Uncatalyzed selective oxidation of liquid cyclohexane with air in a microcapillary reactor

• Published in: Chemical engineering science 2010-08, Vol.65 (16), p.4866-4872
• Main Authors: Fischer, J., Lange, T., Boehling, R., Rehfinger, A., Klemm, E.
• Format: Article
• Language: English
• Subjects: Applied sciences; Chemical engineering; Cyclohexane; Exact sciences and technology; Heat and mass transfer. Packings, plates; Mass transfer; Microreactor; Microstructure; Oxidation; Reaction engineering; Reactors
• ISSN: 0009-2509; 1873-4405
• DOI: 10.1016/j.ces.2010.05.028

The uncatalyzed selective oxidation of cyclohexane with air was performed at high-p,T-conditions in a microcapillary reactor. Operation pressures were between 2.0 and 8.0 MPa and operating temperatures between 453 and 533 K. The measured space-time-yield showed only a slight dependence on the pressure, but a strong dependence on temperature. At 533 K, a space-time-yield of about 6.000 kg/(m 3 h) was reached, which corresponds to a size of 2 m×2 m×2 m (8 m 3) of the microstructured reactor assuming a capacity of 100.000 t/a compared to 500 m 3 total reactor volume realized with a cascade of bubble columns of each about 100 m 3. Unfortunately, selectivity drops at this temperature below 80% which is significantly lower than the selectivity in the conventional process. Passivating the capillary walls with silicon allowed an increase in selectivity. By fitting a simple mass transfer/reaction model for molecular oxygen, the mass transfer coefficient could be determined for T=453 K and a 0.75 mm capillary as k L =3.04×10 −3 m/s. With the help of the Hatta number, mass transfer limitations can be excluded for the microcapillary reactor, whereas the bubble column reactor is weakly limited by the gas/liquid mass transfer of the molecular oxygen. Thus, process intensification by enhancing mass transfer using a microstructured reactor for cyclohexane oxidation with air is quite low.