**6.3. Experimental study of mass transfer efficiency in the system toluene-methylcyclohexane**

relative volatility and becomes the limiting factor in mass transfer efficiency. Therefore, moderately high viscosities of ionic liquids in ED would not limit the mass transfer when combined with high values of relative volatility. However, the relative volatility does not enhance mass transfer efficiency sufficiently in the presence of a very viscous ionic liquid.

**Figure 5.** Tray efficiency profiles along the column for S/F = 1 calculated when the different solvents are added to the

**Figure 6** shows the generated Height Equivalent to a Theoretical Plate (HETP) profiles along

For the structured packing, the values of mass transfer efficiency are represented the HETP, and here the lowest value means the most efficient case. The mass transfer efficiency order is [emim][OAc] > [emim][DCA] ≈ [emim][Cl] > EG in the rectifying section. The observation of these profiles does not produce different conclusions from sieve trays. However, two important points are observed here. Firstly, in contrast to sieve trays, more notorious difference in efficiency is shown here. This is explained by the fact that in packed columns the liquid and vapor flow are in countercurrent and the packing surface allows an intimate vapor-liquid contact. As a result, the packed distillation column operates closer to equilibrium than sieve trays, and thereby the effect of the relative volatility predominates over the increase in liquid phase viscosity. This is the reason why [emim][Cl] produces now better mass transfer efficiencies than EG on contrary to the case of the sieve tray column and this latter solvent presents the lowest mass transfer efficiency (highest *HETP*). It worth to mention that, the results obtained here were previously validated in a pilot plant where that developed rate-based model predicts the performance of the pilot plant within 10% error.

**6.2. Mass transfer efficiency in Mellapack® 250Y structured packing**

column and D/F = 0.4 (mass basis). The column is numbered from the top to the bottom [35].

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

the column for the same operating conditions as the sieve tray column.

The extractive distillation of toluene-methylcyclohexane is an interesting case because the ionic liquid [hmim]TCB] overcome by far the relative volatility of the conventional organic solvent NMP (see **Table 1**). In addition to this, the viscosity of the ionic liquid is not as high as [emim][OAc] for example. Therefore, one would not expect a decrease in mass transfer efficiency as it was concluded before. However, since this is a very nonpolar mixture and the ionic liquid is a solvent of a polar nature, phase split is expected when mixing. Nevertheless, the phase splitting can be solved by increasing the S/F ration at high values [25]. Due to this fact, there could be a decrease in mass transfer efficiency. **Figure 7** shows the ternary map indicating the one-phase region to operate.

As it can be observed in **Figure 7**, to reach the one-phase region, two conditions should be set: first, high concentration of [hmim][TCB] inside the column and high methylcyclohaxene distillate rates (or low reflux ratios) to keep its concentration as low as possible inside the column to avoid phase split. **Figure 8** shows the experimentally obtained liquid phase concentration profiles when NMP (**Figure 8a**) and [hmim][TCB] (**Figure 8b**) where the solvent, respectively.

It can be observed that, due to the high S/F ratios, high solvent concentration was developed inside the ED column for both cases NMP and [hmim][TCB]. However, when using the organic solvent, a little lower liquid phase solvent concentration is observed due to this solvent is volatile. High concentration will lead to high liquid phase viscosities. **Figure 9** shows the viscosity profiles inside the column and the Height Equivalent to a Theoretical Plate (HETP).

**Figure 7.** Ternary diagram for the system toluene-methylcylcohexane-[hmim][TCB] [38].

In **Figure 9a** it can be clearly observed the influence of the viscosity of [hmim][TCB] due to the high S/F ratios. While the separation of the toluene-methylcyclohexane mixture with NMP show low viscosities, [hmim][TCB] exhibit high values reaching the 4 [cP] in the rectifying section leading to a decrease in mass transfer efficiency as observed in **Figure 9b**. The impact of the liquid phase resistance on the mass transfer efficiency is significant even though the use of the first solvent produces much higher relative volatilities. The conclusions made in this work should be used in the selection of the ionic liquids for extractive distillation processes

**Figure 9.** Liquid phase viscosity inside the ED column and HETP for the system toluene-methylcyclohexane using both

Mass Transfer in Extractive Distillation when Using Ionic Liquids as Solvents

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

119

In this study, the mass transfer efficiency of the extractive distillation with ionic liquids has been analyzed for the water ethanol mixtures using a developed rate-based model and in a

The results from the rate-based indicated that the use of ionic liquids as solvents in ED is ben-

The high viscosities of ionic liquids were reflected in the increase of que liquid phase viscosity inside the ED column. However, this viscosity did not decrease the mass transfer efficiency of the ED whereas the relative volatilities are high. On the other hand, when very high viscosities or high solvent to feed ratios are applied to the ED column, a decrease in mass transfer

Project Erasmus Mundus Chile, Project FONDECYT 11150417 and Project RC-130006-CILIS granted by Fondo de Innovación para la Competitividad, del Ministerio de Economía,

Referenced tables and figures were taken from Computer & Chemical Engineering Journal

Fomento y Turismo, Chile are kindly acknowledged for funding this book chapter.

eficial due to the increase in relative volatilities when compared to organic solvents.

efficiency was observed even though having high relative volatilities.

and from Separation & Purification Technology Journal.

along with the studies on relative volatility.

pilot plant scale extractive distillation column.

**7. Conclusions**

solvent NMP and [hmim][TCB] [38].

**Acknowledgements**

**Figure 8.** Concentration profiles (mass fractions) for the ED of toluene-MCH using (a) NMP and (b) [hmim][TCB] as solvents. For cases 1 and 2 [38].

**Figure 9.** Liquid phase viscosity inside the ED column and HETP for the system toluene-methylcyclohexane using both solvent NMP and [hmim][TCB] [38].

work should be used in the selection of the ionic liquids for extractive distillation processes along with the studies on relative volatility.
