Two-stage process produces ... TECHNOLOGY
REACTION
OXIDATION
Off gas
Off air
SEPARATION OF CRUDE PRODUCT
Crude product
Process originally developed to use ethylene extended to propylene and butene
Stripping column
Propylene orbutene Catalyst pump
Air
Wacker Process Can Make Acetone, MEK A new production route to either acetone or methyl ethyl ketone (MEK) by direct oxidation of propylene or butene has been opened up by extending the Wacker process for oxidizing ethylene to acetaldehyde. Reactions involved are similar to those used in the ethylene oxidation, but differ in reaction rates and by-product type and amount, Dr. Juergen Smidt, director of Wacker Chemie, told the Sixth World Petroleum Congress in Frankfurt. According to Wacker, the new processes have several advantages over current routes: • Reactions are one-step, produce crude product (mixed with catalyst solution) directly from starting material. • Reactions are highly selective, produce relatively few by-products. • Yields are h i g h - 9 2 to 94% for acetone, 85 to 88% for MEK vs. less than 80% by other routes. • Energy consumption is relatively low—about 18 kwh. per 100 lb. of ketone. • Feed can be a mixture of olefins and saturated hydrocarbons. • Only one main product is formed and thus there is no by-product marketing problem. However, Wacker points out, advantages depend on plant location and whether or not a new plant is 50
C&EN
JULY
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being considered. These advantages may not be so clear-cut in cases where a company already has an existing plant. Wacker isn't giving details at the moment, but says several firms in the U.S. and Japan are interested in both processes. For now, though, there seems to be little interest in either process in Germany itself. For one thing, there is excess capacity for acetone. For another, MEK doesn't enjoy such wide use as a solvent as it does in the U.S.; the Germans favor esters. Whether the extended Wacker process will pick up as fast as the original ethylene-to-acetaldehyde route is an open question. Celanese brought its 100 million-pound-peryear plant on stream last summer (C&EN, Aug. 6, 1962, page 2 9 ) , with increased capacity to 160 million pounds by 1964 (C&EN, June 24, page 2 3 ) . In the next year or two about 500,000 metric tons annual capacity of Wacker acetaldehyde will be on stream throughout the world. It was only three years ago that Hoechst started up the first acetaldehyde plants using the Wacker process. Direct Oxidation. In the Wacker process, olefins react with an aqueous solution containing palladium (II) and copper (II) chlorides. Active component of the catalyst is the PdCl 2 . This reacts stoichiometrically with the olefin to form the corresponding
carbonyl compound, metallic palladium, and hydrochloric acid. In a simultaneous and instantaneous step, CuClo reoxidizes the palladium to the chloride. Thus only catalytic amounts of PdCl 2 are needed. The over-all reaction is thus olefin plus CuCl 2 and water in the presence of PdCl 2 to form the carbonyl, CuCl, and HC1. In a separate but continuous step, CuCl is reoxidized to the cupric form with oxygen or air and HC1. The net reaction for the whole system is thus a direct oxidation of olefin to the corresponding carbonyl compound in the presence of the two chlorides. The reaction between olefin, PdCl 2 , and water actually occurs in two steps; first a PdCl2-olefin-7r-complex is formed. This complex hydrolyzes to the carbonyl compound, metallic palladium, and HC1. The complex formation is an equilibrium reaction but the hydrolysis is irreversible. Wacker finds that increasing the carbon chain length of the olefin shifts the equilibrium of the complex formation back toward the olefin. It also lowers the rate of hydrolysis. Both factors lower the rate of reaction of olefin with PdCl 2 . Thus reaction rate of propylene is about one third and that of butene-1 about one fourth that of ethylene. Therefore, reaction volumes with acetone and MEK must be correspondingly larger than that with acetaldehyde, Dr. Smidt says. By-Products. In the case of ace-
. . . MEK, which is purified like this ... CRUDE DISTILLATION
LIGHT END DISTILLATION
HYDROGENATION
CAUSTIC SCRUBBING
Low-boiling material
NaOH
FINAL DISTILLATION
DRYING
Back to crude distillation
Pure MEK
H2
Process water
Crude product
Residue
, . .or acetone, which is purified like this EXTRACTIVE DISTILLATION
DRYING
H20 Crude product
Low-boiling material
Pure acetone
Off water
tone, by-products consist of 0.5 to 1.5% propionaldehyde, 2 to 4% chlorinated material, 0.8 to 1.4% carbon dioxide, and 0.5 to 1.5% other substances. For MEK, they consist of 4% n-butyraldehyde, 4 to 6% chlorinated material, 0.5 to 1% carbon dioxide, and 2 to 2.5% other material. The process can be run in either of two ways. Olefin and oxygen can be reacted simultaneously with catalyst solution in a one-stage system. Or olefin can be reacted first with catalyst solution to form the carbonyl compound. Then, after the carbonyl is separated, the reduced catalyst solution is treated separately with oxygen or air in a second stage. The first system requires very pure feed, Dr. Smidt says. The two-stage setup requires less concentrated olefin and would thus generally be favored. Except that reduction volume is larger for MEK, production of crude acetone and MEK are the same. The
processes differ in subsequent purification, though. In making crude acetone or MEK with the two-stage process, olefin is fed to a reactor where it's treated with catalyst solution. Saturated hydrocarbons, small amounts of nitrogen brought in during the oxidation step, carbon dioxide, and unreacted olefin separate as off-gas from the reactor. The reaction mixture in the catalyst solution goes to a stripping column where steam strips off crude product. The catalyst solution is pumped from the stripper to the oxidation reactor and treated with air. Oxidized catalyst solution is recycled to the reaction vessel, where it reacts with more of the olefin. Lifetime of the catalyst is "practically unlimited," Dr. Smidt says. Those parts of the system contacting the catalyst must be made of titanium or must be carbon-lined, since the catalyst is so corrosive.
A two-step distillation is used to purify the crude acetone to a material meeting ASTM standards. Purifying MEK is more complex and expensive. Separating the by-products formed is harder. Also, MEK forms an azeotrope with water. MEK Purified. In purifying MEK, a crude distillation separates an MEKwater mixture from most of the water and chlorobutanone, chlorobutanone coming off as a side stream. The MEK-water mixture is further purified in a separator. A second distillation removes by-products boiling lower than MEK. Among these are n-butyric aldehyde, propionic aldehyde, acetone, and acetaldehyde. The next step, hydrogenation, converts n-butyraldehyde to butanol. The latter doesn't deteriorate the quality of the final product, according to Dr. Smidt, and is a more economic and efficient approach than the complete removal of n-butyraldehyde. The hydrogenation, however, also hydrogenates small amounts of chloro compounds, so the product stream must be scrubbed with aqueous sodium hydroxide to remove hydrochloric acid. After scrubbing, the MEK is dried by azeotropic removal of water along with a certain amount of MEK. The azeotrope is recycled to the crude column. The MEK bottom product is distilled again, and the residue, a small amount of higher-boiling prodducts, is discarded. JULY
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