The design and operation of a circulating fluid catalytic cracking (FCC) riser pilot plant is presented in the paper. Emphasis is given on the effect of riser length on feedstock conversion and coke yield. Riser lengths considered in the study were 1.46 and 9.95 m. The effect of catalyst residence time on feedstock conversion and coke yield at constant feed rate and reactor temperature (526 degrees C) is presented. The catalyst residence time varies with reactor volume and catalyst circulation rate. It is shown that for the same feed conversion, high catalyst circulation rates are required with the lower riser volume. Lower gasoline and lower coke yields are experimentally observed with the short riser length. A second-order kinetic model is used to correlate the variation in conversion and coke yield. Commercial results validate the pilot plant design as a useful tool for guiding commercial operations.