PROCESS FOR PURIFYING CAPROLACTAM

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: PROCESS FOR PURIFYING CAPROLACTAM The invention relates to a 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. Such a process is known from NL-A-77110150 , which describes the so-called benzene extraction of caprolactam from water to benzene and the back extraction from benzene to water. The drawback of this process, however, is that if the columns are more heavily loaded, for example if the caprolactam production is increased, this will adversely affect the extraction yield of the extraction columns employed. The aim of the invention is now to provide a process in which the extraction steps can be more heavily loaded without loss of extraction yield. This aim is achieved because at least a separate pre-extraction takes 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. This makes it possible to increase the load of any extraction step by 20% with only one extra theoretical tray; it moreover even leads to an improved extraction yield. Purifying caprolactam by means of benzene extraction and back extraction often implies a bottleneck in the production of caprolactam, because the extraction columns are unable to process the increasing caprolactam productions. This problem can now be simply solved even without having to install extra equipment,, for example columns, by causing a pre- extraction to take place. New extraction columns are moreover expensive and it is often difficult to incorporate them in an existing installation. This pre-extraction can take place in for example 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 energy is supplied to them, for example by means of a stirrer. This results in the formation of droplets of one of the liquids in the other, a dispersion. The dispersion then remains in the settling part for a sufficient length of time for the droplets to coalesce, preferably with a laminar flow. Suitable mixer/settlers are the box-type mixer/settlers, the IMI, the "General Mills" or the Kemira mixer/settlers, described in "Liquid-Liquid Extraction Equipment" by Godfrey J.C. and Slater M.J., Ed. Wiley, COP (1994), Ch.I, pp. 294-297. It is also possible to place an extension on a column, which extension may optionally also be widened. Caprolactam-containing mixtures can for example be extracted with the aid of extraction columns. Columns fitted with rotating internally installed elements, known as 'rotating disc columns' (RDCs) , and elements for pulsing the liquid column are suitable for use as extraction columns. Pulsing columns with packing bodies, pulsing columns with sieve trays, asymmetrical rotating disc contactors (ARD) , Scheibel columns and Kϋhni columns are also suitable . In general it will suffice to carry out the process in a column comprising 3-25 theoretical trays, depending on the desired degree of purification. It is of course possible to increase the number of trays if so desired. The extraction process is preferably carried out in countercurrent mode, because the extraction will then proceed in the most efficient, and hence an economic, manner . Caprolactam can for example be prepared via a Beckmann rearrangement of cyclohexanone oxime in the presence of sulphuric acid. The Beckmann rearrangement may take place in both the gas phase and the liquid phase. After the rearrangement in the liquid phase the rearrangement mixture is usually neutralised with the aid of for example ammonia. This results in phase separation. One of the phases contains virtually all the caprolactam and the other phase contains virtually all the ammonium sulphate formed. In one embodiment of the invention the caprolactam-containing mixture is, in the benzene extraction, first introduced into for example the mixer/settler as a pre-extraction step, to which the organic solvent obtained from an extraction step, for example a column, is added. Next, the two phases are intensively mixed for some time. With respect to the mixing time it is important that physical equilibrium is almost achieved. The mixing time will usually be between 5 minutes and half an hour. Then the mixture is introduced into the settling part, where the dispersion obtained can settle, which results in a separate water phase, which is subsequently separated, and a separate organic phase. Next, the separated water phase is fed to the top side of the benzene extraction column. 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 at one third of the extraction column's height. 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 entire amount, of the extraction media can optionally be fed into the top (or bottom) third part of the extraction column and/or to the side of the mixer/settler . It is possible to carry out several benzene extractions and/or back extractions. The pre-extraction can be carried out before the benzene extraction or before the back extraction or before both extraction steps. The pre-extraction preferably takes place on the concentrated side(s) of a benzene extraction and/or back extraction. The pre-extraction preferably takes place in a mixer/settler. It is of course also possible to use several mixer/settlers. The temperature at which the pre-extraction step before the first step takes place lies between 20 and 80°C. The temperature of the pre-extraction step before a second extraction lies between 10 and 60°C. In the benzene extraction step also other extraction media can be used. Suitable extraction media for the benzene extraction are benzene, toluene, xylene, chloroform, chlorinated hydrocarbons or higher alcohols, i.e. mono- or polyhydric alcohols with 5-12 C atoms. These extraction media may also contain caprolactam. In the case of the back extraction the extraction medium is usually water or a solution of caprolactam in water. The extraction media are preferably recycled and reused. Optionally this reuse of the extraction media can occur after purification of the used media. An embodiment of the process according to the invention is schematically illustrated in Figure 1, in which A and B represent extraction columns. A caprolactam/water mixture is fed to a mixer/settler 2 via pipe 1. In addition, an organic solvent, for example benzene or benzenic lactam, is fed to the mixer/settler 2, via pipe 4. The pre-extracted aqueous caprolactam-containing feed is fed to column A via pipe 3. Organic solvent is fed into column A in countercurrent mode via pipe 6 to extract the caprolactam from the aqueous solution. An aqueous solution from which the greater part of the caprolactam has been released is discharged via pipe 5. The organic caprolactam solution is returned to mixer/settler 2 via pipe 4, after which it is fed to mixer/settler 8 via pipe 7. Via pipe 9 the organic caprolactam solution is fed to extraction column B, in which back extraction of caprolactam takes place with the aid of water supplied via pipe 11. Solvent from which almost all the caprolactam has been removed is discharged via pipe 10, to be reused as an extraction medium for the caprolactam/water mixture. The solution of caprolactam in water obtained from column B is fed to mixer/settler 8 via pipe 12, and conveyed on from there via pipe 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 wt . % caprolactam was extracted with the aid of benzene in a so-called 'rotating disc column' (RDC) with a length of 4.5 m and a diameter of 7.5 cm; see Figure 2. The discs rotated at a speed of 400 rotations per minute (rpm) . The aqueous caprolactam was fed to the top of the column (7') via feed (1') at a flow rate of 30 1/h. Benzene was fed to the bottom via stream (2') at a flow rate of 84 1/h. The extraction yield was optimized by varying the amount of rotating energy supplied. The outgoing water phase (3 1 ) contained 0.4 wt . % caprolactam, the outgoing benzene phase (4') 22 wt . % caprolactam. The extraction yield is based on the amount of caprolactam in (3'), relative to the amount of caprolactam in the feed (1'), is 99.5% Example I Comparative Experiment A was repeated, only now a 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 residence time in the mixer was 5 minutes. The settler (9") was a rectangular container measuring 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 caprolactam mixture and the benzene were increased by 25%. The flow rate of feed (1") was 37.5 1/h, and that of stream (2") 105 1/h. The outgoing water phase (3") contained only 0.2 wt . % caprolactam, while the outgoing benzene phase (4") still contained 22 wt . % caprolactam. The extraction yield was 99.8%, while the capacity of the extraction- steps combination had moreover been increased by 25%. Comparative Experiment B An aqueous caprolactam mixture containing 71% caprolactam was extracted with the aid of toluene in a 'pulsed packed column' (ppc) 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 1 ) via feed (l 1 ) at a flow rate of 7.6 kg/h; see Figure 2. Toluene was fed to the bottom via stream (2') at a flow rate of 28 kg/h. The pulsation speed in the column varied from 0.005 to 0.02 m/s. The outgoing water phase (3') contained 8 wt . % caprolactam, the outgoing toluene phase (4 ' ) 17 wt . % caprolactam. The extraction yield was based on the amount of caprolactam in (3 1 ) relative 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 1 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 flow rates of feed (1") and stream (2") were increased by 20%. The flow rate of feed (1") was 9.1 kg/h and that of stream (2") 34 kg/h. The outgoing water phase (3") contained only 5 wt . % caprolactam, while the outgoing toluene phase (4") still contained 17 wt . % caprolactam. The extraction yield was 96%, while the capacity of the extraction steps had moreover been increased by 20%. Comparative Experiment C An aqueous caprolactam mixture containing 71 wt . % caprolactam was extracted with the aid 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') via feed (1') at a flow rate of 123 kg/h; see Figure 2. Benzene was fed to the bottom via stream (2') at a flow rate of 243 kg/h. The pulsation rate in the column varied between 0.006 and 0.0125 m/s. The outgoing water phase (3') contained approx. 1.5 wt . % caprolactam, the outgoing benzene phase (4 ' ) approximately 20 wt . % caprolactam. The extraction yield was based on the amount of caprolactam in (3 1 ) relative to the amount of caprolactam 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 flow rates of feed (1") and stream (2") were increased by 25%. The extraction yield was 99.8%. Comparative Experiment D A benzenic caprolactam mixture containing 20 wt . % caprolactam was subjected to back 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/h; see Figure 2. The benzenic caprolactam mixture (2') was fed to the bottom at a flow rate of 103 kg/h. The pulsation rate in de column was 0.02 m/s. The outgoing water phase (3') contained approx. 28 wt . % caprolactam, the outgoing benzene phase (4 ' ) approx. 0.05 wt . % caprolactam. The extraction yield was 99.0%. Example IV Comparative Experiment D was repeated, only now a mixer/settler combination as described in Example I was placed at the bottom of the column. Residence time in the mixer was 7 minutes. Residence time in the settler was 10 minutes. While the pulsation rate remained unchanged the flow rates of streams (1") and (2") were increased to 60 kg/h (1") and 124 kg/h (2") . The outgoing benzene phase contained only 0.02 wt . % caprolactam. The extraction yield was 99.9%.

Claims: C L A I M S 1. 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, characterised in that at least a separate pre-extraction takes 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. 2. Process according to Claim 1, characterised in that the pre-extraction takes place on the concentrated side(s) of a benzene extraction and/or back extraction. 3. Process according to Claim 1 or Claim 2, characterised in that the pre-extraction takes place in a mixer/settler. 4. Process according to any one of Claims 1-3, characterised in that the pre-extraction before the benzene extraction takes place at a temperature of between 20 and 80°C. 5. Process according to any one of Claims 1-3, characterised in that the temperature of the pre- extraction before the back extraction takes place at a temperature of between 10 and 60°C. Process as described in the introduction and elucidated with reference to the examples.

Inventors: GROOT ZEVERT LOUISE ANNEMARIE, SIMONS ANTONIUS JACOBUS FRANCI, GROOT ZEVERT, LOUISE, ANNEMARIE, SIMONS, ANTONIUS, JACOBUS, FRANCISCUS
Applicants: DSM NV, GROOT ZEVERT LOUISE ANNEMARIE, SIMONS ANTONIUS JACOBUS FRANCI, DSM N.V., GROOT ZEVERT, LOUISE, ANNEMARIE, SIMONS, ANTONIUS, JACOBUS, FRANCISCUS
Application year: 1998
Main classification: C07D201
IPC codes: C07D201