Abstract: Butadiene is prepared by directing a C.sub.4 hydrocarbon stream through seven parallel adiabatic beds. The dehydrogenation period and the regeneration period are each about 9 1/5 minutes with evacuation and purge taking about 2 3/5 minutes of the 21 minute cycle. The temperature of the regeneration gas is individually adjusted in response to the summation of deviation of targeted conversion from average conversion and reactor deviation from average conversion amongst all reactors. Moreover, such adjustment minimizes deviations of each reactor from average conversion during the previous 21 minute cycle.The dehydrogenation effluent is mixed and sent through a common quenching zone. The quenched product stream's density varies in response to the extent of conversion, permitting measurement of conversion for each three minutes. Average conversion for each cycle is measured by averaging the conversions for such seven time periods. The density data are directed to a digital computer.