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What is the minimum FCC riser velocity to be maintained to avoid catalyst slumping or back mixing in the riser?
20/04/2015 A: Rajesh Gajera, Essar Oil Limited, rajesh.gajera@essar.com
A gas velocity of about 1.8 meters/sec is usually the minimum velocity for operation of a riser in dilute phase flow that will still provide an adequate flux rate of catalyst. At a gas velocity of 1.8 meters/sec the maximum catalyst flux rate is approximately 5 lb/ft2/ sec. If this catalyst flux were increased without a simultaneous increase in gas velocity, catalyst in the riser would slump back into a dense phase mode,which phase is not in the flow regime essential to this invention. While there is theoretically no maximum gas velocity limitation for the riser treatment zone, various practical limitations exist. With increasing gas velocity,the sheer volume of gas moving through the reactor system and main colum increases. Consequences of the additional volume are higher utility costs and greater vessel size. In addition, higher velocities increase the erosive action of the flowing catalyst stream thereby by reducing equipment. Therefore, gas velocities within the riser will generally not exceed 24 meters/sec. However, a more desirable range of gas velocities for the treatment zone is from 1.8 to 12.2 meters/sec. For this range of velocities, the catalyst flux rate will usually not exceed 200 lb/ft2/sec.