Such a process is disclosed in US-A- 3,264,060. In it, a process is described for the manufacture of lactams, including caprolactam, in which process cycloalkanone oxime is rearranged by Beckmann rearrangement in the presence of sulphuric acid, whereby a lactam-containing rearrangement mixture is obtained. Subsequently, the rearrangement mixture is neutralised in a neutralisation zone with the aid of ammonia, whereby a mixture containing ammonium sulphate solution and aqueous crude lactam is obtained. Next, the ammonium sulphate solution and the aqueous lactam are separated from one another.

The ammonium sulphate solution is passed into an evaporator in which the water present in the ammonium sulphate solution partially evaporates and in which ammonium sulphate crystals are liberated from the solution. In this process mother liquor evolves which comprises the remaining non-evaporated ammonium sulphate solution. Ammonium sulphate crystals are recovered in the next process step by separating them from the mother liquor with the aid of for example a centrifuge. The mother liquor separated here is then diluted by adding water, whereupon the dilute mother liquor is passed to the neutralisation zone. One possibility of accomplishing such dilution is to utilise condensate obtained by condensing

evaporatedwater present in the evaporator. If this option is exercised, the condensate is returned to the production process for lactam.

The aqueous lactam stream is purified by extraction with benzene. This produces purified lactam.

It has been found, however, that the purified lactam may contain a high concentration of organic contaminants that are not removed in the purification step and that the purified lactam may have a high extinction measured in accordance with ISO 7059. The presence of organic contaminants in caprolactam has a highly detrimental effect in that, in the polymerisation of the caprolactam to nylon-6, such contaminants have a highly adverse effect on the quality of nylon-6.

We have now found that the problem of contamination of the purified caprolactam is most acute when the condensate is returned to the production process for caprolactam. If, however, the condensate is not returned to the production process for caprolactam, water from a different source needs to be added to the process. This results in strongly increased water consumption, which entails high costs.

Moreover, high water consumption constitutes an environmental burden, when the aim of for example the Responsible Care programmes of the chemical industry should precisely be to alleviate the environmental burden.

The object of the invention is to supply a process for the preparation of caprolactam in which unnecessary water usage is limited and in which the problem of inadequately pure caprolactam is reduced or

eliminated. This object of the invention is achieved by purifying the condensate being and returning it to the production process for caprolactam. Purification of the condensate, which is returned to the production process for caprolactam, results in caprolactam which is substantially purer than caprolactam produced by a production process in which the condensate is not purified, all other steps being identical. Purifying the condensate in accordance with the invention allows the condensate to be returned to the production process without any serious decline in purity of the caprolactam. Returning the condensate in accordance with the invention strongly reduces water consumption.

There exist various production processes for caprolactam in which an ammonium sulphate solution is formed in a single or in a plurality of process steps. This is possible in for example a production process in which cyclohexanone, cyclohexanone oxime and caprolactam are obtained, in that order, from benzene or toluene. An ammonium sulphate solution can be formed for example during neutralisation of the rearrangement mixture with ammonia or aqueous ammonia, which rearrangement mixture is obtained during the formation of caprolactam from cyclohexanone oxime through a Beckmann rearrangement in the presence of sulphuric acid. An ammonium sulphate solution may also form in the preparation of hydroxylamine used in the reaction of cyclohexanone to form cyclohexanone oxime. The process of the invention is not limited to any production process for caprolactam nor to any process step in which an ammonium sulphate solution is formed.

According to the process of the invention,

ammonium sulphate crystals are obtained, in a crystallisation step, from the ammonium sulphate solution by evaporating the solvent, which solvent usually is water. This is normally effected in a crystalliser. Examples of crystallisers are described in"Perry's Chemical Engineers Handbook"by Don W.

Green and James O. Maloney, 7th edition, McGraw Hill, 1997, Section 18, pages 44-55. The temperature and pressure at which the crystalliser is operated are not critical. The crystalliser usually is operated at a temperature of between 20 and 180°C and at a pressure of between 20 mbar and 8 bar. The crystalliser is preferably operated at a temperature of between 40 and 130°C and at a pressure of between 50 mbar and 2 bar.

In the crystallisation step, the vapour evolving in the crystalliser condenses in which process a condensate is obtained. Such condensation may be effected by cooling the vapour, for example with a heat exchanger.

Subsequently, the condensate is purified.

Such purification may be effected in various ways.

These include separation with activated carbon, oxidative treatments such as ozone treatment or a hydrogen peroxide treatment, W irradiation, an ion exchanger, polymer absorbent, biological purification, membrane separation, extraction or distillation. It is preferred for purification to be effected with the aid of activated carbon or an oxidative treatment. It is also possible to apply a combination of the aforementioned techniques. For example, an ozone treatment can be combined with a hydrogen peroxide treatment or with W irradiation. Aqueous streams can

be purified by the aforementioned techniques in a manner known to those skilled in the art. Many such purification techniques are described in"Handbook of Separation Process Technology"by Ronald W. Rousseau, 1987, John Wiley & Sons Inc. Distillation is described on pages 229-339, extraction on pages 405-467, ion exchangers on pages 697-732, membrane separation on pages 954-981, biological purification on pages 220-221 and purification with activated carbon is described on pages 651-653. The"Chemical Engineer's Handbook"by John H. Perry, 4th edition, McGraw-Hill Book Company covers distillation in Section 13, extraction in Section 14, adsorption and ion exchangers in Section 16 and membrane separation techniques for liquids on page 19 of Section 21.

In the process of the invention, the purified condensate may be returned to the production process for caprolactam in various locations.

"Production process for caprolactam"here means the combination of process steps used for the preparation of caprolactam. Accordingly, the production process for caprolactam comprises also those process steps that are applied for the preparation of intermediate products for caprolactam. Examples of locations to which the purified condensate may be returned are the preparation of hydroxylamine, the preparation of cyclohexanone oxime, the neutralisation of the rearrangement mixture obtained via Beckmann rearrangement and the caprolactam purification.

The process of the invention is not limited to any form of caprolactam. The caprolactam preferably is E-caprolactam.

Surprisingly, the unpurified condensate has been found to contain organic contaminants such as cyclohexanone oxime, aniline and particularly octahydrophenazine. High concentrations of organic contaminants, in particular high concentrations of octahydrophenazine, are found in the non-purified condensate especially where the ammonium sulphate solution is formed in neutralising rearrangement mixture with ammonia or aqueous ammonia, which rearrangement mixture is obtained in the formation of caprolactam from cyclohexanone oxime via a Beckmann rearrangement in the presence of sulphuric acid. For this reason, the process of the invention is particularly advantageous where the ammonium sulphate solution is formed in neutralising rearrangement mixture with ammonia or aqueous ammonia, which rearrangement mixture is obtained in the formation of caprolactam from cyclohexanone oxime via a Beckmann rearrangement in the presence of sulphuric acid.

The presence of the aforementioned contaminants in the condensate is surprising in that they are not to be found in the ammonium sulphate solution supplied to the crystalliser. Without being bound to any scientific theory, it is assumed that precursors of the aforementioned organic contaminants are present in the ammonium sulphate solution supplied to the crystalliser and that the aforementioned organic contaminants are formed in the crystallisation step. It is, in addition, surprising that the condensate is found to contain organic contaminants, such as octahydrophenazine, which has a boiling point of 355- 365°C at a pressure of 1 bar. One would not expect

octahydrophenazine because of its high boiling point, to be present in the condensate at the crystalliser operating temperature.

We have found that purifying the condensate strongly reduces the concentration of organic contaminants, in particular the concentration of unsaturated organic compounds, more in particular the concentration of octahydrophenazine, in the condensate.

Each and any reduction of the concentration of organic compounds is advantageous. It is preferred for the concentration of octahydrophenazine in the condensate to be reduced by purification to less than 0.3 mg/kg, more preferably to less than 0.1 mg/kg, particularly to less than 0.01 mg/kg.

We have also found that it is much more efficient to remove the organic contaminants, in particular to remove octahydrophenazine, from the condensate than to remove these contaminant from a caprolactam-containing stream. Without being bound to any scientific theory, it is assumed that such higher efficiency is due to the fact that, as a rule, organic compounds can be separated more readily from a non- organic stream than from an organic stream.

The invention is now illustrated by the following example without being limited thereto.

Example I In this example the process is embodied as schematically represented by Figure 1. To a neutralisation zone 1 are supplied rearrangement mixture 2 (28.5 tonnes/hour) consisting of 29/57 parts by weight of caprolactam and 28/57 parts by weight of

sulphuric acid, and ammonia 3 (36 tonnes/hour) consisting of 5/36 parts by weight of ammonia and 31/36 parts by weight of water. The temperature in neutralisation zone 1 is 45-55°C. In the neutralisation zone there is obtained a neutralisation mixture that contains ammonium sulphate solution and aqueous crude caprolactam. The pH of the neutralisation mixture is 4- 5. The neutralisation mixture4 (64.5 tonnes/hour) is passed to a separator 5, in which the ammonium sulphate and the aqueous crude caprolactam are separated through phase separation. A stream of aqueous crude caprolactam 6, (19.5 tonnes/hour), consisting of 10/39 parts by weight of water and 29/39 parts by weight of caprolactam, and a stream of ammonium sulphate solution 7 (45 tonnes/hour), consisting of 19/45 parts by weight of ammonium sulphate and 26/45 parts by weight of water, exit from the separator 5. The concentration of octahydrophenazine in the ammonium sulphate solution 7 is less than 0.01 mg/kg. The ammonium sulphate solution 7 then is admitted to a series of crystallisers 8 which are operated at temperatures of between 50 and 110°C and pressures of between 100 mbar and 1.1 bar. Ammonium sulphate crystals formed in the crystallisers exit from the crystallisers in a stream 9 (19 tonnes/hour in total). Water that has evaporated in the crystallisers is caused to condense, in which process condensate is obtained. The condensate 10 (26 tonnes/hour in total) is supplied to a carbon column 11. Carbon column 11 contains 10 m3 of Norit ROX and is operated at a temperature of 90°C. The concentration of octahydrophenazine in the condensate 10 is 0.52 mg/kg

and is reduced to less than 0.01 mg/kg by purification.

Purified condensate 12 (26 tonnes/hour) is discharged from carbon column 11. Demineralised water 13 (5 tonnes/hour) is added to the purified condensate 12, whereupon a stream 14 (31 tonnes/hour) is obtained.

Ammonia 15 (5 tonnes/hour) is added to stream 14, whereupon aqueous ammonia 3 is obtained. The aqueous crude caprolactam 6 is then purified by the technique described in WO-A-9849140. There is obtained caprolactam having an octahydrophenazine concentration of less than 0.01 mg/kg and an extinction, determined in accordance with ISO 7059, of 0.10.

Comparative expriment A The process is carried out as described in Example 1, the sole difference being that the condensate 10 is not purified. There is obtained caprolactam having an octahydrophenazine concentration of 0.95 mg/kg and an extinction of 0.19.

Example II To a series of crystallisers was supplied an aqueous ammonium sulphate solution that had formed in neutralising rearrangement mixture with ammonia or aqueous ammonia, which rearrangement mixture had been obtained in the formation of caprolactam from cyclohexanone oxime via a Beckmann rearrangement in the presence of sulphuric acid. The crystallisers were operated at temperatures of between 50 and 110°C and at pressures of between 100 mbar and 1.1 bar. The vapour evolving in the crystallisers was caused to condense, in which process condensate was obtained. The

octahydrophenazine concentration of the condensate was measured by high-performance liquid chromatography. A concentration of 1.07 mg of octahydrophenazine per kg of condensate was measured.

This condensate was purified with the aid of an activated carbon column (diameter 1.5 cm, length 15 cm). This carbon column had been filled with 5 g of Norit ROX 0.8 carbon over a length of 9.5 cm. The carbon had been activated by boiling in demineralised water for 30 minutes. The condensate was passed through the column at a flow rate of 40 ml of condensate per hour. The octahydrophenazine concentration of the condensate so purified was measured by high-performance liquid chromatography. The measured concentration was less than 0.01 mg of octahydrophenazine per kg of condensate.