Process for the recovery of ε-caprolactam from extract water

Abstract

A process for the recovery of -caprolactam from extract water of polycaprolactam obtained by hydrolytic polymerization, wherein the extract water is concentrated, subsequently contained oligomers are depolymerized, non-depolymerizable impurities are separated, water and low-boiling impurities are removed, wherein for adjusting the purity of the recovered -caprolactam and the energy consumption used for the process a part of the product is removed from the process as intermediate products.

Claims

1. A process for the recovery of -caprolactam from extract water of polycaprolactam obtained by hydrolytic polymerization, comprising the following process steps: a. by single- or multistage distillation the extract water is concentrated by separation of water, wherein a first intermediate product stream is obtained, whose concentration of -caprolactam is increased with respect to the extract water; b. the first intermediate product stream is separated by distillation into a first vaporous -caprolactam-water phase and a liquid oligomer--caprolactam phase; c. into the liquid oligomer--caprolactam phase steam is introduced in the presence of a catalyst, wherein the -caprolactam oligomers are hydrolytically depolymerized to monomeric -caprolactam, and -caprolactam is stripped out of the phase by the steam, wherein a second vaporous -caprolactam-water phase is obtained and wherein non-depolymerizable impurities are discharged from the process as bottom product; d. the first vaporous -caprolactam-water phase formed in step b) and the second -caprolactam-steam phase formed in step c) are combined to a unified -caprolactam-water phase, from which water is separated by distillation and thus an oligomer- and water-free -caprolactam phase is obtained as second intermediate product stream; wherein the -caprolactam phase obtained in step d) as second intermediate product stream is supplied to a distillation comprising at least two stages in a further step e), wherein an end product stream depleted of highly volatile impurities is obtained, and that at least a part of the first intermediate product stream and/or the second intermediate product stream is discharged from the process.

2. The process according to claim 1, wherein the first intermediate product stream, the second intermediate product stream and the end product stream are combined to a unified product stream.

3. The process according to claim 1, wherein phosphoric acid is used as a catalyst in step c) of claim 1.

Description

EXEMPLARY EMBODIMENTS

(1) Further developments, advantages and possible applications of the invention can also be taken from the following description of exemplary embodiments and numerical examples as well as the drawing. All features described form the invention per se or in any combination, independent of their inclusion in the claims or their back-reference.

(2) With reference to the drawing, FIG. 1, the process will now be explained below.

(3) FIG. 1 shows a block diagram of the process according to claim 1.

(4) The extract water 1 produced in the extraction of polyamide 6, which is not shown in FIG. 1, is supplied to a distillation process 2. This distillation can be carried out in one or more stages. Water in the form of steam 3 is separated from the extract water 1. It can be recirculated into the extraction process for reuse. As bottom product concentrated extract water 4 is obtained, which is supplied to a further distillation process 5.

(5) In this distillation process 5, the concentrated extract water is separated into a gaseous -caprolactam-steam phase 7 and a liquid oligomer--caprolactam phase 8.

(6) The oligomer--caprolactam phase 8 is supplied to a depolymerization process 9. In this process 9, the oligomers obtained in phase 8 are hydrolytically depolymerized to -caprolactam by introducing steam 10 and a catalyst 11. As catalyst 11 phosphoric acid is often used. The -caprolactam furthermore is stripped out of the oligomer--caprolactam phase by the steam and, shown as stream 12, supplied to a further distillation process 14 together with the -caprolactam-water phase 7. The constituents of the oligomer--caprolactam not depolymerizable in process 9 are discharged from the process as bottom product 13 and disposed of as waste.

(7) In the distillation process 14, water, in the form of steam, is separated from the phases 7 and 12 as top product 15, and as bottom product 16 -caprolactam poor in water is obtained, which however still contains impurities. To separate the same, this phase 16 is introduced into a further distillation 17 which is carried out in at least two stages. The low-boiling impurities are separated in the first stage or stages as top product 19, and the high-boiling impurities are separated in the last stage as bottom product 20. The recovered, completely purified -caprolactam end product is obtained as top product 21 of the last distillation stage.

(8) To be able to also produce recovered -caprolactam with lower purity and with lower energy consumption, product quantities, represented as stream 6 (first intermediate product stream) and 18 (second intermediate product stream), according to the invention can be branched off already from the concentrated extract water 4 and from the oligomer- and water-free -caprolactam 16. These branched streams 6 and 18 can be mixed separately or with each other in a freely selectable ratio and can be supplied to a polymerization process.

LIST OF REFERENCE NUMERALS

(9) 1 extract water from extraction 2 distillation 3 steam 4 extract water, concentrated 5 distillation 6 extract water, concentrated (first intermediate product stream) 7 -caprolactam-steam phase 8 oligomer--caprolactam phase 9 depolymerization process 10 steam 11 catalyst 12 -caprolactam-steam phase 13 non-depolymerizable impurities 14 distillation 15 steam 16 oligomer- and water-free -caprolactam 17 distillation 18 oligomer- and water-free -caprolactam (second intermediate product stream) 19 low-boiling impurities 20 high-boiling impurities 21 purified -caprolactam (end product stream)