Reaction products recovery . Reactor products are first exchanged to produce steam, then cooled to about 80-90。C, and scrubbed with a 30-35 percent aqueous ammonium sulfate solution with some free sulfuric acid. Here excess ammonia is removed and most of the scrubbed solution recycled. Some sulfuric acid is added and part of the solution withdrawn for ammonium sulfate recovery. The ammonium sulfate solution, for a number of reasons , contains very low organic impurities and after special treatment gives white crystals in conventional equipment.
Hot gases from the neutralizer next are water washed at ambient temperature to absorb acrylonitrile ,acetonitrile hydrogen cyanide and other by products . effluent gases ---mainly nitrogen , carbon oxides and unreacted oxygen and propylene-are vented .
This organic solution is steam stripped and crude acrylonitrile recovered as overhead and the lean water recycled from the bottom to the top of the absorption tower .
Small alkali additions are made to this solution to maintain its ph and avoid corrosion problems .
Acylonitrile purification
After preliminary evaporation to remove high boiling produce-cyanohydrins and a little succinonitrile-purification of acrylonitrile is carried out by a series of distillation step. Succinonitrile is formed in trace amounts by the addition of HCN to the very small amounts of carbonyl compounds-acrolein ,acetaldehyde , acetone-present in the mixture
Hydrogen cyanide is removed first as overhead at more than 99.5 percent purity and can be recovered and used directly or burned in flare .
Next , acetonitrile is removed by extractive distillation with water ;minimum of acrylonitrile is lost with the dilute aqueous solution of acetonitrile as bottoms . this solution is stripped in another column not show in fig.1—and the bottoms water recycled to the extractive distillation step . the homogeneous phase containing up to 70 percent acetonitrile at the top may be disposed of or preferable used for recovering very pure , dry acetonitrile
Acrylonitrile is dehydrated and the overhead , an azeotropic mixture of acrylonitrile and water , carries small amounts of low boiling impurities which are essentially the last traces of acrolein.
The dehydration column bottoms are fed to the final purification step where pure acrylonitrile goes overhead and the bottom containing some acrylonitrile and the high boiling impurities is recycled to the beginning of the purification process.
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