Chemical News

Wednesday, January 23, 2008

Increased Yield of HCN in Acrylonitrile manufacturing

Increasing the output of HCN from acrylonitrile production without diminishing acrylonitrile yield is highly desirable. In the commercial production of acrylonitrile by propylene ammoxidation, propylene, ammonia, and oxygen are passed over a suitable heterogeneous catalyst and acrylonitrile is made in ~77% yield. The major byproduct is HCN: ~0.08 tons per ton of acrylonitrile produced.

The HCN byproduct is highly toxic and must be handled carefully; however, the HCN is also valuable and contributes to the economics of making acrylonitrile. The two largest uses for HCN are as a feedstock for hexamethylenediamine (HMDA) and methyl methacrlyate (MMA) production.

In certain instances, the HCN supply from an acrylonitrile plant falls short of the quantities needed for HMDA or MMA production. The HCN yield can be increased in the plant by changing operating conditions, but this usually comes at the expense of decreased acrylonitrile production. K. R. Ehrhardt, M. Mueller-Eversbusch, and D. Stapf of BASF disclose a procedure for converting other nitrogen-containing byproducts from acrylonitrile production into additional HCN.

After HCN, the next largest byproduct is MeCN. The BASF workers specify conditions that allow MeCN to be autothermally and noncatalytically converted to HCN in high yield. In the patent’s one example, a 1.1-m long aluminum oxide tube was mounted in a burner. The furnace temperature is maintained at 1300 °C and 1.93 kg/h of MeCN and 0.19 kg/h of H2O are metered into two separate thin-film evaporators and vaporized. The vapors are mixed with 0.12 m3/h of oxygen and fed through six axial boreholes. The residence time of the reaction gases at 1300 °C is 0.05 s. The gaseous products are then rapidly cooled and analyzed. The gas contains 29 vol% HCN, along with CO (28 vol%) and H2 (26 vol%). Whereas the model experiment in the patent uses MeCN, this procedure could also be used on heavier waste products from acrylonitrile production to further boost HCN yields.

Source: CAS/ACS