(621dy) Analysis of Transient and Steady-State Hysteresis Effects in Monolith Reactors
Rama Krishna Dadi, Dan Luss and Vemuri Balakotaiah
Department of Chemical and Biomolecular Engineering, University of Houston, TX-77204
Dynamic hysteresis is observed in many laboratory studies of catalytic reactors in which the temperature of the inlet feed gas is ramped up to a certain value (usually past the light-off value) and then ramped down. Lights off curves describing the exit conversion versus inlet gas temperature do not follow the same path during ramp up and ramp down and this leads to hysteresis. This dynamic hysteresis, which could occur even under ideal conditions (e.g. plug flow and negligible heat effects) is fundamentally different from the steady-state hysteresis due to heat effects and thermal feedback. It is important to understand the key differences between dynamic hysteresis and steady state hysteresis in order to interpret laboratory data, scale-up performance as well as explain the light-off behavior under transient conditions. This work presents a comprehensive analysis of the dynamic hysteresis and combined effects of dynamic and steady-state hysteresis in monolith reactors.
We show that dynamic hysteresis loop expands when thermal feedback decreases or Peh increases which is the opposite of the trend observed in steady state hysteresis. We present analytical expressions for the width of the dynamic hysteresis loop as a function of the ramp rate, solid to gas heat capacity ratio, space time and the heat Peclet number, for limiting case of pseudo-homogeneous model with negligible heat effects (DTad=0). Two-phase plug flow model is used to study the impact of interphase gradients on dynamic hysteresis behavior. Numerical simulations are presented to illustrate the combined effect of steady state multiplicity and dynamic hysteresis when heat effects are significant.