Dynamic hysteresis in monolith reactors and hysteresis effects during co-oxidation of CO and C2H6C2H6

• Formulas are presented for width of the dynamic hysteresis loop.
• Impact of inter and intra phase heat transfer on dynamic hysteresis is analyzed.
• Impact of ramp rate on steady-state plus dynamic hysteresis is illustrated.
• Simultaneous and separate ignition of CO and hydrocarbon mixture is illustrated.

In many laboratory catalytic reactors light-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 dynamic hysteresis. This dynamic hysteresis, which may occur even under ideal conditions (e.g. plug flow and negligible heat effects) is fundamentally different from the steady-state hysteresis which is due to heat effects and thermal feedback. We present 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 the limiting cases of pseudo-homogeneous and two phase plug flow models with negligible heat effects (ΔTad=0ΔTad=0). We review the conditions under which steady-state hysteresis can exist for a finite ΔTadΔTad and demonstrate the combined impact of steady-state and dynamic hysteresis effects. We study the light-off behavior and dynamic hysteresis during the co-oxidation of CO+C2H6CO+C2H6 mixture on Pt/Al2O3Pt/Al2O3. It is found that 1% of CO alone ignites at 473 K whereas 500 ppm of C2H6C2H6 alone ignites at 723 K. The exotherm generated by the CO oxidation can reduce the ignition temperature of C2H6C2H6 during the co-oxidation of CO and C2H6C2H6. Bifurcation diagrams of solid exit temperature versus fluid inlet temperature for different inlet compositions illustrate the simultaneous as well as separate ignitions for CO and C2H6C2H6. Each 1% increase in the CO feed mole percentage decreases C2H6 ignition temperature by about 80 °C. A feed of 3% CO is required for simultaneous ignition of a mixture of CO and 500 ppm of C2H6C2H6.

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