Organic compounds -- part of the class 532-570 series – Organic compounds – Unsubstituted hydrocarbyl chain between the ring and the -c-...
Reexamination Certificate
2000-09-19
2001-11-27
Kifle, Bruck (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
06323341
ABSTRACT:
The invention relates to a process for the purification of crude &egr;-caprolactam.
Such a process is known from U.S. Pat. No. 5,496,941. This patent publication describes the continuous purification of crude &egr;-caprolactam by hydrogenation, subsequent treatment in an acid medium and subsequent distillation in an alkaline medium. The treatment in the acid medium can be carried out in two ways:
(1) by passing the hydrogenation product, in a solvent, over an ion exchanger containing terminal acid groups, or
(2) by distilling the hydrogenation product in the presence of sulphuric acid.
The process results in a treated product, from which impurities like cyclic nitrites, amines and imines are removed.
A disadvantage of the process from U.S. Pat. No. 5,496,941 is that crude &egr;-caprolactam prepared by cyclization of alkyl 6-aminocaproate, 6-aminocapronitrile, 6-aminocaproic acid, 6-aminocaproic amide and/or oligomers thereof, such as for instance described in WO-A-9837063, cannot be effectively purified with this method.
We have found that this is caused by the fact that &egr;-caprolactam prepared by cyclization contains small amounts of N-substituted or C-substituted lactams and/or amides and these cannot be effectively removed with the above known method. Other processes to prepare &egr;-caprolactam e.g. according to Beckmann rearrangement result in crude &egr;-caprolactam containing a different pattern of impurities.
An object of the invention is to provide a purification process for crude &egr;-caprolactam prepared by cyclization of alkyl 6-aminocaproate, 6-aminocapronitrile, 6-aminocaproic acid, 6-aminocaproic amide and/or oligomers thereof.
This object is achieved in that the crude &egr;-caprolactam is subjected to a crystallization process.
We have found that according to the invention an increased purity can be obtained, which cannot be attained according to the above known purification process.
It is believed that also other known methods to purify &egr;-caprolactam prepared by cyclization of alkyl 6-aminocaproate, 6-aminocapronitrile, 6-aminocaproic acid, 6-aminocaproic amide and/or oligomers thereof, e.g. by extraction, do not result in sufficiently purified caprolactam. Due to the fact that the type of the impurities of the crude &egr;-caprolactam was unknown and that these impurities are present in low levels in the crude &egr;-caprolactam, at which level their behaviour in purification steps cannot be predicted, the success of the process according to the invention could not be foreseen.
Preferably the crystallization process comprises the following steps:
(1) liquid crude &egr;-caprolactam is fed into a crystallizer
(2) in the crystallizer conditions are set such that &egr;-caprolactam crystals and a mother liquid are formed
(3) a stream from the crystallizer is fed to a separator where the &egr;-caprolactam crystals are separated from the mother liquid
(4) the mother liquid is recycled.
In step (2) the crystallizer is operated such that crystallization of &egr;-caprolactam occurs through cooling. In the crystallizer relatively pure &egr;-caprolactam crystals are formed (solid phase) and a mother liquid, which comprises &egr;-caprolactam, impurities and optionally solvent (liquid or melt phase). The solid phase in the crystallizer can have a different appearance, depending on the way the crystallization is performed. The crystallization in step (2) can be performed either by cooling via a heat exchanging surface (suspension or layer crystallization) or by adiabatic cooling by evaporation of part of the contents of the crystallizer, for instance a solvent, under reduced pressure (crystallization in suspension). The method of crystallization induced by reduced pressure cooling is preferred, since no crystallization on inner surfaces of the crystallizer occurs. In reduced pressure cooling the condensed vapour from the crystallizer may or may not be returned, totally or partially to the contents of the crystallizer. Preferably the crystallizer is operated by evaporating the solvent under reduced pressure.
Preferably solvent is present in the mixture in the crystallizer, although crystallization can also be conducted without solvent. Many solvents are suitable. Examples of suitable solvents are water, alkanes (like n-hexane, n-heptane, iso-octane, cyclohexane), alcohols (like methanol, ethanol, n-propanol, butanol), aromatic hydrocarbons (like benzene, toluene, o-xylene, m-xylene, p-xylene), ammonia, chlorinated hydrocarbons (like tetrachloromethane, chloroform or ethylchloride), ketones (like acetone or methylethyl keton) and esters (like ethyl acetate). Preferably water and aromatic hydrocarbons are used as solvent, since these solvents give large crystals. Most preferred as solvent is water. The solvent will act as a freezing point depressor for the melt in the crystallizer.
The concentration of solvent in the melt in the crystallizer is dependent on the solvent, the amounts of impurities in the feed caprolactam and the way the cooling in the crystallizer is performed. With the preferred solvent water and reduced pressure cooling the concentration water in the melt is usually below 20 weight %, preferably 1-15 weight % and more preferred 2-10 weight %.
A solvent stream may be directly fed to the crystallizer and/or is mixed with the liquid crude caprolactam feed stream prior to being fed to the crystallizer.
The temperature of the mixture in the crystallizer is dependent on the presence and concentration of solvent and impurities in the mixture, but at most 69° C., being the melting temperature of pure &egr;-caprolactam. Preferably the temperature of mixture in the crystallizer is 20-69° C., more preferable 35-67° C. The crystallizer can be operated in batch or in continuous mode. Preferably the crystallizer is operated in a continuous mode.
The separator in step (3) may be any separator that is capable of separating crystals from the mother liquid, e.g. a filter working under forces like gravity, reduced pressure, increased pressure or a centrifuge. Various types of filters and centrifuges can be used. In these separators during or after separation washing of the crystals is possible and preferred. The separator in step (3) is for instance a horizontal vacuum belt filter. This type of solid-liquid separator has an excellent washing efficiency. Another example of a separator is a crystal washcolumn, in which the crystals are compacted into a packed bed which bed is transported with gravity, hydraulic pressure or a mechanical means. An example of a crystal washcolumn in which the crystal bed is transported with a mechanical means is a Niro screw-type wash column system as for example described in ‘European Chemical News’, Jun. 30-Jul. 6, 1997, page 23. A crystal washcolumn has the advantage that an effective separation of the &egr;-caprolactam crystals from the mother liquid is achieved and simultaneously very effective washing of the crystals is performed. A more preferred crystal washcolumn is the so-called TNO-Thijssen hydraulic wash column as described in “Improved procedures for separating crystals for the melt”, D. Verdoes, G. J. Arkenbout et al., Applied Thermal Engineering, 17 (8-10), 1997, 879-888.
In the TNO-Thijssen hydraulic wash column, the purified &egr;-caprolactam crystals are removed from the crystal bed and subsequently molten by a heat exchanger. A part of the molten &egr;-caprolactam crystals is recycled to the crystal washcolumn as washing liquid. The &egr;-caprolactam washing liquid finally crystallizes on the surface of &egr;-caprolactam crystals present in the so-called washfront. This is advantageous because, with a minimum quantity of washing liquid, a very effective separation of the &egr;-caprolactam crystals from the mother liquid and simultaneously a high washing efficiency of the &egr;-caprolactam crystals is achieved. In the TNO-Thijssen hydraulic wash column the purified &egr;-caprolactam is obtained as a liquid melt.
Advantageously the purified &egr;-caprolactam from step (3) is further purified in a second crystallization step (2
Agterberg Frank P. W.
Guit Rudolf P. M.
Haasen Nicolaas F.
DSM N.V.
Kifle Bruck
Pillsbury & Winthrop LLP
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