MXPA00010330A

MXPA00010330A - Improved process for producing pure carboxylic acids

Info

Publication number: MXPA00010330A
Application number: MXPA/A/2000/010330A
Authority: MX
Inventors: John Arthur Turner; Duncan Charles Woodcock
Original assignee: E I Du Pont De Nemours And Company
Priority date: 1998-08-03
Filing date: 2000-10-20
Publication date: 2001-09-07

Abstract

A process for producing pure carboxylic acids by catalytic liquid phase air oxidation of a suitable precursor in a solvent in which the oxidation reaction is carried out in a plug flow reaction zone at a high solvent:precursor ratio and reaction conditions sufficient to maintain the pure acid in solution as it is formed, and product produced from such process.

Description

IMPROVED PROCESS TO PRODUCE PURE CARBOXYLIC ACIDS The present invention discloses an improved process for producing pure carboxylic acids by means of a catalytic liquid phase oxidation of a suitable precursor in a solvent and more particularly, a process for producing a highly pure terephthalic acid according to such a process. the oxidation reaction in a piston-type expense reaction zone in a high solvent: ratio of the precursor, temperature and pressure sufficient to obtain the terephthalic acid in solution as it is formed. Then, the pure terephthalic acid is systematically crystallized from the resulting reaction medium and recovered as pure crystals without the need for separate purification. Terephthalic acid is an important raw material used in the production of poly (ethylene terephthalate), i.e. PET for the conversion into fibers, films and containers, is commercially produced by means of the purification of tereftalic acid technical grade or raw grade. Virtually all technical grade terephthalic acid is produced by oxidation in air catalytic liquid phase of paraxylene. The Ref. 124422 commercial processes use acetic acid as a solvent and a multivalent heavy metal or metals as catalysts. Cobalt and manganese are the most widely used heavy metal catalysts and bromine is used as a renewed source of free radicals in the process. Acetic acid, air (molecular oxygen), paraxylene and catalyst are continuously fed into an oxidation reactor which is maintained at a temperature of 175 ° C to 225 ° C and 1500 to 3000 kPa (ie 15 to 30 atmospheres). The acetic acid: paraxylene feed ratio is typically less than 5: 1. Air is added in quantities in excess of the stoichiometric requirements to minimize the formation of side products. The stoichiometric oxidation reaction and heating is typically removed by allowing the acetic acid to boil. The corresponding vapor is condensed and most of the condensate is refluxed into the reactor. 2 moles of water are formed per mole of reacted paraxylene, and the residence time is typically 30 seconds to 2 hours, depending on the process. The effluent from the reactor is a slurry of crystals of crude terephthalic acid which are recovered by filtration, washed, dried and transported to storage. They are then fed to a separate purification stage. The main impurity is 4-carboxybenzaldehyde (4-CBA), which is incompletely oxidized paraxylene. Although the purity of the crude grade terephthalic acid is typically greater than 99%, it is not a sufficient purity to produce the PET therefrom, to achieve the required degree of polymerization.

BRIEF DESCRIPTION OF THE INVENTION The present invention is an improved continuous process for producing pure carboxylic acids, by means of catalytic liquid phase oxidation of a correspondingly suitable precursor in a solvent comprising an aliphatic carboxylic acid and optionally water which substantially reduces the time of residence in the reactor providing, for the precipitation of pure acid crystals directly from the resulting reaction medium in a systematic sequence, ie defined crystallization, is separated from the oxidation reaction in the case of terephthalic acid, for example, the process of the present invention eliminates the need for separate purification of crude TA crystals. The process comprises: (a) forming a feed stream comprising a solvent and an oxidation catalyst at a pressure in the range of 2000 to 10000 kPa; (b) the dissolution of gaseous oxygen in the feed stream to obtain an oxygen concentration in the range of 0.5% to 3% w / w and optionally the preheating of the feed stream at a temperature in the range of 120 to 180 ° C; (c) continuously and simultaneously feeding the feed stream and a precursor to a piston-type expense reaction zone to form a reaction medium wherein the solvent: precursor ratio is at least about 30: 1 and the carboxylic acid the resultant is maintained in solution as it is formed; (d) the systematic reduction of the pressure of the reaction medium of step (c) while cooling to a temperature in the range of 120 to 180 ° C and thereby precipitating the crystals of the pure acid to form a glass slurry pure in the reaction medium; (e) optionally concentrating the slurry; and (f) the recovery of the pure crystals of the acid from the slurry. The pure crystals of the acid can be recovered from the reaction medium, also referred to herein as "mother liquors", as a wet cake by filtration and washing, and then transported directly to a next reaction step, for example, esterification, or the crystals can be dried and transported for storage. According to another aspect of the invention, precipitation, and crystallization of the pure crystals of the acid from the reaction medium are carried out in a sequence defined by (i) first the reduction of the pressure of the reaction medium to a value in the range of 1000 to 3000 kPa by means of which unreacted oxygen, water, acetic acid and volatile by-products, for example, vaporized oxides of carbon and vapor are discharged from the reaction medium and thereafter ( ii) reducing the pressure of the reaction medium in one or more additional steps to a value in the range of about 300 kPa while cooling the reaction medium to a temperature of about 150 ° C. According to another aspect of the present invention, it is a new composition of matter consisting essentially of substantially pure terephthalic acid in the form of separate rhomboidal crystals which are produced by the process of: (a) the formation of a feed stream comprising of acetic acid and an oxidation catalyst at a pressure in the range of 2000 to 10000 kPa (b) the gaseous oxygen solution in the feed stream to achieve an oxygen concentration in the range of 0.5% to 3.0% p / p and optionally the preheating of the feed stream at a temperature in the range of 120 ° C to 180 ° C (c) feeding continuously and simulataneously the feed stream and the paraxylene to a piston-type expense reaction zone to form the terephthalic acid within a reaction medium wherein the ratio of acetic acid: paraxylene is at least about 30: 1 and the terephthalic acid of this formed way is kept in solution; (d) the systematic reduction of the pressure of the reaction medium of step (c) while cooling to a temperature of 120 ° C to 180 ° C and thereby substantially precipitating the pure crystals of terephthalic acid to form a slurry; (e) optionally concentrating the slurry; and (f) recovery of the pure crystals of terephthalic acid from the slurry. The crystals of the terephthalic acid produced according to the present invention are distinctly of angular structure, for example, rhomboidal, and thus differ from the TA crystals produced according to the prior art, which tend to be round agglomerates of very fine crystals. little ones. The process of the invention results in the production of highly pure carboxylic acid crystals from a simple step single pass stage oxidation reaction sequence, including crystallization and recovery of the product i.e. without the need for a separate additional purification step.

DETAILED DESCRIPTION OF THE INVENTION The present invention is an improved continuous process for catalytic liquid phase oxidation of a suitable precursor, such as paraxylene, in the presence of an aliphatic carboxylic acid solvent, particularly acetic acid to produce a highly pure carboxylic acid. In the case of terephthalic acid (TA), the pure crystals are in the form of angular crystals distinctly separated from a rhomboid structure other than TA acid crystals produced according to known oxidation / purification processes. As used herein in the description of the carboxylic acid crystals produced according to the process of the invention, the terms "pure", "highly pure" and "substantially pure" are used interchangeably and mean that such acid crystals have a purity of at least 99.5% by weight, although the purity can be as high as 99.9% by weight and even higher, for example, of 99.95% by weight. The process will be described by reporting the production of highly pure terephthalic acid, although it is applicable to the production of a range of pure carboxylic acids, such as phthalic acid, isophthalic acid, etc., and mixtures thereof. The process is first carried out by forming a feed stream comprising a solvent i.e. an aliphatic carboxylic acid, which is typically acetic acid or a non-aliphatic organic solvent such as benzoic acid and an oxidation catalyst at an elevated pressure that is in the range of 2000 Kpa to 10000 Kpa. In practice, the feed stream will usually contain some amount of water. The term (solvent) as used herein describing the process of the invention, therefore, means the total amount of (i) water, if present, which may be in a concentration of 3% by weight up to as high as 30% by weight and (ii) the aliphatic carboxylic acid or the non-aliphatic organic acid. The molecular oxygen dissolves in the feed stream to reach a dissolved oxygen concentration of 0.5% to 3.0% w / w and the feed stream can then be heated to a temperature in the range of 120 ° C to 180 ° C before that is introduced in the reaction zone. The oxygen source can be pure oxygen, the air any convenient gas containing oxygen. In practice, the feed stream is fed to a simultaneous piston-type expense reactor and continuously with paraxylene and catalyst to thereby form a reaction medium wherein the resulting ratio of the solvent: paraxylene is at least about 30: 1, although the paraxylene solvent ratio can be as high as 200: 1 with the process filling out the results satisfactorily. In a preferred aspect, the solvent: paraxylene ratio is in the range of 65: 1. The process is carried out in the presence of an oxidation catalyst which can be homogeneous or heterogeneous and selected from one or more heavy metal compounds, such as, for example, cobalt and / or manganese compounds. In addition, the catalyst may also include an oxidation promoter such as bromine or acetaldehyde. The selection of the catalyst and the oxidation promoter and its use and handling of the entire process according to the invention is within conventional practice. The components of the catalyst / oxidation promoter are added to the feed stream in liquid form, as a solution before the feed stream is introduced into the reaction zone and are generally in solution throughout the process. The term "piston-type reactor" is used herein to define a typically tubular reaction zone wherein the radial mixing of the reactants occurs as if they flowed into the tube or conduit. The invention, however, is intended to comprise any reactor configuration where it approximates a piston-type expense reaction zone of a type suitable for carrying out the oxidation reaction according to the process of the invention, ie, performing the Oxidation reaction in liquid phase without boiling. The residence time of the reaction medium within the reaction zone is relatively short, ie, in the order of 5 minutes or less, taking into account the discovery that the reaction is selective and proceeds very rapidly under the conditions of the process. the invention. In practice it has been observed that the oxidation reaction under piston-type run runs at the desired conversions is 0.5 to 2.5 minutes. The oxidation reaction is exothermic in the magnitude of 12.6xl06 J / Kg of reacted paraxylene. Typically this heating has been removed by allowing the acetic acid solvent to boil, with the resulting vapor, being condensed and the condensate in varying amounts being refluxed into the reactor. According to the present invention, however, the choice of the solvent, the solvent: precursor ratio, the temperature and pressure cooperate to maintain the reaction medium, particularly oxygen and the TA in a non-boiling liquid phase, as the carrier medium. reaction passes through the piston-type expense reaction zone. Also in operation the heat of reaction does not necessarily need to be removed from the reaction zone. The pure T A is recovered from the reaction medium in a recrystallization process which is separated from the piston-type expenditure oxidation reaction. Crystallization systematically involves reducing the pressure and temperature of the reaction medium whereby TA precipitates in the reaction medium as substantially pure crystals while the impurities and other byproducts of the reaction remain in the solution. The reaction medium exists in the reaction zone of the piston-type expense at a temperature in the range of 180 ° C to as much as 250 ° C and a pressure in the range of 2000 Kpa to 10000 Kpa and is passed to a stage of crystallization . The crystallization can be carried out by reducing the pressure of the reaction medium to about 300 Kpa in a single stage or in several graduated stages while reducing the temperature to a value in the range of about 150 ° C. In a preferred aspect of the invention, the crystallization of the crystals of pure TA is carried out in two basic steps. In a first stage the pressure of the reaction medium is reduced to a value in the range of 1000 to 3000 Kpa for example, 2000 Kpa whereby the unreacted oxygen, water, acetic acid and volatile by-products, by For example, carbon oxides vaporize and steam is discharged from the reaction medium. Then in a second step, the pressure of the reaction medium is further reduced in one or more additional steps while the reaction medium is cooled to about 150 ° C. the reduction of the pressure can be carried out in any suitable manner such as, for example, by passing in a reaction medium through a pressure reducing valve to a liquid turbine. The crystals of TA precipitate from the reaction medium and form a diluted slurry of 1.0% to 6% w / w of concentration. It has been found that reducing the temperature of the reaction medium according to the present process to a value in the range. 150 ° C for crystallization, primary impurities, such as 4-CBA and undesirable color bodies, which could otherwise precipitate with the remaining TA in the solution. In this way it is possible according to the present invention to produce pure TA crystals via a catalytic liquid phase oxidation of the paraxylene without the need for a separate purification step.

By economy in the operation of the diluted slurry it can be thickened, i.e., concentrated, by any suitable means at a higher acid crystal concentration, above about 60% w / w. then the pure T A crystals can be recovered from the slurry by filtration, washed and optionally dried and sent to storage. The reaction medium which remains after the pure T A crystals is recovered, i.e. the mother liquor can be recycled and fed as a component of the feed stream to the oxidation reaction. In practice, the feed stream to operate the process on a continuous basis will comprise the recycled mother liquor which is supplemented with fresh aliphatic carboxylic acid (eg acetic acid) and fresh liquid catalyst to complete the physical and chemical losses of the stream of original power. The feed stream will be pressurized and oxygenated, and then simultaneously fed with liquid paraxylene in the piston-type expense reaction zone at an inlet temperature of about 120 to 150 ° C and at elevated pressure to reach a solvent ratio: paraxylene up close of 65: 1 for the resulting reaction medium with the result that the reaction proceeds rapidly (ie, 0.5 to 2.5 minutes of residence time of the reactor) without boiling, and the remains of TA in solution as it is formed. Although the reaction can be adiabatic, the cooling media can be used to directly recover and reuse the heat of the reaction. The invention will now be described with reference to the following examples. EXAMPLES Example 1: Oxidations of the piston-type expense reactor. The experiments are performed using a piston-type expense reactor system comprising two feed vessels, a reaction coil and two product collection containers. The simultaneous flow of the feed vessels through the reaction coil to one of the collection containers of the product is established by differential gas pressure and the appropriate valve operations. The first container is charged with a known composition of paraxylene in acetic acid / water solvent. The second feed vessel is charged with a known catalyst composition in the acetic acid / water solvent. The air is introduced into both feed vessels through submerged pipes at a pressure that ensures that the desired amount of oxygen (in excess of the stoichiometric requirement of paraxylene) is in solution. The feed vessels and the reaction coil are immersed in an oil bath to preheat the feeds at the required reaction temperature. The simultaneous flows of the feed vessels through the reaction coil are then established in the first collection vessel of the product marked "without spectrum". After a predetermined time the current is changed to the second product collection container. After a further predetermined time, the product stream is changed back to the "spectrumless" collection vessel. At the end of the experiment all containers are cooled, discharged, washed and drained. The contents of the solid and the liquid of the collection container of the sample product are recovered and analyzed and the composition of the solution leaving the reaction coil is calculated.

In Table 1, the concentrations of paratolualhedide TA precursors (ptolald), paratoluic acid (ptol) and 4-carboxybenzaldehyde (4-CBA) are reported. For experiments where the residence time of the reaction are varied. At this scale, the reactions run under closely exothermic conditions near the temperature of the oil bath of 210 ° C in all respects. The examples demonstrate the effect of residence time on precursor concentrations followed at 4.86 minutes of residence time, the conversion of the paraxylene from one step to TA is greater than 99.5% (the precursors less than 0.5% of the paraxylene fed). At 1.28 minutes of residence time, the conversion of paraxylene to 4-CBA (the intermediate tending to coprecipitate with the TA in conventional processes) is 1% or less in all respects. The conversion of a high pass of paraxylene to the TA is not essential for the process while the concentration of 4-CBA in the reaction stream leaving the reactor is low. During the crystallization stage the T A precursors substantially remain in the solution in the reaction medium, i.e, dissolved in the mother liquor and thereby can be recycled to the piston-type oxidation reactor. Table 1: Oxidation results of the piston-type expense reactor. In all experiments, the following parameters are set (all compositions are p / p); Solvent: Water 5%, acetic acid 95% Paraxylene: 0.5% w / w (200: 1 ratio of the solvent: paraxylene) Catalyst: Co 632 ppm, Mn 632 ppm, Br 1264 ppm + Zr 96 ppm. Oil bath temperature: 210 ° C Reactor Solution (ppm w / w) Example 2 Crystallization To a 2% w / w terephthalic acid solution (TA), 125 ppm 4-CBA, 175 ppm ptol and other oxidation intermediates in it solvent 5% w / w water, acetic acid 95% w / w is prepared at high temperature (210 ° C) and at a high enough pressure to maintain a liquid phase. The solution is passed, continuously, through a valve to reduce the pressure inside a crystallizing vessel whose temperature and pressure are controlled such as the precipitated TA of the solution. The slurry produced in the crystallizer is passed in front of the crystallization vessels where the pressure and temperature are systematically reduced to ambient conditions, and in addition to precipitated TA. During the course of the experiment, the crystals of the first crystallizer (TA filtered in hot) where it is recovered and analyzed for the content of 4-CBA and paratoluic acid (ptol) and the size of. medium particle (using a psd laser diffraction analyzer LS230 Coulter). Crystals from downstream containers (cold-filtered TA) are also recovered and analyzed for proposed references. In Table 2, the hot-filtered TA, the contents of 4-CBA and ptol and the average particle sizes are reported for experiments where the temperature of the first crystallizer, the residence time and the stirring speed are varied. For reference, a cold-filtered TA analysis is also included. Examples 7, 8 and 9 show that, in hot-filtered TA, the contents of ptol and 4-CBA fall when the filtration temperature is reduced from 196 to 148 ° C. The data also shows that the average particle size increases with reduced temperature. In a separate experiment, Examples 10 and 11 show that, in hot-filtered TA, the reduction in filtration temperature from 151 ° C to 126 ° C causes the level of 4-CBA to increase, while the level of ptol and the average particle size is declined. When viewed together, Examples 7 to 11 indicate an optimum crystallizer temperature, with respect to collectively the incorporation of intermediates and the average particle size, in the region of 140 ° C to 160 ° C, particularly around 150 ° C. C. Examples 12 and 13 show that increasing the residence time of the first crystallizer from 9 to 18 minutes benefits both the incorporation of intermediates and the average particle size. Examples 14 and 15, when viewed throughout Example 9, show that increasing the agitator speed of the first crystallizer, from 270 to 1000 rpm, does not have a strong influence on the average particle size, but tends to reduce the incorporation of intermediaries. Table 2 Results of the crystallization experiments. In all experiments the following parameters are set (all comparisons are p / p); Solvent: water 5%, acetic acid 95% Aromatics of the feed solution: Ta 2%, 4CBA 125 ppm, ptol 175 ppm Temperature of the feed solution: 210 ° C It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention. Having described the invention as above, property is claimed as contained in the following:

Claims

CLAIMS 1. A process for producing a pure carboxylic acid by catalytic liquid phase oxidation of the corresponding precursor in a solvent selected from an aliphatic carboxylic acid or a non-aliphatic organic acid and optionally including water characterized in that it comprises: (a) the formation of a feed stream comprising a solvent and an oxidation catalyst at a pressure in the range of 2,000 to 10,000 kPa; (b) dissolving gaseous oxygen in the feed stream to obtain an oxygen concentration in the range of about 0.5% to 3.0% w / w and optionally preheating the feed stream to a temperature of about 120 ° C to 180 ° C; (c) continuously and simultaneously feeding the feed stream and said precursor to a piston-type expense reaction zone to form a reaction medium wherein the ratio of precursor solvent is at least about 30: 1 and the carboxylic acid resulting is maintained in the solution as it is formed; (d) systematically reducing the pressure of the reaction medium of step (c) while cooling to a temperature in the range of about 120 ° C to 180 ° C whereupon the carboxylic acid crystals are precipitated to form a slurry; (e) optionally concentrating the slurry; and (f) recovering the carboxylic acid crystals from the slurry.
2. The process according to claim 1 characterized in that the systematic reduction of the pressure of the reaction medium of step (c) is carried out by (i) first reducing the pressure of the reaction medium to a value in the range of about 1,000 at 3,000 kPa where unreacted oxygen, water, precursor and volatile secondary products vaporize and the vapor is vented from the reaction medium, and then (ii) reduce the pressure of the reaction medium. in one or more additional steps at a value in the range of 300 kPa while cooling the reaction medium to a temperature of about 150 ° C.
3. The process according to claim 1 or claim 2 characterized in that it includes the additional step of recycling the remainder of the reaction medium of step (f) as a component of the feed stream to the reactor.
4. A new composition of matter consisting essentially of terephthalic acid in the form of discrete rhomboid crystals produced by the process of: (a) the formation of a feed stream comprising acetic acid and an oxidation catalyst at a pressure in the range of 2,000 to 10,000 kPa; (b) dissolving gaseous oxygen in the feed stream to obtain an oxygen concentration in the range of about 0.5% to 3.0% w / w and optionally preheating the feed stream to a temperature of about 120 ° C to 180 ° C; (c) continuously and simultaneously feeding the feed stream and the paraxylene to a piston-type expense reactor at a ratio of acetic acid: paraxylene of at least about 30: 1 to form terephthalic acid and keep it in the solution as it is formed; (d) systematically reducing the pressure of the reaction medium of step (c) while cooling said reaction medium to a temperature in the range of about 120 ° C to 180 ° C whereupon the terephthalic acid crystals precipitate pure to form a slurry; (e) optionally concentrating the slurry; and (f) recovering the carboxylic acid crystals from the slurry.
5. The process according to claim 4, characterized in that the systematic reduction of the pressure of the reaction medium of step (c) is carried out by (i) first reducing the pressure of the reaction medium to a value in the range of about 1,000. at 3,000 kPa where the unreacted oxygen, water, precursor and volatile by-products vaporize and the vapor is vented from the reaction medium, and then (ii) reduce the pressure of the reaction medium in one or more additional stages at a value in the range of 300 kPa while cooling the reaction medium to a temperature of about 150 ° C.
6. A process for producing pure terephthalic acid by catalytic liquid phase oxidation of paraxylene in a solvent selected from an aliphatic carboxylic acid and optionally including water characterized in that it comprises: (a) the formation of a feed stream comprising a solvent and a Oxidation catalyst at a pressure in the range of 2,000 to 10,000 kPa; (b) dissolving gaseous oxygen in the feed stream to obtain an oxygen concentration in the range of about 0.5% to 3.0% w / w and optionally preheating the feed stream to a temperature of about 120 ° C to 180 ° C; (c) continuously and simultaneously feeding the feed stream and said paraxylene to a piston-type expense reaction zone to form terephthalic acid in a reaction medium wherein the ratio of solvent: paraxylene is at least about 30: 1 and the terephthalic acid is maintained in the solution as it is formed; (d) systematically reducing the pressure of the reaction medium of step (c) while cooling to a temperature in the range of 120 ° C to 180 ° C whereupon the crystals of pure terephthalic acid are precipitated to form a slurry; (e) optionally concentrating the slurry; and (f) recovering the carboxylic acid crystals from the slurry.
7. The process according to claim 6 characterized in that the systematic reduction of the pressure of the reaction medium of step (c) is performed by (i) first reducing the pressure of the reaction medium to a value in the range of about 1,000 to 3,000 kPa where unreacted oxygen, water, precursor and volatile by-products vaporize and vapor is vented from the reaction medium, and after (ii) reduce the pressure of the reaction medium in one or more additional stages at a value in the range of 300 kPa while the reaction medium is cooled to a temperature of about 150 ° C, and the process includes the additional step of recycling the remainder of the reaction medium from step (f) as a component of the feed stream to the reactor.