Comparison study for the oxidative dehydrogenation of isopentenes to isoprene in fixed and fluidized beds

The oxidative dehydrogenation (ODH) of hydrocarbons is an important route to obtain olefins and diolefins. Fluidized bed is good choice for the highly exothermic ODH of isopentenes to isoprene. A novel Zn-Fe layered double hydroxide catalyst was synthesized for fluidized beds. The catalytic tests were conducted in a micro fluidized-bed reactor with a tapered inner diameter from 4 to 9 mm and a fixed-bed reactor of the same size. In comparison with the fixed bed, the micro fluidized bed showed higher conversion and selectivity with a low ratio of steam to olefin over a wide range of conditions. This unusual results lie in the coupling of the intense back-mixing in fluidized beds with the Mars-van Krevelen ODH mechanism. Our simplified modelling reveals the strong nonlinearity of the kinetics coupled with strong back mixing can achieve better conversion and selectivity in the fluidized bed than the fixed bed at isothermal condition. Our results also suggest that isothermal fixed bed catalyst tests cannot guarantee the best conversion and selectivity for complex ODH reactions. Catalyst tests should be carried out both in fixed beds and micro fluidized beds with back mixing as early as in the stage of catalyst screening and development.