Industrial hydrogenation reactor runaway

In adiabatically operated industrial hydrogenation reactors temperature hot spots have been observed under steady-state conditions. They are attributed to the formation of areas with different fluid residence time due to obstructions in the packed bed. It is shown that in addition to these steady-state effects dynamic instabilities may arise which lead to the temporary formation of excess temperatures well above the steady-state limit if a sudden local reduction of the flow rate occurs. An example of such a runaway in an industrial hydrogenation reactor is presented together with model calculations which reveal details of the onset and course of the reaction runaway. 

Eigenberger, G. and V. Wegerle. Runaway in an Industrial Hydrogenation Reactor. (7th International Symposium of Chemical Reaction Engineering, Boston, 1982) A.C.S. Symp. Series No. 196, 133-143. 

Purely adiabatic fixed-bed reactors are used mainly for reactions with a small heat of reaction. Such reactions are primarily involved in gas purification, in which small amounts of noxious components are converted. The chambers used to remove NO, from power station flue gases, with a catalyst volume of more than 1000 m3, are the largest industrial adiabatic reactors, and the exhaust catalyst for internal combustion engines, with a catalyst volume of ca. 1 L, the smallest. Typical applications in the chemical industry include the methanation of traces of CO and CO2 in NH3 synthesis gas, as well as the hydrogenation of small amounts of unsaturated compounds in hydrocarbon streams. The latter case requires accurate monitoring and regulation when hydrogen is in excess, in order to prevent complete methanation due to an uncontrolled temperature runaway. 

Pure hydrogen and oxygen mixtures are highly explosive. Reactions that involve such mixtures are carried out safely in microchannel reactors. For example, the direct preparation of hydrogen peroxide is obtained with a special catalyst, avoiding the circuitous anthraquinone process, used at the industrial scale. Calculations of explosion limits clearly demonstrate that there is a considerable shift when explosive reactions are carried out in microchannels. The safety of the process is not only due to the avoidance of thermal runaway (because of large surface-area-to-volume ratio), but also due to the fact that radical chains are broken down due to the increased wall collision in the small channels of the reactor. 

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