MXPA98007379A - Process of minimization of residues and recovery of produ - Google Patents

Abstract

A process for the improved recovery of hydrogen cyanide (HCN) obtained from the reactor effluent of an ammoxidation reaction of propylene or isobutylene, including passing the reactor effluent through a quench column, an absorption column, a first column of distillation, a second distillation column, a cooler and an extraction tank, where the improvement includes contacting the vapor phase containing hydrogen cyanide with a water stream

Description

WASTE INITIATION AND PRODUCT RECOVERY PROCESS FIELD OF THE INVENTION The present invention relates to an improved process for the manufacture of acrylonitrile or methacrylonitrile. In particular, the present invention relates to the improvement of the recovery of hydrogen cyanide used during the manufacture of acrylonitrile or methacrylonitrile. The recovery of acrylonitrile / methacrylonitrile produced by the ammoxidation of propylene or isobutylene on a commercial scale has been accomplished by rapidly cooling the reactor effluent with water followed by passing the gaseous stream containing acrylonitrile or methacrylonitrile resulting from rapid cooling to an absorber where the water and the gases are contacted in countercurrent flow to remove substantially all of the acrylonitrile or methacrylonitrile; the aqueous stream containing the acrylonitrile or methacrylonitrile and HCN is then passed through a series of associated distillation columns and decanters for separation and purification of the acrylonitrile or methacrylonitrile produced from a vapor stream containing substantially all of the HCN. Typical recovery and purification systems used during the manufacture of acrylonitrile or methacrylonitrile are described 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 recovery of the HCN by-product in the manufacture of acrylonitrile or methacrylonitrile. Another object of the present invention is to provide an improved process for the manufacture of acrylonitrile or methacrylonitrile which reduces the amount of HCN directed to the incineration or other recovery processes. Another object of the present invention is to provide an improved process for the manufacture of acrylonitrile or methacrylonitrile which includes transporting the reactor effluent obtained during the ammoxidation of propylene or isobutylene to a quench column where the hot effluent gases are cooled by contact with a spraying water, passing the effluent from the reactor cooled above to an absorption column where the crude acrylonitrile or methacrylonitrile is absorbed in water, passing the resulting vapor stream containing the HCN to a product cooler and then to an extraction tank, wherein the improvement includes contacting the cooled HCN product with an aqueous stream. Another object of the present invention is to provide an improved process as described above wherein the aqueous stream is brought into contact with the cooled HCN product before the combined stream is introduced into the extraction tank. A further object of the present invention is to provide an improved process as described above wherein the aqueous stream is brought into contact with the cooled HCN product within the extraction tank. Another object of the present invention is to provide an improved process as described above wherein the aqueous stream is brought into contact with the cooled HCN product before the combined stream is introduced into the extraction tank and into the extraction tank. A further object of the present invention is to provide an improved process as described above where the reactor effluent is obtained from the ammoxidation of propylene, ammonia and oxygen to produce acrylonitrile. Another object of the present invention is to provide an improved process as described above where 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 additional objects, advantages and novel features of the invention will be set forth in part in the following description, and in part will be apparent to those skilled in the art after examination of what follows next or can be learned by putting the invention into practice. The objects and advantages of the invention can be realized and obtained by means of the instrumentalities and combinations indicated in particular in the appended claims. To achieve the above objects and other objects and according to the purpose of the present invention made and described widely herein, the process of the present invention includes transporting the reactor effluent obtained during the ammoxidation of propylene or isobutylene to a cooling column. fast where the hot effluent gases are cooled by contact with an aqueous spray, pass the reactor effluent cooled above to an absorption column where the crude acrylonitrile or methacrylonitrile is absorbed in water, pass the aqueous solution containing the acrylonitrile or methacrylonitrile, plus HCN and other impurities, to a first distillation column (recovery column), where a considerable portion of the water and impurities are removed as a liquid waste product, while HCN, water, a secondary portion of impurities and acrylonitrile or methacrylonitrile they are extracted as upper steam stream. In past operations, said upper steam stream is further cooled using a heat exchanger, and directed to an extraction drum, to separate and condense liquids that are returned to the recovery process, while the remaining steam stream is directed to a torch, incinerator or other waste process. The present invention improves the past operation by adding an aqueous stream to the cooled stream directed to the extraction tank or, alternatively, by adding an aqueous stream using spray nozzles inside the extraction tank to contact the vapor stream. Optionally, aqueous streams can be added to the cooled stream as well as to the spray nozzles at the same time. The aqueous stream may be a portion of the aqueous stream used to cool the reactor effluent in the quench column, a portion of the aqueous stream used to absorb the acrylonitrile or methacrylonitrile in the absorption tower ("lean water"), or another aqueous stream of the process including fresh water or demineralized water. The liquid stream from the waste from the extraction tank, including the recovered HCN, can be redirected to the quench tower by addition to the aqueous stream used to cool the reactor effluent in the quench column, or the absorption tower by adding it to the aqueous stream used to absorb the acrylonitrile or methacrylonitrile in the absorption tower, or at some other point, in the recovery process. In a preferred embodiment of the present invention, the process is performed with the reactor effluent obtained from the ammoxidation of propylene, ammonia and oxygen to produce acrylonitrile. In another preferred embodiment of the present invention, the effluent from the reactor is obtained by the reaction of propylene, ammonia and air in a fluid bed reactor while in contact with a fluid bed catalyst. Conventional fluidized bed ammoxidation catalyst can be used in the practice of the invention. For example, the fluid bed catalyst 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 a more efficient operation during the recovery of HCN. The operation of the HCN product cooler and the HCN extraction tank within the above exposed temperature range results in a higher recovery of HCN in the liquid product from the HCN extraction tank, which can be recovered in the existing process of recovery and purification of acrylonitrile or methacrylonitrile. Said improved recovery means that less HCN is directed to incineration, with the consequent reduction of emissions from the combustion of waste gases. Another advantage of the implementation of the invention is that the operation of the HCN product cooler and the HCN extraction tank in the temperature and pressure bands discussed above leads to a higher recovery of HCN product. BRIEF DESCRIPTION OF THE DRAWING Figure 1 is a schematic representation of the process applied to the manufacture of acrylonitrile and the improved recovery of HCN. DETAILED DESCRIPTION OF THE INVENTION The present invention will now be described in detail with reference to Figure 1. The reactor effluent, obtained by the ammoxidation of propylene or isobutylene, ammonia and oxygen containing gas in the fluid bed reactor (not shown) while is in contact with a fluidised bed ammoxidation catalyst, is transported to a quench column 10 by transfer line 11, where the hot effluent gases are cooled by contact with a water spray 14. The cooled effluent gas containing the desired product (acrylonitrile or methacrylonitrile, acetonitrile and HCN) is then passed to the bottom of an absorption column via line 12, where the products are absorbed into the water entering the absorption column 20 at the top by line 24. The non-absorbed gases pass from the absorber through the tube 22 located in the upper part r of the absorber 20. The aqueous stream containing the desired product is then passed via line 23 from the bottom of the absorber 20 to the upper portion of a first distillation column 30 (recovery column) for further purification of the product. The product is recovered from the upper portion of the recovery column and sent to a second distillation column 40 (concentrate column) by line 32, while water and other impurities are extracted from the recovery column 30 by line 33. In the concentrate column 40, the HCN is taken up and removed from the column by the line 42 is cooled in the upper condenser 80 and the resulting material is directed to the reflow drum 50 by the line 41. The liquid reflux from the reflux drum 20 is returned to the upper portion of the concentrate tower by the line 53. The vapor phase material is withdrawn from the reflux drum 50 via line 52 and cooled in the HCN 90 product condenser. The cooled and partially condensed effluent from the HCN 90 product condenser is directed to the HCN extraction tank by the lines 93 and 61. Optionally, an aqueous stream can be added to the partially condensed effluent of the HCN product condenser via line 71, and the combined stream is directed to the reservoir of ext. HCN ration by line 61. The HCN extraction tank may optionally be provided with one or more spray nozzles 73, to provide for the addition of an aqueous stream to the HCN extraction tank via line 72. A significant portion The HCN is condensed or is absorbed by the added aqueous stream, and is withdrawn from the HCN extraction tank via line 63 and returned to the recovery process. The optional positions where the condensed or absorbed HCN and the added aqueous material, stream 63, can be returned to the recovery process, include cooling water 14 of the quench tower 10, the absorption water 24 of the tower of absorption 20, the recovery column 30. The non-condensed HCN, together with other non-condensable material, is extracted from the HCN extraction tank by line 62, and can be sent directly to the incinerator, or purified and recovered by means conventional ones known in the art. When the process is started by adding an aqueous stream 72 to the spray nozzles of the extraction drum, the aqueous stream is contacted more efficiently as compared to the direct addition of an aqueous stream 71. Depending on the non-condensable products present In the vapor stream entering the extraction tank, and the ratio of aqueous vapor to steam stream, the temperature of the steam leaving the extraction tank, it is possible to recover the hydrogen cyanide from the steam stream in a wide range of recovery efficiencies. During a torch analysis test it was observed that with the activated water addition system in the extraction tank, the amount of HCN going to the torch decreases from 90.6 kg (200 pounds) / hour to 45.3 kg (100 pounds) / hour. The incoming steam stream in the extraction tank had 33.0% by weight of HCN, 58.3% of nitrogen, 4.12% of C02, 4.12% of CO and in this case it had a mass flow of 271 , 8 kg (600 pounds) / hour. A lean water flow of 3 gpm was used for the sprays.

The limit of the addition of water is controlled simply by the viability of the amount of solution to be extracted from the extraction tank, as well as by the limitation of the additional wastewater treatment. The simulations of the process further demonstrate that tripling the water flow would increase the efficiency of HCN recovery through the extraction tank from 50% to 84.5%. If chilled water is used at a temperature of 12.8 ° C (55 ° F), the efficiency of the recovery could be increased to 96%. Preferably, the ammoxidation reaction is carried out in a fluid bed reactor although other types of reactors are contemplated, such as transport line reactors. Fluid bed reactors, for the manufacture of acrylonitrile, are known in the prior art. For example, the design of the reactor 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 known in the prior art, as evidenced by U.S. Patent 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. An oxygen source can be used. However, for economic reasons, it is preferable to use air. The typical molar ratio of oxygen to olefin in the feed should be in the order of 0.5: 1 to 4: 1, preferably 1: 1 to 3: 1. The molar ratio of ammonia to olefin in the feed in the reaction can vary between 0.5: 1 to 5: 1. There is really no upper limit to the ammonia-olefin ratio, but in general there is no reason to overcome a 5: 1 ratio for economic reasons. The reaction is carried out at a temperature between the bands of about 260 to 600 ° C, but the bands preferred are 310 to 500 ° C, with 350 preferred in particular 480 ° C. The contact time, although not critical, is generally of the order of 0.1 to 50 seconds, with a contact time of 1 to 15 seconds being preferred. In addition to the catalyst of U.S. Patent No. 3,642,930, other catalysts suitable for the practice of the present invention are disclosed in U.S. Patent No. 5,093,299, incorporated herein by reference. The conditions in which the absorption column, the recovery column and the concentrate column are maintained range from 1 to 7 psig (26.7 to 43.3 ° C (80 to 110 ° F)), 0.5 to 10 psig (68.3 to 76.7 ° C (155 to 170 ° F)), and -10 to > psig (11.1 to 33.3 ° C (52 to 92 ° F)), respectively. The present invention not only results in an unexpected improvement of HCN recovery rates, but achieves this improvement without increasing the size of the columns used in the recovery and purification section. In addition, the concomitant increase in production rates is obtained without observed deterioration in the quality of the product. As will be apparent to those skilled in the art, various modifications of the invention may be made or followed in the light of the foregoing description and explanation without departing from the spirit and scope of the description or scope of the claims.

Claims (9)

1. CLAIMS 1. A process for the manufacture of acrylonitrile or methacrylonitrile which includes transporting the reactor effluent obtained during the ammoxidation of propylene or isobutylene to a quench column where the hot effluent gases are cooled by contact with an aqueous spray, passing the reactor effluent cooled above to an absorption column where the crude acrylonitrile or methacrylonitrile is absorbed in water, passing the resulting vapor stream containing the HCN to a product cooler and then to an extraction tank, where the improvement includes putting in contact the HCN product cooled with an aqueous stream.
2. 2. The process of claim 1, wherein said aqueous stream is brought into contact with the cooled HCN product before the combined stream is introduced into the extraction tank.
3. 3. The process of claim 1, wherein said aqueous stream is brought into contact with the cooled HCN produced within the extraction tank.
4. 4. The process of claim 1, wherein said aqueous stream is brought into contact with the cooled HCN produced before the combined stream is introduced into the extraction tank and into the extraction tank.
5. 5. The process of claim 1, wherein said aqueous stream is lean water.
6. 6. The process of claim 1, wherein said aqueous stream is fresh water. The process of claim 1, wherein said aqueous stream is demineralized water. The process of claim 1, wherein the reactor effluent is obtained from the ammoxidation of propylene, ammonia and oxygen to produce acrylonitrile. The process of claim 1, wherein 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.