Start-up and wrong-way behavior in a tubular reactor: dilution effect of the catalytic bed

Abstract

The start-up and the wrong-way behavior of a fixed-bed reactor were analyzed through one-dimensional heterogeneous and pseudo-homogeneous models. The simulation work was based on the methanol oxidation to formaldehyde, which takes place in a fixed-bed reactor with two distinct zones. In the first part of the reactor, the catalyst was diluted with inert, and in the second zone the catalyst is pure. This activity profile leads to new features on the start-up and wrong-way behavior of the system when compared with a uniform catalytic bed. For a partially diluted bed, when the inlet temperature is increased (decreased), the final steady state can show a hot spot lower (higher) than the initial one. This behavior is not observed in a one-zone bed, where the final steady-state maximum temperature is always higher (lower) than the initial one if the inlet temperature is submitted to a positive (negative) change. During the dynamic period, the transient profiles are closer to the initial steady states in the case of the two-zone bed, pointing out that the catalyst dilution in the upstream section of the reactor can decrease the system sensitivity in both steady state and dynamic period. The differences between the predictions obtained through the pseudo-homogeneous and the heterogeneous models can be more significant on the transient responses than on the steady state situations and the wall temperature is the most important parameter on the reactor dynamic response. Moreover, significant wrong-way behavior can occur for step changes and ramp variations in feed and wall temperatures.