DE2216208C3 - Process for the continuous production of terephthalic acid - Google Patents

Description

(a) intermittently a part of the oxidizing reaction product from the Oxydat.onsreaktor removed and,. .

Cb) the withdrawn reaction product in a first crystallizer transferred,

(C) the reaction product is oxidatively aftertreated there at almost the same or lower temperature and pressure conditions than them used in the first main oxidative reaction were, taking molecular oxygen-containing gas during a Duration of 1 to 5 minutes initiates and

^{(d) forwards} the main part of the reaction product, which is contained in the first crystallizer, to a subsequent second crystallizer -

^{a5 is} deducted (see Japanese patent _publication 742/19Og. ^^ _{dn continuous}

A * "^ _Solv edium and catalogs

Initially in the reactor together with £% _and the oxygen-containing gas

the AiKy ^ s _{the reaction} con- domestic product _per

^e g ^D [? _{ch is} withdrawn from the bottom of the reactor inuie J «p _ate ntverubl.chung 10119/1964

(see japams ^

^bz J-. ¹ J _{oxidation in} liquid phase of p-dialkyl-In de ^r "J _{Q, to} form vonTerephthal ^{benzenes" 'w} i difficult to control the oxidation reaction acidic isi ^ _{füh and the} thereby obtained with intermediates such as ben _Z aIdehyd, p-toluic acid , and other _ducts contaminated which the

α affects the color quality of the product

Between «8 ™ · _{years the} so-called direct warming process was used (cf. Japanese KKSSSihungen 3168/1958 and 3840/1961). nl; Ai _P c _Pm process is used for high-purity terephthalic, _{or ethylene} oxide

acid direkm ^ ^ _{terephthalic acid with large} .

Si "prf" i _s implemented as it is very economical Bern ^ ^ cte Stdle over conventional supply and ζahlre cne »terephthalic acid poly-

besi drive currently ₁ oei u _{ächst to} dimethyl

£ ^ e! which much easier than can be. However, the station method is a ter-

.. .. _χ Raw material er quality than ^ R ^ ™

, step as _{interim) the} oxy complete

with direct

from

those in the ^^ "

for example ^

40

There have been various processes for the preparation of terephthalic acid by oxidation of

Ä1 SSSÄSTSXJS 5 pounds! To effectively maintain the purity of the product

to increase. . . ,.

To this end, a number of improvements to Vervdtatändigmg oxidation in flu ssige rte ™ · ^ W 1 ^"nt" J ^ ^g has already been

Leads can generally be in the following

Sf ^{- the type of execution B.}

«Ft

ss

Finally, reaction product after the reaction has ended is withdrawn from the reactor (see Japanese Patent Publication 742/1966).

A second group comprises a semi-continuous process in which an alkylbenzene together US a TcH a "mixture" of solvent and catalyst is in certain cases continuously introduced together with an oxygen-containing gas into a reactor, in which the solvent and the oxidation catalyst were introduced, followed by the entire reaction product at once from the reactor after the reaction has ended (cf. GB PS 903677 and I FR Pb IJM ^ this process to achieve a complete oxidation reaction works on is' ^ ^{st it} f ^^ J. part of that, due to the_ formation ^ by-products also be on prolonged treatment with acidic material at higher temperatures al j>, de Hauptoxydat.onsreaktion ™ J ^ß eend "clothes" r oxidation reaction one ™ ** ™ l% * of the sour look and ^^^ ¹⁰ ^ ¹ ^, ^ ™ " ¹ ^ ^ _n ". Substance concentration in the reaction "gh» ^ " ^l ™ ierl the initiation of the oxygen-laden noses" in the system during a longer period of time

3 4

Risk of explosion occurs, whereby the explosion limit is free solvent in the endoxidation zone about 8 vol.% oxygen content in the gas phase when the water formed in this zone is in the steam. form along with the solvent vapor far in US Pat. No. 33 54 202 is a process is removed so that in the end zone the for the production of benzene polycarboxylic acids by 5 solvent concentration at above 90% oxidation can be kept in the liquid phase of polyalkylbenzenes.

indicated in a semi-continuous manner, with US Pat. No. 3,092,658 describing a cond this process continuously air according to the continuous system for oxidizing aromatic bringing the total amount of polyalkylbertzene hydrocarbons with the formation of carbon feed during a period of time corresponding to io acids, the The reaction system comprises at least 2 oxy-vio to vi of the total time required for the loading stages and is introduced into the last oxidation charge of the polyalkylbenzene, so that air is introduced in a controlled amount,
that the oxidation reaction is completed, whereby the main oxidation reaction itself is oxybenzaldehyde in the procedures described above by means of a very pure terephthalic acid, which is only oxybenzaldehyde according to US Pat , contains, can be obtained. When carried out in a plurality of reactors,
However, this method is the quality of the obtained. In a conventional method for carrying out tenen terephthalic acid unsatisfactory for the oxidation reaction in a continuous manner, use as a material for the direct polymer is common, for example that in the US patent specification, since the according to the The above ao 29 62 361 process described above, where US patent specification obtained terephthalic acid has a continuously withdrawn from the reaction product from the optical density (ε 380 ΐημ) of greater than 0.1 and continuously and more than 0.1 ° / o Contains 4-carboxybenzaldehyde as a compound in one or more crystallizers of major impurity. Furthermore, the process of sizing is introduced and effective therein during a foregoing U.S. patent if left for the appropriate period of time to employ the crystalline semi-continuous process to effect cation of terephthalic acid. According to since the oxidation reaction ends only in this US patent specification, the reaction product being achieved when air alone is introduced into the reactor during a given period of time continuously in a certain flow rate, which still contains the reaction ratio from the oxidation reactor which was obtained and then put into the large-sized crystallizer after the oxidation reaction is carried out once, while continuously terminating the reaction after all of the supplied product from the crystallizer was continuously introduced into it in a same methylbenzene flow rate as the introduction However, the process cannot be carried out on one, but it is impossible to apply an off-operation in a continuous manner, 35 reaching effect of the subsequent reaction in which the reaction product is continuously obtained, even if an oxygen-containing one Total gas condensed reaction system during the course of which would be continuously introduced into the crystallizer. The oxidation reaction is withdrawn. The reason for this lies in the fact that the amount of active The continuous process, whereby the feed reaction products, which are to be subjected to the reaction, in particular, alkylbenzene, solvents and catalysts, are introduced continuously to the reactor, equal to the total amount of Reactant while continuously the oxidation reaction in the crystallizer is so low that it is withdrawn due to the product, has a great economic inadequate contact with the introduced borrowed advantage, as there is a much better space-time oxygen can not be oxidized more effectively, thus yield per unit capacity of the reactor and per 45 the explosion limit is not exceeded, ie results in a unit of time and a simplified operation with less than 8 vol.% oxygen concentration compared to the single batch process or the semi-vapor phase of the reactor. Otherwise from continuous operation results. On the other hand, the amount of oxygen introduced in the continuous process has the disadvantage that known processes are too small to obtain a sufficient quality of the terephthalic acid obtained, and intimate contact with the reactants is unsatisfactory due to the increased content, so that the final reaction of unreacted feed material and always insufficiently takes place due to the lack of intermediates, since the proportion of oxygen in the crystallizer. In addition, it is reaction material which is impossible to withdraw the effect of the final oxy-fuel from the reactor without improving a sufficient residence time in the dation by having the oxygen reactor is greater than in the case of a single amount in a considerable amount Way is increased as the batch or a semi-continuous process. Consumption of oxygen is so low that in US Pat. No. 3,064,044 a multi-continuous introduction of the oxygen-containing gas is described as a multi-stage oxidation system for the production of the concentration of oxygen in the system to -dicarboxylic acids, with an aromatic 60 increasing in size so that the hydrocarbon explosive limit is exceeded immediately in several oxidation zones and the industrial reaction with molecular oxygen in the presence of operation becomes dangerous. When the reaction product withdrawn from the oxidation of an oxidation catalyst at elevated pressures is continuous and subjected to temperatures. In particular, when fed to the crystallizer and a final oxidation is to be withdrawn from it below 65, the phenomenon occurs that essentially anhydrous conditions enable part of the reaction fed to the crystallizer. This is done essentially by the product in this not remaining for a sufficient introduction of large amounts of practically water- time period in order to be in contact with the oxygen

To get agitation, but withdrawn from the crystallizer a crystallizer of small size is withdrawn in a state in which the and those contained in the reaction product are active Inadequate response. From ven connections, especially one additional it can be concluded from this phenomenon that the final reaction is subject to, i. H. such final reaction not carried out effectively 5 substances which were in very small amount in the reaction. but a serious one

The object of the invention is therefore to create damage and deterioration in the color value a process for the continuous production and the purity of terephthalic acid formed from High purity terephthalic acid of a fiber quality, cause product if it is converted into by-products which are converted to use as material for direct io or unreacted in reaction polymerization suitable, wherein the terephthalic products are contained, effective in the crystallizer acid directly with ethylene glycol or ethylene of small size with the oxygen-containing gas, oxide and converted to a polyester of high which this for a short period of time in Quality is polymerized. a relatively large amount, but below that

This problem is solved according to the invention, explosive limit is introduced, contacted according to the person by a process for continuous production, thereby producing the final reaction of terephthalic acid, whereby p-xylene is con- ends, with the conversion of the various continuous in an oxidation reactor together with the VeT contained in the reaction product lower aliphatic carboxylic acid as solvent impurities either in terephthalic acid or an agent, in particular acetic acid, in which 2 to more often ao other harmless substances is effected, so that Amount by weight of p-xylene and with an oxyda- it is now possible to make a terephthalic acid of tion catalyst, especially a cobalt, to produce high-purity manganese in good yield, and bromine-containing catalyst, is introduced by the process of the invention

and in the liquid phase with molecular oxygen, on the other hand, terephthalic acid of high purity or a molecular oxygen-containing gas, 35 continuously in good yield without the above especially air, at a temperature of 80 to disadvantages are formed.

250 ° C and an overpressure of 2 to 30 kg / cm ^a The inventive method is applicable to all

during a reaction time of 0.5 to 6 hours, a process for the continuous production of in particular 1 to 3 hours, is oxidized, and the terephthalic acid by oxidation in the liquid phase Terephthalic acid formed in this way from the reaction of p-xylene can be used, but is particularly valuable separated by crystallization, which thereby leads to the production of terephthalic acid from fiber characterized is that one quality, which as material for the direct poly

merization is used, with terephthalic acid

(A) intermittently part of the oxidizing reaction product converted directly with ethylene glycol or ethylene oxide from the oxidation reactor 35 is set.

removed and The oxidation of p-xylene in the liquid phase at

(B) the withdrawn reaction product in a first process according to the invention can after all Crystallizer transferred, usual ways are carried out, however, the

(c) the reaction product there oxidatively according to particularly favorable reaction conditions for this acts at almost the same or lower tem- 40 the following are:

temperature and pressure conditions than they are at the The applied reaction temperature is in the

The main oxidation reaction used was from 80 to 250.degree. C., preferably from 130 to where molecular oxygen is contained Gas is carried out for a period of 1 to, the reaction must be initiated under Uberantmo-5 minutes, and 45 spherical pressure must be carried out so that the p-xylene

(d) the main part of the reaction product, which is contained in and the solvent in the liquid phase in the case of the first crystallizer, are kept at a reaction temperature. Second, reaction-closing crystallizer pressures of 2 to 30 kg / cm ² gauge are generally preferred. The average dwell time

According to the method according to the invention, 50 of the reactants in the oxidation reactor can bcclic Intermediates completely to terephthalic acid takes 0.5 to 5 hours, preferably 1 to 3 hours, are oxidized and the formation of by-products. The solvents to be used include

due to the lack of oxygen prevents lower aliphatic carboxylic acids such as acetic acid, be, with the oxidativc aftertreatment of propionic acid and butyric acids. From this the Reaction product in the first crystallizer, following the use of acetic acid particularly favorably referred to as "final implementation". The amount of the solution to be used only Post-oxidation of the reaction products is used. tcls is at least 2fuche of the weight According to the process according to the invention, p-xylene can be used as a feed mass, but this is Terephthalic acid of high purity in good use in a 3 to 6-fold amount in general can be obtained. 60 mean cheap. The oxidizing agent to be used

The invention will be adjusted based on which consists of molecular oxygen or a mole Zcichmmg explained in more detail. culurcm oxygen-containing gas. The oxygen

The figure of the drawing shows a schematic illustration of Cms comprising those with oxygen concentration flow schemes represents, which an embodiment of the trationcn above 7 vol .- "/», and the application Illustrated method according to the invention. 65 of air is around most convenient and cheapest. the

After completing the procedure, the amount of unused oxygen is 3 to in a defined amount intermittently from the 500 mol, preferably S to 300 mol per mol of p-xylene. Oxidation reactor withdrawn reaction product to comprise the catalysts to be used

Cobalt, manganese and similar heavy metals should be chosen longer than the sum Compounds and compounds containing bromine. As the one containing heavy metals to bring about the final reaction Compounds are inorganic necessary time, which is necessary for the introduction of the reaction salts, Naphthenate and lower aliphatic acid product from the oxidation reactor to the first salts of cobalt and manganese are particularly cheap. The time required as 5 crystallizer and the compounds containing the bromine are hydrogen bromide, bringing the reaction product from the first Kri-Cobalt bromide, Sodium bromide, potassium bromide and stallizer required for the second crystallizer bromine-containing organic compounds, such as tetra-time increases.

bromethane and elemental bromine especially to be used in this way intermittently in certain benefits. It is also possible to use further reaction components withdrawn from the oxidation reactor, such as ketones, aldehydes and the like. The reaction product must be used intermittently in the first. Crystallizer are introduced and in it the reaction product, the oxidation reaction of which closes the reaction, are subjected. None is finished, is intermittently, in a defined manner, the beneficial effect of the final reaction is withdrawn from the reactor and then achieved in parts when the reaction product is fed to the first crystallizer that completes the crystallization. The sator is introduced after a long period of time since the completion of the amount of the reaction pro-oxidation reaction withdrawn at one time, the product can be adjusted to a suitable value manner as the equipped Oxydaschaften the "0 tion reactor contained in the reaction product, since the oxidation reaction crude terephthalic acid, the quality of the final formed takes place at elevated temperature and elevated pressure is terephthalic acid, the effectiveness. The content of the first crystallizer may be considered to be reaction in the first crystallizer, the yield of far less than that of the oxidation reaction terephthalic acid per hour and the like, and it may be transferred from a crystallizer. In general, the withdrawal pass 25 will be of the borrowed type. In particular, at intervals of 2 to 10 minutes each, the content of the crystalline favorably has such a content that it is preferred. In such a procedure, part of the reaction product is withdrawn from the oxidation reactor at once in an amount of reaction product from 30 to 70%, preferably from 50 to corresponding to the sum of p-xylene plus catalyst 70% by volume of the total content of the crystallizer -Solvent-liquid mixture, which takes up to 30 vessels. The use of a Kristallisadem oxidation reactor for a period of time of such a large capacity that only an amount within the range of 2 to 10 minutes of less than 30% by volume of the total content is drawn from the reactor in an amount tes of the reaction product withdrawn at once discharged once to the first crystallizer. Ingested is not favorable, since it is economical in this case the actual amount of application is disadvantageous and insufficient contact at one time is usually far greater than that between the reaction product and the oxygen sum of p-xylene plus catalyst. Solvent-containing gas results, which in turn leads to a bad mixture per hour per application interval on the effectiveness of the final reaction. The reason for the increased volume of the contents in the Vcr- The method according to the invention is therefore the same as the state before entry into the reactor 4 ° characterized in that the introduction and gate. In general, this increase or discharge rate of the reaction product in the expansion of the volume in the oxidation catalyst reaches a terminating rate with defined proportions with an increase in use of 20 to 40% more, although this is carried out in a vessel of relatively small size as a function of the reaction capacity of the crystallizer used The same varies as the initial volume is never obtainable before the successful result obtained, if introduction into the oxidation catalyst is carried out continuously due to the phenomena inevitable the introduction and discharge of the reaction with the inclusion of the product, even if the contents undergo thermal expansion Because a small-sized crystallizer is used for the purpose of the reaction at high temperature, the presence of the reduction in the amount of the gas contained in the first crystalline oxygenator in the slurry, the reaction product contained in the 5 ° sator The formation of 2 moles of water and 1 mole of tercphthalate and part of the acid discharged to the crystallizer from one mole of p-xylene and the like reaction product on contact with the acidic-in too long interval of discharge above the substance-containing gas base A short variation of the range listed above, namely 2 to is prevented and from the crystallizer after 10 minutes, the removal of an excessively complete, final conversion of the reaction product results in a large proportion of the reaction product, which leads to an uneven reaction within once and is not Due to the flow of the entire reaction material, the liquid level in the reactor is reduced, so that a terephthalic acid of only low purity becomes unstable as the oxidation reaction progresses. are.

On the other hand, too short an interval is the discharge operation of the reac

carryover i.ti time period below the above recovery product in the crystallizer is finished, win

guided area, d. H. the application of a vcr- the envisaged brightening implementation through a position cmc

ringertcn mence of the reaction products * at once. Oxygen-containing gas is carried out for this purpose. The off

not favorable, since this is an abbreviation of the Zeitsch icBcndc implementation is enipcratiir with such 1

raumcs the final reaction yields, so Duss ⁶ 5 and pressure conditions executed, the almost glcicl

a sufficient final response prevented or below those at the main

_w j _n j oxidation reduction are feared, lcmperalurc

The distance between the withdrawal of the reaction products is in the range from 80 to 250 ° C., preferably 130 bi

200 ° C. and pressures in the range from 2 to 30 kg / cm ² are particularly favorable. If the temperature is too low, the effect of the final reaction is poor, and if the temperature is too high, there is an adverse effect on the quality of the terephthalic acid formed. If the final reaction in the first crystallizer is carried out at lower pressures and temperatures than in the main oxidation reaction in the oxidation reactor, some of the terephthalic acid in the reaction mixture will crystallize.

The gas containing the molecular oxygen preferably has an oxygen content of at least 7% by volume, and the use of air is most advantageous. The amount of air used is preferably 10 to 50 NM ³ per ton of reaction product, although it varies depending on the quality of the desired terephthalic acid, the concentration of oxygen in the final reaction system and other reaction conditions. The feed rate of the oxygen-containing gas is preferably as high as possible, and the period of time for the final reaction is as short as possible, generally 1 to 5 minutes. Longer periods of time in the final reaction only result in the risk of explosion due to the increase in the oxygen content and also result in damage to the quality of the terephthalic acid formed.

After the subsequent reaction has ended, the reaction product in the first crystallizer is released into the second crystallizer. In this case it is advantageous to create a liquid-closed state inside the vessel in which part of the reaction product is kept within the vessel at all times and not the entire amount of the reaction product is drained off. However, the amount so that it is preferred should the retained in the vessel the reaction product not be large in order to prevent an advance of the effective final reaction, not more than 3O ^and / or, in particular not more than 20 ⁰ Ai, the volume of the to leave the entire reaction product in the first crystallizer.

In the process according to the invention, part of the reaction product contained in the oxidation product is removed intermittently, so that the oxidation reaction must be carried out using a device in which the reaction product can be effectively drained off. All methods or devices can be used for this purpose, provided they are equipped with devices or devices for the intermittent discharge of the reaction product. A preferred example for a usable reaction device is of the following type:

The reactor is equipped with a liquid level measuring device, which is connected to a detector and a signal indicator, through which a control valve of the drainage line is operated in such a way that the valve is closed when it is opened is when the liquid level of the friction coefficient has reached its upper C itciv / nivciui , and the valve is closed when the liquid level of the reaction product has reached its lower level. In this case, the upper (ireir / .e and the lower limit of the liquid level are each determined in a position where the range between the maximum liquid level and the minimum liquid level where the continuous oxidation reaction takes place smoothly. For this purpose, the application is However, all types of liquid measuring devices can of course also be used, even if malfunctions in operation and susceptibility are often caused due to corrosion with the solvent and the presence of crystalline terephthalic acid in the liquid.

An example of an embodiment of the invention is described in detail below with reference to the drawing explained.

In the drawing, the reference number 1 denotes the oxidation reactor, 2 denotes the stirrer, 3 denotes a motor to drive the stirrer, with 4 a line for ao introduction of the p-xylene charge mass, solvent and catalyst, with 5 a line for introducing the oxygen-containing gas, with 6 a line for the removal of the reaction products?;, with 7 a line for discharging the exhaust gas, with 8 a Cooler, with 9 a receiver for the condensed liquid, with 10 a line for recirculation the condensed liquid and 11 denotes a line for discharging the exhaust gases. Reference numbers 12 represent a first crystallizer for effecting the final reaction and the crystallization operation of the withdrawn from reactor 1 Reaction product, 13 a stirrer, 14 a motor for driving the stirrer, 15 a line for introducing an oxygen-containing gas, 16 a line for discharging the reaction product, 17 a gas discharge line, 18 a cooler, 19 a condensed liquid receiver, 20 a line for recirculating the condensed liquid and 21 an exhaust line. Furthermore is with the reference number 22 a an X-ray source, which is connected to the reactor 1, and with 22 /) an X-ray detection device, whose signal of the liquid level to the opening and closing the control valve 24 present in the reaction product discharge line 6 through the Compound 23 is used.

The liquid level measuring devices 22 ″, 22 /) and the control valve 24 are set so that the valve is closed when the liquid level in the actuator reaches the upper limit level 25. The reference number 27 in the first crystallizer 12 represents a liquid level which is reached when the au; The reaction product withdrawn from the reactor 1 is introduced, and a liquid level is designated by 28, at which the reaction product is removed after the end of the final reaction,

When carrying out the continuous production of high-purity terephthalic acid using the device shown in the drawing; a feed mixture, which in each case contains the required amounts of p-xylene solvent U " ¹ catalyst, is fed continuously into the reactor through line 4, and the satinslollhal term gas is continuously introduced from line 5 into ili stirred reaction liquid so that a continuous oxidation in liquid plinse of p-xylene is effected

converted oxygen-containing gas, evaporated solvent, water formed in the reaction and other unreacted reactants is discharged from the discharge line 7, cooled in the cooler 8 and introduced into the receiving tank 9, the non-condensed waste gases from the system being discharged through line 11 and the condensed liquid can be returned to the reactor via line 10. As the oxidation reaction proceeds, the liquid level of the contents of the reactor increases so that the upper limit level 25 is reached. In this case, the liquid level measuring devices 22a and 22b generate the X-ray-type, a signal for actuating the opening movement of the valve 24, which was held closed up to this time. The liquid level in the reactor then gradually decreases as the reaction product is withdrawn therefrom, and when it has reached the lower limit level 26, the liquid level measuring device actuates a signal again to close the control valve. By operating in this way, the reaction product is withdrawn intermittently from the reactor 1 and passed to the first crystallizer 12 via line 6. The volume of liquid when unloading at once is set in such a way that it is equal to the volume amount of the area lying within the area between the upper limit level 27 and the lower limit level 28. The final reaction is then carried out by introducing an oxygen-containing gas from line 15 into the reaction product, which has now reached its upper limit value 27 on transfer from reactor 1 as a batch charge, with stirring for a period of 1 to 5 minutes by means of a Stirrer 13 causes. The reaction product, the final reaction of which has ended, is then withdrawn from line 16 at such a rate that the one-sided portion of the discharge therefrom causes the liquid level to drop down to the value 28 and is fed to the second crystallizer (not shown) and, if necessary, continues to be added a third crystallizer wcitergelitct.

At the time when the liquid level of the contents in the reactor 1 reaches the level 25 after the application of the product of the final reaction from the first crystallizer is completed a further proportion of a period of time of the liquid discharged from the reactor 1 to the first Crystallizer 12 fed and the final reaction repeated in the same way.

The following examples serve to further illustrate the invention.

Example I.

The implementation was carried out using a device as shown in the drawing, which consists of a titanium-lined, thin-walled reactor (content -10 liters), which is connected to a return cooler, stirrer, heating equipment, IUnIaB for the charge mass, IUn I a W for ilns Silent gas, outlet for this reaction product and Gauinui chair-type liquid level mixers was designed in conjunction with a titanium-lined and tie-resistant first Crystalline foil with an internal capacity of 5 liters, the in the same way as the reactor with a reflux condenser, stirrer, heating devices, inlet for the Feed mass, inlet for the oxygen-containing gas and outlet for the reacted product and second and third crystallizers each with a capacity of 40 liters. The reactor was initially filled with 12 kg of acetic acid, 61 g of cobalt acetate, 3 g of manganese acetate and 36 g of sodium bromide charged and the contents introduced in this way at a temperature of 190 ° C

ίο held at a pressure of 20 kg / cm ² . To this reactor, air and p-xylene were introduced at a flow rate of 4.1 NM / kg p-xylene and 2.4 kg / hour, respectively, over a period of 30 minutes. With continued introduction of air and p-xylene at these rates, the reactor was successively filled with a catalyst-acetic acid solution, the ratio of cobalt acetate, manganese acetate and sodium bromide to the acetic acid used being the same as that

ao originally added ratio to the reactor was charged in an amount of 7.2 kg / hour. In the meantime, the reaction product was discharged intermittently and periodically from the reactor and discharged to the subsequent first crystallizer in accordance with the signal from the level meter in an amount of about 2 kg, each at an interval of about 10 minutes. The first crystallizer provided with the reaction product from the reactor was operated at a temperature of 185 "C under a pressure of 14 kg / cm, while air was immediately introduced at a flow rate of 1.6 NM ³ / hour for 2 minutes, so that the oxygen concentration within the system in this case was 5% by volume, so that the final reaction was effected.After completion of the final reaction in the first crystallizer, the contents were then transferred to the second crystallizer, being constant at about The second and the third crystallizer were operated under 7 kg / cm pressure and atmospheric pressure with an average time of about 60 minutes each, so that the crystallization of the terephthalic acid was effected The drain removed from the third crystallizer was then cinei solids-liquid.s-Tr Subjected to cnniing, washing and drying in order to obtain the terephthalic acid, the properties and yields of which are given in the table below.

Comparative Example I.

The procedure of Example I was repeated in the same manner, but using a Tilai Lined vessel with an inner inlet voi 40 liters was used instead of the first crystallizer, in which a large part of the amount of the Reactor withdrawn Renktionsproduktes stand stayed, and the result became intermittent

fiu rend in an amount of 2 kg at a time for this first crystal isolor together with air ii an amount of 1, (1 NM ^{; 1} / stuiule true one / in the system was 5 vol. - ¹ vol., in order to effect the final reaction, while intermittently the reaction product was withdrawn at a rate corresponding to the rate of introduction

14th

Properties and yield of the terephthalic acid obtained are given in the table.

It can be seen from the values that neither the purity nor the color of the terephthalic acid obtained satisfactory in comparison with that obtained according to the procedure of Example 1 Are worth, although the introduction and discharge of the reaction product intermittently in the first Crystallizer takes place. This is obviously due to the fact that the newly introduced Reaction product receives an insufficient final reaction in the large crystallizer, wherein the reaction product containing the active ingredients for further oxidation are dispersed in large quantities in the reaction mass and the final Reaction can be subjected to a greatly reduced concentration.

Comparative example 2

The procedure according to Comparative Example 1 was repeated in full, but the ratio of the introduction of air was increased to 4.2 NM ³ / hour. Properties and yields of the terephthalic acid obtained are given in the table. In this example, the yield and properties of the terephthalic acid were improved to a certain extent, but the process cannot be put into industrial practice because the oxygen concentration reaches values up to 15%, which is well above the maximum explosion limit during operation .

Comparative example 3

The procedure of Comparative Example 1 was repeated in full, but with no air

is fed into the first crystallizer. The results with regard to the properties and quality of terephthalic acid are listed in the table. From these values, it can be seen that the purity and color of terephthalic acid in this example suffer marked deterioration due to the final response not being effected.

example 1 Comparison
example 1 Comparison
example 2 Comparative
example 3 Purity (weight o / o) 99.97 99.96 99.97 99.94 4-carboxybenzaldehyde
hyd (ppm) 300 370 290 570 Molecular extinction
Coefficient ¹ )
(ε 380 mn) <0.01 0.02 <0.01 0.05 Color difference value b ² ) -0.2 + 0.6 -0.2 + 2.0 Yield (mol-%) 96 94 96 92 Oxygen concentration in
first crystallizer
(Vol.-0 / β) 5 5 15th <1

') Value of the absorption at 380 Γπμ determined using a spectrophotometer with a Solution of 5 g of terephthalic acid in 100 ml of aqueous 2η ammonia. The lower the value is, the better the color.

² ) The value indicates the so-called "apparent color" obtained by determining the reflection light of terephthalic acid (solid) using a color difference meter (Color Machine CM-20, manufactured by Color Machine Co.), and the 6 value denotes the yellowing; the lower the value, the better the color.

1 sheet of drawings

Claims (1)

Ij Claim: , 5 Process for the continuous production of terephthalic acid, where p-Xylo! continuously in an oxidation factor together with a lower aliphatic carboxylic acid as solvent, especially acetic acid, in the 2- to 6 times the amount by weight of p-xylene and with an oxidation catalyst, in particular a catalyst containing cobalt, manganese and bromine, is introduced and in the liquid phase with molecular oxygen or a molecular Oxygen-containing gas, especially air, at a temperature of 80 to 250 ° C and an overpressure of 2b.s30kg / cm- during a reaction time of 0.5 to 6 hours, in particular 1 to 3 hours, is oxidized, and the terephthalic acid thus formed by the reaction mixture is separated off by crystallization, characterized in that that he