Ionic liquid entrainers

Activity coefficients can be used as a predictive tool to design the ionic liquid structure to suit a particular application, as previously demonstrated in the screening for an optimal ionic liquid entrainer [85-89],... 

A major highlight is the solubility of cellulose in ionic liquids, which allows functionalisation to give materials suitable for a diverse range of applications, including novel applications such as materials for sensors and specialised composites. Their use as entrainers in extractive distillation is also very promising. 

Ionic liquids are a class of novel solvents with a melting point below 100°C and a negligible vapor pressure, which are interesting entrainers for extractive distillation. Examples of ionic liquids that have been investigated with respect to their potential as entrainers are l-R-3-methyl-imidazoHum-bis(trifluoromethyl-sulfonyl)-imides ([RMIM]+[CE3S02]2N-). 

Figure 3.2-5 shows separation factors at infinite dilution a with NMP and NMP-water mixtures and with various ionic liquids as possible entrainers for the separation of benzene-cyclohexane mixtures. It can be seen that the ionic liquid shows much better performance than NMP. Hence less theoretical stages are needed for the separation, leading to lower energy consumption and lower demand in equipment Conventional extractive distillation processes require an additional column for regenerating the entrainer. 

The ionic liquid is acting as an entrainer. Specific benefits of the ionic liquid are ... 

In many cases, the formation of azeotropes does not allow the separation of two or more compounds by a simple distillation. Very well known azeotropes with an industrial relevance are, for example, water/ethanol or water/THF. Sometimes the azeotrope can be broken by addition of another compound. These compounds are called entrainers. It was found that ionic liquids work as entrainers for a whole range of azeotropic systems [10]. Very high separation factors can be achieved, especially if water is part of the azeotropic mixture. Ionic liquids are usually hygroscopic materials with a strong affinity to water. Obviously, the interactions between ionic liquid and water are much stronger than those between water and the other component of the azeotrope. The ionic liquids literally grabs the water and releases the second compound, which can be distilled off as a pure material. Figure 9.5 displays the... 

To afford a sufficient separation the amount of ionic liquid added has to be in the range of 30 to 50 wt.%. This sounds like a large percentage, but is an even smaller quantity than that usually needed with a classic entrainer like dimethylformamide (DMF). In all cases the entrainer has of course to be recycled. Figure 9.6 shows a generic flow chart for the ionic liquids-based process. 

Table 3. For the pyridinium-based ionic liquids, a much stronger interaction is expected between the pyridinium ring of the cation and the aromatic solute [24], However, it can be observed that the incorrect choice of anion and the alkyl substituents on the pyridinium ring can adversely affect this. Pyridinium ionic liquids based on the [mebupy] cation, such as [mebupy] [BF4], have proven to be quite effective as entrainers, as observed by Meindersma et al. [17] in liquid-liquid equilibria studies.

Ionic liquids were found to be suitable entrainers for the separation of azeotropic (water + alcohol) mixtures by means of extractive distillation or solvent ex-... 

Generally, most hydrophilic ionic liquids are the best entrainers. " A decrease in alkyl chain length of the cation enhances the relative volatility of the alcohol. " " The choice of anion also has a large influence. Ionic... 

Other azeotropic organic mixtures that can be separated by using ionic liquids as entrainers include (alkane + ester), (cycloalkane + ketone) and (benzene + hexafluorobenzene). " Especially the effect of the ionic liquid on the last-mentioned mixture is interesting. The binary (benzene + hexafluorobenzene) is known to have double azeotropes that is, a minimum pressure and a... 

In the same extractive distillation category, recent research developments have shown that it is possible to use nonvolatile hyperbranched polymers or ionic liquid as entrainers in the extractive distillation process. An example in the literature (Seiler et al." ) is for ethanol-water separation using hyperbranched polyglycerol or ionic liquid like [EMIM] [BF4] as entrai-ner. Because the entrainer is nonvolatile, the recovery of entrainer is much easier than saline extractive distillation. 

Continuous Phase Composition Emulsion liquid membrane properties can be significantly influenced by changing the composition of the external aqueous phase. Emulsion stability can be improved by an increase in the viscosity as a result of the decrease in the rate of fluid drainage between the liquid films [88]. An increase in ionic strength of the external phase has been shown to cause a decrease in entrainment phenomena during permeation. This has been attributed to an alteration of the stmcture of the interface between the emulsion and the external phase promoted by the presence of electrolytes in the external phase. A reduction in osmosis also occurs due to a reduction in the chemical potential difference between both sides of the membrane [94,98]. 

This type of adsorption is the basis for a number of important industrial processes, notably the separation of mineral ores by froth flotations (Somasun-daran, 1979), the de-inking of waste paper (Turai, 1982), the ultrapurification of fine powders for the chemical and ceramic industries (Mougdil, 1991), and the production of foamed concrete. In the last case, for the concrete to entrain air, it is not even necessary for the liquid phase to show any foaming. In most of the processes used above, surfactants, such as salts of long-chain carboxylic acids or long-chain amines, that adsorb with their polar or ionic heads oriented toward the solid and their hydrophobic groups oriented away from it, are used to make the surface of the solid hydrophobic. 

In addition both methods and processes are claimed for drying surfaces using similar (but apparently different) technology. They include USPs 5,980,642 and 6,096,240 which involve use of a (per)fluoropolyether coupled with an non ionic additive, and USP 5,733,416 which involves a process wherein water is displaced and entrained in a water-displacement composition comprising an organic liquid and a surfactant with the water evaporated at a rate exceeding the entrainment rate. 

These have the advantage of high membrane flux, which results from the very small thickness of the organic membrane. However, there are a number of operational difficulties. The first of these concerns the osmotic transport of water across the membrane as a result of different ionic concentrations in the two aqueous phases. This causes the membrane drops to swell and ultimately to break down, mixing the strip and feed solutions. Another difficulty arises with the ultimate breaking of the emulsion and separation of the two phases that can give rise to entrainment problems. In addition, the overall process is much more complex than that of the supported liquid membrane. 

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