RU2399610C2 - Method of producing highly pure terephthalic acid - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: terephthalic acid production method involves a step A) for oxidising paraxylene to terephthalic acid with air in the presence of a liquid reaction phase kept at temperature between 180°C and 230°C, where the liquid reaction phase contains paraxylene, acetic acid, water and a catalyst composition, where amount of water is 5-12% of the weight of acetic acid, the mass ratio of acetic acid to paraxylene is not less than 30:1 and must be such that 15-50% of reacted terephthalic acid is present in solid form at oxidation temperature and the catalyst composition contains cobalt, manganese and bromine combined with at least one element selected from a group consisting of zirconium and hafnium, where atomic ratio Co:Mn:Br is in the range 1:0.2-1.0:1.1-2.7, and atomic ratio of cobalt to elements selected from a group consisting of zirconium and hafnium is equal to 1:0.03:0.3, where total mass of Co and Mn is equal to 100-500 mg per 1 kg of the liquid reaction phase; and B) extraction of terephthalic acid through crystallisation at temperature between 150°C and 80°C.

EFFECT: method gives terephthalic high purity without a secondary purification step currently used in practice.

3 cl, 19 ex, 3 tbl

Description

State of the art

Pure terephthalic acid (PTA) is an important raw material used in the production of poly (ethylene terephthalate) (PET) for conversion into fibers, films and containers, commercially obtained by purification of crude - or technical grade - terephthalic acid obtained by catalytic, liquid-phase air oxidation p-xylene (PX). In fact, the entire technical grade PTA is obtained by catalytic, liquid-phase air oxidation of p-xylene.

Commercial methods use acetic acid as a solvent and a polyvalent heavy metal catalyst, most widely based on cobalt and manganese compounds, and an activator with bromide or bromide ions as a renewable source of free radicals.

Acetic acid, air, p-xylene and the catalyst are fed continuously to the oxidation reactor, which is maintained at 175 to 225 ° C under a pressure of 1.5-3.0 MPa. The weight ratio of acetic acid to p-xylene is typically less than 5: 1. Air is added in an amount above stoichiometric requirements to minimize the formation of by-products. The oxidation reaction is exothermic, and the heat is usually removed, allowing the acetic acid solvent to boil. The corresponding steam condenses and most of the condensate is re-routed to the reactor. The residence time is usually 30 minutes - 2 hours, depending on the method.

The effluent from the reactor is a suspension of crystals of crude terephthalic acid, which is purified by filtration, washed, dried and stored. After that, the crystals are sent to separate stages of purification (see US patent 5,350,133). While the main impurity is 4-carboxybenzaldehyde (4-CBA), p-toluic acid (pTA) is also present in significant quantities. Although the purity of the unrefined PTA grade is usually greater than 99%, it is not pure enough for the PET produced from it to achieve the required degree of polymerisation.

US Pat. No. 6,034,269 discloses a method for producing high purity terephthalic acid by catalytic, liquid phase oxidation of p-xylene in a reaction zone of a displacement stream, where a high mass ratio between the solvent (acetic acid) and p-xylene is used and a temperature and pressure sufficient to maintain PTA in solution as it forms.

The acid crystallizes from the resulting reaction medium and is purified without the need for a separate purification. While the purity may be as high as 99.95%, pTA is present in an amount in excess of 80-90 ppm.

US Pat. No. 6,307,099 describes a process for the homogeneous liquid phase oxidation of p-xylene in PTA, where the 4-CBA content of the purified terephthalic acid most preferably does not exceed about 500 ppm, for example 20-300 ppm. No data regarding PTA, color parameters, and catalyst residues are provided. The method requires the circulation of a large volume of solvent and uses a very high concentration of an oxidation catalyst calculated from oxidized p-xylene. The method is controlled under reaction conditions where substantially all of the terephthalic acid obtained in the oxidation reaction remains in solution during the reaction. A detailed description of this patent stipulates the possibility of some precipitation during the reaction, for example, up to 10%, but preferably no more than about 2% by weight of the obtained terephthalic acid can precipitate during the course of the reaction. Patents (US patent 6,034,269 and US patent 6,037,099) do not specify the oxidation catalyst, its concentration and its significant effect on the quality of terephthalic acid. In these patents, the selection of a catalyst and an oxidation activator occurs within standard practice.

SUMMARY OF THE INVENTION

The present invention relates to a method for producing high purity terephthalic acid, comprising the steps of:

A) oxidizing paraxylene to terephthalic acid with air in the presence of a liquid reaction phase maintained at a temperature of from 180 to 230 ° C, where the liquid reaction phase contains paraxylene, acetic acid, water and a catalyst composition, where water is 5-12 percent by weight of acetic acid, the mass ratio of paraxylene to acetic acid is such that 15-50% of the reacted terephthalic acid is present in solid form at the oxidation temperature, and the catalyst composition includes cobalt, manganese and bromine in combination with at least one element selected from the group consisting of zirconium and hafnium, where the atomic ratio Co: Mn: Br: is in the range 1: 0.2-1.0: 1.1-2.7, and the atomic ratio of cobalt to elements selected from the group consisting of zirconium and hafnium, is 1: 0.03-3.0, where the total mass of Co and Mn is 100-500 mg per 1 kg of the liquid reaction phase; and

B) purification of terephthalic acid by crystallization at a temperature in the range from 150 to 80 ° C.

DETAILED DESCRIPTION OF THE INVENTION

It has now been unexpectedly discovered that it is possible to obtain high purity PTA by catalytic, liquid phase oxidation of p-xylene without a purification step. The method described herein leads without a purification step to a PTA that contains small amounts of impurities, preferably less than 40 ppm by weight of total 4-CBA and pTA, and less than 20 ppm pTA, and has excellent color parameters.

Such very high quality PTA can be obtained under the very specific reaction and concentration conditions disclosed in this specification. The catalytic, liquid-phase oxidation of p-xylene must be carried out in acetic acid as a solvent, at such temperature, pressure and mass ratio of acetic acid to p-xylene that 15-50% of the formed PTA is not supported in the solution, and is controlled under specific temperature conditions in the presence of a very specific composition and catalyst concentration. The catalyst should contain compounds that include Co and Mn with Zn and / or Hf compounds, and bromine and / or bromine compounds as activator; where the atomic ratio Co: 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 atomic the ratio of Co: Zr and / or Hf is 1: 0.03-0.3.

The oxidation is carried out in a stirred reactor under stirring conditions suitable for homogenizing the liquid phase and providing similar temperature conditions at all points of the reactor, at a temperature ranging from 195 to 220 ° C or, preferably, according to a temperature profile starting from 180 to 200 ° C and then up to 230 ° C. Preferably, the temperature in the last period is 205-215 ° C. Temperatures above 230 ° C are not recommended as acetic acid reacts and is lost. However, in some cases, it is convenient after the end of the oxidation reaction to heat the reaction mixture for a short time to a temperature of 230-240 ° C in the absence of air flow (for example, for 10-30 minutes).

The concentration of the catalyst, expressed as the total mass of Co and Mn per 1 kg of the liquid reaction phase, is 100-500 mg.

PTA is purified from the reaction phase by crystallization at a temperature of from 80 to 150 ° C. and then washed with acetic acid and / or water. The mother liquor is partially recycled to the oxidation reactor and partially regenerated.

The color L * determined according to the CIE Standard Method on atomized PTA containing particles with an average size of less than 50 μm has a minimum of 95.5.

The impurities obtained from the catalyst, expressed as the metal Co and Mn, amounted to less than 8 parts per million by weight; in particular, Co was less than 4 ppm and Mn less than 2 ppm.

The PTA crystallized from the mother liquor is at least partially in the form of distinctly angular crystals, for example having a rhombic structure, and thus differs from the PTA obtained according to the methods commonly used in the prior art for catalytic heterogeneous liquid-phase oxidation of p-xylene where the crystals tend to be rounded agglomerates of smaller crystals.

Acetic acid, used as a solvent, contains from 1 to 15 wt.% Water, preferably 5-10%.

The mass ratio of acetic acid / p-xylene is not less than 30: 1, but should be such that 12-50%, in particular 20-30 wt.%, PTA after the oxidation reaction was present in the form of a solid phase.

The oxidation reaction is exothermic. Typically, in known methods, heat is removed by boiling an acetic acid solvent to produce condensable vapor, and the condensate, in varying amounts, is recycled to the reactor. Typically, in the method of the present invention, the reaction temperature and pressure are maintained at a level necessary to maintain a preferred temperature profile and achieve a preferred temperature of the last heating step.

The reaction can also be carried out in an ideal displacement reactor.

In an ideal displacement reactor, molecular oxygen is dissolved in the feed stream to achieve an excess of dissolved oxygen concentration above the stoichiometric value. When using a stirred reactor, air is passed through the liquid phase at a rate sufficient to remove heat and not exceed the flammability limit at the top of the reactor. The source of oxygen can be pure oxygen, air, or any convenient oxygen-containing gas.

Examples of cobalt, manganese, zirconium and hafnium compounds suitable for use as a catalyst component are acetates, carbonates, hydroxides and oxides. Examples of bromine or bromine-containing compounds are bromine, HBr, NaBr, KBr and organic bromides, which are known to give bromide ions at an oxidation temperature, such as bromobenzenes, benzyl bromides and tetrabromoethane.

The reaction time in a stirred reactor depends on the reaction conditions and generally ranges from 13 to 45 minutes.

The PTA obtained by the method of the present invention, due to its very low content like 4-CBA and pTA, is particularly suitable for the production of high molecular weight polyalkylene terephthalates and their copolymers.

The following examples are given to illustrate, but not limit, the scope of the present invention.

EXAMPLES

Oxidation conditions

Oxidation of p-xylene with air is carried out in a 250 ml titanium alloy reactor. The reactor is equipped with a magnetic stirring system, operating at 100-3000 rpm, has air access at the base and exit through a condenser equipped with a phase separator, a pressure and temperature regulator, an electrically heated shell and a sample outlet.

P-xylene, a catalyst and a solvent (acetic acid with addition of water) are charged into the reactor and the pressure is increased with nitrogen to 2.5 MPa. Then raise the temperature to the desired temperature within 15-20 minutes. When the temperature of the liquid medium in the reactor reaches the desired temperature, an air flow is established in the liquid phase at 0.5 l / min and a stirring speed of 2200 rpm. The exhaust gas is continuously monitored using an oxygen analyzer. When oxygen consumption ceases, the speed of the mixer is reduced to 200-300 rpm and the heating of the reactor is completed. The temperature of the contents of the reactor was lowered to approximately 85 ° C for 10-60 minutes.

During this cooling period, the PTA crystallizes from the stirred liquid medium. The solids were separated at approximately 80 ° C, washed (3 times 10 ml of acetic acid and 1 time 20 ml of H ₂ O), dried at 80 ° C for 5 hours, weighed, crushed and analyzed. The content of 4-CBA and pTA is determined by high pressure liquid chromatography.

Analytical Definitions

pTA and 4-CBA were measured by liquid chromatography (HPLC) using a Dupont Zorbax NH ₂ column, ammonium phosphate buffer solution (pH adjusted to 4.25 with concentrated NH ₄ OH, if needed, H ₃ PO ₄ if needed, ) and 254 nm absorption detector. A sample of dry PTA 0.2 + 0.0005 g was dissolved in 20 ml of a 3.7 wt.% Solution of ammonium hydroxide; 20 ml of distilled water and 10 ml of benzyl alcohol were added to the PTA sample.

pH was adjusted to 7 (6.8-7.2) concentrated H ₃ PO ₄ .

Before use, the column is initially washed with CH ₃ CN and after 15 minutes with distilled water. The buffer solution was flushed through the column for 16 hours at a rate of 1.3 ml / min to balance the column.

For analysis, a Perkin-Elmer Sigma 10 data processing device and Spectra-Physics computing integrator (or equivalents) are used.

Example 1

2.5 g of p-xylene, 150 g of acetic acid containing 10 wt.% H ₂ O, 0,100 g Co (CH ₃ COO) ₂ ∙ 4H ₂ O and Mn (II) acetate, Zn (IV) acetate were charged into the reactor , HBr (47% solution in H ₂ O) in the atomic ratio Co: Mn: Zr: Br = 1: 0.6: 0.05: 1.7. The mixture was oxidized with air (flow 0.5 l / min) at 195 ° C for 20 minutes, then the temperature was increased to 210 ° C over 3 minutes, and at this temperature, oxidation continued for 22 minutes to achieve complete conversion of p-xylene . 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 instead of zirconium, hafnium was used in an amount corresponding to the atomic ratio Co: Mn: Hf: Br = 1: 0.6: 0.1: 1.7. Terephthalic acid contained 14 ppm of 4-CBA and 5 ppm of p-toluic acid.

Example 3 (Comparative)

The procedure of Example 1 was repeated, except that no zirconium was added. The quality of the obtained terephthalic acid in the absence of Zr or Hf is lower. PTA contained 89 ppm of 4-CBA and 6 ppm of p-toluic acid.

Examples 4-7

The procedure of Example 1 was repeated except that the mixture was oxidized with air at 195 ° C for 15 minutes and at 210 ° C for 7 minutes. The composition of the catalysts in the experiments was changed, as shown in Table 1.

Table 1 Exp. number Atomic ratio
Co: Mn: Zr: Br The purity of terephthalic acid, parts per million 4-CBA p-toluic acid four 1: 1.2: 0.1: 2.3 85 9 5 1: 0.8: 0.1: 2.3 twenty 5 6 1: 0.2: 0.1: 2.3 fourteen four 7 1: 0: 0.1: 2.3 Low content pTA is absent

The results clearly demonstrate the very high effect of the catalyst composition on the purity of the resulting terephthalic acid. A very high PTA purity is achieved if the concentration of manganese varies in a certain range. In the absence of manganese (experiment number 7), the oxidation rate is very low and virtually no PTA is formed.

Examples 8-11

The procedure of example 4 was repeated, except that experimentally changed the composition of the catalysts, as shown in Table 2.

table 2 Exp. number Atomic ratio
Co: Mn: Zr: Br The purity of terephthalic acid, parts per million 4-CBA p-toluic acid 8 1: 1.2: 0.1: 2.3 90 5 four 1: 1.2: 0.1: 2.3 85 9 9 1: 0.6: 0: 2.3 104 four 10 1: 0.6: 0.1: 2.3 6 four eleven 1: 0.2: 0: 2.3 118 8 6 1: 0.2: 0.1: 2.3 fourteen four

The results clearly show that the synergistic effect of zirconium strongly affects the atomic ratio of Co: Mn in oxidation catalysts. When the Co: Mn ratio is 1: 1.2, the synergistic effect of zirconium on the purity of terephthalic acid is negligible.

Example 12

2.5 g of p-xylene, 150 g of acetic acid containing 5% by weight of H ₂ O, 0.120 g of cobalt acetate tetrahydrate and Mn (II) acetate, Zn (IV) acetate, Zn (IV) solution in H ₂ O) in the atomic ratio Co: Mn: Zr: Br = 1: 0.2: 0.1: 1.35. The air flow used for the oxidation of p-xylene was varied during oxidation in the range of 1.5-0.05 L / min depending on the oxygen content in the exhaust gas. The mixture was oxidized at 195 ° C for 7 minutes, then the temperature was increased to 205 ° C over 2 minutes, and at this temperature, oxidation continued for 9 minutes (total oxidation time 18 minutes). The resulting terephthalic acid contained 24 ppm 4-CBA and 6 ppm p-toluic acid.

The experiment demonstrates that under specific reaction conditions and the composition of the catalyst it is possible to obtain high purity PTA at a temperature of 205 ° C in the last stage of oxidation.

Example 13 (comparative)

The procedure of Example 12 was repeated, except that the oxidation of p-xylene was carried out isothermally at 205 ° C for 18 minutes. Terephthalic acid contained 42 ppm of 4-CBA and 6 ppm of p-toluic acid. The color parameter L * was 93.1.

A comparison of the results of experiments 12 and 13 confirms that the isothermal oxidation of p-xylene gives terephthalic acid with a lower purity than stage oxidation at different temperatures.

Examples 14-17

The procedure of Example 4 was repeated except that the composition of the catalysts in the experiments was Co: Mn: Zr = 1: 0.6: 0.1 and the atomic ratio Co: Br was changed as described in Table 3.

Table 3 Exp. number Atomic ratio
Co: Br The purity of terephthalic acid, parts per million 4-CBA p-toluic acid fourteen 1: 1.00 84 four fifteen 1: 1.35 22 four 16 1: 2.7 32 5 17 1: 3.44 52 6

As can be seen from the results, the concentration of bromine in the reaction system must be optimal in order to obtain high purity PTA.

Example 18

5.0 g of p-xylene, 150 g of acetic acid containing 5 wt.% H ₂ O, 0.20 g of cobalt acetate tetrahydrate and Mn (II) acetate, Zn (IV) acetate, HBr (47% solution) were charged into the reactor. in H ₂ O) in the atomic ratio Co: Mn: Zr: Br = 1: 0.6: 0.1: 2.3. The air flow used for the oxidation of p-xylene was varied during oxidation in the range of 1.5-0.05 L / min depending on the oxygen content in the exhaust gas. The mixture was oxidized at 190 ° C for 10 minutes, then the temperature was increased to 220 ° C over 5 minutes, and at this temperature, oxidation continued for 5 minutes. In the next stage, the air flow was stopped and the reaction mixture was heated to 235 ° C for 8 minutes and stirred at this temperature for 20 minutes. The resulting 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 reaction mixture after the oxidation reaction was not heated to 235 ° C. The formed terephthalic acid contains 117 ppm of 4-CBA and 27 ppm of p-toluic acid. The results of Examples 18 and 19 confirm that subsequent heating of the reaction mixture in the absence of air increases its purity.

Claims (3)

1. The method of obtaining terephthalic acid of high purity, comprising stages
A) oxidation of paraxylene to terephthalic acid with air in the presence of a liquid reaction phase maintained at a temperature of from 180 to 230 ° C, where the liquid reaction phase contains paraxylene, acetic acid, water and a catalyst composition, where water is 5-12% by weight of acetic acid, the mass ratio of acetic acid to paraxylene is at least 30: 1 and should be such that 15-50% of the reacted terephthalic acid is present in solid form at the oxidation temperature, and the catalyst composition includes cobalt, marg gold and bromine in combination with at least one element selected from the group consisting of zirconium and hafnium, where the atomic ratio Co: Mn: Br is in the range 1: 0.2-1.0: 1.1-2 , 7, and the atomic ratio of cobalt to elements selected from the group consisting of zirconium and hafnium is 1: 0.03-0.3, where the total mass of Co and Mn is 100-500 mg per 1 kg of the liquid reaction phase; and
B) the allocation of terephthalic acid by crystallization at a temperature in the range from 150 to 80 ° C. 2. The method according to claim 1, in which the oxidation temperature is in the range from 180 to 200 ° C in the first stage of oxidation and in the range from 200 to 225 ° C in the last stage of oxidation, while the degree of conversion of p-xylene to derivatives acid in the first stage of oxidation is in the range from 50 to 80%. 3. The method according to claim 2, in which after the completion of the oxidation step and before the separation step, the reaction mixture is heated for 10-30 minutes at a temperature in the range of 230-240 ° C. in the absence of air.