MXPA98004155A - Methanolysis of distilling residues of the gross ester distillation in the witten-hercules procedure for the obtaining of dimetiltereftal - Google Patents

Abstract

The invention relates to a process for the methanolysis of the distillation residue of the crude ester distillation in the process for the preparation of dimethylterephthalate, according to the Witten-Hercules method, characterized in that methanol is introduced directly into the methanolysis zone (3.5). more than one plane. Advantageously, methanol is introduced in 2 to 10 planes, particularly 3 to 8 planes, to the methanolysis zone (3.

Description

METHANOLYSIS OF DISTILLING RESIDUES FROM THE DISTILLATION OF GROSS ESTER IN THE WITTEN-HERCULES PROCEDURE FOR THE OBTAINING OF DLYMETILTEREFTALATE The invention relates to an improved process for obtaining substances of value from the distillation residue of the crude ester distillation in the Witten-Hercules process for the production of dimethylterephthalate (DMT), separating the high-boiling substances that are not desired . 1. The Witten-Hercules procedure and the methanolysis According to the Witten-Hercules procedure, DMT is obtained by oxidizing p-xylene (PX) in the presence of a catalyst containing cobalt and mangano, in a first step of oxidation first to p-toluyl acid (PTS). ), it is esterified with methanol to p-toluyl acid methyl ester (PTE), which in a second oxidation step is catalytically oxidized to monomethylterephthalate (MMT), which is again esterified with methanol to DMT. The first and second oxidation steps are carried out totally or partially with the substances involved mixed. In this mixture a part of the PTS is also directly oxidized to terephthalic acid (TPS), which is esterified together with the rest of PTS and directly results in DMT. The oxidation process, the so-called oxidation, is carried out with oxygen or with gases containing oxygen, as air, discontinuously or continuously in individual reactors or cascades of multistage reactors. The crude ester mixture produced in the esterification and containing predominantly DMT and PTE is rectified in the so-called crude ester distillation in a column system and divided into three fractions. The easily boiled, fraction containing PTE, is returned to oxidation. The medium boiling is called crude DMT, and is purified in other process steps that can be composed, for example, by distillation and crystallization, or can be used to obtain pure terephthalic acid (TPA), which can be esterified directly with glycols. From the highest boiling fraction, the residue (hereinafter referred to as "distillation residue"), the substances of value contained therein are obtained in a further treatment. The subsequent treatment is usually made up of a so-called methanolysis. From the residue that remains after the methanolysis (hereinafter referred to as "methanol residue"), the catalyst components are usually extracted as acetates with an aqueous solution containing acetic acid and fed back to the oxidation instead of fresh catalyst. Finally, the waste from the extraction is incinerated to obtain energy in a waste incineration equipment. In the methanolysis, the distillation residue together with methanol are brought to the desired elevated temperature, at which the desired reactions take place under pressures which are usually around normal pressure. New substances of value are then formed, as will be illustrated below, which are added to the substances of value which, due to their partial separation, remained in the distillation residue of the crude ester distillation. The valuable substances are distilled with methanol vapor, are defiled and taken to other stages of the DMT process, where they are finally transformed into DMT. The "high boiling substances" that are co-distilled, but not desired in the process or that even cause disturbances, are usually separated distillatively from the substances of value still in the methanolysis, before the feedback of the substances of value , and they end up in the residue of the methanolysis. The methanolysis is very complex, in which the following reactions and the following processes play a role: (1) Esterification with methanol of non-esterified acids that were still in the distillation residue, such as MMT, TPS and residual PTS, as well as of carboxyl groups, which are linked to molecules of high molecular weight, to obtain the corresponding methyl esters; (2) Re-esterification with methanol of esters of high-boiling aromatic alcohols contained in the distillation residue, such as p-toluyl alcohol (pTA) and p-hydroxymethylbenzoic acid methyl ester, aromatic acids occurring in the process, MMT, TPS and PTS, to obtain the corresponding methyl esters and the aromatic alcohols that are released; (3) Etherification with methanol of the aromatic alcohols that are released by re-esterification (2), to obtain the corresponding methyl ethers. As in reactions (1) to (3), substances are obtained that are relatively easily volatilized, that is, that are distilled under the conditions of methanolysis or that are stabilized by the hot methanol vapor, the equilibrium of the reactions of esterification and re-esterification is continuously displaced in a desired manner in the direction of the formation of the substances of desired boiling value desired. In turn, the substances of relatively low volatility value originally present are distilled without chemical modification, or else they are distilled by the hot methanol vapor. These substances include: (4) DMT as well as their precursors of relatively easy volatility, such as PTE, terephthalaldehyde methyl ester (TAE), HMBME, p-methoxymethylbenzoic acid methyl ester (MMBME) and similar substances. In addition to these desired separation reactions and operations, there is a definitely undesired one, namely (5) the stabilizing distillation of substances of relatively easy volatility, but of higher boiling than the substances of value, such as trimellitic acid methyl ester (TMT). ), relatively easy volatility diphenyls and similar residue components ("high boiling distillable substances"), which impede feedback to the other stages of the DMT process and, therefore, must be separated from valuable substances. There is also a series of unwanted secondary reactions, mostly favored by high temperatures, namely: (6) Inverse reactions of the esterification and re-esterification reactions described, which always lead to equilibria: these reactions take place in areas of the equipment poorly mixed or remixed; (7) Condensation reactions, which, under the effect of the cobalt components that act as catalysts of the distillation residue, also occur mostly in poorly mixed, highly tempered areas of the equipment and which lead to the formation of products polynuclear, viscous; (8) Thermolytic dissociation processes (Crack processes) on hot surfaces and in superheated parts of the methanolysis mixture, in which carbon dioxide and easily boiled substances are formed, mostly non-transformable in DMT and therefore unwanted; (9) Formation in highly temperate zones of cobalt compounds of poor dissolution, partly coke type, that were produced by the Crack and re-esterification processes; (10) Precipitation of metallic cobalt by reduction of cobalt compounds by methanol, or contained reducible substances, such as formaldehyde, in hot walls. While the inverse reactions (6) have a negative effect on productivity in particular, the condensation and Crack processes according to (7) and (8) are primarily performance reducers. The reactions according to (9) and (10) influence the cobalt balance in a very unfavorable way, since the sedimented cobalt-containing substances elude the next extraction and the apparent loss of the catalyst that is produced must be replaced by fresh catalyst. In addition, substances that are difficult to dissolve are deposited in the pipe lines and parts of the equipment and thus cause frequent interruptions of operations and higher cleaning costs. The substances that are formed according to (7) remain largely in the residue of methanolysis and therefore have an unfavorable influence on the next extraction, since only with a fluid consistency of the residue does a great recovery of the catalytic metals result. If the viscosity of the methanol residue is too large, large amounts of diluting solution auxiliaries must be added before extraction, which after removal must be separated again, generating costs. Since the residues after extraction are normally incinerated, a high viscosity is also problematic here, since the viscous substances can hardly be pumped, transported or transported elsewhere. 2. Metanolysis equipment according to the state of the art 2.1 Previous equipment of a stage Relying on the esterification processes of similar technique, originally reactors of methanolysis were used as column reactors, heated from the outside with integrated reactor bottom and on top of a part of distillation for the separation of the above-mentioned high-boiling distillable substances. These reactors had in the reservoir part a distributor to discharge the methanol in the form of vapor. The high temperatures in the walls of the reactor, however, favored the side reactions indicated under (6) to (10). Furthermore, it was observed that the hot walls of the reactor were easily crusted and the solids that were exfoliated, together with other existing solids or that formed in the distillation residue, blocked the reaction bottoms, with which interruptions of the operation for cleaning jobs. 2. 2 Improved equipment of one stage An improvement of this original methanolysis with external heater reactor were systems with reactors without reaction bottoms, in which in addition to the introduction of methanol in the tank, there was circulation of methanolysis mixture with an external circulation heat exchanger, possibly introducing more methanol into the circulation. In this system it was possible, with a sufficiently large product circulation, to carefully introduce the heat necessary for the methanolysis reactions and the stabilizing distillation of the valuable substances, without external heating of the reactor. In this way, the disturbing side reactions indicated under (6) to (10) were considerably reduced.

Figure 1 shows a device like this for the performance of a methanolysis. The distillation residue 1.1 is introduced into the circulation-1.4 under the circulating heat exchanger 1.6 and introduced with a mixture of circulating methanolysis in the upper part of the reaction zone 1.5 of the reactor 1.3. The methanol is divided into two partial streams 1.2a, hereinafter referred to as "reaction methanol", and 1.2b, hereinafter referred to as "stabilizing distillation methanol". Of course, the methanol from both partial flows produces both a methanolysis of the distillation residue and also the stabilizing distillation of volatile substances. The denominations chosen only characterize main action points. The reaction methanol 1.2a is introduced directly into reactor 1.3; the stabilizing distillation methanol 1.2b indirectly, by circulation 1.4. In the area of methanolysis 1.5, which covers practically the volume of liquid in the -reactor 1.3, methanolysis takes place in the liquid traversed by the methanol vapor 1.2a, at temperatures of 180 ° C to 300 ° C, under pressure atmospheric or slightly reduced or elevated. The heat necessary for this and for the stabilizing distillation of the volatile substances of value is supplied by the circulating heat exchanger. 1.6, for example by heating with a heat transfer oil, by circulation 1.4, below which the Methanol from the stabilizing distillation 1.2b. The substances of value indicated under (1) to (4), as well as the high-boiling distillable substances according to (5), form with the methanol vapor the vapors flow 1.9a, which leaves the methanolysis zone 1.5 in the distillation part 1.7 and goes to a column without part of separation superimposed. The high-boiling substances that can be distilled according to (5) influence the oxidation in a very disadvantageous manner and therefore must be separated before the refeeding of the substances of value. For this, the vapor flow 1.9a in the distillation part 1.7 is subjected to a separation rectifying operation by the return 1.9b, which is a part of the deflement 1.9c cooled in the defoldering part 1.8. The high-boiling distillable substances, as components of the liquid return of the distillation part 1.7, return again to the methanolysis zone 1.5 and are finally extracted with the residue of methanolysis 1.10, from which, as described, the components are extracted catalytic The pump 1.11 produces the circulation 1.4 of the liquid that is taken out of the reservoir of the reactor 1.3. The pump can be removed, if sufficient stabilizing distillation methanol is fed to circulation 1.4, and thus the necessary volume displacement occurs.

The substances of value are fed back either as liquid valued substances 1.9d or together with methanol vapor in vapors 1.9e to other stages of the DMT process. 2. 3 Two-stage methanolysis The separation of the high-boiling distillable substances in the column on the setting of the distillation part 1.7 is only achieved satisfactorily, when the distillation residue does not contain very large quantities of substances of value, particularly DMT. . Otherwise, the temperature in the methanolysis zone must be raised considerably, in order to reach the high return quantities necessary for the separation of the high-boiling distillable substances. The foregoing promotes, on the one hand, the unwanted side reactions according to (7) to (10). On the other hand, the high amounts of methanol required for the stabilizing distillation, load the system and can not be processed or can not be processed as well in the other stages of the procedure. In these cases, the two-step methanolysis can be used for the separation of the high-boiling distillable substances, described in DE-A1 139 05 586 and shown in FIG. 2. In the first stage in the left section of the figure, the drawing (and the denominations) coincide with figure 1. In the 2.5 I methanolysis zone of this stage temperatures of 180 ° C to 300 ° C are usually taken. The residue 2.10 I taken from the circulation 2.4 I of the first stage is taken to the second stage in the right section of the figure, which again coincides mostly with figure 1, but with a separate distillation part 2.7 II with a working part 2.13, a circulation 2.14 with circulating heat exchanger 2.15 and pump 2.16, as well as, to reduce the temperature of the tank, an own introduction for the stabilizing distillation methanol 2.2c. In the 2.5 l methanolysis zone of the second stage, a temperature of 180 ° C to 300 ° C is generally available. The flow of steams 2.9a II, similar to that in figure 1, is divided into the 2.9h II defilement, which in turn is divided into 2.9b return II and substances of value 2.9d II, as well as in residual vapors 2.9e II. In the two-step methanolysis, the residue from the methanolysis 2.10 I of the first stage is subjected to a new methanolysis in the second stage. The high boiling distillable substances 2.17 arrive from the deposit of the working part 2.13 to the 2.10 II residue and are escludado with the same as residue of methanolysis 2.10.3. OBJECT OF THE INVENTION Although the two-step methanolysis according to DE-A1 39 04 586 works in a careful manner and with a higher recovery of substances of value than the methanolysis described before older designs, it is much more expensive. The above is not only valid for investment costs, but also for operating and maintenance costs. Therefore, a procedure is needed for the methanolysis of the distillation residue, which works at least as carefully and efficiently, but which, in terms of investment, operation and maintenance costs, is more economical than the methanolysis of two stages. 4. Brief description and advantages of the invention Surprisingly it has been found that the methanolysis of the distillation residue from the crude ester distillation in the production of DMT can be considerably improved in this respect with the Witten-Hercules method, if in the methanolysis zone directly introduces methanol in more than one plane. Methanol can be introduced in liquid form, however, it is advantageously applied at least partially as steam. It is advisable to provide at least one circulation with circulating heat exchanger and suitably a pump, because in this way problematic heating of the walls can be eliminated. With a sufficiently high product circulation, the heat necessary for the methanolysis reactions and the stabilizing distillation of the valuable substances is carefully fed. The temperature of the heat transfer agent only then needs to be a little, for example up to about 30 ° C, above the methanolysis temperature. It is also advisable to use this circulation simultaneously as an indirect introduction of methanol. A second, and possibly even a third circulation with indirect introduction of methanol to the methanolysis zone, as well as a direct introduction of assigned methanol, when the distillation residue (for specific reasons of the equipment or due to faults) contains large amounts of DMT and / or high molecular weight substances. It is a basic property of the process according to the invention that methanol is directly introduced into more than one plane in the methanolysis zone. Methanol is advantageously introduced in 2 to 10, particularly 3 to 8 planes. However, it is advisable to provide at least an indirect introduction through a product circulation. In the framework of the investigations that led to the present invention, it was found that the reaction rates of the methanolysis reactions, contrary to the opinion prevailing until then, are proportionally high and, therefore, the residence times in the area of comparatively low methanolysis. The inverse reactions are minimized by the process according to the invention, characterized by the principle of counterflow, stepwise release of methanol and intensive stabilizer distillation. Therefore, the method according to the invention allows a given amount of distillation residue to be processed in a given volume of equipment at low temperatures compared to conventional methanolysis and to optimally exploit the distillation residue without precipitations or blockages worthy of mention. Alternatively, with a given apparatus volume with equal temperature compared to conventional methanolysis, the space-time performance is essentially improved. In the first case, the DMT yield of the whole process is improved by several percentage points, because due to the low temperature, undesired reactions (6) to (10) are widely inhibited. Due to the low temperatures, the high-boiling distillation substances can also be maintained in the methanol residue with lower returns, even by simply superimposed distillation columns. The methanol residue is so fluid at the extraction temperatures, that the extraction of the catalysts is not problematic. In new equipment, by the simple form of the column type reactors with superimposed distillation part, which are particularly suitable for carrying out the process, considerable investment advantages result. However, the method according to the invention can also be carried out without major modifications in all existing equipment. Particularly relevant improvements are obtained in thin methanolysis reactors, column type, if the number of introduction planes can be increased to 4 or 5. However, in the case of less thin methanolysis reactors, which occur mainly in equipment more new, usually only 1 or 2 additional direct introduction plans are convenient, otherwise the introductions have a negative influence on each other. The process according to the invention can also be applied in two methanolysis and optionally several stages. Although the comparatively high investment, operation and maintenance costs are preserved, however, even in the case of loading with significant quantities of distillation residue with concentrations of relatively high value substances, larger parts of said substances are recovered than hitherto.

. DETAILED DESCRIPTION OF THE INVENTION Figure 3 shows a device for carrying out the method according to the invention. It is similar to the equipment of Figure 1 and, therefore, the denominations coincide. Reactor 3.3 has four direct introductions of methanol _ 3.2a.1 up to a.4, as well as two methanol mixture flows 3.4.1 and 3.4.2 with indirect introductions of methanol 3.2.bl and 3.2.b2, the exchangers of circulation heat 3.6.1 and 3.6.2, as well as pumps 3.11.1_ and 3.11.2. The pumps can be completely or partially removed if a sufficient displacement effect is achieved with the introduction into the respective circulation of sufficient quantities of methanol, if any, preheated. The upper product circulation contains the feed for the distillation residue 3.1, which is conducted with the introduced methanol 3.2.bl and the circulating methanolysis mixture to the vapor space under the superimposed column 3.7, which acts as a liquid eliminator. The separated vapors are bound with the vapors leaving the methanolysis zone 3.5 to give the vapors 3.9a and these are subjected in column 3.7 to the separation operation described to remove the high boiling distillable substances. These are added to the liquid that leaves the column 3.7 and, finally, are discharged with the methanol residue 3.10. The methanolysis mixture for the upper circulation 3.4.1 is taken above the direct introduction of higher methanol. The methanolysis mixture for the lower circulation 3.4.2 is taken over the following direct introduction of lower methanol 3.2.a2, mixed with indirect methanol 3.2.b2, is conducted by the circulating heat exchanger 3.6.2 and is delivery in the same area of the methanolysis zone 3.5 above the direct introduction of methanol 3.2.a2. Below this second direct introduction of methanol there are two other 3.2.a3 and 3.2.a4 in the deposit part of the methanolysis zone 3.5. The vapors 3.9a loaded with volatile substances of value, that leave by the head of the column superimposed 3.7, are cooled in the phlegmatizer 3.8 to such a degree, until it separates the disflemado 3.9c, which is fed back partially as return 3.9ba column 3.7 and partially as a liquid value substance 3.9 as well as other stages of the DMT process. The uncondensed residual vapors 3.9e, which still contain valuable substances, are also fed to other stages of the DMT process. It will be clear for the person skilled in the art that the indicated number of direct and indirect introductions of methanol, as well as their arrangement according to figure 3, are only examples and that other embodiments according to the invention are possible, which are all characterized because to the methanolysis zone, methanol is introduced directly and, if necessary, indirectly into more than one plane. It is possible to preheat in a mixed or unmixed manner the substance flows introduced into the circulations, to keep the temperature difference in the circulating heat exchanger as low as possible and, thus, be able to carefully heat the distillation residue. To reduce the thermal load of the distillation residue, it can also be distributed in several circulations, conveniently with indirect methanol. The distillation residue, which in the process according to the invention is subjected to methanolysis, is produced in the distillation of the reaction mixture of the esterification step in the DMT process and contains, in addition to the non-distillable catalytic components and substances of high molecular weight, almost non-distillable, depending on the equipment and its operation, still 2% to 30% of DMT, as well as acids not yet esterified, esters of aromatic alcohols with the acids produced in the oxidation, as well as the other indicated substances low (1) to (5). In the methanolysis, of these substances the reaction products indicated under (1) to (3) are produced, which reach column 3.7 superimposed together with the vapors 3.9a indicated under (4) and (5), and there, due to the low temperatures in the process according to the invention, with low returns 3.9b, they are separated from the unwanted high boiling substances which were co-distilled according to (5) and fed back as liquid value substances 3.9d, or, in residual vapors 3.9a existing stages of the DMT process.

In general, methanol, divided into the various divisions, is used in an amount 0.5 to 10 times the distillation residue. A considerable part, for example 25% to 60% of the total quantity, should fall indirectly through the circulation of immediate higher or higher product arranged accordingly, and a smaller part, for example 15% to 25%, to through the direct introduction that corresponds to him. The rest of the methanol is divided into the other direct and indirect introductions, being convenient that 30% to 60% of this residual amount is distributed in the circulation of immediate lower product, if it exists, and the direct introduction that corresponds to it and the methanol that still remain distributed in the remaining direct and / or indirect introductions. The optimal distribution of the residual methanol depends, among other things, on the geometrical proportions of the reactor and can be determined by varying the amounts. Regardless of the number and type of methanol introductions (direct or indirect), a temperature of 200 ° C to 275 ° C, advantageously from 220 ° C to 260 ° C, conveniently prevails in the methanolysis zone. The temperature, as mentioned above, is advantageously maintained by circulating methanol mixture through one or more external heat exchangers of circulation. As in the known processes, it is conveniently worked at atmospheric pressure or under slightly elevated or reduced pressure of 0.8 to 3.0 bar, preferably 1.0 to 2.0 bar, at the head of the reactor. Although higher pressures are possible, they are technically inconvenient, since they promote the inverse reaction according to (6). In the process according to the invention, the distillation residue is separated into a methanol residue 3.10, which still contains 0.5 to 2.5%, preferably about 1% DMT, as well as high distillable boiling substances, all residues non-distillable high molecular weight organic compounds, as well as the catalytic components, and a volatile fraction containing methanol, from which is separated a 3.9h parallax used as return 3.9b. It is not convenient to design the methanolysis in such a way that the low quantities of DMT contained in the residue of methanolysis 3.10 are also distilled or stabilized. Although the above would be desirable for interest in a high DMT yield, it would render the 3.10 methanolysis residue unnecessarily viscous, so that, as mentioned, it would be difficult to extract and incinerate. For the process according to the invention, in principle all reactors which, independently of the methanol introductions, match the reactors used for the usual methanolysis processes, are suitable. That is, they usually have a diameter D of 0.5m to 5m or more, are cylindrical and have a length of 2 to 10 x D, they have in the lower part that covers approx. 50% or more of the volume, the methanolysis zone, until the introductions of methanol no grafted piece, have in the upper part that are preferably devices for liquid / vapor separation and, depending on the type of construction, a column superimposed or part of work, a separate column with or without part of work or incorporated separation elements of usual construction type to reinforce the effect of distillative separation. The average empty tube velocities in the reactor methanololysis zone should usually not exceed 15 cm / sec, advantageously 10 cm / sec, so that the flow rates are not too unfavorable. The distance of the planes on which methanol is directly or indirectly fed to the reactor must be at least 0.3 and up to 3 x D and preferably 1 to 2 x D. In the case of indirect feeding, the distance between the points is measured in which the circulating methanol mixture loaded with methanol re-enters the reactor and the next point where direct or indirect methanol is introduced. In any case, the circulations must not be cut off from each other (that is, between the intake and supply points of a circuit, no point of intake and / or supply of other circulation must be found).

Claims (13)

NOVELTY OF THE INVENTION Having described the foregoing invention, the content of the following is claimed as property: CLAIMS

1. A process for the methanolysis of the distillation residue of the crude ester distillation in the process for the preparation of dimethylterephthalate according to the Witten-Hercules method, characterized in that methanol is introduced directly into more than one plane in the methanolysis zone.

2. A process according to claim 1, characterized in that methanol is introduced in 2 to 10 planes to the methanolysis zone.

3. A process according to claim 1, characterized in that methanol is introduced in 3 to 8 planes to the methanolysis zone.

4. A process according to one of claims 1 to 3, characterized in that there is at least one circulation of methanolysis mixture with an external heat exchanger circulating, through which the necessary heat for the methanolysis reactions is introduced totally or partially. and for the stabilizing distillation of valuable substances.

5. A method according to claim 4, characterized in that the circulation or circulations are used or used simultaneously as indirect introduction (s) of methanol.

6. A process according to claim 4, characterized in that the methanolysis mixture of the upper or higher immediate circulation is fed back to the methanolysis reactor above the liquid level of the methanolysis mixture.

7. A process according to one of claims 1 to 6, characterized in that the distillation residue is introduced completely or partially into the methanolysis zone through one or more circulations.

8. A process according to one of claims 1 to 7, characterized in that the flows introduced into the methanolysis zone are heated first individually or in a mixture.

9. A process according to one of claims 1 to 8, characterized in that the temperature in the methanolysis zone is from 200 ° C to 275 ° C.

10. A process according to one of claims 1 to 8, characterized in that the temperature in the methanolysis zone is from 220 ° C to 260 ° C.

11. A process according to one of claims 1 to 10, characterized in that the vapors charged with valuable substances and high-boiling distillation substances are conducted through a laying column, without a working part, in which they are separated. the valuable substances of the high-boiling distillable substances, which reach the residue of methanolysis.

12. A process according to one of claims 1 to 11, characterized in that the distance of the planes in which methanol is directly or indirectly introduced is at least 0.3 and up to 3 x D, where D is the diameter of the methanolysis reactor. .

13. A process according to one of claims 1 to 12, characterized in that the methanol is at least partially applied in the form of steam.