(350a) Runaway in MICRO-Channel Reactors

Exothermic reactions occurring in micro-channel reactors bring with them the possibility of runaway. Microreactors are different from conventional tubular reactors due to their larger wall/reactor volume ratio. Consequently, the effect of wall conduction on runaway cannot be neglected. Our aim is to study the influence of wall conduction and heat losses on critical parameters for runaway. Microreactor performance is determined by (i) convection-reaction phenomena within the fluid phase, (ii) heat dissipation along the axial length of the reactor via solid-phase conduction, and (iii) heat losses to packaging via solid-phase conduction. A 1-D model, capturing the three main phenomena, is used.

Conduction Parameter (CP) gives the relative importance of conductive heat transfer compared to the energy carried by the fluid [1]. In this study order of magnitude analysis is employed to reduce the model to two distinct tubular reactors for low and high CP. Influence of wall conduction and heat losses is studied by using parametric sensitivity analysis on the whole model[2, 3].

For intermediate values of CP, it is observed that both packing losses and heat conduction work together. Increasing CP, while keeping the ends insulated restricts the role of conduction because of which the critical Stanton number remains almost constant for practical values of CP. However, as the Biot number is increased the role of conduction is augmented. For low CP and high CP, the critical Stanton number approaches the critical value of the tubular reactor approximations derived from order of magnitude estimates. It is shown that a necessary condition to prevent runaway is that the reactant side Stanton number of the microreactor should always be greater than the critical Stanton number of the High CP approximation. Also, regions in the parameter space are identified which ensure safe operation of the reactor irrespective of the cooling rate.

References

1. Moreno, A., Murphy, K., & Wilhite, B. A. (2008). Parametric study of solid-phase axial heat conduction in thermally integrated microchannel networks. Industrial & Engineering Chemistry Research, 47(23), 9040-9054
2. Morbidelli, M. and A. Varma, A generalized criterion for parametric sensitivity: application to thermal explosion theory. Chemical Engineering Science, 1988. 43(1): p. 91-102.
3. Varma, A., M. Morbidelli, and H. Wu, Parametric sensitivity in chemical systems. 2005: Cambridge University Press.