15 Extractive and Azeotropic DistillationExtractive and azeotropic distillation have the common feature that a substance not normally present in the mixture to be separated is deliberately introduced into the system in order to increase the difference in volatility of the most hard to separate components. Extractive distillation can be defined as distillation in the presence of a substance which is relatively non-volatile compared to the components to be separated, and which, therefore, is charged continuously near the top of the fractionating tower, so that an appreciable concentration is maintained on all plates in the tower below its entry. Azeotropic distillation can be defined as distillation in which the add ed substance forms an azeotrope with one or more of the components in the feed, and by virtue of this are present on most of the plates in the tower above its entry at an appreciable level of concentration.These separation methods find their principal applications in the separation of mixtures whose components boil too close together for the economical use of simple fractionating equipment. These separation methods are particularly applicable when the components to be separated differ in chemical type. The theoretical principles involved are well documented, and will not be further considered here. The processes diff er in the means used to maintain be desired solvent concentration on the plates of the tower. In extractive distillation the high concentration of solvent is maintained by virtue of its non-volatility, and by the fact that it is charged at a high point in the tower. The solvent is, necessarily, removed from the base of the principal tower. In azeotropic distillation, most of the solvent is taken off from overhead, with relatively small amounts (ideally, none) drawn off with the bottoms.Extractive distillation is generally more flexible than azeotropic distillation, a greater variety of solvents and a wider range of operation conditions are available; and the concentration of solvent may be controlled by heat and material balances rather than by the accident of azeotrope composition. Furthermore, since vaporization of the solvent is not required beat loads are usually considerably less. It has been mainly used for the separation of toluene, not benzene. But it is mentioned here forthe sake of completeness.The use of azeotroptc distillation as a means of separation of BTX components from other non-aromatic hydrocarbons has been known and employed for some thirty years. Aceton e is used as an entrainer to purify benzene from similar-boiling non-aromatic hydrocarbons. Toluene can be separated by the use of either methanol, or of methyl ethyl ketone. Ethyl benzene may be separated from styrene either by isobutanol, of by 1-nitro-propane.In a 1966 review paper, further information was made available. Fifty-eight possible entraining agents for separating ethyl benzene ( bp. 136. 2 'C) and para-xylene(bp. 138. 4 ℃）have been examined. It would appear that 2 methyl butanol is the most suceessful agent, requiring a column with only 48 percent of the number of theoretical plates required if no entrainer were used.The separation of para and meta-xylenes (bp. 138. 2。

C and 139. 2℃respectively ) is much more difficult. Of thirty five entrainers examined the best appears to be 2-metbylbutanol, but the change of relative volatility is only from 1. 020 to 1. 029, and hence it can be safely concluded that azeotropic distillation for the separation of the para-and meta xylenes is not an economic proposition.Last, and easiest of the Cs aromatics is the separation of meta-from ortho-xylene ( bp. 139. 2 。

C and 144. 5 'C respectively ) . Twenty-eight entrainers were examined,the best being formic acid, required a column containing only 70 percent of the theoretical plates required of no entrainer is used.Two commercial processes have been developed for the separation of toluene using azeotropic distillation. One using an aqueous solution of methyl ethyl ketone and water and the other using methanol. Both processes operate on a narrow boiling range concentrate. The equilibrium relationships are well established. One commercial plant has been built using the methanol azeotrope which was developed by Socony-Vacuum oil Co. The azeotropic system consists of two towers each containing 36 trays operating at 30. p. s. i. g. The toluene concentrate is mixed with 75 vol per cent of anhydrous methanol for a feed containing 25 vol per cent toluene, and charged to the first tower. Cold reflux of 0. 6 to 0. 7 vols/ vol of feed is employed. Practicallycomplete recovery of toluene is obtained in the form of a 70 vol per cent concentrate from the bottom methanol. Cold reflux of the order4. 0 to 4. 5 vols/vol of feed is employed, and toluene recovery from this tower averages about 95 per cent, while overall yields of about 80 per cent of the toluene present in the initial feed are obtained. Losses are distributed mainly in the azeotrope overhead and in the acid treating and re-running stages.The MEK process, developed by Union Oil Company, has had one commercial plant operating, and handles a wide variety of straight-run and cracked stocks. A preconcentrate feedstock with a toluene content in the range 35 85 vol per cent is charged, together with the solvent (90 vol per cent MEK plus 10 vol per cent H20), to the 22nd tray of a 50 tray tower. The volumetric ratio of solvent +water/hydrocarbon charge varies between 2. 0 to 3. 0 to 1 depending upon the boiling range and type of non aromatics present. Cold reflux in the ratio of 0 to 1 reflux / feed is employed. The azeotropic distillate is composed 0 about 75 vol percent solvent. A small amount of water-rich phase separates from it in the reflux drain (80 vol per cent water, 20 vol per cent MEK) and is recycled back to the feed, 95 97 percent of the toluene present in the fed is recovered as bottoms from the azeotropic tower as 99 percent purity toluene on a solvent-free bases. The removal of MEK from the azeotropic distillate is accomplished by four stages of water-extraction thus reducing the ketone content to approximately 1 vol percent. The water material is then fractionated in an auxiliary tower to remove the remaining ketone. Recovery of MEK from the water extracts is done by distillation of the MEK-water azeotrope from the excess water in a tower with only a few trays and a low reflux ratio. The azeotrope thus obtained is then fed directly to the primary azeotrope tower together with the toluene pre-concentrate feed. The crude toluene so produced, after removal of MEK is treated with sulphuric acid, and followed by redistillation to produce nitration grade toluene. Overall recover of toluene is around 90 percent.。