JPS59116280A - Treatment of phthalic anhydride distillation residue - Google Patents

Abstract

PURPOSE:To dispose the distillation residue of phthalic anhydride obtained by the catalytic vapor-phase oxidation of naphthalene or o-xylene, by mixing the residue with a fraction having lower boiling point than phthalic anhydride and extracting the residue from the distillation pot. CONSTITUTION:Crude phthalic anhydride obtained by the catalytic vapor-phase oxidation of naphthalene or o-xylene with a gas containing molecular oxygen is purified by distillation to separate the low-boiling fraction and phthalic anhydride fraction. The distillation residue produced by the distillation is distilled batchwise in vacuum to recover the phthalic anhydride remaining in the residue. The residue left after the recovery of the residual phthalic anhydride is mixed with a fraction having lower boiling point than phthalic anhydride (e.g. benzoic acid, toluic acid, etc.) and the residue is extracted from the distillation pot. The extraction can be carried out easily by adding 0.1-2pts.wt. of the low-boiling fraction to 1pt.wt. of the residue, thereby lowering the freezing point of the residue preferably to <=130 deg.C. EFFECT:Phthalic anhydride can be recovered from the distillation residue at a high rate of recovery.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for treating phthalic anhydride distillation residue. More specifically, the present invention relates to a method for treating a distillation residue of heptallic anhydride obtained by catalytic gas phase oxidation of naphthalene or ortho-xylene.

Phthalic anhydride is produced by air oxidation of naphthalene or ortho-xylene, but at the same time various compounds are produced as impurities, which must be purified. This purification is usually carried out by distillation, and is carried out by continuous or batch distillation.

In this distillation, a fraction with a lower boiling point than phthalic anhydride and a fraction or residue with a higher boiling point are separated and removed, and the phthalic anhydride fraction is recovered as purified phthalic anhydride. Here, the fraction or residue with a higher boiling point than phthalic anhydride still contains a large amount of phthalic anhydride, so in order to improve the product yield, this is redistilled to recover the phthalic anhydride. That is advantageous. Therefore, the present inventors used a part of the distillation residue (meaning a fraction or residue with a higher boiling point than phthalic anhydride) generated when crude phthalic anhydride was subjected to purification distillation to
We investigated a method for recycling crude phthalic anhydride to the distillation process. However, if crude phthalic anhydride obtained by catalytic gas-phase oxidation of naphthalene is continuously distilled while circulating the column bottoms to the distillation process, unknown impurities may be mixed into the product, resulting in a reduction in product yield. They discovered that problems that could not have been predicted in conventional batch operations, such as deterioration and equipment corrosion, occurred.

Therefore, instead of circulating the distillation residue directly to the distillation process for crude phthalic anhydride, we conducted a separate distillation to distill out phthalic anhydride, which was then recovered as a product, or the distillate was used to produce crude 7-thalic anhydride. We investigated a method of circulating the water to the distillation process. In this case, it was found that the above-mentioned problem did not occur, but if more than a certain amount of phthalic anhydride is recovered from the distillation residue, the freezing point will rise, making distillation difficult and causing the residue remaining in the distillation vessel to rise. It has been observed that even the removal of the substances becomes difficult.

In the case of batch distillation, it is possible to increase the recovery rate of phthalic anhydride from the distillation residue compared to the case of continuous distillation, but when the phthalic anhydride content in the residue falls below a certain amount, the freezing point As a result, it becomes difficult to extract the distillate from the distillation vessel, and at the same time, the problem of clogging in the transfer pipe occurs. Although it varies depending on the composition of the distillation residue, for example, it is a distillation residue of crude phthalic anhydride obtained by catalytic gas phase oxidation of naphthalene, with a phthalic anhydride content of 85% by weight and other raw materials below about 300°C. In the case of a distillation residue consisting of a fraction of 15%, when the phthalic anhydride content is 70% or less, the freezing point of the residue is found to be 160°C or higher, and in this case, recovery of 7thalic anhydride is confirmed. Even getting the rate to 50% proved difficult.

The present invention was made in order to eliminate the various drawbacks of the conventional methods as described above, and purification distillation of crude phthalic anhydride obtained by catalytic gas phase oxidation of naphthalene or ortho-xylene with a molecular oxygen-containing gas. The distillation residue generated when separating the low boiling point fraction and phthalic anhydride fraction by subjecting it to
The phthalic anhydride contained in the distillation residue is recovered by batch distillation under reduced pressure, and the residue after recovering the phthalic anhydride is mixed with a fraction having a boiling point lower than that of phthalic anhydride. This is a method for treating phthalic anhydride distillation residue, which is characterized by extracting the substance from the distillation pot.

Next, the present invention will be explained with reference to the drawings. That is,
In the drawing, crude phthalic anhydride obtained by catalytic gas phase oxidation of naphthalene or ortho-xylene, preferably naphthalene, with air is sent from line 1 to first distillation column 2 after being pretreated, such as heat treatment, if necessary. Heavy materials such as pitch are separated from line 3. The fraction vaporized in the first distillation column 2 is passed from the top of the column through a line 4 to a condenser 5.
There, it is condensed and sent via line 6 to second distillation column 7, where it is subjected to vacuum distillation. At this time, it is also possible to omit the condenser 5 and directly supply the vapor to the distillation column 7. A part of the distillation residue is sent to the evaporator 10 of the high-boiling point distillation column 9 through the line 8 and subjected to vacuum distillation in a batch manner. The fraction from the high-boiling distillation column 9 can be converted into a product, but after passing through the line 11 and condensing in the condenser 12 and refluxing a part of it to the distillation column 9, the remainder can be used in the second distillation column if necessary. It is preferable to circulate it to the distillation column 7. Pure phthalic anhydride is taken out from line 13, condensed in condenser 14, and then
The product is taken out from line 15.

Components with a lower Buddha point than phthalic anhydride are passed through line 1 from the top of the column.
6 to a condenser 17 for condensation, a portion of which is refluxed to the distillation column via line 18, and the remainder is discharged via line 19 and sent to tank 21. A part of it is mixed with a high viscosity residue produced by recovering a predetermined amount of phthalic anhydride from the distillation residue in the evaporator 1o, and the residue is dissolved and discharged to the outside of the thread through a line 20. Ru. Note that the second distillation column may be further divided into two distillation columns, in which case the low boiling point fraction is separated from the top of the column after the first distillation column, and the bottom liquid is separated from the top of the column. It is sent to a distillation column, where pure phthalic anhydride is recovered from the top of the column, and a distillation residue is extracted from the top of the column.

Therefore, the distillation in the high boiling point distillation column 9 is carried out batchwise at a temperature of 200 Torr or less, preferably 50 to 50 Torr.
It is held in Masutashita. This distillation can be carried out until the phthalic anhydride content in the residue becomes 50% by weight or less, but it is preferable to end the distillation when the freezing point of the residue does not exceed 230°C. Note that the distillation residue to be distilled here may be a fraction with a higher boiling point than phthalic anhydride, or the entire residue, but particularly heavy residues such as pitch may be separated and separated as shown in the example shown in the drawing. Treatment is advantageous to increase the recovery rate of phthalic anhydride.

By recovering the phthalic anhydride content in the distillation residue at a high rate in this way, the residue remaining in the evaporator 10 has a freezing point of 130°C, preferably over 160°C, so it can be kept as it is. It is difficult to extract it. Therefore, in the present invention, a fraction having a boiling point lower than that of phthalic anhydride is added to and mixed with this residue. In other words, it was found that when a fraction with a lower boiling point than phthalic anhydride, mainly composed of benzoic acid or toluic acid, was added to this residue, the freezing point and viscosity were significantly lowered, making extraction extremely easy. . Therefore, even if the phthalic anhydride content is recovered until the solidification point of the residue exceeds 160°C, it can be sufficiently extracted.

The low-boiling fraction to be added to and mixed with the residue is preferably a fraction that mainly contains low-boiling components that are produced as by-products during the production of phthalic anhydride such as benzoic acid or toluic acid, and the content of the low-boiling components is 10%. ~100%, preferably 30-9
Approximately 0% is good.

As such a fraction, a low-boiling fraction obtained from the top of the column during purification distillation of crude phthalic anhydride is most preferable.

The amount of the low-boiling point fraction added varies depending on the composition of the residue and the composition of the low-boiling point fraction, but in any case, after adding and mixing, the amount of the low-boiling point fraction added should be such that the freezing point is about 150°C or less, preferably about 130°C or less. Add to. Crude phthalic anhydride obtained by oxidizing naphthalene After recovering phthalic anhydride from the distillation residue during purification distillation
C., a low boiling point fraction containing 50 to 90% benzoic acid and 50 to 10% phthalic anhydride may be added in an amount of 5 to 100 parts by weight or more per 100 parts by weight of the residue.

After the residue and the low-boiling fraction are mixed in the evaporator at a temperature above the freezing point, they are extracted through line 32. Since the line 32 only needs to be kept warm as long as it is above the freezing point of the extracted mixture, keeping it warm with Shimada steam or the like is sufficient. Note that the released mixture is incinerated in an incinerator or used as fuel, if necessary.

As described above, the method for treating phthalic anhydride distillation residue according to the present invention involves subjecting crude phthalic anhydride obtained by catalytic gas phase oxidation of naphthalene or ortho-xylene with a molecular oxygen-containing gas to purification distillation to obtain a product with a low boiling point. The phthalic anhydride contained in the distillation residue is recovered by batch distillation of the distillation residue generated when separating the distillate and the phthalic anhydride fraction, and the phthalic anhydride contained in the distillation residue is recovered. Since the residue is mixed with a fraction having a lower boiling point than phthalic anhydride and the residue is extracted from the distillation vessel, the high boiling point fraction can be sufficiently separated, and the anhydrous The recovery rate of phthalic acid can be increased. In addition, by mixing a fraction with a lower boiling point than phthalic anhydride into the residue from the batch vacuum distillation, the freezing point of the residue is significantly lowered, making extraction work easier and at the same time preventing blockages in the transfer piping. Problems can be solved. Therefore, it becomes possible to increase the recovery rate of phthalic anhydride from the distillation residue to 50% or more. Furthermore, if a low boiling point fraction obtained when the phthalic anhydride fraction is separated by purification distillation of crude phthalic anhydride is used as the fraction having a lower boiling point than the phthalic anhydride, the processing cost will be reduced.

Next, the present invention will be explained in more detail by giving examples.

Example 1 Crude phthalic anhydride obtained by catalytic gas phase oxidation of naphthalene with air was heat treated and then supplied to a first distillation column. The fraction vaporized in the first distillation column was sent to the second distillation column to remove the low boiling point fraction.

The bottom liquid was sent to a third distillation column, where it was distilled under reduced pressure, and pure phthalic anhydride was obtained from the top of the column.

The distillation residual liquid (bottom liquid) of the sixth distillation column contains 8 phthalic anhydride.
It contained 5% by weight and 15% by weight of impurities such as nitrogen-containing compounds, and its freezing point was 130°C. This distillation residue is sent to the evaporator of the high-boiling point distillation column, and the pressure is reduced to 50 Tor.
r, phthalic anhydride purity of 99.8% was recovered by batch distillation at a bottom temperature of 230° C. and a reflux ratio of 2 to 3. In this case, the freezing point of the residual pickles (phthalic anhydride concentration 702%) at a recovery rate of about 50% by weight of phthalic anhydride is 16
The temperature rose significantly to 0°C, and the pipe for extraction from the evaporator was removed from the evaporator.
Even though the tube was kept warm with steam at 0 ki/1yn2G, a problem of clogging inside the tube occurred. Therefore, it was impossible to increase the recovery rate of phthalic anhydride to 50% or more under this condition.

Therefore, the low boiling point fraction obtained from the second distillation column (about 70% benzoic acid, about 30% phthalic anhydride) is added to the obtained residue.
%) was mixed with the residue after recovering phthalic anhydride in the proportions shown in Table 1, its freezing point was as shown in Table 1, and in both cases, there was no problem of removing the residue from the pot or clogging the transfer piping. There was no problem at all, and there was no problem with the incineration process at the incinerator. Note that the recovered phthalic anhydride was circulated to the second distillation column.

Table 1 100 0 160 90 10 117 80 20 109 60 40 90 40 60 88 20 80 B6 Example 2 The method of Example 1 (= the recovery rate of phthalic anhydride from the distillation residue was about 80% by weight) The freezing point of the residue (50% phthalic anhydride concentration) was 183°C, but the second
When low boiling point fractions are mixed in the proportions shown in the table, the solidification points are as shown in Table 2. If 60 parts by weight or more of low boiling point fractions are mixed, problems such as removal of the residue from the pot and blockage in the transfer piping may occur. There was no problem at all, and there was no problem with the incineration process at the incinerator.

1 00 0
1 8390 10 1
73 80 20
1 6660 40
14840 60
9920
80 B6 Example 3 In the method of Example 1 (=), when the recovery rate of phthalic anhydride from the distillation residue is about 90% by weight, the freezing point of q pickles (phthalic anhydride 30%) is 216 ° C. However, when low boiling point fractions were mixed in the proportions shown in Table 3, the freezing points were as shown in Table 3.If the freezing point is below 130°C, remove the residue from the pot or block it in the transfer pipe. There were no problems at all.

Table 5 100 0 216 90 10 208 80 20 195 60 40 172 40 60 107 20 80 10'0

[Brief explanation of drawings]

The drawing is a flow sheet showing one embodiment of the present invention. 2.7.9... Distillation column.

Claims (4)

[Claims] (1) Generated when crude phthalic anhydride obtained by catalytic gas phase oxidation of naphthalene or ortho-xylene with molecular oxygen-containing gas is subjected to purification distillation to separate a low-boiling point fraction and a phthalic anhydride fraction. The phthalic anhydride contained in the distillation residue is recovered by batchwise vacuum distillation of the distillation residue, and a fraction with a boiling point lower than that of phthalic anhydride is mixed with the residue after the phthalic anhydride recovery. 1. A method for treating phthalic anhydride distillation residue, which comprises removing the residue from the distillation vessel. (2) The treatment method according to claim 1, wherein the proportion of the low boiling point fraction mixed into the residue is 0.1 to 2 parts by weight per 1 part by weight of the residue. (3) The treatment method according to claim 1 or 2, wherein the distillation residue contains 95 to 70% by weight of phthalic anhydride. (4) The treatment method according to any one of claims 1 to 3, wherein 10 to 60% by weight of phthalic anhydride remains in the pickled material after phthalic anhydride recovery.