Purification of gases 16





Simulation studies on gas solubilities
Gas solubility
ILs as separating agents


Simulation studies on gas solubilities

Experimentally, it has been found that many gases, including CO2, are highly soluble in ionic liquids. Simulations have been conducted to investigate this, and results indicate that the anion is primarily responsible for high CO2 solubility.

radial distribution function for CO2 associating
with the PF6 anion in 1-n-butyl-2(H/CH3)-3-methylimidazolium
PF6
In Fig. 1, calculations from the paper by Cadena et al. show the radial distribution function for CO2 associating with the PF6 anion in 1-n-butyl-2(H/CH3)-3-methylimidazolium PF6. The blue curve shows the anion-CO2 g(r) for the case when the carbon at the 2-position on the ring is bonded to a hydrogen, and the red curve shows when this group is a methyl group. The curves are virtually indistinguishable, despite the fact the this subtle change greatly affects the organization of the anion about the cation in the bulk liquid.(bmim)(PF6) simulation
Experimental solubility measurements confirm that the two ionic liquids have roughly the same CO2 solubility. These results suggest that the cation plays a relatively minor role in controlling CO2 solubility (at least for these compounds).

Fig.1: Radial distribution function for CO2 associating with the PF6 anion in 1-n-butyl-2(H/CH3)-3-methylimidazolium PF6


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Gas solubility

The solubility of gases in ionic liquids is important, since many applications focus on the use of these liquids as replacement solvents for reactions involving gaseous species. It can be measured using a gravimetric technique, by means of a microbalance, originally developed for measuring adsorption on solids. Due to the non-volatile nature of the ionic liquids, we can use this balance to measure gas solubility.

Water isotherms in
(bmim)(PF6) as a function of
temperature.Fig.2: Water isotherms in (bmim)(PF6) as a function of temperature.

Fig. 2 shows a typical set of isotherms for water in (bmim)(PF6). Note that the ionic liquid takes up a large amount of water (on a mol fraction basis), even at relatively low pressure.

Isotherms for various gases in (bmim)(PF6) at 25C. Notice the enhanced solubility of CO2 relative to other gases.

Fig. 3 shows a series of isotherms for various gases at 25C. Except for CO2, all the gases remain in the Henry's Law regime up to 13 bar. CO2 shows some non-linearity, indicating some degree of saturation and possible CO2-CO2 interactions in the liquid.




Fig.3: Isotherms for various gases in (bmim)(PF6) at 25 °C.
Notice the enhanced solubility of CO2 relative to other gases.


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Ionic liquids as separating agents

A simple gas absorption setupIf an ionic liquid can be found to preferentially dissolve certain gaseous species, then it can be used in conventional gas absorption applications. The non-volatile nature of ionic liquids plays two important roles. First, there will be no cross-contamination of the gas stream by the solvent during operation. This means no solvent loss and no air pollution. Second, regeneration of the solvent is easy. A simple flash or mild distillation step is all that is required to remove the gas from the solvent, again with no cross-contamination. Fig. 4 shows a simple absorber schematic.


Fig 1: A simple gas absorption setup. Contaminated gas is fed to the bottom of the absorber, and ionic liquid is fed in the top in counter-current fashion. Purified gas is removed from the column (uncontaminated by the non-volatile solvent), and the absorbed gas is removed in the liquid effluent and treated in another unit operation. The ionic liquid is recycled to the top of the column.



In addition to their use as conventional absorbents, ionic liquids may also be immobilized on a support and used in a supported liquid membrane (SLM).
The membrane will work if a gas preferentially dissolves in the liquid. SLMs may be used in a continuous separation process without a regeneration step. SLM technology has been around for a while, but one of the weaknesses of these membranes is that the liquid in which the gas dissolves eventially evaporates, thus rendering the membrane useless. Since ionic liquids are completely non-volatile, this problem is entirely eliminated.

Studies for the separation of CO2 from N2 and CH4, as well as the separation of ethane from ethylene, have been carried out. The results to date are encouraging, and show that selectivity generally follows a solution-diffusion mechanism, with solubility the dominant factor. It results that the selectivity of CO2 over N2 is greater than 400, indicating that CO2 could be almost completely removed from air using a continuous SLM setup. [Group]

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