CS254648B1 - Process for processing the residue in the production of dimethyl terephthalate after rectification - Google Patents

Abstract

The solution concerns a method of processing distillation residues from the distillation of mother liquors after DMT crystallization. The essence is that the residue is subjected to distillation and, after distilling off DMT, DMI and trimellitic acid ester, the residue is subjected to methanolysis at a temperature of 170 to 180 °C, while methanol vapors are introduced below the level of the distillation residue.

Description

Dimethyl terephthalate (DMT) is produced by the catalytic oxidation of a mixture of p-xylene and circulating methyl p-toluylate under pressure and pressure esterification of the resulting acids with methanol. The resulting reaction mixture is subjected to distillation under reduced pressure to separate off the non-volatile fractions, the distillate is separated on a column of methyl p-toluylate fraction, which is returned to oxidation and crude DMT, which is purified by double crystallization, suspension from methanol and distillation under reduced pressure.

The esterification reaction mixture contains, in addition to methyl p-toluylate and DMT, other substances convertible into these products, such as p-formylbenzoic acid methyl ester and p-xylyl esters of all acids present in the oxidate, as well as substances not convertible to DMT or its intermediates, as catalyst and substances resulting from undesired side reactions in oxidation and esterification such as trimethyl ester of trimellitic acid and high molecular weight resinous substances. Also present are substances formed in the oxidation and esterification of the isomeric xylenes which accompany p-xylene in small amounts, i.e., dimethyl-o-phthalate and dimethylisophthalate (DMJ). The level of all undesirable substances in the production chain must be kept at a level that does not interfere with the ongoing processes. Undesirable substances must be regularly removed from the production chain at the lowest possible loss in recoverable products. Both of these groups of substances are difficult to separate from each other and loss of useful products occurs when removing unwanted impurities.

The nonvolatile undesirable products leave the production chain in the form of distillation residues. These are primarily residues from the distillation under reduced pressure of the reaction product from the esterification of the so-called crude ester, containing mainly catalyst and resinous substances, less volatile than DMT. This stream is further treated with methanol vapors from which the distillate is returned to the production process, the residue is subjected to catalyst extraction and then removed from the production.

The second stream, containing undesirable impurities, is the residue from pressure distillation, where methanol is separated from the mother liquors after crystallization of crude DMT. In addition to DMT and DMJ, it contains, on the one hand, other substances, which undergo distillation of the crude ester under reduced pressure together with the DMT fraction, on the other hand, by-products which are formed mainly by distillation of the mother liquors by thermal exposure. This stream is 85% by weight in the production process. 15% by weight must be withdrawn from the process so that the level of impurities is kept constant.

The recovered product is rectified under reduced pressure to separate methyl p-toluylate, methyl p-formylbenzoate and the bulk of DMT with DMJ as distillate. The distillation conditions are chosen so that undesirable by-products, in particular trimethyl ester of trimellitic acid, are not passed through and that there is enough DMJ in the distillation residue to be destroyed so that its level in the production chain does not increase.

The distillation residue to be disposed of contains, in addition to trimethyl trimellitate, DMJ and resinous substances, DMT which cannot be completely separated from DMJ and substances which are cleavable to DMT or its intermediates and which are wasted in the current production process.

This disadvantage is overcome by the process of the present invention, wherein the residue after rectification under reduced pressure of a portion of the digestion residue from the distillation of the mother liquors after crystallization of the crude DMT to be withdrawn from the production process is subjected to distillation at 5-10 kPa. to 250 ° C to remove the remainder of DMT and DMJ together with the bulk of trimethyl trimellitate. The residue is subjected to methanolysis by heating to 170 to 280 ° C with methanol vapors at a temperature of 65 to 280 ° C, which are brought below the level of the distillation residue. The leaving volatiles are returned to the production chain either after condensation or directly. Preferably, the methanolysis is carried out together with the residue from the distillation of the crude ester so that the distillation residue is returned to the production chain before distillation of the crude ester.

In the process according to the invention, a portion of the distillation residue is withdrawn in a conventional manner under reduced pressure. In this process, methyl p-toluylate, methyl p-formylbenzoate and the bulk of DMT and DMJ are distilled off. This rectification is carried out through a 22 bell tray column operating at a reflux ratio of 1: 1 to 1: 3. At a pressure of 5 to 10 kPa and a reboiler temperature of up to 250 ° C, only a small amount of trimethyl ester of trimellitic acid passes through the column. Along with it, a portion of DMT and DMJ remains in the digester in addition to the high-boiling resinous substances. The size of the digester residue is controlled by the DMT: DMJ ratio in the mother liquors after crystallization of the crude DMT, which is between 1: 1 and 1: 2. If the DMT: DMJ ratio is close to 1: 2, a larger distillation residue is intentionally left. The distillate is returned to the production chain.

After rectification over the column, further distillation is performed only through a transducer installed parallel to the column, which is closed during rectification. Under the same conditions, the residue of DMT and DMJ is transferred to the distillate under these circumstances, together with the vast majority of trimethyl trimellitate. This fraction is collected separately and excluded from further production.

High-molecular-weight resinous substances remain in the digestive residue, which are partially capable of cleaving to DMT and intermediates in its production under the influence of temperature in the presence of methanol vapor. The residue is therefore treated with methanolysis, whereby with the vapors of methanol the cleavage products are returned to the production process. The methanolysis product can be introduced into the production chain directly in the form of vapors, eg into an apparatus which serves as an intermediate reservoir of the oxidate before it is sprayed into the esterification and at the same time they leave the esterification of methanol vapor through it. It is also possible to condense the outgoing product and then return it to the production process with or without methanol. The first variant is more advantageous because it does not need any additional apparatus.

30 to 50 wt. and thus the yield of the entire DMT production process is increased by 1 to 1.5% by weight.

From the handling point of view, it is advantageous not to perform methanolysis of the DMT residue,

DMJ and trimellitic acid trimethyl ester alone, but return it to a suitable point in the production chain either by adding it to the oxidate or to the crude ester. With the main production stream, the remainder is then distilled through the crude ester and transferred to the methanol with the remainder. This procedure requires no new equipment to operate.

In order to further elucidate the nature of the invention, the following examples are provided where the present process is compared with the process of the invention.

He did

In the present process, 4 460 kg of the residue from the distillation of mother liquors after the crystallization of crude DMT containing 5.3 wt. % methyl p-formylbenzoate, 26.8 wt. DMT, 23.3 wt. DMJ and 14.6 wt. trimethyl trimellitate. Distil at a pressure of 6 kPa through a 22-bed tray column at a reflux ratio of 1: 2 up to a reboiler temperature of 240 ° C. 2,300 kg of distillate are obtained, which contains 5.8% by weight of distillate. % methyl p-formylbenzoate, 47.8 wt. DMT, 41.5 wt. DMJ and 2.8 wt. trimethyl trimellitate. The residue of 2160 kg contains 0.4 wt. % methyl ester of p-formylbenzoic acid, 4.4 wt. DMT, 3.9 wt. DMJ and 26.8 wt. trimethyl trimellitate. This residue is excluded from production.

Example 2

In the process according to the invention, the residue of Example 1 weighing 2 160 kg, which is otherwise excluded from production, is subjected to further distillation on the same apparatus and under the same conditions, except that distillation is carried out via a transducer installed parallel to the column; which was closed during rectification in Example 1. The rectification column is discarded by opening the transducer. 770 kg of distillate, containing 0.6 wt. % methyl ester of p-formylbenzoic acid, 12.1 wt. DMT, 10.8 wt. DMJ and 74.2 wt. trimethyl trimellitate. This fraction is excluded from production.

The distillation residue, weighing 1,390 kg, is transferred to 2,500 l of a thermoformed boiler, where it is heated to a temperature of 250 ° C and then brought below the steam level of methanol at 80 ° C at 1,200 kg / h. The distillate obtained is condensed after condensation, methanol is distilled off to obtain 650 kg of a product which contains 0.5% by weight of the product. % methyl p-formylbenzoate, 24.4 wt. DMT, 6.3 wt. DMT; and esters of p-xylyl alcohol with acids, which occur as intermediates in the production of DMT. This product returns to oxidation in the main chain. A residue of 740 kg is excluded from production.

This procedure gives an additional 650 kg of recoverable products over the process described in Example 1.

Example 3

The procedure was as in Example 2, but the methanolysis was not performed on a separate device. The residue from the distillation of the DMT, DMJ and trimellitic acid trimethyl ester fractions weighing 1 390 kg is added to the crude ester from the esterification in the main production chain, subjected to distillation under reduced pressure, and passed to a digester residue which is treated with methanolysis.

Claims (1)

SUBJECT OF THE INVENTION Process for the treatment of the residue of dimethyl terephthalate after recrystallization under reduced pressure of a portion of the digestion residue from the distillation of the mother liquors after crystallization of the crude dimethyl terephthalate to be displaced from the dimethyl terephthalate production process by oxidizing the p-xylene and circulating methyl p-toluylate mixture. methanol, characterized in that the residue is distilled at a pressure of 5 to 10 kPa at a boiling point of 200-250 ° C to remove dimethyl terephthalate and dimethylisophthalate together with trimethyl trimellitate and the residue is subjected to methanolysis by heating to 170-180 ° C by treatment with methanol vapors at a temperature of 65 to 280 ° C, which are brought below the distillation residue, leaving the volatiles either after condensation or directly returned to the production chain, preferably the methanolysis of the residue is carried out together with the residue from the distillation The crude ester is purified by returning the distillation residue to the production chain prior to distillation of the crude ester.