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Publisher Summary The metals passivation is the process of mitigating the deleterious effects of contaminant metals, thereby improving the catalyst activity and/or selectivity to more desired products. Metals passivation is accomplished today in two ways: (1) metering the metals passivation agent, usually via the fresh feed, into the cracker where it deposits on the catalyst or (2) incorporating additives during manufacture into the catalyst formulation or into a separate carrier particle, which is blended with the catalyst. Passivation additives are normally referred to as “metal traps.” In the case of passivation of vanadium, the average life of the cracking catalyst particle is prolonged. With nickel, passivation decreases dehydrogenation activity. The refiner usually has several ways to utilize the benefits of reduced yields of hydrogen and coke achieved through metals passivation. One economically attractive option for many refiners has been to crack resid blended with the traditional gas oil feed.

The mixing and bulk circulation characteristics of a fluidized bed of wheat flour having a relatively high aspect ratio were determined with a 6 in. diam. column fitted with a slowly revolving paddle just above the gas distributor. In this system the bulk of the solids moved down the column in stick-slip flow which was readily observed because of the appearance of relatively stable horizontal cracks in the bed. At lower gas velocities the behavior of the system was characterized by zone fluidization which led to erratic and unreproducible mixing performance. Operation at greater than twice the minimum fluidization velocity overcame this problem and provided consistent mixing. An increase in gas velocity increased the bulk circulation rate and reduced the batch mixing time. The solids next to the wall of the column seemed to move at the same rate as the core of the bed, at least at lower gas velocities. The downward velocity of these solids was directly proportional to gas velocity and to bed height and a complex function of flour moisture content. The velocity also depended on vertical position but not on the speed of the rotating paddle. Adding an anti-agglomerant promoted smooth fluidization and reduced the minimum fluidization velocity, and for the same relative gas velocity it increased the bulk circulation rate and reduced the batch mixing time.