and was assumed to be constant.     

Measuring the extent of phase separation during polymerization of composite latex particles using modulated temperature DSC

The morphological features of composite latex particles predominantly develop during the polymerization process and depend upon a significant number of variables. In this study, we have concentrated on the relative polarities of the two polymers in the particles and the rate at which we added the monomers during semibatch reactions containing the seed polymer latex. Our particular interest was to develop data that could reveal the extent of polymer phase separation as a function of the amount of monomer fed, and to characterize the morphology resulting from it. While TEM is the most common analytical technique employed, we show in this paper that modulated temperature DSC can generate data that allows us to follow the phase separation process as the monomer feed progresses. By considering the possibilities of having “phases” within the particles of pure polymer, homogeneously mixed (but nonequilibrium) polymers, gradient and interfacial polymer, we have been able to quite successfully simulate the DSC data. This results in quantitative estimates of the relative amounts of these “phases” and their polymer compositions. Combining these results with TEM photos showing the spatial characteristics of the morphology, we can achieve a much greater understanding of the physical structure of the composite latex particles. In many cases we find that phase separation is far from complete at the end of the reaction process. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2790–2806, 2005     

Length separation of Zwitterion-functionalized single wall carbon nanotubes by GPC

The length-fractionation of shortened (250 to 25 nm), zwitterion-functionalized, single wall carbon nanotubes (SWNTs) has been demonstrated via gel permeation chromatography (GPC). The UV-Vis spectrum of each fraction indicates an apparent "solubilization", as evident by the direct observation of all predicted optically allowed interband transitions between the mirror image spikes in the density of states of both metallic and semiconducting SWNTs with various tube diameters. As evident by the presence or absence of the 270 nm, pi-plasmon absorption, this "solubilization" is a dynamic process and leads to re-aggregation if left undisturbed for a couple of weeks or upon dissociation of the pendant octadecylamine groups. This non-destructive and highly versatile separation methodology opens up an array of possible applications for shortened SWNTs in nanostructured devices.     

Gas chromatographic separation of anaesthetic gases on a single column

Summary Gas chromatography has been employed in the separation of mixtures of air, carbon dioxide, nitrous oxide and a volatile anaesthetic (halothane, isoflurane or enflurane) on a single Chromosorb 101 column by temperature programming from room temperature. Calibration over the required range for the analysis of exhaled air, demonstrated good linearity with a repeatability for test mixtures of about 1%.     

Proton permeation into single vesicles occurs via a sequential two-step mechanism and is heterogeneous

This article describes the first single-vesicle study of proton permeability across the lipid membrane of small (approximately 100 nm) uni- and multilamellar vesicles, which were composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC). To follow proton permeation into the internal volume of each vesicle, we encapsulated carboxyfluorescein, a pH-sensitive dye whose fluorescence was quenched in the presence of excess protons. A microfluidic platform was used for easy exchange of high- and low-pH solutions, and fluorescence quenching of single vesicles was detected with single-molecule total internal reflection fluorescence (TIRF) microscopy. Upon solution exchange and acidification of the extravesicular solution (from pH 9 to 3.5), we observed for each vesicle a biphasic decay in fluorescence. Through single-vesicle analysis, we found that rate constants for the first decay followed a Poisson distribution, whereas rate constants for the second decay followed a normal distribution. We propose that proton permeation into each vesicle first arose from formation of transient pores and then transitioned into the second decay phase, which occurred by the solubility-diffusion mechanism. Furthermore, for the bulk population of vesicles, the decay rate constant and vesicle intensity (dependent on size) correlated to give an average permeability coefficient; however, for individual vesicles, we found little correlation, which suggested that proton permeability among single vesicles was heterogeneous in our experiments.