DE10211907A1 - Oxidation of o-xylene to o-tolylic acid and other oxidation products by staggered supply of an oxygen-containing gas - Google Patents

Abstract

A process is described for producing oxidation products of o-xylene which can be further oxidized to phthalic acid, in which an o-xylene-containing liquid stream is passed through at least one reaction zone and an oxygen-containing gas at at least two points spaced apart in the direction of flow of the liquid stream bubbles turbulently into the liquid stream.

Description

The present invention relates to a method for manufacturing of oxidation products of o-xylene, which lead to phthalic anhydride are further oxidizable by oxidation of o-xylene in liquid Phase.

Phthalic anhydride is one of the most important technically aromatic compounds. It is e.g. B. in the synthesis of Alkyd resins, unsaturated polyester resins, paints, plastics, Plasticizers, phthalocyanine dyes, insect repellents, Denaturing agents and for the production of numerous fine chemicals used.

Phthalic anhydride is produced industrially by the oxidation of o-xylene or naphthalene. Two oxidation processes are common. On the one hand, the oxidation in the gas phase is carried out on a heterogeneous catalyst, generally a vanadium-containing catalyst. In this process, however, about 20 mol% of the starting material is converted to CO _x and is therefore lost as a yield.

Because of the better selectivity of the oxidation reaction is as Alternatively, a two-stage reaction procedure is proposed been in the o-xylene with a oxygen-containing gas, mostly air, under homogeneous catalysis Intermediates on the way to phthalic anhydride, mostly o-tylenic acid, 2-methylbenzaldehyde, 2-methylbenzyl alcohol and phthalide, is oxidized. These intermediates are isolated, then evaporated and in the gas phase in a second stage Phthalic anhydride oxidized.

Such a two-stage process is described in EP-A 0 256 352 described. Carrying out the o-xylene oxidation in liquid Phase is illustrated on a stirred tank reactor. As Oxidizing agent serves compressed air that is introduced through a dip tube becomes.

Wang Y., Xie Y. and Zhang J. report in Chinese scientific journal "Petroleum Refining and Chemical Industrie "2000, Band 31, about" Investigation of the Oxidation of o-xylenes in the liquid phase for the production of o-tolylic acid ". The influences of the Reaction conditions such as B. reaction temperature, reaction pressure, Air flow rate and response time to which Yield of the product o-tolylic acid examined. The authors come to concluded that there was an ideal reaction temperature, one optimal pressure of the reaction system and an ideal reaction time gives. As the amount of air increases, the yield of o-toluic acid.

Cheng Z., Li G., Li W., Dai L., Niu Y., Hu J., Gu J., Mao X. and Li M. report in the Chinese journal "Journal der Fuel Chemistry "(Ranliao Huaxue Xuebao), 1981 Volume 9, Issue 2, Pp. 152-162 on a "Study of the Kinetics of Oxidation reaction of o-xylene ".

DE 100 02 807 and DE 100 02 810 also relate to methods for the oxidation of o-xylene with oxygen.

The two-stage reaction of the oxidation of o-xylene to phthalic anhydride generally has a better selectivity than the one-stage oxidation in the gas phase. Nevertheless, there is also a risk of over-oxidation of the o-xylene in the case of liquid phase oxidation with the formation of CO _x , acetic acid, formic acid or other products which arise from the loss of one or more carbon atoms from the o-xylene and can no longer be converted into phthalic anhydride by further oxidation ,

The invention is therefore based on the object of a method to state the liquid phase oxidation of o-xylene, if possible selectively leads to such oxidation products that Phthalic anhydride are further oxidizable.

According to the invention, this object is achieved by a method in which an o-xylene-containing liquid stream is passed through leads at least one reaction zone, and at least two in Flow direction of the liquid stream at spaced locations Bubbles oxygen-containing gas in the liquid stream.

If the liquid stream is passed through more than one Reaction zone, e.g. B. by two or more connected in series Guided reactors, there is no between the reaction zones gas / liquid phase separation. The last reaction zone becomes a Liquid containing oxidation products of o-xylene Reaction mixture and an oxygen-depleted gas stream (hereinafter also "exhaust gas").

The oxygen-containing gas usually contains oxygen one or more inert gases, such as e.g. B. nitrogen, argon or Carbon dioxide. The oxygen volume fraction of the oxygen-containing Gases is preferably 10 to 90 vol .-%, preferably 15 to 35 vol%; the rest is made up of inert gas. The usage of air as the oxygen-containing gas is particularly preferred. The oxygen-containing gas is preferred under conditions bubbled in an intimate mixture of the gas and liquid phases allow.

According to the inventive method with high Selectivity in addition to a small amount of phthalic acid Obtained oxidation products of o-xylene, which further to phthalic anhydride are oxidizable, especially o-tolylic acid, methylbenzaldehyde, Methylbenzyl alcohol, phthalide, di (methylbenzyl) phthalate, Methylbenzylmethylbenzoate, mono (methylbenzyl) phthalate, Methylbenzylformylbenzoate, formylbenzylmethylbenzoate, monomethylphthalate, Methylbenzyl formate and methylbenzyl acetate, of which o-tolylic acid den Forms the main part. As far as the "selectivity" of the Oxidation reaction is addressed, the selectivity with respect to the sum of the above compounds.

There is a risk of undesired over-oxidation of the o-xylene Occurrence of local oxygen concentration maxima. Therefore takes place in the method according to the invention oxygen-containing gas in a way that is local Largely avoids oxygen concentration maxima. At the first dossier the bubbled oxygen-containing gas reacts under Oxygen consumption with xylene, the dispersed gas phase of oxygen impoverished. This can be done at the second and further dosing points bubbled oxygen-containing gas with the dispersed oxygen-poor gas phase with the formation of gas bubbles with a coalesce average oxygen content.

So that a sufficient mass transfer of oxygen from the Gas phase takes place in the liquid phase, the Oxygen content in the gas phase does not fall below a critical value. This is preferably achieved in that the Oxygen volume fraction of the exhaust gas is monitored. The is preferably Oxygen volume fraction of the oxygen-depleted gas 0.01 to 7.5% by volume, in particular 0.05 to 6% by volume, particularly preferably 1 up to 6 vol .-%.

Arbitrary are for carrying out the method according to the invention Reaction apparatus suitable in which by appropriate organs, such as Gas distributor or nozzles, a gas phase into a liquid phase is dispersed. Suitable examples are Stirred tank reactors and bubble columns.

In a preferred embodiment are in the reaction zone Fixtures provided that are essentially in one level extend perpendicular to the direction of flow of the liquid stream and are spaced from each other. The internals divide the Reaction zone in individual chambers by the liquid stream to be passed one after the other. The internals prevent one Backmixing in the direction of flow. Preferably you pearl it oxygen-containing gas at at least one point between two successive internals. The reaction zone is preferably elongated and in particular has a ratio of Length to diameter of more than 5, especially more than 10 on. The number of those separated by internals Sections can e.g. B. 2 to 15 or 5 to 10. suitable such internals are such. B. perforated plates or valve bottoms.

The reaction zones or through are advantageously Internally separated sections of a reaction zone Provided with a packed bed at least in some areas. The Packing causes a (re) dispersion of the gas phase and good cross-flow mixing.

The oxidation of the o-xylene is exothermic. The emerging Heat of reaction is preferably removed. You can do this in the Reaction zones arranged or on the outer reactor jacket attached or in thermal connection Provide heat exchangers by a suitable cooling medium be flowed through. So it is z. B. possible in the reaction zones to provide arranged cooling coils or cooling fingers. at Embodiments of the method according to the invention, in which a Partial stream of the liquid removed from the last reaction zone Reaction mixture is returned before the first reaction zone, a preferred way of removing the heat of reaction is to cool the recycle stream by z. B. by a Heat exchanger conducts. Of course, different types heat dissipation can be combined.

The pressure is preferably 10 to 30 bar, in particular 15 to 25 bar. If you have several in series Reaction zones used is the pressure in the individual reaction zones preferably substantially the same (except for a small one Pressure difference required for the transport of the mixture of liquid Stream and dispersed gas phase from a reaction zone to next is required). The reaction temperature is preferably 120 to 210 ° C, especially 138 to 180 ° C, particularly preferably 140 to 165 ° C. The dwell time of the liquid stream in the reaction zone or the accumulated residence time in the Reaction zones when using reaction zones connected in series is preferably 0.5 to 4.5 hours.

The process according to the invention is preferably carried out in the presence of a transition metal-containing catalyst.

On the one hand, the catalysts are soluble in o-xylene Transition metal salts or complexes, such as cobalt and / or in particular Manganese salts or complexes, suitable. Aliphatic cobalt salts, alicyclic or aromatic carboxylic acids are preferred. Examples include naphthenates, acetates, propionates, butanoates, Pentanoates, hexanoates, heptanoates, octanoates, 2-ethylhexanoates, Nonanoates, isononanoates, decanoates, laurinoates, palmitates, Stearates, benzoates, tolylates, phthalates and citrates. examples for other molecules that have a solubility of transition metals cause are acetylacetonate, porphyrins, salcomin, salen or Phthalocyanines. A suitable catalyst is e.g. B. Cobalt naphtenate. The o-xylene-containing feed is preferably outside the Reaction zone with the catalyst, suitably in Form is metered in a catalyst stock solution. This will the coincidence of local high catalyst concentrations with the reactants o-xylene and oxygen avoided, which otherwise could lead to undesired overoxidation. Generally speaking the catalyst in an amount of 2 to 500 ppm, preferably 5 up to 50 ppm, calculated as metal, based on o-xylene-containing Inflow, used. Higher catalyst concentrations can increase lead to undesirable precipitations in the reactor.

Alternatively, heterogeneous catalysts, i.e. H. in the liquid Electricity insoluble or hardly soluble transition metal salts or complexes suitable. They can be in the form of a suspension in the liquid stream or be fixed in the reaction zones, e.g. B. as Filling or coating the inner walls. As a more heterogeneous The catalyst is e.g. B. Cobalt (II) oxalate is suitable.

After leaving the reaction zone, the reaction discharge suitably fed from a gas-liquid separator in which the liquid reaction mixture and the oxygen-depleted Gas can be separated from each other. As a gas-liquid separator z. B. a gravity separator, d. H. one with lower Flow velocity through the vessel in which the Gas phase and the liquid phase due to their different Separate density from each other, suitable. Alternatively, a Swirl separator, d. H. a cylindrical container in which the mixed-phase reaction discharge tangential to the container wall is used.

In preferred embodiments, the separated liquid Reaction mixture at least partially in front of the reaction zone recycled. The return of part of the liquid Reaction mixture allows the maintenance of a sufficiently large one Liquid volume flow in the reaction zones. At the same time with the recycled liquid reaction mixture is preferred a fresh feed containing o-xylene into the reaction zone fed. The ratio of feed to recycled liquid The reaction mixture is generally 1:10 to 1:200, preferably 1: 50 to 1: 100. Expediently, one is the feed stream appropriate amount of that removed from the reaction zone liquid reaction mixture discharged as a product stream.

The product stream can be prepared using conventional methods, in particular by distillation, the oxidation products of o-xylene, in particular o-tolylic acid, methylbenzaldehyde, methylbenzyl alcohol, Phthalide, methylbenzylmethylbenzoate and monomethylbenzylphthalate, be isolated. The separated, unreacted o-xylene is expediently returned to the oxidation reaction.

For the production of phthalic anhydride, the separated Oxidation products evaporated and in a second stage known manner in the gas phase in the presence of a oxidized gas oxidized to phthalic anhydride. to The second stage is generally used one Fixed bed reactor, preferably a tube bundle reactor. As Catalysts are suitable for those with an active mass Titanium dioxide in the form of its anatase modification and / or Vanadium pentoxide.

The invention is illustrated by the accompanying figures and the the following examples are explained in more detail.

Fig. 1 shows a schematic representation of an apparatus suitable for carrying out the method according to the invention with a device divided by means of internals reaction zone.

Fig. 2 shows a schematic representation of an apparatus suitable for carrying out the method according to the invention apparatus having two successive reaction zones.

According to FIG. 1, o-xylene 3 is premixed with a catalyst stock solution 2 and the feed containing o-xylene is fed to reactor 1 . The reactor 1 is divided into chambers by perforated plates 10 a-d arranged transversely to the direction of flow. Via lines 4 a- 4 e is in the reactor 1, an oxygen-containing gas, eg. B. air introduced. After leaving the reactor 1 , the reaction discharge is fed to the gas-liquid separator 6 . The liquid phase is cooled in the heat exchanger 8 and fed to the pump 9 . A partial stream of the liquid reaction mixture is discharged via line 5 . The oxidation products of o-xylene formed are isolated therefrom by methods known per se. The other partial stream of the liquid reaction mixture is introduced again into the reactor 1 via line 11 .

Referring to FIG. 2 o-Xylene 3 is premixed with a catalyst stock solution 2 and the o-xylene-containing feed supplied to the reactor 1 a. The order from the reactor 1 a is fed to the reactor 1 b via line 12 without phase separation. B via lines 4 a, 4 a in the reactors 1, 1 b an oxygen-containing gas, for. B. air introduced. After leaving the reactor 1 b, the reaction discharge is fed to the gas-liquid separator 6 . The liquid phase is cooled in the heat exchanger 8 and fed to the pump 9 . A partial stream of the liquid reaction mixture is discharged via line 5 . The oxidation products of o-xylene formed are isolated therefrom by methods known per se. The other partial stream of the liquid reaction mixture is introduced again into the reactor 1 a via line 11 .

Examples 1 and 2

A device with two reactors and a circulating pump connected in series, as shown in FIG. 2, was used. The two reactors are vertically arranged high-pressure reaction tubes with a length of 2 m and a diameter of 30 mm. A metering point for air was arranged near the lower end of the reaction tubes. A continuous liquid oxidation of o-xylene was carried out at a pressure of 20 bar. The supplied o-xylene was mixed with cobalt naphtenate before being introduced into the first reactor. An amount of reaction mixture corresponding to the o-xylene fed was continuously discharged from the second reactor. The composition of the oxidation products was analyzed by high performance liquid chromatography. The operating parameters and analysis results are summarized in the following table.

Example 3 (comparison)

A device was used as in the previous examples, but comprising only one reactor.

You can see that the overall selectivity is higher if the Air is introduced at two dosing points.

Claims (8)

1. Process for the preparation of oxidation products o-xylene, which can be further oxidized to phthalic acid, in which one an o-xylene-containing liquid stream through at least one Reaction zone leads and at least two in Flow direction of the liquid stream at spaced locations Bubbles oxygen-containing gas in the liquid stream. 2. The method according to claim 1, wherein one in the reaction zone Fixtures that are essentially in one level extend perpendicular to the direction of flow of the liquid stream and are spaced from each other. 3. The method according to claim 2, wherein the oxygen-containing Gas at at least one point between two successive internals. 4. The method according to any one of the preceding claims, wherein the Volume fraction of oxygen in that from the reaction zone dissipated oxygen-depleted gas flow 0.01 to 7.5 vol .-%. 5. The method according to any one of the preceding claims, wherein one a transition metal salt or complex as a catalyst used. 6. The method according to claim 5, wherein as a catalyst in liquid stream soluble cobalt salt of an aliphatic, alicyclic or aromatic carboxylic acid used. 7. The method according to any one of the preceding claims, wherein one the liquid withdrawn from the reaction zone Some of the reaction mixture is returned to the reaction zone. 8. The method of claim 7, wherein the volume ratio of o-xylene feed to recycled liquid reaction mixture 1:10 is up to 1: 200.