**1. Introduction**

Azeotropic and close boiling point mixtures cannot be separated by normal fractional distillation. Extractive distillation (ED) is an energy efficient technology that enables the separation of these complex mixtures by using a high boiling point solvent added at the top of the column. With this, the activity coefficients at the liquid phase are modified improving the

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

relative volatiles. As a result, high purity products are obtained at the top of the distillation column with low energy demand. **Figure 1** depicts a common extractive distillation process including the solvent recovery column.

An important parameter in the ED process is the solvent-to-feed ratio which is defined by the ratio of the solvent added to the column and the feed stream added to the column on mass basis:

$$\mathbf{S}/\text{F} = \text{solvent feed stream/feed stream} \tag{1}$$

However, ionic liquids also show high viscosities. For example, one of the less viscous ionic liquids, 1-ethyl-3-metylimidazolium dicyanamide ([emim][DCA]), shows viscosities around 15 mPa·s at 298.15 K [27–30]. Nevertheless, ionic liquids containing halides as anion, such as chloride [I] or bromide [Br], exhibit much larger viscosities than the previous anion. For instance, the ionic liquid 1-octyl-3-methylimidazolium bromide ([omim][Br]) displays a viscosity of around 5000 mPa·s at 298.15 K [31] while 1-octyl-3-methylimidazolium chloride ([omim][Cl]) a value of 20.000 mPa·s at 298.15 K [32, 33]. These viscosity values could bring a

Mass Transfer in Extractive Distillation when Using Ionic Liquids as Solvents

http://dx.doi.org/10.5772/intechopen.76544

109

decrease in mass transfer efficiency when using ionic liquids in extractive distillation.

effects of ionic liquid on the separation need to be evaluated.

equipped with structured packing.

**separation**

Because of this, an ED column with more separation stages due to the mass transfer limitations will be required masking the above gained advantages provided by ionic liquids. However, a good point is that, the viscosity of ionic liquids drastically decreases with increasing the temperature [27] and since ED is a thermal separation process, the real mass transfer

Therefore, the main aim of this work is to evaluate the decrease in mass transfer throughout the mass transfer efficiency concept when using ionic liquids in the extractive distillation of two important cases: separation of water ethanol and methylcyclohexane-toluene mixtures. The first represents the industrial case of dehydration of ethanol and the second the separation of aromatics from aliphatic in the petrochemical industry. On the other hand, the mass transfer efficiency will be evaluated using rate-based modeling or non-equilibrium stage modeling [34] since, on contrary of the equilibrium model, this one considers a real separation stage which is limited by the mass transfer. Therefore, the effect of the viscosity of ionic liquids of mass transfer efficiency would be quantified by this model. Finally, an experimental evaluation of the decrease in mass transfer efficiency is performed in an ED pilot plant

**2. Ionic selection for water: ethanol and toluene: methylcyclohenene** 

experimentally determined relative volatilities and viscosities for both case studies.

As it was mentioned above, ionic liquids show better performance than organic solvents in terms of vapor–liquid equilibrium. Therefore, the selection of the ionic liquids for a certain case study is based on the increase in relative volatility. The work of Ge et al. [23] shows an experimental selection of ionic liquids for water-ethanol separation. On the other hand, Gutierrez-Hernandez et al. [25] carried out a selection of ionic liquids for methylcyclohexane-toluene separation based on liquid-liquid extraction experiments. Nevertheless, the high viscosities of ionic liquids could limit the mass transfer in the ED column. **Table 1** contains

It can be observed in **Table 1** that the selected ionic liquids for the separation of the waterethanol mixture, 1-ethyl-3-methylimidazolium chloride ([emim][Cl) and 1-ethyl-3-methylimidazolium acetate ([emim][OAc]) showed higher produced relative volatilities than the conventional organic solvent ethylene glycol (EG). For the second case, the ionic liquid

The common solvents used in industry are normally of organic nature such as ethyleneglycol for separating water and ethanol [1–3], phthalic anhydride [4], *N*-methyl-pyrrolidone (NMP) and sulfolane [5–7] for the separation of aromatic from aliphatic and 1,3 butadiene from C<sup>4</sup> hydrocarbons, 1,2 propanediol [8, 9], 1,4 butanediol [10] or dimethyl sulfoxide [11] for the dehydration of tetrahydrofuran among other separations. These organic solvents show several drawbacks. Since the solvent is added at the top of the column and the organic solvents are volatile, they reduce the product purity. Besides this, large amounts of solvent are required to achieve certain separation. Lately, a new class of solvents called ionic liquids have been proposed as a novel solvent for ED due to their properties such us negligible vapor pressure and high selectivity in separation processes. Therefore, a non-volatile solvent is added to the column producing free solvent products in the condenser. Lately, many works have been published regarding the separation of azeotropes and close boiling point mixtures using ionic liquids as solvent, most of them are related to vapor-liquid equilibrium evaluation of ionic liquids for improving relative volatilities [12–22]. Here, it has been demonstrated that, in general, ionic liquids can produce better relative volatilities than organic solvents [22–25]. This advantage results in less solvent needed to perform the separation which brings less operational cost, or an ED column with less separation stages or a decrease in capital cost with regard a conventional ED column operating with organic solvents [26].

**Figure 1.** Scheme of a conventional extractive distillation unit and the solvent recovery step.

However, ionic liquids also show high viscosities. For example, one of the less viscous ionic liquids, 1-ethyl-3-metylimidazolium dicyanamide ([emim][DCA]), shows viscosities around 15 mPa·s at 298.15 K [27–30]. Nevertheless, ionic liquids containing halides as anion, such as chloride [I] or bromide [Br], exhibit much larger viscosities than the previous anion. For instance, the ionic liquid 1-octyl-3-methylimidazolium bromide ([omim][Br]) displays a viscosity of around 5000 mPa·s at 298.15 K [31] while 1-octyl-3-methylimidazolium chloride ([omim][Cl]) a value of 20.000 mPa·s at 298.15 K [32, 33]. These viscosity values could bring a decrease in mass transfer efficiency when using ionic liquids in extractive distillation.

relative volatiles. As a result, high purity products are obtained at the top of the distillation column with low energy demand. **Figure 1** depicts a common extractive distillation process

108 Heat and Mass Transfer - Advances in Modelling and Experimental Study for Industrial Applications

An important parameter in the ED process is the solvent-to-feed ratio which is defined by the ratio of the solvent added to the column and the feed stream added to the column on mass

S/F = solvent feed stream/feed stream (1)

The common solvents used in industry are normally of organic nature such as ethyleneglycol for separating water and ethanol [1–3], phthalic anhydride [4], *N*-methyl-pyrrolidone (NMP) and sulfolane [5–7] for the separation of aromatic from aliphatic and 1,3 butadiene

the dehydration of tetrahydrofuran among other separations. These organic solvents show several drawbacks. Since the solvent is added at the top of the column and the organic solvents are volatile, they reduce the product purity. Besides this, large amounts of solvent are required to achieve certain separation. Lately, a new class of solvents called ionic liquids have been proposed as a novel solvent for ED due to their properties such us negligible vapor pressure and high selectivity in separation processes. Therefore, a non-volatile solvent is added to the column producing free solvent products in the condenser. Lately, many works have been published regarding the separation of azeotropes and close boiling point mixtures using ionic liquids as solvent, most of them are related to vapor-liquid equilibrium evaluation of ionic liquids for improving relative volatilities [12–22]. Here, it has been demonstrated that, in general, ionic liquids can produce better relative volatilities than organic solvents [22–25]. This advantage results in less solvent needed to perform the separation which brings less operational cost, or an ED column with less separation stages or a decrease in capital cost with

regard a conventional ED column operating with organic solvents [26].

**Figure 1.** Scheme of a conventional extractive distillation unit and the solvent recovery step.

hydrocarbons, 1,2 propanediol [8, 9], 1,4 butanediol [10] or dimethyl sulfoxide [11] for

including the solvent recovery column.

basis:

from C<sup>4</sup>

Because of this, an ED column with more separation stages due to the mass transfer limitations will be required masking the above gained advantages provided by ionic liquids. However, a good point is that, the viscosity of ionic liquids drastically decreases with increasing the temperature [27] and since ED is a thermal separation process, the real mass transfer effects of ionic liquid on the separation need to be evaluated.

Therefore, the main aim of this work is to evaluate the decrease in mass transfer throughout the mass transfer efficiency concept when using ionic liquids in the extractive distillation of two important cases: separation of water ethanol and methylcyclohexane-toluene mixtures. The first represents the industrial case of dehydration of ethanol and the second the separation of aromatics from aliphatic in the petrochemical industry. On the other hand, the mass transfer efficiency will be evaluated using rate-based modeling or non-equilibrium stage modeling [34] since, on contrary of the equilibrium model, this one considers a real separation stage which is limited by the mass transfer. Therefore, the effect of the viscosity of ionic liquids of mass transfer efficiency would be quantified by this model. Finally, an experimental evaluation of the decrease in mass transfer efficiency is performed in an ED pilot plant equipped with structured packing.
