Esterification: Methyl Acetate and Other Esters
Methyl acetate is another high-volume commodity chemical that is manufactured commercially using RD. It is used as an intermediate in the manufacture of variety of polyesters such as photographic film base, cellulose acetate, Tenite cellulosic plastics, and Estron acetate. The conventional processes used multiple reactors with a large excess of one of the reactants to achieve high conversion of the other. The product was difficult to purify because of the formation of methyl acetate-methanol and methyl acetate-water azeotropes. Different means of dissociating the methyl acetate-methanol azeotrope were employed, such as the use of several atmospheric and vacuum distillation columns or extractive distillation. A typical process contained two reactors and eight distillation columns, making it complex and capital intensive.
Eastman Kodak developed an RD process for the manufacture of high-purity and ultra-high-purity methyl acetate : the traditional process and the RD process are shown in Fig. 1.3. The remarkable fact is that, in spite of the reaction having an unfavorable equilibrium limitation, a high-purity product is obtained using a near-stoichiometric ratio of methanol and acetic acid in the RD column. The whole process is integrated in a single column eliminating the need for a complex distillation column system and recycling of the methanol-methyl acetate azeo-
Reactive distillation process
trope. A single RD column at Eastman Kodak's Tennessee plant produces 180 000 tons/year of high-purity methyl acetate.
Methyl acetate production with a heterogeneous catalyst such as ion-exchange resins is also feasible and an excellent review of various investigations into this subject has been published . This gives a rigorous analysis of the reacting system in the RD column with the help of experiments and modeling. For all practical purposes, chemical and phase equilibria can be assumed to explain the experimental findings; at high reflux ratios the system may deviate slightly from this assumption. The role of reflux ratio is quite crucial. It not only dictates the capital cost and energy requirements but also influences the extent of reaction taking place in the column. At low reflux ratio, separation is poor whereas at high reflux ratio the reactant methanol tends to get separated through enrichment of the azeo-trope of methanol and methyl acetate. Sufficient conversion is possible only through a double-feed strategy. This has also been demonstrated, in a different context, by Doherty and co-workers for the same system .
Apart from methyl acetate, RD may be used for the production of other esters such as ethyl acetate, isopropyl acetate, and butyl acetate [13, 15]. The reaction may also be used as tool for recovery of acetic acid and other carboxylic acids from aqueous solutions, detailed discussion of which is in the section on recovery of chemicals with RD.
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