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

The start-up and the wrong-way behavior of a fixed-bed reactor were analyze d 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 pa rt of the reactor, the catalyst was diluted with inert, and in the second z one the catalyst is pure. This activity profile leads to new features on th e start-up and wrong-way behavior of the system when compared with a unifor m catalytic bed. For a partially diluted bed, when the inlet temperature is increased (decreased), the final steady state can show a hot spot lower (h igher) than the initial one. This behavior is not observed in a one-zone be d, 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 cl oser 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 stale and dynamic period. Th e differences between the predictions obtained through the pseudo-homogeneo us and the heterogeneous models can be more significant on the transient re sponses than on the steady state situations and the wall temperature is the most important parameter on the reactor dynamic response. Moreover, signif icant wrong-way behavior can occur for step changes and ramp variations in feed and wall temperatures. (C) 2000 Elsevier Science Ltd. All rights reser ved.