Handling of Heat in Reactors

Some of the most common methods of heat transfer with catalytic reaction will be briefly reviewed. 

Heat exchanger-like, multi-tube reactors are used for both exothermic and endothermic reactions. Some have as much as 10,000 tubes in a shell installed between tube sheets on both ends. The tubes are filled with catalyst. The larger reactors are sensitive to transient thermal stresses that can develop during startup, thermal runaways and emergency shut downs. 

Coolant flow is set by the designed temperature increase of the fluid and needed mass velocity or Reynolds number to maintain a high heat transfer coefficient on the shell side. Smaller flows combined with more baffles results in higher temperature increase on the shell side. Reacting fluid flows upwards in the tubes. This is usually the best plan to even out temperature bumps in the tube side and to minimize temperature feedback to avoid thermal runaway of exothermic reactions. 

Crystallographic investigation revealed nickel-spinnel formation of the catalyst with the alumina carrier. This needed one hundred or more degrees Centigrade higher temperature than any thermocouple ever indicated. 

In reactor construction, the tubesheet is the most expensive part and the possibility of tubes getting loose from the tubesheet is the most dangerous problem. Therefore, large reactors have some limitations on the rate at which they can be heated (for example, 25°C/hour), to avoid developing temperature differences in the body which lead to transient thermal stresses. These stresses can cause tubes to bend and break loose from the tube sheets. 

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