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 wather/THF. Sometimes the azeotrope can be broken by addition of another compound. These compounds are called entrainers. Ionic liquids do also act as entrainers for a great variety of azeotropic systems. Especially if water is one component of the azeotrope very high separation factors can be achieved. Ionic liquids usually are quite hygroscopic materials which shows their strong affinity to water. Obviously the interactions between ionic liquid and water are much stronger than between water and the other component of the azeotropes. The ionic liquids literally graps the water and releases the second compound which can be distilled off as pure material. In other words the ionic liquid is acting as an extractant for water and the process is then called extractive distillation. Figure 1 displays the classic equilibrium phase diagram for THF/water. The dots indicate the change after adding the ionic liquid.
Figure 1: Equilibrium phase diagram for the system THF/Water. The solid line shows the classic azeotropic mixture. The dots indicate how the azeotrope has been broken after addition of the ionic liquid. The amount of THF in the vapor phase is always larger than in the liquid phase.
To afford a sufficient separation the amount ionic liquid added has to be in the range of 30 to 50 w%. This sounds like a large percentage, but is an even smaller quantitiy than what is usually needed with a classic entrainer like dimethylformamide (DMF). In all cases the entrainer has of course to be recycled. Figure 2 shows a generic flow chart for the ionic liquids based process.
Figure 2: Generic flow chart for the ionic liquid based process. The ionic liquid is acting as an extractant washing down compound B. Compound A is released and can be distilled of as pure compound at the head of the column. B is removed at the bottom. The ionic liquid is recycled into the process after having removed residual B in a falling film evaporator.
The advantage of an ionic liquid over a classic entrainer is that ionic liquids have no vapor pressure. This means that the entrainer itself does not need to be distilled and hence a second distillation column is not necessary and a significant amount of energy can be saved. A benchmark calculation has revealed a saving potential of 37% for energy cost and 22% for the investment. BASF has run an extractive distillation process in a pilot plant continously for 3 months. Although the ionic liquid faced severe thermal exposure of about 250°C in the recycling step its performance was unchanged without any purge. This again underlines how thermally stable ionic liquids are and that extremely high recycling rates are possible without a decrease in performance.
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