Hydrogenation of 1,5,9-cyclododecatriene in fixed-bed reactors: down- vs. upflow modes

Abstract

Performance of trickle-bed and up-flow reactors was studied experimentally and theoretically for an exothermic multistep hydrogenation of 1,5,9-cyclododecatriene (CDT) in n-decane as solvent over 0.5% Pd/alumina catalyst. Intrinsic kinetics was studied in a batch stirred slurry reactor using the powdered catalyst, and a Langmuir-Hinshelwood-type rate model is proposed. Using this rate equation, a trickle-bed-reactor model was developed that incorporates contributions of partial wetting and stagnant liquid holdup, in addition to the external and intraparticle mass transfer for the gas-phase reactant (hydrogen). It was also modified to describe the behavior of the upflow reactor. Experimental data were obtained in both upflow (trickle-bed) and down-flow modes at different liquid velocities, pressures, and inlet feed concentrations at 373-413 K. Reactor performance of the two modes was compared in terms of global hydrogenation rate, CDT conversion, selectivity to cyclododecene and the maximum temperature rise observed in the catalyst bed. The conversions and global hydrogenation rate were significantly higher in a trickle-bed reactor than in the upflow reactor. Similarly, a significant temperature rise in the catalyst bed was observed for the downflow operation compared to the upflow mode, which is explained from wetting characteristics of the catalyst bed. Model predictions for both reactors agreed well with experimental data.