MXPA97000631A - Improved acrilonitr recovery process - Google Patents

Abstract

The present invention relates to a process for the recovery of acrylonitrile or methacrylonitrile obtained from the reactor effluent of an ammoxidation reaction of propylene or isobutylene comprising the passage of the reactor effluent through an absorber column, a first decanter, a column of recovery, a second decanter and a separating column where the improvement comprises the maintenance of the interior temperature of the first and second decanter between approximately 32øF to 75

Description

IMPROVED ACRYLONITRYL RECOVERY PROCESS BACKGROUND OF THE INVENTION Field of the Invention The present invention is directed to an improved process for the preparation of acrylonitrile or methacrylonitrile. In particular, the present invention is directed to refinement in the recovery processes used during the preparation of acrylonitrile or methacrylonitrile. In addition, the improved process of the present invention improves the quality of the resulting product by minimizing peroxide impurities. The recovery of acrylonitrile / methacrylonitrile produced by the ammoxidation of propylene or isobutylene on a commercial scale has been carried out by rapidly cooling the reactor effluent with water followed by the passage of the gaseous stream containing acrylonitrile or methacrylonitrile resulting from quenching to an absorber where water and gases are contacted in a countercurrent flow to remove substantially all of the acrylonitrile or methacrylonitrile, the aqueous stream containing substantially all of the acrylonitrile or methacrylonitrile is then passed through a series of distillation columns and associated decanters for the separation and purification of the produced acrylonitrile or methacrylonitrile.

Typical recovery and purification systems that are used during the preparation of acrylonitrile or methacrylonitrile are disclosed in U.S. Patent Nos. 4,234,510 and 3,885,928, assigned to the assignee of the present invention and incorporated herein by reference. SUMMARY OF THE INVENTION The primary object 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 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 the amount of impurities in the resulting final product. Additional objects, advantages and novel features of the invention will be set forth in part in the description that follows and, in part, will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention can be carried out and obtained by means of the instruments and combinations particularly indicated in the appended claims. To achieve the foregoing and other objects and in accordance with the purpose of the present invention as incorporated herein and widely described, the process of the present invention comprises the transportation of the reactor effluent obtained during the ammoxidation of propylene or isobutylene to a column of tempering where the hot effluent gases are cooled on contact with an aqueous spray, the reactor effluent being cooled to the top towards an absorbing column where the crude acrylonitrile or methacrylonitrile is absorbed in water, passing the aqueous solution containing the acrylonitrile or methacrylonitrile to a first decanter where a first aqueous layer and a first organic layer containing acrylonitrile and methacrylonitrile are formed, transferring the first organic layer to a second decanter where a second layer of water and a second organic layer are formed, and recovering the acrylonitrile or methacrylonitrile of the second layer organic wherein the improvement comprises maintaining the first and second decanter at a temperature of about 32 ° F to about 75 ° F and maintaining the second decanter at a temperature of about 32 ° F to about 75 ° F. Preferably, the temperature of the first decanter is maintained at about 32 ° F to 70 ° F, with 32 ° F to 65 ° F being especially preferred. Preferably, the temperature of the second decanter is maintained between about 35 to 68 ° F, with 40 to 65 ° F being especially 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 still preferred embodiment of the present invention, the reactor effluent is obtained by the reaction of propylene, ammonia and air in a fluid bed reactor while in contact with a fluid bed catalyst. The conventional fluidized bed ammoxidation catalyst can be used in the practice of the invention. For example, the fluid bed catalyst as described in U.S. Patent Nos. 3,642,930 and 5,093,299, incorporated herein by reference, may be used in the practice of the present invention. The present invention allows the most efficient operation during the recovery and purification of acrylonitrile or methacrylonitrile produced. The operation of the decanters within the temperature range establishes the previous results in the improved phase separation of the water, thereby increasing the volume of water phase in the decanters. This improved separation means that larger product volumes can be treated during the subsequent recovery and purification steps (i.e., distillation and drying) without any increase in the size of any equipment. Another advantage of the practice of the invention is that the operation of the decanter in the above-mentioned temperature range leads to the minimization of water-soluble impurities such as peroxide, which are harmful to the users of the final product. BRIEF DESCRIPTION OF THE DRAWING Figure 1 is a schematic representation of the process that is applied to the preparation of acrylonitrile. DETAILED DESCRIPTION OF THE INVENTION The present invention will now be described in detail with reference to the figure. The reactor effluent obtained by the ammoxidation of propylene or isobutylene, ammonia and oxygen containing gas in a fluid bed reactor (not shown) while being kept in contact with a fluidised bed ammoxidation catalyst is transported to a quenching column ( not shown) where the hot effluent gases are cooled by contact with water spray. Typically, any excess ammonia contained in the effluent is neutralized upon contact with sulfuric acid in the quench to remove the ammonia as ammonia sulfate. The cooled effluent gas containing the desired product (acrylonitrile or methacrylonitrile, acetonitrile and HCN) is then passed to the bottom of an absorber column 5 through line 1 where the products are absorbed into water entering the column absorber 5 from the top through the pipe 3. The unabsorbed gases pass from the absorber through the tube 7 located in the upper part of the absorber 5. The aqueous stream containing the desired product then passes through the pipeline 9 from the bottom of the absorber 5 towards the upper portion of a first distillation column 13 (recovery column) for the subsequent purification of the product. The product is recovered from the upper portion of the recovery column 13 and sent to the decanter 19 through the pipe 15. The aqueous solution containing crude acrylonitrile is allowed to separate into a first layer containing water and a second layer comprising product organic (for example, crude acrylonitrile and HCN) in the decanter 19. The product layer is then transferred through line 23 to a second distillation column 27 for the subsequent purification and recovery of produced acrylonitrile or methacrylonitrile. The lower stream obtained from the column 13 can be sent directly to an incinerator or a distillation separator column 21 through the pipe 17 to recover the crude acetonitrile as a vapor leaving from the top through the pipe 25. The current The lower stream obtained from the column 21 can be sent to an incinerator through the pipes 41 and 43. The product stream entering the distillation column 27 is distilled and the produced acrylonitrile is recovered as a side stream from the column 27 through the pipe 31. This acrylonitrile product is stored in a second decanter 33 which is maintained at a temperature between about 32 ° F to 70 ° F until a second phase of separation of the produced acrylonitrile and water takes place. The acrylonitrile produced is then removed from the decanter 33 through line 35 and introduced into the bottom of column 27 to exit through line 39 for final purification and recovery. The co-product HCN is recovered from column 27 as a stream from the top through line 29 and either incinerated or purified and recovered by conventional means known in the art.

Preferably, the ammoxidation reaction is carried out in a fluid bed reactor although other types of reactors are envisaged such as transport line reactors. Fluid bed reactors for the production of acrylonitrile are well known in the prior art. For example, the reactor design set forth in U.S. Patent No. 3,230,246, incorporated herein by reference, is suitable. The conditions for the ammoxidation reaction to occur are well known in the prior art as is apparent from U.S. Pat. Nos. 5,093,299; 4,863,891; 4,767,878 and 4,503.001; incorporated herein by reference. Typically, 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. However, for economic reasons, it is preferred to use air. The typical molar ratio of oxygen and 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 and olefin in the feed in the reaction can vary from 0.5: 1 to 5: 1. There is not really an 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, the preferred ranges being 310 ° to 500 ° C, with 350 ° to 480 ° C being especially preferred. The contact time, although not critical, is generally in the range of 0.1 to 50 seconds, preferably with a contact time of 1 to 15 seconds. In addition to the catalyst of the patent No. 3,642,930, other catalysts suitable for the practice of the present invention are set forth in U.S. Patent No. 5,093,299, incorporated herein by reference. The conditions under which the absorbent column is maintained, the recovery column and the separating column vary between 5 to 7 psig. (80 ° F to 110 ° F), 1 to 4.5 psig. (155 ° F to 170 ° F), and 7 to 13 psig. (170 ° F to 210 ° F), respectively. The present invention not only results in an unexpected improvement in the production proportions but it obtains this improvement without increasing the size of the columns used in the recovery and purification section. In addition, the concurrent increase in the proportions of production does not come with any deterioration observed in the quality of the product. In fact, the quality of the product improves with the practice of the present invention due to the decrease in peroxide impurity levels found in the final product. As will be apparent to those skilled in the art, various modifications of this invention may be made or followed in the light of the foregoing discussion and discussion without departing from the spirit and scope of the disclosure or the scope of the claims.

Claims (7)

1. NOVELTY OF THE INVENTION Having described the present invention is considered as a novelty and therefore claimed as property described in the following claims. l. A process for the preparation of acrylonitrile or methacrylonitrile comprising the transport of the reactor effluent obtained during the ammoxidation of propylene or isobutylene to a tempering column where the hot effluent gases are cooled on contact with an aqueous spray, passing the effluent of reactor cooled to the top towards an absorber column wherein the crude acrylonitrile or methacrylonitrile is absorbed in water, the aqueous solution containing the crude acrylonitrile or methacrylonitrile to a first decanter where a first aqueous layer and a first organic layer are formed. they contain acrylonitrile and methacrylonitrile, transferring the first organic layer to a second decanter where a second layer of water and a second organic layer are formed, and recovering the acrylonitrile or methacrylonitrile from the second organic layer wherein the improvement comprises the maintenance of the first and second decanter at a temperature in temperature from about 32 ° F to about 75 ° F.
2. 2. The process according to claim 1, characterized in that the interior temperature of the first decanter is maintained at about 32 ° F to 70 ° F.
3. 3. The process according to claim 2, characterized in that the interior temperature of the first decanter is maintained between 32 ° F to 65 ° F.
4. 4. The process according to claim 1, characterized in that the interior temperature of the second decanter is maintained between about 35 to 68 ° F.
5. 5. The process according to claim 4, characterized in that the interior temperature of the second decanter is between about 40 to 65 ° F. The process according to claim 1, characterized in that the reactor effluent is obtained from the ammoxidation of propylene, ammonia and oxygen to produce acrylonitrile. The process according to claim 1, characterized in that the reactor effluent is obtained by the reaction of propylene, ammonia and air in a fluid bed reactor while in contact in a fluid bed catalyst.