T h i s paper describes the adaptation of a semicommercial scale high pressure hydrogenation pilot plant to the production of approximately 8000 pounds per day of xylidines by the hydrogenation of mixed isomeric nitroxylenes. Included are a description of the process and equipment involved, personnel requirements, types of data taken, and results obtained. Optimum operating data previously obtained i n smaller laboratory soale equipment were checked and found to be essentially satisfactory for larger scale operation. This conclusion was later confirmed by actual commercial operation of the process on a scale approximately twenty times larger than the semicommercial plant scale.OhlPA4NIOS papers (1, 3 ) outline the factors on which the C production of xylidines was based, general development of the Esso Laboratories process for this purpose, and the properties and characteristics of the finished xylidines produced. Work on the nitroxylene hydrogenation process development ( 1 ) was initiated in batch autoclaves of , 1-liter capacity, proceeded through the usual laboratory and semicommercial pilot plant development in continuous units of three different sizes, small, medium, and large (production rates equivalent to approximately 3, 30, and 8000 pounds of xylidines per day), and finally culminated in less than 10 months after the first autoclave runs in the commercial hydrogenation of nitroxylenes to produce the equivalent of 150,000 pounds per day of recoverable specification xylidines.Continuous pilot unit equipment of small and medium size, with production rates equivalent to approximately 3 and 30 pounds of xylidines per day has been described ( 2 ) . This type of equipment is suitable for the laboratory development of a process with respect to catalyst and feed stock testing and the effect of process variables on feed stock conversion, product yields, hydrogen consumption, etc. Because of its relatively small size, however, equipment of this type is usually overdesigned with respect to piping and vessel sizes as compared with commercial scale equipment. In small scale equipment, mass velocities are lower; pressure drop in the system is not as much of a problem. Heat losses, on the other hand, are much greater and, for highly exothermic or endothermic reactions, more nearly isothermal operation may be obtained. Adiabatic conditions, which prevail in commercial high pressure hydrogenation reactors of conventional design, are difficult to approximate in small scale equipment. I n cases where heat of reaction is of significant influence on product yield or even on operating conditions necessary to satisfactory operation of a given process, it is difficult to predict the results to be obtained in commercial scale operation based on laboratory scale operation alone.