Help with this flue gas reaction rate

[Engr Trav]

I been running flue gas cleanup simulations and I'm hitting a wall with the reaction rate consistency, specifically with SO₂ oxidation in the presence of varying O₂ concentrations, its a fixed bed catalyst setup, but once temps drop below 752 F, the conversion rate drops off faster than expected, even under steady flow, trust me, I've triple checked the feed composition and residence time, and while the baseline kinetics hold up at higher temps, the reaction rate sensitivity seems exaggerated once things cool down, more when the gas mix fluctuates slightly (which it always does) by the way
Tired of typing, that's the major problem, need help here please

 

[Iam2much]

So below 752°F, SO₂ oxidation becomes highly temp-sensitive, minor O₂ or temp shifts throw things off fast, so you may need to tweak catalyst loading or preheat strategy

 

[gusfring0609]

It sounds like your SO₂ oxidation is hitting a kinetic threshold, and @Iam2much is right.. Even minor gas mix fluctuations can throw things off due to the non-linear kinetics at lower temps. Also, check for any mass transfer limitations that could be more pronounced at lower temps.

 

[ch3m3ist_]

I been running flue gas cleanup simulations and I'm hitting a wall with the reaction rate consistency, specifically with SO₂ oxidation in the presence of varying O₂ concentrations, its a fixed bed catalyst setup, but once temps drop below 752 F, the conversion rate drops off faster than expected, even under steady flow, trust me, I've triple checked the feed composition and residence time, and while the baseline kinetics hold up at higher temps, the reaction rate sensitivity seems exaggerated once things cool down, more when the gas mix fluctuates slightly (which it always does) by the way
Tired of typing, that's the major problem, need help here please

Same idea as @gusfring0609. Have you looked into mass transfer limitations becoming dominant as temperatures decrease? At lower temperatures, the reaction rate itself slows, but the rate of diffusion of reactants to and products from the catalyst surface can become the bottleneck. Even slight fluctuations in O2 could then have a disproportionate effect if the system is already mass-transfer limited. Consider modeling the Thiele modulus or effectiveness factor for your catalyst pellets across your operating temperature range, especially around that 752F mark, and see if the effectiveness factor drops significantly. This might reveal whether diffusion is starting to play a more prominent role than you initially anticipated based on higher-temperature kinetics.