Catalyst dilution to improve dynamic controllability of cooled tubular reactors

The dynamics of cooled tubular reactors are strongly affected by the overall heat-transfer coefficient U, which depends primarily on the velocity through the tubes. Higher velocities produce larger U's but increase pressure drops. Design involves trade-offs among tube diameter, tube length and number of tubes to achieve a specified pressure drop and per-pass conversion.This paper points out that another design optimization variable is available to improve the dynamic stability of cooled tubular reactors. If the catalyst activity is high, the size of the required reactor is small, which implies small heat-transfer area, and the reactor can be uncontrollable. If the catalyst is diluted with an inert solid, a bigger reactor will be required that will have more heat-transfer area and provide improved dynamic stability. This catalyst dilution strategy is explored for two reaction systems: a hypothetical chemical process and the carbonylation of dimethyl ether to produce methyl acetate.

► Catalyst dilution requires a larger reactor for the same conversion.
► Larger reactors provide more heat-transfer area for same tube diameter.
► Larger area results in smaller differential temperature driving forces, which raises the required coolant temperature for a given reactor temperature.
► For a given minimum coolant inlet temperature, a higher required coolant temperature means a larger change in the differential temperature driving force can be made, which means larger changes in heat removal can be achieved.
► Catalyst dilution improves dynamic controllability (ability to handled larger disturbances).