Manufacture of Phthalic Anhydride
Phthalic anhydride (PAN) production in the United States in 1972 was 0.9 billion pounds per year; this total is estimated to increase to 2.2 billion pounds per year by 1985. Of the current production.
The most common method for production of phthalic anhydride is by oxidation of o-xylene. Phthalic anhydride is used in the manufacture of plasticizers (additives to polymers to give them more flexibility) and polyesters, among other applications.
The raw materials are air and o-xylene. The o-xylene feed, which contains 5 wt% inert impurities is vaporized in unit V-701.
Air, which may be assumed to contain only O2 and N2, is mixed with recycle, if there is any recycle, and heated. The hot air and vaporized o-xylene are mixed and sent to a packed bed reactor. The contents of Stream 7 must be below the LFL of oxylene, which is 1 mole%. In this reactor, essentially 100% of the o-xylene is reacted. Most goes to form phthalic anhydride, but some complete and incomplete combustion of o-xylene occurs, some maleic anhydride is formed, and a heavy impurity is also formed. The selectivities are given later.
The reactor effluent enters a complex series of devices known as switch condensers. The net result is that all light gases and water leave in Stream 9, with small amounts of both anhydrides, and the phthalic anhydride, maleic anhydride, inerts, and heavy impurity leave in Stream 10. The “dirty air” in Stream 9 must be treated before it can be vented, and this is an additional expense. It is also possible to recycle some of the “dirty air.” Any “dirty air” not recycled must be sent to a scrubber, in which the anhydrides are scrubbed into water. The water is then sent to an on-site waste water treatment plant, and an operating charge is assessed. The contents of Stream 10 are sent to a series of two distillation columns which produce liquid waste (Streams13 and 16) which is burned for fuel. No economic credit is allowed. The product in Stream 15 must be 99.9% phthalic anhydride. This process must produce 75,000 metric tons/year of phthalic anhydride.
The selectivity for the phthalic anhydride reaction is 70%, for the complete combustion of o-xylene is 15%, for the incomplete combustion of o-xylene is 5%, for maleic anhydride is 9%, and for the heavy impurity is 1%. The heavy impurity consumes a negligible amount of oxygen and produces a negligible amount of light gases.
Switch Condensers (SC-701): These are a complex set of condensers. Phthalic anhydride is first desublimated and then melted. There are three condensers, one in the desublimation mode, one in the melting mode, and one in stand-by mode.
Distillation Column (T-701): Here, 99% of the phthalic anhydride and all of the heavy impurity goes to stream 14. All of the inert and enough of the maleic anhydride to allow stream 15 to satisfy its purity requirement go to Stream 13.
Distillation Column (T-702): Here, 99.9% of the phthalic anhydride, and any remaining maleic anhydride go to stream 15, and all of the heavy impurity goes to stream 16.
Fig 1.: Phthalic anhydride (PAN) production
Process Details
Feed Streams
Stream 1: air, consisting of 79% N2 and 21% O2 - free
Stream 2: o-xylene with 5 wt % inert impurity
Equipment
Compressor (C-701): increases pressure of air feed from 1 atm to 3 atm
Vaporizer (E-701): vaporizes o-xylene feed which is already above 3 atm
Fired Heater (H-701): heats air to reaction temperature
Reactor (R-701): the following reactions occur:
C8H10 + 3O2 → C3H4O3 + 3H2O
C8H10 + 7.5O2 → C4H2O3 + 4H2O + 4CO2
Apllication of Phthalic anhydride (PAN)
The primary use of phthalic anhydride (PA) is as a chemical intermediate in the production of plastics from vinyl chloride. Phthalate esters that function as plasticizers are derived from phthalic anhydride.
Phthalic anhydride has another major use in the production of polyester resins and other minor uses in the production of alkyd resins used in paints and lacquers, certain guys, insect repellents, and urethane polyester polyols.
Phthalic anhydride has also been used as a rubber scorch inhibitor and retarder.
The second largest outlet for PA is in unsaturated polyester resins (UPR) which are usually blended with glass fibers to produce fiberglass-reinforced plastics. Principal markets are construction, marine and transportation.
The third largest outlet is PA-based alkyd resins that are used in solvent-based coatings for architectural, machinery, furniture and fixture applications.
Small volume uses for PA include the manufacture of dyes and pigments, detergents, herbicides and insecticides, fire retardants, saccharin and polyester resin cross-linking agents.