Abstract
As a result of their high tuneability and low volatility, room temperature ionic liquids have been proposed as replacement solvents in a wide range of industrial applications. They are particularly well-suited for use as an entrainer (or solvent) in extractive distillation processes to separate close boiling and azeotropic mixtures. The limiting selectivity is a common, fundamental parameter used to screen and rank entrainer candidates. In the present study, we present a detailed thermodynamic analysis to understand the basis for its use along with the necessary, underlying assumptions. We find that, while for many cases the limiting selectivity can correctly rank ionic liquid entrainer candidates for homogeneous extractive distillation processes, it is not always able to capture the correct phase behavior. We, instead, recommend the use of composition dependent activity coefficients.
Highlights
The separation of azeotropic and close boiling mixtures via distillation is not without great challenges
We find that the smaller the value of the relative volatility, the greater the number of required stages, and in the limit that the relative volatility goes to unity, the number of stages goes to infinity
For all of the systems studied, we use binary interaction parameters for the NRTL equation provided in the original publications, allowing us to calculate both composition dependent and limiting activity coefficients for the most volatile component (MVC) and least volatile component (LVC) in the ternary mixture formed by the addition of ionic liquids (ILs) [28]
Summary
The separation of azeotropic and close boiling mixtures via distillation is not without great challenges. If we take the IL (entrainer) and MVC (or LVC) to be a binary mixture, we suspect similar behavior may exist, causing the activity coefficient of the LVC (or MVC) to go through an extremum at an intermediate IL composition The implication of this would be the appropriateness of S∞ to screen and rank ILs as entrainer candidates. In the present study, we investigate the ability to use S∞ (or limiting activity coefficients) to screen ILs as entrainer candidates for homogeneous extractive distillation processes. For all of the systems studied, we use binary interaction parameters for the NRTL (non-random, two-liquid) equation provided in the original publications, allowing us to calculate both composition dependent and limiting activity coefficients for the MVC and LVC in the ternary mixture formed by the addition of IL [28]. This may be in part due to the limitation of using S∞ as will be explored here, and is due to the importance of the inclusion of additional properties
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