Heteroazeotropic column

Figure 2.13. (a) A heteroazeotropic column with decanter, (b) the distillation trajectory of heteroazeotropic column for separation of benzene(l)-isopropil alcohol(2)-water(3) mixture (benzene-entrainer). xli and xl2, two liquid phases xp, initial feed xp+p, total feed into column region of two liquid phases is shaded. 

Experimental verification of the separation of the mixture u-hexane-ethyl acetate by heteroazeotropic batch distillation in a bench scale rectification column... 

The analysis in the RCM suggests that a RD process may be feasible, as the trajectories converge to the ester vertex, the highest boiler and stable node. The heteroazeotrope of water-alcohol is an unstable node, while the heteroazeotrope water-acid is a saddle. At total conversion, the temperatures at the column s ends at atmospheric pressure would be 373 K in top and about 713 K in bottom. Clearly, the last value is excessive. Here we assume a temperature below 473 K to avoid thermal decomposition, a condition that can be realized working under vacuum at 32kPa and diluting with about 12mol% alcohol. 

Benzene dning, either by azeotropic or preferably heteroazeotropic distiHation, or on molecular sieves. To obtain a residual water content less than 30 ppm, the distillation column must have at least 15 trays. 

For heterogeneous batch distillation a new double column configuration operating in closed system is suggested. This configuration is investigated by feasibility studies based on the assumption of maximal separation and is compared with the traditional batch rectifier. The calculations are performed for a binary (n-butanol - water) and for a ternary heteroazeotropic mixture (isopropanol - water + benzene as entrainer). Keywords heteroazeotrope, batch distillation, feasibility studies. 

If components of a mixture form a heteroazeotrope or by the addition of an entrainer (E) a heteroazeotrope can be formed, the azeotropic composition can be crossed by decantation. In the pharmaceutical and fine chemical industries batch processes including the batch heteroazeotropic distillation (BHD) are widely applied. As far as we know the BHD was exclusively applied in the industry in batch rectifiers (equipped with a decanter) in open operation mode (with continuous top product withdrawal). The batch rectifier (BR) was investigated with variable decanter holdup by Rodriguez-Donis et al. (2002) and with continuous entrainer feeding by Modla et al. (2001, 2003) and Rodriguez-Donis et al. (2003), respectively. Recently the BHD was extensively studied for the BR and multivessel columns by Skouras et al. (2005a,b). 

Product separation and purification the first distillation column is designed to produce a cut enriched with acetic acid by the removal of the lighter and heavier components (methyl iodide, methyl acetate, etc.. This-cut is then dehydrated by heteroazeotropic distillation. The aqueous fraction recovered at the top is refractionated to remove excess water. The heavy stream is treated in a finishing column which produces glads acetic add in the distillate, while the residual acetic add at the bottom is also recovered in a complementary fractionation that separates the heavy products such as propionic add. These high-alloy steel columns each have between 35 and 45 actual trays. 

The recovered acetic acid (unconverted acetic add) is reconcentrated in two columns (45 and 55 trays). The first removes excess water at the top in the form of a heteroazeotrope with p-xylene, for example. The organic phase obtained by condensation and settling serves as a reflux, and the aqueous phase is partly purged. The second column removes the polymerization products at the bottom and produces gladal acetic add at the top, which is recycled. 

The dehydrator is a column with-about 30 trays, designed to separate the water from the phenol by heteroazeotropic distillation in the presence of benzene and feed-and make-up toluene. The benzene/water azeotrope (bplJ>13 69 C, water content 8.8 per cent weight) and toluene/water azeotrope (bpl013 = 84 Q water content 13.5 per cent weight) leaving at the top are cooled rod condensed to yiddjwo phases ... 

Purification. The crude ethylene dichloride is then purified. It is first dehydrated by heteroazeotropic distillation (10 trays). With water, ethylene dichloride in fact forms an azeotrope (bp1013 = 72.3°C, water content 92 per cent weight) which settles to yield an organic phase that serves as a reflux, and an aqueous phase added to that produced-by the hot quench. It is then rid of the light components (ethylene dichloride, dichloro-ethyiene, trichloroethylene, etc.) and heavy components (1,1,2-trichloroethane, penta-chloroethane, perchloroethyiene, etc.) in two distillation columns with 45 and 55 actual trays respectively. This operation also produces a small amount of ethylene dichloride, which forms an azeotrope with trichloroethylene (bp1>0u = 82.1 C, ethylene dichloride content 56.5 per cent weight). 

The mode of operation and the dimensioning of a heteroazeotropic distillation as exemplified by the separation of the system water-acetic acid has been described by Wolf et al. [61b]. Morozova and Platonov [61c] analyzed the structure of phase diagrams of multicomponent mixtures using a digital computer. They studied the requirements for the separation of azeotropic mixtures. In order to achieve optimum column combinations Serafimov et al. [58 c] studied the ternary mi.xture isopropanol/ benzene/water on the basis of a mathematical treatment of the separation of heteroazeotropic mixtures. In another paper [58 d] a procedure was presented for the separation into its components of the water-containing mixture with acetone, ethanol, benzene and butyl acetate by means of the thermodynamic and topological analysis of the phase diagram structure. 

Processes for completely fractionating binary mixtures with heteroazeotropes consist of two distillation columns and one decanter (Fig. 11.3-1). As the azeotrope lies within the miscibility gap of the liqnid the azeotrope can be broken by decantation. The two fractions from the decanter are at different sides of the azeotropic point. Purification of these two rather impnre fractions is performed by distillation. The pure products are recovered as bottoms from the distillation columns C-1 and C-2. 

Further increase in nonideality and transition to heteroazeotropes makes it again possible to separate mixtures, not using just a distillation column, but a column with decanter complex. Cases e and / occur, but very seldom therefore, we will not consider them further. 

We examine separation of the mixtures, concentration space of which contains region of existence of two hquid phases and points of heteroazeotropes. It is considerably easier to separate such mixtures into pure components because one can use for separation the combination of distillation columns and decanters (i.e., heteroazeotropic and heteroextractive complexes). Such complexes are widely used now for separation of binary azeotropic mixtures (e.g., of ethanol and water) and of mixtures that form a tangential azeotrope (e.g., acetic acid and water), adding an entrainer that forms two liquid phases with one or both components of the separated azeotropic mixture. In a number of cases, the initial mixture itself contains a component that forms two liquid phases with one or several components of this mixture. Such a component is an autoentrainer, and it is the easiest to separate the mixture under consideration with the help of heteroazeotropic or heteroextractive complex. The example can be the mixture of acetone, butanol, and water, where butanol is autoentrainer. First, heteroazeotropic distillation of the mixture of ethanol and water with the help of benzene as an entrainer was offered in the work (Young, 1902) in the form of a periodical process and then in the form of a continuous process in the work (Kubierschky, 1915). 

This uses the previously stated general theory of section trajectory bundles in columns with one and two feeds for analysis of mixtures separation in the complexes of heteroazeotropic and heteroextractive distillation. 

Figure 6.16e shows separate usage of a distillation column and a decanter at the bottom product when binary heteroazeotrope is saddle. The example can be separation of the mixture butanol(l)-acetone(2)-water(3) (Pucci, Mihitenho, Asselineau, 1986). Sections trajectories do not differ from trajectories at separation of homogeneous mixture of the same type. Figure 6.16f shows joint usage of the distillation column and decanter for the same mixture. The decanter is installed at the side product. Water is withdrawn from the decanter, and the organic phase is returned into the column. The bottom product of the column is butanol. 

In all the above-mentioned cases of application of heteroazeotropic or heteroextractive distillation, the second product point of distillation column (of the second product, obtained at that column end, where there is no decanter) should belong to the possible product region at the corresponding boundary element of concentration simplex (of bottom product Reg at Fig. 6.16a d and at Fig. 6.17a,b or of top product Regi) at Fig. 6.16e,f). 

The location of intermediate sections trajectories of columns with two feeds, including those at extractive, heteroazeotropic, and heteroextractive distillation, has fundamental distinctions from that of section trajectories of the simple columns. At sharp extractive or heteroextractive distillation, pseudoproduct point x), of the intermediate section should be located at the continuation of the boundary element, to which components of top product and of entrainer belong. If this condition is valid, the whole trajectory bundle of the intermediate section including trajectory tear-off point x[ from the mentioned boundary element is located in the region Reg where the top product components are more volatile and the entrainer components are less volatile than the rest of components. The trajectory tear-off point of the intermediate section is the stable node x[ = A+). The conditions of intermediate section trajectory tear-off in different points of trajectory tear-off region Reg allow to determine limit modes of extractive distillation for each mixture - the mode of minimum flow rate of the entrainer min, and for the... 

In the listed cases, one chooses the split at which a vapor close in composition to heteroazeotrope yo N az column to the decanter, one of the... 

Some examples of separation of three-component mixtures using curvature of separarix fines are described in Chapter 3. Examples of the application of entrainers forming heteroazeotropes and of columns with decanters for het-eroazeotropic and heteroextractive distillation are given in Chapter 6. 

Batch time (energy) requirements are provided for the separation of ternary zeotropic and heteroazeotropic mixtures in three closed batch column configurations. Two multivessel column modifications (with and without vapor bypass) and a conventional batch colunm operated under the cyclic policy, were studied. The multivessel column performs always better than the conventional column and the time savings vary from 24% up to 54 %. Moreover, by eliminating the vapor bypass in the multivessel, additional time savings of 26% can be achieved for a zeotropic mixture. However, the multivessel with the vapor bypass should be used for the heteroazeotropic mixtures. 

Batch time comparisons are provided for the separation of one zeotropic and two heteroazeotropic systems. We consider batch time as a direct indication of energy consumption since the boilup is constant for all columns. The columns are operated as closed systems. In the multivessel a ternary mixture is separated simultaneously in one such close operation and the final products are accumulated in the vessels (Wittgens et al, 1996). In the cyclic column the products are separated one at each time and for a ternary mixture a sequence of two such closed operations is needed. An indirect level control strategy based on temperature feedback control is implemented as proposed by Skogestad et al (1997). 

Multivessel column For separating a heteroazeotropic mixture of this topological class a decanter has to take the place of the middle vessel. The mixture is separated simultaneously in one closed operation with an initial built-up period. During this period the composition profile is built-up and the heteroazeotrope accumulates in the middle vessel (Fig. 4a). At the second (decanting) period the heteroazeorope is decanted and the organic phase is refluxed back in the column. The aqueous phase accumulates in the middle vessel, while methanol and 1-butanol are accumulated in the top and bottom vessel, respectively, as shown in Fig. 4b. 

Cyclic column The separation is performed in two cycles with a built-up period in between. During Cycle 1, methanol is accumulated in the top vessel (Fig 5a). Then a built-up period is needed where the heteroazeotrope accumulates in the top. Cycle 2 is a heteroazeotropic distillation with a decanter taking the place of the top vessel. The aqueous phase is gradually accumulated in the top vessel (see Fig. Sb) and the organic phase is refluxed back in the colunm. The still is getting enriched in 1-butanol (Fig. 5b). 

However, a modified multivessel for the separation of heteroazeotropic mixtures is problematic from the practical point of view. It is not practical to have a decanter where a vapor phase is bubbled through. Also the decanter is operated in a temperature lower than that of the column and a hot vapor stream entering the decanter is not very wise. 

---