Other Separation Methods

Membrane separation processes are discussed in Chapter 8. Liquid-phase mass transfer rates at the surface of membranes - either flat or tubular - can be predicted by the correlations given in this chapter. 

A variety of gas-liquid contacting equipment with mechanical moving elements (e.g., stirred (agitated) tanks with gas sparging) are discussed in Chapters 7 and 12, including rotating-disk gas-liquid contactors and others. 

A number of researchers have studied high-efficiency membrane filtration techniques as they apply to surfacant micelles. This process is called ultrafiltration, microfiltration, and nanofiltration, depending on the pore size of the membranes. These techniques are applied both to isolation of surfactants themselves and, in micelle enhanced ultrafiltration, to separation of other compounds that are trapped in surfactant micelles so that they are too large to permeate the membrane (99). 

Ultrafiltration with membranes of typical pore sizes (0.1-20 nm) would not be expected to remove molecules of less than 1000 MW from aqueous solution. Surfactants, however, form micelles of large enough size that ultrafiltration can be effectively used. This was demonstrated for a nonylphenol 9-mole ethoxylate, where more than 90% of the surfactant was removed from solution (100). The separation is incomplete because indi- 

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Hydrophobic ultrafiltration membranes allow the above phenomenon to be used in reverse, permitting surfactant to permeate the membrane but holding back soil and oil (102,103). In this case, the surfactant concentration is enriched in the permeate, presumably because adsorption of the surfactant (alcohol ethoxylates) to the membrane aids in permeation. It is important to control the temperature below the cloud point of the surfactant (103). 

Nanofiltration has been demonstrated effective on the pilot scale in separating al-kanesulfonates from sodium chloride with high selectivity (104). Nanofiltration of a surfactant is a complex process and requires optimization of the membrane and ionic strength of the medium (99). 

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Modes of Operation There is a close analogy between sedimentation of particles or macromolecules in a gravitational field and their elec trophoretic movement in an electric field. Both types of separation have proved valuable not only for analysis of colloids but also for preparative work, at least in the laboratoiy. Electrophoresis is applicable also for separating mixtures of simple cations or anions in certain cases in which other separating methods are ineffectual. 

Advantages to Membrane Separation This subsertion covers the commercially important membrane applications. AU except electrodialysis are pressure driven. All except pervaporation involve no phase change. All tend to be inherently low-energy consumers in the-oiy if not in practice. They operate by a different mechanism than do other separation methods, so they have a unique profile of strengths and weaknesses. In some cases they provide unusual sharpness of separation, but in most cases they perform a separation at lower cost, provide more valuable products, and do so with fewer undesirable side effects than older separations methods. The membrane interposes a new phase between feed and product. It controls the transfer of mass between feed and product. It is a kinetic, not an equihbrium process. In a separation, a membrane will be selective because it passes some components much more rapidly than others. Many membranes are veiy selective. Membrane separations are often simpler than the alternatives. 

Separation Factor The separation factor, a, is defined consistent with other separation methods. It is important to recall that in membranes, a is the result of differing rates, and that it has no eqm-librium implications. The convention in membrane separations is to define the separation so that a > I. 

The isotope effect (i.e. the difference in the rates of evolution of hydrogen from H20 and D20) on hydrogen evolution is very important for theoretical and practical reasons. The electrolysis of a mixture of H20 and D20 is characterized, like in other separation methods, by a separation factor... 

Nondestructive hyphenated methods of analysis require follow-on spectro-photometric methods. The most common spectrophotometric analysis is FTIR, and it is most commonly associated with GC. It is, however, possible to combine it with other separation methods under specific conditions. 

Other separation methods mentioned by Fisk and Remeika (1989) include centrifuging off low-melting fluxes, evaporation of a high-vapour pressure metal, cutting crystals from the melt, etc. In the conclusion of their review, Fisk and Remeika... 

Very few, if any, recent biomedical publications describe the use of ion-impact mass spectrometry without the use of GC or some other separation method because most biological samples are chemically complex. The production of clean and useful El mass spectrometric signals requires the substance of interest to be very pure, and thus direct El experiments are usually confined to preliminary studies of highly purified biomolecules or to studies on the metabolism of pure materials. Two publications that describe direct El methods applicable to biochemical analysis and neuropharmaceutical studies are those of Costa et al. (1992) and Karminski-Zamola et al. (1995). 

The lanthanide group of elements (Table 11.7) is very difficult to separate by traditional methods because of their similar chemical properties. The techniques originally used, like the precious metals, included laborious multiple fractional recrystallizations and fractional precipitation, both of which required many recycle streams to achieve reasonably pure products. Such techniques were unable to cope with the demands for significant quantities of certain pure compounds required by the electronics industry hence, other separation methods were developed. Resin ion exchange was the first of these... 

The separation in the isobaric decaffeination processes is executed with absorption of caffeine, that means, the caffeine dissolved in CO2 is carried over into water by means of a packed washing column, or by adsorption with activated charcoal, but without recovery therefrom. Other separation methods under investigation are the use of membranes, since the difference in molecular weight between extract and solvent is high enough, or by the addition of substances of low solvent power. It is questionable whether the advantage of the possible isobaric process can compensate for the investment for the additional process steps required. 

The advantages of this technique over other separation methods can be summarized as follows. 

Hydrotalcite is often too fine grained to produce treatment columns with suitable permeability. As an alternative, the sorbent may be mixed with contaminated water in a tank (Lazaridis et al, 2002). The spent sorbent is then separated from the treated water by flocculation, flotation, or other separation methods (see Section 7.2.4). Lazaridis et al. (2002) investigated the use of surfactants with dispersed-air flotation to separate spent hydrotalcites from treated water. At ionic strengths of 0.1 M using KNO3, effective flotation and separation could be obtained by using a mixture of dodecylpyridinium chloride, sodium dodecylhydrogen sulfate, and a cetyltrimethyl ammonium bromide frother (Lazaridis et al., 2002,322,323). 

Macrocyclic antibiotics such as teicoplanin and vancomycin have been used in chiral stationary phases separations of amino acids, drugs, and other species using HPLC and other separations methods. These applications have been reviewed in a number of recent sources, including a 2004 monograph on chiral separations. [97, 101-107]... 

Equation 4.38 is, of course, essential to design effective TG columns. However, it has a much wider use in aiding our understanding of the role of natural convection in other separation methods. 

Randomly overlapping component peaks in chromatography have been studied by Rosenthal [36], by Davis and this author [33,34], and by Guiochon and co-workers [37,38]. The results may be assumed to apply to other separation methods as well. 

Chromatography, as compared to other separation methods based on phase equilibrium, stems from a notoriously heterogeneous physical system. Even in partition chromatography, where supposedly partition between bulk phases is predominant, the different kinds of high-area interfaces often lead to surface effects. In the most general case, we assume that there are several mechanisms of retention, both bulk and interfacial. In this case the numerator of the last equation must be enlarged to incorporate the other mechanisms... 

Distillation remains the most reliable separation technique, and is the most efficient in a large number of cases. Therefore, it should be tried first and other separation methods should be rated against it. 

By a hybrid separation, distillation is combined with other separation methods, such as L-L extraction, adsorption, crystallization and membranes. It is mainly... 

The basis of the electrofocusing mechanism lies in the properties of the charge-bearing constituents of proteins. The information thus provided by IEF is very useful and complements information obtained for other physical parameters. In comparison to some other separation methods, IEF is easy to... 

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