Organic compounds -- part of the class 532-570 series – Organic compounds – Unsubstituted hydrocarbyl chain between the ring and the -c-...
Reexamination Certificate
1998-08-17
2001-02-20
Shah, Mukund J. (Department: 1624)
Organic compounds -- part of the class 532-570 series
Organic compounds
Unsubstituted hydrocarbyl chain between the ring and the -c-...
Reexamination Certificate
active
06191274
ABSTRACT:
The invention relates to a process to separate &egr;-caprolactam from an aqueous mixture containing &egr;-caprolactam and at least 0.5 wt % oligomers.
Such a process is known from U.S. Pat. No. 5,495,016. In this patent publication a process is described in which 6-aminocapronitrile is converted into crude &egr;-caprolactam by reaction with water. The resulting aqueous mixture comprises ammonia, &egr;-caprolactam and oligomers. By first separating ammonia by distillation an aqueous mixture is obtained comprising &egr;-caprolactam and said oligomers. Subsequently &egr;-caprolactam is separated from this mixture by distillation, in which the &egr;-caprolactam and water is recovered as the top product and oligomers are recovered in a concentrated mixture as bottom product.
A disadvantage of this process is that at the high reboiler temperatures of the distillation column &egr;-caprolactam will easily convert to more oligomers (2 wt. % absolute according to Example I of U.S. Pat. No. 5,495,016). Furthermore fouling of pipes and other process equipment because of solidification of the oligomers cannot be easily avoided.
The object of the invention is a process for the separation of &egr;-caprolactam from aqueous mixtures comprising also oligomers in which the above mentioned disadvantages are avoided.
This object is achieved in that the separation is performed by extraction using an organic extraction agent.
It has been found that extraction is an efficient method of separating &egr;-caprolactam from oligomers in aqueous mixtures. Furthermore the oligomers are obtained in an aqueous mixture which can be easily further processed to yield more &egr;-caprolactam.
Extraction is a known method of separating &egr;-caprolactam from the reaction mixture obtained by the Beckmann rearrangement of cyclohexane oxime with sulfuric acid or oleum. After neutralization of the discharged rearrangement reaction mixture with ammonia, the &egr;-caprolactam is separated from the ammonium sulfate by extraction with an organic solvent, such as benzene. No oligomers are formed in this process.
In addition to the Beckmann rearrangement of cyclohexanone oxime to &egr;-caprolactam, there are further synthesis routes leading to &egr;-caprolactam. Possible routes which yield an aqueous mixture are the cyclization of 6-aminocaproic acid as described in U.S. Pat. No. 4,730,040 and the reaction of 6-aminocapronitrile as described in the earlier mentioned U.S. Pat. No. 5,496,941. Because the by-product spectrum of the here described processes differ completely from that of &egr;-caprolactam prepared by a Beckmann rearrangement, it was not to be expected that extraction in general would be a favorable method of separating &egr;-caprolactam from 6-aminocaproic acid and from oligomers. U.S. Pat. No. 5,495,016 only mentiones distillation as separation method.
U.S. Pat. No. 4,730,040 mentions the extraction of &egr;-caprolactam from aqueous mixtures containing &egr;-caprolactam using chloroform. This patent is however silent if &egr;-caprolactam can also be separated from aqueous mixtures also containing oligomers.
U.S. Pat. No. 3,485,821 describes a process for the preparation of &egr;-caprolactam by heating an aqueous mixture of 6-aminocaproic acid. In the examples &egr;-caprolactam is isolated by extraction with chloroform. Only in Example 13 of this patent publication some polyamide is also present in the aqueous &egr;-caprolactam containing mixture. No extraction is performed in this example to isolate &egr;-caprolactam. The polyamide was separated from the aqueous mixture by cooling and filtration.
U.S. Pat. No. 4,013,640 describes the extraction of water soluble amides, such as &egr;-caprolactam, n-butyramide and n-valeramide, from an aqueous mixture using alkyl phenol solvents as the extraction agent. This patent publication mentions a wide range of linear and cyclic amides which can be extracted. It was therefore not to be expected that &egr;-caprolactam could be succesfully separated from other amides such as oligomers.
The oligomers are generally oligomers (dimers and trimers mostly) of 6-aminocaproic acid or of 6-aminocaproamide.
The concentration oligomers in the aqueous mixture is higher than 0.5 wt. %. Preferably not more than 10 wt. % and most preferably not more than 5 wt. % of oligomers are present in the aqueous mixture.
The invention relates especially to the separation of &egr;-caprolactam from aqueous mixtures obtained in (a) a process to prepare &egr;-caprolactam in which 6-aminocapronitrile is converted into crude &egr;-caprolactam by reaction with water as described in for example U.S. Pat. No. 5,495,016, (b) a process to prepare &egr;-caprolactam by cyclization of 6-aminocaproic acid in water as described in for example U.S. Pat. No. 4,730,040 or (c) in a comparable process as under (b) in which the starting mixture comprises 6-aminocaproic acid and 6-aminocaproamide.
The aqueous mixture obtained in processes (a) and (c) will also contain ammonia which is a side-product of the reaction to &egr;-caprolactam starting from 6-aminocapronitrile or 6-aminocaproamide. It is advantageous to separate the ammonia prior to the extraction, for example by distillation or steam stripping. In the distillation any unconverted 6-aminocapronitrile (in process (a)) and part of the water will generally be separated as well. The concentration of the &egr;-caprolactam, optionally 6-aminocaproic acid, optionally 6-aminocaproamide and oligomers in the aqueous mixture obtained after such an ammonia separation will be preferably higher than 10 wt %.
In the event that the aqueous mixture is obtained starting from 6-aminocaproic acid or mixtures of 6-aminocaproic acid and 6-aminocaproamide, the conditions of such process are preferably as described below.
Preferably the concentration of ammonia and ammonia equivalents is below 5 wt. %. With ammonia equivalents is meant that every mol of a compound having a —C(O)—NH
2
group is calculated as a mol NH
3
. The total molar amount of NH
3
thus calculated can be expressed in a weight % of NH
3
. This number should be below 5 wt. % and preferably below 3 wt. %.
The concentration of &egr;-caprolactam, 6-aminocaproic acid, 6-aminocaproamide and oligomers is preferably between 5-50 wt. % and more preferably between 10-35 wt. %. The concentration of &egr;-caprolactam is preferably between 5-30 wt. %.
The temperature is preferably between 270 and 350° C., more preferably higher than 290° C.
The pressure is preferably between 5.0 and 20 MPa. Normally the pressure will be greater than or equal to the resulting pressure of the liquid reaction mixture and the temperature employed.
The reaction to &egr;-caprolactam can be performed continuously. The aqueous mixture obtained in the extraction according to the invention is preferably recycled to the cyclization processes as (a)-(c) here described. It has been found that the compounds, for example 6-aminocaproic acid, 6-aminocaproamide and oligomers which are present in this mixture can react to &egr;-caprolactam in a high yield. Thus by using extraction to isolate &egr;-caprolactam a valuable recycle stream is also obtained which can be succesfully used to prepare more &egr;-caprolactam.
The aqueous phase obtained in the extraction will contain oligomers and optionally 6-aminocaproamide and/or 6-aminocaproic acid.
The extraction agent is preferably an organic solvent which is substantially immiscible with water. By substantially immiscible is here meant that the mixture of organic solvent and the aqueous mixture results in two segregated phases at the extraction temperature. Preferable the mutual solubility under the conditions of the extraction is not higher than 30 wt. % and more preferably less than 20 wt. %.
Possible extraction agents are aromatic solvents, for example benzene, toluene, xylene; ethers, for example diethylether, methyl tert-buthylether; Preferably chlorinated hydrocarbons with 1 to 10 carbon atoms are used. Examples are chloroform, dichloromethane and 1,1,1-trichloroethane.
Another class of preferred extraction agents
Frentzen Yvonne H.
Guit Rudolf P. M.
DSM N.V.
Kifle Bruck
Pillsbury & Winthrop LLP
Shah Mukund J.
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