Page 82 - Chemical and process design handbook

Speight_Part II_A 11/7/01 3:16 PM Page 2.23

ACROLEIN

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Acrolein (2-propenal, CH =CHCHO, freezing point: –87 C, boiling point:
2
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52.7 C, density: 0.8427, flash point: –18 C) is the simplest unsaturated
aldehyde. The primary characteristic of acrolein is its high reactivity due
to conjugation of the carbonyl group with a vinyl group.
Acrolein is a highly toxic material with extreme lachrymatory properties.
At room temperature acrolein is a liquid with volatility and flammability
somewhat similar to those of acetone, but, unlike acetone, its solubility in
water is limited. Commercially, acrolein is always stored with hydroquinone
and acetic acid as inhibitors.
The first commercial process for manufacturing acrolein was based on
the vaporphase condensation of acetaldehyde and formaldehyde.

HCH=O + CH CH=O → CH =CHCHO + H O
3 2 2
Catalyst developments led to a vapor-phase processes for the production of
acrolein in which propylene was the starting material.
CH CH=CH + [O] → CH =CHCHO + H O
3 2 2 2
The catalytic vapor-phase oxidation of propylene (Fig. 1) is generally
carried out in a fixed-bed multitube reactor at near atmospheric pressures
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and elevated temperatures (ca 350 C); molten salt is used for temperature
control. Air is commonly used as the oxygen source and steam is added to
suppress the formation of flammable gas mixtures. Operation can be sin-
gle pass or a recycle stream may be employed.
The reactor effluent gases are cooled to condense and separate the
acrolein from unreacted propylene, oxygen, and other low-boiling compo-
nents (predominantly nitrogen). This is commonly accomplished in two
absorption steps where (1) aqueous acrylic acid (CH =CHCO H) is con-
2 2
densed from the reaction effluent and absorbed in a water-based stream
and (2) acrolein is condensed and absorbed in water to separate it from the
propylene, nitrogen, oxygen, and carbon oxides. Acrylic acid may be
recovered from the aqueous product stream if desired. Subsequent distilla-

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