Purification caprolactam

After purification, caprolactam is polymerized by two different methods. 

The rotating-disk contactor (RDC), developed in the Netherlands (158) in 1951, uses the shearing action of a rapidly rotating disk to interdisperse the phases (Eig. 15b). These contactors have been used widely throughout the world, particularly in the petrochemical industry for furfural [98-01-1] and SO2 extraction, propane deasphalting, sulfolane [126-33-0] extraction for separation of aromatics, and caprolactam (qv) [105-60-2] purification. Columns up to 4.27 m in diameter are in service. An extensive study (159) has provided an excellent theoretical framework for scale-up. A design manual has also been compiled (160). Detailed descriptions and design criteria for the RDC may also be found (161). 

Caprolactam Extraction. A high degree of purification is necessary for fiber-grade caprolactam, the monomer for nylon-6 (see Polyamides). Cmde aqueous caprolactam is purified by solvent extractions using aromatic hydrocarbons such as toluene as the solvent (233). Many of the well-known types of column contactors have been used a detailed description of the process is available (234). 

The infrared spectmm of caprolactam has been given (3). Melting point data for the caprolactam—water system, as shown in Eigute 1, ate indicative of successful purification of caprolactam by crystallization from aqueous solution such purification is very effective for separating and rejecting polar impurities. 

Cyclohexanone oxime is converted quantitatively to caprolactam by Beckmann rearrangement in the presence of oleum, which is of sufficient strength to consume the several percent water in the molten oxime. The reaction mass is neutralized with aqueous ammonia to a cmde caprolactam layer and a saturated solution of ammonium sulfate. Approximately 1.5 kg of the total 4.4 kg ammonium sulfate per kilogram of caprolactam is produced in this step. Purification is by multistage vacuum crystallization from aqueous solution in neatly quantitative yield. 

The formation of oxime and rearrangement to caprolactam are conventional. The rearrangement produces 1.5 kg of the total 2.4 kg by-product ammonium sulfate per kilogram of caprolactam. Purification is accompHshed by vacuum distillation. A similar caprolactam process is offered by Inventa (11). 

The oxime is converted to caprolactam by Beckmann rearrangement neutralization with ammonia gives ca 1.8 kg ammonium sulfate per kilogram of caprolactam. Purification is by vacuum distillation. A no-sulfate, extraction process has been described, but incineration of the ammonium bisulfate recovers only sulfur values and it is not practiced commercially (14). 

Toray. The photonitrosation of cyclohexane or PNC process results in the direct conversion of cyclohexane to cyclohexanone oxime hydrochloride by reaction with nitrosyl chloride in the presence of uv light (15) (see Photochemical technology). Beckmann rearrangement of the cyclohexanone oxime hydrochloride in oleum results in the evolution of HCl, which is recycled to form NOCl by reaction with nitrosylsulfuric acid. The latter is produced by conventional absorption of NO from ammonia oxidation in oleum. Neutralization of the rearrangement mass with ammonia yields 1.7 kg ammonium sulfate per kilogram of caprolactam. Purification is by vacuum distillation. The novel chemistry is as follows ... 

A process for depolymerizing nylon-6 and polyester-nylon-6 mixed scrap was patented by Allied Chemical Corporation in 19656 and 1967.7 Ground scrap was dissolved with high-pressure steam at 125-130 psig (963-997 kPa) pressure and 175-180°C for 0.5 h in a batch process and then continuously hydrolyzed with superheated steam at 350°C and 100 psig (790 kPa) to form -caprolactam at an overall recovery efficiency of 98%. The recovered monomer could be repolymerized without additional purification. 

The cyclic oligomers are only slightly soluble in water and dilute solutions of caprolactam. They tend to separate out from die extracted waste during die process of concentration and chemical purification of die caprolactam. The cyclic oligomers tend to form on the walls of the equipment used in die process equipment. 6-Aminocaproic acid or sodium 6-aminocaproate may also be found in die oligomeric waste, especially if sodium hydroxide is used to initiate die caprolactam polymerization. 

The caprolactam obtained must meet die specifications of permanganate number, volatile bases, hazen color, UV transmittance, solidification point, and turbidity in order to be used for repolymerization alone or in combination witii virgin CL.5 Reported CL purification methods include recrystallization, solvent extraction, and fractional distillation. One solvent extraction technique involves membrane solvent extraction. Ion exchange resins have been shown to be effective in the purification of aqueous caprolactam solutions. In one such process,... 

AlliedSignal s Infinity, Forever Renewable Nylon, prepared by recycling of polyamide-6, is briefly described. The polyamide is treated by depolymerisation, purification of the caprolactam monomer and repolymerisation. The new resin is said to exhibit the same properties as those of virgin polyamide-6. ALLIEDSIGNAL... 

The rearrangement was done in similar ways by different caprolactam producers, and the differences can only be found in the purification processes. With the formation of ammonium sulfate being the most important problem for the producers of e-caprolactam, and due to the rising costs of its removal, many companies searched for new possibilities to produce caprolactam. There are some important industrial processes avoiding the cyclohexanone oxime as an intermediate product. 

Alternative purification protocols are available. Zhaolin Sun of Lanzhou University reports (Tetrahedron Lett. 45 2681,2004) that the ionic liquid TISC was specifically designed to promote Beckmann rearrangement. TISC is not soluble in water, so the product caprolactam was easily removed from the ionic liquid by extraction with water. 

N20 emission at chemical plants and the methods of its abatement have been considered in several reviews [185-187]. The major emission is related to the preparation of nitric add and its use in oxidation processes, like those involved in the production of adipic acid, caprolactam, glyoxal, acrylonitrile, and so forth. Of them, the biggest emission is the offgases of adipic acid about 1 M MT N 20 per year with a concentration of 30-40%. Recovery and purification of N20 from these offgases for use in the oxidation of benzene to phenol are described by Uriarte [188], Some companies use these off-gases to obtain medical-grade nitrous oxide. 

After purification, the lactam is polymerized by heating at elevated temperatures in an inert atmosphere. During self-condensation, the ring structure of the lactam is opened so that the monomer acts as an epsilon-aminocaproic acid radical. Unlike that of nylon 66, the polymerization of caprolactam is reversible the polymer remains in equilibrium with a small amount of monomer. As with nylon 66, nylon 6 is extruded in thin strands, quenched, and cut into chips for subsequent spinning, or the molten polymer is pumped directly to the spinning equipment. 

Water purification, including removal of phenol, halogenated compounds, pesticides, caprolactam, chlorine... 

A few polymers can be converted back to their monomers for purification and repolymerization. Polymers formed by ring-opening polymerization fall in this class, as shown in the foregoing by the conversion of nylon 6 back into caprolactam. When ethyl cyanoacrylate is used as a binder for metal and ceramic powders, it can be recovered for reuse by pyrolysis at 180°C.184 The monomer can be obtained by pyrolysis of polymethyl methacrylate in 92-100% yield,185 poly(a-methylstyrene) in 95-100% yield, and polytetraflu-oroethylene in 97-100% yield.186 Polystyrene can be de-polymerized to styrene containing some styrene dimer by heating with solid acids or bases at 350 400°C.187 It is pos sible that the dimer could be recycled to the next run to produce more monomer. The best yield (> 99% styrene) was obtained by passing polystyrene through a fluidized bed of a solid catalyst at 400 700°C, with a contact time of more than 60 s.188... 

To meet commercial specifications (permanganate number and volatile base content in particular), the crude caprolactam must undergo a rather complex purification. This operation includes the following treatment extraction by benzene and water, passage over activated charcoal and ion exchange resins, selective hydrogenation is the presence of caustic soda, etc... 

The rotating disc contactor (B), developed in the Netherlands in 1951, uses the shearing action of a rapidly rotating disc to interdisperse the phases. These have been used in the petrochemical industry for furfural and SOj extraction, propane deasphalting, sulfolane extraction for the separation of aromatics from aliphatics, and caprolactam purification. Columns up to 4.3 m in diameter are in service. 

The above-discussed acid retardation and base retardation in the immobiUzed Uquid phase could be compared with the so-called salting-out effects. However, this term is hardly applicable to the case of salt retardation, the first example of which was demonstrated by a successful removal of small amounts of NH4CI from a concentrated brine of (NH4)2S04. This practically important problem arises in the manufacture of caprolactam, where large amounts of sulfuric acid are converted into ammonium sulfate used for the preparation of the crystalline fertilizer. The new process of ISE on nanoporous NN-381 resin allowed an effective purification of very large volumes of concentrated sulfate brine, due to the fact that small ions of NH and Cl are efficiently squeezed into and retained in the finest pores of the sorbent [172]. We consider this salt retardation process as a convincing proof of our interpretation of the mechanisms of the new electrolyte separation process. 

Triaryl isocyanurates are useful as activators for the continuous anionic polymerization and postpolymerization of c-caprolactam to nylon-6 possessing a low unreacted monomer content and a highly stable melt viscosity [93]. Although a wide variety of catalysts for the trimerization of aryl isocyanates to triaryl isocyanurates are known, purity of trimerized product (a requirement for nylon of good quafity) is problematic and purification results in product... 

The cyclohexanone oxime obtained by any of these processes (1, 2,4, 6, and 7, in Figure 2.11) is converted to caprolactam by the Beckmann rearrangement in oleum. The resulting caprolactam sulfate is neutralized with ammonia and purified. The neutralization process yields about 2 + 0.2 kg of ammonium sulfate/kg of caprolactam. The lactam purification may entail extraction with organic solvents (toluene, benzene, chlorinated aliphatic hydrocarbons) followed by extraction of the organic solution with water and subsequent isolation of the lactam by either crystallization or distillation. Newer developments are concerned with the gas-phase catalytic rearrangement of cyclohexanone oxime using a boric acid on carbon catalyst in a fluidized-bed operation [122],... 

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