MXPA97006000A - Improved process for the recovery of acrilonitr - Google Patents

Abstract

A process for the recovery of acrylonitrile or methacrylonitrile obtained from the reactor effluent of a reaction of ammoxidation of propylene or isobutylene, which comprises passing the effluent from the reactor through an absorber column, a recovery column and a scrubbing column, in where the improvement comprises increasing the pressure of the upper part of the recovery column, by mechanical means, to increase it by approximately 0.25 to 5 psi and to improve the hydraulic capacity of the recovery and depuration columns.

Description

IMPROVED PROCESS FOR THE RECOVERY OF ACRYLONITRIL BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention is directed to an improved process for the manufacture of acrylonitrile or methacrylonitrile. In particular, the present invention is directed to the improvement in the recovery processes used during the manufacture of acrylonitrile and methacrylonitrile. The recovery of acrylonitrile / methacrylonitrile produced by the oxidation of propylene or isobutylene on a commercial scale has been effected by rapid cooling of the reactor effluent, with water, followed by the passage of the gaseous stream containing acrylonitrile or methacrylonitrile resulting from cooling to a absorber, wherein the water and gases are contacted in a countercurrent flow to extract substantially all of the acrylonitrile or methacrylonitrile, the aqueous stream that substantially contains all the acrylonitrile or methacrylonitrile is then passed through a series of distillation columns and associated decanters for the separation and purification of the acrylonitrile or methacrylonitrile product.

Typical recovery and purification systems that are used during the manufacture of acrylonitrile or methacrylonitrile are presented in U.S. Patent Nos. 4,234,510 and 3,885,928, assigned to the assignee of the present invention and referred to herein.

SUMMARY OF THE INVENTION The main objective of the present invention is to provide an improved process for the manufacture of acrylonitrile or methacrylonitrile. Another object of the present invention is to provide an improved recovery and purification process for use during the manufacture of acrylonitrile or methacrylonitrile. A further object of the present invention is to provide an improved process for the manufacture of acrylonitrile or methacrylonitrile which reduces waste gas and improves product yield, recovery efficiency and product quality by reducing the amount of organic impurities in the product. the resulting final product. The additional advantages and novelty features of the invention will be set forth in part in the description that follows and, in part, will be apparent to those skilled in the art upon examination of the following disclosure or may be learned by practice of the invention. The objects and advantages of the invention can be achieved or obtained by means of the instrumentation and combinations particularly indicated in the appended claims. To achieve the above objectives, as well as other objectives, and in accordance with the purpose of the present invention as it is incorporated and described in a general manner herein, the process of the present invention comprises the transportation of the effluent of the reactor obtained during the oxidation of propylene or isobutylene to a fast cooling column, where the hot effluent gases are cooled by contact with an aqueous spray, the cooled effluent coming from the reactor then passes through the upper part towards an absorbing column, where the acrylonitrile or the methacrylonitrile is absorbed in water, the aqueous solution containing the acrylonitrile or methacrylonitrile is then passed through a recovery distillation column and a scrubbing distillation column to recover the acrylonitrile or methacrylonitrile product, wherein the improvement comprises the increase in pressure in the upper part of the column of recovery distillation, by mechanical means, in approximately 0.1 to 5 psi, to improve the hydraulic capacity of the recovery and purification columns. For example, normally the pressure at the top of the recovery column, in the recovery and purification section of an acrylonitrile plant, is designed to operate or operate from about 1 to less than 5 psig. The present invention is directed to mechanically increase the pressure of the upper part of the recovery column, for example by adding a pressure valve, above the designed pressure, exceeding it by 0.1 to 5 psi, preferably 0.25-0.5 to 5.0, especially from 1.0 to 5.0 psi. In a preferred embodiment of the present invention, the pressure of the top of the recovery column is maintained between 5 to 10 psig, preferably greater than 5 to 10 psig, especially 5.5 to 7.5 psig is preferred. In a preferred embodiment of the present invention, the process is carried out with the reactor effluent obtained from the ammoxidation of propylene, ammonia and oxygen to produce acrylonitrile. In a preferred embodiment of the present invention, the effluent from the reactor is obtained by the reaction of propylene, ammonia and air, in a fluidized bed reactor, while it is in contact with a fluidized bed catalyst. The conventional fluidized bed catalyst for ammoxidation can be used in the practice of the invention. For example, the fluidized bed catalyst as described in U.S. Patent Nos. 3,642,930 and 5,093,299, mentioned herein by reference, may be used in the practice of the present invention. In conventional recovery and purification processes for the recovery of acrylonitrile or methacrylonitrile, it has been found that jet flooding in the recovery and debugging columns usually establishes or fixes the maximum amount of spent water that can be circulated through the columns. absorbers, recovery and purification. The present invention allows the operation of the recovery and purification columns at an increased pressure, thus increasing the production at which the jet flood occurs, thus allowing an increase in the operating or operating regime of the reactor. and lower losses of exhaust gas from the absorber. This is achieved by increasing the flow of spent water from the cooling and purification columns to the absorber, with the surprising discovery that the total recovery procedure loses none of the operating efficiency. The practice of the invention allows higher reaction yield regimes, while maintaining minimum requirements such as the ratio of water depleted to product (at least 11: 1). The practice of the present invention is very beneficial when used in combination with a recovery and purification process operating at a ratio or ratio of water depleted to product of 12: 1 to 11: 1 in the absorber column.

DETAILED DESCRIPTION OF THE INVENTION Next, the present invention will be described in detail. The reactor effluent obtained by the ammoxidation of propylene or isobutylene, ammonia and an oxygen-containing gas, in a fluidized bed reactor, while there is contact with the fluidised bed ammoxidation catalyst, is transported to a quench column , where the hot effluent gases are cooled by contact with a spray of water. Normally, any excess ammonia contained in the effluent is neutralized by contact with sulfuric acid in the rapid cooling, to extract the ammonia as ammonium sulfate. The cooled effluent gas containing the desired product (acrylonitrile or methacrylonitrile and HCN) is then passed to the bottom of an absorber column, where the products are absorbed in water entering the column from the top. The unabsorbed gases pass from the absorber through a tube located in the upper part of the absorber. The aqueous stream containing the desired product is then passed from the lower part of the absorber to the upper portion of a first distillation column (recovery column) for further purification of the product. The product recovered from the upper portion of the recovery column is then sent to a second distillation column for further purification and recovery of the acrylonitrile or methacrylonitrile product. The bottom stream obtained from the recovery column is sent to a scrubbing distillation column to recover crude or crude acetonitrile which is a valuable co-product. In the practice of the present invention, an automatic pressure control valve is installed in the elevated line, in the recovery column, to allow the pressure of the top of the recovery column to be increased by approximately 0.1 to 5 psi, above the design pressure of the top of the recovery column. Normally, the pressure at the top of the recovery column is between 1 to less than 5 psig, normally, between 4 to 4.5 psig. In the preferred practice of the present invention, the pressure of the top of the recovery column is operated or operated at a constant pressure which may vary from 5 psig to approximately 10 psig, preferably from 5.5 to 7.5 psig. The improvement of the present invention is applicable to separate the designs of the recovery and depuration towers, as well as the designs of stacked recovery / depuration towers for the recovery of acrylonitrile or methacrylonitrile, provided that the hydraulic system of the tower is limited by the flood by jet. The pressure control valve used can be obtained from any manufacturer of control valves. It can be activated automatically or controlled manually. Preferably, the ammoxidation reaction is carried out in a fluidized bed reactor, although other types of reactors have been designed, such as, for example, transport line reactors. Fluidized bed reactors, for the manufacture of acrylonitrile, are well known in the prior art. For example, the reactor design disclosed in U.S. Patent No. 3,230,246 referred to herein as a reference is suitable.

The conditions for the ammoxidation reaction to occur are also well known in the prior art as set forth in U.S. Patent Nos. 5,093,299; 4,863,891; 4,767,878 and 4,503.001; mentioned herein as reference. Normally, the ammoxidation process is carried out by contacting propylene or isobutylene in the presence of ammonia and oxygen with a fluid bed catalyst, at an elevated temperature, to produce the acrylonitrile or methacrylonitrile. Any source of oxygen can be used. Nevertheless, for economic reasons, the use of air is preferred. The typical molar ratio of oxygen to olefin in the feed should vary from 0.5: 1 to 4: 1, preferably from 1: 1 to 3: 1. The molar ratio of ammonia to olefin in the feed to the reactor can vary from 0.5: 1 to 5: 1. There is really no upper limit for the ammonia-olefin ratio, but generally there is no reason to exceed a 5: 1 ratio for economic reasons. The reaction is carried out at a temperature between the ranges of about 260 ° to 600 ° C, but preferred ranges are from 310 ° to 500 ° C, especially from 350 ° to 480 ° C. The contact time, although not critical, is generally in the range of 0.1 to 50 seconds, a contact time of 1 to 15 seconds is preferred. In addition to the catalyst of the Patent of the United States No. 3,642,930, other catalysts suitable for the practice of the present invention are disclosed in U.S. Patent No. 5,093,299, mentioned herein by reference. The conditions under which the absorbent column, recovery column and scrubbing column are maintained vary between 5 to 7 psig (80 ° F to 110 ° F), 1 to 4.5 psig (155 ° F to 170 ° F) and from 7 to 13 psig (170 ° F to 210 ° F), respectively. The improvement of the present invention differs over the prior art in operating the recovery column by mechanical adjustment to a pressure that is above its design pressure, during the practice of the ammoxidation process for the manufacture of acrylonitrile / methacrylonitrile . Conventional procedures utilize a pressure on the top of the recovery column of less than 5 psig. In the preferred practice of the present invention, the pressure of the top of the recovery column is normally operated or operated at a pressure ranging from 5 to 10 psig, preferably from 5.5 to 7.0 psig. The operation of the recovery and purification process of the present invention results in the ability to obtain high production rates through the reactor, without making any modification to the recovery and purification system, which includes capital expenditures. The present invention results not only in an unexpected improvement in production regimes but achieves this improvement without increasing the size of the towers used in the recovery and purification section. In addition, the accompanying increase in production regimes is not accompanied by any deterioration observed in the performance of the absorber column, during the recovery of acrylonitrile or methacrylonitrile. The present invention results in improved operating efficiency, improved production, an increase in productivity without the need for capital expenditures. It will be apparent to those skilled in the art that various modifications of this invention can be made or derived, in light of the foregoing discussion and discussion, without departing from the spirit and scope of the disclosure or scope of the claims.

Claims (1)

NOVELTY OF THE INVENTION Having described the present invention, it is considered as a novelty and, therefore, the content of the following CLAIMS is claimed as property: 1. A process for the recovery of acrylonitrile or methacrylonitrile obtained from the effluent of the reactor of a reaction of ammoxidation of propylene or isobutylene, which comprises passing the effluent from the reactor through an absorber column, a recovery column and a scrubbing column, wherein the improvement comprises the increase in the pressure of the top of the the recovery column, by mechanical means, increasing it by approximately 0.1 to 5 psi, to improve the hydraulic capacity of the recovery and purification columns. 2. The process according to claim 1, wherein the reactor effluent is obtained from the reaction of propylene, ammonia and an oxygen-containing gas in a fluidized-bed reactor that is in contact with an ammoxidation catalyst for a fluid bed. 3. The process according to claim 1, wherein the reactor effluent is obtained from the reaction of isobutylene, ammonia and an oxygen-containing gas in a fluid bed reactor, while in contact with a bed ammoxidation catalyst. fluid 4. The process according to claim 1, wherein the recovery column is maintained at a top pressure of between about 5 to about 10 psig 5. The process according to claim 1, in the part pressure The top of the recovery column is maintained between about 5.5 to about 7.0 psig 6. The process according to claim 1, wherein the pressure of the top of the Recovery column is increased by between about 0.25 to about 5 psi. The process according to claim 1, wherein the pressure of the top of the recovery column is increased by between about 0.5 to about 5 psi. The process according to claim 1, wherein the pressure of the upper part of the recovery column is increased by between about

1.0 to about 5 psi.