a shelf is incorporated into a typical fcc riser that permits the tips of the nozzles to extend past the wall of the riser while preventing the tips from promoting coke build - up and protecting the tips from erosion . the shelf can be part of an angled section that often appears in a transition zone for increasing the internal diameter of the riser to accommodate the volumetric expansion of the feed . the shelf section reduces the non - uniformity in the mixing of the catalyst and feed and minimizes backmixing of the feed injection . the shelf of this invention accomplishes these objectives without recessing the feed injectors into the riser wall which can interfere with the spray pattern . the shelf provides a location for the tips of the upwardly directed feed injectors which virtually eliminates quiescent areas that were the sites for riser coking and that resulted from the protrusion of nozzles and / or nozzle protection devices into the riser . the uninterrupted flow path about the nozzles can replenish catalyst and erode away coke if it does form downstream of the reaction zone . this invention is particularly suited for small diameter risers where nozzle projection has the most disrupting influence on the catalyst and oil flow through the riser . this invention may also be used in conjunction with a contoured riser cross - section that shapes the interior of the riser downstream of the nozzles into a polygonal cross - section that matches the spray pattern of the nozzles .