Page 87 - Chemical and process design handbook

Speight_Part II_A 11/7/01 3:16 PM Page 2.28

ACRYLONITRILE

Acrylonitrile (2-propenonitrile, propene nitrile, vinyl cyanide, CH =CHCN;
2
o
o
freezing point: –83.5 C, boiling point: 77.3 C, density: 0.806) used to be
manufactured completely from acetylene by reaction with hydrogen
cyanide.
HC≡CH + HCN → CH =CHCN
2
There was also a process using ethylene oxide as the starting material
through addition of hydrogen cyanide (HCN) and elimination of water.
[
CH CH O + HCN → HOCH CH CN
2 2 2 2
HOCH CH CN → CH =CHCN + H O
2 2 2 2
The presently used process focuses on the ammoxidation (ammonoxida-
tion or oxyamination) of propylene that involves reaction of propylene,
o
ammonia, and oxygen at 400 to 450 C and 7 to 29 psi (48 to 200 kPa) in a
fluidized bed Bi O ·nMnO catalyst (Fig.1).
2 3 3
2CH =CHCH + 2NH + 3O → 2CH =CHCN + 6H O
2 3 3 2 2 2
The effluent is scrubbed in a countercurrent absorber and the acrylonitrile
is purified by fractionation.
In one version of this process, the starting ingredients are mixed with
steam and preheated before being fed to the reactor. There are two main
by-products, acetonitrile (CH CN) and hydrogen cyanide (HCN), with
3
accompanying formation of small quantities of acrolein (CH =CHCHO),
2
acetone (CH COCH ), and acetaldehyde (CH CHO). The acrylonitrile is
3 3 3
separated from the other materials in a series of fractionation and absorption
operations. A number of catalysts have been used, including phosphorus,
molybdenum, bismuth, antimony, tin, and cobalt.
The most important uses of acrylonitrile are in the polymerization to
polyacrylonitrile. This substance and its copolymers make good synthetic
fibers for the textile industry. Acrylic is the fourth-largest synthetic fiber

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