Process for producing epsilon caprolactam from 6-aminocaproic acid

Abstract

The present invention relates to a novel method for producing epsilon caprolactam (CPL) from 6-aminocaproic acid (6-ACA), which can be obtained from conventional petrochemical processes or from biochemical processes. In the case of the proposed method, the reaction time for the conversion of 6-aminocaproic acid to nylon 6 monomer is shorter and significant energy savings are possible, which is conducive to industrial-scale production. The conversion of 6-aminocaproic acid to nylon 6 monomer is carried out at atmospheric pressure and epsilon caprolactam is obtained in the final product without significantly formed oligomers.

Description

[0001] The project that gave rise to this application has received funding from the Biobased Industries Joint Undertaking (JU) under the European Union's Horizon 2020 research and innovation programme under grant agreement No 792195. JU receives support from the European Union's Horizon 2020 research and innovation programme and the Biobased Industries Joint Undertaking.

[0002] The present invention relates to a new process for producing epsilon caprolactam (CPL) from 6-aminocaproic acid (6-ACA), wherein the 6-aminocaproic acid is converted into epsilon caprolactam as a monomer for the synthesis of nylon 6. The process is applicable to starting materials containing 6-aminocaproic acid (i.e., bio-based 6-ACA) produced from petroleum raw material sources or from renewable sources such as sugars (including first-generation and / or second-generation types). The final product obtained is an aqueous solution of epsilon caprolactam free of oligomers, the properties of which correspond to the properties of epsilon caprolactam obtained by conventional means (i.e., by the reaction known as "Beckmann rearrangement"). Moreover, in the case of the proposed process, the reaction time for the conversion of 6-aminocaproic acid into nylon 6 monomer is shorter, and significant energy savings are possible, which is advantageous for industrial-scale production. The solution of epsilon caprolactam thus produced can further be subjected to known purification steps to obtain monomers suitable for polymerization into nylon 6.

[0003] Caprolactam is the lactam of 6-aminocaproic acid (or 6-ACA or 6-aminohexanoic acid). Since caprolactam is a monomer used to produce polyamide 6 (commonly known as nylon 6), its production and purification are of great importance.

[0004] Caprolactam is generally obtained by a reaction known as the "Beckmann rearrangement", by which cyclohexanone oxime in liquid form is converted into caprolactam by means of a mixture of sulfuric acid / SO3 (i.e. oleum). After neutralization with ammonia and separation of the ammonium sulfate salt, the caprolactam solution thus obtained is further subjected to known purification steps in order to obtain monomers suitable for polymerization into nylon 6.

[0005] There is literature describing different processes in which caprolactam is produced from 6-aminocaproic acid or 6-aminocaproamide or 6-aminocaproic acid esters or mixtures thereof.

[0006] U.S. Pat. No. 6194572 describes a process for preparing epsilon caprolactam by treating 6-aminocaproic acid, 6-aminocaproic acid esters or 6-aminocaproamide or mixtures thereof in the presence of superheated steam, wherein a gaseous mixture comprising epsilon caprolactam is reportedly obtained. The process is carried out at a temperature of 250° C. to 400° C. and a pressure of 0.5 MPa to 2 MPa (i.e. above atmospheric pressure) in the absence of a catalyst.

[0007] U.S. Pat. No. 3,485,821 reports the production of caprolactam by heating 6-aminocaproic acid or 6-aminocaproamide with water or an aqueous solution, the concentration of the starting material being 5 to 25 wt % and the temperature being 150° C. to 350° C. The production of caprolactam with a quantitative yield at a high conversion with substantially no contamination due to the polymer is reported; however, the proposed method is applicable to starting materials of low concentration and therefore has no industrial efficiency, and furthermore, 6-aminocaproic acid cannot be completely converted into nylon 6 monomer.

[0008] U.S. Patent No. 4,599,199 reports a process in which ε-aminocaproic acid is introduced into a fluidized alumina bed, and caprolactam is obtained by treating the ε-aminocaproic acid with steam at elevated temperature in the presence of a catalyst, wherein γ-alumina having a particle size of 0.2 mm to 1 mm is reported to be a particularly useful catalyst, and the treatment is carried out in the presence of steam at 290° C. to 400° C.

[0009] U.S. Patent No. 4767856 reports the preparation of caprolactam by heating 6-aminocaproic acid, 6-aminocaproic acid ester or 6-aminocaproamide or a mixture thereof in the presence of an inert reaction medium which is liquid under the reaction conditions and has a boiling point higher than that of caprolactam, the claimed improvements comprising: using hydrocarbons as the reaction medium, maintained at a temperature of 150° C. to 350° C., charging 6-aminocaproic acid, 6-aminocaproic acid ester, 6-aminocaproamide or a mixture thereof at a rate matching its conversion, and separating caprolactam from the reaction mixture at a rate matching its formation rate. The process uses reduced pressure. It is suggested that an acid catalyst be used in addition with an inert reaction medium which is a liquid hydrocarbon (e.g. a mineral oil fraction).

[0010] In U.S. Pat. No. 3,658,810, the technical problem was to develop a new process for preparing ε-caprolactam without forming ammonium sulfate, which was a major problem at the time. The patent reports the production of caprolactam by contacting steam 6-aminocaproic acid or caproamide, preferably using a non-volatile acid catalyst, at a temperature of 150° C. to 400° C. All the experiments presented were conducted at laboratory scale using a few grams of material. As can be seen from its examples, when atmospheric pressure or slightly reduced pressure is used, if a starting material containing an aqueous solution of 6-aminocaproic acid is used, the reported concentration never exceeds 30% by weight, and if a starting material containing 6-aminocaproic acid in discrete form is used, the reported yield is generally less than 90%. It is reported that the yield is only slightly increased at elevated pressure, but 6-aminocaproic acid is still not as effectively converted into nylon 6 monomer as the process according to the invention described herein. In view of the above facts, including limited laboratory small-scale work, the method reported in U.S. Pat. No. 3,658,810 is considered to be more similar to scientific literature and has not proven its use in actual industrial scale.

[0011] In view of the disadvantages of the above-mentioned cyclization methods, there is still a need for an efficient process for the industrial preparation of epsilon caprolactam. The above-mentioned processes require low concentrations of 6-ACA (the aqueous solution typically contains no more than 30% by weight, more typically about 10% by weight, based on the total mass of the starting material), these processes require the use of high temperatures and / or high pressures or reduced pressures (above or below atmospheric pressure) and / or the use of catalysts, typically metals or metal oxides, or heterogeneous catalysts and / or the use of organic solvents. Even when a catalyst is used as described in U.S. Pat. No. 3,658,810, the reported yields are far less than 100%, typically at the level of 70% to 80%, and in the final recovered product, high levels of unconverted 6-aminocaproic acid and its oligomers as well as many other unknown by-products are reported. 6-aminocaproic acid is not completely converted into nylon 6 monomer as in the process according to the present invention as described herein.

[0012] Recently, 6-ACA has become industrially available from conventional petrochemical processes and also from renewable sources, ie bio-based 6-ACA. For example, in WO 2005 / 068643 and WO 2010 / 129936, processes for preparing 6-aminocaproic acid in the presence of enzymes are disclosed.

[0013] In WO 2011 / 078668, the preparation of caprolactam from a starting material comprising 6-aminocaproic acid obtained in a fermentation process in the presence of superheated steam is disclosed. This application is a significant improvement over the preparation methods disclosed in U.S. 6194572 or U.S. 3658810. However, when the weight to weight ratio of carbohydrate to 6-aminocaproic acid in the starting material is 0.03 or lower, the method gives a satisfactory yield. When there is a higher amount of carbohydrate, the reported yield is less than 70%. In addition, the method reports the use of higher pressures, higher than atmospheric pressure.

[0014] It is now becoming important and interesting to come up with processes for the conversion of 6-aminocaproic acid which can be used on an industrial scale.

[0015] Therefore, the object of the present invention is to provide a method for producing ε caprolactam from 6-aminocaproic acid on an industrial scale, the 6-aminocaproic acid can be obtained from traditional petrochemical processes, or can be obtained from biochemical processes, which overcomes the disadvantages of known methods. When the 6-aminocaproic acid obtained from the biochemical process is used as the starting material, ε caprolactam derived from carbohydrates is produced. Therefore, another object of the present invention is also a method for preparing ε caprolactam derived from carbohydrates, wherein a mixture containing 6-aminocaproic acid is recovered from a culture medium containing biomass, wherein the culture medium contains one or more carbohydrates and pollutants from fermentation during the production of bio-based 6-ACA. Another object of the present invention is to provide a method for producing ε caprolactam from 6-aminocaproic acid using atmospheric pressure, thereby converting 6-aminocaproic acid, preferably completely converted into ε caprolactam without significantly forming oligomers (neither from the initial linear 6-ACA nor from the cyclic ε caprolactam obtained) in the final product (i.e., in an aqueous solution of ε caprolactam). Furthermore, in the case of the proposed process, the reaction time for the conversion of 6-aminocaproic acid to nylon 6 monomer is shorter, and significant energy savings are possible, and no organic solvent is used, which is beneficial for industrial-scale production.

[0016] The present invention will be explained in more detail below and presented in Figure 2 middle, Figure 2 A block diagram of a process according to the invention for producing epsilon caprolactam from 6-aminocaproic acid is shown.

[0017] The method for producing epsilon caprolactam from 6-aminocaproic acid comprises the following steps:

[0018] - step (i) - pre-treating the starting material comprising 6-aminocaproic acid to prepare it for step (ii) - cyclization, whereby the starting material is preheated to a temperature within or near the temperature of the cyclization reactor of step (ii) to accelerate the cyclization reaction in step (ii); this may provide the advantage of avoiding by-product formation and yield loss;

[0019] - step (ii) - feeding the pretreated starting material obtained in step (i) at a controlled flow rate into a cyclization reactor and continuously contacting said starting material with a constant flow of superheated steam in the presence of a catalyst, wherein cyclization of 6-aminocaproic acid to epsilon caprolactam occurs, wherein the cyclization reactor is at a pressure and temperature favorable for cyclization and stripping, and stripping of the vapor mixture comprising epsilon caprolactam and water is continuously performed with superheated steam, whereby 6-aminocaproic acid is converted to epsilon caprolactam at equal to or greater than 95%, 96%, 97%, 98%, 99%, 99.5%, 99.9% or 100%, whereby the vapor mixture comprising epsilon caprolactam and water is free of oligomers;

[0020] - Step (iii) - The vapor mixture comprising ε caprolactam and water obtained from step (ii) is condensed to obtain an aqueous solution of ε caprolactam, which is optionally further concentrated according to known methods (e.g. by distillation) and optionally further purified. Free of oligomers means that the weight % of oligomers is less than or equal to 5%, 4%, 3%, 2%, 1%, 0.5%, 0.1% or 0%, depending on the conversion percentage of 6-ACA to ε caprolactam, or is not detected by the analytical methods described herein.

[0021] The pretreatment of the parent material comprising the 6-aminocaproic acid in step (i) is advantageous because the parent material is preheated to the desired temperature before entering the cyclization step (ii) in the step. Therefore, the cyclization reaction in step (ii) is accelerated and almost starts just after the pretreated parent material enters the cyclization reactor. Due to the preheating of the parent material, there is no time delay before the actual start of the cyclization reaction, and the situation of the time delay is if the parent material comprising the 6-aminocaproic acid is directly fed to the cyclization step (ii) without the situation of the pretreatment step (i). Another advantage is that, in the case of pretreatment, it is easy to control the flow of the pretreated material fed to step (ii) from step (i), so that the amount of the linear 6-aminocaproic acid inside the cyclization reactor is maintained at the desired value, thereby avoiding possible side reactions and yield losses as explained below.

[0022] Since the starting material comprising 6-aminocaproic acid used in the proposed method can be in the form of an aqueous solution of 6-aminocaproic acid in which the concentration of 6-aminocaproic acid is at least 50 wt.-% based on the total mass of the starting material, or the starting material comprises 6-aminocaproic acid in the form of a discrete powder, there are two different ways of pretreating the starting material comprising 6-aminocaproic acid.

[0023] about Figure 2 , a starting material comprising 6-aminocaproic acid is pretreated in a pre-reactor R1a, wherein the 6-aminocaproic acid is in the form of an aqueous solution in which the concentration of 6-aminocaproic acid based on the total mass of the starting material is at least 50% by weight. The aqueous solution of 6-aminocaproic acid in which the concentration is at least 50% by weight is loaded from a storage container V2 into the pre-reactor R1a, wherein the aqueous solution of 6-aminocaproic acid is preferably maintained at a temperature in the range of 50° C. to 80° C. In another embodiment, 6-aminocaproic acid and water in a solid state are fed directly into the pre-reactor R1a in an amount that provides the concentration of the above-mentioned 6-aminocaproic acid aqueous solution. A suitable pre-reactor is any reactor that allows the temperature of the feed solution to be raised to a desired temperature, preferably a closed adiabatic reactor in which an adiabatic isolation system is provided in the reactor. The starting material is then preheated to a temperature within or near the temperature of the cyclization reactor of step (ii), preferably preheated to a temperature in the range of 170° C. to 260° C., more preferably preheated to a temperature in the range of 190° C. to 200° C. Owing to there is no material exchange with the outside, therefore the formation of steam causes pressure to increase to up to 8 bar to 10 bar.When satisfying the above conditions, solution keeps at least 30 minutes under these conditions.In the solution prepared in this way, when described solution is fed in the cyclization reactor of step (ii), water is in the condition of flash distillation immediately and will be converted into steam.Because parent material has been preheated, therefore the cyclization in step (ii) starts almost immediately, even if its residence time of 6-aminocaproic acid before the cyclization in the experience step (ii) is minimized, and avoids the loss of possible by-product formation (mainly oligomer) and productive rate.

[0024] These conditions ensure that all linear 6-aminocaproic acid, including some potential oligomers that may be formed from linear 6-aminocaproic acid in such a way that both linear 6-aminocaproic acid itself and potential oligomers are readily converted to cyclic caprolactam, are dissolved in water at atmospheric pressure in the following step (ii).

[0025] The pretreatment can be carried out in batch mode or in continuous mode. For a continuous process, the flow rate regulation of the aqueous solution "in" and "out" of the prereactor will be maintained in such a way that the residence time of the aqueous solution of 6-aminocaproic acid under the conditions defined above is at least 30 minutes.

[0026] The material thus obtained is transferred to the subsequent step (ii) into a cyclization reactor where cyclization occurs.

[0027] The pre-reactor can be made of any suitable material that can withstand the temperature, pressure and acid conditions of the reactor interior. Preferably, the pre-reactor is made of stainless steel. A metering system for feeding the starting material into the reactor is provided, preferably on the top of the reactor, and an outlet opening for conveying the preheated solution to the cyclization reactor of step (ii) by a pipeline under controlled flow, preferably on the side of the bottom. The top of the reactor that keeps the gas phase is connected with a pressure safety valve to avoid pressure increase exceeding fixed limits. In order to provide the heat required for preheating, the reactor is heated from the outside by a suitable means, for example, by a heating jacket or a heating coil with a suitable heating medium. In a preferred embodiment, the outer wall of the reactor is covered with an outer jacket with hot oil.

[0028] Optionally, if necessary, a removable filter is provided inside the reactor or in the outlet opening to filter out and remove some of the undissolved starting material from time to time. This is mainly the case when bio-based 6-aminocaproic acid is used in the starting material, which may contain some residual materials from the fermentation process, i.e. residual renewable raw materials from the bioprocess or other 6-aminocaproic acid contaminants.

[0029] When the starting material comprises 6-aminocaproic acid in the form of discrete powders, as an alternative pretreatment method, the pretreatment is carried out in a premelter R1b. The 6-aminocaproic acid in the form of discrete powders is fed directly as a powder (e.g., using a screw system) into the premelter and then preheated to a temperature higher than the melting point of the 6-aminocaproic acid, i.e., higher than 205°C, preferably preheated to a temperature in the range of 210°C to 260°C, thereby obtaining molten 6-aminocaproic acid free of water. It is not necessary to keep the premelter under pressure, but a slight nitrogen pressure is preferably used to avoid the undesirable effects of the presence of air, i.e., oxidation of impurities (e.g., valeramide, adipic imide), which may affect the quality of caprolactam in its final use. Preferably, the starting material can be continuously fed into the premelter and also continuously transferred to the cyclization reactor of step (ii).

[0030] The material thus obtained is transferred to the subsequent step (ii) into a cyclization reactor where cyclization takes place.

[0031] In a preferred embodiment, the starting material comprises 6-aminocaproic acid in the form of an aqueous solution in which the concentration of 6-aminocaproic acid based on the total mass of the starting material is at least 50 wt %. As already mentioned, the 6-aminocaproic acid can be obtained from a traditional petrochemical process, or can be obtained from a biochemical process, wherein a higher amount of carbohydrates and other pollutants from fermentation during the production of bio-based 6-ACA are included in the 6-aminocaproic acid. In the case of the proposed method, based on the mass of 6-aminocaproic acid, the 6-aminocaproic acid starting material can optionally comprise at least 12.5 wt %, 10 wt %, 5 wt %, 4 wt %, 3.5 wt %, 3.4 wt %, 3 wt %, 2 wt % or 1 wt % and up to 15 wt %, 12.5 wt %, 10 wt %, 5 wt % or 4 wt % carbohydrates.

[0032] In step (ii), the cyclization of the 6-aminocaproic acid contained in the starting material continues to occur in the cyclization reactor R2. By contacting the 6-aminocaproic acid with a stream of superheated steam and in the presence of a catalyst, a mixture vapor of ε caprolactam and water is formed, and the mixture vapor is continuously stripped using the stream of superheated steam, so the cyclization reactor is not a closed system. The cyclization reactor is operated at atmospheric pressure or under a slightly elevated pressure (i.e., in the range of 1.0 bar to 1.5 bar) and at a temperature in the range of 220 ° to 350 °. Preferably, the cyclization reactor is operated at atmospheric pressure or under a slightly elevated pressure (i.e., in the range of 1.0 bar to 1.5 bar). The temperature of the superheated steam is preferably maintained in the range of 300 ° C to 450 ° C.

[0033] That is, inside the cyclization reactor, many chemical species can be present in the mixture, but three are the most critical: 6-aminocaproic acid (6ACA), caprolactam (CPL) and oligomers.

[0034] They pass Figure 1 The reactions shown in are related to each other, with arrows indicating possible reactions:

[0035] k1: cyclization of 6ACA to CPL;

[0036] k -1 : Opening of CPL (to obtain 6ACA);

[0037] k2: polymerization of 6ACA to oligomers;

[0038] k -2 : Depolymerization of oligomers to 6ACA;

[0039] k3: polymerization of CPL into oligomers;

[0040] The depolymerization of oligomers back to caprolactam must be carried out via 6-aminocaproic acid.

[0041] Of course the reaction rates are different, but all reactions can occur simultaneously. Thus, in a closed system, all three chemical species are present together in the reactor.

[0042] It is also convenient and necessary to maintain the conditions inside the cyclization reactor in such a way as to minimize or preferably eliminate the formation of liquid water (H2O) in the reaction mass, since the reaction of caprolactam back to 6-ACA is defined by the following equilibrium:

[0043]

[0044] In the proposed process, the system in the cyclization reactor is not a closed system because superheated steam continuously flows into the cyclization reactor and the vapors of caprolactam and water are continuously stripped out from the cyclization reactor, so that by constantly removing caprolactam, the equilibrium between caprolactam and linear 6-aminocaproic acid is continuously and completely shifted to the right, so that the formation of caprolactam is fast and the linear 6-aminocaproic acid is preferably completely converted into caprolactam.

[0045] The cyclization reactor may be made of any suitable material resistant to strong acids at high temperatures. Preferably, the cyclization reactor is made of stainless steel (such as titanium-containing stainless steel) or some alloys such as Hastelloy or Inconel.

[0046] The cyclization reactor is equipped with a metering system, preferably a nozzle, for feeding the catalyst, preferably a liquid and more preferably a concentrated aqueous solution of an acidic compound, into the reactor. The nozzle is preferably arranged at or near the top of the cyclization reactor. An outlet for introducing a vapor mixture of caprolactam and water into step (iii) via a pipeline is arranged at or near the top of the cyclization reactor. Optionally, a device for cleaning is arranged at or near the bottom of the cyclization reactor to remove sludge / tar from heavy objects and possible degradable pollutants from organic materials (mainly in the case of the bio-based 6-aminocaproic acid used in the method) when necessary.

[0047] In order to provide the necessary heat for the cyclization process, the reactor is heated by suitable means, for example by a heating jacket or heating coils with a suitable heating medium, preferably external heating. In a preferred embodiment, the outer wall of the reactor is covered by an outer jacket with hot oil.

[0048] The cyclization reactor is also equipped with a safety valve to control and maintain the pressure at atmospheric values ​​or at slightly elevated pressure (ie in the range of 1.0 to 1.5 bar).

[0049] Of course, the cyclization reactor can be equipped with all necessary instrumentation for continuous temperature measurement, heating, flow, level control, etc.

[0050] The cyclization reactor is also equipped with a metering system and a distribution system for superheated steam. In a preferred embodiment, the superheated steam enters the cyclization reactor through a nozzle at the top of the cyclization reactor and continuously passes through a pipe inside the reactor to a distribution system implemented at the bottom of the cyclization reactor to achieve and maintain homogeneity and continuous stirring of the mixture, which is composed of a liquid / molten phase and a gas phase. Thus, continuous stirring of the mixture is achieved by the superheated steam, which provides and maintains a completely homogenous system required for a smooth and controlled cyclization process, so that no mechanical stirring system is required.

[0051] In a preferred embodiment, superheated steam is continuously fed through an annular series of steam jets located at the bottom of the cyclization reactor. This arrangement ensures effective stirring inside the cyclization reactor.

[0052] In a preferred embodiment, when the cyclization reactor is fed with the solution obtained in the prereactor R1a, the solution is fed into the cyclization reactor via a nozzle arranged at the bottom, preferably at the side, of the cyclization reactor. Ie the solution comprises water and flash evaporation of the water should generally take place inside the cyclization reactor, preferably in the mixture at the bottom of the cyclization reactor.

[0053] In another embodiment, when the molten material from the premelter R1b is fed into the cyclization reactor, this can be done through a nozzle disposed at the top of the cyclization reactor since the molten material does not contain water and flash evaporation of water does not occur.

[0054] Preferably, in order to allow the cyclization reaction to proceed smoothly and in a controlled manner, the cyclization reactor is brought to operating conditions, i.e. to a selected temperature, a selected superheated steam flow rate and a selected catalyst concentration for the start-up phase, before the pretreated starting material comprising 6-aminocaproic acid from step (i) is fed into the cyclization reactor of step (ii).

[0055] The cyclization reactor is fed with an initial charge of caprolactam solution and catalyst for the start-up phase. For example, the caprolactam solution is obtained in the depolymerization of nylon 6 and usually comprises caprolactam, water and oligomers. The amount of catalyst in the initial charge is calculated based on the mass of the initial charge, and is preferably fed directly to the cyclization reactor of step (ii) through its own pipeline and metering system before the pretreated starting material comprising 6-aminocaproic acid from step (i) is fed into the cyclization reactor.

[0056] In a continuous approach, this initial loading is only required at start-up, for example, no more than once or twice per year.

[0057] The cyclization reactor with the initial charge is heated at atmospheric pressure or at a slightly elevated pressure (i.e. in the range of 1.0 bar to 1.5 bar), preferably by the action of oil in the outer jacket and superheated steam. When the cyclization reactor reaches operating conditions, preferably T of about 220° C. to 350° C., the pretreated starting material comprising 6-aminocaproic acid from step (i) is fed to the cyclization reactor at a controlled flow rate in order to keep the amount of linear 6-aminocaproic acid in the cyclization reactor at a low level in order to avoid any side reactions. The flow rate is controlled in such a way as to keep the temperature in the cyclization reactor within the selected reaction temperature range, ideally at a constant value, with the aim of also keeping the reaction kinetics constant. The constant flow rate is defined each time according to the volume of the cyclization reactor used in the plant and should be defined in such a way that a possible temperature drop inside the cyclization reactor is minimized.

[0058] In a batch pilot test such as described below in the Examples, a pretreated aqueous solution of 6-aminocaproic acid was fed to the cyclization reactor over about 15 to 30 minutes (at an approximate rate of 1 to 2 kg of solution per minute), causing the temperature to decrease, but after an additional time of 15 to 30 minutes, the temperature in the cyclization reactor recovered and was maintained at the selected constant value.

[0059] However, the flow rate of the pretreated aqueous 6-aminocaproic acid solution is not critical for the reasons cited below.

[0060] If the process is carried out batchwise, the concentration of linear 6-aminocaproic acid in the material inside the cyclization reactor generally decreases until complete conversion of 6-aminocaproic acid to caprolactam and water is achieved, preferably within a time range of 3.5 hours to 5.5 hours, depending on the conditions selected inside the cyclization reactor, i.e. temperature, flow rate of superheated steam and catalyst concentration.

[0061] If the process is carried out continuously, the flow of the starting material comprising 6-aminocaproic acid from step (i) into the cyclization reactor of step (ii) is preferably controlled in such a way that selected stable conditions are maintained inside the cyclization reactor of step (ii), i.e. the selected temperature, the selected flow of superheated steam and the selected catalyst concentration are to be kept at constant levels, by which is meant stable conditions and constant residence time depending on the process kinetics.

[0062] Furthermore, if the concentration of 6-aminocaproic acid in the reactor is too high (i.e. the flow rate of pretreated material from step (i) into the cyclization reactor is too high), oligomer formation occurs (via the k2 reaction). This is only a temporary problem, as the constant removal of caprolactam from the reactor leads to a decrease in the 6-aminocaproic acid concentration, eventually causing the equilibrium between oligomers and 6-aminocaproic acid to shift to this last reaction.

[0063] Basically, if the flow of pre-treated material from step (i) is too high (concentration of 6-aminocaproic acid is too high), not all of it will be directly converted into caprolactam, so some oligomers will be formed. In principle, the concentration of 6-aminocaproic acid inside the reactor will be higher for a period of time, and reactions to oligomers may occur. The oligomers will then depolymerize, so that eventually all 6-aminocaproic acid will be converted into caprolactam even over a longer period of time.

[0064] The only macroscopic effect that can be observed is lower productivity, and one solution for similar deviations is to temporarily increase the temperature.

[0065] Preferably, the temperature of the superheated steam is in the range of 300° C. to 450° C. The superheated steam prepared separately in the preheater E3 continuously enters the cyclization reactor and has a dual role:

[0066] - it strips the caprolactam and water formed in the cyclization reactor with steam; and

[0067] - it, together with the heat supplied to the cyclization reactor by the external hot oil, provides sufficient heat for step (iii), i.e. for the concentration of the aqueous caprolactam solution, so that no additional heating means are required in step (iii) as will be explained below. Surprisingly, the use of superheated steam as described herein does not negatively affect the process due to its potential to increase the water content in the reactor.

[0068] The flow rate of superheated steam is continuous and kept constant and within the following range: ensuring that the ratio of water to caprolactam in the vapour leaving the cyclisation reactor and entering step (iii) is kept within the range of 65 w / w to 35 w / w, preferably within the range of 55 w / w to 45 w / w. Thus the flow rate of superheated steam is preferably within the range of 1.3 kg to 1.8 kg per kg of linear 6-aminocaproic acid entering the cyclisation reactor.

[0069] A constant continuous flow of superheated steam entering the reactor is important because due to the constant flow of superheated steam the cyclization reactor is not a closed system and the vapors of caprolactam and water formed in the cyclization reactor are continuously stripped, thereby preventing equilibrium in the cyclization reactor.

[0070] The reaction of 6-aminocaproic acid to caprolactam also results in the formation of one molecule of water per molecule of 6-aminocaproic acid. Under the proposed process conditions (i.e. higher T and atmospheric pressure, water evaporation), this is therefore a further factor preventing the formation of equilibrium in the cyclization reactor.

[0071] As already mentioned, the catalyst is required to suppress the formation of oligomers from 6-aminocaproic acid and to convert all oligomers (which may be formed or present in the initial feed to the cyclization reactor) back into caprolactam, so that all 6-aminocaproic acid contained in the material from step (i) is converted into caprolactam and no oligomer formation occurs in the final product.

[0072] The catalyst is present in the cyclization reactor in an amount of 1 to 10 wt%, 1 to 8 wt%, preferably 3 to 4 wt%, based on the total mass in the cyclization reactor.

[0073] Since the catalyst is partially consumed during the cyclization process, for example by some alkaline compounds originally present in the charge material or also formed during the process or by some losses caused by clearing heavy sludge from the bottom of the cyclization reactor, it is desirable to keep the concentration of catalyst inside the reactor at a preferred level.

[0074] The preferred amount of catalyst inside the cyclization reactor is maintained by regularly adding fresh catalyst based on regular analysis of samples taken from the reactor. Ideally, in a steady-state continuous process, catalyst is fed to the cyclization reactor from time to time only when needed, i.e. when chemical analysis shows that the concentration of the catalyst inside the reactor is decreasing, for example, when the total mass calculation in the cyclization reactor has dropped to less than 2 wt %. For this reason, the reaction mass is sampled and analyzed regularly. In this way, it is ensured that there are usually enough catalysts available for the cyclization reaction, and preferably all 6-aminocaproic acid is converted into caprolactam.

[0075] Suitable catalysts are:

[0076] - phosphorous acid, short-chain polyphosphoric acids, ammonium phosphates and generally any phosphates, including metal salts, capable of delivering a strong acid group -P-OH under process conditions;

[0077] - boric acid and its salts;

[0078] - p-toluenesulfonic acid and its salts;

[0079] -Phosphotungstic acid.

[0080] Preferred catalysts are:

[0081] - Phosphoric acid and short chain polyphosphates

[0082] - Monoammonium phosphate, diammonium phosphate and triammonium phosphate

[0083] -Sodium monophosphate and potassium monophosphate

[0084] -Boric acid and its monoammonium salt

[0085] - p-Toluenesulfonic acid and its monoammonium salt

[0086] -Phosphotungstic acid.

[0087] Phosphoric acid is the preferred one because it is the most effective, easy to handle and metered in liquid form as a 30% to 85% aqueous solution and is also relatively cheap.

[0088] The gas phase of caprolactam and water is introduced from the top of the cyclization reactor into step (iii), in which the gas phase of caprolactam and water is condensed. Optionally, in step (iii), the aqueous caprolactam solution is also concentrated.

[0089] Optionally, when necessary or at the end of the process, the molten phase at the bottom of the cyclization reactor R2, which contains possible residual heavy by-products (e.g. salts, oligomers, spent catalyst), is removed from time to time through a bottom valve. In particular, when the starting material comprises 6-aminocaproic acid obtained from renewable resources such as biomass or sustainable sugars, it is desirable to remove the heavies from the bottom of the cyclization reactor. Thus, the proposed method also enables the use of bio-6ACA obtained in a bioprocess without the need for a previous long and difficult purification step of linear bio-6ACA and is not affected by the amount of carbohydrate content in the mixture.

[0090] In a preferred embodiment, condensation and concentration are carried out simultaneously in a condensation-rectification column C1, wherein an aqueous solution of ε caprolactam is collected at the bottom of the condensation-rectification column and low-pressure steam is collected at the top of the condensation-rectification column. The vapor of caprolactam and water from step (ii) is cooled and condensed to obtain a liquid solution richer in caprolactam than the original vapor phase and stored in a storage container V1.

[0091] The condensation-rectification tower can be made of any suitable material that resists corrosion due to possible acidity at the elevated temperature used; preferably, it is made of stainless steel. The internal number of stages of the condensation-rectification tower is necessarily defined in such a way that only water as low-pressure steam is separated together with trace caprolactam at the top of the condensation-rectification tower, and a liquid caprolactam solution having a concentration richer in caprolactam than in the gas phase can be separated at the bottom of the condensation-rectification tower. Preferably, the concentration of the caprolactam solution is in the range of 25% to 85% by weight, more preferably in the range of 65% to 80% by weight. The internal stage is preferably a combination of standard trays and structured packing. The vapor phase flows upward in countercurrent inside the tower, while the condensed liquid solution of caprolactam flows downward.

[0092] The condensation-rectification column operates at atmospheric pressure. In a preferred embodiment, the temperature profile at the bottom of the column is maintained in the range of 115°C to 125°C and at the top of the column is maintained at about 100°C to 102°C.

[0093] At the top of the column there is a steam condenser E1 in order to cool and condense the vapors of caprolactam and water from the cyclization reactor of step (ii), thus recovering a large amount of the heat of condensation of the water which is reused in the plant, for example for heating water for processing caprolactam as a pumpable liquid or, for example, for concentrating the starting material in the form of a solution to contain at least 50% by weight of 6-aminocaproic acid, based on the total mass of the starting material.

[0094] No heating means is required in the bottom of the condensation-rectification column; the heat required for concentration is supplied by the vapor in step (ii), so that considerable energy savings are possible.

[0095] The final product from step (iii) is an aqueous solution of crude caprolactam having properties corresponding to the properties and quality of crude ε-caprolactam obtained by conventional means before its final purification, ie by the reaction known as "Beckmann rearrangement".

[0096] In a preferred embodiment, the starting material comprises 6-aminocaproic acid in the form of an aqueous solution in which the concentration of 6-aminocaproic acid is at least 50% by weight, based on the total mass of the starting material.

[0097] As already mentioned, the 6-aminocaproic acid can be obtained from conventional petrochemical processes, or can be obtained from biochemical processes, wherein higher amounts of carbohydrates and other contaminants from fermentation during the production of bio-based 6-ACA are contained in the 6-aminocaproic acid. In the case of the proposed method, the 6-aminocaproic acid starting material can optionally contain at least 12.5 wt%, 10 wt%, 5 wt%, 4 wt%, 3.5 wt%, 3.4 wt%, 3 wt%, 2 wt%, or 1 wt%, and up to 15 wt%, 12.5 wt%, 10 wt%, 5 wt% or 4 wt% carbohydrates based on the mass of 6-aminocaproic acid, so that a method for preparing ε caprolactam derived from carbohydrates is also an object of the present invention. In bio-6ACA, there are some possible residual carbohydrates in the starting material, i.e. sugars, such as the monosaccharides glucose, fructose and the disaccharides maltose, isomaltose and / or sucrose.

[0098] The process comprises cyclising a mixture comprising 6-aminocaproic acid and a carbohydrate in a cyclisation reactor in the presence of superheated steam and an acid catalyst under conditions providing continuous and complete conversion to epsilon caprolactam, optionally whereby 6-aminocaproic acid is converted to epsilon caprolactam at a rate of equal to or greater than 95%, 96%, 97%, 98%, 99%, 99.5%, 99.9% or 100%, thereby forming vapour of epsilon caprolactam and water. The acid catalyst is present in an amount of at least 2% by weight, preferably in an amount of 3% to 4% by weight, calculated on the total mass in the cyclisation reactor. Suitable catalysts are: phosphorous acid, short-chain polyphosphoric acid, ammonium phosphate, and generally any phosphate, including metal salts, capable of delivering a strong acid group -P-OH under process conditions; boric acid and its salts; p-toluenesulfonic acid and its salts; and phosphotungstic acid. Preferred catalysts are: phosphoric acid and short chain polyphosphoric acids; ammonium monophosphate, diphosphate, triphosphate; sodium and potassium monophosphate; boric acid and its monoammonium salts; p-toluenesulfonic acid and its monoammonium salts; and phosphotungstic acid. Phosphoric acid is most preferred because it is most effective, easy to handle and metered in liquid form in the form of a 30% to 85% aqueous solution, and is also relatively cheap. The cyclization reactor operates at atmospheric pressure or at a slightly elevated pressure (i.e. in the range of 1.0 bar to 1.5 bar) and at a temperature in the range of 220° to 350°. The temperature of the superheated steam is in the range of 300° to 450°C and the flow rate of the superheated steam is in the range of ensuring that the ratio of water to caprolactam in the steam leaving the cyclization reactor is maintained in the range of 65w / w to 35w / w, preferably 55w / w to 45w / w. The conditions of the above steps are also as described throughout the present application.

[0099] As described in the above step (i), the method may additionally include pre-treating the mixture comprising 6-aminocaproic acid and carbohydrate. In step (i), the mixture is preheated to a temperature in or near the cyclization reactor temperature so that the cyclization reaction is accelerated. The conditions of the above steps are also as described throughout the application.

[0100] Optionally, the cyclization reactor is brought to a pressure and temperature favorable for cyclization and stripping for a start-up phase by feeding the cyclization reactor with an initial charge of caprolactam solution and catalyst before the preheated mixture from step (i) is fed into the cyclization reactor. The conditions for the above steps are also as described throughout this application.

[0101] The method further comprises condensing the vapor mixture comprising epsilon caprolactam and water obtained in the cyclization reactor and further concentrating the epsilon caprolactam aqueous solution, thereby simultaneously condensing and concentrating in a condensation-rectification column as described in step (iii) above, wherein an aqueous solution of epsilon caprolactam having a concentration richer in caprolactam than in the gas phase is collected at the bottom of the condensation-rectification column, and low-pressure steam is collected at the top of the condensation-rectification column. The conditions for the above steps are also as described throughout the present application.

[0102] The features and advantages of the method according to the present invention will now be further demonstrated by examples so as to enhance the understanding of the method itself. These examples should not be considered as limiting the types and quantities of the compounds used, the range of operating parameters, etc. Example

[0103] Example 1 (One-step process in a closed system at elevated T and elevated pressure without catalyst: standard method, reference for innovative methods)

[0104] The following materials were introduced into a pilot reactor made of stainless steel having a volume of 170 liters and provided with an outer jacket (unit R1a):

[0105] - 6-ACA, powder: 12.5 kg (80 wt % based on the total mass);

[0106] - Water: 3.125 kg (20% by weight based on the total mass);

[0107] The reactor is then heated adiabatically from room temperature and atmospheric pressure until the following conditions are achieved, i.e., the holding conditions:

[0108] - Pressure: 8.5 bar;

[0109] -Liquid temperature: 193°C;

[0110] - Steam temperature: 204°C.

[0111] After a hold time of 30 minutes, the heater was turned off, whereupon the temperature and, with it, the pressure, dropped.

[0112] Once the temperature dropped below 100°C, the reactor was opened. The solid "white" material obtained was analyzed (ie: quantitative gas chromatography, dry matter, titration of end groups)

[0113] Based on the analysis of the samples, the composition found was the following:

[0114] - Caprolactam: 0,0kg 0,0% by weight

[0115] -6-ACA: 4,1 kg 26.2 wt%;

[0116] - oligomers: 7,25 kg 46,6 wt. %;

[0117] - Water: 4,25 kg approx. 27,2 wt. %.

[0118] The conversion of 6-ACA is incomplete and no caprolactam is formed:

[0119] -6-ACA conversion: 67.4%;

[0120] -6-ACA yield to CPL: 0%.

[0121] The experiments showed that, based solely on high temperature and relatively high pressure, caprolactam was not formed, but 6-ACA was partially converted to oligomers. The conversion to oligomers produced 1 mol of H2O per mol of 6-ACA, so in the closed system used for the experiments in the described example, the original amount of water was increased with water from the reaction.

[0122] Example 2a (Three-step method, no catalyst)

[0123] Step (i)

[0124] The following materials were introduced into a pilot reactor made of stainless steel having a volume of 170 liters and provided with an outer jacket (unit R1a):

[0125] - 6-ACA, powder: 25 kg (80 wt % based on the total mass);

[0126] - Water: 6.25 kg (20% by weight based on the total mass).

[0127] The reactor is then heated adiabatically from room temperature and atmospheric pressure until the following conditions are achieved, i.e., the holding conditions:

[0128] - Pressure: 8.2 bar;

[0129] -Liquid temperature: 190°C;

[0130] - Steam temperature: 203°C.

[0131] The mixture was maintained under these conditions for a hold time of 30 minutes before all material was transferred to the cyclization reactor in step (ii).

[0132] During the holding time, 6-ACA is completely dissolved in water, and the vapor formed by the initially loaded liquid water simultaneously generates an inert atmosphere, avoiding thermal degradation of 6-ACA, and even if a portion of the linear material (i.e., 6-ACA) is converted into oligomers as shown in Example 1, when entering the cyclization reactor of step (ii), the total mixture is in a liquid state at a temperature close to the temperature required to achieve the cyclization reaction of step (ii), and the water is in a condition for immediate flashing.

[0133] Step (ii)

[0134] A cyclization reactor R2 made of stainless steel with a volume of 190 liters and an outer jacket for hot oil circulation was prepared and brought to working conditions. Agitation inside the reactor was ensured by continuous feeding of superheated steam via a distributor located at the bottom of the reactor R2. The reactor R2 was loaded with an initial charge of a caprolactam solution consisting of caprolactam, oligomers and water (see Table 1 showing a schematic mass balance calculation at the end of Example 2a).

[0135] Reactor R2 is heated to T = 238°C at atmospheric pressure by the action of oil and superheated steam in the outer jacket (in this example, the main heat supply is hot oil).

[0136] The mixture from step (i) is fed to the cyclization reactor R2.

[0137] Flashing of water resulted in an initial temperature drop to 184°C briefly, which was then increased to 254°C by heating while superheated steam was continuously fed to the cyclization reactor at a constant rate of 40 l / hour.

[0138] 6-ACA then undergoes a cyclization reaction with simultaneous production of CPL; polymerization to oligomers may occur as a side reaction. Once formed, the CPL is stripped by superheated steam to step (iii).

[0139] Step (iii)

[0140] Steam, which is a mixture of water vapor and caprolactam from the 6-ACA cyclization reaction, is continuously sent to the distillation column C1. In the pilot unit, the distillation column is a packed bed distillation column made of stainless steel with a condenser (without any heat recovery in the pilot unit) and without a boiler, because the heat for distillation is recovered from the energy transferred in the previous step (ii). As a cooling and condensing medium, some liquid water is fed to the upper part of the distillation column.

[0141] From the top of the distillation column C1, water vapor at T100°C to 102°C substantially free of any caprolactam is sent to be condensed in a separate condenser E1, while from the bottom of the distillation column, a concentrated caprolactam aqueous solution at T110°C to 115°C is collected and stored in a storage container V1.

[0142] In the pilot unit, the distillation-condensation column was initially built for several routine research activities, and since the internal bed was made using some simple conventional Raschig rings, some small amounts of caprolactam were present in the vapor.

[0143] At the end of the run the residual oligomers were removed from the bottom of the cyclization reactor R2.

[0144] Taking into account the difference between input and output, the overall balance is as follows:

[0145] - Caprolactam: +3.7kg;

[0146] -Oligomers: +17.9kg;

[0147] -6-ACA: -25kg (converted to 21.6kg of caprolactam)

[0148] The following parameters were calculated from the mass balance:

[0149] -6-ACA conversion: 100%;

[0150] -6-ACA yield to CPL: 17.1%;

[0151] -Oligomers: 82.9%.

[0152] The quality of the final crude CPL is similar to that of conventional crude caprolactam.

[0153] Specifically:

[0154] -GC purity (% area): 99.4%;

[0155] - GC peaks: 12 (CPL + 7 lights and 4 heavy ones).

[0156] - Visual aspect: CPL solution is transparent

[0157] Schematic mass balance calculations at the end of Example 2a are shown in Table 1. In both cases, the original 6ACA loses H2O to become CPL or oligomers, for which reason the 25 kg of 6ACA in the feed becomes 21.6 kg.

[0158] Table 1

[0159]

[0160] Yield to CPL: 17.2%

[0161] Yield to oligomers: 83.0%

[0162] 6ACA conversion rate: 100%

[0163] Example 2b (Three-step process, without catalyst but using sugar to simulate bio-based 6-ACA with residual original feedstock)

[0164] The substance 6-ACA converted into the cyclic monomer caprolactam can be produced from a petroleum raw material source or from a renewable source, i.e. from a culture medium containing biomass, wherein the culture medium contains one or more carbohydrates such as sugars (first generation and / or second generation type), in this application, the substance 6-ACA is referred to as bio-6ACA. In bio-6ACA, there are some possible residual carbohydrates, i.e. sugars, such as monosaccharides glucose, fructose and disaccharides maltose, isomaltose and / or sucrose, in the starting material. The three-step process without the use of a catalyst was tested to simulate the effect of residual sugar contamination present in 6-ACA on the yield and quality of the produced caprolactam.

[0165] Step (i)

[0166] The following materials were introduced into a pilot reactor (R1a) made of stainless steel having a volume of 170 liters and provided with an outer jacket:

[0167] - 6-ACA, powder: 25 kg (80 wt % based on the total mass);

[0168] -Sugar mixture: 0.85 kg (3.4 wt. % based on dry 6-ACA)

[0169] - Water: 6.25 kg (20% by weight based on the total weight excluding the sugar mixture).

[0170] Notes on sugar mixture: The mixture selected was:

[0171] Glucose: 0,05kg

[0172] Fructose: 0,175kg

[0173] Maltose: 0,625kg

[0174] Considering that glucose and fructose are the starting renewable monosaccharide raw materials and maltose can be a disaccharide derived as a byproduct during the process of producing bio-6ACA, this mixture was selected to simulate the usual fermentation process. The residual sugar after the fermentation and purification process should generally be about 0.3 wt% (referring to the total fermentation broth), and in order to check the applicability of the method, a concentrated mixture more than 3 times was used.

[0175] The reactor R1 is then heated adiabatically starting from room temperature and atmospheric pressure until the following conditions are reached, i.e. the holding conditions:

[0176] - Pressure: 8.2 bar;

[0177] -Liquid temperature: 188°C;

[0178] - Steam temperature: 206°C.

[0179] The mixture was maintained under these conditions for a hold time of 30 minutes before all material was transferred to the cyclization reactor in step (ii).

[0180] During the holding time, 6-ACA is completely dissolved in the mixture, vapor is formed from the initially charged liquid water, an inert atmosphere is generated to avoid thermal degradation, and even if a portion of the linear species is converted into oligomers as shown in Example 1, the overall mixture is liquid at a temperature close to the temperature required to achieve the cyclization reaction of step (ii), while water is in a condition to flash immediately.

[0181] Step (ii)

[0182] A cyclization reactor (R2) made of stainless steel with a volume of 190 liters and an outer jacket for hot oil circulation was prepared and brought to working conditions. Agitation inside the reactor was enabled as explained in Example 2a. The reactor was fed with an initial charge of a caprolactam solution consisting of caprolactam, oligomers and water (see Table 2 showing a schematic mass balance calculation at the end of Example 2b).

[0183] The reactor was heated to T = 257°C at atmospheric pressure by the action of oil and superheated steam in the outer jacket (in this example, the main heat supply was hot oil).

[0184] The mixture from step (i) is fed to a cyclization reactor.

[0185] Flashing of water resulted in an initial temperature drop to 205°C briefly, which was then increased to 262°C by heating while superheated steam was continuously fed to the cyclization reactor at a constant rate of 40 l / hour.

[0186] 6-ACA then undergoes a cyclization reaction with simultaneous formation of CPL; polymerization to oligomers may occur as a side reaction. Once formed, the CPL is stripped by superheated steam to step (iii).

[0187] Step (iii)

[0188] The vapor of the mixture of water vapor and caprolactam as a result of the cyclization reaction of 6-ACA is continuously sent to a packed bed column (C1) made of stainless steel, which has a condenser (without any heat recovery in the pilot unit) and no boiler, because the heat for rectification is recovered from the energy transferred in the previous step (ii). As a cooling and condensing medium, some liquid water is fed to the upper part of the column.

[0189] From the top of the column, water vapor at 100 to 102° C., which does not practically contain any caprolactam, is sent for condensation in a separate condenser, while a caprolactam aqueous solution rich in caprolactam (such as the vapor phase from the cyclization reactor at 110 to 115° C.) is collected from the bottom of the column and stored in a storage container.

[0190] In the pilot unit, the rectification-condensation column was the same as described in Example 2a.

[0191] At the end of the run the residual oligomers were removed from the bottom of the cyclization reactor.

[0192] Taking into account the difference between input and output, the overall balance is as follows:

[0193] - Caprolactam: +4.4 kg;

[0194] -Oligomers: +17.2kg;

[0195] -6-ACA: -25kg (converted to 21.6kg of caprolactam).

[0196] The following parameters were calculated from the mass balance:

[0197] -6-ACA conversion rate: 100%;

[0198] -6-ACA yield to CPL: 20.4%;

[0199] -Oligomers: 79.7%.

[0200] The quality of the final crude CPL is similar to that of conventional crude caprolactam.

[0201] Specifically:

[0202] -GC purity (% area): 98.7%;

[0203] - GC peaks: 11 (CPL + 7 lights and 3 heavy ones).

[0204] - Visual aspect: CPL solution color is dark yellow

[0205] The experiments show that in the case of the three-step process without any catalyst, the conversion rate of 6-ACA, the yield to caprolactam and the quality of the monomer are not affected by the original raw materials such as sugar at all, and in fact the same results are obtained as for the single petroleum linear 6-ACA (see Example 2a). See Table 2.

[0206] Table 2

[0207]

[0208] Yield to CPL: 20.4%

[0209] Yield to oligomers: 79.8%

[0210] ACA Conversion Rate: 100%

[0211] The total amount of sugars is equal to 0.85 / 25=>3.4 wt. %, based on the mass of 6-aminocaproic acid.

[0212] Example 3 (Three-step process, using 1.33 wt% catalyst based on the total mass in the cyclization reactor)

[0213] Step (i)

[0214] The following materials were introduced into a pilot reactor made of stainless steel having a volume of 170 liters and provided with an outer jacket (unit R1):

[0215] - 6ACA, powder: 25 kg (80 wt % based on the total mass);

[0216] - Water: 6.25 kg (20% by weight based on the total mass).

[0217] The reactor is then heated adiabatically from room temperature and atmospheric pressure until the following conditions are achieved, i.e., the holding conditions:

[0218] - Pressure: 8.2 bar;

[0219] -Liquid temperature: 191°C;

[0220] - Steam temperature: 198°C.

[0221] The mixture was kept under these conditions for a hold time of 30 minutes and then all materials were transferred to the cyclization reactor in step (ii) in the same manner and with the same effect as explained previously in Example 2a.

[0222] Step (ii)

[0223] A cyclization reactor R2 made of stainless steel with a volume of 190 liters and an outer jacket for hot oil circulation was prepared and brought to working conditions. Agitation inside the reactor was ensured by continuous feeding of superheated steam via a specific distributor located at the bottom. The reactor was fed with an initial charge of a caprolactam solution consisting of caprolactam, oligomers and water, and with phosphoric acid in an amount of 1.33% by weight, based on the total mass in the cyclization reactor (see Table 3 showing a schematic mass balance calculation at the end of Example 3).

[0224] From time to time (when chemical analysis showed that the catalyst concentration inside the reactor dropped below 2 wt. %), liquid catalyst H3PO4 at a concentration of 75% in water was fed to the cyclization reactor to ensure its constant presence in the reaction mixture during the test.

[0225] The reactor was heated to T = 263°C at atmospheric pressure by the action of oil and superheated steam in the outer jacket (in this example, the main heat supply was hot oil).

[0226] The mixture from step (i) is fed to a cyclization reactor.

[0227] Flashing of water resulted in an initial temperature drop to 190°C briefly, which was then raised to 266°C by heating while superheated steam was continuously fed to the reactor at a constant rate of 40 l / hour.

[0228] 6ACA then undergoes a cyclization reaction with simultaneous production of CPL; polymerization to oligomers may occur as a side reaction. Once formed, the CPL is stripped by superheated steam to step (iii).

[0229] Step (iii)

[0230] The vapor of the mixture of water vapor and caprolactam as a result of the cyclization reaction of 6ACA is continuously sent to a packed bed column (unit C1) made of stainless steel, which has a condenser (without any heat recovery in the pilot unit) and no boiler, because the heat for the distillation is recovered from the energy transferred in the previous step (ii). As a cooling and condensing medium, some liquid water is fed to the upper part of the column.

[0231] From the top of the column, water vapor at 100 to 102° C. practically free of any caprolactam is sent for condensation in a separate condenser, and from the bottom of the column, a concentrated caprolactam aqueous solution at 110 to 115° C. is collected and stored in a storage container.

[0232] In the pilot unit, the rectification-condensation column was the same as described in Example 2a.

[0233] At the end of the run the residual oligomers were removed from the bottom of the cyclization reactor.

[0234] Taking into account the difference between input and output, the overall balance is as follows:

[0235] - Caprolactam: +15.0kg;

[0236] -Oligomer: +6,6kg

[0237] -6ACA: -25kg (converted to 21.6kg of caprolactam).

[0238] The following parameters were calculated from the mass balance:

[0239] -6ACA conversion: 100%;

[0240] -6ACA yield to CPL: 69.4%;

[0241] -Oligomers: 30.6%.

[0242] The quality of the final crude CPL is similar to that of conventional crude caprolactam.

[0243] Specifically:

[0244] -GC purity (% area): 99.3%;

[0245] -GC peaks: 10 (CPL + 5 light and 4 heavy);

[0246] - Visual aspect: CPL solution is transparent

[0247] Table 3

[0248]

[0249] Yield to CPL: 69.6%

[0250] Yield to oligomers: 30.4%

[0251] 6ACA conversion rate: 100%

[0252] Example 4 (Three-step process, using 4.0 wt% catalyst based on the total mass in the cyclization reactor)

[0253] Step (i)

[0254] The following materials were introduced into a pilot reactor made of stainless steel having a volume of 170 liters and provided with an outer jacket (unit R1):

[0255] - 6ACA, powder: 25 kg (80 wt % based on the total mass);

[0256] - Water: 6.25 kg (20% by weight based on the total mass);

[0257] The reactor is then heated adiabatically from room temperature and atmospheric pressure until the following conditions are achieved, i.e., the holding conditions:

[0258] - Pressure: 8.3 bar;

[0259] -Liquid temperature: 185°C;

[0260] - Steam temperature: 189°C.

[0261] The mixture was kept under these conditions for a hold time of 30 minutes and then all materials were transferred to the cyclization reactor in step (ii) in the same manner and with the same effect as explained previously in Example 2a.

[0262] Step (ii)

[0263] A cyclization reactor R2 made of stainless steel with a volume of 190 liters and an outer jacket for hot oil circulation was prepared and brought to working conditions. Agitation inside the reactor was ensured by continuous feeding of superheated steam via a specific distributor located at the bottom. The reactor was fed with an initial charge of a caprolactam solution consisting of caprolactam, oligomers and water, and with phosphoric acid in an amount of 4.0% by weight, based on the total mass in the cyclization reactor (see Table 4 showing a schematic mass balance calculation at the end of Example 4).

[0264] From time to time (when chemical analysis showed that the catalyst concentration inside the reactor dropped below 2 wt. %), the liquid catalyst H3PO4 at a concentration of 75% in water was fed to the cyclization reactor to ensure its constant presence in the reaction mixture during the test.

[0265] The reactor was heated to T = 266°C at atmospheric pressure by the action of oil and superheated steam in the outer jacket (in this example, the main heat supply was hot oil).

[0266] The mixture from step (i) is fed to a cyclization reactor.

[0267] Flashing of water resulted in an initial temperature drop to 208°C briefly, which was then raised to 268°C by heating while superheated steam was continuously fed to the reactor at a constant rate of 40 l / hour.

[0268] 6ACA then undergoes a cyclization reaction with simultaneous production of CPL; polymerization to oligomers may occur as a side reaction. Once formed, the CPL is stripped by superheated steam to step (iii).

[0269] Step (iii)

[0270] The vapor as a mixture of water vapor and caprolactam from the cyclization reaction of 6ACA is continuously transferred to a packed bed distillation column C1 made of stainless steel, which has a condenser (without any heat recovery in the pilot unit) and no boiler because the heat for distillation is recovered from the energy transferred in the previous step. As a cooling and condensing medium, some liquid water is fed to the upper part of the column.

[0271] From the top of the column, water vapor at 100 to 102° C. practically containing no caprolactam is sent to be condensed in a condenser, and from the bottom of the column, a concentrated caprolactam aqueous solution at 110 to 115° C. is collected and stored in a storage container.

[0272] In the pilot unit, the rectification-condensation column was the same as described in Example 2a.

[0273] At the end of the run the residual oligomers were removed from the bottom of the cyclization reactor.

[0274] Taking into account the difference between input and output, the overall balance is as follows:

[0275] - Caprolactam: +24.8kg;

[0276] -Oligomer:-3,2kg

[0277] -6ACA: -25kg (converted to 21.6kg of caprolactam).

[0278] The following parameters were calculated from the mass balance:

[0279] -6ACA conversion rate: 100%;

[0280] -6ACA yield to CPL: 100%.

[0281] -Oligomers: 0%

[0282] The quality of the final crude CPL is similar to that of conventional crude caprolactam.

[0283] Specifically:

[0284] -GC purity (% area): 99.8%;

[0285] -GC peaks: 8 (CPL + 6 light and 1 heavy);

[0286] - Visual aspect: CPL solution is transparent

[0287] Table 4

[0288]

[0289] Yield to CPL: 114.8%

[0290] Yield to oligomers: -14.8%

[0291] 6ACA conversion rate: 100%

[0292] Net yield of 6-ACA: 100%

[0293] Examples 5 to 9

[0294] Following the same procedure as used in Example 4, using the same amount of material and based on the total mass of the initial load, the amount of catalyst was 4 wt %, repeated at different temperatures and some tests were performed to evaluate the effect on the total time required for the complete conversion of 6ACA to converted caprolactam. The results are shown in Table 5.

[0295] Table 5

[0296]

[0297] For each run, the overall distribution of incoming mass and outgoing mass was the same or similar to that shown in the mass balance of Example 4.

[0298] Examples 10 to 11

[0299] As already described in Example 2b, the substance 6-ACA can be produced from a petroleum raw material source or from a renewable source, i.e. from a culture medium comprising biomass, wherein the culture medium comprises one or more carbohydrates, such as sugars (first and / or second generation type), in this application the substance 6-ACA is referred to as bio-6ACA. In bio-6ACA, there are some possible residual carbohydrates present in the starting material, such as the monosaccharides glucose, fructose and the disaccharides maltose, isomaltose and / or sucrose.

[0300] The process of the invention (using a catalyst) was tested to simulate the effect of residual sugar contamination present in Bio-6ACA on the yield and quality of the produced caprolactam.

[0301] For this purpose, the following mixture of sugars is added to 6-ACA in step (i) of the process:

[0302] - Glucose: based on 6-ACA, Ex10 0.02 wt %; Ex11a 0.2 wt % and Ex11b 0.6 wt %;

[0303] - Fructose: based on 6-ACA, Ex10 0.07 wt %; Ex11a 0.7 wt % and Ex11b 2.1 wt %;

[0304] - Maltose: based on 6-ACA, Ex10 0.25 wt %; Ex11a 2.5 wt % and Ex11b 7.5 wt %;

[0305] This is equivalent to:

[0306] - in the case of Example 10, total sugars 0.34 wt.%;

[0307] - in the case of Example 11a, total sugars 3.4 wt.%;

[0308] - In the case of Example 11b, total sugars 10.2 wt%.

[0309] The same procedure as used in Example 4 was used to conduct experiments using sugars as contaminants to evaluate the conversion and yield of 6-ACA to caprolactam in the process of the present invention. The quality of crude caprolactam was also checked. The results are shown in Table 6.

[0310] Table 6

[0311]

[0312] In case of loading of the mixture of total sugars in reactor R1 in step 1, the overall distribution of incoming and outgoing masses was the same or similar to that shown in the mass balance of Example 4 for each trial.

[0313] The caprolactam produced in each test was collected in a container and analyzed by gas chromatography.

[0314] The results are shown in Table 7.

[0315] Table 7

[0316]

[0317] As can be gathered from the results shown, the conversion and yield of 6ACA to caprolactam are not negatively affected in the process of the invention and the quality of the caprolactam is also maintained at a typical level: a slightly yellowish colour at higher contaminant concentrations may indicate the presence of a few ppm of by-products from sugars.

[0318] Examples 12 to 15

[0319] The substance 6-ACA to be converted into the cyclic monomer caprolactam is produced from renewable sources such as sugars (primary and / or secondary type) and requires some salts as nutrients during the fermentation process, both inorganic and organic (the latter also being produced directly during the biological process).

[0320] The process of the invention was also tested with combinations of these additional potential contaminants of bio-6ACA.For this purpose, as shown in Table 8, the following salt mixture was added to 6-ACA in step (i) of the process.

[0321] Table 8

[0322]

[0323] Tests using inorganic and organic salts as contaminants were conducted to evaluate the conversion and yield of 6-ACA to caprolactam in the process of the present invention using the same procedure as used in Example 4. The quality of the crude caprolactam was also checked.

[0324] The results are shown in Table 9.

[0325] Table 9

[0326] Experimenting with salt

[0327]

[0328] In case the reactor R1 in step (i) was loaded with a mixture of total sugars, the overall distribution of incoming and outgoing masses was the same or similar to that shown in the mass balance of Example 4 for each trial.

[0329] The caprolactam produced in each test was collected in a container and analyzed by gas chromatography.

[0330] The results are shown in Table 10.

[0331] Table 10

[0332] Example Example 8 Reference Example 12 Embodiment 13 Embodiment 14 Embodiment 15 Salt 0 1.67％ 5.01％ 3.24％ 9.72％ CPL Color transparent Light yellow Light yellow Light yellow Light yellow GC purity 99.5％ 99.2％ 99.1％ 99.4％ 98.6％ Peak of Light Objects 5 9 7 8 6 Peak of heavy objects 2 3 5 4 3 Total number of peaks 7 12 12 12 9

[0333] As can be seen from the results shown, even if possible contaminants are present in the starting material at levels much higher than the usual inorganic and organic salt content, the conversion rate and yield of 6-ACA to caprolactam are not negatively affected in the process of the present invention. In addition, the quality of caprolactam remains at the usual level: the yellowish color that appears at higher contaminant concentrations can indicate the presence of several ppm of by-products from the contaminants.

[0334] The lower yield observed in Example 15 appears to be more dependent on the lower temperature than the higher amount of salt.

[0335] Example 16

[0336] An additional test was conducted using a combination of all types of possible contaminants from fermentation during the production of bio-based 6-ACA. A combination of sugars, inorganic salts and organic salts was achieved at the maximum level of each family of contaminants.

[0337] In Table 11, the mixture of all pollutants is summarized.

[0338] Table 11

[0339]

[0340] The same procedure as used in Example 4 was used to conduct a test using a mixture of all contaminant sugars, inorganic salts and organic salts to evaluate the conversion and yield of 6-ACA to caprolactam in the process of the present invention. The quality of the crude caprolactam was also checked. The results are shown in Table 12.

[0341] Table 12

[0342] Tests using all contaminants

[0343]

[0344] In the case of the mixture of total salts loaded in the preparation reactor R1 in step (i), the overall distribution of incoming and outgoing masses was the same or similar to that shown in the mass balance of Example 4 for each test.

[0345] The produced caprolactam was collected in a container and analyzed by gas chromatography. The results are shown in Table 13.

[0346] Table 13

[0347] Example Example 8 Reference Example 16 sugar 0 3.40％ Inorganic salt 0 1.67％ Organic salt 0 3.24％ Total pollutants 0 8.31％ CPL color, visual aspect transparent Light Brown GC purity 99.5％ 99.4％ Peak of Light Objects 5 11 Peak of heavy objects 2 4 Total number of peaks 7 15

[0348] As can be seen from the results shown, the conversion and yield of 6-ACA to caprolactam are also not negatively affected in the process of the invention even with the admixture of all possible types of contaminants. In addition, the quality of caprolactam remains at the usual level: a brown color at higher contaminant concentrations can indicate the presence of several ppm of by-products from the contaminants.

[0349] Examples 17 to 20

[0350] Following the same procedure as used in Example 4, some tests were carried out using different catalysts instead of phosphoric acid to evaluate their catalytic effect in the process of the present invention.

[0351] The evaluation is performed by measuring:

[0352] -6ACA conversion rate;

[0353] - Yield of 6-ACA to caprolactam;

[0354] - The quality of the crude caprolactam obtained.

[0355] Each catalyst was added in an amount equal to the molar amount of phosphoric acid used in the procedure of Example 4. The results are shown in Table 14.

[0356] Table 14

[0357]

[0358] The caprolactam produced in each test was collected in a container and analyzed by gas chromatography and pH. The results are shown in Table 15.

[0359] Table 15

[0360]

[0361] From the results shown it can be concluded that the process of the invention also works using different catalysts.

[0362] In the case of diammonium hydrogen phosphate, the reaction proceeds in the same way as for phosphoric acid; moreover, the influence of temperature on the kinetics and therefore on the reaction time is again verified.

[0363] In the case of boronic acids, the yields are lower because the reaction proceeds but with slower kinetics.

[0364] With regard to quality, the main difference lies in the pH of the crude caprolactam obtained using diammonium hydrogen phosphate which is alkaline rather than acidic.

[0365] Embodiment 21

[0366] (Three-step continuous process at industrial scale, as in Examples 4 to 20, the amount of catalyst was kept at 4 wt % to evaluate the scale-up of the process described in Examples 2a to 20).

[0367] Step (i)

[0368] In the 9m 3 In an industrial reactor made of stainless steel, called a pre-reactor, of a volume of 1.37 W and equipped with temperature control (i.e., unit R1), a larger amount of 6-ACA than in the pilot test was fed.

[0369] The reactor feed had the following total materials:

[0370] -6ACA, powder: 975kg

[0371] - Caprolactam / water solution with 75% by weight of caprolactam: 496 kg.

[0372] The total composition loaded in the industrial reactor is:

[0373] Table 16

[0374]

[0375] The solid material was 72 wt% 6-ACA and 28 wt% caprolactam; water was about 8 wt% of the total weight.

[0376] The reactor is then heated adiabatically to achieve the following conditions, i.e., the holding conditions:

[0377] - Pressure: 9.4 bar;

[0378] -Temperature: 202℃.

[0379] The mixture was kept under these conditions for a hold time of 30 minutes and all materials were transferred to the cyclization reactor in step (ii) in the same manner and with the same effect as explained previously in Example 2a.

[0380] Once the transfer in the cyclization reactor is completed, the above-described operation is repeated in the same manner to increase the amount of 6-ACA in a multi-step sequence to obtain a continuous process of step (ii). However, in this example, only two cycles in step (i) are considered.

[0381] Step (ii)

[0382] Prepare to have 22m 3 A cyclization reactor made of stainless steel (i.e. unit R2) with a capacity of 1000 m and a device for thermal control was prepared and brought to working conditions.

[0383] Agitation inside the reactor is ensured by continuous feeding of superheated steam through an annular series of steam jets located in the bottom.

[0384] The reactor was fed an initial charge of CPL oligomer to maintain its level at a known measured and controlled optimum, and phosphoric acid in an amount of 4.0 wt% based on the total mass in the cyclization reactor (see Table 17 showing a schematic mass balance calculation at the end of Example 21).

[0385] The liquid catalyst H3PO4 at a concentration of 85 wt% in water was fed to the cyclization reactor, the flow rate being varied continuously according to the results of chemical analysis so as to always have about 4 wt% of catalyst inside the reactor, thus ensuring its presence in the reaction mixture during the test.

[0386] The reactor was heated to T = 256°C by circulation of diathermic oil in an outer jacket maintained at atmospheric pressure.

[0387] The flash evaporation of water during material transfer resulted in an initial temporary temperature drop (to 237°C) which was then raised by heating (to 252°C) while superheated steam was continuously fed to the reactor at a constant rate via a controlled flow of approximately 1300 kg / hour.

[0388] 6-ACA inside the reactor undergoes a cyclization reaction while producing CPL: once formed, CPL is stripped by superheated steam to step (iii).

[0389] After gradually transferring material from R1 to the cyclization reactor, the level in R2 increases. The cyclization reaction is considered complete when the level returns to the same level as before transferring material from R1.

[0390] Step (iii)

[0391] The vapor as a mixture of water vapor and caprolactam from the cyclization reaction of 6ACA is continuously sent to a packed bed distillation column (i.e., unit C1) which has a condenser and no boiler because the heat for distillation is recovered from the energy transferred in the previous step. Some liquid water is fed to the upper part of the column as a cooling and condensing medium.

[0392] As described in step (ii), during the reaction, from the top of the tower, water vapor at 100° C. to 105° C. substantially free of any caprolactam is transmitted to be directly condensed through a tower water condenser, while from the bottom of the tower, a concentrated caprolactam aqueous solution at 110° C. to 120° C. is collected and stored in a storage container until the cyclization reaction is completed.

[0393] The mass balance and analytical results are summarized in the following table:

[0394] Mass balance

[0395] Table 17

[0396]

[0397] Data confirms:

[0398] a) cyclization of 6-ACA to crude CPL with 100% yield and 100% conversion, also on an industrial scale;

[0399] b) In certain cases, also the small amount of CPL oligomers initially loaded for reactor start-up was converted into crude CPL even though the kinetics were lower than for 6-ACA in the chosen conditions.

[0400] Analytical examination of the residual material in R2 revealed that the initial composition of the CPL oligomer had 3% to 4% of the catalyst H3PO4.

[0401] analyze

[0402] Table 18

[0403]

[0404] From the results it follows that the cyclization process for obtaining 6-ACA to caprolactam as described can be operated even on an industrial scale and with water amounts in the feed of less than 20% by weight (8% by weight in this case).

[0405] Embodiment 22 (Three-step continuous process on an industrial scale, as in Example 21, but using 100% 6ACA)

[0406] An additional run was performed following a procedure similar to that used in Example 21 by feeding 6ACA and water to the prereactor without feeding caprolactam.

[0407] Load the following materials:

[0408] -6ACA, powder: 1150kg;

[0409] - Water: 200 kg (about 15 wt. % of the total weight in the pre-reactor).

[0410] The total composition loaded in the industrial reactor is:

[0411] Table 19

[0412]

[0413] The mass balance and analytical results are summarized in the following table:

[0414] Mass balance

[0415] Table 20

[0416]

[0417] Furthermore, using only 6-ACA in the feed, the data again confirmed that cyclization of 6-ACA resulted in 100% conversion to crude CPL in 100% yield at industrial scale.

[0418] Analytical examination of the residual material in R2 revealed that the initial composition of the CPL oligomer had 3% to 4% of the catalyst H3PO4.

[0419] analyze

[0420] Table 21

[0421]

[0422] The results of this run confirm what was reported in Example 21: the cyclization process of 6-ACA to obtain caprolactam as described can operate even on an industrial scale and with water amounts in the feed below 20% (15% in this case) and by using 100% 6ACA as solid material in the feed.

Claims

1. A method for producing epsilon caprolactam from 6-aminocaproic acid, comprising the following steps: - step (i) - pre-treating the starting material comprising 6-aminocaproic acid to prepare it for step (ii) - cyclization, whereby the starting material is preheated to a temperature within or near the temperature of the cyclization reactor of step (ii) to accelerate the cyclization reaction in step (ii); - step (ii) - the pretreated starting material obtained in step (i) is fed at a controlled flow rate into a cyclization reactor and continuously contacted with a constant flow of superheated steam in the presence of a catalyst, wherein cyclization of 6-aminocaproic acid to epsilon caprolactam occurs and stripping of the vapour mixture comprising epsilon caprolactam and water occurs continuously with said superheated steam, wherein the cyclization reactor is at a pressure and temperature conducive to cyclization and stripping; - step (iii) - condensing the vapor mixture comprising epsilon caprolactam and water obtained from step (ii) to obtain an aqueous solution of epsilon caprolactam; Wherein in step (i), the starting material comprises: 6-aminocaproic acid in the form of an aqueous solution, wherein the concentration of 6-aminocaproic acid is at least 50% by weight, based on the total mass of the starting materials; wherein the cyclization reactor is at a pressure in the range of 1.0 bar to 1.5 bar, and at a temperature in the range of 220°C to 350°C.

2. The method according to claim 1, wherein the temperature of the pretreatment in step (i) is in the range of 190°C to 200°C.

3. A process according to claim 1 or 2, wherein the formation of steam results in an increase in pressure in step (i) to 8 to 10 bar.

4. The process according to claim 1 or 2, wherein the aqueous solution is maintained at a temperature of 170° C. to 260° C. and a pressure of 8 bar to 10 bar until the water in the pretreated aqueous solution is in a condition to immediately flash and be converted into steam when the pretreated aqueous solution is fed into the cyclization reactor of step (ii).

5. The method according to claim 4, wherein the aqueous solution is maintained at a temperature of 190 to 200°C and a pressure of 8 to 10 bar.

6. The method of claim 4, wherein the aqueous solution is maintained in the conditions for at least 30 minutes.

7. The process according to claim 1 or 2, wherein before feeding the pretreated starting material from step (i) into the cyclization reactor, the cyclization reactor is brought to a pressure and temperature favorable for cyclization and stripping for a start-up phase by feeding an initial charge of caprolactam solution and catalyst into the cyclization reactor.

8. The process of claim 7, wherein the catalyst concentration in the initial charge is 3 to 4 wt. %, calculated on the mass of the initial charge.

9. The process of claim 1 or 2, wherein the cyclization reactor is at atmospheric pressure.

10. The method according to claim 1 or 2, wherein the temperature of the superheated steam is in the range of 300°C to 450°C.

11. The process according to claim 1 or 2, wherein the superheated steam is continuously fed through an annular series of steam jets located at the bottom of the cyclization reactor.

12. The process according to claim 1 or 2, wherein the catalyst is present in the cyclization reactor in an amount of 1 wt% to 10 wt%, calculated on the basis of the total mass in the cyclization reactor.

13. The process according to claim 1 or 2, wherein the catalyst is present in the cyclization reactor in an amount of 1 wt% to 8 wt%, calculated on the basis of the total mass in the cyclization reactor.

14. The process according to claim 1 or 2, wherein the catalyst is present in the cyclization reactor in an amount of 3 wt% to 4 wt%, calculated on the total mass in the cyclization reactor.

15. The process according to claim 1 or 2, wherein the required amount of catalyst in the cyclization reactor is maintained by regular addition of fresh catalyst based on regular chemical analysis of samples of the reaction mass taken from the reactor, wherein fresh catalyst is added when the concentration of the catalyst inside the reactor drops below 2 wt. %, calculated on the total mass inside the cyclization reactor.

16. The method according to claim 1 or 2, wherein the catalyst is selected from: Phosphoric acid and short-chain polyphosphoric acids; ammonium monophosphate, diphosphate and triphosphate; Sodium and potassium monophosphate; Boric acid and its monoammonium salt; p-Toluenesulfonic acid and its monoammonium salt; Phosphotungstic acid.

17. The method of claim 16, wherein the catalyst is phosphoric acid.

18. The process according to claim 1 or 2, wherein the flow rate of the superheated steam in step (ii) is within the range that ensures that the ratio of water to caprolactam in the steam leaving the cyclization reactor is maintained within the range of 65 w / w to 35 w / w.

19. The process according to claim 18, wherein the flow rate of the superheated steam in step (ii) is within the range that ensures that the ratio of water to caprolactam in the steam leaving the cyclization reactor is maintained in the range of 55 w / w to 45 w / w.

20. The process according to claim 18, wherein the flow rate of the superheated steam is in the range of 1.3 kg to 1.8 kg per kg of 6-aminocaproic acid entering the cyclization reactor.

21. The process according to claim 1 or 2, wherein in step (iii) a concentration of the aqueous solution of epsilon caprolactam is additionally carried out.

22. The method according to claim 1 or 2, wherein condensation and concentration are simultaneously performed in a condensation-rectification tower, wherein an aqueous solution of epsilon caprolactam having a concentration richer in caprolactam than in the gas phase is collected at the bottom of the condensation-rectification tower, and low-pressure steam is collected at the top of the condensation-rectification tower.

23. The method according to claim 22, wherein the concentration of the caprolactam solution is in the range of 25 wt% to 85 wt%.

24. The process according to claim 22, wherein the condensation-rectification column is operated at atmospheric pressure and the temperature profile at the bottom of the column is maintained in the range of 115°C to 125°C and at the top of the column is maintained at about 100°C to 102°C.

25. The process according to claim 1 or 2, wherein the heat of condensation of water recovered by condensing the vapors of caprolactam and water from the cyclization reactor of step (ii) is reused in a plant.

26. The method of claim 21, wherein the heat required for concentration is supplied by the vapor in step (ii).

27. The method according to claim 1 or 2, wherein the 6-aminocaproic acid is obtained from a conventional petrochemical process or from a biochemical process, wherein the 6-aminocaproic acid contains a relatively high amount of carbohydrates.

28. The method according to claim 27, wherein the carbohydrate concentration in the 6-aminocaproic acid is up to 15% by weight, based on the mass of 6-aminocaproic acid.

29. The method of claim 27, wherein the carbohydrate concentration in the 6-aminocaproic acid comprises at least 1 wt%, and up to 15 wt% carbohydrates based on the mass of the 6-aminocaproic acid.

30. The method of claim 29, wherein the carbohydrate concentration in the 6-aminocaproic acid comprises at least 2 wt% carbohydrates based on the mass of the 6-aminocaproic acid.

31. The method of claim 29, wherein the carbohydrate concentration in the 6-aminocaproic acid comprises at least 3 wt% carbohydrates based on the mass of the 6-aminocaproic acid.

32. The method of claim 29, wherein the carbohydrate concentration in the 6-aminocaproic acid comprises at least 3.4 wt% carbohydrates based on the mass of the 6-aminocaproic acid.

33. The method of claim 29, wherein the carbohydrate concentration in the 6-aminocaproic acid comprises at least 3.5 wt% carbohydrates based on the mass of the 6-aminocaproic acid.

34. The method of claim 29, wherein the carbohydrate concentration in the 6-aminocaproic acid comprises at least 4 wt% carbohydrates based on the mass of the 6-aminocaproic acid.

35. The method of claim 29, wherein the carbohydrate concentration in the 6-aminocaproic acid comprises at least 5 wt% carbohydrates based on the mass of the 6-aminocaproic acid.

36. The method of claim 29, wherein the carbohydrate concentration in the 6-aminocaproic acid comprises at least 10 wt% carbohydrates based on the mass of the 6-aminocaproic acid.

37. The method of claim 29, wherein the carbohydrate concentration in the 6-aminocaproic acid comprises at least 12.5 wt% carbohydrates based on the mass of the 6-aminocaproic acid.

38. The method of claim 29, wherein the carbohydrate concentration in the 6-aminocaproic acid comprises up to 12.5 wt% carbohydrates based on the mass of the 6-aminocaproic acid.

39. The method of claim 29, wherein the carbohydrate concentration in the 6-aminocaproic acid comprises up to 10 wt% carbohydrates based on the mass of the 6-aminocaproic acid.

40. The method of claim 29, wherein the carbohydrate concentration in the 6-aminocaproic acid comprises up to 5 wt% carbohydrates based on the mass of the 6-aminocaproic acid.

41. The method of claim 29, wherein the carbohydrate concentration in the 6-aminocaproic acid comprises up to 4 wt% carbohydrates based on the mass of the 6-aminocaproic acid.

42. The method of claim 1 or 2, wherein the method is performed batchwise or continuously.

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