Title of Invention	A PROCESS FOR PREPARING HIGH PURITY TEREPHTHALIC ACID
Abstract	The present invention relates to a process for preparing high purity terephthalic acid by catalytic, liquid phase oxidation of p-xylene utilizing a partially homogeneous reaction in the presence of a very specific catalyst. The process produces a high purity terephthalic acid without the secondary purification step currently practiced.

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A Process For Preparing High Purity Terephthalic Acid Background Pure terephthallc acid (PTA), an important raw material used in the production of poly (ethylene terephthalate) (PET) for conversion into fibers, films and containers, is ccHianerciaLLly produced by purifying crude-or technical-grade terephthallc acid produced by catalytic, liquid phase air oxidation of p-xylene (PX). Practically all technical-grade PTA is produced by catalytic, liquid phase air oxidation of p-xylene. Commercial processes use acetic acid as a solvent and a' mul¬tivalent heavy metal catalyst, most widely based on cobalt and manganese confounds, and a promoter, with brcanine or bro¬mide ions as the renewable ebturce of free radicals. Acetic acid, air, p-xyl«ne and catalyst are fed continuously into an oxidation reactor that Is maintained at frcan 175 «C to 2250c under pressure of 1.5-3.0 MPa. The feed weight ratio of acetic acid to p-xylene is typically less than S:l. Air is added in amounts in excess of stoichiometric requirements to minimize formation of by-products. The oxidation reaction is exothermic, and heat is typically removed by allowing the acetic acid solvent to boil. The corresponding vapor is con¬densed and most of the condensate is refluxed to the reactor. The residence time is' typically 30 minutes to 2 hours, de¬pending on the process. The effluent from the reactor is a slurry of crude terephthallc acid crystals which are recovered by filtration, washed, dried and conveyed to storage. The crystals- are thereafter fed to a separate purification steps (See llnited, States Patent No. 5,350,133). While the main ±impurity is 4-carboxybenzaldehyde (4-CBA.), p-toluic acid (pTA.) is also pre¬sent in relevant amount. Although the purity of .crude-grade PTA is typically greater than 99%, it is not ptire enough for the PET made from it to reach the required degree of polym¬erization. From United States Patent No. 6,034,269, a process is known for production of high purity teraphthalic acid by catalytic, liquid phase oxidation of p-xylene carried out in a plug flow reaction zone, wherein a high weight ratio between the sol¬vent (acetic acid) and p-xylene and temperature and pressure sufficient to maintain PTA in solution as it is formned are used. The acid is crystallized from the resulting reaction medium and recovered without the need for separate purification. While the purity can be as high as 99.95%, pTA is present in amount.higher than 80-90 ppm. In United States Patent No. 6,307,099 a process for homogene¬ous liquid phase oxidation of p-xylene to PTA is described wherein the 4-CBA content of the recovered terephthalic acid is most preferably no more than about 500 pjaa, e.g., 20 to 300 ppm. No data concerning pTA, the color parameters and catalyst residues are reported. The process requires one to circulate a large voliame of the solvent and uses a veary high concentration of oxidation catalyst, calculated to the oxi¬dized p-xylene. The process is operated under reaction condi¬tions, where substantially all of the terephthalic acid pro¬duced in the oxidation reaction remains in the solution dur- ing the reaction. The detailed description of this patent specifies, the possibility of scxne precipitation dioring the reaction, e.g. up to 10 % but desirably no more than about 2% by weight of the terephthalic acid produced may precipitate during the course of the reaction. The patents (XJloited States Patent Nuittbers 6,034,269 and United States Patent Numbers 6,037,099) do not specify the oxidation catalyst, its concen¬tration, and its significant influence on the quality of terephthalic acid. Uhder these patents the selection of the catalyst and oxidation promoter is within conventional prac¬tice. Suamary of the invention The present invention relates to a process for preparing highly pure terephthalic acid comprising the steps of A) oxidizing para-xylene to terephthalic acid with air in the presence of a liquid reaction phase maintained at a temperature between 180"C and 230"C, wherein the liquid reaction phase comprises para- xylene, acetic acid, water, and a catalyst composition, wherein the water is 5 to 12 percent by weight of the acetic acidj the weight ratio of para-xylene to acetic acid is such that 15 to 50% of the reacted terephthalic acid is pre¬sent as a solid at the oxidation ten^perature, and the catalyst composition comprises Cobalt, Manganese, and Bromine in conibination with at least one element se¬lected from the group consisting of Zirconium and Haf¬nium wherein the atomic ratio of Co: Mn: Br; is in the range of 1:0.2 - 1.0:1,1 - 2.7 and the atomic ratio of Cobalt to the elements selected frcsti the group consist- Ing of Zirconltun and Hafnium is I:0.03 - 3.0, wherein the total weight of Co and Mn; is i00-500nvg per 1 kg of the liquid reaction phase; and B) recovering the terephthalic acid by crystallization at a temperature in the range from 150C to 80®C. Detailed Description It has now unexpectedly been foimd that it is possible to produce by catalytic;, liquid phase oxidation of p-xylene a highly pure PTA without the purification step. The process described herein produces, without a purification step, a PTA which contains small amounts of in^jurities, preferably less than 40 ppm by weight in total of 4-CBA and pTA, and less than 20 ppm of pTA, and having excellent color parameters. Such very high quality of PTA can be obtained under very spe-cific reaction and concentration conditions disclosed in this specification. The catalytic, liquid phase oxidation of p-xylene must be conducted in acetic acid as solvent, at tem¬perature, pressure and with acetic acid to p-xylene weight ratio such that 15 to 50 % of the formed PTA is not main¬tained in solution, and operating under specific temperature conditions in presence of very specific composition and con¬centration of a catalyst. The catalyst roust contain com¬pounds that comprise Co and Mn with Zr and/or Hf compounds, and, as a promoter, brcanine and/or bromine compounds; wherein the atomic ratio Cos Mn: Br is in the range of 1:0,2-1.0:1.1-2.7, preferably 1:0.3-0.8:1.1-1.8 and the atomic ratio of Co:Zr and/or Hf is 1:0,03-0.3. The oxidation is conducted in a stirred reactor under stir- ring conditions suitable to hcanogenize the liquid plaase and provide similar temperature conditions in all points of the reactor, at a teitperature con5>rised in the range from 1950C to 2200c or, preferably, according to a temperature profile starting frara ISO.o to 200»C and then up to 230oC. Preferably the temperature. in the last period is of 205-215oc. Tem¬peratures higher than 230«»C are not recommended because ace¬tic acid reacts and is lost. However, in some oases it is convenient after finishing the oxidation reaction to heat the reaction mixture for a short time to the temperature 230-240 The concentration of the catalyst, expressed ?is total weight of Co and Mn per 1 kg of liquid reaction phase, is of 100 -500 mg. FEA is recovered from the reaction phase by crystallization . at temperatures frcaa 80" to 150oc, and then washed with ace¬tic acid and/or water. The mother liquor is in patrt recycled to the oxidation reactor and in part regenerated. The color L* determined according to the CIB Standard method on pulverized PTA having average particle size of less than 50 ijm has a minimum of 95.5. The impurities derived from the catalyst expressed as Co and Mn metal were in total less than 8 ppm by weight; in particu¬lar Co was less than 4 ppm and Mn less than 2 ppm. PTA crystallized from the reaction liquor, at least in part, is in the form of crystals distinctly angular, e.g. having a rhomboid structure and thereby different frcao the PTA pro- duced according to the commonly vised prior art processes of catalytic heterogeneous liquid phase oxidation of p-xylene, wherein the crystals tend to be rounded agglcanerates of smaller crystals. The acetic acid used as the solvent contains from 1 to 15 wl:% water, preferably 5 to 10%. •die acetic acid / p-xylene weight ratio is not less than 30:1, but must be such that the 12 to 50%, particularly 20 -30 wt% of the PTA after the oxidation reaction is present as solid phase. ' . The oxidation reaction is exothermic. Typically in the known processes the heat has been removed by allowing the acetic acid solvent to boil, with the resulting vapor being con¬densed smd the condensate, in varying amount, refluxed to the reactor. Typically, in the process of the present invention the reaction temperature and pressure are maintained at the level necessary to maintain the preferred temperature profile and to reach the preferred temperature of the last" heating step. The reaction can also be conducted in a plug flow reactor. In the plug flow reactor, molecular oxygen is dissolved in the feed stream to achieve a concentration of dissolved oxy¬gen in excess on the stoichiometric value. Using a stirred reactor, air is passed through the liquid phase with a flow rate sufficient to remove heat and to not exceed the inflam¬mability limit in the top of the reactor. The source of oxy¬gen can be pure oxygen, air, or any convenient oxygen- containing gas. Examples of cobalt, manganese, .zirconium and hafnium ccHti-pounds usable as catalyst component are the acetates, carbon¬ates, hydroxides and oxides. Examples of bromine or bromine containing compounds are brcanlne, HBr, NaBr, KBr, and organic bromides which are known to provide bromide ions at the tem¬perature of oxidation, such as bromobenzenes, benzylbromide and tetrabrcanoethane. The reaction time in a stirred reactor depends on the reac¬tion conditions and is generally from 13 to 45 minutes. The PTA obtained with the process of the present invention, thanks to its very low content of both 4-CBA and pTA, is par-ticularly suitable for the production of high molecular weight polyalkylene terephthalates and copolymers thereof. The following examples are given to Illustrate and not to limit the scqpe of the present invention. EXAMPLES Oxidation conditions Oxidation of p-xylene with air is carried out in a 250 ml re¬actor of a titanium alloy. The reactor is fitted with a mag¬netic stirred system operating at 100-3000 irpm, has an air inlet at the bottom and an outlet through a condenser equipped with a phase separator, pressure and temperature regulator, electric heating mantle and outlet for a products probe. The reactor ie charged with p-xylene, catalyst and solvent (acetic acid added with water) and pressurized with nitrogen to 2.5 MPa. The ten5>erature is then raised to the desired temperature over 15-20 minutes. When the temperature of the liquid medium inside the reactor reaches the desired teirpera- " ture, the flow of air into the liquid phase is established at 0.5 l/iriin. and the stirred speed at 2200 rpm. The outlet gas is continuously monitored using an oxygen analyzer. When the oxygen consumption stops, the stirrer speed is lowered to 200-300 rpm and the heating of the reactor is terminated. The temperature of the reactor contents is cooled to approxi¬mately 85*0 in 10-60 min. During this cooling period, PTA crystallizes frcsn the stirred liquid medium. The solid is isolated at about 80 Analytical Determiiiaticms pTA and 4-CBA are measured by liquid chromatography (HPLC) using a Du Pont "Zorbax" NH2 column, an ammonium phosphate buffer solution (pH adjusted to 4.25 with concentrated NH4OH if it has to be raised', with H3PO4 if it ,has to be lowered) and a 254 nm absorbance detector. A sample of dry PTA of 0.2 + 0.0005 g is dissolved with 20 ml of a 3.7 wt% ammonium hy¬droxide-solution; 20 ml of distilled water and 10 ml benzyl alcohol are added to the PTA sample. The pH is adjusted to 7 (6.8-7.2) with concentrated H3PO4. Before use, the colum is flushed initially with CH3CN, and after 15 minutes with distilled water. The buffer solution is run through the column for 16 h at a rate of 1.3 ml/min to stabilize the column. •' A Perkin-Blmer Sigma 10 data processor and Spectra-Physics computing integrator (or equivalents) are used for analysis. Example 1 The reactor was charged with 2.5 g p-xylene, 150 g acetic acid containing 10 wt% HaO, 0.100 g Co(CH3CX)0)2.4H20 and Mn(II)ace-tate, Zr(IV)acetate, HBr (47% solution in HjO) in the atomic ratio Co:Ma:Zr:Br = 1:0.6:0.05:1,7. The mixture was oxidized with air (flow 0.5 1/min) at 195«C for 20 minutes, then during 3min the temperature was increased to 210 *»C and at this tem-perature oxidation proceeded for 22 min to reach complete con¬version of p-xylene. The HPLC analysis of terephthalic acid determined that it contained 11 ppm of 4-CBA and 4 ppm of p- . toluic acid. The color parameter L* was 95.8. Example 2 The procedure of Example 1 was repeated except that hafnium was employed Instead of zirconium in the amount corresponding to the atonic ratio Co:Mn:Hf:Br = 1:0.6:0.1:1.7. The terephthalic acid contained 14 ppm of 4-CBA. and S ppm of p-toluic acid. Example 3 (Comparative) The procedure of Example 1 was repeated except that no zirco-nium was added. The quality of produced terephthalic acid in the absence of 2r or Hf is lower. The PTA contained 89 ppm of 4-CBA and 6 ppm of p-toluic acid. Examples 4 to 7 The procedure of Example 1 was repeated except that the mix¬ture was oxidized with air at 195 "C for 15 min and at 210 *>C for 7 min. The CMcposition of the catalysts in the experi¬ments was changed as illustrated in Table 1. Table 1 The results unambiguously denjonstrate the very high influence of the catalyst composition on tlie purity of the formed terephthalic acid. A very high purity of PTA is reached only if the concentration of manganese varies in a certain range. In the absence of manganese (Exp. No. 7) the rate of oxida¬tion is very low and PTA is practically not formed. Exanples 8 to 11 The procedure of example 4 was repeated except that the com¬position of the catalysts in the experiment was changed as is illustrated in Table 2 These reetilts clearly show that the synergistic effect of zirconitim is strongly influenced by the atomic ratio of Co:Ma in the oxidation catalysts. When the ratio of CorMn is 1:1.2, the synergistic effect of zirconium on the purity of terephthalic acid is negligible. Exaniple 12 The reactor was charged with 2.5 g p-xylene, 150 g acetic acid containing 5 wt% HaO, 0.120 g cdbalt acetate tetrahy-drate and Mn(II)acetate, Zr(IV)acetate, HBr (47% solution in HaO) in the atomic ratio Co:Mn:Zr:Br = 1: 0.2:0.1:1.35. The flow of air used for oxidation of p-xylene was diiring the oxidation changed in the range 1.5 to 0.05 1/min depending on the oxygen content in the off-gas. The mixture was oxidized at 195oc for 7 min, then during 2 min the temperature was in¬creased to 20S*C and at this temperature oxidation proceeded for 9 min (total reaction time 18 min). The obtained te¬rephthalic acid contained 24 ppm of 4-CBA and 6 ppm of p-toluic acid. . The escperiment dancxistrates that at specific reaction condi¬tions and composition of the catalyst it is possible to ob¬tain high purity PTA at the ternperature 205 C in the last stage of oxidation Example 13 (comparative) The procedure of Bxanrple 12 was repeated except that oxida¬tion of p-xylene was carried out isothermally at 205C for 18 min. The terephthalic acid contained 42 ppm of 4-CBA.and 6 ppm of p-toluic acid. The color parameter L* is 93.1. The conparison of results of experiments 12 and 13 confirms that isothermal oxidation of p-xylene produces terephthalic acid with lower purity than step-wise oxidation at different temperatures. Examples 14 to 17 The procedure of Example 4 was repeated except that the com¬position of the catalysts in experiments was Co:Ito:Zr = 1:0.6J0.1 gmd the atomic ratio Co:Br was changed as is de¬scribed in Table 3. As is seen from the results, the concentration of branine in the reaction system must be optimal in order to obtain high purity PTA. Example 18 The reactor was charged with 5.0 g p-xylene, 150 g acetic acid containing 5 wt% water, 0.20 g cobalt acetate tetrahy-drate and Mn(II) acetate, Zr(IV)acetate, HBr (47% solution in HaO) in the atomic ratio Co:Mn:Zr:Br « 1:0.6:0.1:2.3. The flow of air was during oxidation changed in the range 1.5 to 0.05 1/min depending on the oxygen content in the off-gas. . The mixture was oxidized at 190*>C for 10 min, then during 5 min the temperature increased to 220oc and at" this tempera-ture oxidation proceeded for 5 min. In the next step the inlet of air flow was stopped and during 8- min the reaction mixture is heated to 235 'C and at this temperature stirred for 20 min. The formed terephthalic acid contained 33 ppm of 4-CBA and 5 ppm of p-toluic acid. Example 19 (Comparative) The procedure of Example 18 was repeated except that the re¬action mixture stfter the cxicidation reaction was not heated to 235C. The formed terephthalic acid contains 117 ppm of 4-CBA and 27 ppm of p-toluic acid. The results of Bxairples 18 and 19 confirm that the stibsequraat heating of the reaction mix¬ture in the absence of air increases its purity. CLAIMS l.A process for preparing highly pure tarephthalic acid com¬prising the steps o£ A) oxidizing para-xylene to terephthalic acid with air in the presence of a liquid reaction phase maintained at a temperature between 180C and 230C, wherein the liquid reaction phase comprises para- xylene, acetic acid, water, and a catalyst composition, wherein the water is 5 to 12 percent by weight of the acetic acid, the weight ratio of para-xylene to acetic acid is such that 15 to 50% of the. reacted terephthalic acid is pre¬sent as a solid at the oxidation temperature, and the catalyst composition comprises Cobalt, Manganese, and Bromine in combination with at least one element se¬lected from the group consisting of Zirconium and Haf¬nium wherein the atomic ratio of Co; Mn: Br; is in the range of 1:0.2 - l.Otl.l - 2.7 and the atomic ratio of Cobalt to the elements selected from the group consist¬ing of Zirconium and Hafnium is 1:0.03 - 3.0, wherein the total weight of Co and Mn is 100- 500 mg per 1 kg of the liquid reaction phase; and B) recovering the terephthalic acid by crystallization at a temperature in the range from 150C to 80C. 2. The process according to claim 1 wherein the oxidation temperature is in the range of 180C to 200C in a first oxi¬dation stage and is in the range from 200C to 225°C in a last oxidation stage, while the degree of conversion of p-xylene to acid derivatives in the first oxidation stage is within the range from 50 to 80 percent. 3. The process according to claim 2, wherein after finish¬ing the oxidizing step and prior to the recovering step, the reaction mixture is heated for 10-30 min in the temperature range of 230 - 240 C in the absence of the air.

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