MXPA00001954A - Process for preparing acrylic acid - Google Patents

Abstract

This invention relates to a process for preparing acrylic acid which utilizes an aqueous stream which includes recycled wastewater, at least part of which is stripped of undesirable components in a stripping column prior to being recycled to an acrylic acid absorber.

Description

PROCESS FOR PREPARING METACRILIC ACID The present invention relates to a process for preparing methacrylic acid. In particular, the invention relates to a process for preparing methacrylic acid using an aqueous stream that includes waste water recycled in the process to separate the methacrylic acid from a mixed gas produced. Generally, methacrylic acid is prepared by the catalytic oxidation of at least one hydrocarbon material. For example, methacrylic acid can be prepared from propylene or acrolein in one or two step processes. In a first step, the propylene is oxidized in the presence of oxygen, inert diluent gases, water vapor and the appropriate catalysts to produce acrolein according to equation (I): C3He + 02 = > C2H3CHO + H20 + heat (I) Next, the acrolein is oxidized, in a second step, in the presence of oxygen, inert diluent gases, water vapor and the appropriate catalysts to form acrylic acid according to equation (II): C2H3CHO + 02 = > C2H3COOH + heat (II) Acrolein can be provided as starting material in a step of reaction (II) to produce acrylic acid. Alternatively, propane can be used as starting material. Propane is oxidized using suitable catalysts, for example, as described in U.S. Patent 5,380,933 to form acrylic acid. Ethacrylic acid is prepared in a similar manner by the catalytic oxidation of isobutylene or isobutane. The methacrylic acid prepared using said catalytic oxidation reactions is present in a mixed gas produced leaving the reactor. In general, the mixed gas produced is cooled and contacted with an aqueous stream in the absorption tower, thus providing an aqueous solution of acrylic acid which is then dehydrated in a distillation step to provide a crude solution of acrylic acid. The crude solution of acrylic acid can be used to produce various acrylic esters or further purified to provide various grades of purified acrylic acid which can be used, for example, in the production of super absorbent polymer products. Generally, in chemical manufacturing processes, including processes for the production of acrylic acid, a large waste load occurs. Said waste charges assume the form of waste product gases and wastewater streams. The waste products gases can be generated at various points in the manufacturing process of acrylic acid. Of particular interest is the remainder of the gas of the mixed product leaving the absorber from which the acrylic acid produced after contact with the aqueous stream present in the absorber has been absorbed. This remainder of the mixed gas produced, known as the waste gas from the absorber or waste gas from the absorber, usually goes through the same type of treatment waste as thermal oxidation, incineration or catalytic oxidation before being released into the air to fulfill with the applicable requirements of environmental protection. Waste water is also generated in the acrylic acid production process as well as in other manufacturing processes. A typical acrylic acid production facility can generate up to two pounds of wastewater per pound and acrylic acid produced, depending on the particular process followed. Of particular interest is the residual water recovered in dehydration processes, for example, in the dehydration of acrylic acid. In general, wastewater should also be treated in some way to comply with applicable environmental protection standards before releasing it into the environment. Consequently, since the treatment and elimination of waste gases and waste water constitutes a considerable expense for the manufacturer of acrylic acid, a constant goal of manufacturers is to achieve alternative applications and treatments that add value to the manufacturing process or reduce costs. Several methods have been devised in the art to achieve this goal. It is known in the art that at least part of the waste gas from the absorber can be recycled back to the reactor or reactors. This recycling fulfills several purposes, among which are the provision of an inert diluent and vapor gas to the reagent composition and reduce the wastewater generated in the process by reducing the amount of steam fed into the process. In addition, the small amounts of unreacted propylene and acrolein contained in the waste gas are given another opportunity to react and, therefore, improve the overall yield of acrylic acid by increasing the conversions of propylene and acrolein. The recycling of certain waste generated in an acrylic acid process for its new use in the absorption step of the process has been taught in the art. For example, Japanese Patent Application Kokai (Open to the public) Number 246941/1993 shows the recycling of a recovered acetic acid solution for reuse in an absorber within an acrylic acid absorption tower. However, the application notes that the recovery of the acrylic acid is inefficient in the absorber because no solvent or substantially no acrylic acid is present. U.S. Patent No. 5,785,824 discloses the recycling of wastewater to the absorber of an acrylic acid process in which the stream of recycled wastewater has a specific composition of acetic acid (1-10% by weight), acrylic acid ( 0.5-5.0% by weight) and solvent distillation (0.01-0.5% by weight). It is stated that said recycle stream, which contains these determined amounts of acetic acid, acrylic acid and distillation solvent allows the collection of acrylic acid in the absorber with great efficiency. However, a problem that is foreseen with the recycling of wastewater is that the recycled wastewater may contain an unacceptable concentration of undesirable elements such as volatile organic compounds (VOCs). Such high concentrations and elements n? Desirable in the stream of recycled waste water may cause a higher waste concentration, eg, acrylic acid and organic distillation solvent, in the waste gas of the absorber. Consequently, when part of the waste gas from an absorbent or all the gas is recycled in the reactor, it will include a higher level of undesirable components. Such materials can be detrimental to the catalytic oxidation reactions that form acrolein or acrylic acid in the reactor. In particular, it is believed that the activity of the oxidation catalysts can be reduced by the presence of the higher level of volatile organic components. Likewise, certain organic elements such as toluene can compete effectively with the initial materials, in such a way that a smaller amount of the initial materials reacts and more derived products appear in the acrylic acid produced. The preparation and isolation of methacrylic acid proceeds in similar steps. Consequently, methacrylic acid manufacturers suffer similar problems. The present inventors have now discovered that by removing undesirable components from the waste water with a scrubbing gas that includes the waste gas from the process solves the problem of inadequate levels of undesirable components that are recycled by the absorber. In addition, the invention allows more value to be obtained from the process waste water and waste gas streams. This is done while maintaining an adequate acrylic acid collection capacity in the absorber. Accordingly, a novel process for preparing acrylic acid is described herein in which the following advantages are provided: (1) a smaller number of undesirable components in the waste gas of the absorber leading to a lower amount of potentially harmful components recycled again in the reactor; (2) additional value is obtained from waste gas streams from the process when used to distill the wastewater stream from the process; (3) distilled wastewater with a lower level of potentially harmful components for the catalytic reactions in the reactor are recycled to the absorber for the absorption of acrylic acid, thus reducing the load of wastewater in the facility; (4) The distillation gas streams sent to the thermal oxidizers for treatment have a higher concentration of organic material from the steam distillation process, thus reducing the fuel requirement of the thermal oxidizer. In one aspect of the present invention, a process for preparing methacrylic acid is provided, which includes the steps of: (A) feeding an absorption tower (i) a mixed gas produced by the catalytic oxidation of at least one hydrocarbon material with a gas containing molecular oxygen, and (ii) an aqueous stream that includes recycled waste water and less than 3.0 percent by weight of acetic acid; (B) contacting the mixed gas produced with the aqueous stream in the absorption tower to form an aqueous stream of methacrylic acid; and (C) feeding the aqueous stream of methacrylic acid to a distillation column, where it is subjected to azeotropic distillation in the presence of at least one solvent to form a methacrylic acid solution substantially free of water. In a second aspect of the present invention, there is provided a process for preparing methacrylic acid, which includes the steps of (A) feeding to an absorption tower a mixed gas produced by the catalytic oxidation of at least one hydrocarbon material with a gas containing molecular oxygen, and an aqueous stream comprising recycled waste water and less than 3.0 percent by weight of acetic acid, where, before being added to the aqueous stream, at least a portion of the recycled waste water is extract the undesirable components in a steam distillation column, using a distillation gas that includes a waste gas stream; (B) contacting the mixed gas produced with the aqueous stream in the absorption tower to form an aqueous stream of methacrylic acid; (C) feeding the aqueous stream of methacrylic acid to a distillation column, where it is subjected to azeotropic distillation in the presence of at least one distillation solvent to form a methacrylic acid solution substantially free of water. In a third aspect of the present invention, a process for preparing methacrylic acid is provided which includes the steps of: (A) extracting at least a current day portion of recycled waste water in a steam distillation column, using a gas of distillation including a waste gas from the absorber, to form an aqueous stream having less than 3.0 percent by weight of acetic acid, (B) feeding (i) a mixed gas produced by the catalytic oxidation of at least one material of hydrocarbon with a gas containing molecular oxygen and (ii) the aqueous stream to an absorption tower; (C) contacting the mixed gas produced with the aqueous stream in the absorption tower to form an aqueous stream of methacrylic acid; D) feeding the aqueous stream of methacrylic acid to a distillation column, where it is subjected to azeotropic distillation in the presence of at least one distillation solvent. to form a methacrylic acid solution substantially free of water. In a fourth aspect of the present invention, there is provided a process for preparing methacrylic acid, which includes the steps of (A) feeding to an absorption tower a mixed gas produced by the catalytic oxidation of at least one hydrocarbon material with a gas containing molecular oxygen, and an aqueous stream comprising recycled waste water and less than 3.0 by weight of acetic acid, where before being added to the aqueous to at least a portion of the stream of recycled waste water is extracted the undesirable components in a steam distillation column using a distillation gas that includes a waste gas stream; (B) contacting the mixed gas produced with the aqueous stream in the absorption tower to form an aqueous stream of methacrylic acid; (C) extracting the light ends of the aqueous stream of methacrylic acid, and (D) feeding the aqueous stream of methacrylic acid to a distillation column, where it is subjected to azeotropic distillation in the presence of at least one distillation solvent to form a methacrylic acid solution substantially free of water. Figure 1 represents a flow diagram showing an embodiment of the process of the present invention. Figure 2 shows a flow diagram showing a second embodiment of the process of the present invention. Figure 3 shows a flow diagram showing a third embodiment of the process of the present invention. Throughout the present specification and the claims, unless otherwise indicated, references to percentages are by percentage by weight and all temperatures are in degrees centigrade. It is further understood that for the purposes of the present specification and the claims the limits of the range and proportion, presented herein, are combinable. For example, if intervals 1-20 and 5-15 are presented for a particular parameter, it is understood that intervals 1-15 or 5-20 are also contemplated. Furthermore, it is understood that the term "significant amount" means more than 50 percent by weight of the total composition. It is understood that the term "minor amount" means less than 50 percent by weight of the total composition. It is understood that the term "residual water" means any stream of water containing impurities or additives. Likewise, it is understood that the term "waste gas" means a gas or mixture of gases that contains or contains impurities or additives. It is understood that the term "methacrylic acid" encompasses both acrylic acid and methacrylic acid. The process of the present invention will be described initially with reference to Figure 1. Further references to Figures 2 and 3 will be made to describe various other embodiments of the invention. In addition, although the present invention is described following the terms of a process for preparing acrylic acid, it should be understood that the invention also encompasses the preparation of methacrylic acid. As noted above, the process of the present invention for preparing methacrylic acid includes feeding a mixed gas produced by the catalytic vapor phase oxidation of at least one hydrocarbon material with a molecular oxygen-containing gas to a tower. absorption 2. The produced mixed gas is obtained by means of the catalytic vapor phase oxidation of a hydrocarbon material containing gas in the presence of a suitable oxidation catalyst. The catalytic vapor phase oxidation of a hydrocarbon material to acrolein or acrylic acid as well as the reactors, catalysts and processes for carrying out the same are usually known in the art and are described, for example, in US Pat. Nos. 4,203,906; 4,256,783; 4,326,087; 4, 873.368; 5, 161, 605; 5,177,260; 5,198,578; 5,739,391; 5,821,390 and the pending joint patent application of the United States 09/244182. Depending on the reactants fed to the reactor, the mixed gas produced generally includes inert gases, including, but not limited to, nitrogen, helium, argon, etc; acrylic acid; unreacted hydrocarbon reagents, including, but not limited to, propylene, acrolein, propane, etc .; steam and reagents containing molecular oxygen, including, but not limited to, acetic acid, formaldehyde, maleic acid and other organic elements; as well as C02, CO and H20. In general, the composition of the mixed gas produced fed 1 includes from 5 to 30 by weight of acrylic acid, from 0.1 to 3.0 percent by weight of acrylic acid, from 0.02 to 0.2 percent by weight of acrolein, from 30 to 95 percent by weight of inert gases and 1 to 30 percent by weight of steam. The process of the present invention for preparing acrylic acid also includes feeding an aqueous stream 3 including recycled waste water to the absorber2. The waste water can be any suitable waste water to be used in the absorption of acrylic acid from a mixed gas produced from acrylic acid and can be from any source. Consequently, it is not necessary that the waste water be derived from the same acrylic acid process stream within which it is recycled. Rather, the wastewater can be derived from an acrylic acid process stream and recycled into another. Suitable examples of wastewater are, but are not limited to, wastewater derived from the dehydration of acrylic acid, other distillates and aqueous raffinates. Similarly, it is not necessary for waste water to be derived from a waste water stream of acrylic acid. Accordingly, the waste water can be derived from waste water streams from other chemical processes, for example, from a process stream of methacrylic acid. In addition, waste water can be derived from a natural source such as a river, well, spring or similar. The aqueous stream 3 can include any amount of recycled wastewater up to 100 percent by weight of recycled wastewater. Typically, the aqueous stream 3 will be a mixture of a wastewater stream 22 from the acrylic acid manufacturing process and an essentially pure water stream 5. In one embodiment, the aqueous stream 3 includes a major amount of wastewater . In another embodiment, the aqueous stream 3 includes from 0.1 to 100 percent by weight of wastewater. Preferably, the aqueous stream 3 contains 100 percent by weight of residual water. Regardless of how much recycled wastewater is used, the aqueous stream will contain a major amount of water and minor amounts of at least one of the following elements: acrylic acid, acetic acid or distillation solvents. In general, the aqueous stream contains less than 3.0, preferably less than 2.0 and more preferably less than 1.5 percent by weight of acetic acid. In another embodiment, the aqueous stream is substantially free of distillation solvents or acrylic acid. As indicated in Figure 1, all or a portion of the waste water stream 4 can be removed from the undesirable components before it is introduced into the aqueous stream 3. In one embodiment, the wastewater stream is extracted from it. the undesirable components in a steam distillation column 6 using a stream of distillation gas 7, which includes waste gas, to produce a stream of distilled waste water 29. The waste water stream 29 is then fed to the aqueous stream 3 in the form of a waste water stream 22. Alternatively, undesirable components are extracted to a part of the waste water stream 4 to form the distilled waste water stream 29, the undisturbed portion of the waste water stream being combined 4. with the distilled wastewater stream to form the wastewater stream 22 which is then fed to the aqueous stream 3. Finally, in another embodiment, all the waste water stream 4 is introduced into an aqueous stream 3 as residual water stream 22 without distillation. The quantity of distilled or undistilled wastewater stream 4 will vary according to the amount of undesirable components in the wastewater stream 4. The distillation gas stream 7 may include, but is not limited to, combustion air and fresh air, as well as waste gases. The waste gas stream may be any suitable for extracting the undesirable components from a waste water stream. Among the suitable examples of gas streams derived from the absorber, compressor suction ventilation gas streams stand out. Preferably, the residual gas stream is a stream of residual gas absorber. Said embodiment of the present invention is illustrated in Figure 2 wherein the absorbing gas stream 8 of the absorber 2 is fed to the steam distillation column 6 in the form of distillation gas 7 to remove the undesirable components of the gas. the waste water stream 4 in the steam distillation column 6. The waste gas stream from the absorber 8 is typically divided such that a portion, steam 30, is recycled in an oxidation reactor using the remainder as gas of distillation 7. The waste gas stream 31 of the extractor 6 is generally sent to a residual treatment for incineration or oxidation and then released to the atmosphere. In general, the residual gas with steam distillation has a water content of from 0 to 100, preferably from 5 to 30, more preferably from 8 to 20 percent by weight and a temperature from 20 to 250, preferably from 45 to 125. , more preferably from 50 to 90 degrees centigrade. The use of a part of the waste gas stream of the absorber 8 as the distillation gas 7 is advantageous because the waste gas from the absorber emerges from the absorber with sufficient heat and water content to adequately remove the undesirable components from the waste water. Accordingly, the treatment of a distillation stream with potential waste gas vapor stripping, ie, heating and adding or removing water, is avoided. However, it may sometimes be desirable to use other distillation gas stream, such as air, combustion air, etc., either alone or in combination with a waste gas stream from the absorber. This may require additional heating of the distillation gas to obtain the proper operating temperature range. In addition, additional heat may be required to remove the organic compounds from the wastewater stream. Accordingly, a live steam sprinkler, internal or external reboiler, steam distillation gas feed pre-heater, or other methods known in the art for supplying additional heat to a steam distillation column may be applied. In addition, there may be instances where additional transfer time is required within the steam distillation column. For example, in case there is an increase in pressure drop inside the column and the pressure of the absorber is insufficient to provide adequate momentum transfer. Consequently, devices such as a blower can be used to increase moment transfer. The steam stripping distillation column 6 can be any column suitable for the distillation of undesirable components from the waste water. Said columns are known in the art and include packed columns and columns containing trays. In a further embodiment, the aqueous stream 3 includes a polymerization inhibitor introduced into the polymerization inhibitor feed 12. The polymerization inhibitor can include a water-soluble or alcohol-soluble polymerization inhibitor. They stand out among the right examples, but are not limited to these, hydroquinone; 4-methoxyphenol; 4-ethoxyphenol; 1,2-dihydroxybenzene; catechin monobutyl ether; pyrogallol; 4-aminophenol; 2-mercaptophenol; 4-mercaptophenol; 4-hydroxy-2, 2,6,6,6-tetramethylpiperidinyloxy, free radical; 4-oxo, 2, 2, 6, 6-tetramethylpiperidinyloxy, free radical; 4-amino-2, 2,6,6,6-tetramethylpiperidinyloxy, free radical; isomers of it; derived from it; mixtures of two or more of them; or mixtures of one or more of the foregoing with molecular oxygen.

In the absorber, the produced mixed gas 1 is contacted with the aqueous stream 3 to form an aqueous stream of acrylic acid 9. The aqueous stream of acrylic acid 9 generally includes from 20 to 95, preferably from 35 to 90 and more preferably from 50 to 80 weight percent acrylic acid; from 80 to 5, preferably from 65 to 10, more preferably from 50 to 20 percent by weight of water; and up to 8, preferably 6, more preferably 6 weight percent acetic acid. Generally, the produced mixed gas 1 is fed to the absorber at a temperature of from 165 to 400, preferably from 200 to 350, more preferably from 250 to 325 degrees. The aqueous stream 3 is fed to the absorber at a flow rate of 0.1 to 1.0 aqueous pounds per one pound of hydrocarbon material fed to the reactor, depending on the desired concentration of acrylic acid that will be recovered from the lower parts of the absorber 2. Absorber 2 can have any suitable absorber design known in the art. The aqueous stream of acrylic acid 9 is then fed to the distillation column 10 where it is subjected to azeotropic distillation in the presence of at least one distillation solvent to form a stream of acrylic acid 11. The distillation column can be any column of distillation known in the art. For example, a screen tray, a double flow tray design, or a packed column. The solvent or the distillation solvents can be any suitable solvents for the azeotropic distillation of a stream of acrylic acid. In one embodiment, the solvent is substantially insoluble in water, generally having a solubility in water at room temperature of 0.5 percent by weight or less, preferably 0.2 percent by weight or less. Suitable examples of said water-insoluble solvent are, but are not limited to, heptane; heptene; cycloheptane; cycloheptene; cycloheptatriene; methylcyclohexane; ethylcyclopentane; 1,2-dimethylcyclohexane; ethylcyclohexane; toluene; ethylbenzene; ortho-, meta, or paraxylene; trichlorethylene; trichloropropene; 2, 3-dichlorobutane; 1-chloropentane; 1-chlorohexane and 1-chlorobenzene. In another embodiment, the solvent is selected from ethyl acetate, butyl acetate, dibutyl ether, hexane, heptane, ethyl methacrylate, diethyl ketone, methyl propyl ketone, methyl isobutyl ketone and tert-ketone. -methyl butyl. In a further embodiment, the distillation solvent is a mixed solvent that includes at least two solvents. Suitable examples of solvent useful in the mixed solvent are, but are not limited to, diethyl ketone, methyl propyl ketone, methyl isobutyl ketone, methyl tert-butyl ketone, isopropyl acetate, n-acetate. propyl, toluene, heptane and methylcyclohexane. The preferred distillation solvent is toluene. In an embodiment, illustrated in the figure 3, the aqueous stream of acrylic acid 9 is fed to a steam distillation column of light ends 13 before being fed to the distillation column 10. The distillation column of light ends 13 distills the light ends, even, but not limited to, acrolein, formaldehyde, acetaldehyde, propionaldehyde, methyl ether and methyl vinyl ketone, of the aqueous stream of acrylic acid 9. Generally, the distillation gas used is steam. From the bottom of the column of light ends 13 arises a stream of acrylic acid 14 which is substantially free of light ends. The acrylic acid stream is introduced into the distillation column 10. The vapor 15 leaving the top of the light ends column 13 is recycled back to the absorber 2 where part of the acrolein is recovered in the waste gas of the absorber and recycled back to the oxidation reactor thus improving the performance of the acrylic acid. In a further embodiment, aqueous stream 9 or 14 includes a polymerization inhibitor in the polymerization inhibitor feeds 20 and 21. Suitable inhibitors are described above. In one embodiment, the polymerization inhibitor is 4-hydroxy-2, 2,6,6,6-tetramethylpiperidinyloxy, free radical; derivatives thereof or mixtures of 4-hydroxy-2, 2,6,6,6-tetramethylpiperidinyloxy, free radical with molecular oxygen. In an alternative embodiment, the polymerization inhibitor is a mixture of 4-hydroxy-2,2,6,6-tetramethylpiperidinyloxy, free radical; derivatives thereof or mixtures of 4-hydroxy-2,2,6,6-tetramethylpiperidinyloxy, free radical with at least one other inhibitor and molecular oxygen. Certain distillation columns, such as a sieve tray column, require the use of a polymerization inhibitor in the vapor phase. Suitable examples of suitable vapor phase inhibitors, useful in the present invention, are, but are not limited to, N-nitrosophenylhydroxylamine and salts thereof, nitric oxide, nitrosobenzene and p-benzoquinone. A stream of acrylic acid 11 substantially free of water emanates from the lower parts of the distillation column 10. Generally, the acrylic acid stream 11 has less than 1000, preferably less than 800, more preferably less than 500 ppm of water. The acrylic acid stream may also contain unsubstantial amounts of at least one of the following: acetic acid, propionic acid, β-acryloxypropionic acid (AOPA), acrolein, furfural, benzaldehyde, maleic acid, anhydride maleic, protoanemonin and acetaldehyde. The acrylic acid stream 11 is generally sent to be used as a raw material in the production of acrylic ester or acrylate polymer. The acrylic acid can be used as found or further processed, including, but not limited to, additional distillation to remove specific impurities and further processed to form acrylic acid of various qualities. The stream of the upper distillate 23 emanating from the top of the distillation column 10 generally includes, but is not limited to, azeotropes of water, acetic acid or acrylic acid with the distillation solvent. For example, if toluene were used as the distillation solvent, azeotrope toluene and water, toluene and acetic acid and toluene and acrylic acid would be taken in the upper part in a two-phase liquid system. The upper distilled stream 23 is phase separated into organic and aqueous. Phase separation can be done through means known in the art. In one embodiment, the upper distilled stream 23 is introduced into a tank 24 and the phase is allowed to separate into an organic phase 25 and an aqueous phase 26. The organic phase 25 predominantly includes the distillation solvent. The aqueous phase 26 includes, but is not limited to, acrylic acid, acetic acid, the distillation solvent and water. In one embodiment, the organic phase 25 is recycled back to the distillation column by means of a solvent feed tube 27, such that the distillation solvent can be used again. In addition, as indicated in Figure 1, at least a portion of the aqueous phase 26 can be recycled 28 as waste water directly into the aqueous stream 3. Alternatively, as also indicated in Figure 1, at least one portion of the aqueous phase 26 can be recycled 28 as wastewater directly to the distiller 6 and then in the aqueous stream 3 after the distillation. In another embodiment, at least a portion of the aqueous phase 26 can be recycled 28 as wastewater directly to an aqueous stream 3 and at least a portion of the aqueous phase 26 can be recycled 28 as wastewater directly to the distiller 6 and then to the aqueous stream 3 after distillation. In this embodiment, the distilled wastewater stream 29 and the undistilled wastewater stream 4 are combined as wastewater stream 22 and introduced into the aqueous stream 3. It should be understood that the aqueous phase 26 can be recycled, in part or completely, to another waste water stream in another acrylic acid manufacturing process. Alternatively, part or both of the complete organic and aqueous phases can be diverted or treated and released into the environment. The following examples are provided as an illustration of the present invention. EXAMPLE (a) Azeotropic distillation with toluene solvent An extended run of azeotropic distillation column with toluene was carried out under operating conditions with a 1-inch diameter Oldershaw column, with 30 trays, mounted in a lower reboiler vessel cooled with jet. of air at a flow rate of 30 cc / min. The tray was the tray 15 and control tray 18, both from the bottom. Distillation was operated under the following conditions: • maximum pressure 215 mm Hg • total aqueous AA feed flow 155 g / hr • toluene reflux flow rate 333 g / hr • control tray temperature 75 ° C composition of acrylic acid feed to the distillation column in tray 15 and base in the bottoms of 200 ppm of 4-hydroxy-2, 2,6,6,6-tetramethylpiperidinyloxy, free radical, 216 ppm of hydroquinone and 650 ppm of p-benzoquinone. The distillation worked smoothly for 99 hours. At the end of the run, the column and the container were clean, that is, the polymerization of monomers was not detected. Analysis by gas chromatography showed: • 280 ppm of Hac in the lower parts • 0.2 ppm of toluene in the lower parts «4 percent by weight of AOPA in the lower parts • 96 percent by weight of acrylic acid in the lower parts • less than 300 ppm of water in the lower parts • 1.6 percent by weight of AA in the aqueous distillate Accordingly, an adequate stream of crude acrylic acid was achieved. The aqueous distillates of this process were collected and used in part b below, (b) Distillation of waste water by waste gas entrainment A stream of dry air at room temperature (6050 cc / min - 433 g / hr) was heated to 67 ° C to form a stream of hot dry air. The stream of hot dry air was then bubbled upwards through a 1-inch diameter Oldersha column with 10 trays and contacted, countercurrently, with hot water (67 ° C) running in a downward direction of the column. Approximately 97 g / hr of water were absorbed into the hot air stream and the resulting effluent from this column (air saturated with water (18% water) at 67 ° C) produced a waste gas stream from the absorber (AOG). The AOG was then fed to the bottom of a 1-inch diameter Oldershaw column with 10 trays and contacted, countercurrently with 76 g / hr of aqueous toluene column distillate fed to the top of the column. column. The aqueous distillate was collected from a toluene distillation process according to part a of the present example. The composition of the aqueous distillate was 88.8 percent by weight of water, 2.0 percent by weight of AA, 7.5 percent by weight of Hac, 1.1 percent by weight of formaldehyde and 0.6 percent by weight of formic acid. The aqueous distillate was collected (84 g / hr) and analyzed by means of gas chromatography. The analysis showed a collected aerated distillate containing 87.3 percent by weight of water, 0.2 percent by weight of AA, 2.1 percent by weight of Hac, 0.2 percent by weight of formaldehyde and 0.1 percent by weight of formic acid. Similarly, the distiller's steam effluent was distilled and the condensed liquid was collected (88 g / hr) and analyzed, showing a liquid condensate containing 94.4 percent by weight of water, 0.9 percent by weight of AA , 3.9% Hac, 0.6 percent by weight of formaldehyde and 0.2 percent by weight of formic acid. The analysis of the material balance showed that 79% of AA, 66% of Hac, 84% of formaldehyde and 56% of formic acid of the aqueous distillate of the toluene column were removed.

Claims (10)

REVINDICATIONS

1. A process for preparing methacrylic acid comprising the steps of: (A) feeding an absorption tower (i) a mixed gas produced by the catalytic oxidation of at least one hydrocarbon material with a gas containing molecular oxygen, and ( ii) an aqueous stream comprising recycled waste water and less than 3.0 weight percent acetic acid; (B) contacting the mixed gas produced with the aqueous stream in the absorption tower to form an aqueous stream of methacrylic acid; and (C) feeding the aqueous stream of methacrylic acid to a distillation column, where it is subjected to azeotropic distillation in the presence of at least one distillation solvent to form a methacrylic acid solution substantially free of water. The process of claim 1, wherein at least a portion of the recycled waste water is distilled from the undesirable components in a distillation column using a distillation gas comprising a waste gas stream. 3. The process of claim 2, wherein the distillation gas is a stream of residual gas absorber. 4. The process of claim 1, wherein the aqueous methacrylic acid stream is distilled in a light ends distiller before being fed to the distillation column. 5. The process of claim, wherein the aqueous stream is 100 percent by weight of wastewater. The process of claim 1, wherein at least one distillation solvent is toluene. The process of claim 1, further comprising: (i) phase separation of the upper portions of the distillation column; (ii) recycle the organic phase back into the distillation column; and (iii) recycling at least a portion of the aqueous phase as wastewater into the aqueous stream. 8 The process of claim 2 further comprising: (i) phase separation of the upper portions of the distillation column; (ii) recycle the organic phase back into the distillation column; and (iii) recycling at least a portion of the aqueous phase as wastewater to the distillation column and then as wastewater to the aqueous stream. The process of claim 1, wherein at least one polymerization inhibitor is selected from the group of hydroquinone; 4-methoxyphenol; 4-ethoxyphenol; 1,2-dihydroxybenzene; catechin monobutyl ether; pyrogallol; 4-aminophenol; 2-mercaptophenol; 4-mercaptophenol; 4-hydroxy-2, 2,6,6,6-tetramethylpiperidinyloxy, free radical; 4-oxo, 2, 2, 6, 6-tetramethylpiperidinyloxy, free radical; 4-amino-2, 2,6,6,6-tetramethylpiperidinyloxy, free radical; isomers of it; derived from it; mixtures of two or more of them; or mixtures of one or more of the above with molecular oxygen are added to the absorber. The process of claim 1, wherein at least one polymerization inhibitor is selected from the group of hydroquinone; 4-methoxyphenol; 4-ethoxyphenol; 1,2-dihydroxybenzene; 2-methoxyphenol; p-benzoquinone; phenothiazine; pyggalol; t-butylcatechol; 4-aminophenol; 2-aminophenol; di-t-butyl nitroxide; 2,2,6,6-tetramethylpiperidinyloxy, 4-hydroxytetramethylpiperidinyloxy, free radical; 4-oxo, 2, 2, 6, 6-tetramethylpiperidinyloxy, free radical; 4-amino-2, 2,6,6,6-tetramethylpiperidinyloxy, free radical; isomers of it; derived from it; mixtures of two or more of them; or mixtures of one or more of the above with molecular oxygen is added to the distillation column.