Production of Phthalic Anhydride from o-Xylene
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. Additional information on phthalic
anhydride, it uses, and its manufacture are available.1 The purpose of this project is to determine
the “best” process configuration for a phthalic anhydride from o-xylene process subject to
constraints which will be defined later.
A suggested process flow diagram is in Figure 1. You should use this as a starting point.
However, any change that you can justify that does not violate the laws of nature is allowed. Your
assignment is to develop a “best” case, where “best” is dependent upon economic
considerations. The primary issue is how much recycle is necessary/desirable in order to satisfy
the flammability limit constraint described below. However, there may be other alternatives
which improve process economics which you are left on your own to consider.
Process Description
The raw materials are air and o-xylene. The o-xylene feed, which contains 5 mole% inert
impurities is vaporized in unit E-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 fluidized 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 (Streams 13 and 16) which
is burned for fuel. No economic credit is allowed. The product in Stream 15 must be 99.9 mole%
phthalic anhydride. This process must produce 40,000 metric tons/year of phthalic anhydride.
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Process Details
Feed Streams
Stream 1: air, consisting of 79% N2 and 21% O2 – free
Stream 2: o-xylene with 5 mole % 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:
phthalic anhydride reaction:
