Where is acetic anhydride produced

Methyl acetate carbonylation is a second route. Some acetic acid is produced as a co-product in the methyl acetate carbonylation process. Acetic anhydride, largest application is for the conversion of cellulose to cellulose acetate, which is a component of photographic film and other coated materials.

Acetic anhydride is used in the production of aspirin, acetyl salicylic acid, which is prepared by the acetylation of salicylic acid and it is also used as a wood preservative.

The primary use of acetic anhydride is used in the manufacture of cellulose acetate for films and plastic goods; about 75 percent of the acetic anhydride produced annually in the United States is used for this purpose. Approximately 1. Acetic anhydride other uses include the manufacture of industrial chemicals, pharmaceuticals, perfumes, plastics, synthetic fibers, explosives, and weed killers.

Acetic anhydride reacts with water, it is sometimes used as a dehydrating agent in reaction mixtures where removal of water is necessary. Most acetic anhydride is used captively in the production of cellulose acetate for filter tow and fiber. Eastman capacity includes 1, million pounds of material based on synthesis gas from coal, and million pounds based on the ketene process. About 42 percent of acetic anhydride is consumed in cellulose acetate flake used to produce filter tow.

Acetic anhydride is a mature product with over 85 percent of production going into cellulose acetate flake, which in turn, serves well-established markets in filament yarn, cigarette filter tow and cellulose ester plastics. In this specification it is stated that the reaction is carried out in the absence of any noticeable quantities of water.

In that case the process yields not only acetic anhydride, but also acetic acid. The acetic acid can be easily separated from the main product, acetic anhydride, eg by distillation. It is stated that the process is of particular effectiveness and suitability for the dehydration of methyl acetate to be used as feed to a carbonylation reaction where a substantially anhydrous methyl acetate is desired, eg for the preparation of acetic anhydride as described in, inter alia, GB referred to above.

Thus if anhydrous methyl acetate or methyl acetate containing less than an equimolar amount of water is required it becomes necessary to remove some or all of the co-produced water. These methods along with others in general involve modified distillation techniques since 1 the water and methyl acetate are miscible and form a single phase, and 2 the methyl acetate forms a constant boiling point mixture with the water an azeotrope which makes removal of the azeotroped water difficult.

Furthermore, since the feedstock used is a mixture of acetic acid and a C l compound derived from methanol the process also takes advantage of the favourable low prices of methanol. The two products are readily separated e. It is a particularily important feature of this invention that the hydrogen iodide is recoverable since it is able to react quantitatively with methanol to form methyl iodide and water.

Since during the reaction all the iodide is recycled within the process, the overall reaction is. However, unlike the methyl acetate and water mixture which forms a single phase separable only by distillation, methyl iodide and water mixtures from two separate phases, easily separable by low energy methods such as decanting.

Furthermore, although small amounts of methyl iodide can dissolve in water no azeotrope is formed and therefore separation by distillation is easy. In this case, stage 2 can consist of several steps which allow a the unreacted reactants to be isolated and recycled to stage 1 ; b the hydrogen iodide to be isolated and fed to stage 3 and c the acetic anhydride to be recovered for sale or secondary uses.

Carbon monoxide via line 4 and acetic acid via line 12 are fed to the carbonylation reactor 2 along with recycled methyl iodide and acetic acid via line 8 where reaction occurs.

In the first part of the separation stage, which is conveniently integral with the carbonylation reactor 2 two streams are removed. The higher boiling stream 16, consisting of acetic anhydride and some acetic acid, is fed to a distillation column 18 where the residual acid is removed overhead. The lower boiling stream 14, consisting of carbon monoxide, hydrogen iodide, methyl iodide and residual acetic acid is fed to a scrubber In the scrubber the carbon monoxide is removed overhead and returned to the carbonylation reactor via line 6 and the methyl iodide and resiudal acetic acid removed at the bottom and returned to the reactor 2 via line 8.

The hydrogen iodide is removed at an intermediate point in the scrubber and fed to an iodination unit 22 in which it is reacted with methanol via line 24 to produce methyl iodide and water. The methyl iodide and water may be conveniently separated by decanting in unit 22 and the methyl iodide returned to the scrubber. The water, which may contain small amounts of unreacted methanol and trace amounts of water may be fed to astripper if desired to remove the methyl iodide and methanol as overheads.

In a further embodiment of the invention described however the acetic acid is generated in situ in the carbonylation stage from methanol and carbon monoxide using the carbonylation catalyst.

Thus in a further embodiment of the invention a process is provided for the production of acetic anhydride from methyl iodide, methanol and carbon monoxide characterised by reacting the methyl iodide and acetic acid with carbon monoxide under carbonylation conditions in the presence of an effective amount of a carbonylation catalyst.

It can be supplied in almost any chemical form such as the metal, a simple inorganic salt, for example the halide or nitrate, a simple organic salt, e. The catalyst concentration can be from to ppm and can conveniently be from to ppm based on the total weight of reactants.

Suitable Lewis bases include amines, phosphines and arsines which are capable of undergoing a quaternisation reaction with methyl iodide to form a quaternary ammonium salt which is soluble in the reaction mixture.