GB1577544A - Process for the preparation of iso- or terephthalic acid - Google Patents

Description

(54) PROCESS FOR THE PREPARATION OF ISO- OR TEREPHTHALIC ACID (71) We, STANDARD OIL COMPANY, a corporation organized and existing under the laws of the State of Indiana, United States of America of 200 East Randolph Drive, Chicago, Illinois 60601, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to the preparation of iso- or terephthalic acid by the oxidation of m- or p-xylene with molecular oxygen at elevated temperatures and under liquid phase conditions in the presence of catalysis provided by a combination of one or more transition metal oxidation catalysts and a source of bromine.

The unique catalysis provided by a combination of one or more transition metal oxidation catalysts and a source of bromine has been taught by United States Patent No. 2,833,816 and the commonly derived foreign patent counterparts as generally applicable for the preparation of aromatic carboxylic acids at elevated temperatures in the range of 50 to 2750C and under pressures to maintain liquid phase conditions for the oxidation with a source of molecular oxygen of aromatic hydrocarbons having at least one substituent oxidizable to a carboxylic acid substituent. Said patents also taught the use of C2-Cs carboxylic acids as useful solvents for such catalytic liquid phase oxidations.

Such catalytic liquid phase oxidation has been developed into the predominant process for the world-wide commercial production of isophthalic acid, terephthalic acid and trimellitic acid by the respective air oxidations of m-xylene, p-xylene and pseudocumene in the presence of acetic acid. Such catalytic liquid phase oxidation has also been applied to the commercial production of benzoic acid, but to a lesser extent, by the air oxidation of toluene in the presence of benzoic acid as solvent.

The use of acetic acid as solvent in such commercial production of the benzene di- and tricarboxylic acids, although providing a most economically advantageous process, does have the disadvantage of co-oxidation of acetic acid to the extent of from 80 to 160 kilograms per metric ton (MT) of product produced.

Acetic acid, the most refractory of the C28 aliphatic acid class, is oxidatively converted in such processes to oxides of carbon, water and methyl acetate.

Temperature control of such oxidations conducted in the presence of acetic acid solvent presents no commercial operating problem because of the relatively close boiling points of acetic acid and by-product water and the water concentrations, 3 to 18 weight percent of solvent, involved. Such acetic acid-water solvent system for practical consideration is a one component system of miscible liquids and has from phase rule relationships but one degree of freedom. Thus, for constant volume operation, setting operating pressure as a constant provides a constant operating temperature.

In 1963, a continuous process was disclosed for the preparation of benzene diand tricarboxylic acids, especially terephthalic acid, by the catalytic liquid phase air oxidation of the appropriate methyl-substituted benzene in the presence of benzoic acid, in place of acetic acid, as reaction solvent and in the presence of the earlier disclosed unique combination of one or more transition metal oxidation catalysts and a source of bromine. British Patent Specification No. 1,088,183, published 25 October 1967, is directed to and contains details of such oxidations conducted in the presence of benzoic acid solvent. According to said patent, benzoic acid was selected over the other C2-C8 monocarboxylic acids because its aeration at temperatures in the 50--275"C range produced a medium less corrosive to metals available for fabrication of oxidation vessels than aerated acetic acid at such temperatures, it was less volatile than acetic acid, and by-product water vapor was more readily separated from benzoic acid vapor than from acetic acid vapor at the reaction site. For example, selective condensation of benzoic acid vapors from a vapor mixture thereof with by-product water can be accomplished by simple partial condensation but separation of a mixture of acetic acid and by-product water vapors requires fractionation.

Further benefits from the use of benzoic acid solvent, according to the British Patent, come from the use of high, 170 to 2750 C, oxidation temperatures which provide high oxidation rates; the use of high, 21 to 35 kg/cm2, operating gauge pressures which provide high oxygen concentrations in the liquid phase reaction medium; the required removal of by-product water as it is formed; and the independence of operating temperature on such operating pressure.

The above British Patent provides three continuous air oxidations of p-xylene to terephthalic acid wherein its yields of 85-95 mole percent (% of theory) and purity of 99-99.9 weight percent are illustrated.

However, in spite of such promised high yields and purity of terephthalic acid prepared in the presence of liquid benzoic acid solvent, we have found a major problem associated with the conduct of the continuous process of said British Patent.

Said problem was found to occur during initiation of the p-xylene oxidation at the operating gauge pressure of 21 to 35 kg/cm2 while removing by-product water as it was formed. At such oxidation initiation conditions the operating temperature could not be controlled and the benzoic acid and/or p-xylene or its partial oxidation products were quite rapidly over oxidized to a carbonaceous residue. The lack of stirring of the liquid phase composition, the illustrative examples in the British Patent did not use a stirred-tank type oxidation vessel, could lead to inefficient dispersion of air and distribution of heat of reaction in the liquid phase and thus provide localized "hot spots," the isolated high oxidation rate resulting from highly localized conditions of high temperature and oxygen concentration. But unstirred tubular reaction vessels had been successfully used for other oxidations; for example, the preparation of benzoic acid by air oxidation of toluene in the presence of benzoic acid solution of a combination of one or more transition metal and a source of bromine, without encountering such sudden charring of reactant, product or benzoic acid solvent. Hence, the lack of stirring was not the controlling effect with respect to the problem of rapid charring due to the inability to control temperature during initiation of p-xylene oxidation.

We found that the effects causing the rapid charring during oxidation initiation were associated with the required high operating gauge pressure of 21 to 35 kg/cm2 and removal of by-product water as it is formed. This discovery resulted from our investigation of the effect of retention or lack thereof of small amounts of water in the benzoic acid solvent on the control of initial operating temperature at the 21 to 35 kg/cm2 operating gauge pressure required according to the process of the British Patent.

By experimentation it was found that by going from 5 weight percent to 0 weight percent water concentration in liquid benzoic acid under a gauge pressure of 25-35 kg/cm2 and an initial temperature of 205"C a temperature increase of as much as 110"C was observed without change of pressure.

Substantiation of such effects causing the rapid charring can be understood from the results of the following investigations. Benzoic acid (boiling point of 249"C at 760 mm Hg) with a water content of 5 weight percent was heated to an initial temperature of 205"C under a gauge pressure set at 21 to 35 kg/cm2. The temperature of the liquid benzoic acid was measured as water was boiled off and its water content went from the initial 5% down to 0%. By going from 5% to 0% water the temperature of the liquid benzoic acid increased as much as 110 C without causing an increase in the set gauge pressure. This indicated a unique temperature sensitivity with respect to water content of liquid benzoic acid solvent and accounted for the rapid drastic over oxidation to a charred product during initiation of p-xylene oxidation at a gauge pressure of 21 to 35 kg/cm2 following the required operating conditions of the British Patent.

Based on our discovery of the dramatic temperature increases found during our foregoing experiments we concluded that removing by-product water as rapidly as formed to maintain a near 0 water content and conducting at least the oxidation initiation under a pressure as high as 21 to 35 kg/cm2 could not lead to successful temperture control necessary for commercial operation of the production of iso- or terephthalic acid from the air oxidation of m- or p-xylene in the presence of liquid benzoic acid as reation solvent.

Another problem arose after initiation of the continuous oxidation of m- or pxylene with air in the presence of liquid benzoic acid as a solvent according to the operating conditions of the British Patent. After the oxidation had been successfully initiated in the stirred liquid phase with good temperature control provided by the proper initial and retained water content of the liquid benzoic acid, there were still wide fluctuations in reaction temperature caused by changes in the amount of water removed. This and the foregoing experimental results led us to conclude that the 21 to 35 kg/cm2 operating pressure imposed by the British Patent was too high and the required removal of by-product water as rapidly as formed was unnecessary and would not provide a commercially acceptable continuous process.

We have found that control of operating temperature for both initiation and lined-out (i.e. smoothly operating, steady state) operation for a commercially acceptable continuous air oxidation of m- or p-xylene can be achieved by the use of a solvent system consisting essentially of from 85 up to 97 weight percent liquid benzoic acid and from 15 down to 3 weight percent water provided that the operating gauge pressure is within the range of from 6 up to 25 kg/cm2 and removal of by-product water does not cause the water content of such solvent system to exceed 15% or to go below 3%. At such operating conditions an operating oxidation temperature in the range of from 175--235"C can and should be used.

For such continuous oxidation the unique catalysis of a combination of one or more transition metal oxidation catalyst and a source of bromine is provided by dissolving suitable sources of the components in the solvent system.

The present inventive continuous process is conducted in a stirred oxidation zone to provide efficient dispersion of air in and distribution of heat of reaction through the liquid phase in the oxidation zone.

Thus according to the present invention there is provided a process of preparing iso- or terephthalic acid by the continuous or semi-continuous liquid phase molecular oxygen oxidation of m- or p-xylene, in which the oxidation is conducted in a stirred oxidation zone in the presence of catalyst components comprising a source of bromine and one or more transition metal oxidation catalysts in a solvent system comprising 85 to 97 weight per cent liquid benzoic acid and 15 to 3 weight percent water and at an oxidation zone gauge pressure within the range of 6 to 25 kg/cm2, the temperature being maintained substantially constant within the temperature range of 175 to 2350C by removing by-product water as vapor and controlling the amount of water returned to the reactor.

The transition metal component is preferably selected from manganese, cobalt and cerium and most preferably comprises maganese or a combination of manganese with one or both of cobalt and cerium.

Preferably the catalyst components provide from 0.2 to 1.5 weight percent total bromine based on xylene and from 0.2 to 1.5 weight percent total metal based on xylene. The weight ratio of bromine to total metal is preferably in the range from 0.5 to 2.5.

In a particularly preferred manner of operation wherein p-xylene is oxidized with air, the transition metal catalyst component is provided by manganese and cobalt in the Mn/Co weight ratio of from 1:1 to 6:1 and the sum of the weights of Mn and Co are from 0.5 to 1.5 weight percent of the weight of the p-xylene. Most preferably the solvent system consists essentially of 90% benzoic acid and 10% water, the weight ratio of the solvent system to p-xylene is 2 to 7:1, preferably 3-5:1, the catalyst components are in the concentrations of 0.015 to 0.1% cobalt, 0.08 to 0.2% manganese and 0.02 to 0.3% bromine based on said solvent, the oxidation zone temperature is from 205 to 226.50C, the oxidation zone gauge pressure is from 1424.6 kg/cm2, and the oxygen content of the exhaust gas is from 6-10 volume percent. In another particularly preferred manner of operation wherein p-xylene is oxidized with air, the solvent system consists essentially of 90% benzoic acid and 10% water, the weight ratio of solvent system to p-xylene is 3-5:1.0, and the transition metal catalyst component is manganese and the ratio of bromine to manganese is from 0.8 to 1.5:1.0 and the manganese concentration is from 0.15 to 0.2 weight percent of solvent.

In carrying out the process of the invention the heat of reaction causes vaporization of a little benzoic acid and mainly water from the oxidation zone.

Little or no xylene is vaporized because it is oxidized to products which do not tend to be vaporized at the operating conditions. Satisfactory temperature control is achieved for substantially constant operating conditions by regulating the water content of the liquid reflux (mainly water) to the oxidation zone after the condensation of said vapors to remove heat of reaction. The water content of the liquid reflux, generally 9095 weight percent, can be regulated by varying the operating temperature of the reflux condenser. Change in oxidation temperature from a selected constant operating temperature can be corrected by varying the rate of addition of water (since the condensate has only 5-10 weight % benzoic acid) returned to the oxidation zone. For example, the rate of water return by way of the reflux liquid is decreased or increased in response to a decrease or increase of oxidation zone temperature from the selected constant temperature. Means for such variation of water content of liquid reflux and rate of water return to the oxidation zone are hereafter described. Such means can keep the change of oxidation zone temperature within 5"C above or below the selected constant temperature for the oxidation zone.

Variation of water content in such returned liquid should not decrease the water content of the solvent system below 3 weight percent because at such condition the uncontrolled wide temperature fluctuation conditions again occur.

Variation of water content such returned liquid should not increase the water content of the solvent system above about 15 weight percent, e.g. to 18 weight percent, because such amount of water deactivates the catalyst system partially or completely to make the oxidation rate commercially unattractive.

For the purposes of this invention the m- or p-xylene oxidation is preferably carried out with a weight ratio of the benzoic acid-water solvent system to xylene in the range of 2:1 to 10:1. Also, the system of catalysis is preferably provided by a source of bromine in combination with one or more of manganese, cabalt, or cerium as transition metal oxidation catalyst. Also, it is preferred to use an amount of air which provides from 2 up to 10 volume percent oxygen in the exhaust gas (benzoic acid-free basis) to minimize the amount of partial oxidation products and color body inpurities in the iso- or terephthalic acid product recovered.

Operation of the present inventive process is conducted on a continuous basis for the combination of 6-25 kg/cm2 gauge pressure and the particular benzoic acid-water solvent system to provide for efficient control of substantially constant temperature in the oxidation zone in the operating temperature range of 175-2350C.

The continuous operation of the present inventive process does not have relatively high xylene concentration at start-up; i.e. initiation of oxidation, as will be understood from the start-up procedure for continuous operation. Said start-up differs from batchwise operation in that there is initially charged to the stirred oxidation zone the components of the unique catalysis and solvent system. The resulting solution is stirred and heated to a temperature at which oxidation is initiated, e.g. 160--1700C, but preferably to operating temperature of 175--235"C.

When such heating is only to oxidation initiation temperature, then m- or p-xylene is pumped into the stirred liquid in the oxidation zone and air is injected into said stirred liquid at about 1000-1500 N liters per kilogram of xylene until the oxidation zone temperature reaches the operating temperature in the range 175--235"C. Thereafter the xylene is pumped into the oxidation zone and air rate is increased to the range of from about 3800 to about 5900 N liters per kilogram of xylene to provide the 3-10 volume percent oxygen in the exhaust gas (benzoic acid and water-free basis). After the weight ratio of originally charged solvent system to the total xylene charged reaches the selected ratio in the range of 2-10:1.0, the solution of catalyst components in the benzoic acid-water solvent system is pumped in at a rate with respect to continued xylene pumping to preserve the selected solvent to xylene weight ratio as the injection of air is continued at said 3800-5900 N l/kg xylene providing such 3-10 volume percent oxygen in the exhaust gas. The time for such initiation to complete continuous feed operation is relatively short but does provide time to adjust the operation of the condenser to which the exhaust gas is conducted for removing heat of reaction and adjust the rate of water condensate returned with liquid condensate refluxed to the stirred oxidation zone.

Upon reaching the operating aerated liquid volume of the oxidation zone, the fluid oxidation reaction mixture is then withdrawn from the oxidation zone to supply feed to the separation of iso- or terephthalic acid product from liquid benzoic acidwater solvent system.

Although it is preferable not to operate throughout the period from initiation through complete continuous operation in the manner described above and introducing reactants and solvent system solution of catalysis and withdrawing fluid reaction mixture, it does serve as a useful start-up procedure to gain experience of the unique temperature sensitivity with respect to water content of the solvent system, which as described, has the most pronounced effect on successful control of operating temperature.

After such expenence is gained, then the start-up of the present inventive process can be simplified by resorting to the preferred start-up conditions. For the preferred start-up, the solvent system solution of components of catalyst needed to reach operating volume of the reactor is charged and stirred and heated to the selected operating temperature under the selected operating pressure. Thereafter the xylene is pumped in and air is injected into the stirred solvent at continuous operating rates with the condenser reflux system operating in response to the temperature of the oxidation zone. Removal of fluid is started when the total xylene charged provides, with respect to initial solvent system charged, the selected weight ratio of solvent system to xylene.

The continuous operation of the present inventive process can be conducted in an oxidation vessel having a stirrer, a reflux condenser operated at a temperature of about the melting point of benzoic acid (121.7"C) to condense benzoic acid as a liquid for reflux to the stirred oxidation zone and a side arm-type condenser operated to condense water vapor for removal of by-product water. A pressure control valve can be used between said condensers or preferably after the water vapor condenser, for example at the exit therefrom. Uncondensed gases can be discharged from said condensate collecting vessel through the pressure reducing valve. The reaction vessel should be fitted with temperature and pressure measuring devices and means for charging reactants, solvent solution of components of catalysis, and withdrawing fluid oxidation effluent. With such combination of apparatus elements control of water content of the benzoic acidwater solvent system can be readily monitored and controlled within the 3-15 weight percent water content on the basis of a water material balance. Only a metered amount of water condensate is discarded which is equivalent to the amount of by-product water produced and the remainder of the condensate is pumped back into the reaction zone to control the oxidation zone at constant pressure.

Product iso- or terephthalic acid, relatively insoluble in the solvent system, can be separated from the fluid oxidation effluent by any solid-liquid separation means such as by filtration or centrifugation, at a temperature at which benzoic acid in said effluent remains liquid. Since such fluid effluent is at a temperature of 175--2350C (well above the temperature of benzoic acid solidification), and a gauge pressure of 6-25 kg/cm2, such product separation can be accomplished by decompression with attendant cooling of such effluent. Such decompression can be to ambient or subatmospheric pressure. The cooled effluent but still fluid effluent is pumped to said solid-liquid separation. Preferably such effluent is decompressed to subatmospheric pressure to avoid any flash evaporation in the means for solidliquid separation.

The separated crystalline product can be washed with hot, fresh liquid benzoic acid and then with xylene, toluene or a combination thereof to remove adhering benzoid acid. The washed product is dried and the xylene or xylene-toluene mixture removed by drying is recovered for reuse. The xylene and/or toluene wash of product can be made in the means for solid-liquid separation but is preferably accomplished by suspending the benzoic acid washed solid product in the aromatic hydrocarbon. The foregoing separating and washing of iso- or terephthalic acid product provide the benefits of higher product purity because the product is recovered at a higher temperature than would be possible when acetic acid was the solvent. Such higher temperature separation leaves more of the impurity oxidation intermediates in the mother liquor. Further impurity removal is enhanced by the xylene and/or toluene washing because such aromatic hydrocarbons are better solvents for the impurities than is acetic acid.

Additional benefits resulting, in general, from the present inventive process are reduced combustion of the organic component of the solvent system; cleaner vent streams; less potential corrosion in the oxidation reactor, transfer lines and product recovery apparatus; and lower cost of oxidation reactor because of lower reaction pressure.

With respect to combustion of benzoic acid component of the solvent system, this is reduced to 50 percent of that experienced with acetic acid solvent for the same weight ratios of solvent to xylene and same operating temperatures.

Moreover, the benzoic acid combustion products are only water and oxides of carbon thus eliminating venting of the ester (methylacetate) or separating it from solvent for recycle use as is needed when acetic acid is used as reaction solvent.

Also, no appreciable amount of solvent vapor is vented with non-condensibles as is the case when acetic acid is the reaction solvent. This leads in the practice of the present inventive process to cleaner discarded gases.

Aerated, wet (315% water) benzoic acid is substantially less corrosive at operating temperatures even when containing the bromine component of catalysis than aerated, wet (510% water) acetic acid containing such bromine component.

Such substantially less corrosivity associated with the benzoic acid-water solvent permits the use of less expensive stainless steels of the SS316 type for process apparatus, especially the condenser, rather than the rather expensive metals such as titanium used when acetic acid is the solvent.

Capital investment and operating cost for the commercial practice of the present invention would be lower than when acetic acid or anhydrous benzoic acid are used as reaction solvents. Such lower costs result directly from the use of less expensive metals for process apparatus, lower operating pressure in the oxidation reaction vessel and its auxiliary condensers and condensate receiver, the. elimination of exhaust gas scrubbing, and the elimination of the later process steps of crystallization (generally two or three stages when acetic acid is the solvent), and solvent fractionation to recover solvent for recycle to the oxidation.

The present inventive process, unlike processes using acetic acid solvent, is not retarded rate-wise by the presence of more than 5 weight percent water in the solvent system but there is a reaction rate penalty above 20 weight percent water.

As mentioned before the conditions essential for the conduct of the present inventive process are the combination of operating pressure at 6--25, preferably 14-20, kg/cm2 gauge with the solvent system of benzoic acid (85-97 wt %) and water (15-3 wt %) to achieve control of substantially constant operating temperature selected from the range of 175--235"C inclusive. The illustrative Examples will provide combinations of operating pressure and solvent compositions to obtain good control of substantially constant temperature (not more than +5 C) from the average operating temperature from which those skilled in this art can make a selection or devise other useful combinations.

Although the use of a stirred reaction zone is important with respect to good air dispersion in the liquid reaction mixture and heat removal therefrom, only the ordinary skill of stirring design is involved. For example, the ordinary design criteria of reactor and stirrer-blade geometry, agitation pattern and stirrer power to keep product crystals suspended in the solvent system need be taken into account to provide the necessary stirring. Those criteria can be readily calculated on the basis of published formulae or from empirical data redily obtainable by simple experiments with small scale apparatus.

The useful weight ratio of solvent system to m- or p-xylene is, as before stated, in the range of 2-10:1.0. The illustrative Examples will provide a basis for the process design engineer to either choose therefrom a ratio for specific process design purpose or select a different ratio to meet the particular design devised.

Heat of reaction can be removed as hereinafter illustrated by the addition of water to the reaction zone for evaporation therefrom. The evaporation of 5 kilograms of water per kilogram of xylene oxidized will remove the heat of reaction. Heat of reaction can also be removed by known means of internal indirect heat exchange with heat exchange fluid; for example, such fluid flowing through a tube coil in the reaction zone. Also heat of reaction can be removed by any one of the known external fluid loop heat exchangers wherein liquid reaction mixture flows from the oxidation zone to the external heat exchange loop for indirect heat exchange, for example to exchange heat with water and generate steam, and the cooled reaction mixture is pumped back into the stirred reaction zone. Many other known useful means for removing heat of reaction can be used.

Not previously mentioned are the preferred parameters of components of catalysis which are from 0.3 to 1.5 weight percent of total metal and 0.3 to 1.5 weight percent bromine, both calculated as the element, based on the xylene. Heat of reaction in the preferred mode will be removed by evaporation of reactor solvent, which would be condensed by a three stage condenser. The H2O of reaction would be removed from last stage of the condenser, other condensed liquid returned to reactor based on the xylene.

The suitable source of Mn, Co and Ce can be salts of such metals soluble in the benzoic acid-water solvent system. Such salts include the carboxylates:acetates, propionates, butyrates, naphthenates and benzoates; the acetylacetonate and ethylenediamine tetraacetate complexes as well as the bromides.

The source of bromine can be elemetal bromine; ionic forms of bromine including ammonium bromide; bromides of the transition metal components of catalysis, hydrogen bromide per se or as hydrobromic acid, sodium or potassium bromide or sodium or potassium bromates; and organic bromides including tetrabromoethane, dibromoethylene, benzylbromide, and bromobenzene. Such sources of bromine are known from United States Patent No. 2,833,816 and its commonly derived foreign counterpart patents.

The following thirteen Examples illustrate the conduct of the present inventive process by semi-continuous oxidation wherein terephthalic acid (TA) is produced by air oxidation of p-xylene (PX). The oxidation apparatus is a cylindrical oxidation vessel having a stirrer to agitate its reaction zone which, from the amount of solvent system used and product suspended therein after all the p-xylene had been charged and aerated, comprises about 60 volume percent of the total volume of the vessel.

The remaining 40 volume percent provides disengagement of gases and vapors from the stirred fluid in the reaction zone. Said oxidation vessel is also fitted with separate means for introducing air into the lower portion of the oxidation zone, p-xylene and water into the upper portion of the oxidation zone, discharging fluid oxidation effluent from the bottom of said vessel, discharging the disengaged mixture of gases and vapors from the top of the vessel, and means for sealing the vessel for its operation at pressures above ambient pressure. The top discharge means is connected to a reflux condenser for condensing benzoic acid for its liquid reflux and uncondensed gases and vapors discharge to a side arm type condenser operated to condense water. The water vapor condenser is connected to a receiving vessel which collects water condensate and discharges uncondensed gases (a mixture of nitrogen, oxygen and oxides of carbon together with some water vapor) from the top through a pressure control valved vent line. A water freeze-out trap is between said pressure control valve (adjustable) and apparatus for sampling and analyzing the gases to be vented. The p-xylene is fed to the oxidation zone by a metering pump from a pressurized feed tank. Provision is also made to charge water to the oxidation zone by means of a combination of presssurized feed tank (3.5 kg/cm2 gauge above oxidation zone pressure), metering needle valve and rotameter but an additional by-pass through a ball valve is also provided to add a large amount of water to the reaction zone to quench the reaction quickly in the event of an otherwise uncontrollable sudden rise of reaction temperature occurring. Adjustable means is provided for heating or cooling the stirred liquid or fluid contents in the oxidation zone system initially charged to the reaction vessel and maintaining said solution at operating temperature up to initial introduction of p-xylene and air and, if need be, after introduction of p-xylene is complete. The reflux condenser is heated by steam, 7 kg/cm2 gauge pressure maximum, introduced through an air-pressurized steam regulator, by regulation of the air pressure, the control of the steam pressure over the range of 07 kg/cm2 to the condenser, hence control of its operating temperature is achieved. Said air pressure is controlled in turn in response to the temperature in the reaction zone. For such operation of the condenser temperature in response to the oxidation zone temperature, the steam pressure is increased when reaction temperature decreases and steam pressure is decreased when reaction temperature increases. The attendant increase in steam pressure reduces water content of the solvent system and decrease in steam pressure increases water content of the solvent system. In the illustrative Examples provided hereafter the described means for controlling reaction temperature by control of reflux condenser temperature (steam feed pressure) was sufficiently precise that it did not become necessary to add water in large quenching amounts. However, when the exit vapor flow rates were low and temperature control of vent gas was difficult, more precise control of temperature was more facile with the injection of small amounts of water.

TABLE I p-XYLENE OXIDATION IN BENZOIC ACID-WATER SOLVENT Example No. 1 2 3 4 5 6 7 8 Materials & Conditions p-Xylene, g 218 170 153 273 127 127 127 127 Pump rate, g/hr. 327 170 166 193 90 90 90 90 Pumping time, min. 40 60 55 85 85 85 85 85 Solvent water, wt. % 10 10 10 10 10 10 10 10 Water, g 81 90 90 90 90 90 90 90 Benzoic acid, g 729 810 810 810 810 810 810 810 Co on Solvent, wt. % 0.04 0.04 0.06 0.10 0.03 0.03 0.06 0.03 Mn on Solvent, wt. % 0.08 0.08 0.12 0.02 0.09 0.09 0.09 0.18 Br on Solvent, wt. % 0.20 0.20 0.30 0.02 0.18 0.18 0.18 0.18 Solvent/p-xylene, wt. ratio 3.7 5.3 5.85 3.3 7.1 7.1 7.1 7.1 Operating temp., C. 218 216 218 205 215 205 205 205 Operating gauge pres., kg/cm2 14 16.5 14.5 24.6 19.1 20.1 19.7 19.7 O2 in exhaust, vol. % 6-10 3-9 5-9 6-8 9-12 9-12 9-12 8-14 Total reaction time, min. 45 63 60 90 90 90 90 90 Total reaction Effluent(*) 4-CBA,ppm (**) 2700 430 610 2000 290 400 300 770 p-Toluic acid, wt. % 0.03 0.013 1.27 0.093 0.13 0.12 0.13 p-Xylene, wt. % 0.042 0.033 0.001 ND ND 0.005 ND Terephthalic acid, wt. % 20.4 19.3 30.21 22.3 21.6 22.1 22.3 Terephthalic acid Yield, Mol. % 95.7 95.8 90.7 95.6 96.6 95.7 96.3 Filter cake, 4-CBA, ppm 390 310 3700 290 490 350 1000 ND is none detected.

*Total Reaction Effluent obtained by depressuring to ambient pressure. Hence water content was flash evaporated.

**4-CBA = 4-carboxybenzaldehyde.

The p-xylene oxidations of following Examples 9-14 are conducted in the same manner and in the same equipment as described with respect to Examples 1--8, except that in Example 14 the reaction solvent is made up by combining 80 percent of benzoic acid mother liquor from Example 13 with water, fresh benzoic acid, and manganese acetate tetrahydrate. Hence, the accumulation of intermediate products 4-CBA and p-toluic acid and the increase in color of the recovered product.

TABLE II p-XYLENE OXIDATION IN BENZOIC ACID-WATER SOLVENT Example No. 9 10 11 12 13 14 Materials and Conditions p-Xylene, g. 127 127 127 184 184 184 Pump rate, g/hr. 90 90 90 184 184 184 Pumping time, min. 85 @85 85 60 60 60 Solvent water, wt.% 10 10 10 10 10 10 Water, g. 90 90 90 90 90 90 Benzoic acid, g. 810 810 810 810 810 810 Co on Solvent, wt. % 0.015 0.03 0.03 0 0 0 Mn on Solvent, wt. % 0.09 0.09 0.09 0.2 0.15 0.16 Br on Solvent, wt. % 0.18 0.18 0.18 0.3 0.18 0.13 Solvent/p-xylene, wt. ratio 7.1 7.1 7.1 4.9 4.9 4.9 Operating temp., C. 205 205 205 221 218 218 Operating gauge pres., kg/cm2 19.7 19.7 19.7 19.7 18.3 18.3 O2 in exhaust, vol. %* 7-14 9-13 9-13 7-12 8-13 10-12 Total reaction time, min. 90 90 90 60 60 60 Total reaction Effluent: 4-CBA, ppm. 680 670 480 1900 1400 3100 p-Toluic acid, wt. % 0.14 0.063 0.058 0.20 0.093 0.47 p-Xylene, wt. % 0.055 0.065 0.086 ND 0.005 0.056 Terephthalic acid, wt. % 23.4 16.5 19.3 26.9 30.5 29.3 Terephthalic acid Yield, Mol. % 97.9 97.0 98.0 96.8 95.8 94.0 Filter cake: 4-CBA, ppm. 1000 690 770 1600 1400 3000 *Low-High.

The following p-xylene oxidations illustrate continuous conduct of the present inventive oxidation process. These continuous oxidations are conducted in the same type of apparatus elements as described for the oxidations of the preceding fourteen Examples with the exception that the apparatus elements are larger. This is reflected in the greater amounts of p-xylene fed per hour. The continuous oxidations are started in the same manner as the semi-continuous and operated with p-xylene pumping until the weight ratio of solvent (initially charged) to pxylene is reached. Thereafter a solvent solution of the components of catalysis is also pumped into the reaction zone at a rate to maintain such solvent/xylene ratio and TRE is withdrawn at the rate to provide the residence (hold) time, shown in Table III.

TABLE III p-XYLENE OXIDATION IN BENZOIC ACID-WATER SOLVENT Example Comparative I and II 15 Materials and Conditions Xylene Pump Rate, g/hr. 1510 1420 1050 Solvent water, wt. % 15 8 12 Co on Solvent, wt. % 0 0 0.03 Mn on Solvent, wt. % 0.20 0.20 0.09 Br on Solvent, wt. % 0.30 0.30 0.18 Solvent/p-xylene, wt. ratio 3.0 3.0 4.0 Operating temp., QC. 217 217-226 227 Operating gauge pres., Eg/cm2 22.1 15.5 25.0 2 in exhaust, vol. % 6.9 5.5 5.5 Residence time, min. 45 45 50 Mole CO2/mole xylene 0.29 0.72 0.79 Total reaction effluent: 4-CBA, ppm. 15800 7400 630 p-Toluic acid, wt. % 6.55 2.65 0.037 p-Xylene, wt. % 0.086 0.0005 ND Terephthalic acid, wt. % 8.48 18.0 22.7 Terephthalic acid yield, Mol. % 48.8 79.3 93.8 Washed filter cake: 4-CBA, wt. % 1.49 0.71 0.053 In the conduct of p-xylene oxidation according to Comparative Example I, byproduct water was not removed. That is, the reflux condenser is operated at a temperature to return all water and benzoic acid condensate to the oxidation zone.

This would permit the benzoic acid-water solvent system to increase in water concentration from 10% to 18% by weight at steady state operation. Such 18% water content is above the 15% upper limit permitting an acceptable rate of oxidation. Also 225 grams of p-xylene is added to the initial charge of solvent system containing the components of catalysis and p-xylene pumping is delayed until the time (calculated) for conversion of the initially charged p-xylene by the introduction of air. Under these modified operating conditions, temperature in the stirred oxidation zone could not be controlled at the planned 2l70C, but rather the oxidation zone temperature cycled considerably above the below said temperature.

Also the production of oxides of carbon and oxygen content of exhaust gas fluctuated substantially which indicates not only poor control of temperature of reaction but indicates conditions of too high and too low oxygen concentration in the oxidation zone even through the air input was constant. The sum of effects adverse to control of reactivity and constant temperature can cause build-up of aromatic co- and by-products to concentrations which significantly reduce the desired rate of oxidation to TA.

For the oxidation of Comparative Examples II, the temperature of the reflux condenser was increased to permit removal of by-product water in a gas-vapor mixture at a temperature of 121"C and maintain a 10% water concentration in the solvent system. But p-xylene (908 grams) is again precharged. However, the temperature in the oxidation zone again cycles, a constant temperature of 218"C could not be maintained and the oxygen consumption, in general, is low. The oxidation zone temperature reached a maximum of 226.5"C and at this point the oxygen consumption increases sharply as indicated by an attendant 75% drop in oxygen content of exhaust gas. Also there was an accumulation of p-xylene condensate in the cold traps preceding the exhaust gas sampling and analyzing apparatus. Thus a substantial amount of p-xylene was vaporized as it entered the oxidation zone maintained at 14.06 kg/cm2 gauge pressure and did not oxidize.

Close control of reaction temperature was not possible because the temperature of the reflux condenser could not be closely maintained.

For the conduct of Example 15, the temperature of operation of the reflux condenser is maintained by water heated with a regulatable flow of steam added to the hot water input to the condenser. In this way, a close control of the reflux condenser's temperature in the range of 121 + 0.5"C is achieved. With this close control of the temperture of operation of the reflux condenser and by the use of the indicated reaction zone pressure, close control of reaction temperature at 226.5"C is accomplished, and oxygen consumption reached a good steady state.

The preparation of similarly high yields of good color quality and purity isophthalic acid in the 8595% benzoic acid and 155% water system can be obtained by substituting m-xylene for p-xylene in the foregoing illustrative Examples.

Claims (17)

WHAT WE CLAIM IS:

1. A process of preparing iso- or terephthalic acid by the continuous or semicontinuous liquid phase molecular oxygen oxidation of m- or p-xylene, in which the oxidation is conducted in a stirred oxidation zone in the presence of catalyst components comprising a source of bromine and one or more transition metal oxidation catalysts in a solvent system comprising 85 to 97 weight per cent liquid benzoic acid and 15 to 3 weight percent water and at an oxidation zone gauge pressure within the range of 6 to 25 kg/cm2, the temperature being maintained substantially constant within the temperature range of 175 toi 2350C by removing by-product water as vapor and controlling the amount of water returned to the reactor.

2. A process according to Claim 1 wherein the transition metal catalyst component is selected from manganese, cobalt and cerium.

3. A process according to Claim I wherein the transition metal catalyst component comprises manganese or a combination of manganese with one or both of cobalt and cerium.

4. A process according to any preceding claim wherein the amount of water returned to the reactor is controlled so as to limit fluctuations of oxidation zone temperature to l50C.

5. A process according to any preceding claim in which the weight ratio of the solvent system to the xylene is from 2 to 10:1.0.

6. A process according to any preceding claim in which the catalyst components provide from 0.2 to 1.5 weight percent total metal based on the xylene.

7. A process according to any preceding claim in which the catalyst components provide from 0.2 to 1.5 weight percent total bromine based on the xylene

8. A process according to any preceding claim in which the weight ratio of bromine to total metal is in the range from 0.5 to 2.5.

9. A process according to any preceding claim in which the ratio of air to xylene fed to the oxidation zone provides an exhaust gas containing 3 to 10 volume percent oxygen.

10. A process of preparing iso- or terephthalic acid by the liquid phase oxidation of m- or p-xylene with air in an oxidation zone at an elevated temperature in the presence of a monocarboxylic acid solution of catalysis components comprising a source of bromine and one or more transition metal oxidation catalyst maintained as a liquid at said temperature by elevated oxidation zone pressure, characterized by conducting the air oxidation of m- or p-xylene in a semi-continuous or continuous manner in a stirred oxidation zone containing said catalysis wherein the transition metal component is manganese or a combination of manganese with one or both of cobalt and cerium in a solvent system consisting essentially of liquid benzoic acid and water and at an oxidation zone temperature maintained substantially constant at a selected temperature within the temperature range of 175 to 2350C by maintaining (a) the solvent system components within the range of 85 to 97 weight percent benzoic acid and 15 to 3 weight percent water, (b) the oxidation zone gauge pressure within the range of 6 to 254 kg/cm2, and (c) the removal of by-product water as vapor by cooling the exhaust from the oxidation zone to condense benzoic acid from reflux thereto and varying the amount of water returned to limit fluctuation of oxidation zone temperature to +50C from the selected temperature wherein in said oxidation zone the weight ratio of such solvent system to said xylene is in the range of 2 to 10:1.0, said components of catalysis in the solvent system are present in the amounts of 0.2 to 1.5 weight percent total metal and 0.2 to 1.5 weight percent bromine based on the xylene with a weight ratio of bromine to total metal in the range of 0.5.to 2.5 weight parts of bromine for each part by weight total metal and the ratio of air to xylene fed to said zone provide an exhaust gas therefrom containing 3 to 10 volume percent oxygen.

11. A process according to any preceding claim wherein p-xylene is oxidized with air and the transition metal catalyst component is provided by manganese and cobalt in the Mn/Co weight ratio of from 1:1 to 6:1 and the sum of the weights of Mn and Co are from 0.5 to 1.5 weight percent of the weight of the p-xylene.

12. A process according to Claim 11 wherein the solvent system consists essentially of 90% benzoic acid and 10% water, the weight ratio of the solvent system to p-xylene is 2 to 7:1, the catalyst components are in the concentrations of 0.015 to 0.1% cobalt, 0.08 to 0.2% manganese and 0.02 to 0.3% bromine based on said solvent, the oxidation zone temperature is from 205 to 226.50C, the oxidation zone gauge pressure is from 14-24.6 kg/cm2, and the oxygen content of the exhaust gas is from 6-10 volume percent.

13. A process according to any of Claims 1 to 10 wherein p-xylene is oxidized with air, the solvent system consists essentially of 90% benzoic acid and 10% water, the weight ratio of solvent system to p-xylene is 3-5:1, and the transition metal catalyst component is manganese and the ratio of bromine to manganese is from 0.8 to 1.5:1.0 and the manganese concentration is from 0.15 to 0.2 weight percent of solvent.

14. A process according to any preceding claim wherein the operating temperature fluctuation is limited to not more than +5"C from a selected value by varying the rate of water condensate returned to the oxidation zone.

15. A process for producing iso- or terephthalic acid according to Claim 1 and substantially as hereinbefore described.

16. A process for producing iso- or terephthalic acid which is substantially as described herein as a specific embodiment of the process defined in Claim 1.

17. Iso- or terephthalic acid whenever produced by a method according to any preceding claim.