Systematic Method for Screening Ionic Liquids as Extraction Solvents Exemplified by an Extractive Desulfurization Process

Abstract

For the selection of industrially suitable ionic liquids (ILs) as extraction solvents, a systematic method combining phase equilibrium calculation, physical property prediction, and process simulation is presented. The conductor-like screening model for real solvents is used to predict the liquid–liquid equilibria of the systems composed of the target mixture to be separated and different ILs at the specific global composition of interest, thereby prescreening ILs with higher mass-based distribution coefficient and selectivity as well as lower solvent loss. Group contribution methods are then employed to estimate the key physical properties of the prescreened ILs and further suggest candidates meeting certain physical property constraints. Afterward, the performance of the top IL candidates in a continuous process is analyzed by Aspen Plus to identify finally process-based optimal solvents. The proposed method is illustrated with an extractive desulfurization case study and two most promising ILs for this process are consequently determined.

Rational design of double salt ionic liquids as extraction solvents: Separation of thiophene/n‐octane as example

“…It should be mentioned that the mole based C ∞ and S ∞ are used in Section 2 so that one can recognize the nonideal “mixing” properties of DSILs by avoiding the effect of molecular weights of ions involved in DSILs, and thus analyze the different tendencies of them from the molecular point of view. However, in our previous study, the mass based and have been proven to be more efficient than the molar based C ∞ and S ∞ [see Equations and ] for quickly prescreening promising IL solvents. The reason is that the molecular weights of ILs usually vary in a large range and are generally much higher than those of conventional organic solvents, which also holds for DSILs screening.…”

“…In other words, even two small sets of cations and anions can give rise to a huge number of possible DSILs, making the experimental trial‐and‐error method unrealistic for DSIL design. Moreover, besides the C ∞ and S ∞ of DSILs, their physical and thermodynamic properties at specific conditions of interest should also be evaluated to identify practically attractive extraction solvents . In this context, a systematic DSIL design method is proposed as illustrated in Figure , which consists of four steps.…”

“…For this purpose, in the example of thiophene/ n ‐octane separation, the infinite dilution capacity and selectivity of different DSILs, that is, C ∞ and S ∞ , are calculated by COSMO‐RS as follows: where and stand for the infinite dilution activity coefficient of thiophene and n ‐octane in DSILs, respectively. The C ∞ and S ∞ can offer a simple and quick estimation of the separation performances of DSILs for the extraction of thiophene from n ‐octane . In the calculation, 20 common cations of the imidazolium, pyridinium, and pyrrodinium families are covered together with 25 widely reported anions.…”

“…A large number of experimental and theoretical studies have been reported on the identification of suitable ILs for different separation processes . Particularly, several research groups have employed COSMO‐based activity coefficient models or the UNIFAC‐IL model to perform computational screening and design of ILs as separation solvents. Because of the predictive character of these thermodynamic models, the separation performance of ILs that have not been experimentally covered can be estimated and thus the identification of suitable solvent candidates in a large range is possible.…”

“…Despite the progress made, the selection of practically attractive IL for a specific process is still challenging as in many cases very few ILs can satisfy different desirable properties simultaneously. For instance, when screening ILs as extractive desulfurization solvent, Song et al found that only 831 of the initial 36,260 cation–anion combinations meet the thermodynamic property requirements, among which only 15 satisfy the physical property constraints . Conceivably, if more restrictions such as easy preparation, commercial availability, low cost, and so on of ILs are considered, the number of feasible solvents will be further reduced and in some cases even no candidates could retain.…”