MXPA97004864A - Process for the continuous purification of capillactama impura prepared from 6-amicapronitr - Google Patents

Abstract

The present invention relates to impure capronitrile which is purified by hydrogenation, a subsequent treatment in a medium acid and subsequent distillation in an alkaline medium, by a process in which (a) 6-Amicapronitrile is converted to impure caprolactam by the reaction with water, (b) compounds with high boiling point and compounds with high boiling points and compounds with low boiling points are separated from the impure caprolactam of step (b) treated with hydrogen at a temperature of 50 at 150 ° C and from 1.5 to 250 bar in the presence of a hydrogenation catalyst and, if desired, a solvent to give a mixture A. (d) mixture A in a solvent is passed, from 30 to 80 ° C and from 1 to 5 bar, by a bionic exchanger containing terminal acid groups to give a mixture B1, or (d2) a mixture a is distilled in the presence of sulfuric acid, any solvent present is removed before the addition of sulf acid. Furu, to give a mixture B2 and (e) the mixture B1 or the mixture B2 are distilled in the presence of a base to give caprolactam pu

Description

PROCESS FOR THE CONTINUOUS PURIFICATION OF CAPILLACTAMA IMPURA PREPARED FROM 6-AMICAPRONITRILO The present invention relates to a process for the continuous purification of impure caprolactam by hydrogenation, subsequent treatment in an acid medium and subsequent distillation in an alkaline medium. It is known that caprolactam can be prepared by a Bec man rearrangement of cyclohexanone oxime with sulfuric acid or oil. After neutralization of the reaction mixture of the discharged rearrangement with ammonia, the released caprolactam is separated from ammonium sulfate by extraction with an organic solvent. Depending on the method of preparation for cyclohexanone oxime the starting materials, ie, cyclohexanone and the hydroxylammonium salt and the oximation and rearrangement method, the impure caprolactam prepared by the Beckman rearrangement contains impurities that differ in type and amount. The purity of caprolactam as a raw material in fiber has to meet high requirements. Therefore, a separate and optimized purification process is necessary for each specific process of the preparation of caprolactam from cyclohexanone oxime. In this way, German Patent 1, 253, 716 describes a process in which caprolactam is hydrogenated in the presence of hydrogenation catalysts in suspension or by the percolator bed process with the addition of acids such as sulfuric acid. In a similar process described in German Patent 1, 253, 715, alkali is added during hydrogenation. In another process described in German Patent 1, 004, 616, the caprolactam to be purified is treated first with active carbon and then with ion exchangers and then hydrogenated in the presence of hydrogenation catalysts in suspension or by the bed process percolator, after which the hydrogenated caprolactam is treated with ion exchangers. In addition, the East German patent 75, 083 describes a process for the purification of caprolactam, in which the caprolactam is first distilled and then dissolved in an organic solvent such as water, hydrogenated in the presence of catalysts is lethal in bed fixed, after which the hydrogenated caprolactam is treated with ion exchangers. European Patent 411, 455 shows that the characteristics important for the quality of caprolactam, ie the number of permanganate and the content of volatile bases can be kept simultaneously low if the impure caprolactam is continuously hydrogenated by the liquid phase process. In addition to the Beckman rearrangement of cyclohexanone exempts caprolactam, there are other ways of synthesis that give rise to caprolactam: in this way, it is known that 6-a-inocapronitrile can be reacted with water in gaseous or liquid phase in the presence or absence of a catalyst with release of ammonia to give caprolactam : when aqueous solutions are heated to 10-25% concentration of 6-aminocapronitrile in liquid phase at 250-290 ° C, caprolactam is formed in a yield up to 76% (US 2 301 964). In addition, Patent FR-A 2, 029, 540 describes the cyclization of solutions of 6-aminocapronitrile at 25-35% concentration at 220 ° C in liquid phase in water with the addition of organic solvents in the presence of, for example, compounds of zinc, copper, lead and mercury. In this case, yields of caprolactam up to 83% are obtained. The cyclisation of 6-aminocapronitrile can also be carried out in the gas phase. Starting from aqueous solutions at 80% concentration, the caprolactam yields of about 92% are obtained at 305 ° C using alumina as catalyst (US 2 357 484). It is also possible to convert 6-aminocapronitrile to caprolactam, for example, on copper / vanadium catalysts in the gas phase at 290 ° C in the presence of hydrogen, water and ammonia in a yield of about 77% (EP-A 150 295) .

The 6-aminocapronitrile necessary for cyclization can be prepared, for example, by the partial catalytic hydrogenation of adiponitrile in the presence of ammonia as a solvent: for example, suspended catalysts, such as rhodium in magnesium oxide (US 4 601 859), Raney nickel catalyst (US 2 762 835, Freidlin et al., Russ Chem. Rev. 33 (1964), WO 92/21650), nickel on alumina (US 2 208 598) or fixed bed catalysts, such as copper spinels / cobalt / zinc or iron / cobalt spinels (DB 848 654), cobalt on silica gel (DB 954 416, US 2 257 814) or iron (DE 42 35 466) can be used in this reaction. According to WO 92/21650, aminocapronitrile yields of 60% (conversion 70%, selectivity 86%) and yields of hexamethylenediamine of 9% are obtained, for example in the presence of Raney nickel. At an 80% conversion, the aminocapronitrile yield is 62% (selectivity 77%). A process for the purification of impure caprolactam prepared from 6-aminocaprolactam until now was unknown, however, since this impure caprolactam has a spectrum of by-products that it completely digests from the caprolactam prepared by the Beckman rearrangement, it is not possible to employ the purification methods for caprolactam obtained by a Beckman rearrangement.

In this manner, the impure caprolactam prepared from adiponitrile by means of 6-aminocapronitrile contains, for example, open chain and cyclic nitriles, amines and imines as by-products, which are not found in the impure caprolactam obtained from of the Beckman rearrangement. An object of the present invention is to provide a process for the purification of caprolactam prepared from 6-ammocapronytrile, which process is less costly and gives rise in a safe manner to on-spec caprolactam. We have found that this goal is achieved by a process for the continuous purification of impure caprolactam by hydrogenation, the subsequent treatment in an acid medium and the subsequent distillation in an alkaline medium, where (a) 6-ammocapronitrile is converted to impure caprolactam by the reaction with water, (b) substances with high boiling point and low boiling point are separated from the impure caprolactam from (a), (c) the impure caprolactam from step (b) is treated with 50% hydrogen. at 150 ° C and d 1.5 to 250 bar in the presence of hydrogenation catalyst and, if desired from a solvent to give a mixture A, (di) mixture A in a solvent is passed, from 30 to 80 ° C and from 1 to 5 bar, on an ion exchanger containing terminal acid groups to give a mixture Bl, or (d2) the mixture A is distilled in the presence of sulfuric acid, any solvent is removed before the addition of the sulfuric acid to give a mixture Bl, and (e) the mixture Bl or the mixture B2 is distilled in the presence of a base to give pure caprolactam. according to the invention, the 6-aminocapronitrile is reacted in the liquid or gas phase in the presence of water to give caprolactam. Processes for the cyclization of 6-aminocapronitrile are described, for example, in US 2 245 129, US 2 301 964, EP-A 150 295 or FR-A 2 029 540, so that further information in this regard is unnecessary. The 6-aminocapronitrile which is used according to the invention as starting material is generally obtained by hydrogenating adiponitrile by the known process, for example as described in DE-A 836 938, DE-A 848 654 or US 5 151 543. In a preferred embodiment, the 6-aminocapronitrile is reacted with water in the liquid phase using heterogeneous catalyst. The reaction is carried out in the liquid phase in general from 140 to 320 ° C, preferably from 160 to 280 ° C; the pressure is in general from 1 to 250, preferably from 5 to 150 bar, it being necessary to ensure that the reaction mixture is predominantly liquid under the conditions that are used. The times of stay are in general from 1 to 120, preferably from 1 to 90, in particular from 1 to 60 minutes. In some cases, the stay times of 1 to 10 minutes have proven to be completely sufficient. In general, at least 0.01, preferably from 0.1 to 20, in particular from 1 to 5 moles of water are used per mole of 6-aminocapronitrile. It is advantageous to use 6-aminocapronitrile in the form of a solution of 1-50, in particular of 5-50, particularly preferably 5-30% by weight of concentration in water (in which case the solvent is at the same time a reactant ) or in a water / solvent mixture. Examples of the solvents are Alkanes, such as methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol and tert-butanol, hydrocarbons such as petroleum ethers, benzene, toluene and xylene, lactams such as pyrrolidone and caprolactam and lactate substituted with alkyl such as N-methylpyrrolidone, N-caprolactam and N-ethylene caprolactam and carboxylates preferably of carboxylic acids of 1 to 8 carbon atoms. Ammonia may also be present in the reaction. Of course you can also use mixtures of organic solvents. Water and alkane mixtures in a water / alkanol weight ratio of 1-75 / 25-99, preferably 1-50 / 50-99, have proven to be particularly advantageous in some cases. In principle, it is also possible to use 6-aminocapronitrile as a reagent and at the same time as a solvent. Examples of heterogeneous catalysts that may be used are acidic, basic or amphoteric oxides of the elements of the second, third or fourth main groups of the Periodic Table, such as calcium oxide, magnesium oxide, boron oxide, alumina, tin or silica in the form of pyrogenic silica, silica gel, kieselguhr, quartz or mixtures thereof and metal oxide of the second to sixth subgroup of the Periodic Table, as amorphous titanium oxide, or as anatase or rutile, zirconium oxide, zinc oxide, manganese oxide or mixtures of these. The oxides of the lanthanides and actinides such as cerium oxide, thorium oxide, praseodymium oxide, samarium oxide, mixed rare earth oxides or mixtures thereof with the aforementioned oxides can also be used. Examples of other catalysts can be: vanadium oxide, niobium oxide, iron oxide, chromium oxide, molybdenum oxide, tungsten oxide or mixtures thereof, mixtures of the aforementioned oxides with another oxide are also possible. Some sulphides, selenides, and tellurides can also be used, such as zinc teluriide, tin selenide, molybdenum sulfide, sulfide of tungsten and nickel, zinc and chromium sulphides. The aforementioned compounds can be doped with compounds of the first and seventh major groups of the Periodic Table or can contain these compounds. Zeolites, phosphates and heteropolyacids and acidic and alkaline exchangers, for example Naphioni, are other suitable catalysts. If required, these catalysts can contain up to 50% by weight of copper, tin, zinc, manganese, iron, cobalt, nickel, ruthenium, palladium, platinum, silver or rhodium. Depending on the composition of the catalyst, it can be used as a catalyst without support or supported catalyst. For example, titanium dioxide can be used as a titanium dioxide extrudate or as titanium dioxide applied in a thin layer on a carrier. All the methods described in the literature can be used to apply titanium dioxide to a carrier such as silica, alumina or zirconium dioxide. In this way it is possible to apply a thin layer of titanium dioxide by hydrolysis of titanium organils, such as titanium isopropylate or titanium butylate, or by hydrolysis of TiCl 4 or other inorganic compounds containing titanium. Sols containing titanium dioxides can also be used. Other suitable compounds are zirconyl chloride, nitrate of aluminum or cerium nitrate. Suitable carriers are powders, extrudates or granules of the aforementioned oxides or other stable oxides, such as silica. The carriers that are used can be made macroporous to improve mass transport. In another preferred embodiment, the 6-aminocapronitrile is cyclized in liquid phase with water at elevated temperatures in the absence of catalyst by heating an aqueous solution of 6-a-incapronitrile in the liquid phase without the addition of catalyst in a reactor to give a mixture I consisting of mainly in water, caprolactam and a fraction with high boiling point (high boiler). In this preferred embodiment, the water is preferably used in excess, particularly preferably from 10 to 150, in particular from 20 to 100 moles of the water used per mole of 6-aminocapronitrile and an aqueous solution of 6-aminocapronitrile is obtained . In another preferred embodiment, generally from 5 to 25 moles of water are used per mole of 6-aminocapronitrile and the solution can in general also be diluted to 5-25% by weight of 6-aminocapronitrile by adding an organic solvent. Examples of suitable solvents are: Alcadan of Cj-C4 as methanol, ethanol, n-propanol, isopropanol and butanols, glycols such as ethylene glycol, diethylene glycol, triethylene glycol, and tetraethylene glycol, ethers such as ethyl butyl ether and diethylene glycol diethyl ether, C 1 -C 6 alkanes, such as n-hexane, n-heptane, n-octane, n-nonane and n-decane and cyclohexane, benzene, toluene, xylene, lactams such as pyrrolidone and caprolactam, and N-alkyl (of d.-C lactams, N-methylpyrrolidone, N-methyl caprolactam and N-ethylcaprolactam.) In another embodiment, from 0 to 5, preferably from 0.1 to 2% by weight of ammonia, hydrogen or nitrogen may be added The reaction is preferably carried out at from 200 to 370 ° C, preferably from 220 to 350 ° C, particularly preferably from 240 to 320 ° C. In general, the reaction is carried out under pressure The pressure, as a general rule, is chosen in the range from 0.1 to 50, preferably from 5 to 25 MPa, so that the reaction mixture is preferably as a liquid phase.The duration of the reaction depends mainly on the parameters chosen for the process and is in general from 20 to 180 and preferably 20 to 90 minutes in the continuous process. As a general rule, the conversion decreases in the case of shorter reaction times and observations to date have shown that annoying oligomers are formed in the case of longer reaction times. The preferred cyclization is carried out in a continuous, preferably in a tube reactor, a stirred kettle or a combination thereof. Cyclization can also be carried out batchwise. The reaction then, in general, takes 30 to 180 minutes. As a general rule, the discharged mixture consists mainly of 50 to 98, preferably 80 to 95% by weight of water and 2 to 50, preferably 5 to 20% by weight of a mixture containing mainly 50 to 90% by weight. , preferably from 65 to 85% by weight of caprolactam and from 10 to 50, preferably from 15 to 35% by weight of a fraction with a high boiling point. (high boilers). In step (b) of the novel process, the fractions with high boiling point and the tractions with low boiling point eliminate the impure caprolactam obtained in step (a), through the separation of ammonia, any solvent present, such as those mentioned above, in particular alcohols, excess water and unconverted 6-aminocapronitrile and any of the byproducts with boiling point of impure caprolactam by distillation, preferably by the top, and then the separation of the impure caprolactam from the high-boiling fractions, such as the oligomers of 6-aminocaproic acid, by distillation, preferably by the top. The observations to date have shown that if the fractions with low boiling use are separated before the high-boiling fractions or the latter before the previous ones or if the two separate simultaneously is not crucial to the success of the invention. According to the invention, the impure caprolactam pre-purified in step (b) is treated with hydrogen, using the pure caprolactam in the form of a molten substance, preferably dissolved in a solvent. Preferred solvents are those which are inert under the conditions of hydrogenation and treatment with an ion exchanger. The following are particularly suitable: C1-C3 alkanols, such as methanol, ethanol, n-propanol and isopropanol, preferably ethanol, and particularly preferably water. In a preferred embodiment, the solvent used is that of cyclization of 6-aminocapronitrile, provided an alcohol or water has been used. Generally, 50 to 95, preferably 70 to 95%, by weight of impure caprolactam solutions are used in the hydrogenation, the percentages being based on the solution. When the solvent of the cyclization step is employed, it may be necessary to add a solvent or distill a solvent to reach the desired concentration. According to the invention, the treatment with hydrogen is carried out at 50 to 150 ° C, preferably at 60 to 95 ° C, particularly preferably at 70 to 90 ° C, in the liquid phase. The pressure is chosen as a function of temperature, so as to maintain the liquid phase, according to the invention, the pressure is from 1.5 to 250, preferably from 5 to 100, particularly preferably from 5 to 20 bar. In general, hydrogen is used in amounts of 0.0001 to 5.0, preferably 0.001 to 0.7, particularly preferably 0.03 to 0.3 mol per mol of caprolactam. As a general rule, the time of stay is from 10 to 300, preferably from 15 to 200 minutes. In general, the space velocity of the catalyst is selected in the range from 1 to 6, preferably from 1.5 to 4 kg of caprolactam per liter of catalyst per hour. The hydrogenation can be carried out in suspension or in a fixed bed, in the latter case preferably a solution of caprolactam together with hydrogen, upstream or downstream, is passed over a fixed bed catalyst in a tubular zone. Observations to date have shown that hydrogenation catalysts that can be used preferably are those based on a metal that is selected from the group consisting of iron, nickel, cobalt, ruthenium, rhodium, palladium, osmium, iridium and platinum. , particularly preferably cobalt, nickel and palladium, very particularly preferably palladium, in the form of unsupported catalysts or supported catalysts, preferably the latter. In a preferred embodiment, supported palladium catalysts are used which contain from 0.01 to 10, preferably from 0.05 to 5, particularly preferably from 0.1 to 2% by weight based on the palladium catalyst. The carriers that are preferably used are activated carbon, alumina, zinc oxide, silica, titanium oxide, lanthanum oxide or zirconium dioxide or mixtures thereof. In another preferred embodiment, supported nickel catalysts are used which contain from 1 to 80, preferably from 5 to 50% by weight based on the nickel catalyst. In addition, the supported nickel catalyst may contain activating additives based on zirconium, manganese, copper or chromium elements, these additives being present, in general, in the form of oxides, in amounts from 0.1 to 20, preferably from 1 to 5% by weight based on the amount of nickel used. The preferred carriers used are aluminum oxide, silica gel, aluminas, active carbon, magnesium silicates, aluminum phosphate and boron phosphate, particularly preferably magnesium silicates, aluminum phosphate, boron phosphate and aluminum oxide.

The preparation of these precipitated or impregnated catalysts is described, for example, in Ullmann's Encyclopedia of Industrial Chemistry, Volume A5, pages 348-350, fifth edition completely revised. In another preferred embodiment, supported catalysts are used in which the catalytically active metals are concentrated on the surface. These catalysts are generally obtained by methods known per se by treating the preformed carriers containing the aforementioned substances in the form of granules, beads or extrudates with an aqueous solution of the metal salts, for example, of the nitrates, for after dry them, calcine them and then activate them with hydrogen. In a preferred embodiment, supported palladium or nickel catalysts are arranged so that they are fixed, for example, in the form of a bed, in a tubular zone, for example having a length to diameter ratio of 10: 1 to 50: 1, and the solution of caprolactam (impure) and hydrogen are passed over the fixed bed catalyst by the liquid phase or percolator bed process. Observations to date have shown that particularly the UV index and the permanganate titre number (NTP) of the impure caprolactam improves as a result of the hydrogen treatment.

The cooling and lowering of the pressure gives a mixture A consisting mainly of caprolactam and solvent, of which one is used. If the hydrogenation is carried out in a solution of caprolactam, the discharged hydrogenation mixture is usually dissolved in one of the solvents mentioned above for hydrogenation, preferably water, before treatment with the ion exchanger. According to the invention, a mixture A in a solvent is passed, in step (di), from 30 to 80 ° C, preferably from 50 to 60 ° C and from 1 to 5, preferably from 1 to 2 bar , on an ion exchanger containing terminal acid groups to give a mixture Bl, which is discharged. Ion exchangers which are preferably used are highly acidic, ie ion exchangers containing sulfo in the H form. Suitable ion exchangers are available commercially, for example, as AmberliteS, Dowex®, or Lewatit® (see, for example, Encyclopedia of Industrial Chemistry, volume A14, fifth edition completely revised, page 451). The ion exchanger charge is chosen, as a rule, in the range of 1 to 15, preferably 1 to 10 kg of caprolactam per 1 of ion exchanger per hour. Observations to date have shown that the UV index is also improved as a result of the treatment with the cation exchanger. The charged ion exchanger can usually be regenerated by washing with an aqueous mineral acid, such as sulfuric acid or phosphoric acid, and in general, the basic compounds fixed in the ion exchanger can be removed as aqueous solutions of the corresponding salts. according to the invention, the treatment with the ion exchanger can be replaced by a distillation in the presence of sulfuric acid (step (d2)), removing any solvent present before the addition of the sulfuric acid. In a preferred embodiment, any solvent present removes in a distillation column having from 2 to 4, particularly preferably 2 or 3 theoretical plates, at a lower temperature of not more than 145 ° C. The pressure is chosen as a function of the selected temperature. In general, the pressure is chosen in the range of 35 to 75, preferably 40 to 60 mbar (measured at the top of the distillation column) when the lower temperature is 145 °. According to the invention, the sulfuric acid in general from 0.1 to 0.5, preferably from 0.2 to 0.3% by weight based on the amount of caprolactam, of sulfuric acid (calculated as 100% concentration by weight of sulfuric acid) is added to the impure caprolactam thus obtained or to the impure caprolactam which is already free of the solvent. The distillation is then carried out to give a mixture B2, and the residue of the distillation containing sulfuric acid is advantageously fed to a cleavage plant for sulfuric acid. In a preferred embodiment, the distillation is carried out in a distillation column with 12 to 18, preferably 14 to 16 theoretical plates, at a higher pressure of 3 to 6, preferably 3 to 4 mbar and a lower temperature not higher of 145 ° C. The mixture Bl or B2 obtained in the treatment in the acid medium, either by treatment with an ion exchanger or by treatment with sulfuric acid, is distilled according to the invention (step e) in the presence of a base. Generally, alkali metal or alkaline earth metal compounds, such as hydroxides or water-soluble carbonates, are used as the base, for example, lithium hydroxide, sodium hydroxide, potassium hydroxide, magnesium hydroxide, calcium hydroxide, carbonate. of sodium or mixtures thereof, particularly sodium hydroxide in the form of a sodium hydroxide solution is preferred, the amount chosen as a general rule in the range of 0.5 to 0.9, preferably 0.1 to 0. 8 mole percent, based on caprolactam. In a preferred embodiment, from 0.05 to 0.25, preferably from 0.1 to 0.15% by weight of a sodium hydroxide solution (calculated with 100 of concentration by weight) is used. The distillation can be carried out in a manner known per se, separating the solvents from the caprolactam, fractions with a low boiling point and fractions with a high boiling point. In a preferred embodiment, the solvent, in particular water, is first distilled by the upper part of the mixture Bl or B2 to which a base has been added, as a rule, in a distillation column at a lower temperature not higher than 140. ° C being chosen and the pressure adjusted accordingly, preferably a pressure of 35 to 65, preferably 40 to 60 mbar (average at the top of the distillation column) is employed. The lower product is advantageously fed to a second distillation column. The lower product of the first distillation column is distilled, in general, in another distillation column, as a general rule from 4 to 6, preferably 4 mbar (measured at the top of the column) and at a lower temperature not higher than 145 ° C. In this distillation stage, fractions with low boiling point. The lower product is preferably fed to a third distillation column. The lower product of the second distillation column is fed, as a rule, to another distillation column, generally using a pressure of 4 to 6, preferably 4 mbar and a lower temperature not higher than 145 ° C. Observations to date have shown that the superior product consists of pure on-spec caprolactam. In another preferred embodiment, the lower product of the third column can be fed to a falling film evaporator making it possible to separate more caprolactam, which is advantageously recycled to the first distillation column. The variant in which a sodium hydroxide solution is used as the base is also preferred. In this case, the lower product containing sodium from the third column or from the falling film evaporator can be fed to an incineration plant, to obtain sodium carbonate and steam. It is also possible to combine the separation operations of the second and third columns in a single operation using only one distillation column. In this case, fractions with a low boiling point are usually separated by the top, fractions with a high boiling point at the bottom and caprolactam through a lateral stream. A bleeding vapor from the fractions with a low boiling point is advantageously recycled in stage © (hydrogen treatment). Observations to date have shown that the UV index is also reduced as a result of handling by distillation in the presence of a base. The sequence of the purification step comprising hydrogenation, treatment in an acidic medium and distillation in the presence of a base makes it possible, in the novel process, to prepare a pure lactam that completely satisfies the specifications of pure caprolactam obtained by rearrangement Beckman in terms of the characteristics of the permanganate absorbance number (NAP) the permanganate titration number (NTP); Free bases, volatile bases (BV) and UV index (UV). The content of the impurities detectable by gas chromatography is, as a general rule, from 100 to 150 ppm, based on caprolactam. Since some impurities in the range of 10 ppm or less can make it impossible to stick to the characteristics, and the structure of many impurities in an amount of 10 ppm or less and their chemical behavior in the purification stages are not known, success of the novel process was not predictable.

Examples Purification was carried out using impure caprolactam which was obtained by cyclization of a 10% solution of ethanolic 6-aminocapronitrile (ACN) in the presence of 2 moles of water per mole of ACN: A solution of 6-aminocapronitrile) in water and ethanol (10% by weight of ACN 6.4% by weight of water, the rest ethanol) was passed, at 100 bar in a heated tube reactor with a capacity of 25 ml (diameter 6 mm, length 800 mm) and filled with dioxide of titanium (anatase) in the form of extrudates of 1.5 mm, with a reaction temperature of 240 ° C and a residence time of 30 minutes. The product stream leaving the reactor was analyzed by gas chromatography and high pressure liquid chromatography (CLAP). Conversion: 100% performance: 88%. The reaction mixture was separated from the high-boiling substances and the low-boiling substances by fractional distillation. The impure caprolactam thus obtained had a purity of 99.5% according to the gas chromatographic analysis. 1000 g of impure caprolactam were dissolved in 250 g of water. 3.5 g of 5% palladium by weight on active carbon as carrier was added to the aqueous solution in an autoclave, and the stirring mixture was hydrogen 4 hours at 80 ° C / 5 bar. After having allowed the autoclave to cool down and let it stand still, the catalyst is filtered. The filtrate was passed over an L of a highly acidic ion exchanger (Amberlite® IR 120, form H) at 50 ° C and atmospheric pressure for 0.6 hours by the percolating bed method. 4 g of a 25% aqueous sodium hydroxide solution was added to the ion exchanger discharge. The water is distilled in a distillation column with two theoretical plates, at a pressure higher than 50 mbar and at a lower temperature of 135 ° C. Substances with a low boiling point were distilled from the bottom product of the first column in a second column with 5 theoretical plates, at a pressure higher than 3.5 mbar and a lower temperature of 140 ° C. The lower product of the second column is distilled in a third column with 15 theoretical plates. At a pressure higher than 4 mbar and a lower temperature of 145 ° C, a total of 990 g of caprolactam was distilled through the upper part (99%, based on the impure caprolactam used). According to the analysis by gas chromatography, the resulting impure caprolactam contained only 140 ppm of impurities and no compounds were found that could adversely affect the polymerization of caprolactam to nylon 6. The characteristics of pure lactation were: NAP: 1.5 NTP: 1.2 Free bases: < 0.05 meq / kg volatile bases: < 0.5 meq / kg UV: 2.5 Caprolactam prepared from 6-aminocapronitrile thus meets the specifications required Beckman caprolactam. The improvement in the UV index and the permanganate titration number (NTP) by the individual purification steps is shown in Table 1.

Table 1 Permanganate titration number (NTP) The stability of caprolactam with potassium permanganate was determined by titration. The permanganate titration number (NTP) corresponds to the consumption of a solution of potassium permanganate 0.1 N in me, based on 1 kg of caprolactam, which was found during the titration of a solution containing sulfuric acid.

Absorbance number of permanganate (NAP) The stability of caprolactam to potassium permanganate was determined by photometry (see also ISO 8660 method). this purpose, equal amounts of 0.01 N potassium permanganate solution were added to an aqueous solution of caprolactam at 3% (m / m) and to a white sample (distilled water). After 10 minutes the absorbances E at 420 nm of the caprolactam sample and the blank sample were compared. The absorbance number of permanganate was calculated from the absorbance measured Volatile bases (BV) (Determination in a pam apparatus, see also the method ISO 8661 Caprolactam industrial use - Determination of the content of volatile bases). During a distillation in an alkaline medium volatile bases were released from the sample (Kjeldahl apparatus), taken in 0.01 N hydrochloric acid and determined by titration with a 0.01 N sodium hydroxide solution, the weight of the caprolactam sample being 20 ± 0.1 g. BV = (B - A) X 0.01 X 1000 meq / kg A = consumption of sodium hydroxide solution 0.01 NB = consumption of sodium hydroxide solution 0.01 N a blank determination, UV index (UV) The specific absorbances of an aqueous solution of caprolactam 50% (m / m) to 270, 280, 290, 300, 310, 320, 330, 340, 350 and 360 nm were determined in a 10 cm cell. The sum of the absorbances was multiplied by 2 and the UV index was obtained, based on 100% caprolactam. To determine the free bases, 150 ml of distilled water free of C02 and gasified with nitrogen was brought exactly to pH 7.0 with a 0.01 N sodium hydroxide solution, and 50 ± 0.1 g of caprolactam was added. The preparation was then brought to 20 ° C with 0.01 N hydrochloric acid at pH 7.0. the amount of free base could then be calculated using the following ula, where A (mi) is the hydrochloric acid consumption 0.01N: Free bases = 0.01 * A * 1000/50 = 0.2 * A meq / kg

Claims (1)

1. REINVINDICATIONS A process for the continuous purification of impure caprolactam by hydrogenation, subsequent treatment in an acid medium and subsequent distillation in an alkaline medium, where (a) 6-aminocapronitrile is converted to impure caprolactam by reaction with water, (b) Substances with high boiling point and low boiling point are separated from the impure caprolactam from step (a), (c) the impure caprolactam from step (b) is treated with hydrogen at 50 to 150 ° C and 1.5 at 250 bar in the presence of hydrogenation catalyst and, if desired, of a solvent to give a mixture A, (di) mixture A in a solvent is passed, from 30 to 80 ° C and from 1 to 5 bar, on an ion exchanger containing terminal acid groups to give a mixture Bl, or (d2) the mixture A is distilled in the presence of sulfuric acid, removing any solvent before the addition of the sulfuric acid to give a mixture Bl, and (e) the Bl mix or mix B2 is distilled in presence of a base to give pure caprolactam. process, in accordance with claim 1, in where the solvent used is water, SUMMARY OF THE INVENTION The present invention relates to impure capronitrile which is purified by hydrogenation, the subsequent treatment in an acidic medium and the subsequent distillation in an alkaline medium, by a process in which: (a) 6-aminocapronitrile is converted to impure caprolactam by the reaction with water, (b) Substances with high boiling point and low boiling point are separated from the impure caprolactam from step (a), (c) the impure caprolactam from step (b) is treated with 50% hydrogen. at 150 ° C and d 1.5 to 250 bar in the presence of hydrogenation catalyst and, if desired, a solvent to give a mixture A, (di) mixture A in a solvent is passed, from 30 to 80 ° C and from 1 to 5 bar, on an ion exchange which contains terminal acid groups to give a mixture Bl, or (d2) the mixture A is distilled in the presence of sulfuric acid eliminating any solvent before the addition of the sulfuric acid to give a mixture Bl, and (e) Blend Bl or the B2 mixture is distilled in presence of a base to give pure caprolactam.