JPH0726014A - Process for recovering lactam - Google Patents

Abstract

PURPOSE:To recover a high-purity lactam at a high yield by a simple process. CONSTITUTION:A process for recovering a lactam from a crude lactam obtd. from extraction water produced as a by-product at a 6-nylon polymn. plant comprises a step for distilling the crude lactam in the absence of an alkali to obtain a purified lactam, a step for heating the resulting distillation residue and at least a half as much polycaprolactam in the presence of phosphoric acid or its deriv. to 260 deg.C or higher to cause the ring-opening polymn. of the lactam and oligomers in the residue to give polycaprolactam, a step for depolymerizing polycaprolactam and recovering the resulting crude lactam monomer, and a step for oxidizing the crude lactam monomer and rectifying it under an alkaline condition to obtain a pure lactam.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for recovering a distillation residue containing a crude lactam oligomer obtained from extraction water by-produced from a 6-nylon polymerization plant as a lactam monomer (hereinafter simply referred to as lactam).

[0002]

2. Description of the Related Art Usually, oligomers are contained in a large amount in, for example, water extracted from nylon chips and crude lactam obtained from the depolymerized product of polymer scraps. It is economically important to recover this oligomer by returning it to lactam. is there. In the case of oligomers in the extracted water, the method of concentrating and cooling the extracted water to crystallize the oligomers and separating them by filtration and then treating them is also possible. In the case of distilling into a mixture, a method of treating an oligomer remaining as a distillation residue is carried out, and various methods have been proposed for a long time. However, the cyclic dimer, which is the main component in the oligomer, is thermally and chemically stable, and therefore has various problems when it is returned to the lactam and recovered. For example, a method of decompressing or steam distilling an oligomer under phosphoric acid or an alkali catalyst (Japanese Patent Publication No. 46-
24388 and Japanese Patent Publication No. 46-31537), the sublimation of the oligomer is so severe that line clogging easily occurs and the yield is low. A method of heating an oligomer in the presence of phosphoric acid to depolymerize it after polymerization (Japanese Patent Laid-Open No. 59-706).
No. 62) has a problem that it takes a long time for the oligomer to reach the dissolution / polymerization equilibrium, and the amount of sublimation of the oligomer increases if depolymerized before the equilibrium is reached. When using an oligomer of the alkaline distillation residue in this method,
There is also a problem that the amount of phosphoric acid used increases because the activity of phosphoric acid is inhibited by the alkali in the residue. Further, in the method of heating an oligomer in the presence of an alkali catalyst and depolymerizing it after the polymerization (Japanese Patent Publication No. 59-6849), the quality of the crude lactam obtained is low because an alkali is used as a catalyst until a satisfactory quality is obtained. The purification is complicated and the loss of lactam is high as a result.

Further, in order to attach importance to economical efficiency, when lactam is also recovered from an oligomer in a residue obtained by distilling a crude lactam obtained from a depolymerized product of polymer waste, impurities often derived from the polymer waste are often removed. The quality of the recovered lactam deteriorates significantly, and it may be expensive to purify it until it can be used as a raw material.The relatively small amount of crude lactam obtained from the depolymerization product of polymer scraps in the distillation residue Since the lactam is recovered from the oligomer, there is a case where the refining cost is rather increased and the economical efficiency is deteriorated.

[0004]

SUMMARY OF THE INVENTION It is an object of the present invention to solve the problems of the above-mentioned conventional methods, that is, to suppress the sublimation of oligomers and to economically recover lactams of high quality.

[0005]

[Means for Solving the Problems] In order to solve the above problems, the present inventors have earnestly studied and studied a method for recovering lactam from a distillation residue containing an oligomer, and as a result, finally completed the present invention. Came to. That is, the present invention provides a method for recovering a lactam from a crude lactam obtained from extraction water by-produced from a 6-nylon polymerization plant, a step of distilling the crude lactam in the absence of an alkali to obtain a purified lactam, which is produced in the step. Heating the distillation residue to 260 ° C. or higher in the presence of phosphoric acid or its derivative together with 0.5 times or more the amount of the polycaprolactam of the distillation residue, and subjecting the lactam and the oligomer in the distillation residue to ring-opening polymerization to obtain polycaprolactam. A method for recovering lactam, which comprises a step of depolymerizing the polycaprolactam to recover a crude lactam monomer, and a step of obtaining a purified lactam by rectifying the crude lactam monomer under alkaline conditions after oxidation. Is.

In the present invention, the crude lactam obtained from the water extracted as a by-product from the nylon polymerization plant is purified by distillation without adding an alkali such as sodium hydroxide or potassium hydroxide, and the lactam is removed from the oligomer in the distillation residue. It is a method of collecting. Generally, an alkali is added to the crude lactam obtained from the water extracted as a by-product from the nylon polymerization plant and distilled, but this distillation residue contains an alkaline component and inhibits the activity of phosphoric acid and its derivatives as a catalyst. Therefore, it is not suitable for the present invention. If the quality of the lactam obtained as a distillate is insufficient, it is possible to further purify after distillation.

Thus, the distillation residue obtained by distilling the crude lactam obtained from the water extracted as a by-product from the nylon polymerization plant without adding an alkali gives 0.5 times the distillation residue in the presence of phosphoric acid or its derivative as a catalyst. The mixture is heated to 260 ° C. or higher together with an amount of polycaprolactam to cause ring-opening polymerization of lactam and oligomer in the distillation residue. Preferred specific examples of phosphoric acid or its derivative as a catalyst in the present invention include orthophosphoric acid, pyrophosphoric acid, metaphosphoric acid, polyphosphoric acid, triphosphoric acid, phosphorous acid, hypophosphorous acid, and ammonium salts, potassium salts and sodium salts of phosphoric acid. , Phosphoric acid derivatives such as organic phosphonates, organic phosphites or phosphorus oxides and sulfides. The addition amount is appropriately determined depending on the amount of distillation residue mixed with polycaprolactam, but 0.01-of the total amount of distillation residue and polycaprolactam
5.0% is preferable, and if it exceeds 5.0%, the amount of oligomer at the polymerization equilibrium increases, which is not preferable.

The polycaprolactam to be added to the distillation residue in the present invention may be polymer scraps generated from processing steps such as spinning and molding, but according to the present invention, the lactam and oligomer in the distillation residue are opened together with the polycaprolactam. It is also possible to use a part of the ring-polymerized polycaprolactam as it is. The addition amount is preferably 0.5 times the amount of the distillation residue or more, and when it is 0.5 times the amount or less, the sublimation amount of the oligomer in the distillation residue increases during the process of the present invention, or it takes a long time to dissolve by heating. There is a problem such as having. The reaction temperature is 260 ° C or higher, preferably 280 ° C or higher. Further, if the temperature is 350 ° C. or higher, the thermal decomposition products are increased in the produced lactam and the quality is degraded, which is not preferable.

The polycaprolactam obtained by the ring-opening polymerization as described above is depolymerized by subsequently blowing in superheated steam or distilling it under heating and reduced pressure. This depolymerization reaction may be carried out in a batch system in which superheated steam is directly blown into the reaction vessel in which ring-opening polymerization has been carried out, or the depolymerization is carried out after transferring the ring-opening polymerized polycaprolactam to a separately prepared depolymerization reaction vessel. It is also possible. Furthermore, the ring-opening-polymerized polycaprolactam is introduced into a reaction vessel for depolymerizing polymer scraps generated from processing steps such as spinning and molding of polycaprolactam, and mixed with these polymer scraps for dissolution. It may be polymerized.

When the ring-opening polymerized polycaprolactam is transferred to the depolymerization reaction container, the whole amount is not transferred and a part is left in the ring-opening polymerization reaction container, and this is added to the distillation residue in the present invention. It may be used as a polycaprolactam to which the following distillation residue and a catalyst are added, and the ring-opening polymerization of the present invention may be repeated. Further, the crude lactam thus obtained is concentrated, then oxidized with an oxidizing agent and distilled under alkaline conditions for purification. Examples of the oxidizing agent to be used include known ones such as permanganic acids, chromic acids, chloric acids, organic and inorganic peroxides and ozone, but hydrogen peroxide and ozone are preferable in that no residue is generated during the treatment. Further, if this purification alone is insufficient as a raw material, it is of course possible to use ion-exchange resin treatment or activated carbon treatment together.

[0011]

EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. In the examples, "parts" means "parts by weight". Example 1 Distillation residue of crude ε-caprolactam obtained from extraction water by-produced from 6 nylon polymerization plant (Oligomer 6
To 100 parts of 8% and 32% of monomer), 100 parts of poly ε-caprolactam waste thread generated in the spinning step and 1.0 part of orthophosphoric acid were added, and after substituting with nitrogen, the mixture was heated to 300 ° C. After 5 hours, the distillation residue was completely dissolved in poly ε-caprolactam and reached equilibrium. Immediately in this 32
Depolymerization was carried out by blowing superheated steam at 0 ° C., and the produced monomer was discharged. The yield of crude monomer was 92% (based on the total amount of the residue charged and the waste fiber added). Further, the obtained monomer-containing water was concentrated to a lactam concentration of 60%, 400 ml of this was put into a gas absorption bottle, and ozone was blown at a rate of 40 mg / min (ozone + air = 2 l / min) at 30 ° C. Oxidized for a minute. After the oxidation, a 20% aqueous sodium hydroxide solution was added thereto until the pH reached 12, water was evaporated, and distilled under reduced pressure to obtain purified ε-caprolactam. The quality of ε-caprolactam obtained by the above method was such that the permanganate value was 3400 seconds and the concentration of free basic substance was 0.03 meq / kg.

Example 2 In the same manner as in Example 1, the distillation residue was dissolved in polyε-caprolactam and ring-opening polymerized. With 120 parts of the polymer in a molten state, the polymer is discharged into a depolymerization reaction kettle for depolymerizing polymer waste generated from processing steps such as spinning and molding, and the polymer generated from processing steps such as spinning and molding is charged. Scraps (250 copies in total,
Steam was blown together with orthophosphoric acid (1.0 part) to effect depolymerization. The yield of crude monomer was 93%. Further, the obtained crude monomer-containing water was concentrated to a lactam concentration of 60%, passed through an ion exchange resin column packed with a strong cation exchange type resin, and then 0.005 per 100 parts of the monomer-containing water. 35% hydrogen peroxide solution was added at a ratio of 1 part, and the mixture was oxidized at 30 ° C. for 30 minutes. After the oxidation, a 20% aqueous sodium hydroxide solution was added thereto until the pH reached 12, water was evaporated, and distilled under reduced pressure to obtain purified ε-caprolactam. The quality of ε-caprolactam obtained by the above method was such that the permanganate value was 3000 seconds and the concentration of free basic substance was 0.01 meq / kg.

Example 3 To the remaining 80 parts of the polymer obtained in Example 2, 120 parts of the residue obtained by distilling the crude monomer obtained from the extracted water by-produced from the 6 nylon polymerization plant used in Example 1 were added. After adding 0.6 part of orthophosphoric acid and replacing with nitrogen, the mixture was heated to 300 ° C. After 5 hours, the distillation residue was completely dissolved in poly ε-caprolactam and reached equilibrium. While 120 parts of this polymer was kept in a molten state, it was put into a depolymerization reaction kettle for depolymerizing polymer waste generated from processing steps such as spinning and molding in the same manner as in Example 2, and processing such as spinning and molding. Depolymerization was carried out by blowing steam with polymer scraps (total 500 parts, orthophosphoric acid 2.0 parts) generated from the process. The same operation as above was repeated 5 times in total for the remaining 80 parts of the polymer. The average yield of ε-caprolactam after 5 purifications was 89.6%.
The average quality of ε-caprolactam after 5 purifications was a permanganate value of 3100 seconds and a free basic substance concentration of 0.01 meq / kg.

Comparative Example After adding 0.5 parts of orthophosphoric acid to 100 parts of a residue obtained by distilling crude ε-caprolactam obtained from the water extracted as a by-product from the 6-nylon polymerization plant used in Example 1, and replacing with nitrogen, Heated to 300 ° C. Even after 7 hours, 20 in the reaction system
% Of the oligomer remained and the equilibrium was not reached. In the same manner as in Example 1, superheated steam was blown in to carry out depolymerization, and the produced monomer was allowed to flow out. The yield of the crude monomer was 70%, and the oligomer which could not reach the equilibrium and a part of ε-caprolactam remained in the reaction vessel as a depolymerization residue.

[0015]

When the method of the present invention having the above-mentioned constitution is adopted, the yield is high and the residue in the process is small. Further, since it is not necessary to crystallize and separate the oligomer from the extracted water, the process can be simplified, which is economically advantageous. Further, since the lactam monomer obtained in the present invention does not originate from the residue obtained by distilling the crude lactam obtained from the depolymerized product of the polymer waste, it has few impurities and can be easily purified thereafter, which contributes to the industry. That's a big deal.

Claims (1)

[Claims] 1. A method for recovering a lactam from a crude lactam obtained from extraction water by-produced from a nylon 6 polymerization plant, a step of distilling the crude lactam in the absence of an alkali to obtain a purified lactam, which is produced in the step. Heating the distillation residue to 260 ° C. or higher in the presence of phosphoric acid or its derivative together with 0.5 times or more the amount of the polycaprolactam of the distillation residue, and subjecting the lactam and the oligomer in the distillation residue to ring-opening polymerization to obtain polycaprolactam. A method for recovering lactam, which comprises a step of depolymerizing the polycaprolactam to recover a crude lactam monomer, and a step of obtaining a purified lactam by rectifying the crude lactam monomer under alkaline conditions after oxidation. .