MXPA00006246A - Preparation of (meth)acrylic acid - Google Patents

Abstract

This invention relates to a process for preparing (meth)acrylic acid, whereby turndown control is utilized to maintain optimal distillation column performance in the dehydration of aqueous (meth)acrylic acid to provide a (meth)acrylic acid solution.

Description

PROCESS FOR PREPARING ACRYLIC ACID (MET) This invention relates to a process for preparing (meth) acrylic acid. In particular, the present invention deals with a process for preparing acid (met) acrylic which utilizes reduction control to maintain the optimum performance of the distillation column in the dehydration of aqueous t (meth) acrylic acid to provide pure (meth) acrylic acid. The acrylic acid is generally prepared by the catalytic vapor phase oxidation of at least one hydrocarbon material. For example, acrylic acid can be prepared from propylene and / or acrolein in a one or two step process. In a first step the propylene is oxidized in the presence of oxygen, inert diluent gases, water vapor, and appropriate catalysts to produce acrolein according to equation (I): C3H + 02 = - > C2H3CH0 + H20 + heat (I). The acrolein is then oxidized, in a second step in the presence of oxygen, inert diluent gases, water vapor and appropriate catalysts to form acrylic acid according to equation (II): C2H3CH0 +% 02 = C2H3C00H + heat (II) . Acrolein can also be provided as a raw material in a one step reaction (II) to produce acrylic acid. Alternatively, propane can be used as a raw material. The propane is oxidized using appropriate catalysts, for example, as described in U.S. Patent 5,380,933 to form acrylic acid product. Methacrylic acid is prepared in a similar manner by the catalytic oxidation of isobutylene and / or isobutane. The acrylic acid which is prepared using said catalytic oxidation reactions with vapor phase is present in a gas of combined products leaving the reactor. In general, the mixed product gas is cooled and comes into contact with a liquid stream in an absorption tower, thereby providing a solution of aqueous acrylic acid which is then dehydrated in a distillation step to provide an acid stream. pure acrylic. The stream of pure acrylic acid can be used to produce various acrylic esters or to be further purified and provide various grades of purified acrylic acid which can be used subsequently, for example in the production of superabsorbent polymer products or various other polymer materials. Typically in the manufacture of acrylic acid there are occasions where the rate of feed of aqueous acrylic acid to the distillation column and / or the composition of the feed stream of aqueous acrylic acid fed to the distillation column changes. For example, when the feed rate and / or composition of the aqueous acrylic acid feed stream is reduced below the maximum capacity of the system by low demand or as a result of variations in the output of the oxidation unit. In a similar manner, the rate of aqueous acrylic acid feed may be increased as a result of the increase in demand or as a result of variations in the output of the oxidation unit. As a result, a change of increase or concentration of aqueous acrylic acid is supplied to the distillation column, this thereby leads to more or less aqueous acrylic acid dehydrated in the distillation column and thereby reduces or increases the amount of pure acrylic acid that is produced. This can cause problems with respect to maintaining optimal column performance, including proper separation characteristics. Typically, at start-up, a particular column is adjusted to operate at a predetermined optimum steam velocity that is selected to maintain adequate performance of the column. The optimum vapor velocity depends on the proportion of distillation solvent to water and the distillation solvent feed rate for the system and is generally established by maintaining a predetermined proportion of distillation solvent to the water and the solvent feed rate of the solvent. distillation. As well, an adequate amount of heat must be provided to the column to boil most of the distillation solvent and water in the upper part. As it is generally assumed that the process will be operated to its full capacity, an appropriate vapor velocity is typically determined for the optimum operation of the distillation column at almost 100 percent of the column capacity. Any change in the use of the predetermined capacity, i.e., changes in the feed or composition of the aqueous acrylic acid, can result in problems in the performance of the distillation column. For example, when the feed rate of the aqueous acrylic acid for a distillation column is reduced and the feed rate of the distillation solvent is kept constant, the speed of the distillation column vapor is reduced and the proportion of distillation solvent increases the water. As a result, since the vapor velocity changes from its predetermined optimal value, the performance of the column, including the separation characteristics, is reduced. An additional problem can occur when product demand is reduced. Generally, if reduction control is not available, the manufacturer must stop completely to avoid excessive stagnation of the acrylic acid product that brings storage problems, such as polymerization threat, space use and space availability. In addition, starting and stopping procedures should be initiated more frequently. As indicated above, the preparation and separation of methacrylic acid have similar steps. Consequently, methacrylic acid manufacturers have similar problems. Distillation methods for removing water and impurities from aqueous methacrylic acid solutions are a known technique. For example, U.S. Patent No. 5,785,821 discloses the dehydration of an aqueous acrylic acid solution using a water-insoluble solvent, for example, toluene. The patent discloses recycling of waste water to the absorber of an acid process Acrylic where the stream of recycled wastewater has a specific composition of acetic acid (3-10% by weight), acrylic acid (0.5-5.0% by weight) and distillation solvent (0.01-0.5% by weight). Said recycling stream, which contains these specific amounts of acetic acid, acrylic acid and distillation solvent allows the collection of the acrylic acid in the absorber with great efficiency. However, this reference does not indicate the problem of reducing control of the distillation column in a process for preparing (meth) acrylic acid, which uses distillation to separate (meth) acrylic acid from water and impurities. The inventors of the present invention have now discovered a process for preparing (meth) acrylic acid having reduction control in response to feed rate and / or composition changes in the stream of aqueous (meth) acrylic acid which is fed to the distillation column. Said reduction control is achieved by controlling the amount of water or distillation solvent or both for feeding the column to maintain a predetermined optimum vapor velocity that provides optimum column performance. Furthermore, in one aspect, the invention allows the additional value of the waste water streams of the process to be obtained by using said streams to adjust the rate of feed of the water to the distillation column. This is carried out while maintaining the proper performance characteristics of the distillation column. Accordingly, a novel process for preparing (meth) acrylic acid is described herein wherein the following advantages are provided: (1) the ability to maintain performance characteristics of the column with response to feed rate and / or the fluctuations of the composition in the aqueous (meth) acrylic acid fed to the distillation column, including the ability to avoid total stoppage during low demand operating at reduced speeds; (2) additional value of the wastewater streams from the process can be obtained by using them to adjust the water feed rate in the distillation column including the recycled wastewater recovered from the fixed charges of the distillation column directly for reduction control therefore reducing the load of residual water in the installation; and (3) reduce the production loss in the fixed charges of the distillation column by recycling in residual water recovered from the fixed charges of the distillation column directly for reduction control. In one aspect of the present invention there is provided a process for preparing (meth) acrylic acid, including the steps of (A) feeding an acid stream aqueous (meth) acrylic acid including (meth) crylic acid to a distillation column; (B) distill the acid stream aqueous (meth) acrylic, at a predetermined vapor velocity, in the presence of at least one distillation solvent substantially insoluble in water, to form a stream of pure (meth) acrylic acid and (C) maintain the predetermined vapor velocity in response to the fluctuation of aqueous (meth) acrylic acid (i) by monitoring a proportion of distillation solvent to water during distillation and (ii) adjusting at least a part of the amount of water and the amount of distillation solvent fed to the distillation column to maintain the predetermined steam velocity. In a second aspect of the present invention, there is provided a process for preparing (meth) acrylic acid, including the steps of (A) feeding a stream of aqueous (meth) acrylic acid including (meth) acrylic acid to a distillation column; (B) distilling the stream of aqueous (meth) acrylic acid, at a predetermined steam rate, in the presence of at least one distillation solvent, substantially insoluble in water, to form a stream of pure (meth) acrylic acid and (C) maintaining the predetermined vapor velocity in response to the fluctuation of aqueous (meth) acrylic acid (i) by monitoring a ratio of distillation solvent to water during distillation and (ii) adjusting the amount of water fed to the column of distillation to maintain the predetermined steam velocity, wherein at least a portion of the amount of water adjustment includes recycled waste water.

In a third aspect of the present invention, there is provided a process for preparing (meth) acrylic acid, including the steps of (A) feeding an acid stream (met) aqueous acrylic including (meth) acrylic acid to a distillation column; (B) distill the acid stream aqueous (meth) acrylic, at a predetermined vapor velocity, in the presence of at least one distillation solvent, to form a stream of pure (meth) acrylic acid and (C) maintain the predetermined vapor velocity in response to the fluctuation of aqueous (meth) acrylic acid (i) by monitoring a proportion of distillation solvent to water during distillation and (ii) adjusting at least a quantity of water and the amount of distillation solvent fed to the distillation column to maintain the default steam speed. In a fourth aspect of the present invention, there is provided a process for preparing (meth) acrylic acid, including the steps of (A) feeding an acid stream (met) aqueous acrylic which includes (meth) acrylic acid and at least one polymerization inhibitor selected from 4-hydroxy-2,2,6,6-tetramethyl-piperidinyloxy and derivatives thereof to a distillation column; (B) distilling the stream of aqueous (meth) acrylic acid, at a predetermined steam rate, in the presence of at least one distillation solvent, to form a stream of pure (meth) acrylic acid and (C) maintaining the predetermined vapor velocity in response to the fluctuation of aqueous (meth) acrylic acid (i) by monitoring a ratio of distillation solvent to water during distillation and (ii) adjusting at least a quantity of water and the amount of distillation solvent They are fed to the distillation column to maintain the predetermined steam velocity. Figure 1 depicts a process flow diagram of (meth) acrylic acid showing an embodiment of the process of the present invention. Figure 2 depicts a process flow diagram of (meth) acrylic acid showing a second embodiment of the process of the present invention. Figure 3 depicts a process flow diagram of (meth) acrylic acid showing a third embodiment of the process of the present invention. Figure 4 depicts a process flow diagram of (meth) acrylic acid showing a fourth embodiment of the process of the present invention. Throughout this specification and claims, unless otherwise indicated, the references made to the percentages are by percentage by weight and all temperatures are in degrees centigrade. It is also understood that for the purposes of this specification and claims the limits of variation and proportion, mentioned herein, may be combined.

For example, if the margins of 1-20 and 5-15 are mentioned for a particular parameter, it is understood that the margins of 1-15 or 5-20 are also contemplated. The term "waste water" is understood as any water stream having impurities and / or additives therein. The term "(meth) acrylic acid" is understood as both acrylic acid and acrylic acid and likewise the term (meth) acrylates is understood as acrylates and methacrylates. In addition, the term "principal amount" is understood to be greater than 50 percent by weight of the total composition. The term "lower amount" is understood as less than 50 percent by weight of the total composition. The term "reduction control", as used herein, is understood within scope control in response to both reduction and elevation of the acid Aqueous acrylic (met) acrylic and / or positive or negative changes for the composition of the aqueous acrylic acid stream that is fed to the distillation column. The term "vapor velocity" is understood as the velocity of the steam flow in the distillation column.

This term is known in the art and is used in the present as a generally accepted meaning.

The term "proportion of water distillation solvent", as used herein, means the proportion of the total distillation solvent fed to the distillation column from any source and fed anywhere to the column for the total of water fed to the distillation column from any source and fed anywhere to the column. The term "aqueous (meth) acrylic acid fluctuation" is used herein to include within its scope both the change in the feed rate of the aqueous (meth) acrylic acid to the distillation column and the change in composition of the aqueous (meth) acrylic acid stream fed to the distillation column. The term "aqueous (meth) acrylic acid feed stream composition" as used herein is understood as the concentration of (meth) acrylic acid in the aqueous stream. The process of the present invention will initially be described with reference to Figure 1. Further reference to Figures 2, 3 and 4 will be made to describe other embodiments of the invention. Furthermore, although the present invention is described as being followed in terms of a process for preparing acrylic acid, it should be understood that the invention also includes a process for the preparation of (meth) acrylic acid.

As mentioned above, the process of the present invention for preparing acrylic acid includes feeding a stream of aqueous acrylic acid 1 formed by absorbing the acrylic acid from a finished product gas to a distillation column 2. The finished product gas generally is obtained, by catalytic vapor phase oxidation of a hydrocarbon material with a molecular oxygen containing gas in the presence of a suitable oxidation catalyst. The catalytic vapor phase oxidation of a hydrocarbon material for acrolein and / or acrylic acid, as well as the reactors, catalysts and processes for carrying them out are generally known in the art and are described, for example in the patents of the United States 4,203,906; 4,256,783; 4,365,087; 4,873,368; 5,161,605; ,177,260; 5,198,578; 5,739,391; 5, 821, 390, EP 911313, and copending United States patent application 09/244182. Depending on the reactants fed to the reactor, the finished product gas generally includes acrylic acid as well as inert gases, including but not limited to, nitrogen, helium, argon, etc .; hydrocarbon reagents without reaction, including but not limited to, propylene, acrolein, propane, isobutane, isobutylene, etc .; steam, and molecular oxygen containing reagents including, but not limited to, air, oxygen, etc .; reaction by-products, including, but not limited to, acetic acid, formaldehyde, maleic acid, and other organic; as well as C02, CO and H20. The gas of finished products is fed to an absorber where it makes contact with an aqueous stream thereby producing a stream of aqueous acrylic acid 1. The stream of aqueous acrylic acid 1 generally includes from 20 to 95, preferably 35 to 90, and even better 50 to 80 percent by weight of acrylic acid, from 80 to 5, preferably from 65 to 10, but much better from 50 to 20 percent by weight of water; and up to 8, preferably up to 6, but better still up to 5 percent by weight of acetic acid. As shown in Figure 1, the stream of aqueous acrylic acid 1 is fed to a distillation column 2 where it is subjected to distillation in the presence of at least one distillation solvent to form a stream of pure acrylic acid 3. The stream of pure acrylic acid 3 includes acrylic acid and also includes varied amounts of at least one of the following: water, acetic acid, propionic acid, β-acryloxypropionic acid (AOPA), acrolein, furfural, benzaldehyde, maleic acid, anhydride maleic, protoanemonin, acetaldehyde and distillation solvents. The stream of pure acrylic acid 3 generally includes from 90 to 99.9, preferably from 93 to 99.9, but better still from 95 to 99.9 percent by weight of acrylic acid. In one embodiment, the stream of pure acrylic acid 3 is substantially free of water, that is, it has less than 1000, preferably less than 800, but still better less than 500 ppm of water. In one embodiment, which is illustrated in Figure 2, the stream of aqueous acrylic acid 1 is fed to a light-end purifying column 43 before the distillation column 2 is fed. The column of light ends 43 purifies the ends luminous, including, but not limited to, acrolein, formaldehyde, acetaldehyde, propionaldehyde, methyl ether, and methyl vinyl ketone, from the aqueous acrylic acid stream 1. Emerging from the bottom of the column of luminous ends 43 find the aqueous acrylic acid stream 44 which is substantially free of said luminous ends. The aqueous acrylic acid stream 44 generally has the same concentration of a-acrylic acid as mentioned above for the current of acrylic acid 1. The stream of aqueous acrylic acid 44 is then introduced into the distillation column 2. The stream 20 emerging from the upper part of the luminous end cluster 43 is sent for disposal or recycling back to the absorption operation where some of the purified acrolein is recovered in the absorber out of the gas and recycled back to the absorber. oxidation reactor thereby improving the production of acrylic acid. Streams of aqueous acrylic acid 1 or 44 can also be treated with a basic compound such as, but not limited to, sodium hydroxide, potassium hydroxide or calcium carbonate to react with maleic acid impurities. The basic compound is added to a stoichiometric ratio to the maleic acid impurity. Other additives such as the oxazolidine derivatives can be added to the aqueous acrylic acid to reduce the level of aldehydes present in the pure acrylic acid product. Any distillation method known in the art can be used, including, but not limited to, simple distillation, multistage distillation, azeotropic distillation, and steam distillation. In addition, the distillation column can be any suitable distillation column that is known in the art. Suitable examples include, but are not limited to, screen tray, double flow tray or packaging distillation columns. In one embodiment, azeotropic distillation is used. In another embodiment, the double flow tray design is used. Drainage holes can also be provided for a distillation column design for the removal of polymerizable liquids, eg, acrylic acid, from the trays. Polymerizable liquids can be collected in the trays of the distillation column and tend to polymerize, thus a more frequent stop is required to clean the column of the polymer materials. These drainage holes are known in the art and are described for example in U.S. Patent Nos. 4,442,048; 4,374,000; 3,717,553; and EP 856343. In one embodiment, a double flow tray distillation column having drainage holes is used. The solvent or distillation solvents may be any solvent suitable for the distillation of an acrylic acid stream. Suitable examples of the distillation solvent useful in the present invention include but are not limited to ethyl acetate, butyl acetate, dibutyl ether, ethyl acrylate, methyl methacrylate, ethyl methacrylate, diethyl ketone, methyl propyl ketone, methyl isobutyl ketone, methyl tert- butyl ketone, isopropyl acetoate, n-propyl acetate, heptane, heptene, cycloheptane, cycloheptene, cycloheptadiene, cycloheptatriene, methylcyclohexane, ethyl cyclopentane, dimethylcyclohexane, ethylcyclohexane, toluene, ethylbenzene, xylene, trichlorethylene, trichloropropene, dichlorobutane, chloropentane, chlorohexane, chlorobenzene, and mixtures thereof. In one embodiment, the distillation solvent is substantially insoluble water; generally with 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 this water-insoluble distillation solvent include, but are not limited to heptane, heptene, cycloheptane, cycloheptene, cycloheptadiene, cycloheptatriene, methylcyclohexane, ethylcyclopentane, dimethylcyclohexane, ethylcyclohexane, toluene, ethylbenzene, xylene, trichlorethylene, trichloropropene, dichlorobutane, chloropentane, chlorohexane, chlorobecene, and mixtures thereof. In an alternative embodiment, the distillation solvent is a mixing solvent that includes at least two solvents. Suitable examples of the solvents useful in said mixed solvent include, but are not limited to, diethyl ketone, methyl propyl ketone, methyl isobutyl ketone, methyl tert-butyl ketone, isopropyl acetate, n-propyl acetate, toluene, heptane and methylcyclohexane. The preferred distillation solvent is toluene. The distillation solvent can be fed completely into the upper tray of the distillation solvent or alternatively it can be fed to the column at several points at the same time. As mentioned above, the process of the present invention presents reduction control of the distillation column which compensates for fluctuations in the feed rate and / or the composition of the aqueous acrylic acid which is fed to the distillation column 2 so that the performance of the column, including the separation characteristics, remains at optimum levels. Generally, the performance of distillation column 2 is optimized in response to fluctuations in the aqueous feed rate and / or the composition by monitoring the ratio of distillation solvent to water and adjusting the distillation solvent and / or water that is feed the distillation column 2 to maintain a predetermined value for the vapor velocity. In one embodiment, which is described in Figure 1, the reduction control of the distillation column 2 is achieved as follows. A feed speed controller of the distillation solvent 6 which controls the distillation solvent feed stream 7 is set at a predetermined speed at the start of the process. A water distillation solvent proportionator controller 4 controlling the preparation water stream 5 and the aqueous acrylic acid feed stream 1 is set, at the start of the process, to a predetermined water distillation solvent ratio. The preparation water stream 5 and the acrylic acid feed stream 1 combine to form the total feed stream of aqueous acrylic acid 15. Once the solvent ratio is fixed. of distillation to water and the speed of feeding of the distillation solvent, the vapor velocity of the column is fixed at an optimum, predetermined value that provides the performance of the column. Typically, the good performance of the column is evident by means of a good separation, the produced product that is within the specifications, minimum loss of performance in the distillate streams of the column. Generally, the goal is to maintain a constant vapor velocity and the vapor position of the appropriate fixed charge, for example, if azeotropic distillation is used an appropriate azeotrope composition. For purposes of describing this embodiment, it is assumed that the distillation solvent feed rate and the distillation solvent ratio to the water in the distillation column 2 are set to operate at maximum feed rates, ie, 100 percent. of capacity, so that predominantly all the water requirements of the distillation column 2 at the start are provided by means of the feed stream of aqueous acrylic acid 1. However, the preparation water stream 5 is also used for control the minor variations in the feed stream of aqueous acrylic acid 1, which are not associated with the deliberate reduction or major fluctuations of the system. Once the ratio of the water distillation solvent and the distillation solvent feed rate are set at start-up to the optimum values, the internal vapor flow, i.e. the vapor velocity, of the distillation column is fixed. The adjustment of the default values will vary according to the type of distillation column that is used and will of course explain the capacity utilization of the column. For example if a double flow tray column is used, for a good performance of double flow tray the ratio of solvent of distillation to water and the speed of feeding of the distillation solvent are set to promote a good constant steam velocity at through the trays to a particular capacity use. The determination of the pre-set values of the ratio of the water distillation solvent and the feed rate of the distillation solvent is within the capabilities of those skilled in the art and is not discussed further herein. In this embodiment, the distillation solvent feed rate is controlled by a single flow control loop. As indicated, the feed speed controller of the distillation solvent 6 is set at a predetermined distillation solvent feed rate. The distillation solvent feed speed controller 6 monitors a signal 9 that it receives from the distillation solvent feed feed stream flow meter 10 relative to the flow velocity of the distillation solvent feed stream. The distillation solvent feed speed controller 6 determines whether an adjustment in the flow velocity of the distillation solvent feed stream 7 is necessary to maintain the speed and predetermined distillation solvent supply signals 27 and the valve of distillation solvent control 11 in compliance. In this embodiment, the aforementioned control loop is mainly used to correct minor fluctuations in the distillation solvent feed stream 7 so as to maintain a constant distillation solvent feed rate. The control of the proportion of distillation solvent to water is more complex in this embodiment. As indicated above, the water distillation solvent proportion controller 4 is adjusted to maintain a predetermined vapor velocity. The water distillation solvent proportion controller 4 receives: (1) a signal 12 from the distillation solvent feed stream flow meter 10 with respect to the flow velocity of the distillation solvent feed stream.; (2) a signal 13 from the total aqueous acrylic acid feed flow meter 14 with respect to the flow rate of the total aqueous a-crical acid feed stream 15; and (3) a signal 16 of the water composition meter 17 with respect to the water composition of the total aqueous acrylic acid feed stream 15. In response to signals 12, 13, and 16 the solvent ratio control of distillation to water 4 determines whether an adjustment in the flow velocity of the aqueous acrylic acid feed stream 1 and / or the preparation water stream 5 is necessary to maintain the predetermined steam velocity and accordingly signals the water acrylic acid feed stream control 18 (signal 28) and / or preparation water feed current control valve 19 (signal 29). It will be apparent to those skilled in the art that the water distillation solvent proportion control 4 can directly control the aqueous acrylic acid feed stream control valve 18 and / or the water feed stream control valve of preparation 19 or can indirectly control these by sending signals 18 and / or 29 as regulation points to independent flow controllers. Said independent loops may, for example, be a flow control loop as described above for controlling the feed stream of distillation solvent 7 in the distillation column 2. In that case, the distillation solvent proportion controller water 4 would send the signals 28 and / or 29 to the independent controllers which would then adjust the acrylic acid feed current control valve 18 and / or the preparation water feed flow control valve 19 (signal 29) in This is to provide the appropriate water flow to the distillation column 2. Chemical process controllers, flow meters and composition meters are known in the art and any suitable controller and meter can be used in the present invention. Chemical process controllers include but are not limited to proportional controllers (P), more integral proportional controllers (PI), proportional plus derivative (PD) controllers, more integral plus derivative (PID-) proportional controllers, and fuzzy logic controllers and neural network. Suitable flow meters include, but are not limited to, the Dahl flow tube, the Kennison flow nozzle, the Pitot tube, the Pitot tube with static intake, the venturimeter, the ultrasound, the turbine and the orifice flow meter. . Suitable composition meters include, but are not limited to, the sound velocity meter, the ultraviolet analyzer, the infrared analyzer, the mass spectrometer, the X-ray absorption, the hydrostatic and the like. While the present invention is described in terms of adjusting water content of the total aqueous acrylic acid feed stream 15, it should be understood by those skilled in the art that a number of system parameters can be adjusted to maintain the speed of default steam. For example, instead of adjusting the amount of water-fed to the distillation column, to maintain the required vapor velocities the amount of distillation solvent fed to the distillation column 2, the production rate of the acrylic acid can be adjusted pure, the aqueous consistency of the aqueous acrylic acid streams 1 or 44, the handling of both the feed amounts of the water and the distillation solvent to the distillation column 2 and / or any combination thereof.

The water in the preparation water stream 5 can come from any suitable source, including city water, deionized water (DI) and waste water or mixtures thereof. In one embodiment, the water is DI water. In one embodiment, at least a portion of the preparation water stream 5 is recycled waste water. The wastewater can be any wastewater suitable for use in a dehydration operation of acrylic acid and can come from any source. Consequently, it is not necessary that the residual water derive from the same process in which it is recycled. For example, waste water can be derived from one process of (meth) acrylic acid and recycled into another. Suitable examples of wastewater include, but are not limited to, wastewater derived from dehydration of (meth) acrylic acid, other aqueous distillates, steam condensates and refined. In the same way, it is not necessary for the residual water to be derived from a waste water stream of (meth) acrylic acid. As a result, waste water can be derived from other wastewater streams from chemical processes, for example, from a (meth) acrylic process stream. In addition, the wastewater can be derived from a natural source such as a river, a well, a spring or equivalent. The preparation water stream 5 includes any suitable amount of recycled waste water, that is, from 0 to 100 percent by weight of recycled waste water. Typically, the preparation water stream 5 will be a mixture of a stream of recycled waste water from an acrylic acid manufacturing process and a stream of essentially pure water, eg, deionized water. In one embodiment, the preparation water stream 5 includes a larger amount of waste water. In another embodiment, the preparation water stream 5 includes from 0.1 percent by weight to 100 percent by weight of wastewater. Preferably, the preparation water stream 5 contains 100 percent by weight of waste water. Regardless of how much recycled waste water is used, the preparation water stream 5 will contain a larger amount of water and smaller amounts of impurities derived from a (meth) acrylic and / or methacrylate acid manufacturing process. In another embodiment, the aqueous stream is substantially free of distillation solvents. In one embodiment, which is shown in Figure 3, at least a portion of the preparation water stream 5 is a recycle wastewater stream derived from the fixed charge vapor stream 21 emanating from the upper of the distillation column 2. The fixed charge steam stream 21 generally includes, but is not limited to, water, acrylic acid, acetic acid and / or distillation solvent. The fixed charge steam stream 21 is condensed and phase separated into organic and aqueous phases. The separation by phase can be carried out by the known technique. In the embodiment of Figure 3, the fixed charge steam stream 21 is condensed and introduced to a phase separation tank 22 and allowed to phase separate into an organic phase 23 and an aqueous phase 24. The phase organic 23 predominantly includes the distillation solvent. The aqueous phase 24 includes, but is not limited to, acrylic acid, acetic acid, distillation solvent and water. In this distillation form, at least a portion of the aqueous phase 24 is recycled for the preparation water stream for use in reduction control. As indicated above, it is understood that the aqueous phase 24 can be recycled in part, or totally for the preparation water stream 5. If the distillation solvents having a greater solubility in water are used, the direct recycling of the phase water 24 back to the preparation water stream 5 can be counterproductive, since the aqueous phase 24 will contain a larger amount of distillation solvent. Then, an increased amount of distillation solvent will be returned to the lower portion of the distillation column 2. Consequently, this will result in an undesired increase in the amount of distillation solvent that appears in the dehydrated acrylic acid. In addition, before recycling the concentration of the distillation solvent in the aqueous phase 24, which must sometimes be reduced. This can be carried out by any number of methods known in the art, for example, by diluting an aqueous stream or having no distillation solvent or by processing methods such as a purifying column or equivalent. In an embodiment of the present invention, as illustrated in Figure 4, this problem is pointed out. A refiner purifier 30 can be used to purify the distillation solvent from the aqueous phase 24. The refiner purifier 30 can be any purifying column that is known in the art. The refiner purifier 30 receives the aqueous phase 24 from the tank 22 through the feed 31 and purifies the distillation solvent from the aqueous phase 24 using a purifying gas 32. The purifying gas 32 can be generated by the contents of a unit of heat transfer attached to the purifying column or it may be a current from any source. The refined purifier typically operates at a temperature of from 80 ° C to 120 ° C and at an atmospheric pressure. Preferably the refined purifier operates at a temperature of from 5 ° C to 110 ° C. The vapor stream from the top 33 is condensed and introduced to a tank 34 and allowed to phase separate into an organic phase 35 and an aqueous phase 36. The organic phase 35 predominantly includes the purified distillation solvent. The aqueous phase 36 includes residual water substantially free of distillation solvent. In this embodiment, at least a portion of the aqueous phase 36 is recycled 37 back to the purifying column 30 or can be recycled for another use or sent for disposal. From the bottom of the purifying column 30 emanates a waste water stream 38 at least a portion of which is recycled to the preparation water stream 5. alternatively, some of the waste water stream 38 can be recycled for another use or it can be deleted. For example, the waste water stream can be used as a water feed in absorbers for different acrylic acid production units, or as indicated as all or part of the preparation water stream 5, or it can be eliminated in a wastewater treatment plant. The organic phase 35 is sent, via feed 42, to a tank of recycle solvents 39 which can be used to generate new inhibitor feed streams or recycle 40 for the feed stream of distillation solvent 7.

The organic phase 23 can also be recycled back to the distillation column by a distillation solvent feed stream 7 so that the distillation solvent can be reused or the organic phase 23 can be used to form a stream of inhibitor preparation for .facilitate the feed polymerization inhibitor for the distillation column. In another embodiment, the stream of aqueous acrylic acid 1 or 44 includes at least one polymerization inhibitor. Suitable inhibitors include, but are not limited to, hydroquinone; para-benzoquinone; phenothiazine. 4-methoxy phenol; 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, 2,6,6,6-tetramethylpiperidinyloxy, free radical; 4-amino-2, 2,6,6,6-tetramethylpiperidinyloxy, free radical; 4 -propoxyphenol; 4-butoxyphenol; 4-heptoxyphenol; hydroquinone monobenzylether; 1.2 -dihydroxybenzene; 2-methoxyphenol; 2, 5-dichlorohydroquinone; 2,5-di-tert-butylhydroquinone; 2-acetylhydroquinone, hydroquinone-onobenzoate; 1,4-dimercaptobenzene; 1,2-dimercaptobenzene; 2, 3, 5-trimethylhydroquinone; 2 -aminophenol; 2-N, N-dimethylaminophenol; 4'-ethylaminophenol; 2,3-dihydroxyacetophenone; 1,2-dimethyl ether; 2-methylthiophenol; t-butyl pyrocatechin; di-tert-butyl nitroxide; di-tert-amylnitroxide; 2, 2, 6, 6-tetetramethyl-piperidinyloxy; 4-dimethylamino 2, 2, 6, 6-tetramethyl-piperidinyloxy; 4-amino-2,2,6,6-tetramethyl-piperidinyloxy; 4-ethanoyloxy-2,6,6,6-tetramethyl-piperidinyloxy; 2, 2, 5, 5 -tetramethyl-pyrrolidinyloxy; 3-amino-2, 2,5,5-tetramethyl-pyrrolidinyloxy; 2,2,5,5-tetramethyl-l-oxa-3-azacyclopentyl-3-oxy; 2,2,5,5-tetramethyl-3-pyrrolinyl-1-oxy-3-carboxylic acid; 2,3,3,5,5,6,6-octamethyl-l, 4-diazacyclohexyl-1,4-dioxy; copper compounds such as copper dimethyldithiocarbonate; diethyldi copper iocarbamate, - copper salicylate; isomers thereof; derivatives thereof; mixtures of two or more thereof; or mixtures of one or more of the foregoing with molecular oxygen. In one embodiment, the polymerization inhibitor is 4-hydroxy-2,2,6,6-tetramethyl-piperidinyloxy, derivatives thereof or mixtures of 4-hydroxy-2, 2,6,6-tetramethyl-piperidinyloxy with oxygen molecular. In an alternative embodiment, the polymerization inhibitor is a mixture of 4-hydroxy-2,2,6,6-tetramethyl-piperidinyloxy, derivatives thereof or mixtures of 4-hydroxy-2,2,6,6-tetramethyl -piperidinyloxy with hydroquinone and molecular oxygen. If a distillation column design that requires the use of a vapor phase polymerization inhibitor is used, such as a screen tray, suitable vapor phase inhibitors include N-nitrosophenylhydroxylamine and salts thereof. The acrylic acid stream 3 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 it is or can be further processed to obtain purer grades of the acrylic acid before use. The following examples are provided as an illustration of the present invention.

Example 1 Azeotropic distillation with toluene solvent at 100% capacity - A prolonged operation of an azeotropic toluene distillation column was conducted under operating conditions using a 1 inch diameter column, with 30 Oldershaw trays installed in a waste heat transfer unit cooled with air at a rate of 30 cm3 per minute. The feeding tray was a tray 15 and the control tray was in tray 18, both of the lower part. The distillation was operated under the following conditions: • upper pressure of 215 mm Hg • feed rate of 155 g / hr of aqueous AA • reflux rate of 333 g / hr of toluene • 75 ° C of control tray temperature One Aqueous acrylic acid feed composition was fed to the distillation column in tray 15 and reflux of toluene was fed to the upper tray at the indicated rate. The aqueous acrylic acid feed composition contained 67% by weight of acrylic acid, 1% by weight of β-acryloxypropionic acid (AOPA), 28% by weight of water and 3% by weight of acetic acid, and 1% by weight of other secondary components such as formaldehyde, formic acid, maleic acid, and the hydroquinone polymerization inhibitor. Hydroquinone was obtained from Aldrich Chemical Co. From Milwaukee, Wisconsin. In addition an aqueous solution of 0.08% by weight of 4-hydroxy-2,2,6,6-tetramethyl piperidinyloxy, liberation radical polymerization inhibitor was obtained from Aldrich Chemical Co. of Milwaukee, Wisconsin, was fed into the acid feed aqueous acrylic at a rate of 5 g / hr and 0.24% by weight of aqueous solution of p-benzoquinone vapor phase inhibitor, available from Aldrich Chemical Co. of Milwaukee, Wisconsin, was fed to the upper tray at a rate of 10 g / hr. In addition, an additional stream of 0.34% by weight of aqueous solution of 4-hydroxy-2,2,6,6-tetramethylpiperidinyloxy free radical was fed to the upper tray at a rate of 5 g / hr. Inhibitor supplies resulted in levels of inhibitors in the column, based on residues, of 200 ppm of -hydroxy -2, 2,6,6-tetramethylpiperidinyloxy, free radical, 316 ppm of hydroquinone, and 650 ppm of p benzoquinone. The distillation operates uniformly for 99 hours. At the end of the operation, the column and the pot were clean, that is, no monomer polymerization was detected. Analysis by gas chromatography showed that the effluent streams had the following compositions: Residues (105.7 h / hr): • 96% by weight of AA • 4% by weight of AOPA • 280 ppm of HAc • 0.2 ppm toluene Distillate aqueous (54.3 g / hr): • 91.0% by weight of H20 • 1.6% by weight of AA • 7.4% by weight of HAc • 393 ppm of toluene The loss of production of acrylic acid through the aqueous distillate was 0.82% Example 2 Az azotropic distillation with 50% strength d.toluene solvent A prolonged azeotropic distillation operation was conducted as described in Example 1. Distillation was reduced to operate at a 50% reduction rate, ie 50 % water feed speed. A calculated amount of DI water was added to the aqueous solution feed as preparation water so as to maintain the desired vapor velocity. The operating conditions were as follows: • upper pressure of 215 mm Hg • feed rate of 77.5 g / hr of aqueous AA • feed rate of 22.9 g / hr of DI water • reflux rate of 300 g / hr of toluene • 75 ° C control tray temperature An aqueous acrylic acid feed composition was fed to the distillation column in the tray and the reflux of toluene was fed to the upper tray at the indicated speed. The aqueous acrylic acid feed composition contained 67% by weight of acrylic acid, 1% by weight of β-acryloxypropionic acid (AOPA), 28% by weight of water and 3% by weight of acetic acid, and 1% by weight of other secondary components such as formaldehyde, formic acid, maleic acid, and the hydroquinone polymerization inhibitor. Hydroquinone was obtained from Aldrich Chemical Co. of Milwaukee, Wisconsin. In addition 4-hydroxy -2, 2,6,6,6-tetramethylpiperidinyloxy, free radical polymerization inhibitor was obtained from Aldrich Chemical Co. of Milwaukee, Wisconsin, was fed as 0.13% by weight aqueous solution in the aqueous acrylic acid feed at a rate of 5 g / hr and p-benzoquinone vapor phase inhibitor, available from Aldrich Chemical Co. of Milwaukee, Wisconsin, as a toluene solution of 0.12% by weight and an additional stream of 4-hydroxy -2 , 2, 6, 6-tetranethylpiperidinyloxy, free radical as a toluene solution of 0.26% were fed to the upper tray at a rate of 10 g / hr. Inhibitor supplies resulted in inhibitor levels in the column, based on residues, of 200 ppm of 4-hydroxy-2,2,6,6-tetramethylpiperidinyloxy, free radical, 316 ppm of hydroquinone and 650 ppm of -benzoquinone . The experiment operated uniformly for 18 hours. At the end of the operation, the column and the pot were clean, that is, no polymerization of monomers was detected and the influent streams had the following compositions: .Residues (52.6 h / hr): • 96% by weight of AA • 4% by weight of AOPA • 107 ppm of HAc • 0.8 ppm of toluene The aqueous distillate (47.8 g / hr): • 94.4% by weight of H20 • 1.3% by weight of AA • 4.3% by weight of EAc • 263 ppm of toluene The production loss of the acrylic acid through the aqueous distillate was 1.24% Example 3 Azeotropic distillation with toluene solvent at a 50% reduction A prolonged azeotropic distillation was carried out as described in Example 1. Distillation it was reduced to operate at 50% of the reduction rate, that is, 50% of the aqueous feed rate. A calculated amount of aqueous distillate recycling was added to the aqueous feed as preparation water to maintain the desired vapor velocity. The operating conditions were as follows: • upper pressure of 215 mm Hg • feed rate of 77.5 g / hr of aqueous AA • feed of 4.7 g / hr of recycled aqueous distillate • reflux rate of 300 g / hr of toluene • 70 ° C control tray temperature An aqueous acrylic acid feed composition with recycled aqueous distillate was fed to the distillation column in the tray 15 and the toluene reflux was fed to the upper tray at the indicated rates. The aqueous acrylic acid composition containing 67% by weight of acrylic acid, 1% by weight of β-acryloxypropionic acid (AOPA), 28% by weight of water and 3% by weight of acetic acid, 1% by weight of others minor components such as formaldehyde, formic acid, maleic acid, and the hydroquinone polymerization inhibitor. Hydroquinone was obtained from Aldrich Chemical Co. of Milwaukee, Wisconsin. Also, 4-hydroxy-2,2,6,6-tetramethylpiperidinyloxy, free radical polymerization inhibitor, was obtained from Aldrich Chemical Co. of Milwaukee, Wisconsin, was fed as an aqueous solution of 0.13% by weight in the feed of Aqueous acrylic acid at a rate of 5 g / hr and the vapor phase inhibitor of p-benzoquinone, Aldrích Chemical Co. of Milwaukee, Wisconsin, was obtained as 0.12% by weight of toluene solution and an additional stream of 4 - hydroxy-2, 2,6,6-tetramethylpiperidinyloxy, free radical as 0.26% by weight of toluene solution were fed to the upper tray at a rate of 10 g / hr. Inhibitor feeds resulted in inhibitor levels in the column, based on the residues, of 200 ppm of 4-hydroxy-2,2,6,6-tetramethylpiperidinyloxy, free radical, 316 ppm of hydroquinone and 650 ppm of p-1. benzoquinone. Error! Marker not defined. The experiment worked runiformmente during 20 hours. At the end of the operation, the column and the pot were clean and the effluent streams had the following compositions: Residues (53.5 g / hr): • 96% by weight of AA • 4% by weight of AOPA • 360 ppm of HAc • 1.1 ppm of toluene The aqueous distillate (24.7 g / hr recycled, 24.0 g / hr residual): • 91.2% by weight of H20 • 1.2% by weight of AA • 7.6% by weight of HAc • 295 ppm of toluene The loss of production of acrylic acid through the aqueous distillate was 0.53%. Examples 1, 2 and 3 demonstrated the maintenance performance of the distillation column, including the good separation characteristics, including a reduction using DI water (example 2) and a reduction using recycled aqueous distillate (example 3) . Such maintenance of column performance is evident by the equivalent recovery of acrylic acid in the residues of the distillation column, ie 96% by weight of acrylic acid, despite the reduction of the aqueous acrylic acid feed. , as seen in example 3, the use of the aqueous distillate from the top of the distillation column to adjust the water feed to the distillation column to compensate for the reduction of the aqueous acrylic acid feed reduces the water load In addition, when examples 2 and 3 are compared, it can be seen that the reproduction loss of acrylic acid in the upper part of the distillation column is halved, i.e., 1.24% in Example 2 to 0.53% in Example 3 due to the 50% recycling of the residual water load of the aqueous distillate from the column to the distillation column.

Claims (20)

1. CLAIMS 1. A process for preparing methacrylic acid, comprising the steps of: (A) feeding a stream of aqueous methacrylic acid comprising methacrylic acid to a distillation column; (B) distilling the aqueous methacrylic acid stream at a predetermined vapor velocity, in the presence of at least one distillation solvent substantially insoluble in water, to form a stream of pure methacrylic acid; and (C) maintaining the predetermined vapor velocity in response to fluctuation of aqueous methacrylic acid by (1) monitoring a ratio of distillation solvent to water during distillation, and (2) adjusting at least one of the amounts of water and the feed amount of the distillation solvent for the distillation column to maintain the predetermined vapor velocity. The process of claim 1, further comprising the high phase separation portions of the distillation column in an organic phase and an aqueous phase wherein at least a portion of the organic phase is recycled back to the column of distillation. 3. The process of claim 1, wherein the amount of water fed to the column is adjusted using recycled waste water. The process of claim 1, further comprising the high phase separation portions of the distillation column in an organic phase and an aqueous phase, wherein at least a portion of the aqueous phase is used to adjust the water to the column. The process of claim 1, wherein the aqueous methacrylic acid stream is purified from light ends before the distillation column is fed. 6. The process of claim 1, wherein at least one distillation solvent substantially insoluble in water is selected from the group consisting of heptane, heptene, cycloheptane, cycloheptene, cycloheptadiene, cycloheptatriene, methylcyclohexane, ethylcyclopentane, dimethyclohexane, ethylcyclohexane, toluene , ethylbenzene, xylene, trichlorethylene, trichloropropene, trichlorobutane, chloropentane, chlorohexane, chlorobenzene, and mixtures thereof. The process of claim 1, wherein at least one distillation solvent substantially insoluble in water is toluene. 8. The process of claim 1, wherein the distillation column is a double flow tray column. The process of claim 1, wherein at least one polymerization inhibitor is selected from the group and hydroquinone; 4-methoxyphenol; 4-ethoxyphenol; 1,2-dihydroxybenzene; 2-methoxyphenol; p-benzoquinone; phenothiazine; pyrogallol; t-butylcatechol; 4-aminophenol; 2 -aminophenol; di-t-butylnitroxide; 2, 2, 6, 6-tetramethylpiperidinyloxy, free radical; 4-hydroxy-2, 2,6,6,6-tetramethylpiperidinyloxy, free radical; 4 -oxo-2, 2,6,6,6-tetramethylpiperidinyloxy, free radical; 4-amino-2, 2,6,6,6-tetramethylpiperidinyloxy, free radical; 4-ethanoyl-2,2,6,6-tetramethylpiperidinyloxy, free radical; 2, 2, 5, 5-tetramethylpyrrolidinyloxy, free radical; isomers thereof; derivatives thereof; mixtures of two or more thereof; or mixtures of one or more of the above with molecular oxygen are added to the distillation column. The process of claim 8, wherein the double flow tray column contains trays having drainage holes to prevent retention of polymerizable liquid in the trays. 11. A process for preparing methacrylic acid, comprising the steps of: (A) feeding a stream of aqueous methacrylic acid comprising methacrylic acid and at least one polymerization inhibitor selected from 4-hydroxy-2, 2, 6, 6-tetramethylpiperidinyloxy and derivatives thereof for a distillation column; (B) distilling the aqueous methacrylic acid stream at a predetermined vapor velocity, in the presence of at least one distillation solvent, to form a stream of pure methacrylic acid; and (C) maintaining the predetermined vapor velocity in response to the feed rate fluctuation of the aqueous methacrylic acid by (i) monitoring a proportion of distillation solvent to water and (ii) adjusting at least a quantity of water and the amount of distillation solvent fed to the distillation column to maintain the predetermined vapor velocity 12. The process of claim 11, further comprising the high phase separation portions of the distillation column in an organic phase and a phase aqueous wherein at least a portion of the organic phase is recycled back to the distillation column. The process of claim 11, wherein the amount of water fed to the column is adjusted using recycled waste water. 14. The process of claim 11, further comprising the high phase separation portions of the distillation column in an organic phase and an aqueous phase, wherein at least a portion of the aqueous phase is used to adjust the water fed in. the spine. 15. The process of claim 11, wherein the aqueous methacrylic acid stream is purified from light ends before the distillation column is fed. The process of claim 11, wherein at least one distillation solvent is selected from the group consisting of heptane, heptene, cycloheptane, cycloheptene, cycloheptadiene, cycloheptatriene, methylcyclohexane, ethylcyclopentane, dimethylcyclohexane, eticyclohexane, toluene, ethylbenzene, xylene , trichlorethylene, trichloropropene, bichlorobutane, chloropentane, chlorohexane, chlorobenzene, ethyl acetate, butylacetate, dibutyl ether, ethyl acrylate, methyl methacrylate, ethyl methacrylate, diethyl ketone, methylpropyl ketone, methyl isobutyl ketone, methyl tert-butyl ketone, acetate isopropyl, n-propyl acetate and mixtures thereof. The process of claim 14, wherein the portion of the aqueous phase that is used to adjust the water fed to the column is purified from the distillation solvent before being used to adjust the water fed to the column. 18. The process of claim 11, wherein the distillation column is a double flow tray column. The process of claim 11, wherein at least one polymerization inhibitor is selected from the hydroquinone group; para-benzoquinone; phenothiazine; 4-methoxyphenol; 4-ethoxyphenol; 1,2-hydroxybenzene; monobutyl etherdecatecol; pyrogallol; 4-aminophenol; 2-mercaptophenol; 4-mercaptophenol; 4-hydroxy-2, 2,6,6,6-tetramethylpiperidinyloxy, free radical; 4-oxo-2, 2,6,6,6-tetramethylpiperinyloxy, free radical; 4-amino-2, 2, 6, 6-tetramethylpipridinyloxy, free radical; 4-propoxyphenol; butoxyphenol; ethoxyphenol; monobenzylether-dehydroquinone; 1.2-dihydroxybenzene; 2-methoxyphenol; 2,6-dichlorhydroquinone; 2,5-di-tert-butylhydroquinone; 2-acetylhydroquinone; monobenzoatodehydroquinone; 1,4-dimercaptobenzene; 1,2-dimercaptobenzene; 2, 3, 5-trimethylhydroquinone; 2 -aminophenol; 2-N, N-dimethylaminophenol; 4-etlaminophenol; 2,3-dihydroxyacetophenone; 1,2-dimethyl ether; 2-methylthiophenol; p-butylcatechol; di-tert-butylnitroxide: di-tert-amylnitroxide; 2, 2, 6, 6-tetramethylpiperidinyloxy; 4-dimethylamino-2,2,6,6-tetramethylpiperidinyloxy; 4-amino-2, 2,6,6-tetramethyl-piperidinyloxy, 4-ethanoyloxy-2,2,6,6-tetramethylpiperidinyloxy; 2, 2, 5, 5-tetramethyl-pyrrolidinyloxy; 3-amino-2, 2,5,5-tetramethyl-1-oxa-3-azacyclopentyl-3-oxy; 2, 2, 5, 5-tetramethyl-pyrrolinyl-1-oxy-3-carboxylic acid; 2,2,3,3,5,5,6,6-actamethyl-l, 4-diazacyclohexyl-1,4-dioxy; copper compounds such as copper dimethyldithiocarbamate, copper diethyldithiocarbamate; copper salicylate; isomers thereof; derivatives thereof; mixtures of two or more thereof; or mixtures of one more of the above with molecular oxygen are added to the distillation column. The process of claim 11, wherein the double flow tray column has trays containing drainage holes to prevent retention of polymerizable liquid in the trays.