This column is based on “Acrylonitrile Production from Propylene,” a report published by Intratec. It can be found at: www.intratec.us/analysis/acrylonitrile-e11a.
The highly reactive compound acrylonitrile (AN) contains a carbon-carbon double bond conjugated with a nitrile group. The chemical became important mainly after the 1930s, when it started to be used in the manufacture of synthetic fibers and rubbers.
The following describes a process for acrylonitrile production from propylene (Figure 1).
Ammoxidation. Chemical-grade propylene, ammonia and compressed air are fed to a reactor, where an ammoxidation reaction (oxidation of propylene in the presence of ammonia and catalyst) occurs in the vapor phase, over fluidized catalysts. Internal coils remove reaction heat, used for generating steam.
Separation. The reactor effluent is sent to a quench column, where unreacted ammonia is neutralized with sulfuric acid. The cooled effluent gas is partially condensed and transferred to a recovery column. The effluent gases are directed to a nitrile absorber, where acrylonitrile is absorbed in a chilled aqueous stream and transferred to the recovery column.
In the recovery column, acetonitrile and water are separated from acrylonitrile and hydrogen cyanide. While partially purified acrylonitrile product is recovered from the column top, condensed and sent to the first decanter, most of the water is removed from the column bottom and recycled to the quench.
Acetonitrile recovery. A liquid side stream is withdrawn from the recoery column, cooled and returned to the column. Because this stream contains mostly an acetonitrile-water azeotrope, part of it is sent to the acetonitrile stripper, where the acetonitrile-water mixture is distilled under vacuum to separate water and heavy organics, which are recycled to the quench, and a gaseous top draw comprising acetonitrile and water. This acetonitrile-water azeotrope is then distilled at high pressure into three phases: a bottoms product comprising mainly heavy impurities (this is discarded); an overhead stream where water is recovered; and an aqueous stream containing some acetonitrile. This stream is withdrawn from the top of the column and recycled to acetonitrile stripping. Here, high-purity acetonitrile is drawn off as a liquid sidestream.
Purification. The condensed overhead product from the recovery column is fed to a decantation drum and separated into a higher-density phase, which is recycled to the column, and a less-dense phase, which is fed into the HCN column for the separation of HCN from acrylonitrile. The acrylonitrile-rich stream is transferred into the second decanter, in which the denser phase (mainly water) is withdrawn from the decantation drum and recycled to the recovery column. The less dense phase is transferred to acrylonitrile purification, where fiber-grade acrylonitrile product is withdrawn from a side-draw.
Most modern acrylonitrile production is based on vapor-phase ammoxidation of propylene, but there are alternative pathways (Figure 2).
The total operating cost (raw materials, utilities, fixed costs and depreciation costs) estimated to produce acrylonitrile was about $1,800 per ton of acrylonitrile in the second quarter of 2014. The analysis was based on a plant constructed in the U.S. with the capacity to produce 300,000 metric tons per year of acrylonitrile.
Edited by Scott Jenkins
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