Separating efficiency, prediction

The PVP K-90 and K-120 peaks are not distorted at the high molecular weight end for the GM-PWxl column, even though the GM-PWxi column has less separation efficiency for high molecular weight PEO standards than tht GM-PW column. This is another example that the separation efficiency for these four columns for PEO standards cannot always be used to predict which... 

Liibberstedt [64] tested three different hydrocyclones for HeLa cell separation a 7 mm Bradley [67], a 10 mm Mozley (Richard Mozley Ltd., Redruth, UK), and a 10 mm Dorr-Oliver (Dorr-Oliver GmbH, Wiesbaden, Germany) (the dimension quoted here is the diameter of the cylindrical part of each hydrocyclone). The best results were obtained with the Dorr-Oliver hydrocyclone (Fig. 3), which produced a cell separation efficiency of 81 % when working at a pressure drop of 300 kPa and a flow rate of 2.8 L min When operating with two 10 mm Dorr-Oliver connected in series (the overflow of the first as feed for the second) at 200 kPa, the global efficiency of the arrangement was 94% [65]. These experimental values confirm the computational fluid dynamics (CFD) predictions that high levels of efficiencies for mammalian cells could be achieved with small diameter hydrocyclones [46]. 

Triboelectric separators for plastics have been described in detail (46,47). The separation efficiency in a triboelectric separator can be enhanced by adding media against which the components of the mixture will charge. As a result, random charging between the components of the mixture is reduced and controlled, and a predictable charging is achieved (47). 

Fig. 4. Separation efficiency vs degree of crosslinking. The solute is vitamin B-12 and the gel is hydrolyzed polyacrylamide. The solid line in the inset is the prediction from Flory excluded volume theory (Eq. 3)...

The main argument for making MIP CEC is to combine the selectivity of the MIPs with the high separation efficiency of CEC. This argument appears to fail, however, if the adsorption isotherm of the MIP is nonlinear, which seems to be the rule. In the case of nonlinear isotherms, the peak shapes depend mainly on the isotherm, particularly so if the separation system is otherwise very efficient (has low theoretical plate height, see Fig. 1). In the case of ionized analytes the situation is more complex. If an ionized analyte is not adsorbed at all on the MIP, then it is separated only due to electrophoresis, and its peak will not be widened due to the nonlinear effect. In this case, however, the MIP is merely behaving like an inert porous material. In intermediate cases an ionized analyte may participate in both separation mechanisms and for this case we do not have exact predictions of the peak shape. 

Since most other modeling techniques for polymers are extremely demanding, the limited capabilities of COSMO-RS for efficient prediction of solubilities in polymers can be of great help in practical applications when suitable polymers with certain solubility requirements are desired. One application may be the selection of appropriate membrane polymers for certain separation processes. Predictions of drug solubility in polymers are sometimes of interest for pharmaceutical applications. Furthermore, it is most likely that COSMO-RS can also be used to investigate the mutual compatibility of polymers for blends. This aspect, and many other aspects of the potential of COSMO-RS for polymer modeling, still awaits systematic investigation. 

Hydrocyclones are very simple devices and always operate with a flow ratio Rf > 0. They may be easily designed to give a desired separation efficiency (Castilho and Medronho, 2000), and their performance may also be easily predicted (Coelho and Medronho, 2001). In the last few years, it has been shown either theoretically or experimentally that hydrocyclones may be used in animal cell separations (Luebberstedt et al., 2000 Medronho et al., 2005 Elsayed et al., 2006 Pinto et al., 2007) aimed mainly at mammalian cell retention in perfusion bioreactors (Jockwer et al., 2001 Elsayed et al., 2005). 

The various effects that have been predicted for electrochromatography with perfusive flow may allow for a strong increase of the separation efficiency. The most pronounced effects are expected at high values of to, which can be created by the use of large-pore-sized particles and the use of high-ionic-strength mobile phases. 

Unfortunately, the results proved to be rather ambiguous. While the beneficial effects of pore flow on separation efficiency could be demonstrated, the increase in efficiency was only approximately 30% at conditions at which fully perfusive behavior can be expected. The highest efficiency that was reported was a reduced plate height of 1.3. This improvement is much less as predicted. 

If the geometry of an FFF channel is known exactly and a parabolic flow profile in the channel can be assumed (see Sect. 1.2), it is possible to make exact predictions about the separation of the sample as well as the separation efficiency. In this section, only the general theoretical expressions universally applicable to all FFF techniques operating in the normal mode are provided. Specialities of the different FFF methods are given during their detailed discussion in Sect. 2. 

S.6 Choice of Organic Modifier. Selection of the organic modifier type could be viewed as relatively simple The usual choice is between acetonitrile and methanol (rarely THF). In Chapters 2 and 4 the principal difference in the behavior of methanol and acetonitrile in the column is discussed. In short, methanol shows more predictable influence on the analyte elution, and the logarithm of the retention factor shows linear variation with the concentration of methanol in the mobile phase. Often for the effective separation of complex mixtures of related compounds, this ideal behavior is not a benefit and greater effect of the type and organic concentration on the separation efficiency is required. Acetonitrile as an organic modifier may offer these variations due to the introduction of a dual retention mechanism. The dual retention mechanism was discussed in Chapter 2. 

Wu et al. (1993] have developed a mathematical model based on Knudsen diffusion and intermolecular momentum transfer. Their model applies the permeability values of single components (i.e., pure gases) to determine two parameters related to the morphology of the microporous membranes and the reflection behavior of the gas molecules. The parameters are then used in the model to predict the separation performance. The model predicts that the permeability of carbon monoxide deviates substantially from that based on Knudsen diffusion alone. Their model calculations are able to explain the low gas separation efficiency. Under the transport regimes considered in their study, the feed side pressure and pressure ratio (permeate to feed pressures) are found to exert stronger influences on the separation factor than other factors. A low feed side pressure and a tow pressure ratio provide a maximum separation efficiency. 

The terms for the various contributions to the peak broadening combined in Eq. (24) give the impression that they are independent of each other. In practice, however, an interdependence exists between these terms. This leads to a much smaller decrease in separation efficiency than predicted by the simplifying van Deemter theory. 

A mathematical model to be solved numerically has been developed and used to predict the separation effects caused by nonstationary conditions for a bulk liquid membrane transport. Numerical calculations compute such pertraction" characteristics as input and output membrane selectivity (ratio of respective fluxes), concentration profiles for cations bound by a carrier in a liquid membrane phase, and the overall separation factors all being dependent on time. The computations of fluxes and separation factors as dependent on time have revealed high separation efficiency of unsteady-state pertraction as compared with steady or near-steady-state process (with reactions near equilibrium). 

There are numerous models to predict the separation efficiencies in hydrocyclones. One example is (Besendorfer 1996) ... 

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