Chemicals & Materials Principle Animation:Fixed-Bed Catalytic Reactor Lab

Reaction rate rises exponentially with temperature (Arrhenius): at one or two hundred degrees almost nothing happens, and only above three hundred does it pick up—so the feed is preheated to the salt temperature first. Too hot a feed maxes out conversion but enters the runaway zone.

Near the inlet the feed is richest and releases heat fastest, faster than the salt can take it away, so a hot spot forms; further down there is less feed, heat release slows and the temperature falls back toward the salt. The hot spot is where catalyst is most likely to be damaged or coked.

Reactant molecules cross the gas film, enter the pores, adsorb on active sites and react with oxygen; products then desorb. When it is cooler, reaction is slow and molecules reach the pellet centre; at the hot spot reaction is so fast that feed is used up near the pore mouths, the inner catalyst sits idle and the effectiveness factor η drops.

A hotter hot spot means faster reaction and more heat—once this feedback can no longer be held, the reactor runs away. Raising the feed by only a few degrees can make the hot spot jump from tens to a couple of hundred kelvin. Diluting the inlet section with inert balls or cooling harder pushes the runaway line back.

Side reactions coke the catalyst fastest at the hot spot, so that section deactivates first; the hot spot drifts downstream and flattens and outlet conversion slowly drops. Regeneration feeds 1% oxygen: a burn front sweeps the bed, the coke is burned off and activity returns. The oxygen must be dilute, or the burn-off itself would damage the catalyst.