CA1062279A - Secondary oxidation of fluid effluent from primary oxidation of meta-or para-xylene - Google Patents

Abstract

SECONDARY OXIDATION OF FLUID EFFLUENT
FROM PRIMARY OXIDATION OF META- OR
PARA-XYLENE
ABSTRACT OF THE DISCLOSURE
Primary catalytic oxidation of m- or p-xylene with air in the presence of acetic acid solution of metal and bromine com-ponents of catalysis at temperatures from 130 to 250°C and at elevated pressure to maintain a liquid phase of the acetic acid solution produces a fluid oxidation effluent which is a suspen-sion of crystalline iso- or terephthalic acid in liquid acetic acid mother liquor containing, in addition to dissolved components of catalysis, some dissolved phthalic acid product and smaller amounts of both oxygen-containing precursors of such product and oxygen-containing aromatic co-products.
By the present inventive concept such fluid effluent is subjected to continuous secondary air oxidation under liquid phase conditions as it is being cooled to precipitate additional phthal-ic acid product. Such secondary oxidation permits lowering the concentration of catalyst components in the primary oxidation, improves yield and quality of the phthalic acid product, signifi-cantly lowers acetic acid oxidation, and permits recycle to the primary oxidation of a major portion of solids content of acetic acid mother liquor as a source of components of catalysis.

Description

BACKGROUND OF THE INVENTION
United States Patent No. 3,064,044 provided the first dis-closure of improving the quality of terephthalic acid product by the sequential use of a primary oxidation of p-xylene with air in a liquid phase of acetic acid solution of a metal oxidation cata-lyst and a source of bromine as components of catalysis followed by a secondary oxidation with air of the fluid effluent of such primary oxidation. Said disclosed combination of primary and .~, " >

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., : , :' lO~;Z;~79 secondary air oxidations is conducted in two series connected, stirred-oxidation zones wherein p-xylene and acetic acid solution of components of catalysis and air are continuously charged to the first zone for the primary oxidation and fluid oxidation effluent of the primary oxidation, air and substantially anhydrous acetic acid (2-5% water) is charged to the second zone for the secondary oxidation. The primary oxidation is conducted at a temperature in the range of 150 to 205C and the secondary oxida-tion is conducted at a higher temperature range of from 185 to 225C. The exhaust gas containing water and acetic acid vapor mixture from the secondary oxidation is charged back to the pri-mary oxidation as a means for maintaining the liquid phase in the secondary oxidation substantially anhydrous. Exhaust gas-vapor mixture from the primary oxidation is cooled to condense water and acetic acid and the condensate is fractionated to recover sub-stantially anhydrous aeetic acid for reuse. The residence times in the primary oxidation and secondary oxidation are substantially -~ equal, preferably 60 minutes in each oxidation. The fluid effluent from the secondary oxidation is a suspension of crystalline tere-phthalic acid in acetic acid mother liquid containing some dis-solved terephthalic acid.
Such fluid effluent from secondary oxidation is, aecording to the patent, cooled in two or more stages to crystallize addi-tional terephthalic acid~ Crystalline terephthalic acid is separated from the cooled acetic acid mother liquor. The patent indicates that the substantially anhydrous acetic acid mother liquor can be recycled to the primary oxidation zone to provide its acetic acid solvent and catalysis requirements.
The foregoing eoneept for the use of a eombination of pri-mary and seeondary oxidations has the undesirable effect of morethan doubling the burning of aeetie aeid solvent which would oecur in a single oxidation zone because of the higher temperature of :

, 106i2279 operation in the secondary oxidation using substantially anhydrous acetic acid solvent and using in the secondary oxidation concen-trations of catalyst components suitable for the lower tempera-ture operation in the primary oxidation zone but unsuitable for the higher temperature oxidation. The increase in acetic acid burning is due to the increase of secondary oxidation operating temperature, metal oxidation catalyst concentration, residence time, and decrease in water content of acetic acid solvent.
In general, catalytic activity is known to vary directly with temperature and/or catalyst concentration. Thus for equi-valent catalytic activity at two different liquid phase oxidation operating temperatures, the catalyst metal concentrations should be varied inversely with temperature. However, in the foregoing combination of primary oxidation and secondary oxidation wherein the latter is operated at a higher temperature with the metal catalyst concentration more suited to the lower operating tempera-ture of the primary oxidation, the catalyst concentration in the secondary oxidation is inherently too high. Accordingly, at the temperature and catalyst concentration conditions of secondary oxidation the acetic acid burning is inherently greater than in the primary oxidation. Such greater acetic acid burning could be avoided only by removing catalyst metal from the fluid effluent going to secondary oxidation. But such removal of catalyst metal is, as a practical matter, not feasible without dilution of the effluent by added acetic acid and enlarging the secondary re-action vessel.
United States Patent No. 3,859,344, issued 7 January 1975 also teaches a combination of primary air oxidation of p-xylene in the presence of acetic acid solution of metal oxidation catalyst and bromine as components of catalysis and secondary air oxidation of fluid effluent from the primary oxidation. The primary oxida-lO~;Z279 tion is conducted continuously at a temperature in the range from80 to 250C, preferably from 130 to 200C, at a gauge pressure of from 2 to 30 kg/cm2 to maintain liquid phase conditions and at a residence time of from 0.5 to 5, preferably 1 to 3 hours. How-ever, to overcome the drawbacks of secondary oxidation of Patent No. 3,064,044, the secondary oxidation of the later patent is conducted intermittently and batchwise in the first of two or more, preferably thru series connected, stirred crystallization zones for a reaction time of 1 to 5 minutes at a reaction tempera-ture equal to or below primary oxidation temperature. This isaccomplished, according to said 1975 patent, by intermittently, every 2-10 minutes, withdrawing fluid oxidation effluent from the primary oxidation and charging the withdrawn effluent to the first stirred vessel which has a fluid operating volume of from 30% to 70%, preferably 50-70%, of the total volume of such first vessel.
Such first vessel has a total volume of about one-eighth that ordinarily used for continuous oxidation.
Following such intermittent and batchwise secondary oxida-tion of the effluent, preferably 70-80% thereof, is withdrawn and charged to the second and, if any, succeeding stirred crystalliza-tion zones. The temperature profile over the two or more crystal-lization zones is, of course, that of decreasing temperature from the first to last zone.
The main objective of such combination of continuous primary oxidation and intermittent batchwise secondary oxidation was, according to said patent, to obtain a terephthalic acid product of .
higher quality than would be obtained by continuous primary oxi-dation alone~ The example illustrating such combination of con-tinuous primary oxidation and intermittent, batchwise secondary oxidation establishes that such objective was met.

lO~;Z;~79 ~ oweveI~, said patent s-tates that terephthalic acid product of unavoidably higher color would result from continuous secondary oxidation conducted with fluid effluent from the primary oxidation in a zone conventionally sized -to receive the volume of continu-ously withdrawn primary oxidation fluid effluent and such secon-dary oxidation would likely produce a gaseous exhaust having an oxygen content in the explosive range.
Such combination of continuous primary oxidation and inter-mittent, batchwise secondary oxida-tion has immediately recognizable technical disadvantages. One disadvantage is that it defeats, *o a substantial extent, the reported (Patent No. 3,064,044) advan-tages of the combined continuous primary and secondary oxidations in two series connected oxidation zones. A second recognizable disadvantage comes from smooth through flow of the repeated volume decrease-increase cycle in the primary oxidation zone by the intermittent withdrawal therefrom which can effect cycling air demand therein and heat removal therefrom.
There are other disadvantages inherent to such combination of continuous primary oxidation and intermittent batchwise secon-dary oxidation not immediately recognizable. For the success ofsuch combination of primary and secondary oxidations there must be used in the primary oxidation a relatively high cobalt metal con-centration based on p-xylene, for example, 21.6 mg atom Co per gram made p-xylene, a very high gram atom ratio of cobalt to manganese of 20:1.0 and high gram atom ratio of bromine to total metals of 1.36 to 1Ø Each of such 20:1.0 gram atom ratio of cobalt to manganese, high Br to total metal, or high cobalt to p-xylene gram atom per gram mole ratio of 21.6:1.0 can cause excessive burning of acetic acid. But when all three are used in 39 combination, we have found, the total acetic acid burned amounts to 140-160 grams per kilogram of terephthalic acid produced. The 10t;2Z79 value of acetic acid burned is an operating cost added to the cost of producing terephthalic acid product.
Another charge to the cost of producing terephthalic acid product comes from the high, 21.6 milligram atom of cobalt per gram mole of p-xylene, which is also 21.6 milligram atoms of cobalt per gram mole of terephthalic acid produced, or 7O68 grams cobalt per kilogram of terephthalic acid.
The present inventive combination of continuous primary air oxidation of m- or p-xylene and continuous secondary oxidation of the fluid effluent from primary oxidation conducted in one or more of the subsequent series connected crystallization zones operated at successively lower temperatures and pressures avoids the dis-advantages of the prior combinations of primary and secondary oxidations which were: excessive acetic acid burning, operation of the secondary oxidation zone with an oxygen concentration in the explosive range, use of the unconventionally small sized first crystallization zone for secondary oxidation, continuous operation interrupted by intermittent withdrawal of fluid effluent from primary oxidation, intermittently batchwise conducted secondary oxidation, and high cobalt usage. Also the present inventive com-bination of continuous primary and secondary oxidations provides the surprising beneficial technical effects of using lower con-centrations of cobalt, manganese and bromine in the primary oxi-dation and recycle of a major portion of acetic acid mother liquor or its solids content to the primary oxidation. The continuous operation of the secondary oxidation provides the further benefit of partial digestion of the originally suspended crystalline phthalic acid product by the acetic acid mother liquor to remove from such suspended solids a substantial portion of the precursors occluded therein. The secondary oxidation does, as expected, con-vert the originally dissolved precursors, m- or p-toluic acid and lQ~27~

m- or p-formylbenzoic acid, by oxidation to iso- or therephthalic acid. Said digestion adds the originally occluded precursors back to the acetic acid mother liquor as solutes for oxidation. Thus the re- and continued growth of phthalic acid product during the dual function secondary oxidation and crystallization occurs in a mother liquor environment having a reduced solute content of said precursors. This digestive phenomena combined with secondary oxidation allows the catalyst concentration to be reduced which improve product quality and lower acetic acid burning.
Thus the present inventive concept of combining continuous primary oxidation with continuous secondary oxidation in one or more of the crystallization zones will increase yields of iso- or terephthalic acid product from the same initial quantity of m- or p-xylene fed to the primary oxidation, decrease acetic acid con-sumption by burning, decrease catalyst metal use per use per kilo-gram of the phthalic acid product, and reduce the concentration of intermediates in the solid phthalic acid product, that is provide a phthalic acid product of substantially improved purity.
Summary of the Invention Briefly stated, the present inventive concept is applied to the continuous production of iso- or terephthalic acid by the process steps and apparatus disclosed and illustrated by either of United States Patents Nos. 2,962,361 or 3,092,658 or 3,170,768, which, respectively, use three, two and one stirred crystalliza-tion zones following the primary oxidation zone to which is fed the xylene to be oxidized, air and acetic acid solution of cobalt, manganese and a source of bromine. The continuous secondary oxi-dation is conducted by injection of air into one or more but at least the first, of said crystallization zones at their conventional operating temperature, pressure and volume loading by such air injection rate that the gaseous exhaust from any crystallization 10~ 79 zone does not contain or form an oxyyen to acetic acid gas vapor mixture in the explosive range, that is, an exhaust gas containing 8 to 10 or more volume percent oxygen on acetic acid and water free basis. While the oxidation can be conducted with a high boil-up of acetic acid to remove heat of reaction and provide, with respect to unused oxygen an acetic acid vapor rich gaseous exhaust which is not in the flammable or explosive range, such flammable or explosive range mixture can form as acetic acid vapor is con-densed as first step for recovery of acetic acid. Air injection into both primary and secondary oxidation is controlled so that the oxygen content of the gaseous exhaust therefrom is within the range of from 1 to 8, volume percent oxygen on an acetic acid, water free basis to avoid an exhaust containing or forming a mix-ture in the flammable or explosive range. Preferably the primary oxidation is conducted with an air input to provide a gaseous ~ -exhaust having an oxygen content (acetic acid, water free basis) of 2-6 volume percent and the secondary oxidation is operated with an air input to provide a gaseous exhaust having (same basis) from

2 to 6 volume percent.
The conventional operating volume loading of each of the series connected stirred-crystallization zones is in the volume ratio range of crystallization zone volume to oxidation zone fluid effluent operating volume of from about 0.5 to 2.0:1Ø
The primary air oxidation of m- or p-xylene is conducted - continuously in a single stirred oxidation zone operated at a temperature in the range of from 170 to 225C., preferably from 190 to 210C., and at a gauge pressure within the range of 10 to ., , 35 kg/cm . The feed to such oxidation zone comprises the xylene and an acetic acid solution containing dissolved cobalt and manga-nese, generally as their acetic acid soluble salts such as their acetates and a dissolved bromine-containing compound. The ~eight 10~;~279 ratio of acetic acid solution to m- or p-xylene is in the range from 2 to 6:1.0 ancl preferably 2.3 to 3.5:1Ø The acetic acid solution contalns cobalt (calculated as the metal) in the ranye from 0.8 -to 1.75 milligram atom (mga) per gram mole of the xylene, manganese (calculated as the metal) in the gram atom ratio of cobalt to manganese in the range of 0.125 to 1.0:1.0, and bromine (calculated as the ion) in the gram atom ratio of Br: total of Co and Mn in the range of from 0.5 to 1.5:1.0 to minimize acetic acid burning and not sacrifice rate of xylene oxidation.
Each of cobalt and manganese can be provided in any of its known acetic acid soluble ionic or combined forms, for example, carbonate acetate tetrahydrate, and/or bromide. Because of the 0.5 to 1.5:1.0 gram atom ratio of Br to total Co and Mn and the fact that their bromides each have a Br to metal gram atom ratio of 2:1, the catalysis is not provided by use of bromides of both Co and Mn. Rather the catalysis can be provided by appropriate ratios of the bromide salts and acetic aeid soluble forms contain-ing no bromine, e.g., aeetates. As a praetieal matter the 0.125 to 1.0:1.0 Co to Mn ratio is provided by use of their aeetie aeid soluble forms other than bromides, e.g., both as aeetate tetra-hydrates, and the 0.5 to 1.0:1.0 Br to total metal gram atom ratio is provided by a souree of bromine. Sueh bromine sourees inelude elemental bromine (Br2), or ionie bromine (e.g., HBr, Na or KBr, NH3Br, ete.), or organie bromides which are known to provide bromide ions at the operating temperature of the primary oxidation (e.g., bromobenzenes, benzylbromide, mono- and di-bromoacetic acid, bromoaeetyl bromide, tetrabromoethane, ethylenedibromide, etc.).
The total bromine in Br2 and the ionic bromide is used to deter-mine satisfaction of the Br to total Co and Mn gram atom ratio of 0.5-1.5:1Ø The bromide ion released from the organic bromides ~ _ g _ 10~;2Z79 at primary oxidation operating conditions can be readily deter-mined by known analytical means. Tetrabromoethane, for example, at operating temperatures such as 170 to 225C has been found to yield about 3 effective gram atoms of bromine per gram mole.
The air fed to the primary oxidation zone should provide an exhaust gas-vapor mixture containing ~measured on acid, water free basis) of from 2 to 5 volume percent oxygen. Air volume feed rate (calculated at 25C and 760 mm Hg pressure) per kilogram of xylene fed to the primary oxidation in the range of .25 to 1.0 normal liters (nl) air/kg xylene will provide such 2-8 vol.~ 2 content exhaust gas.
The secondary oxidation is conducted in one or more, at least the first, of the series connected, two or more stirred crystallization zones receiving primary oxidation fluid effluent or a cooled concentrate thereof at the conventional design operating conditions for said crystallization zones. The first stirred crystallization zones is sized for normal continuous crystallization operation to have an operating volume in the range - of from 0.5 to 2.0 times the volume of the fluid effluent from the primary - 9(a) -lO~Z;~79 oxidation. For example, the operating volume of the first stirred crystallization zone is 0.5 to 2.0 times the operating volume of one primary oxidation zone or times the sum of the operating volumes of two or more primary oxidation zones operating in parallel and simultaneously providing continuous fluid effluent feed to the first oxidation zone. The operating volumes of the second or other stirred (two or more) crystallization zones subsequent to the first are usually, but not necessarily, the same as the operating volume of the first stirred crystallization zone. The two or more series connected stirred crystallization zones are substantially equal in operating volume to accommodate the expansion of their fluid contents by acetic acid boiling as their operating pressures are sequentially decreased to provide cooling by acetic acid vaporization at the lower pressures.
In one aspect of this invention there is provided a continuous production and recovery of iso- or terephthalic acid by the continuous steps of (A) introducing of air, m- or p-xylene and an acetic acid solution containing as components of catalysis a source of dissolved cobalt, manganese and bromine 2U into a single, stirred primary oxidation zone operated at a temperature of from 170 to 225C and a gauge pressure of 10 to 35 kg/cm to provide therein a weight ratio of 2-6:1.0 of such acetic acid solution to said xylene; from 0.8 to 1.75 milligram atoms of cobalt per gram mole of the xylene, manganese in the gram atom ratio of cobalt to manganese in the range of from 0.125 - to 1.0:1.0, and from 0.5 to 1.5 gram atom of bromine per gram atom total of cobalt and manganese; and an air to xylene ratio to provide a gaseous exhaust from the oxidation zone containing from 2 to 7 volume percent oxygen on an acetic acid and water free basis; (~) withdrawing from the oxidation zone fluid 'X! - 10 -j ~i ,~. ~

~Of~Z'~79 effluent comprising a suspension of crystalline iso- or terephthalic acid product in acetic acid mother liquor containing dissolved iso- or terephthalic acid, precursors thereof and reaction co-products; tC) cooling and depressuring said fluid effluent step-wise in two or more series connected stirred zones to a first temperature of from 150 to 200C and a gauge pressure of from 3 to 15 kg/cm and to a final temperature of from 60 to 120C and gauge pressure of from 0 to 2 kg/cm2 in the last series connected zone, wherein at least the first stirred cooling and depressuring zone has a fluid operating volume of from 0.5 to 2.0 times the volume of fluid effluent withdrawn from the oxida-tion zone; and (D) separating iso- or terephthalic acid product from the suspension in acetic acid mother liquor withdrawn from the last stirred cooling and depressuring zone. The yield of iso- or terephthalic acid product is increased and the iso- or terephthalic acid precursor content of both the acetic acid mother liquor and the iso- or terephthalic acid product is decreased by continuously introducing air into at least the first of the stirred cooling and depressuring zones to provide an exhaust therefrom containing oxygen on an acetic acid and water free basis of 1 up to 8 volume percent as the fluid effluent flows through said first zone.
The following operating pressures and resulting temperatures in such series connected, stirred crystallization zones will illustrate their normal operation associated with fluid effluents produced by primary oxidation. Operations I and II illustrate broad ranges for such combined operations with, respectively two and three series connected stirred crystalliza-tion zones. Operation III illustrates a typical combined operation of a typical primary oxidation of p-xylene and typical - 10ta) -.

10f~2279 operation of each of three series connected, stirred crystalliza-tion zones.
TABLE I
Primary Oxidation Operation I Operation II Operation III
temperature, C2 163 to 246 170 to 205 220C
pressure, kg/cm gauge 3.5 to 35 11.3 to 28.1 28 First crystallization pressure, kg/cm2 gauge 3.5 to 15 4.2 - 6.3 9.8. temperature, C. 150 to 200 150 - 180 188 10 Second Crystallization pressure, kg/cm2 gauge -0.5 to 2 0.1 - 5.0 2.8 temperature, C. 65 to 120 107 to 130 165 Third crystallization:
pressure, kg/cm2 gauge - -0.5 to 2 0 (ambient) temperature, C. - 60 - 110 107 .

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For the conduct of the continuous secondary oxidation of the present inventive combination of continuous primary and secondary oxidations, such secondary oxidation is conducted in the first stirred crystallization zone at a temperature in the range from 150 to 210C and a pressure in the ranye from 3.5 to 15 kg/cm2 with an air feed to provide from 1 to 8 volume percent oxygen (acetic acid and water free basis) in the exhaust gas-vapor mix-ture therefrom. When additional beneEits are desired from diges-tion of iso or terephthalic acid crystalline product and diminu-tion of product precursors concentration in acetic acid motherliquor, secondary oxidation can be further conducted in the second (of two or three) stirred crystallization zones. Very little di-gestion will occur in a third stirred crystallization zone because it is operated at a temperature (65-110C) where no significant amount of the iso- or terephthalic acid product dissolves. How-ever, secondary oxidation of product precursors dissolved in mother liquor can occur in the third stirred crystallization zone ; thereby reducing product precursor content in the acetic acid mother liquor, increasing crystalline acid product yield and improving the acetic acid mother liquor quality for recycle to the primary oxidation.
The Drawing The accompanying drawing is a flow sheet illustration of the present inventive combination of continuous primary and secon-dary oxidations suitable for the continuous manufacture of iso-phthalic acid (IA) or terephthalic acid (TA) from m- or p-xylene.
In said flow sheet there are illustrated the four chemical engineering unit processes comprising continuous catalytic liquid phase primary oxidation conducted in stirred-tank type oxidation vessel 10; continuous crystallization of iso- or terephthalic acid product conducted in three stages wherein each thereof of 10~i2;~9 crystallization vessels 20, 30 and 40 cooling is provided by evaporation of acetic acid and water at pressures successively lower than the pressure in the primary oxidation: crystalline IA or TA product is recovered from acetic acid mother liquor and washed with acetic acid in a solid-liquid separator shown as centrifuge 50: and acetic acid mother liquor from solid-liquid separation is stripped of all its water and 70-95% of its acetic acid content in mother liquor stripper 70.
Heavy weight lines are provided in the drawing of said flow sheet illustration to facilitate following of essential feeds into and the flow of product through and out of the depicted process.
The present inventive secondary oxidation is shown in the drawing as being conducted in the first stage of crystallization, a preferred embodiment, by the introduction of compressed air through valved air lines 131 and 132 whose valves are open. The valves in valved air lines 131a, 133 and 134 are closed. Valved air supply lines 131a, 133 and 134 are shown for the purpose of illustrating additional conduct of secondary oxidation in the ; second crystallizer 30 and/or third crystallizer 40 when such ; 20 operation is needed or desired.
The continuous unit processes shown in the drawing are here-after described in their steady-state operation rather than first ~; describing the start-up of each process unit which can be accom-plished in manners well understood by the chemical engineer ; experienced in such unit process operations.
- The primary oxidation process is conducted in stirred-tank type oxidation vessel 10 having a stirred oxidation zone into which via liquid charge line 19 there are continuously charged xylene (m- or p- isomer) from xylene feed line 11 and acetic acid solution of catalyst components from acetic acid solution line 12 through liquid charge line and compressed air from source 13 via valved iZ~79 conduit 13a. Said oxidation vessel has associated therewith Eor removal of heat of reaction reflux condenser 15 into which there is charged gaseous exhaust from the vapor space of said oxidation vessel 10 and from which uncondensed gas flows via vent line 16 and pressure control valve 16a and acetic acid-water condensate of high water content flows via reflux line 17 into the stirred re-action zone. Acetic acid and water vapors are condensed by indirect heat exchange between said vapors and heat exchange fluid entering reflux condenser 15 by line 15a and exiting via line 15b.
Fluid oxidation effluent flows from the oxidation zone in said oxidation vessel 10 via slurry transfer conduit 18 and flow -~ control valve 18a into the stirred slurry of iso- or terephthalic acid in stirred crystallization vessel 20. The fluid oxidation effluent comprises a suspension of iso- or terephthalic acid crystals in acetic acid solution of water (byproduct of oxidation), catalyst components, iso- or terephthalic acid, and oxygen-contain-ing aromatic compounds which are co- and by-products of the oxidation of xylene feed.
A portion of dissolved iso- or terephthalic acid product is crystallized from solution in crystallization vessel 20 operated at a pressure below that of the primary oxidation but well above atmospheric pressure. The outer portion of the crystalline pro-duct is removed by digestion and serves as a site for crystal growth as said dissolved product comes out of solution. Compres-sed gas is introduced from source 13 via open valved conduits 131 and 132 into the stirred slurry in crystallization vessel 20 to provide the secondary oxidation. Acetic acid and water vapors generated by flash evaporation to the lower pressure and by heat of reaction in crystallization vessel 20 and gases (2' N2, CO2 and CO) are charged via exhaust conduit 21 to reflux condenser 22 cooled by indirect heat exchange with heat exchange fluid entering ':

106;2279 by line 22a and leavin{T by line 22b. From reflux condenser 22 uncondensed gases are discharged via line 26 and pressure control valve 27 and condensate is discharged by line 23 either entirely into crystallization vessel 20 or, if desired, in part thereto with the remainder being withdrawn through valved conduit 23 for fractional distillation to remove water and to concentrate the acetic acid to a water content of 3~5 weight percent as part of the solvent for dissolving catalyst components to provide the acetic acid solution thereof for the primary oxidation.
The slurry produced in stirred crystallization vessel 20 flows therefrom to stirred crystallization vessel 30 via slurry transfer 25 and flow control valve 25a into the stirred slurry contained in said crystallization vessel 30 operated at a pressure between the pressure in crystallization vessel 20 and atmospheric pressure. The pressure drop between operating pressures of crystallization vessels 20 and 30 causes flash evaporation of water and acetic acid thereby causing cooling, and optionally con-centration, of the slurry from crystallization vessel 20 and further precipitation of product acid. The water-acetic acid vapor mixture generated exhausts via vapor exhaust line 31 to re-flux condenser 32 cooled by indirect heat exchange with heat exchange fluid entering via line 32a and leaving via line 32b. The resulting water-acetic acid condensate flows frorn reflux condenser 32 wholly or in part to crystallization vessel 30 via condensate reflux 33 from which, if desired, the non-recycled condensate is charged to the before mentioned acetic acid concentration step to provide the 95-98% acetic acid solvent for catalyst component dis-solution and feed to primary oxidation.
The slurry produced in stirred crystallization vessel 30 flows via slurry transfer 35 and flow control valve 35a into the slurry in stirred crystallization vessel 40 operated at atmospheric - . . ..

lO~Z~79 pressure. ~inal crystallization of iso- or terephthalic acid is accomplished by flash evaporation of acetic acid and water at the lower pressure. The water-acetic acid vapor mixture flows from said crystallization vessel 40 via vapor exhaust 41 to reflux con-denser 42 cooled by indirect heat exchange with cooling fluid entering via line 42a and leaving via line 42b. The condensate thus produced flows into said crystallization vessel 40 via reflux line 43 or, if desired, a part of the condensate can be removed by valved line 44 for concentration of acetic acid and recycle as catalyst solvent to primary oxidation as before described.
The slurry of iso- or terephthalic acid produced in stirred crystallization vessel 40 is withdrawn therefrom via slurry trans-fer conduit 45 by suction side of a suitable slurry pump 46 which discharges said slurry into a means for solid-liquid separation such as centrifuge 50 shown in the drawing for separating acetic acid mother liquor from crystalline iso- or terephthalic acid product. Any solid-liquid separation means such as a filter press can be used in place of centrifuge 50. Wet iso- or terephthalic acid product is discharged from centrifuge 50 through solids dis-charge 54 after being washed by acetic acid entering centrifuge 50by valved wash line 57. The separated acetic acid mother liquor is discharged from centrifuge 50 via valved conduit 53 to the suction side of pump 54 to valved conduit 56 to collection drum 60. A portion, 50 to 90~ of the mother liquor, if desired, can be withdrawn by valved conduit 57 and sent (means not shown) to pri-mary oxidation to provide part of acetic acid solvent, metal cata-; lyst components and bromine in the primary oxidation by entering stirred-tank type oxidation vessel 10 via line 12 and charge line 19. Acetic acid wash liquor flows from centrifuge 50 via valved line 52 also to dissolve catalyst to enter said oxidation vessel via line 12 and charge line 19.

106ZZ7~
Acetic acid mother liquor in collection drum 60 is withdrawn therefrom via line 61 by the suction side of pump 62 and discharged therefrom into line 63 as feed to anchor stirred mother liquor stripper 70 wherein water and acetic acid are eva-porated, (e.g., at an operating temperature of 190C and pressure of 0.7 kg/cm2) by heat supplied by reboiler 75 which heats the bottoms fraction flowing from lines 73 and 74 thereto and returned via live 76 to said stripper 70. All of the residue from said stripper 70 flows through valved conduit 77 to the suction side of residue pump 80 and is discharged into residue transfer line 81 - from which a 10-90% purge is withdrawn by valved conduit 82 for -discard, preferably after extraction of catalyst therefrom by known means. The 90-10% of the residue not discarded can be charged to the primary oxidation conducted in stirred-tank type oxidation vessel 10 via lines 81 and 84 through charging line 19.
From the foregoing, any chemical engineer skilled in the design and/or operation of such catalytic liquid phase oxida-tion processes together with details of secondary oxidation and ~ the results thereof hereinafter presented can suitably modify any ' '~ 20 process design such as the process design shown in the accompanying ` drawing or the process designs illustrated in the drawings of the ~ United States Patents Nos. 2,962,361, 3,092,658 or 3,170,768 to ; satisfy particular plant operation requirements.
' Specific Embo'd'ime'nts o'f 'the Invention '~ Both the continuous primary and continuous secondary oxidations of this invention are conducted in stirred reaction ~' zones wherein a liquid phase of acetic acid solution of components of catalysis is maintained at the respective operating tempera-- tures. Since the primary oxidation and preferred secondary oxida-tion in the first crystallization zone are operated at temperatures ' well above the normal boiling point (118C at 760 mm Hg) temperature of acetic acid, superatmospheric pressures are used in the primary . : ~ '~ , . . .
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106iZ;279 and secolldary oxidation zones to maintain liquid phase of the acetic acid solution. The operatiny supera-tmospheric pressure ranges hereinbefore associated with the operating temperatures ranges for the present inventive combinations of primary oxidation and preferred secondary oxidation are above the minimum pressures to maintain a liquid phase of acetic acid during operation of such oxidations. Such higher than minimum operating pressures are important because they provide higher oxygen partial pressures than would otherwise be available. The higher oxygen partial pressures in turn provide higher oxygen concentrations in the liquid phase reaction site. Stirring enhances mass transfer and uniform dis-tribution of the oxygen in the liquid phase reaction medium. Supply of oxygen in excess of the stoichiometric requirement by maintaining in the exhaust gas-vapor mixture from 1 to 8 volume percent of ; oxygen (acetic acid, water free basis) in combination with the effects of higher than minimum operating pressure and stirring mini-mize incremental oxygen starvation conditions in the liquid phase reaction site. Oxygen starvation in such reaction site lead to competing reactions, such as thermal coupling of partially oxygen-ated xylenes and free radical coupling, which produce colored im-purities (e.g., compounds having the fluorenone and benzil struc-tures which are typically yellow), and fluorescing impurities (e.g., stilbenes) as well as the partially oxygenated precursors, toluic acid and formyl benzoic acid.
Operations Common to Comparative and!Illustratlue Examples Said combination of use of pressure stirring and excess oxygen are used in each of the following comparative base case examples wherein no secondary oxidation is conducted and in each of the illustrative examples wherein the combination of primary and secondary oxidations are conducted.

10f~ 79 ~ lso al] the primary oxidations are conducted under opera-ting conditions whereby the water content of acetic acid in the reaction zone is controlled by known means within the range of 10 to 14 weight percent.
Further with respect to the comparative and illustrative examples to follow, -the terephthalic acid product production from air oxidation of p-xylene was conducted in a commercial installa-tion wherein the terephthalic acid, after drying to remove adher-ing acetic acid wash liquid, is fed to a purification system designed to operate with terephthalic acid having 0.14 to 0.18 weight percent p-formylbenzoic acid content. In such purification system the p-formylbenzoic acid is catalytically reduced to mainly p-toluic acid in an aqueous medium at elevated temperature and pressure and thereafter highly pure terephthalic acid is recovered from the aqueous medium by crystallization in a quality suitable for direct reaction with ethylene glycol for polyester fiber and ; filament production. Hence the first illustrated uses of the ; present inventive combination of continuous primary and secondary oxidations were conducted, not to minimize p-formylbenzoic acid precursor production, but rather to meet the purification design conditions with respect to p-formylbenzoic acid content of tere-phthalic acid feed.
The following examples comprise the comparative Base Opera-tions - wherein no secondary oxidation is practiced and four illustrative Examples 1, 2, 3 and 4 carried Ollt in the sequence and duration shown in TABLE I.

iO~;2Z79 TABLE I
OXIDATION SEQUENCE AND DURATION
Sequen_ Duration Base Operation I 172 hours Example 1 300 hours Example 2 188 hours Base Operation II 232 hours Example 3 216 hours Example 4 164 hours Base Operation III 160 hours The Base Operations were conducted at normal operating con-ditions of the commercial installation known to provide a washed and dried terephthalic acid crystalline product of about 0.16 weight percent p-formylbenzoic acid (also known as 4-carboxyben-zaldehyde, abbreviated as 4-CBA) content. Such operation includes recycle of about 50 weight percent of the residue from acetic acid stripper 70 via valved line 73, pump 80 and line 81 to valved . charge line 19 to the primary oxidation in oxidation vessel 10.
The remaining 50% of such residue is purged via valved purge line 82.
Other conditions for the Base Operations listed in TABLE II.
TABLE II
AVERAGE CONDITIONS FOR BASE OPERATIONS
__ ~ Operating Conditions in Oxidation Vessel 10:

Temperature 225C

Pressure 26 kg/cm2 gauge Feed rate 51.1 liters/minute Feed Composition, weight percent:
28% p-xylene ::.

tO6Z279 65.8% Acetic acid of 4.3 weight percent water 0.0235% Cobalt as metal 0.0635% Manganese as metal 0.089% Bromine as ion 1.50 milligram atom cobalt per gram mole p-xylene 2.9 gram atom Mn per gram atom of Co.
0.65 gram atom Br per gram atom of total metals Air Rate to provide 1.8-2.0 volume percent 2 in exhaust Aerated liquid phase volume 70% of oxidation vessel .~ 10 Residence time of 45 minutes Operating Conditions of three crystallizers:
; Crystallizer 20 Temperature 188C
Pressure 9.8 kg/cm gauge Volume 50% of total volume Residence time 23 minutes Crystallizer 30 Temperature 165C
: Pressure 2.8 kg/cm2 Volume 43% of total volume - Crystallizer 40 Temperature 107C
Pressure 0 kgm/cm2 gauge .~ Volume 40% of total volume In the four examples illustrating the present invention the primary oxidation conducted in oxidation vessel 10, except for adjustment of components of catalysis in an attempt to maintain the same 4-CBA content in terephthalic acid product, and operation of crystallizers 30 and 40 are as shown in TABLE III. Therefore, those operating conditions which are the same are not repeated in TABLE III which also contains, for convenient reference, compar-able operating conditions for Base Operations.

lO~ZZ79 TABLE II

AVER~GE OPERATING CONDITIONS FOR ILLUSTRATIVE EXAMPLES
= . . _ . . .
Base Example No.
Oxidation Vessel 10: Operation 1 2 3 4 mg atom Co: gm mole 1.5 1.315 1.0961.059 0.953 p-xylene Br: Total metal, gram atom ratio 0.65 0.68 0.62 0.82 0.96 Operation of Crystal-lizer 20:
Temperature, C188 197 194 186 188 Pressure, kg/cm2 gauge9.8 9.8 9.8 9.8 9.8 Volume, ~ of total 40.5 40.4 69.7 40.2 68.9 Residence time, min-utes 23 23 39 23 39 O Content of exhaust 2gas, vol.~ 0 5 5 5 5 For both the Base Operations and the conduct of Examples 1-4 analyses are performed on feed mixture to oxidation vessel 10 sampled from charge line 19, the acetic acid mother liquor from centrifuge 50 sampled from valved line 57 and recovered tere-phthalic acid product sampled from centrifuge discharge 54. Such samples are taken every four hours but the particular analyses of interest are performed on the time schedules and for the deter-minations indicated in TABLE IV. Hereafter terephthalic acid will be abbreviated as "TPA". Chemical components of the analytical determinations are reported in TABLE IV in weight percent. The other values are explained following said table.

10~2~79 TABLE IV
_nalyses of _ Time Period TPA Product for 4-CBA (1) Every 4 Gardner b-value Every 4 F.I.(2) Every 4 p-Toluic Acid (3) Every 8 Optical Density (4) Every 24 Aromatic By-Products Every 8 Cobalt Every 8 Acetic Acid Mother Liquor for:
Water Every 4 Total Solids Every 4 4-CBA Every 8 p-Toluic Acid Every 8 Cobalt Every 8 Aromatic By-Products Every 8 Feed Mixture to Oxidation Vessel 10 for:
Cobalt Every 4 Bromine Every 4 p-Xylene Every 4 Acetic Acid Every 4 - Water Every 4 -(1) "Gardner b-Value" is from a TPA industry standardized measure-ment of the yellowness of TPA wherein the numerical values are associated with the yellow color intensity. Thus as the numerical b-value increases or decreases from sample to sample the yellow color intensity likewise respectively increases or decreases.
(2) "F.I." is Fluorescence Intensity and is a measure of impuri-~30 ties in TPA which imparts fluorescence not only to TPA but also to -polyester (e.g. polyethyleneterephthalate) textiles. The numerical value is not an absolute value but rather is the value of measured fluorescence emission from a standard solution of 0.4 gram TPA sample in 10 milliliters of dimethyl sulfoxide ex-posed to ultraviolet light at about 330 mm relative to the measured fluorescence emission from a standard solution of 1.0 ug quinine sulfate per milliliter of 0.1 N sulfuric acid exposed at the same time to the same ultraviolet light. An increase or decrease of the numerical value is associated with an increase or decrease, respectively, of fluorescence intensity.

(3) Optical density numerical value is the transmitted 380 nm light through a 4 cm cell containing 4 weight percent TPA dissolved in ammonium hydroxide (0.88 g NH3/1.0 ml. water) relative to 380 nm light transmitted through a 4 cm cell containing ammonium hydroxide (0.88 gm NH /1.0 ml. water). This is a qualitative measurement of impuri~y content of TPA wherein increase or decrease of total impurities is associated with an increase or decrease respectively, of total impurities.

, ;ZZ79

(4) "Aromatic Impurities" is the sum of the weight percent of each of (A) benzoic acid, Methyl-substituted phthalic acids and tri-milletic acid determined every eiyht hours and (B) 18 other aromatic oxygen-containing compounds higher boiling (separation determined by gas chromatography on esterified sarnple TPA product or mother liquor solids) than trlmellitic acid (the ester of this acid since it dehydrates to an anhydride before it boils) deter-mined every 24 hours. Such 18 "high boilers" range in ester (same ester) boiling points from bibenzoic acid isomers up to a carboxybenzyl dicarboxybenzoate and also includes cis- and trans-4,4-dicarboxy stilbene, a 312 M.W. lactone, 2,5-dicarboxyfluore-none and a dicarboxyanthraquinone which are formed either by oxidative condensation and/or free radical reactions during primary oxidation in isolated oxygen-rich or oxygen-starred portions of the liquid phase.
The gaseous exhaust from both the primary oxidations and the secondary oxidation, when used, were measured and analyzed for their CO, CO2 and 2 contents. From the total of carbon oxides (hereafter (''COx'') from both primary and secondary oxidation and the total TPA produced, the ratio of total moles of Cx per mole of TPA produced is calculated. Such ratio accounts for both over-oxidation of p-xylene, decarboxylation, decarbonylation, and acetic acid burning in the primary oxidation and acetic acid burning (said over-oxidation, decarbonylation and decarboxylation occur only in the primary oxidation) in the secondary oxidation~ The difference between such ratio for any one Illustrative Example and such ratio for Base Operation accounts for reduction in said xylene and sol-vent burning as well as in decarboxylation and decarbonylation.
; TABLE V which follows provides data, averaged over the duration of the oxidations, pertinent to the content of 4-CBA and p-toluic acid in the mother liquor, the quality improvement of TPA product and TPA product yield increased resulting from the combination of continuous primary and secondary oxidation according to the present invention. Comparable data also pre-sented for the Base Operations:

lO~Z279 TABLE V
AVERAGE: RESPONSES TO SECONDARY OXIDATIONS

Illustrative Examples Base 1 2 3 4 Operations Mother Liquor:
4-CBA, wt.% 0.0372 0.0372 0.0455 0.0509 0.1041 p-Toluic Acid, Wt% 0.0336 0.0270 0.0489 0.0454 0.2725 TPA Product:
4-CBA, wt% 0.143 0.144 0.147 0.146 0.160 p-Toluic Acid, wt% 0.0249 0.0228 0.0210 0.0228 0.0383 Gardner b-value 7.94 7.18 7.27 7.81 8.39 Optical Density 2.40 2.01 2.27 2.31 2.70 Fluorescence Intensity 5.47 4.47 4.83 4.92 5.94 Increased TPA Yield, lb/day 1985 2048 2333 1888 Total moles CO /mole TPA 0.565 0.550 0.597 0.563 0.623 Reduction of moles COx/mole TPA, ~ 9.31 11.72 4.17 9.63 Such data indicate the trend of beneficial responses obtainable from the present inventive concept of combining continu-ous primary and continuous secondary oxidation. The indicated beneficial trends are the increased TPA yield and quality especially with respect to TPA color and fluorescence. Lowering of concentra-tions of components of catalysis at otherwise constant operation of the primary oxidation was done to decrease activity of catalysis and provide, on the basis of prior experience, TPA products of 4-CBA contents equal to that of TPA of Base Operations. This also would have provided TPA products of the same p-toluic acid content.
It was unexpected, therefore, that in spite of such experience base the trends of 4-CBA and p-toluic acid contents of TPA products from Examples 1-4 were materially lower.
However, such data, average of the over-all oxidations for the 172-232 hours of operation involved, wherein there were significant changes of process variables such as catalyst metals concentrations with respect to p-xylene (at constant MniCo ratio), and bromine to total metals in the primary oxidation, does not make possible absolute comparisons of measured responses from the added step of continuous secondary oxidation. Also such absolute comparison is not possible without taking into account the fact that more definitive responses occurred after the analyses made following such changes of process variables. Accordingly, to find the more definitive responses it is desirable to determine the extent of the direct effect contributed by the added continuous secondary oxidation. This was done by a complex, involved statis-tical approach making use of all the foregoing analytical data from the regularly scheduled samplings and the operation logs from the Base Operations and Examples 1-4.
To determine the direct effects of the added continuous secondary oxidation and its operating conditions (e.g., temperature and loading volume), it is necessary to remove effects related to variations in levels of all controllable variables throughout the test operations, including the changes made intentionally. This ; is done by correcting each response measured at its specified time interval to some common level for all variables important for that response. Then any differences remaining in the responses from the conduct of Examples 1-4 compared to the Base Operations and between the conduct of Examples 1-4 will be due to temperature, loading volume and residence time of secondary oxidation, presence or absence of secondary oxidation; and a combination of all sources of error impinging on any given response measured. The effects of temperature, and volume loading of secondary oxidation can be then studied by comparing those corrected responses and the results will be confounded only with random (White noise) errors.
The statistical techniques used followed data analysis described by C. Daniel and F. S. Wood in their publication FITTING EQUATIONS TO DATA, Wiley and Sons, New York (1971). The lO~Z7~

techniques ~Ised for modellincl of discrete transfer function models (model showin~ the dynamic effect of a process variable on a response) are described by G.E.P. Box and G.M. .~enkins in their publication TIME SERIES ANALYSIS: FORECASTING AND CONTROL; Holden-Day, San Francisco (1970). The computer programs used to identify and fit the transfer function models were those of P.E. Ashworth - et al., Digital Computer Programs for Box Jenkins Time Series Anal~sis, Forecasting and Control:User Specifications, ISCOL pro-gram Products, Lancaster, U.K. (1971). The purpose of the transfer function models is to provide some explicit direction in applying response time delay to input variables. A response time delay -~ equal, respectively, to the 4 or 8 hour sampling time (e.g., 4 hour response delay for the every 4 hour sampling period) was used.
Selection of temperatures and volume loadings for the ; added secondary oxidation of Illustrative Examples 1-4 provided a two-level factorial experimental design. In such experimental design each combination of temperature and volume loading condi-tions in the secondary oxidation are held substantially constant for long (164-300 hours) time periods. Cobalt concentration (hence total metal oxidation catalyst) and ratio of bromine to total metal were rapidly changed as needed. The resulting data obtained is characterized in general by statisticians and data analysts as nested data. For the purposes of the foregoing statistical tech-niques the substantially constant conditions are considered as "nesting variables" and the rapidly changed conditions are con-sidered as variables "nested within" the nesting variables. Be-cause the nesting variables are subject to more extrinsic sources ; of variability than are the nesting within variables, the latter produce changes in response with greater variability than the nested variables. This dichotomy of variability required two fittings of equations to the data. First, each response is fitted "

106;2Z7~
in terms of the nested variables and each response is corrected to a common level of the nested variable. Finally, such corrected responses are analyzed to determine the effect of the nesting factors.
Ultimate response to change of the nesting within vari-ables at time t were not reflected at time t because such change of the nesting within variables were made in the primary oxidation and the residence times in both primary and secondary oxidations, process dynamics, caused a delay of attendant reflected ultimate responses measured in the acetic acid mother liquor and/or TPA
product. This is so because the over-all process dynamics has a finite as against non-zero residence time. Such delay of ultimate response was taken into account for the foregoing fitting of the equations to test data.
The same statistical techniques for dynamic data analysis were also applied to the Base Operations to obtain corrected ; response results therefor.
TABLE VI, which follows, presents corrected, correlated response data for each of the Base Operations and Illustrative Examples 1-4 as bases for more definitive response comparisons which are presented in TABLE VII.

TABLE VI
CORRECTED AVERAGES OF RESPONSES

Illustrative Examples Base Operations Mother Liquor:
4-CBA, Wt.~ .0229 .0414 .0570 .0588 .1065 .1043 1016 p-Toluic Acid,Wt.% .0275 .0269 .0648 .0564 .1975 .2039 1924 TPA Product:
4-CBA, Wt.% .141 .132 .139 .130 .170 .174 .163 p-Toluic Acid,wt.% .0189 .0155 .0171 .0274 .0331 0349 .0355 Gardner b-Value 7.66 7.43 7.77 8.01 8.05 8.00 7.93 Optical, Density 2.37 2.41 2.38 2.50 2.50 2.50 2.33 Fluorescence Intensity 5.33 4.97 4.89 5.48 5.50 5.42 5.34 ; - 27 -tOf~Z~79 T~BLE VII presents data with respect to percent savings in catalysis to primary oxidation for both actual operations and that to be expected from operations corrected to constant 0.17 weight percent 4-CBA in TPA product and savings in total burning (total mole Cx per mole TPA) for both actual operations and that to be expected from operations corrected to the constant 0.17 - weight percent 4-CBA in TPA product. The corrections to said constant 4-CBA content are made on the basis of the statistical analysis d~scribed before.

TABLE VII
SAVINGS IN CATALYSTS AND BURNING FROM
ADDED CONTINUOUS SECONDARY OXIDATION

Base Illustrative ExamPles Operations 1 2 3 4 Average Co in feed, wt.% 0.02350.02110.01830.01730.0149 Average Br in feed, wt.% 0.0890.077 0.060 0.074 0.075 Savings in Catalyst, % 12.5 23.8 27.9 37.9 Corrected Co in feed, Wt.% 0.02350.01850.01680.01480.0140 ; Additional Catalyst Savings, % 10.4 6.2 10.4 3.1 -Total Catalyst Savings, % 22.9 30.0 38.3 41.0 Total burning, CO~/TPA 0.6230.565 0.550 0.597 0.563 Savings in burning, % 0.058 0.073 0.026 0.060 Corrected Total Burning, COX/TPA 0.6230.548 0.565 0.537 0.520 Total Expected Savings in burning,% 0.075 0.058 0.086 0.103 ~ From the measured air to and gaseous exhaust from the con-; tinuous primary and secondary oxidations it was found that about 98.7% of the total burning occurred in the primary oxidation and 7~

about 1.3~ of the total burning occurred in the secondary oxida-tion. Acetic acid burning amounts to about 0.18 kg in the primary oxidation and 0.002 kg in the added secondary oxidation per kilo-gram TPA product of the present inventive process.
As before mentioned, an added benefit from the added con-tinuous secondary oxidation comes from digestion of crystalline TPA produced in the primary oxidation and removal therefrom of especially 4-CBA. The extent of such digestion amounts to about 51, 59, 70 and 67 percent, respectively, in Illustrative Examples 1, 2, 3 and 4 and decreases the 4-CBA content in TPA product, respectively by 37, 48, 51, and 51 percent. Thus the total 4-CBA
(mother liquor and TPA product) oxidized in the secondary oxida-tions of said examples, respectively, amounts to a total of 50.3, 61.2, 59.5, and 60.7 percent of the 4-CBA present from the primary oxidation.
The present inventive process comprising combining con-tinuous primary and continuous secondary oxidations has been illustrated with respect to the production of TPA from p-xylene.
The production of isophthalic acid from m-xylene carried out according to the present inventive process is illustrated by Examples 5 and 6 to follow.
Illustrative Examples 5 and 6 The present inventive combination of continuous primary oxidation of m-xylene with continuous secondary oxidation in the first stirred zone of cooling the fluid effluent of the primary oxidation zone comprise Illustrative Examples 5 and 6. The opera-ting conditions for such illustrative processes are reported in TABLE VIII.

.

l~Z~7~
TABL~ VIII

Operating Conditions Illustrative Example Oxidation Vessel 10: 5 6 Residence Time, minute 63 45 Aerated Liquid Phase, % of total volume 70 70 Feed, liters per minute 7.76 11.96 Air, Nm3/liter feed .73 .78 Exhaust 2' vol.% 2.5 3.0 10 Midpoint Temperature, C 222 232 Pressure, kg/cm2 24.6 28.9 Feed Composition, wt%:
Cobalt as Metal .012-.017 .011-.015 Manganese as Metal .034-.048 .031-.042 Bromine as Bromide ion .055-.060 .07-.10 m-xylene 18-19 20.5-21.2 Acetic Acid 76-77 73-74 Water 4.5-5.5 5.6-6.3 Crystallizer 20: -20 Temperature, C 182 182 Pressure, kg/cm2 8.44 9.19 Volume, % of Total 45 45 Exhaust 2' vol.~ 5.0 5.0 The weight percent of isophthalic acid and its precursors 3-carboxybenzaldehyde and m-toluic acid contents of the total solids, dissolved as well as suspended, in the resulting fluid effluents from the last stirred cooling zone (i.e., feed in slurry transfer line 47 going to centrifuge 50) are reported in TABLE IX.
The same data are shown for production of isophthalic acid (Comparative Examples IV and V) from m-xylene oxidation at com-parable oxidation conditions of primary oxidation of Examples 5 and 6 but with no secondary oxidation.

~-lOl~iZZ79 TABLE I~

Component,Illustrative Exa~ples Comparative Examples Wei~ht percent 5 6 IV V
Isophthalic Acid98.6 96.5 93.9 91.7 3-Carboxybenzaldehyde 0.1 0.15 0.66 0.94 m-Toluic Acid 0.17 0.27 1.65 2.84 The data in TABLE IX show tha~ by the present inventive combinations of primary oxidation of m-xylene with secondary oxi-dation of fluid effluent from the primary oxidation there is a decrease of total 3-carboxybenzaldehyde of 85 percent (Example 5 v. Comparative Example IV) and 84% (Example 6 v. Comparative Example V) and of total m-toluic acid of 89.6 and 90.3 percent on the same comparative basis. Such decreases of precursors of iso-phthalic acid result, as such data indicate, in higher isophthalic acid content of the total solids. Said higher product acid con-tents represent substantial yield increases over the absence of secondary oxidation in the Comparative Examples.
TABLE X
DRIED-WASHED ISOPHTHALIC~ ~PRODUCT

Illustrative Exam~le Comparative Example

5 6 IV V
3-Carboxybenzaldehyde 0.26 0.17 0.64 0.71 m-Toluic Acid 0.066 0.040 0.135 0.075 Illustrative Example 7 In this process the primary oxidation operating conditions of the Base Operations for p-xylene oxidation is employed but the effluent therefrom is subjected to the secondary oxidation operating condi-tions of Illustrative Example 1. The present inventive process ~ when so conducted can be expected to improve the washed and dried - 30 terephthalic acid yield as indicated by Illustrative Examples 1-4.

But more important the quality improvement for such dried product is indicated by typical 4-CBA content of 0.04-0.07 weight percent and p-toluic acid content of 0.019-0.02 weight percent.

Claims (3)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A continuous production and recovery of iso- or tere-phthalic acid by the continuous steps of (A) introducing of air, m- or p-xylene and an acetic acid solution containing as components of catalysis a source of dissolved cobalt, manganese and bromine into a single, stirred primary oxidation zone operated at a tempera-ture of from 170 to 225°C and a gauge pressure of 10 to 35 kg/cm2 to provide therein a weight ratio of 2-6:1.0 of such acetic acid solution to said xylene; from 0.8 to 1.75 milligram atoms of cobalt per gram mole of said xylene, manganese in the gram atom ratio of cobalt to manganese in the range of from 0.125 to 1.0:1.0, and from 0.5 to 1.5 gram atom of bromine per gram atom total of cobalt and manganese; and an air to xylene ratio to provide a gaseous exhaust from said oxidation zone containing from 2 to 7 volume percent oxygen on an acetic acid and water free basis; (b) with-drawing from said oxidation zone fluid effluent comprising a sus-pension of crystalline iso- or terephthalic acid product in acetic acid mother liquor containing dissolved iso- or terephthalic acid, precursors thereof and reaction co-products; (C) cooling and de-pressuring said fluid effluent step-wise in two or more series connected stirred zones to a first temperature of from 150 to 200°C
and a gauge pressure of from 3 to 15 kg/cm2 and to a final tempera-ture of from 60 to 120°C and gauge pressure of from 0 to 2 kg/cm2 in the last series connected zone, wherein at least said first stirred cooling and depressuring zone has a fluid operating volume of from 0.5 to 2.0 times the volume of fluid effluent withdrawn from the oxidation zone; and (D) separating iso- or terephthalic acid product from the suspension in acetic acid mother liquor with-drawn from the last stirred cooling and depressuring zone; increas-ing the yield of iso- or terephthalic acid product and decreasing the iso- or terephthalic acid precursor content of both the acetic acid mother liquor and the iso- or terephthalic acid product by continuously introducing air into at least the first of said stirred cooling and depressuring zones to provide an exhaust there-from containing oxygen on an acetic acid and water free basis of 1 up to 8 volume percent as the fluid effluent flows through said first zone.

2. The process of Claim 1 wherein the cobalt concentration in the primary oxidation zone is decreased in the range of from 23 to 41 percent and there is recycled thereto from 10 to 90 weight percent of the solids content of the acetic acid mother liquor separated in the recovery of iso- or terephthalic acid.

3. The process of Claim 2 wherein the primary oxidation is conducted at a temperature of 255°C and a pressure of 26 kg/cm2, the feed thereto contains 2.2 weight parts of acetic acid per weight part of xylene and a cobalt concentration of from 0.95 to 1.32 milligram atoms per gram mole of xylene, the gaseous exhaust therefrom contains on an acetic acid and water free basis from 1.8 to 2.0 volume percent oxygen, and 40-65 weight percent of said acetic acid mother liquor solids is recycled thereto; and the fluid effluent therefrom is charged to the first of three series con-nected stirred crystallization zones operated at a gauge pressure of 9.8 kg/cm2, at a temperature in the range of from 186 to 197°C
at a fluid operating volume of from 1.0 to 1.75 times the volume of such fluid effluent, and at an air input to provide in the gaseous exhaust therefrom an oxygen content on acetic acid free basis of 5 volume percent.