MXPA99008578A - Process for purifying caprolactam - Google Patents

Abstract

Process for purifying an aqueous caprolactam mixture by extracting the caprolactam with the aid of an organic caprolactam solvent that is not miscible with water in a benzene extraction, and releasing the caprolactam from the organic solvent by means of extraction with water, with the formation of an aqueous caprolactam solution, in a back extraction, with at least a separate pre-extraction taking place before the benzene extraction and/or the back extraction, in which, in a pre-extraction before the benzene extraction, an organic solvent that is not miscible with water is used for the extraction and, in a pre-extraction before back extraction, water is used for the extraction.

Description

PROCEDURE TO PURIFY THE CAPROLACTAMA The invention relates to a process for purifying an aqueous caprolactam mixture, extracting the caprolactam with the aid of an organic solvent for caprolactam that is not miscible with water in an extraction with benzene, and releasing the caprolactam from the organic solvent by extraction with water, forming an aqueous solution of caprolactam, in a reverse extraction. Said process is known from NL-A-77110150, which describes the so-called extraction of caprolactam with benzene from water to benzene, and the reverse extraction of benzene to water. However, the drawback of this method is that if the columns are loaded more strongly, for example, if the production of caprolactam is increased, this will adversely affect the extraction performance of the extraction columns used. It is an object of the invention now to provide a method in which the extraction steps can be loaded more strongly, without loss of extraction performance. This objective is achieved because at least one separate pre-extraction takes place, before extraction with benzene and / or reverse extraction, where, in a pre-extraction before extraction with benzene, it is used for the extraction. extraction an organic solvent that is not miscible with water; and it is used for water extraction, in a pre-extraction before the reverse extraction. This makes it possible to increase the load of any extraction step by 20% with only an extra theoretical tray; in addition, it even leads to an improved extraction performance. Purifying caprolactam by means of benzene extraction and inverse extraction frequently implies a bottleneck in the production of caprolactam, because the extraction columns are not capable of processing the increasing productions of caprolactam. This problem can now be solved in a simple way, even without having to install extra equipment, for example columns, causing a pre-extraction to take place. The new extraction columns are additionally expensive and it is often difficult to incorporate them into an existing installation. This pre-extraction can be carried out, for example, in a mixer / settler. A mixer / settler is an apparatus comprising a mixing part and a settling part. The liquids are combined in the mixing part and energized, for example, by means of a stirrer. This results in the formation of drops of one of the liquids in the other: a dispersion. Then the dispersion remains in the half-feeder portion for a sufficient length of time for the droplets to coalesce, preferably with a laminar flow. Suitable mixers / settlers are box type mixers / settlers, I MIs, "General Mills" or Kemira mixers / settlers, described in Liquid-Liquid Extraction Equipment by Godfrey JC and Slater MJ, Ed. Wiley, COP ( 1994), chapter I, pages 294-297. It is also possible to place an extension in a column, and said extension can also be extended, optionally. For example, mixtures contai caprolactam can be extracted with the help of extraction columns. The columns equipped with rotating elements, installed internally, known as "rotating disc columns (RDC, acronym for its English designation: Rotating Disc Columns) and elements to press the liquid column, are suitable for use as extraction columns. pulsers with packing bodies, pulsating columns with sieve trays, asymmetric rotary disc contactors (ARD, asymmetrical rotating disc), Scheibel columns and Kühni columns are also suitable. , it will suffice to carry out the procedure in a column comprising 3 to 25 theoretical trays, depending on the degree of purification desired.Of course it is possible to increase the number of trays, if so desired. The preferred extraction process is carried out in a countercurrent mode, since the extraction will then proceed in the most efficient and, therefore, most economical manner. Caprolactam can be prepared, for example, by rearrangement of Beckmann cyclohexanone oxime in the presence of sulfuric acid. The Beckmann rearrangement can be carried out both in the gas phase and in the liquid phase. After the rearrangement in liquid phase, the rearrangement mixture is usually neutralized with the aid, for example, of ammonia. This results in phase separation. One of the phases contains virtually all of the caprolactam and the other phase contains virtually all of the ammonium sulfate formed. In one embodiment of the invention, the mixture containing caprolactam, in the extraction with benzene, is first introduced, for example, into the mixer / settler, as a pre-extraction step, to which the organic solvent obtained from a step of extraction, for example, in a column. The two phases are then intensively mixed for a certain time. With regard to the mixing time, it is important that almost the physical balance is achieved. The mixing time will usually be between 5 minutes and a half hour. The mixture is then introduced into the settling portion, where the obtained dispersion can settle, which results in a separate aqueous phase, which is subsequently separated, and a separate organic phase. The separated aqueous phase is then fed to the upper side of the extraction column with benzene. It is also possible to feed a portion of the dispersion from the mixer / settler, to the top of the extraction column, and another portion to the column, at a point, for example one third of the height of the extraction column. The extraction medium is fed to the bottom of the extraction column.

It is not necessary to feed the (total) amount of the extraction media to the top (or bottom) of the column or the mixer / settler. A portion, or the total amount, of the extraction means may optionally be fed to the third upper (or lower) part of the extraction column and / or to the mixer / settler side. It is possible to carry out several extractions with benzene and / or several inverse extractions. Pre-extraction can be carried out before extraction with benzene or before inverse extraction, or before both extraction steps. Preferably, pre-extraction takes place on the concentrate side (s) of a benzene extraction and / or a reverse extraction. Pre-extraction is preferably carried out in a mixer / settler. Of course, it is also possible to use several mixers / settlers. The temperature at which the pre-extraction step takes place before the first step is between 20 and 80 ° C. The temperature of the pre-extraction step before the second extraction is between 10 and 60 ° C. Other extraction media can also be used in the extraction step with benzene. The extraction media suitable for extraction with benzene are: benzene, toluene, xylene, chloroform, chlorinated hydrocarbons or higher alcohols, that is, monohydric or polyhydric alcohols with 5 to 12 carbon atoms. These extraction media can also contain caprolactam.

In the case of reverse extraction, usually the extraction medium is water or a solution of caprolactam in water. Preferably the extraction means is recirculated and reused. Optionally this repeated use of the extraction means may occur after purifying the media used. In FIG. 1, one embodiment of the method according to the invention is illustrated schematically; and in it A and B represent extraction columns. A mixture of caprolactam / water is fed to a mixer / settler 2, by means of a tube 1. Additionally an organic solvent, for example benzene or benzene lactam, is fed to the mixer / settler 2, by means of tube 4. It is fed the feed containing aqueous caprolactam, previously extracted, to column A by means of tube 3. Organic solvent is fed into column A, in countercurrent mode, by means of tube 6 to extract the caprolactam from the aqueous solution. An aqueous solution from which most of the caprolactam has been discharged is discharged via tube 5. The organic solution of caprolactam is returned to the mixer / settler 2 by means of tube 4, after which it is fed to the mixer / settler 8 by means of tube 7. By means of tube 9 the organic caprolactam solution is fed to the column of extraction B, in which the reverse extraction of caprolactam takes place, with the help of the water supplied by means of the tube 11. The solvent from which almost all of the caprolactam has been removed is discharged by means of the tube 10, to be used again as extraction medium for the caprolactam / water mixture. The solution of caprolactam in water, obtained from column B, is fed to the mixer / settler 8, by means of tube 12 and transported therefrom, by means of tube 13, for further processing. The invention will be further demonstrated with reference to the following non-limiting examples.

COMPARATIVE EXPERIMENT A An aqueous caprolactam mixture containing 71.9% by weight of caprolactam was extracted, with the aid of benzene, in a so-called "rotating disk column (RDC), having a length of 4.5 m and a diameter of 7.5 cm, see Figure 2 The discs rotated at a speed of 400 revolutions per minute (rpm) The aqueous caprolactam was fed to the upper part of the column (7 ') by feeding (1 *) at a flow rate of 30 L / The benzene was fed to the lower part by means of the current (2 '), at a flow rate of 84 L / hour The extraction performance was brought to the optimum point by varying the amount of rotary energy supplied. (3 ') aqueous exit contained 0.4% by weight of caprolactam; the exit benzene phase (4 '), 22% by weight of caprolactam. The yield of the extraction is based on the amount of caprolactam in (3 ') with respect to the amount of caprolactam present in the feed (1'), which is 99.5%.

EXAMPLE I Comparative experiment A was repeated, only now the mixer / settler combination was placed before the column; see figure 3. The mixer (8") was a stirred vessel with a height of 15 cm and a diameter of 30 cm The dwell time in the mixer was 5 minutes The settler (9") was a rectangular vessel that measured 0.6 by 0.3 by 0.2 m. The residence time in the settler was 10 minutes. The rotation speed in the RDC was 700 rpm. The flow rates of the aqueous mixture of caprolactam and benzene were increased by 25%. The feed flow rate (1") was 37.5 L / hour and that of the current (2") was 105 L / hour. The aqueous outlet phase (3") contained only 0.2% by weight of caprolactam, while the exit benzene phase (4") still contained 22% by weight of caprolactam. The extraction yield was 99.8%, while the capacity of the combination of extraction steps was further increased by 25%.

COMPARATIVE EXPERIMENT B An aqueous mixture of caprolactam containing 71% caprolactam was extracted, with the help of toluene, in a "packed, pulsating column" (PPC, acronym for its designation in English Pulsed Packed Column), with a length of 5 m and a diameter of 5 cm The aqueous caprolactam mixture was fed to the top of the column (7 ') by means of the feed (1') at a flow rate of 7.6 kg / hour; see figure 2. Toluene was fed to the bottom by means of stream (2 ') at a flow rate of 28 kg / hour. The velocity of pulsation in the column varied from 0.005 to 0.02 m / s. The aqueous exit phase (3 ') contained 8% by weight of caprolactam; the toluene exit phase (4 '), 17% by weight of caprolactam. The extraction yield was based on the amount of caprolactam in (3 ') with respect to the amount of caprolactam in the feed (1') and was 93%.

EXAMPLE II Comparative experiment B was repeated, only now a mixer / settler combination as described in Example I was placed before the column; see figure 3. The residence time in the mixer was 6 minutes. The residence time in the settler was 10 minutes. The feed flow (1") and current (2") regimes increased by 20%. The feed flow rate (1") was 9.1 kg / hour and that of the current (2") was 34 kg / hour. The aqueous outlet phase (3") contained only 5% by weight of caprolactam, while the toluene exit phase (4") still contained 17% by weight of caprolactam. The yield of the extraction was 96%, while the capacity of the extraction steps had increased additionally by 20%.

COMPARATIVE EXPERIMENT C An aqueous mixture of caprolactam containing 71% by weight of caprolactam, with the help of benzene, in a PPC with a height of 6 m and a diameter of 0.23 m. The aqueous caprolactam mixture was fed to the top of the column (7 ') by means of the feed (1') at a flow rate of 123 kg / hour, see figure 2. Benzene was fed to the lower part by means of current (2 ') at a flow rate of 243 kg / hour. The pulsation velocity of the column varied between 0.006 and 0.0125 m / s. The aqueous exit phase (3 ') contained approximately 1.5% by weight of caprolactam; the leaving benzene phase (4 '), approximately 20% by weight of caproiactam. The yield of the extraction was based on the amount of caprolactam present in (3 ') with respect to the amount of caprolactam present in the feed (1') and was 99.4%.

EXAMPLE III Comparative experiment C was repeated, only now a mixer / settler combination, as described in example I, was placed before the column; see figure 3. The residence time in the mixer was 5 minutes. The residence time in the settler was 11 minutes. The feeding (1") and current (2") regimes increased by 25%. The yield of the extraction was 99.8%.

COMPARATIVE EXPERIMENT D A benzene mixture of caproiactam containing 20% by weight of caprolactam was subjected to reverse extraction, using water in a PPC with a height of 8 m and a diameter of 0.10 m. The water was fed to the top of the column (7 ') at a flow rate of 50 kg / hour; see figure 2. The benzene mixture of caprolactam (2 ') was fed to the lower part, at a flow rate of 103 kg / hour. The pulse rate in the column was 0.02 m / s. The aqueous exit phase (3 ') contained approximately 28% by weight of caprolactam; the leaving benzene phase (4 ') approximately 0.05% by weight of caprolactam. The yield of the extraction was 99.0%.

EXAMPLE IV Experiment D was repeated, only now a mixer / settler combination, as described in Example I, was placed in the lower part of the column. The residence time in the mixer was 7 minutes. The residence time in the settler was 10 minutes. While the pulsation rate remained unchanged, the flow regimes of currents (1") and (2") increased to 60 kg / hour (1") and 124 kg / hour (2"). The exit benzene phase contained only 0.02% by weight of caprolactam. The yield of the extraction was 99.9%.

Claims (6)

NOVELTY OF THE INVENTION CLAIMS

1. - Procedure for purifying an aqueous caprolactam mixture by extracting caprolactam with the aid of an organic solvent for caprolactam that is not miscible with water in an extraction with benzene, and releasing the caprolactam from the organic solvent by means of extraction with water, with formation of a aqueous solution of caprolactam in a reverse extraction; characterized in said process because at least one separate pre-extraction is carried out before extraction with benzene and / or from the reverse extraction, in which, in a pre-extraction before extraction with benzene, a solvent is used for the extraction organic that is not miscible with water and, in a pre-extraction before the reverse extraction, water is used for extraction.

2. Method according to claim 1, further characterized in that pre-extraction is carried out on the concentrated side (s) of a benzene extraction and / or a reverse extraction.

3. Method according to claim 1 or 2, further characterized in that the pre-extraction is carried out in a mixer / settler.

4. Method according to any of claims 1-3, further characterized in that pre-extraction is carried out prior to extraction with benzene, at a temperature between 20 and 80 ° C.

5. Method according to any of claims 1 to 3, further characterized in that the pre-extraction is carried out prior to the reverse extraction at a temperature between 10 and 60 ° C. 6.- Procedure as described in the introduction and clarified with reference to the examples.