JPS6158467B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for recovering methacrylonitrile (hereinafter abbreviated as MAN), and more particularly to a method for recovering MAN from an ammoxide reaction gas such as isobutylene or tert-butyl alcohol. MAN is produced by a gas phase contact reaction between isobutylene, etc., ammonia and oxygen, that is, an ammoxidation reaction. The product of this reaction is
In addition to MAN, it contains methacrolein, hydrocyanic acid, isobutyronitrile, etc., and is obtained in the form of a high-temperature gas.
In order to separate and purify MAN from this reaction gas, the reaction gas is absorbed with absorption water, a reactant containing MAN as a main component is recovered, and this absorption liquid is distilled to separate by-products to obtain purified MAN. was. In this MAN purification process, the reaction gas is absorbed by water, as shown in the figure, in the absorption tower 1.
, the reaction gas is sent from the conduit 2 at the lower end, and the absorbed water is sent from the conduit 3 at the upper end,
The rising reaction gas is brought into contact with the falling absorption water, the reactant containing MAN as a main component is absorbed into the absorption water, the unabsorbed inert gas is removed from the top of the column as an off-gas, and the absorption liquid is extracted from the bottom of the column. It was transferred to the distillation process and obtained a purification MAN. Conventionally, in absorption towers, MAN in off-gas
It is operated under conditions where the concentration is, for example, 100 ppm or less, and for this purpose, it is said that the amount of absorbed water will not be less than the amount corresponding to the saturated solubility of MAN in water. required at least 38.2 tons of absorbed water. Therefore, the amount of liquid and distillation in the subsequent MAN purification process increased, resulting in an increase in equipment scale and energy consumption. This invention was made in view of the above-mentioned problems, and its purpose is to reduce the amount of absorbed water and reduce the burden on equipment and energy in the purification process. The gist of this is that when absorbing reactants from an ammoxide reaction gas containing methacrylonitrile as the main component using absorbed water, the temperature of the absorbed water must be lowered and the amount of absorbed water must be increased to dissolve MAN at a saturation concentration. This is a method for recovering methacrylonitrile, which is characterized in that the amount of water is less than or equal to the amount of water, and the methacrylonitrile is recovered as two layers: a saturated aqueous solution layer of MAN and an oil layer. This method increases the solubility of MAN in the absorbed water by lowering the temperature of the absorbed water, and at the same time lowers the temperature of the reaction gas, causing MAN to condense near the bottom of the absorption tower, and increasing the amount of absorbed water to dissolve MAN at a saturation concentration. In this method, the amount of water is less than that required for MAN, and it is recovered as two layers: a saturated aqueous solution layer and an oil layer. Absorbed water temperature is fed reactant gas temperature,
Although it varies depending on the MAN concentration, etc., under normal conditions, such as a gas concentration of 5 to 10 vol% and a temperature of 30 to 50°C, the temperature is 0 to 20°C, more preferably 10° or less. Note that the amount of water required to dissolve 1 ton of MAN at a saturation concentration is 29 tons, so the amount of water absorbed will be less than (29 tons/1 ton of MAN). According to this method, the amount of absorbed water is reduced, that is, the amount of absorbed liquid is reduced, and the burden on equipment and energy in subsequent purification steps is reduced. Also, MAN
It is now possible to handle the liquid by separating it into two layers: a saturated aqueous solution layer and an oil layer, and the aqueous solution and oil layer are separately fed to a purification process selected according to their respective compositions, where they are distilled or extracted. Therefore, rationalization of the purification process can be achieved. This will be explained below with reference to examples. Absorption tower consisting of 52 trays, MAN7.2% by volume
(Others are inert gases) containing 40℃ reaction gas at 4.6
It was supplied at a rate of ×10 ³ Nm ³ /ton MAN. At the same time, absorbed water was fed in, and the amount and temperature of the absorbed water were varied so that the MAN concentration of the off-gas (pressure 0 to 0.5 Kg/cm ² ·G) emerging from the top of the column was 100 ppm. The results were as shown in the table below.

[Table] The temperature of absorbed water is 0 to 20℃, more preferably 10℃ or less, and if it is 10℃, the amount of absorbed water is 21.8T/T・MA
N, 38.2T/T・M when using absorbed water at 25℃
Compared to AN, the amount of absorbed water is reduced by 57% (21.8/
38.2), reducing the burden of subsequent purification steps.

[Brief explanation of the drawing]

The drawing is a schematic diagram showing the flow of reaction gas and absorbed water in the absorption tower. 1... Absorption tower, 2, 3... Conduit.

Claims (1)

[Claims] 1. When absorbing reactants from an ammoxide reaction gas containing methacrylonitrile as a main component using absorbed water, lower the temperature of the absorbed water and increase the amount of absorbed water necessary to dissolve methacrylonitrile at a saturation concentration. A method for recovering methacrylonitrile, characterized in that the amount of water is less than or equal to the amount of water, and the methacrylonitrile is recovered as two layers: a saturated aqueous solution layer of methacrylonitrile and an oil layer.