Fluid cracking product distribution

Table 9.5. Approximate product distributions of fluid catalytic cracking for amorphous silica-alumina and zeolite catalysts.

A model for the riser reactor of commercial fluid catalytic cracking units (FCCU) and pilot plants is developed This model is for real reactors and feedstocks and for commercial FCC catalysts. It is based on hydrodynamic considerations and on the kinetics of cracking and deactivation. The microkinetic model used has five lumps with eight kinetic constants for cracking and two for the catalyst deactivation. These 10 kinetic constants have to be previously determined in laboratory tests for the feedstock-catalyst considered. The model predicts quite well the product distribution at the riser exit. It allows the study of the effect of several operational parameters and of riser revampings. 

Murphy, J. R., Air Products-HRI, The Development of Feed and Air Distribution Systems in Fluid Catalytic Cracking, presented at the 1984 Akzo Chemicals Symposium, Amsterdam, The Netherlands... 

Table 7 shows the yield distribution of the C4 isomers from different feedstocks with specific processing schemes. The largest yield of butylenes comes from the refineries processing middle distillates and from olefins plants cracking naphtha. The refinery product contains 35 to 65% butanes olefins plants, 3 to 5%. Catalyst type and operating severity determine the selectivity of the C4 isomer distribution (41) in the refinery process stream. Processes that parallel fluid catalytic cracking to produce butylenes and propylene from heavy cmde oil fractions are under development (42). 

The influence of the temperature distribution on selectivity varies according to the reaction scheme. Among such schemes, the ccmsecutive reaction (A —B — C) qualitatively represents many organic reactions with by-products. As shown in the previous section, the use of dilute phase is recommended for endothermic reactions, but prohibited for exothermic reactions. This conclusion agrees with the development of fluid bed reactors for partial oxidations (exothermic) and cracking (endothermic). This knowledge may help one to design or develop new fluid bed contactors. 

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