GB736111A - Process for the production of acrylonitrile - Google Patents

Abstract

In a continuous process for the production of acrylonitrile by passing acetylene and hydrocyanic acid together in the gas phase at elevated temperature through a reaction chamber in the presence of a solid known to be capable of catalysing the reaction of acetylene with hydrocyanic acid in the gas phase to form acrylonitrile, the catalyst is continuously introduced into, and withdrawn from, the reaction chamber, and moves continuously through the reaction chamber in countercurrent to the reaction gases throughout the period of the reaction. It is preferred to operate with the catalyst moving downwardly through the reaction chamber, the speed of the reaction gases in the reaction chamber being preferably 200 to 1500 times the speed of migration of the catalyst through the reaction chamber. The reacting gases are expediently heated to temperatures up to a maximum of 200-220 DEG C. and may be diluted with an inert gas, e.g. nitrogen, which is preferably preheated and supplied to the reaction chamber entirely or partially separate from the reacting gases. The diluent gas can be preheated by heat exchange with the hot gaseous reaction products or by allowing it to flow through the spent catalyst before the latter is withdrawn from the reaction chamber. To avoid overheating in the reaction zone a cold inert flushing gas, preferably nitrogen, may be supplied intermittently to the reaction zone and the supply of the reacting gases may also be shut off or reduced. The supply of the flushing gas and of the reacting gases may be automatically controlled by temperature sensers arranged in the reaction zone. The acetylene and hydrocyanic acid may be employed in the form of a synthesis gas mixture obtained by reaction of hydrocarbons, especially methane, with excess nitrogen in the electric arc. This synthesis gas mixture is preferably cooled to below 650 DEG C. before use and may be supplied to the reaction zone without previous purification. The catalyst should enter the reaction zone at 450-580 DEG C. and separate heating means may be necessary to attain this temperature. Preferred catalysts are active carbons, e.g. activated charcoals such as wood charcoal. Active carbons which have been formed by steam activation and particularly those which have been activated in the presence of alkali metal compounds, especially potassium compounds, are particularly suitable. The catalyst may also comprise an active carbon, e.g. wood charcoal, which has been moulded with an alkali metal compound as binding agent and tar before activation in the presence of an alkali metal compound, said carbon still containing 10 per cent or more of an alkali metal compound after activation and being used without previous washing. A suitable active carbon can be obtained by pressing wood charcoal dust into shaped elements with alkali lye and tar and activating with hydrogen at 800-1000 DEG C. after low temperature carbonization at 400 DEG to 600 DEG C. The active carbons may also be impregnated before use with an alkali metal compound, especially an alkali metal hydroxide, and if desired additionally treated with an alkaline earth metal compound, e.g. a barium salt. Mercury vanadate is also stated to have a good catalytic action. When active carbons are used they are first deaerated by subjecting to a reduced pressure treatment, if desired at elevated temperature, and then introduced into the reaction chamber in an atmosphere of inert gas, preferably being conveyed pneumatically. The apparatus used should preferably be constructed of a low-alloy steel containing up to 0.2 per cent carbon, 1.5 to 9 per cent chromium and 0.5 to 1.5 per cent aluminium. These alloys may also contain silicon and/or titanium in an amount of 0.3 to 1.5 per cent in each case. Two types of apparatus for carrying out the process are described and illustrated. They comprise essentially a reaction chamber provided with heating means and gas supply means. The chamber is connected at one end with a hopper for continuously charging the catalyst to the chamber and is provided at the other end with a bucket wheel or a horizontally rotating perforated base-plate for the continuous discharge of the catalyst in regulated quantities. In each case the catalyst migrates downwardly through the chamber which in one embodiment is divided into a plurality of reaction tubes from which the catalyst is discharged through the horizontally rotating perforated base-plate.