The Oxidation of Cyclohexane in a Capillary

1. O H O O 2 H N O Caprolactam + 3 + & Adipic acid >120 C >120 C ~15 bars KA - mixture ~8-15 bars KA oil adipic acid Motivation Results Mass transfer correlation for Taylor flow: Bercic and Pintar, 1997 Van Baten and Krishna, 2004 The Oxidation of Cyclohexane in a Capillary 0.001s-1 to 0.08 s-1 VR=50 ml D =2.1 mm • If smaller (than predicted by correlations from the literature) mass transfer coefficient is used, agreement between model and experimental results gets better. • Mass transfer might be the reason for the discrepancies between the model and the experimental results Nylon -6,6 Source: http://www.uni-regensburg.de • Traditional cyclohexane oxidation (“1st step”) process operates at: • 3-8% cyclohexane conversion • 85% selectivity to cyclohexanol and cyclohexanone • adiabatic condition Experimental set up:capillary reactor (D=2.1 mm), T= 1600C, P=15 atm, QL=0.1-1.0 ml/min The interest in cyclohexane oxidation has not diminished in years: • Concentrations of the products obtained experimentally are an order of magnitude lower that those obtained by PFR model (conversion at 20 min (model)=36% conversion (exp) = 4%). 2003-2006: 61 papers; 32 in Chinese and 21 in English Mass transfer correlation for Taylor flow in a capillary used Figure 2. Experimental and modeling results for cyclohexanol (ROH) and cyclohexanone (RO) concentrations obtained in the capillary reactor at 160ºC and 15 atm. Mass transfer coefficient used in the model was an order magnitude lower then the one predicted from the correlations available in the literature. * Source: SciFinder Kinetics from Kharakova et al, 1989 R. Jevtic, P.A. Ramachandran, M. P. Dudukovic • Two possible modifications to improve the current process: • Selectively oxidize cyclohexane directly to adipic acid in one step, or • Increase volumetric productivity in the first step without sacrificing selectivity toward cyclohexanol and cyclohexanone Figure 1.Comparison of experimental and modeling results for cyclohexanol (ROH) and cyclohexanone (RO) concentrations in cyclohexane oxidation in the capillary at 160ºC and 15 atm without the use of a catalyst. Summary • Taylor flow in a capillary: • 3 different mixers used. • Similar results observed-Taylor flow erratic and almost independent of the gas and the liquid flow rates used • Design, set up and the experimental study in the capillary reactor is completed. • There is discrepancy between model and experimental results, which is, most likely, due to poor mass transfer in the capillary • Better mixing of gas and liquid is needed. • A small improvement in the product yield can lead to significant impact on the process economics. 3 2 1 • References • Schaefer, R.; Merten, C.; Eigenberger, G., Autocatalytic Cyclohexane Oxidation in a Bubble Column. The Canadian Journal of Chemical Engineering 2003, 81, (741-748). • Bercic, G.; Pintar, A., The role of gas bubbles and liquid slug lengths on mass transport in the Taylor flow through capillaries. Che. Eng. Sci. 1997, 52, (21/22), 3709-3719. • Kreutzer, M. T.; Du, P.; Heiszwolf, J. J.; Kapteijn, F.; Moulijn, J. A., Mass transfer characteristics of three-phase monolith reactors. Chem. Eng. Sci. 2001, 56, (21-22), 6015-6023. • van Baten, J. M.; Krishna, R., CFD simulations of mass transfer from Taylor bubbles rising in circular capillaries. Chemical Engineering Science 2004, 59, (12), 2535-2545. Goals Improved understanding and quantification of Gas flow rate: 1.2 ml/min Liquid flow rate: 3.6 ml/min (nylon tubing, 1/8’’ OD) the effect of the reactor type the effect of oxygen availability on rates and selectivity in cyclohexane oxidation Chemical Reaction Engineering Laboratory