Process for producing a high purity caprolactam

Disclosed is a process for producing a high purity caprolactam which comprises converting cyclohexene obtained by the partial hydrogenation of benzene into cyclohexanol through hydration, converting the cyclohexanol into cyclohexanone through dehydrogenation, converting the cyclohexanone into cyclohexanone oxime through oximation and converting the cyclohexanone oxime into caprolactam through the Beckman rearrangement, characterized by comprising isolating and purifying the methylcyclopentanol from the cyclohexanol prior to use of such cyclohexanol in dehydrogenation and feeding the isolated methylcyclopentanol directly to oximation in order that the methylcyclopentanol is not fed to dehydrogenation. The process of the invention advantageously provides an economic method for producing a caprolactam with greater purity.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a process for producing a high purity
 caprolactam, and more particularly, to an efficient process for producing
 a high purity caprolactam, which comprises separating methylcyclopentanol
 contained in cyclohexanol prepared by the hydration of cyclohexene, which
 is obtained through partial hydrogenation of benzene, and removing
 efficiently methylcyclopentanone contained in cyclohexanone to be provided
 subsequently in oximation.
 2. Description of the Prior Art
 Cyclohexanone which is used in the manufacture of high purity caprolactam
 is prepared generally by dehydrogenation of cyclohexanol. The yield of
 this reaction depends on the various reaction conditions and a kind of a
 catalyst used, but in general is 40-90% of conversion at a temperature of
 from 100 to 400.degree. C., and an atmospheric pressure of 0-10. At this
 reaction, the catalyst may be used in powder form or in particulate form,
 but a particulate form catalyst is more desirable so as to obtain a better
 result. In general, these catalysts are used alone without using a
 carrier, or in conjunction with a known carrier, if necessary.
 On the other hand, the general process for producing cyclohexanol includes
 a process comprising oxidation of cyclohexane prepared by hydrogenation of
 benzene to provide a mixture of cyclohexanol and cyclohexanone and a
 process comprising converting benzene into cyclohexene through partial
 hydrogenation, and subsequent hydration of cyclohexene.
 In the former process, the oxidation product of cyclohexane that is
 prepared in the form of a mixture of cyclohexanol and cyclohexanone is
 produced by oxidation of cyclohexane using a gas including a molecular
 oxygen as an oxidant in a liquid phase. In this response, conversion and
 selectivity may be controlled by using a supported or non-supported
 catalyst system. However, there is economic disadvantage in recovering
 cyclohexanol because of a low conversion of the reaction. In addition, the
 ratio of the resultant alcohol to ketone must be controlled carefully upon
 preparing a mixture of cyclohexanol and cyclohexanone obtained through
 oxidation of cyclohexane.
 In common, the ratio of alcohol exceeds the ratio of ketone.
 In the latter process, partial hydrogenation of benzene by using a
 transition metal catalyst and co-catalyst system in an aqueous solution
 phase produces cyclohexene which is then hydrated by the inorganic solid
 acid catalyst to provide cyclohexanol. Advantageously, the partial
 hydrogenation reaction is carried out in such a way that benzene is mainly
 converted into its main reaction product, cyclohexene, while production of
 cyclohexane, which is a byproduct of the reaction, is suppressed by
 contacting the benzene with hydrogen gas in the presence of any catalyst
 selected form the catalysts described hereinafter.
 EP 552, 809 A1 discloses a particulate hydrogenation catalyst comprised
 mainly of metallic ruthenium, and more particularly, a mixture of a
 ruthenium catalyst using a zinc compound as its co-catalyst and an oxide
 or a hydroxide of a metal such as silica, alumina, zirconium, or hafnium
 or the like which is used as a dispersing agent for increasing selectivity
 and accomplishing stability of the catalyst. On the other hand, examples
 of a catalyst for hydration of cyclohexanol include an inorganic
 acid(British Patent Nos. 1,381,149 and 1,542,996), hetero
 polyacid(Japanese Patent Publication SHO 58-1089), organic acid(Japanese
 Patent Publication SHO 43-16125) or zeolite(Japanese Patent Publication
 SHO 194828), or the like. Out of the above-mentioned catalysts, zeolite is
 desirable because it can provide advantages such as separation of catalyst
 and product and suppression of a side reaction.
 A process for producing cyclohexanone through a dehydrogenation of
 cyclohexanol is more advantageous as compared to a process for producing
 cyclohexanone through a dehydrogenation of a mixture of cyclohexanol and
 cyclohexanone prepared by an oxidation of cyclohexane in that it can
 provide savings in production cost and more stability in production
 process. Accordingly, much attention is paid to the former process.
 In spite of the above-mentioned advantages, the process for the production
 of cyclohexanol by the partial hydrogenation of benzene and subsequent
 hydration of the cyclohexene has a drawback that leads to a formation of
 undesired impurities such as methylcyclopentanol, cyclohexyl-cyclohexene
 isomer, and dicyclohexyl ether in cyclohexanol. These impurities are
 produced in an amount of 0-1000 ppm according to process conditions and
 are known to be produced by an isomer reaction or a dimerization or an
 etherification reaction between the partial hydrogenation and hydration.
 Cyclohexyl-cyclohexene isomer and dicyclohexyl, out of the aforementioned
 impurities, are high-boiling point compounds, so they can be easily
 removed during the cyclohexanol dehydrogenation process or by a column for
 removing a high-boiling point compound and a low-boiling point compound
 which is provided in front of a distillation column for separating a
 mixture of cyclohexanol and cyclohexanone. In contrast, there is
 significant difficulty in removing the methylcyclopentanol because its
 boiling point is almost the same as that of the content of a reactor. In
 the case that the methylcyclopentanol is converted to methylcyclopentanone
 and is fed to an oximation process, a purity of caprolactams produced
 through a Beckmann rearrangement and the quality of an ammonium sulfate
 by-product will tend to be deteriorated.
 As a method for overcoming these problems, International Patent Publication
 97/WO03956(Japanese Patent No. 9031052) describes a process for the
 production of .epsilon.-caprolactam capable of reducing the
 methylcyclopentanone content of the cyclohexanone to be converted into the
 oxime to 400 ppm or less by providing an additional distillation column to
 a conventional distillation process or by adopting a strict distillation
 condition so as not to subject methylcyclopentanone to be fed subsequently
 in oximation. However, this process suffers from considerable technical
 shortcomings, since providing the additional distillation column and
 adopting the strict distillation condition entail high maintenance costs
 and operation costs. Moreover, the quality of a caprolactam produced by
 this process cannot meet the quality requirement since the
 methylcyclopentanone is not removed completely.
 Thus, the present inventors have repeated studies in order to overcome the
 above problems encountered in the prior art, keeping in mind that the
 methylcyclopentanol contained in the cyclohexanol was converted to the
 methylcyclopentanone, and if the methylcyclopentanone was fed to
 oximation, it became difficult to remove and affected adversely the
 subsequent process and quality of the product, whereas methylcyclopentanol
 itself did not affect the subsequent process and quality of the product
 unlike methylcyclopentanone. Consequently, we discovered that removal of
 the methylcyclopentanol from the cyclohexanol prior to use of such
 cyclohexanol in dehydrogenation so as not to form methylcyclopentanone
 through dehydrogenation improves the overall efficiency of the process and
 quality of the caprolactam.
 SUMMARY OF THE INVENTION
 Therefore, it is an object of the invention to overcome the problems
 encountered in the prior art and to provide a process for producing a high
 purity caprolactam which is easy to carry out industrially and produces
 only very small amounts of impurities which are difficult to remove.
 We found that the above object is achieved by a process for producing a
 high purity caprolactam which comprises converting cyclohexene obtained by
 a partial hydrogenation of benzene into cyclohexanol through hydration,
 converting the cyclohexanol into cyclohexanone through dehydrogenation,
 converting the cyclohexanone into of cyclohexanone oxime through oximation
 and converting the cyclohexanone oxime into caprolactam through a Beckman
 rearrangement, characterized by comprising isolating and purifying the
 methylcyclopentanol from the cyclohexanol prior to use of such
 cyclohexanol in subsequent dehydrogenation and feeding the isolated
 methylcyclopentanol directly to oximation without passing dehydrogenation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 The term "methylcyclopentanol", when used herein, includes
 1-methylcyclopentanol, 2-methylcyclopentanol and 3-methylcyclopentanol. As
 a rule, the boiling points of cyclohexanol, cyclohexanone,
 methylcyclopentanol isomer, and methylcyclopentanone isomer are
 161.degree. C., 155.degree. C., 136-152.degree. C., and 139-145.degree.
 C., respectively. These differences in boiling points are utilized as an
 important motive in the present invention.
 FIG. 1 illustrates a conventional method for producing caprolactam, wherein
 cyclohexanol 1 starting material is fed just before dehydrogenation 2, and
 then converted to a mixture of cyclohexanol and cyclohexanone through
 dehydrogenation. The resulting reaction mixture is then separated into
 high-boiling point cyclohexanol and low-boiling point cyclohexanone
 through fractional distillation 3. The high-boiling point cyclohexanol
 flows downwardly into a lower portion of a distillation column, whereas
 the low-boiling point cyclohexanone is vaporized upwardly to an upper
 portion of the column and is separated. The cyclohexanol which is
 recovered in a lower portion of a distillation column is recycled back to
 the first reaction process and is fed to the dehydrogenation 2. At this
 time, methylcyclopentanol-based impurities contained in the cyclohexanol
 are converted into methylcyclopentanone via dehydrogenation, and then are
 provided to oximation in conjunction with the cyclohexanone. As a result,
 in this prior art process, it is not possible to obtain a high purity
 caprolactam due to the methylcyclopentanol-based impurities. In order to
 overcome this problem, one can provide an additional distillation column
 to the entire reaction system or recover a mixture of cyclohexanol and
 cyclohexanone at a strict distillation condition so as to prevent
 methylcyclopentanone from being fed to oximation process. However, this
 method may result in a substantial increase in production cost.
 As shown in FIG. 2, the present invention is featured by feeding of
 cyclohexanol 1 carried out posterior to dehydrogenation 2 and just prior
 to separation of the cyclohexanol and cyclohexanone 3.
 In the separation process of the cyclohexanol and the cyclohexanone 3 of
 the invention, cyclohexanol which is added in the cyclohexanol feeding
 process 1 and a mixture of cyclohexanol and cyclohexanone returned from
 the preceding dehydrogenation process 2 are subjected to fractional
 distillation. At this process, the cyclohexanone with a low boiling point
 is vaporized toward an upper part of the distillation column, and then fed
 to oximation, whereas the cyclohexanol with a high boiling point is
 provided to dehydrogenation 2 in a liquid phase.
 The cyclohexanol to be fed to a separation process 3 from the cyclohexanol
 feeding process 1 includes methylcyclopentanol derivatives. These
 derivatives are mostly vaporized in conjunction with cyclohexanone, and to
 be introduced, not to dehydrogenation 2, but to oximation 3 since they
 have a lower boiling point as compared to the cyclohexanol. This alcohol
 compound does not perform oxination with hydroxylamine and does not affect
 the subsequent processes.
 Therefore, the methylcyclopentanol is not introduced in the dehydrogenation
 2 or if any, only 400 ppm or less of the cyclohexanol is introduced. At
 this process, the reactants are converted to a mixture of cyclohexanone
 and unreacted cyclohexanol at the conversion of 40-70%. The mixture of the
 cyclohexanone and the cyclohexanol passing through dehydrogenation 2 is
 mixed with a newly added cyclohexanol comprising methylcyclopentanol
 impurities, and then is introduced to the separation process 3. The
 dehydrogenation 2 is desirable to be carried out in the presence of Fe/ZnO
 or copper/silica mixture catalyst.
 The separation process 3 of the cyclohexanol and cyclohexanone may be
 carried out under various pressures. Preferably, the process is performed
 under an atmospheric pressure of 10-760 mmHg, more preferably, 30-70 mmHg.
 In the present invention, a distillation column consists of 50 to 100
 stages.
 An additional distillation column can be provided before and after the
 separation process 3 of the cyclohexanol and cyclohexanone under reduced
 pressure or the ordinary atmospheric pressure in order to remove a
 low-boiling point compound (for example, water and hydrocarbon derivatives
 of 6-9 carbons) and a high-boiling point compound (dicyclohexyl ether,
 cyclohexylcyclohexene, phenol and other high-boiling point compound).
 Controlling of temperature and atmospheric pressure upon installing the
 additional distillation column depends largely on a composition of a
 reactant. In general, the low-boiling point compound is separated at a
 pressure of 100-760 mmHg, while the high-boiling point compound is
 separated at a pressure of 10-100 mmHg. Separation is carried out at the
 optimum condition that allows the distillation cost to be minimum.
 The above-mentioned cyclohexanone without methylcyclopentanone is converted
 to cyclohexanone oxime through oximation 4. This reaction is carried out
 by a reaction of cyclohexanone with hydroxylamine, wherein the
 hydroxylamine is preferably utilized in the form of a sulfate salt or a
 hydrochloric acid salt since it is not stable under ordinary conditions.
 For example, the cyclohexanone is reacted with the hydroxylamine sulfate
 in the aqueous solution phase or non-aqueous solution phase. The oximation
 for producing cyclohexanone oxime can be carried out by any known method
 which is suitable for this purpose. For example, the oximation may be
 carried out by reacting cyclohexanone with nitrogen monoxide and hydrogen
 in the presence of a noble metal catalyst or by reacting cyclohexanone
 with ammonia in the presence of hydrogen peroxide.
 Finally, the resulting cyclohexanone oxime is converted into caprolactam by
 Beckmann rearrangement 5.
 The Beckmann rearrangement 5 is carried out by reacting cyclohexanone oxime
 with oleum or concentrated sulfuric acid at a suitable temperature, and
 then followed by neutralization with a basic compound such as aqueous
 ammonia to give a crude caprolactam. In case that the cyclohexanone oxime
 is converted in the presence of oleum, the ratio of sulfuric acid to
 cyclohexanone oxime is preferably 1.0-2.0, by mole ratio. At this process,
 oleum is more useful than sulfuric acid, and as a rule, the oleum having
 an SO.sub.3 content of 10-30% by weight is used. The Beckmann
 rearrangement of cyclohexanone oxime with oleum is desirably performed at
 a temperature from 60.degree. C. to 100.degree. C. At low temperature,
 side reaction is suppressed, but a viscosity of a reactant is increased,
 and the reverse phenomena occur at a high temperature, so the temperature
 has to be controlled carefully considering a yield and efficiency of the
 process. Cooling is needed in Beckmann rearrangement so as to remove the
 heat of the reaction.
 In the present invention, advantageously a ruthenium catalyst is used in
 hydrogenation of benzene, and a solid acid catalyst is used in hydration
 of cyclohexene.
 The crude caprolactam obtained by the Beckmann rearrangement can be
 separated and purified by any suitable method, such as, for example,
 extraction by an organic solvent and subsequent distillation under reduced
 pressure to provide caprolactam. The process for producing a high purity
 caprolactam according to the present invention may be carried out on a
 batch basis or continuously.
 Although the preferred embodiments of the invention have been disclosed for
 illustrative purposes, those skilled in the art will appreciate that
 various modifications, additions and substitutions are possible, without
 departing from the scope and spirit of the invention as disclosed in the
 accompanying claims. In the following examples, a composition of a
 solution is analyzed quantitatively by a gas chromatography using a
 capillary column.
 EXAMPLE 1
 400 ppm of methylcyclopentanol was added to 500 g of cyclohexanol without a
 methylcyclopentanol and a methylcyclopentanone, and then 500 g of
 cyclohexanone was added thereto. The resulting mixture of cyclohexanol and
 cyclohexanone was separated according to the separation process 3 of a
 method depicted in FIG. 2 to provide cyclohexanol. The above separation
 was carried out in a distillation column, wherein a temperature of an
 upper stage of the distillation column was set to 73.degree. C. and the
 first stage comprised of 30 stages of distillation stages, the second
 stage comprised of 20 stages and the third stage comprised 30 stages. The
 purity of the purified cyclohexanone was 99.6%, and methylcyclopentanol
 content of cyclohexanone was 380 ppm, and methylcyclopentanol content of
 cyclohexanol was less than 20 ppm.
 EXAMPLE 2
 A cyclohexanol was prepared by the same method described in example 1
 except that 800 g of methylcyclopentanol was added. The purity of the
 purified cyclohexanone was 99.5%, and methylcyclopentanol content of
 cyclohexanone was 770 ppm, and methylcyclopentanol content of cyclohexanol
 was less than 30 ppm.
 EXAMPLE 3
 A cyclohexanol was prepared by the same method described in example 1
 except that 1200 ppm of methylcyclopentanol was added. The purity of the
 purified cyclohexanone was 99.6%, and methylcyclopentanol content of
 cyclohexanone was 1150 ppm, and methylcyclopentanol content of
 cyclohexanol was less than 40 ppm.
 EXAMPLE 4
 400 ppm of methylcyclopentanol was added to a 500 g of cyclohexanol without
 a methylcyclopentanol to be fed newly, and then a mixture solution of the
 cyclohexanol and the cyclohexanone which is prepared by dehydrogenation of
 1000 g of cyclohexanol without methylcyclopentanol was added thereto.
 Thereafter, a cyclohexanone was purified by the same method as Example 1.
 The dehydrogenation was carried out in a gas phase by vaporizing
 cyclohexanol in the presence of a catalyst via pretreating and activating
 in a reactor. After completion of the dehydrogenation, the resulting
 mixture solution of the cyclohexanol and the cyclohexanone was introduced
 to a separation process. The dehydrogenation process was proceeded by
 using a copper/silica as catalyst under the condition of a temperature of
 240.degree. C. and a pressure of 760 mmHg. LHSV(Liquid Hourly Space
 Velocity) was 0.7 l/g cat.hr. The LHSV was mainly controlled by
 controlling a flow rate of an inlet of a reactor and a conversion of the
 reaction was 50%. The purity of the purified cyclohexanone was 99.7%, and
 methylcyclopentanol content of cyclohexanone was 390 ppm, and
 methylcyclopentanol content of cyclohexanol was less than 10 ppm.
 EXAMPLE 5
 A cyclohexanol was prepared by the same method described in example 4
 except that 800 ppm of methylcyclopentanol was added. The purity of the
 purified cyclohexanone was 99.5%, and methylcyclopentanol content of
 cyclohexanone was 780 ppm, and methylcyclopentanol content of cyclohexanol
 was less than 10 ppm.
 EXAMPLE 6
 A cyclohexanol was prepared by the same method described in example 4
 except that 1200 ppm of methylcyclopentanol was added. The purity of the
 purified cyclohexanone was 99.6%, and methylcyclopentanol content of
 cyclohexanone was 1160 ppm, and methylcyclopentanol content of
 cyclohexanol was less than 15 ppm.
 EXAMPLE 7
 200 g of cyclohexanone obtained in the example 4 and 20% aqueous ammonia
 solution were simultaneously added dropwise to a 20% aqueous solution of
 hydroxylamine sulfate, while keeping pH 4-4.5, and then an additional
 large amount of hydroxylamine was added thereto and reacted for an
 additional 30 minutes. An oil layer was removed and then dehydrated under
 reduced pressure to give cyclohexanone oxime.
 The resulting cyclohexanone oxime and 25% oleum (1.5 equivalents based on
 sulfuric acid) were introduced in a reactor for the Beckmann rearrangement
 at 80-100.degree. C. for 1 hour. Cooling was performed so as to suppress a
 local heating. The resulting mixture was neutralized with a 10% by weight
 aqueous ammonia to give a caprolactam reactant, while adjusting a reaction
 condition of pH 6-7 and a temperature of 70-80.degree. C.
 The resulting neutralized solution was extracted with toluene three times
 in a separatory funnel. At this stage, a concentration of caprolactam
 contained in toluene has to be kept so as not to exceed 20%. The resulting
 organic layer was removed and distilled under reduced pressure so as to
 remove the toluene and minor amounts of moisture content to give a crude
 caprolactam. Thereafter, a suitable amount of a sodium hydroxide was added
 to the crude caprolactam, and then the resulting product was purified by
 distillation under a high vacuum to provide 3-parts caprolactams comprised
 of 10% of the first part, 70% of the middle part and 20% of the remainder.
 The caprolactam of the middle part was considered as a purified
 caprolactam and its purity was analyzed. For the purified caprolactam, PZ
 and volatile bases were determined and the results obtained were shown in
 Table 1.
 EXAMPLE 8
 A purified caprolactam was prepared by the same method described in example
 7 except that 200 g of cyclohexanone which is obtained in example 5 was
 added. For the purified caprolactam, PZ and volatile bases were determined
 and the results obtained were shown in Table 1.
 EXAMPLE 9
 A purified caprolactam was prepared by the same method described in example
 7 except that 200 g of cyclohexanone which is obtained in example 6 was
 added. For the purified caprolactam, PZ and volatile bases were determined
 and the results obtained were shown in Table 1.
 TABLE 1
 Example 7 Example 8 Example 9
 Pz 15300 13100 12800
 Volatile 0.6 0.6 0.7
 bases
 [TEST METHOD]
 PZ(Permanganate-zahl: a Number of Permanganate:
 To determine the PZ, 1 g of a caprolactam sample was dissolved in 100 ml of
 water and treated with 1 ml of aqueous solution of 0.01 N potassium
 permanganate. Thereafter, the time period was determined on a second basis
 that it takes until a color of a reaction solution became the same as that
 of standard solution due to an oxidizable material.
 Volatile Bases [mEq/kg]:
 To determine the Volatile bases, 30 g of caprolactam was dissolved in 400
 ml aqueous solution of a sodium hydroxide. After boiling the resulting
 product for 1 hour, the producing decomposition gas and distilled water
 were added in 500 ml of deionized water which is prepared by dissolving 4
 ml of a 0.02 N aqueous solution of hydrochloric acid therein, followed by
 titrating with a 0.1 N sodium hydroxide. The volatile bases are equivalent
 to the value that a reduced portion of a hydrochloric acid is converted to
 a value for ammonia.
 COMATIVE EXAMPLE 1
 In order to illustrate the effects of the present invention as compared to
 a conventional process for producing caprolactam, cyclohexanone was
 purified by the method depicted in FIG. 1. 400 ppm of methylcyclopentanol
 was added to 500 g of cyclohexanol free of methylcyclopentanol and
 methylcyclopentanone and the dehydrogenation process of Example 4 was
 repeated. The mixture of the cyclohexanol and the cyclohexanone obtained
 by the preceding dehydrogenation was separated by the same method as
 Example 1. The conversion of cyclohexanone was 53%, and the purity of the
 cyclohexanone obtained was 99.4%, and methylcyclopentanone content of
 cyclohexanone was 610 ppm, and methylcyclopentanol content of cyclohexanol
 was less than 10 ppm.
 COMATIVE EXAMPLE 2
 The cyclohexanone was purified by the same method described in comparative
 example 1 except that 800 ppm of methylcyclopentanol was added. The
 conversion of cyclohexanone was 49%, and the purity of the cyclohexanone
 obtained was 99.4%, and methylcyclopentanone content of cyclohexanone was
 1275 ppm, and methylcyclopentanol content of cyclohexanol was less than 15
 ppm.
 COMATIVE EXAMPLE 3
 The cyclohexanone was purified by the same method described in comparative
 example 1 except that 1200 ppm of methylcyclopentanol was added. The
 conversion of cyclohexanone was 55%, and the purity of the cyclohexanone
 obtained was 99.4%, and methylcyclopentanone content of cyclohexanone was
 1970 ppm, and methylcyclopentanol content of cyclohexanol was less than 20
 ppm.
 The results obtained in examples 1-6 and comparative examples 1-3 were
 shown in Table 2.
 TABLE 2
 A B C D
 EXAMPLE 1 400 380 &lt;20 --
 EXAMPLE 2 800 770 &lt;30 --
 EXAMPLE 3 1200 1150 &lt;40 --
 EXAMPLE 4 400 390 &lt;10 --
 EXAMPLE 5 800 780 &lt;10 --
 EXAMPLE 6 1200 1160 &lt;15 --
 Comparative 400 -- &lt;10 610
 Example 1
 Comparative 800 -- &lt;15 1275
 Example 2
 Comparative 1200 -- &lt;20 1970
 Example 3
 A: An amount of methylcyclopentanol added (ppm)
 B: Methylcyclopentanol content of cyclohexanone (ppm)
 C: Methylcyclopentanol content of cyclohexanol (ppm)
 D: Methylcyclopentanone content of cyclohexanone (ppm)
 As can be seen from Table 2, according to the present invention, the
 methylcyclopentanol which is contained in the cyclohexanol as an impurity
 may be readily removed and not provided to a dehydrogenation process,
 thereby little methylcyclopentanone which affects adversely to oximation
 is produced. As a result, the purified cyclohexanone free of
 methylcyclopentanone impurity is fed to the subsequent process.
 Accordingly, the novel process has the advantage that quality of the
 caprolactam is substantially improved and the production cost is reduced.