Process for Distillation and Recovery of Olefinic Nitriles

Recovery and purification systems for acrylonitrile and methacrylonitrile obtained by the ammoxidation of propylene or isobutylene are known, see for example U.S. Pat. Nos. 3,433,822; 3,399,120; and 3,535,849. The systems used involve the direct contact of the reactor effluent with an aqueous quenching liquid in a quenching system. In the quenching system, the reactor effluent is usually cooled to a temperature between about 90.degree. to about 230.degree.F.

The gases from the quenching system are then normally conducted to an absorber where the acrylonitrile or methacrylonitrile and water-soluble by-products are absorbed in water. The aqueous solution formed in the absorber is then treated in various distillation columns to obtain the desired product. The total system for recovery and purification may vary widely in the make-up of the individual components. Usually, however, they involve at least four components: one that removes acetonitrile; one that removes hydrogen cyanide; one that dries the acrylonitrile or methacrylonitrile product; and one that recovers the product. Many systems also use an ammonium sulfate recovery component. The particular components in the total recovery and purification system are not critical in the present invention.

It has now been discovered according to the present invention that substantial capital and operating cost savings can be realized along with improved recovery of acrylonitrile or methacrylonitrile by the recycle of the product column bottoms to form at least part of the quench liquid in the quench system. More specifically, this improvement in the recovery and purification of acrylonitrile or methacrylonitrile produced by the ammoxidation reaction of propylene or isobutylene, molecular oxygen and ammonia in the presence of an ammoxidation catalyst applies to the known system which consists of the steps of (a) contacting the ammoxidation reactor effluent with a quench liquid in a quench system to produce a gaseous quench effluent from said quench system having a temperature from about 90.degree.F. to about 230.degree.F.; (b) absorbing the gaseous quench effluent in water to form a solution and removing most of the by-products produced in the ammoxidation reaction and most of the water from the solution to obtain a crude acrylonitrile or crude methacrylonitrile product; and (c) distilling the crude acrylonitrile or crude methacrylonitrile to obtain an overhead stream of product-quality acrylonitrile or methacrylonitrile and a bottoms stream containing acrylonitrile or methacrylonitrile and impurities. The invention as applied to this known recovery and purification comprises recycling at least part of the bottoms stream obtained in step (c) and using this bottoms stream as at least part of the quench liquid in step (a). This improvement of the present invention saves the capital cost of a fractional distillation column that has been employed in the art and saves the operating expense of this eliminated column. Moreover, the efficiency of the recovery and purification system is improved.

The central aspect of the present invention is the recycle of the product column bottoms to form at least part of the quench liquid in the quenching step of the recovery and purification of acrylonitrile or methacrylonitrile.

The process of the present invention is applicable to any system that has two components: the first of which is a quench system and the second of which is a final product distillation column. Of course, all recovery and purification systems contain a number of intermediate separation and purification steps, but the exact nature of these steps is not critical in the present invention.

The first critical point of the recovery and purification is the quench system. The quench system in the recovery and purification scheme directly contacts a quench liquid with the effluent from the ammoxidation reactor. The reactor effluent gas susually has a temperature between about 700.degree. and 900.degree.F. Normally, this reactor effluent gas is cooled to a temperature of about 450.degree.F. in a gas cooler. Cooling to gas temperatures below 450.degree.F., in indirect contact coolers however, is undesirable because of deposits that form on the cooler surfaces.

In the quench system, the effluent from the ammoxidation reactor is cooled to a temperature of about 90.degree. to about 230.degree.F. In the preferred practice of the invention, the reactor effluent is cooled to a temperature of about 100.degree. to about 210.degree.F. and in the most preferred practice of the invention, the reactor effluent is cooled to about 200.degree.F. These temperatures are attained by controlling the flow rate and temperature of the quench liquid in relation to the flow rate and temperature of the gas quenched.

The quench system effluent goes to an intermediate system that is not critical in the present invention. The intermediate system normally consists of an absorber where the water-soluble products are dissolved to form a solution, and a number of distillation columns where the components of the solution are separated and purified. This intermediate system is not critical to the present invention because this recovery, separation and purification can be conducted by a number of known techniques.

In the present invention, however, the most preferred separation and purification of the solution formed in the absorber consists of the steps of (i) passing the solution into a distillation column where distillation is conducted to produce a gaseous overhead stream containing acrylonitrile or methacrylonitrile, HCN and water and a liquid bottoms stream containing acetonitrile and water; (ii) condensing said gaseous stream and allowing an aqueous layer and an organic layer to form; (iii) transferring the organic layer to a distillation column and distilling the organic layer to produce a gaseous overhead product that contains substantially all of the HCN, and a liquid bottoms stream that contains acrylonitrile or methacrylonitrile and water; and (iv) transferring the bottoms stream from (iii) to a distillation column wherein the water is removed as overhead and crude acrylonitrile or crude methacrylonitrile is produced in a bottoms stream. This preferred intermediate treatment could be replaced by any one of a number of known systems for producing a crude nitrile product from the absorber solution.

The second critical step in the recovery and purification is the step where the crude acrylonitrile or crude methacryloni35000000000000000000000000000000000000000000000000000000000000 0000