Process for the purification of crude caprolactam

Abstract: Disclosed is a process for recovering and purifying crude caprolactam from crude caprolactam-containing wastewaters consisting essentially of the steps: (a) obtaining a wastewater stream containing crude caprolactam from at least one process which by-produces a crude caprolactam-containing wastewater stream selected from caprolactam polymerization, catalytic cracking of polymers and oligomers of caprolactam, and thermoplastic processing of polycaprolactam; (b) mixing the crude caprolactam-containing wastewater stream with between 0.01 to 5% by weight, based on the crude caprolactam in the wastewater stream, of an acid; (c) subjecting the mixture of crude caprolactam-containing wastewater stream and acid obtained according to step (b) to purification distillation at a temperature of between 100 to 180°C and a pressure of between 50 to 3000 Pa; and (d) obtaining purified crude caprolactam as a distillate from step (c).

Description: The present invention relates to a process for the purification of crude caprolactam, more specifically it relates to the purification of crude caprolactam by distillation in the presence of an acid. There are a multiplicity of sources for crude caprolactam like wastewater from the manufacture and thermoplastic processing of polycaprolactam (nylon 6) or catalytic cracking of oligomers and polymers of caprolactam. Extractions of wastewater from the manufacture of polycaprolactam contain oligomers, polymers and monomers of epsilon-caprolactam, hereinafter referred to as caprolactam. There are several methods known for the recovery of caprolactam from these extractions, which contains oligomers and polymers of caprolactam. DE-A 950,726 and DE-A 1,272,287 disclose a depolymerization of polycaprolactam with phosphoric acid. The crude caprolactam is distilled by a steam distillation. The distilled caprolactam does not meet all quality requirements and the phosphoric acid containing residues are difficult to dispose. DD-A 5310 discloses a depolymerization of polycaprolactam with sodium hydroxide. U.S. Pat. No. 5,169,870 discloses a process for the recovery of epsilon-caprolactam from nylon 6 carpet by catalytical depolymerization of the polycaprolactam, followed by vacuum distillation. Object of the present invention was to provide a process for the purification of crude caprolactam which produces caprolactam in high purity and yield. The object of the present invention could be achieved by a process for recovering and purifying crude caprolactam from crude caprolactam-containing wastewaters consisting essentially of the steps: (a) obtaining a wastewater stream containing crude caprolactam from at least one process which by-produces a crude caprolactam-containing wastewater stream selected from caprolactam polymerization, catalytic cracking of polymers and oligomers of caprolactam, and thermoplastic processing of polycaprolactam; (b) mixing the crude caprolactam-containing wastewater stream with between 0.01 to 5% by weight, based on the crude caprolactam in the wastewater stream, of an acid; (c) subjecting the mixture of crude caprolactam-containing wastewater stream and acid obtained according to step (b) to purification distillation at a temperature of between 100 to 180°C and a pressure of between 50 to 3000 Pa; and (d) obtaining purified crude caprolactam as a distillate from step (c). One source for crude caprolactam is the wastewater from the polymerization of caprolactam. During the manufacture of polycaprolactam wastewater is produced which contains from 1 to 20%. by weight of solids, preferably 2 to 10, most preferred 3 to 8% by weight. The solids contain from 1 to 70% by weight of caprolactam, from 0.1 to 10% by weight of oligomers of caprolactam and from 1 to 99% by weight of polycaprolactam, based on the total amount of solids. This wastewater may be concentrated by distilling off the water to a solid concentration of from 50 to 90% by weight, preferably 60-85% by weight. The oligomers and polymers of the caprolactam preferably are catalytically cracked by introducing the wastewater to a fluidized bed of aluminum oxide at a temperature of from about 270°C to about 400°C, preferably from about 290 to 360°C. The introduction into the fluidized bed reactor may occur through a nozzle with or without an inert gas. Suitable aluminum oxides are alpha- or gamma-aluminum oxide, preferred is gamma-aluminum oxide as catalyst. The catalyst usually is held in the form of a fluidized bed by an inert gas like carbon dioxide, argon, nitrogen, preferred is nitrogen. Generally the inert gas is introduced into the fluidized bed reactor at a temperature of from 290°C to 400°C. The gas mixture which exits the fluidized bed reactor preferably is condensed to an aqueous solution of crude caprolactam. The water of the aqueous solution of crude caprolactam preferably is evaporated by distillation or thin film evaporation. Suitable thin film evaporators for example are horizontal units with a tampered rotor, using steam jets to provide vacuum. They usually consist of a dehydrator to remove water, a first distillation unit, and a second distillation unit and they are commercially available from for example Artisan Industries. After the water has been removed, the remaining crude caprolactam contains in general from 85 to 99% by weight, preferably from 90 to 99% by weight, most preferred from 95 to 99% by weight of caprolactam, based on the total amount of crude caprolactam. Another source for crude caprolactam is the wastewater of thermoplastic processing of polycaprolactam, which usually contains after distilling off the water from 85 to 99% by weight preferably from 90 to 99% by weight of caprolactam. Another source for crude caprolactam is the catalytic cracking of polycaprolactam from sources like nylon 6 carpet, waste strands or other nylon 6 parts in accordance with the process described above or the process disclosed in U.S. Pat. No. 5,169,870. The crude caprolactam from the different sources described above is distilled according to the present invention in the presence of an inorganic acid like sulfuric acid or oleum or an organic acid like p-toluolsulfonic acid or an ion exchange resin like a crosslinked polystyrene with sulfonic acid groups. Preferred is sulfuric acid. A suitable amount of acid ranges from 0.01 to 5% by weight, preferably from 0.1 to 1% by weight based on the crude caprolactam. The amount of ion exchange resin usually is from 1 to 30 by weight, preferably from 2 to 10% by weight, based on the crude caprolactam. The distillation is usually performed under a pressure of from 50 Pa to 3000 Pa, preferably from 100 to 2000 Pa. The temperature is from 100 to 180°C, preferably from 120 to 150°C. The distillate consists of usually from 95 to 99,9% by weight caprolactam, preferably from 96 to 99.9% by weight. The remaining sump after the distillation (destillation bottoms) may be neutralized with for example ammonia, NaOH or CaO and incinerated thereafter. Example 1: (Comparative): 500 g of crude caprolactam, obtained by depolymerization of polycaprolactam, which has been recovered from chip wash water, with a solid content of 95% by weight, which consists of 95% by weight of caprolactam and 5% by weight of water. The crude caprolactam is heated to between 165 - 180°C and then distilled at 666 - 1066 Pa. 450 g of a colorless distillate is obtained. The distillate was 99% by weight caprolactam with the properties shown in Table 1. Example 2: 500 g of crude caprolactam, obtained by depolymerization of polycaprolactam, which has been recovered from chip wash water, with a solid content of 95% by weight, which consists of 95% by weight of caprolactam and 5% by weight of water, is mixed with 1% by weight of 95-98% sulfuric acid. The mixture is heated to between 165 - 180°C and then distilled at 666 - 1066 Pa. 450 g of a colorless distillate is obtained. The distillate was 99% by weight caprolactam with the properties shown in Table 1. Example 3: 500 g of crude caprolactam, obtained by depolymerization of polycaprolactam, which has been recovered from chip wash water, with a solid content of 95% by weight, which consists of 95% by weight of caprolactam and 5% by weight of water, is mixed with 0.1% by weight of 95-98% sulfuric acid. The mixture is heated to between 165 - 180°C and then distilled at 5-8 mm Hg. (666 - 1066 Pa). 450 g of a colorless distillate are obtained. The distillate was 99% by weight caprolactam with the properties shown in Table 1. Examples Permanganate Number 1) Fluorescence Number 2) Crude Caprolactam 30.5 26827 1 (Comparative) 4.5 307 2 4.6 226 3 2.1 247 It is seen from the data in Table I that the quality is improved due to the purification using acid. 1) Determination of permanganate Absorbance Number in caprolactam The permanganate reduced by caprolactam is determined photometrically. Equal amounts of a 0.01 N potassium permanganate solution are added to a 3% (m/m) aqueous caprolactam solution and to a distilled, oxygen free, pH 6.2-6.5, water blank. After 10 minutes, the absorbance of the solutions is measured at λ = 420 nm. The permanganate absorbance number is calculated from: PAN = (A 1 - A 0 ) * 100/3 A 1 = Absorbance number of the permanganated caprolactam solution against water. A 0 = Absorbance of the permanganated water blank solution against water 2) Determination of Fluorescence Number of Caprolactam The fluorescence number is determined by scanning the prepared caprolactam sample on a Simadzu RF-5000 spectrometer. The wavelength from 400 nm to 650 nm is used. The sample is prepared by dissolving 5 grams of caprolactam in 43 grams of deionized water. A 1 cm quartz cell is filled with the sample solution. The cell is placed in the instrument and held for 1 minutes for stabilization. The sample is scanned from λ = 374 nm to λ = 600 nm. The peak centered at 430 nm is the quantitative measure of the sample fluorescence. The area between 400 nm and 490 nm is integrated. This number is reported as the fluorescence number.

Claims: A process for recovering and purifying crude caprolactam from crude caprolactam-containing wastewaters consisting essentially of the steps: (a) obtaining a wastewater stream containing crude caprolactam from at least one process which by-produces a crude caprolactam-containing wastewater stream selected from caprolactam polymerization, catalytic cracking of polymers and oligomers of caprolactam, and thermoplastic processing of polycaprolactam; (b) mixing the crude caprolactam-containing wastewater stream with between 0.01 to 5% by weight, based on the crude caprolactam in the wastewater stream, of an acid; (c) subjecting the mixture of crude caprolactam-containing wastewater stream and acid obtained according to step (b) to purification distillation at a temperature of between 100 to 180°C and a pressure of between 50 to 3000 Pa; and (d) obtaining purified crude caprolactam as a distillate from step (c). The process as in claim 1, wherein prior to step (b), there is practiced the step of (a 1 ) concentrating the crude caprolactam in the wastewater stream to between 85 to 99% by weight. The process according to claim 2, wherein said step (a 1 ) includes evaporating water from said crude caprolactam-containing wastewater stream. The process according to claims 1 to 3, wherein the acid is at least one inorganic acid selected from the group consisting of sulfuric acid and oleum. The process according to claims 1 to 3, wherein the acid is at least one organic acid selected from the group consisting of p-toluolsulfonic acid and an ion exchange resin having sulfonic acid groups. The process according to claims 1 to 5, further comprising neutralizing distillation bottoms obtained according to step (c), and incinerating said neutralized distillation bottoms.

Inventors: FUCHS HUGO DR, JOHNSON DONALD R, LOSIER THOMAS P, NEUBAUER GERALD DR, RITZ JOSEF DR, FUCHS, HUGO. DR., JOHNSON, DONALD R., LOSIER, THOMAS P., NEUBAUER, GERALD, DR., RITZ, JOSEF, DR.
Applicants: BASF CORP, BASF CORPORATION
Application year: 1995
Main classification: C07D201
IPC codes: C07D201