**6.1. Mass transfer efficiency in sieve tray column**

**Figure 3** shows the generated relative volatility profile along the ED column that accounts for how good the separation is.

Before analysis these results, it has to be clarified that, the concentration of the solvent, either the organic solvent or the ionic liquids, is higher in the rectifying section than in the stripping section. It means from stage 2 to 12 in our ED column. This is because the solvent feed is at the top of the column and below the solvent is diluted by the feed stream at stage 12. Having mentioned this, it can be observed in **Figure 3** that the relative volatility profiles, in general, follow the same trend as **Table 1** where the ionic liquid [emim][Cl] showed the best values being the most promising solvent for water-ethanol, separation. In spite of these results, this ionic liquid would not be used in a real ED column due that it is a corrosive fluid [45] and the melting point is 87°C [46]. Anyhow, this ionic liquid will be kept for the analysis. Therefore, [emim][OAc] becomes the most promising ionic liquid which exhibits the second best relative volatilities. On the other hand, EG produces the lowest relative volatility values as indicated in **Table 1**.

Next, **Figure 4**, shows the viscosity profile at the liquid phase when using the different solvent. It is clearly seen that the increase in the liquid phase viscosity inside the ED column is directly influenced by the solvent viscosity. This explains why when using [emim][Cl] as the solvent the highest viscosity values are observed inside the column and the rest of the solvent follow the trend in **Table 1**. Therefore, the trend observed in this figure is as follows: [emim] [Cl] > [emim][OAc] > [emim][DCA] > EG. In the stripping section the liquid viscosity drops for all four solvents as the solvent concentration is reduced by dilution with the feed stream.

**Figure 3.** Relative volatility profiles along the extractive distillation column for S/F = 1.

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**Figure 4.** Viscosity profiles along the column for S/F = 1 formed when the different solvents are added to the column and

D/F = 0.4 (mass basis) [35].

As a summary, ionic liquids produced higher relative volatilities that the organic solvent, but higher viscosities as well. **Figure 5** shows the mass transfer efficiency profiles along the column for all solvents.

**Figure 5** shows the tray efficiency profiles over the ED column calculated using Eq. (2). The rectifying section shows lower mass transfer efficiencies than the stripping section due to the effect of solvent viscosity. At S/F = 1, the mass transfer efficiency order is [emim] [OAc] > [emim][DCA] > EG > [emim][Cl]. This order does not follow exactly the expected trend from **Figure 4**. Therefore, the viscosity is not the only important effect in calculating the mass transfer efficiency as observed in Eq. (2). Hence, the relative volatility values should play an important role as well. Ionic liquids are able to outperform the relative volatilities of the common organic solvents as it has previously mentioned. In **Table 1** it is observed that very good relative volatilities are produced by [emim][OAc]. This property enhances the mass transfer efficiency even though having relatively high viscosity. However, for then case of [emim][Cl], this ionic liquid shows the highest relative volatility and also it exhibits high viscosities. Here, due to the high viscosities this property is more important than Mass Transfer in Extractive Distillation when Using Ionic Liquids as Solvents http://dx.doi.org/10.5772/intechopen.76544 115

**Figure 3.** Relative volatility profiles along the extractive distillation column for S/F = 1.

**6. Results and discussion**

how good the separation is.

in **Table 1**.

column for all solvents.

**6.1. Mass transfer efficiency in sieve tray column**

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

**Figure 3** shows the generated relative volatility profile along the ED column that accounts for

Before analysis these results, it has to be clarified that, the concentration of the solvent, either the organic solvent or the ionic liquids, is higher in the rectifying section than in the stripping section. It means from stage 2 to 12 in our ED column. This is because the solvent feed is at the top of the column and below the solvent is diluted by the feed stream at stage 12. Having mentioned this, it can be observed in **Figure 3** that the relative volatility profiles, in general, follow the same trend as **Table 1** where the ionic liquid [emim][Cl] showed the best values being the most promising solvent for water-ethanol, separation. In spite of these results, this ionic liquid would not be used in a real ED column due that it is a corrosive fluid [45] and the melting point is 87°C [46]. Anyhow, this ionic liquid will be kept for the analysis. Therefore, [emim][OAc] becomes the most promising ionic liquid which exhibits the second best relative volatilities. On the other hand, EG produces the lowest relative volatility values as indicated

Next, **Figure 4**, shows the viscosity profile at the liquid phase when using the different solvent. It is clearly seen that the increase in the liquid phase viscosity inside the ED column is directly influenced by the solvent viscosity. This explains why when using [emim][Cl] as the solvent the highest viscosity values are observed inside the column and the rest of the solvent follow the trend in **Table 1**. Therefore, the trend observed in this figure is as follows: [emim] [Cl] > [emim][OAc] > [emim][DCA] > EG. In the stripping section the liquid viscosity drops for all four solvents as the solvent concentration is reduced by dilution with the feed stream. As a summary, ionic liquids produced higher relative volatilities that the organic solvent, but higher viscosities as well. **Figure 5** shows the mass transfer efficiency profiles along the

**Figure 5** shows the tray efficiency profiles over the ED column calculated using Eq. (2). The rectifying section shows lower mass transfer efficiencies than the stripping section due to the effect of solvent viscosity. At S/F = 1, the mass transfer efficiency order is [emim] [OAc] > [emim][DCA] > EG > [emim][Cl]. This order does not follow exactly the expected trend from **Figure 4**. Therefore, the viscosity is not the only important effect in calculating the mass transfer efficiency as observed in Eq. (2). Hence, the relative volatility values should play an important role as well. Ionic liquids are able to outperform the relative volatilities of the common organic solvents as it has previously mentioned. In **Table 1** it is observed that very good relative volatilities are produced by [emim][OAc]. This property enhances the mass transfer efficiency even though having relatively high viscosity. However, for then case of [emim][Cl], this ionic liquid shows the highest relative volatility and also it exhibits high viscosities. Here, due to the high viscosities this property is more important than

**Figure 4.** Viscosity profiles along the column for S/F = 1 formed when the different solvents are added to the column and D/F = 0.4 (mass basis) [35].

**Figure 5.** Tray efficiency profiles along the column for S/F = 1 calculated when the different solvents are added to the column and D/F = 0.4 (mass basis). The column is numbered from the top to the bottom [35].

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.

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

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

**Figure 6.** HETP profiles along the column for S/F = 1 (mass basis) calculated when the different solvents are added to the

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column and D/F = 0.4 (mass basis). The column is measured from the bottom (0 m) to the top (6 m) [35].

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

**toluene-methylcyclohexane**

Plate (HETP).

indicating the one-phase region to operate.

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

**Figure 6** shows the generated Height Equivalent to a Theoretical Plate (HETP) profiles along the column for the same operating conditions as the sieve tray column.

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.

**Figure 6.** HETP profiles along the column for S/F = 1 (mass basis) calculated when the different solvents are added to the column and D/F = 0.4 (mass basis). The column is measured from the bottom (0 m) to the top (6 m) [35].
