W O R K B O O K
COSTS F E A T U R E
by Walter L. Hardy, Foster D. Shell, Inc.
Acetic Anhydride The most economical process for the commercial production of acetic anhydride is the one which uses the raw materials at hand
EVIOUS columns in this series have discussed synthesis of the basic chemicals from which aspirin is made. O n e column [IND. E N G .
covered the synthesis of salicylic acid from phenol, caustic soda, and carbon dioxide, and an earlier column [IND.
E N G . C H E M . 49,
(1957) ] described the manufacture of phenol from benzene. Salicylic acid is converted to aspirin by acetylation with acetic anhydride. Approximately 0.57 pound of acetic anhydride reacts with 0.77 pound of salicylic acid for each pound of aspirin produced. T h e 15,100,000 pounds of aspirin produced in 1955 represented a market for 8,600,000 pounds of acetic anhydride. Pharmaceutical products, however, consume only a small fraction of annual acetic anhydride production. According to the U. S. Tariff Commission, 841,668,000 pounds of acetic anhydride were produced in 1955— other sources report even higher figures. Of this quantity, the bulk, or about 7 5 % , is used to make cellulose acetate. Acetic anhydride may be made by several commercially feasible processes. An early method, which is now considered obsolete, involved
the reaction of sodium acetate with sulfur dichloride and chlorine. 8CH,COONa + SC12 + 2C12 - * 4(CH 3 CO) 2 0 + 6NaCl + NaiSO,
T h e acetic acid formed is separated from the by-product salts and purified by vacuum distillation. Acetaldehyde may be oxidized in the liquid phase to form acetic anhydride. Unstable peracetic acid is formed as an intermediate in this reaction and reacts in turn with acetaldehyde to form acetic anhydride. A catalyst is used to keep the amount of peracetic acid in the reactor below an explosive concentration. CH,CHO + 0 2 — CH,COOOH CH 3 COOOH + CH3CHO — (CH,CO)20 +
Excess aldehyde, water, and solvent used as a diluent are removed from the batch by vacuum distillation. Acetic anhydride and acetic acid in the still bottoms are separated by distillation. Acetylene may be made to react with glacial acetic acid to form ethylidene acetate which is converted to acetic anhydride by distillation in the presence of a catalyst.
C 2 H 2 + 2CH3COOH - • CH3CH(OCOCH,)Î CH 3 CH(OCOCH 3 ) 2 - • ( C H 3 C O ) 2 0 + CHjCHt)
Acetaldehyde is produced by-product.
Production by Catalytic Oxidation A large proportion of acetic anhydride production is made by catalytic oxidation of glacial acetic acid. Ketene is formed as an intermediate product, and*is reacted with glacial acetic acid to form acetic anhydride. CH 3 COOH — ( ^ , = 0 = 0 +
CH 3 COOH + C H 2 = C = 0 — (0Η30Ο)2Ο
T h e crude acetic anhydride is refined by fractionation. Acetic anhydride is also produced in significant amounts by oxidation of propane and butane. This is a petrochemical operation which yields a mixture of organic acids and aldehydes, which must be separated. T h e selection of an acetic anhy dride process will depend on the capacity desired and the availability of raw materials. T h e acetylene and hydrocarbon oxidation process requires a nearby source of raw material gases and would appear VOL. 49, NO. 8 ·
A Workbook Feature
Net Raw Material Cost of Acetic Anhydride Raw Material
0.11 0.35 0.14 Net raw material cost
0.1320 0.0084 0.0022 $0.1426
Gross raw material cost
0.2080 0.0101 0.0088 $0.2269
Acetaldehyde Manganese acetate (catalyst) Diluent loss at 1% (as acetic acid)
1.2 0.024 0.016 SODIUM ACETATE PROCESS
1.6 0.252 0.175
Sodium acetate Sulfur dichloride Chlorine By-products Sodium chloride Sodium sulfate
0.13 0.04 0.05
0 350 f Negligible unless separated or special market found Net raw material cost
Net raw material cost
Gross raw material cost
0.0781 0.1380 0.0342 $0.2503
Net raw material cost
ACETIC ACID PROCESS
Acetic acid (glacial)
1.165 1.380 0.006
Calcium carbide Acetic acid (glacial) Catalyst (mercuric oxide) By-product Acetaldehyde
feasible only as a part of a m a n u facturing complex producing these gases as a by-product. Sources Must Be N e a r b y
T h e older sodium acetate process requires an economical source of chlorine. This presupposes a near by chlorine producer from which the gas could be piped or availability of captive chlorine. Sulfur dichloride must be available or fhanufactured. By-product salts are difficult to separate a n d m a y have to be dis carded where special markets are not available. T h e acetaldehyde method may be appropriate when this material, but not acetic acid, is available. Acetic acid oxidation, on the other hand, may prove the most economic road to acetic anhydride when acetic acid is easily obtained at low cost. T h e acetylene process also requires glacial acetic acid. These com ments are particularly applicable to foreign plants where local avail ability of r a w materials will play a large part in determining the process selected. W h e n a n acetic anhydride plant is erected as part of a manufacturing 52 A
complex, with all or most of the r a w materials available at the cost of cap tive production, what m a y otherwise be an unprofitable venture, can be justified. An independent acetic an hydride producer w h o will purchase his r a w materials on the open market m a y have a more limited choice. T h e table shows the net r a w material cost of 1 pound of acetic anhydride for the acetaldehyde, sodium acetate, and acetic acid processes. T h e acetylene process is considered as well, on the basis of captive acetylene production from calcium carbide. This is offered more as a curiosity than as a practical suggestion. Inadequate Margin
T h e current cost of acetic anhy dride is 15.5 cents per pound in car load d r u m quantities, delivered. T h e margin of 2 cents per pound above the r a w material costs of the acetic acid process, the least expen sive of the four processes covered in the table, is inadequate to cover production costs. N o commercial acetic anhydride plant could survive in the United States on the basis that all r a w materials be purchased at current market prices.
INDUSTRIAL AND ENGINEERING CHEMISTRY
0.067 0.100 5.70
Costs a n d Profits
T h e estimated cost of a plant with a capacity of 10,000,000 pounds p e r year of acetic anhydride by t h e acetic acid oxidation process is $650,000. Costs of production, in cluding amortization^ labor, utili ties, a n d maintenance, will be 2 to 3 cents per p o u n d of product. A n additional cent per pound will cover the cost of packaging a n d sales. Subtracted from the sales price of 15.5 cents, this leaves 11.5 to 12.5 cents to cover the cost of r a w materials as well as profits a n d administrative overhead. Ideally, raw materials should be available at a cost of about 8 cents per pound of acetic anhydride for profitable operation.
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