MXPA00012649A - Process for recovery of olefinically unsaturated nitriles - Google Patents

Abstract

A process for the recovery of acrylonitrile from an ammoxidation reactor effluent stream containing acrylonitrile, water and organic impurities including passing an absorber bottoms stream through a single recovery/stripper column (80), generating an acrylonitrile-rich overhead stream (96), a lean water side stream (84), and a recovery/stripper bottoms stream (92) containing organic impurities, wherein the acrylonitrile-rich overhead stream is passed into condenser (98) and into a decanter (100) to separate water from the acrylonitrile.

Description

PROCEDURE FOR THE RECOVERY OF OLYMPICLY INSATJRATED NUCLEARS BACKGROUND OF THE INVENTION The present invention relates to a process for the recovery and purification of an olefinically unsaturated nitrile. of an effluent stream from a reactor. The process for the production of olefinically unsaturated nitriles by catalytic reaction of ammonia and an olefin are well known. For example, acrylonitrile and methacrylonitrile can be produced by the catalytic vapor phase oxidation of propylene and isobutylin, respectively, in the presence of ammonia. In commercial processes for the preparation of acrylonitrile from propylene, ammonia and oxygen, the reactor effluent contains, in addition to the desired acrylonitrile product, considerable amounts of the by-product hydrogen cyanide, acetonitrile and other impurities such as succinonitrile and other nitrates. The exact composition of the effluent and the by-products and impurities it contains can vary considerably depending on the reaction conditions of ammoxidation and catalysts. Process reactor effluents to produce other olefinically unsaturated nitriles in a similar manner contain various by-products or impurities.

Some methods for treating reactor effluents of the type described for separating and recovering acrylonitrile product from byproducts or impurities are known. For example, see patents of E.U.A. 3,399,120, 3,433,822, 3,936,360, 4,059,492, 4,166, 008, and 4,404,064 sue are included herein by way of reference. Typically, these procedures include introducing the reactor effluent into an extinguishing chamber where it is contacted with water (generally containing sulfuric acid to neutralize excess ammonia in the reaction) -to cool the effluent and remove some contaminants such as polymers produced in the reactor. The cooled effluent gases from the cooling stream flow to an absorption column where they are contacted with water. The liquid stream at the bottom of the absorption column contains most of the nitriles produced in the reaction and some impurities, and is sent to an extractive distillation column, which is also called the recovery column. Most acrylonitrile in the extractive distillation column is obtained in the head (distillate) of the recovery column while water and impurities make up most of the bottom stream of the recovery column. The lower stream is typically supplied to a secondary distillation column or separation column to separate the acetonitrile and water in a higher stream while the secondary column bottoms containing water and various impurities are recirculated for example to the quench column.

The use of two distillation columns, the recovery column and the separation column is effective to achieve the separation and recovery of the product that is required in commercial operations. However, this system is expensive due to the cost of the equipment in question (not only the two distillation columns, but also the related pumps, piping, heat exchangers, etc.) and operating costs such as the use of energy from the two columns. There is a need for better procedures that can achieve the desired recovery at a low cost.

BRIEF DESCRIPTION OF THE INVENTION A process for recovering acrylonitrile from a stream containing acrylonitrile, water and organic impurities comprises the steps of (a) cooling an effluent stream from an ammoxidation reactor comprising acrylonitrile, water and organic impurities with an aqueous quench stream, producing thus a stream of cooled reactor effluent; (b) passing the stream of cooled reactor effluent through an absorption stream, thereby generating a lower stream comprising water, acrylonitrile, and organic impurities; and (c) passing the lower absorption stream through a single recovery / separation column, generating an acrylonitrile-rich top stream, a light water side stream, and a bottom recovery / separation stream comprising organic impurities.

In a specific embodiment of the invention, the top stream rich in acrylonitrile is passed through a decanter to remove water from the acrylonitrile. In another specific embodiment of the process, the lateral stream of light water is recirculated for use in the absorption column. The present invention is more economical than the acrylonitrile processes of the prior art. Since it can achieve the desired level of product recovery without requiring a recovery distillation column and a separation distillation column, both capital and operating costs are reduced.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a flow chart of an acrylonitrile recovery process of the prior art. Figure 2 is a flow diagram of an acrylonitrile recovery process of the present invention.

DESCRIPTION OF THE PREFERRED MODALITIES The processes for producing olefinically unsaturated nitriles are well known in the art. For example, an ammoxidation process for producing acrylonitrile is described in the U.S.A. No. 4,590,011, which is included herein by way of reference. The production of acrylonitrile generally involves supplying propylene, ammonia, an oxygen source such as air and an inert gas such as nitrogen to a fluidized bed reactor zone in which the reagents contact an ammoxidation catalyst. The typical molar ratio of oxygen and olefin in the supply is from 0.5: 1 to 4: 1, preferably from 1: 1 to 3: 1. The molar ratio of the ammonia and the olefin in the supply in the reaction is generally between 0.5: 1 to 5: 1. The conditions for the ammoxidation reaction that will take place are well known in the prior art as shown in the patents of E.U.A. Nos. 5,093,299, 4,863,891, 4,767,878 and 4,503,001, included herein by way of reference. The reaction is typically carried out at a temperature between about 260 ° to 600 ° C, preferably 310 ° to 500 ° C and most preferably 350 ° to 480 ° C. The contact time, although it is not determinant, is generally in the range of 0.1 to 50 seconds, preferring a contact time of 1 to 15. Preferably, the ammoxidation reaction is carried out in a fluid bed reactor although they can also be use other types of reactors. Fluid bed reactors for the manufacture of acrylonitrile are well known in the prior art. For example, the reactor design established in the U.S.A. No. 3,230,246, which is included herein by way of reference, is adequate.

The catalysts to be used in the reaction zone are well known in the art. In the patents of E.U.A. Nos. 3,642,930 and 5,093,299, included herein by way of reference, some suitable catalysts are disclosed. The reactor effluent will contain acrylonitrile, the desired product, in addition to organic impurities such as acetonitrile, as well as a certain amount of excess reagents, all generally in the gaseous state and at a temperature between about 450 to 480 ° C. The effluent from the reactor is transported to a cooling column (not shown) in which the hot effluent gases are cooled in contact with the water spray. Typically, any excess ammonia contained in the effluent is neutralized upon contact with sulfuric acid on cooling to remove the ammonia as ammonium sulfate. The cooled effluent gas containing the desired product (acrylonitrile) is then passed to the bottom of an absorption column (not shown) where the product is absorbed in water entering the column from the top. The unabsorbed gases pass from the absorption column to a tube located at the top of the absorption column. The lower aqueous stream of the absorber containing the desired product is then processed for further purification. Figure 1 shows a prior art process for purification of the acrylonitrile-containing aqueous stream from the bottom of the absorber. The aqueous stream 10 enters a first distillation column or recovery column 12, which generates an upper stream 14 containing water and acrylonitrile and a lower stream 16 containing water and various impurities, but relatively little acrylonitrile. The upper stream 14 of the first distillation column 12 passes through a condenser 18 to a decanter 20 in which water and acrylonitrile are separated. The water stream 22 of the decanter can be recirculated for use at another time of the process. The acrylonitrile stream of product 24 can be stored or further purified if desired. The lower stream 16 of the first distillation column 12 is pumped to a second distillation column or separation column 30. A portion of the upper stream 32 of this column 30, after passing through a condenser 34, is recirculated. to column 30, while another part 38 of the upper stream is sent to waste treatment. The lower stream 40 of the second column 30 is also sent to waste treatment 42. A side stream 44 of the second column 30 is a light stream of water that can be recirculated to the absorber (not shown). A second sidestream 46, which contains water and a relatively small amount of acrylonitrile, is recirculated to the first distillation column 12, passing through a cooler 50. The heat exchangers 26 and 48 provide energy for the distillation. Although the recovery system of Figure 1 will achieve the desired separation, it does so at a relatively high cost, due to the two distillation columns 12 and 30, heat exchangers 26 and 48, the corresponding pumps 60, 62, 64 and 66 , the related tubes, and the like, as well as the steam, cooling water and other expenses that are required to operate the recovery and purification process. Figure 2 shows one embodiment of an improved recovery system of the present invention. The supply 10, again coming from the lower part of the absorber, is supplied to a single distillation column 80. While the first distillation column 12 and the second distillation column 30 of FIG. 1 can, for example, have approximately 70 and 50 trays, respectively, the single distillation column 80 of Figure 2 may have for example about 110 trays. Of course the present invention is not limited to columns having a particular number of trays or limited to columns having trays. Fill columns can be used instead. A first sidestream 82 contains mainly water and impurities and is sent to waste treatment. A second sidestream 84 is used as a light stream of water to be recirculated to the absorber. A third side stream 86 passes through a cooler 88 and is recirculated to the top 90 of the column 80. A lower stream 92 is sent to waste treatment. A heat exchanger 94 supplies heat for distillation. The upper stream 96 of the combined recovery / separation column 80 passes through a condenser 98 to a decanter 100, producing an acrylonitrile stream of product 102 and a stream of water 104 that can be recirculated upon cooling, the absorber or the current 10. The operating conditions for distillation column 80 may vary depending on the products to be recovered and the degree of recovery desired. For example, the column can be operated at approximately atmospheric pressure at the inlet of condenser 98. Temperatures and pressures in other parts of the column can be determined by the type of column interiors and heat load in the heat exchangers. The process can be operated with the heat exchanger load adjusted so that, for example, 99.9% by weight of acrylonitrile and 99.4% of free hydrogen cyanide in the lower stream of the absorber 10 can be recovered in the upper stream 96. In addition, by adjusting the flow rate and temperature of the stream 86 which is recirculated to the top 90 of the column, the upper stream 96 may contain as little as 0.3% of the acetonitrile of stream 10. In other words, 99.7% The acetonitrile in the lower stream of absorber 10 can be extracted and sent to waste treatment by means of stream 82. The mode of operation described in the previous paragraph is suitable in situations where hydrogen cyanide is a valuable by-product and this is intended to be recovered to a practical degree. In situations where it is not necessary to recover this compound, the procedure could be operated with a reduced heat exchanger load. The amount of process equipment required for the recovery procedure of Figure 2 is considerably less than for the procedure of Figure 1. Recovery of each acrylonitrile is still possible despite reductions in equipment requirements and the resulting reduction is operating costs. In particular, the elimination of condenser and reflux required by a second distillation column decreases the use of total steam. In addition, since a condenser is removed, the use of water for cooling is reduced. The above description of the specific embodiments of the present invention is not intended to be a complete list of all possible embodiments of the invention. Those skilled in the art will recognize that modifications can be made to the specific embodiments described herein that could be within the scope of the present invention.

Claims (4)

NOVELTY OF THE INVENTION CLAIMS

1. - A process for recovering acrylonitrile from a stream of reactor effluent comprising acrylonitrile, water and organic impurities, comprising the steps of: cooling an ammoxidation reactor effluent stream comprising acrylonitrile, water and organic impurities with an aqueous stream of cooling, thereby producing a stream of cooled reactor effluent; passing the stream of cooled reactor effluent through an absorption column, thereby generating a lower stream of absorber comprising water, acrylonitrile and organic impurities; and passing the lower absorber stream through a single recovery / separation column, generating an acrylonitrile-rich top stream, a light water side stream and a bottom recovery / separation stream comprising organic impurities.

2. The process according to claim 1, further characterized in that the top stream rich in acrylonitrile is passed through a decanter to separate water from acrylonitrile.

3. The process according to claim 1, further characterized in that the side stream of light water is recirculated for use in the absorption column.

4. - The process according to claim 1, further characterized in that the ammoxidation reactor effluent stream is produced by catalytic reaction of ammonia and propylene.