Process for the production of isophorone

#The present invention provides a process for the production of isophorone by the alkali-catalysed condensation of acetone.

#The production of isophorone by the alkali-catalysed condensation of acetone and its separation from the reaction products is a long established process described in German Patent Specifications Nos. 1,144,269; 1,165,018; 1,205,525 and 1,095,818. One method of producing isophorone commercially is to feed acetone and an aqueous solution of an alkali catalyst to the upper section of a pressure distillation reactor which contains a refluxing mixture of acetone/water azeotrope (80:20% w/w respectively) and alkali catalyst (<0.1% w/w) at a temperature in the range 150.degree. to 300.degree. C, usually 200.degree. to 240.degree. C and a pressure in the range 30-35 bar. The lower part of the pressure still operates as a stripping section, from where unconverted acetone passes as an overhead azeotrope product with water and is condensed at the top of the column to produce the reflux. Additionally small amounts of isoxylitone co-produced with the isophorone are hydrolysed back to isophorone and acetone in the lower part of the pressure still. The crude isophorone product leaving the bottom of the pressure still is decanted from the water phase and purified by topping free from water and low-boiling substances and by tailing from the high-boilers in two vacuum stills.

#The isophorone produced by this process tends to be contaminated to an undesirable degree with colour-formers which are virtually inseparable from isophorone by simple distillation. Furthermore, the product tends to contain a significant quantity of the beta-isomer of isophorone, the alpha-isomer being the desired product. Whilst it is known to decompose colour-forming contaminants in the bulk product into higher boiling compounds, thereby rendering them more readily separable, by heating with a strong acid, this treatment involves the whole of the product and moreover increases the concentration of the undesirable beta-isomer. Therefore, the treatment of the bulk product with strong mineral acids is undesirable.

#The invention will now be illustrated by the following Examples which are described with reference to the accompanying FIGURE showing a flow-diagram of the plant used.

#With reference to the FIG. 1 is a pressure still containing 40 bubble cap plates, the upper 10 plates functioning as a reaction zone and the lower 30 plates functioning as a hydrolysis zone; 2 is a distillation column; 3 is an acid treatment vessel; 4 is a toluene-para-sulphonic acid inlet pipe; 5 is a distillation column; 6 is a decantation vessel; 7 is an aqueous phosphoric acid inlet piep; 8 and 9 are distillation columns.

#A mixture comprising 65% w/w acetone, 34.7% w/w water and 0.3% w/w KOH was fed to the 3rd plate from the top of the reaction section of the pressure still 1 at such a rate as to maintain a refluxing azeotropic mixture of acetone an water (ca. 80:20 w/w) at ca. 205.degree. C/32 bar and a potassium hydroxide concentration in the reflux of <0.1% w/w. The acetone was converted to isophorone and by-products including colour-forming compounds which passed down the column in the descending reflux into the hydrolysis section of the column where the unconverted acetone was stripped from the isophorone, the stripped acetone in the form of its water azeotrope passing back up the colum into the reaction section where it finally condensed at the top of column to produce the reflux. A small amount of isoxylitone by-product in the isophorone was hydrolysed back to isophorone an acetone in the hydrolysis section of the column 1. The volatility of the colour-forming compounds in water is intermediate between that of the acetone/water and isophorone/water azeoptropes, resulting in their concentration at a point in he hydrolysis section where the acetone constant is less than 10% w/w.

#A side-stream was collected at the 12th plate from the base of the hydrolysis section of the pressure still 1.* This side stream comprised equal amounts by weight of oil and water phase. The oil phase comprised 63% w/w isophorone, 17% w/w mesitylene, 75% w/w of high-boiling substances, 3% w/w acetone and 2% w/w of water.