Distillation pinch

The concept of minimum reflux is more complex in azeotropic distillation, because of the high non-ideal behaviour and distillation boundaries. For the special case of ternary distillation, the analysis may be simplified. It is useful to mention that the minimum reflux is linked with the concept of distillation pinch. This represents a zone of constant phase composition, so that the driving force becomes very small. Consequently, the number of necessary stages for separation goes to infinite. Similarly, there is a minimum reboil rate. In this respect, three classes of limiting separations may be distinguished (Stichlmair and Fair, 1999). Figures 9.36 to 9.38 present concentration profiles obtained by simulation with an ideal system benzene-toluene-ethyl-benzene. 

Example 6.1 The flowsheet for a low-temperature distillation process is shown in Fig. 6.19. Calculate the minimum hot and cold utility requirements and the location of the pinch assuming AT, m = 5°C. 

The consequences of placing distillation columns in different locations relative to the pinch will now be explored. There are two possible ways in which the distillation column can be integrated. The reboiler and condenser can be integrated either across the pinch or not across the pinch. 

Distillation across the pinch. This arrangement is shown in... 

All these arguments can be summarized by a simple statement The appropriate placement for distillation is not across the pinch. ... 

Let us now consider a few examples for the use of this simple representation. A grand composite curve is shown in Fig. 14.2. The distillation column reboiler and condenser duties are shown separately and are matched against it. Neither of the distillation columns in Fig. 14.2 fits. The column in Fig. 14.2a is clearly across the pinch. The distillation column in Fig. 14.26 does not fit, despite the fact that both reboiler and condenser temperatures are above the pinch. Strictly speaking, it is not appropriately placed, and yet some energy can be saved. By contrast, the distillation shown in Fig. 14.3a fits. The reboiler duty can be supplied by the hot utility. The condenser duty must be integrated with the rest of the process. Another example is shown in Fig. 14.36. This distillation also fits. The reboiler duty must be supplied by integration with the process. Part of the condenser duty must be integrated, but the remainder of the condenser duty can be rejected to the cold utility. 

Starting with an inappropriately placed distillation, if it is shifted above the pinch by increasing its pressure, the condensing stream. 

If the distillation column will not fit either above or below the pinch, then other design options can be considered. One possibility is... 

The composite curves for this flowsheet are shown in Fig. 14.86. The composite curves are dominated by the reboilers and condensers of the two distillation columns and the feed vaporizer for the acetone feed. It is immediately apparent that the two distillation columns are both inappropriately placed across the pinch. Linnhoflf and Parker ... 

Figure 14.8 Simplified fiowsheet for the acetic anhydride process. The composite curves show both distillation columns to be across the pinch. (From Smith and Linnhoff, Trans. IChemE, ChERD, 66 195, 1988 reproduxxd by permission of the Institution of Chemical Engineers.)...

The appropriate placement of distillation columns when heat integrated is not across the pinch. The grand composite curve can be used as a quantitative tool to assess integration opportunities. 

The concept of the appropriate placement of distillation columns was developed in the preceding chapter. The principle also clearly applies to evaporators. The heat integration characteristics of distillation columns and evaporators are very similar. Thus evaporator placement should be not across the pinch. ... 

Minimum Reilux with Pinch Zone. There are some distillations where the minimum reflux does not occur at the intersection of the upper and lower operating lines and the q line. These cases arise when the equiUbrium is skewed from positive activity coefficients and when the operating line intersects the equiUbrium line in a 2one of constant composition, a pinch 2one, which is not at the line intersection. Figure 14 illustrates such a case. An example of such a pinch 2one in an ethanol—water column is available (37). 

Extractive distillation is defined as distillation in the presence of a miscible, high boiling, relatively nonvolatile component, the solvent, that forms no azeotropes with the other components in the mixture (23). It is widely used in the chemical and petrochemical industries for separating azeotropic, close-boiling, and other low relative volatiUty mixtures, including those forming severe tangent pinches. 

In distillation operations, separation results from differences in vapor-and liquid-phase compositions arising from the partial vaporization of a hquid mixture or the partial condensation of a vapor mixture. The vapor phase becomes enriched in the more volatile components while the hquid phase is depleted of those same components. In many situations, however, the change in composition between the vapor and liquid phases in equihbrium becomes small (so-called pinched condition ), and a large number of successive partial vaporizations and partial condensations is required to achieve the desired separation. Alternatively, the vapor and liquid phases may have identical compositions, because of the formation of an azeotrope, and no separation by simple distillation is possible. 

In normal applications of extractive distillation (i.e., pinched, closeboiling, or azeotropic systems), the relative volatilities between the light and heavy key components will be unity or close to unity. Assuming an ideal vapor phase and subcritical components, the relative volatility between the light and heavy keys of the desired separation can be written as the produc t of the ratios of the pure-component vapor pressures and activity-coefficient ratios whether the solvent is present or not ... 

The variable that has the most significant impact on the economics of an extractive distillation is the solvent-to-feed (S/F) ratio. For closeboiling or pinched nonazeotropic mixtures, no minimum-solvent flow rate is required to effect the separation, as the separation is always theoretically possible (if not economical) in the absence of the solvent. However, the extent of enhancement of the relative volatihty is largely determined by the solvent concentration and hence the S/F ratio. The relative volatility tends to increase as the S/F ratio increases. Thus, a given separation can be accomplished in fewer equihbrium stages. As an illustration, the total number of theoretical stages required as a function of S/F ratio is plotted in Fig. 13-75 7 for the separation of the nonazeotropic mixture of vinyl acetate and ethyl acetate using phenol as the solvent. 

A 250-ml flask is charged with 28 g (0.20 mole) of 2-acetyIcyclohexanone and 25 g (0.22 mole) of 30% hydrogen peroxide in 100 ml of /-butyl alcohol. The solution is refluxed for 3 hours, cooled, and a pinch of palladium on charcoal (10%) is cautiously added. The mixture is refluxed for an additional hour to decompose excess peroxide. The cooled mixture is then filtered through celite, and the volume is reduced by removal of /-butyl alcohol and water at reduced pressure. Distillation of the residue affords about 85% of cyclopentanecarboxylic acid, bp 59-6271 rnm, 123727 mm, 215-21671 atm. 

Figure 8-25. Example of typical pinch point for critical region for high-pressure distillation. Used by permission, Wichterle, I., Kobayashi, R., and Chappeiear, P. S., Hydrocarbon Processing, Nov. (1971) p. 233, Gulf Publishing Co., all rights reserved.

Cyclopentadiene was prepared2 by heating dicyclopentadiene (purchased from Eastman Organic Chemicals) and a pinch of hydro-quinone (1,4-benzenediol) under a column of glass helices or a Vigreux column at 175° and collecting the distillate in a receiver cooled with a 2-propanol-dry ice bath. The monomer was dried over Linde 4A Molecular Sieves at —20° and could be stored at this temperature for several weeks without excessive dimerization. 

Install distillation columns above or below the pinch. 

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