1 Introduction

1.1 Background

Caprolactam, as an important organic chemical raw material and engineering plastic as well as the monomer of nylon-6 fibers, has been widely used in textile, automobile, electronics and other fields. Cyclohexanone oxime is the key intermediate for the production of caprolactam. More than 90% of caprolactam worldwide is manufactured by the rearrangement of cyclohexanone oxime.

SINOPEC has developed an innovative process for the production of cyclohexanone oxime by continuous ammoximation of cyclohexanone in single-reactor (CACS). CACS is the core of an integrated environment-friendly technology for the green production of caprolactam. This technology applies the original particles of SINOPEC’s proprietary hollow titanium silicate (HTS) molecular sieve as catalyst and microfiltration membranes for recycling the catalyst. Compared to the conventional process, cyclohexanone oxime can be produced by a one-step reaction with high selectivity and conversion in which cyclohexanone reacts with NH3 and H2O2 catalyzed by HTS. The main by-product is only water.

1.2 Process description

The main process of CACS includes 6 units: reactor, catalyst separation, solvent recovery, extraction, distillation and wastewater stripping. As shown in Figure 1, cyclohexanone, NH₃, H₂O₂ and solvent are continuously fed into the slurry reactor loaded with HTS zeolites. The mixture containing products and catalyst slurry are continuously withdrawn and introduced into the membrane filtration system for separation, where the concentrated catalyst slurry is recovered to the reactor while the reaction liquid to the solvent recovery unit. The solvent is recovered and the cyclohexanone oxime-water mixture goes into the extraction unit. The extractant is distilled in the extractant recovery tower while the cyclohexanone oxime further goes to the distillation tower and the refined product is obtained at the bottom. The wastewater after oxime extraction goes into the wastewater stripper, where the small amount of organics is recovered, and the bottoms are discharged into the wastewater treatment system.

1.3 Operating conditions

Reaction temperature: 80℃ - 85℃

Reaction pressure: 0.3 MPa (g) - 0.4 MPa (g)

n(H₂O₂) /n(cyclohexanone) ratio: 1.08 - 1.15

Figure 1 CACS Process

2 Technology characteristics

2.1 Process characteristics

An innovative process combines the slurry-bed reactor and the membrane separators. The catalyst in the size of submicron is separated and recycled continuously, leading to the stable operation of the whole process.

By using a unique technology to prevent the catalyst from deactivation, the lifetime of the catalyst is prolonged.

The consumption of the catalyst is reduced significantly based on the unique catalyst regeneration technology.

CACS process reacts in mild conditions, discharges less wastes and no NOx/SOx emission. It is a green and clean process without worries of equipment corrosion and environment pollution in comparison with conventional technologies.

2.2 Feed adaptability

This process can use cyclohexanone produced by various processes as the feed, including cyclohexane oxidation, phenol hydrogenation and cyclohexene hydration.

2.3 Performance

The total conversion of cyclohexanone ≥ 99.9% and the selectivity of cyclohexanone oxime ≥ 99.5%. The consumption of catalyst < 0.2 kg/t oxime. The main specifications of cyclohexanone oxime are shown in Table 1.

Table 1 Quality specifications of cyclohexanone oxime

2.4 Operation cycle

Operating cycle of cyclohexanone ammoximation unit is 2 - 3 years.

3 Supporting catalyst

The HTS molecular sieve is used as cyclohexanone ammoximation catalyst in the CACS process. The catalyst has high activity and selectivity. The main properties of the catalyst are given in Table 2.

Table 2 Properties of HTS molecular sieve as cyclohexanone ammoximation catalyst

4 Commercial Applications

The (CACS) technology has been used in 9 commercial plants with the largest capacity of 200 kt/a.