Catalytic cracking converts heavy petroleum fractions into gasoline, distillates, and light olefins. It revolutionized refining over 50 years ago and has been evolving continuously since. Catalytic cracking was commercialized in fixed, moving, and fluid-bed reactors (fluid catalytic cracking, FCC, has created fine powder fluidization). The design aspects of this important process, in three major reactor types, constitute a fascinating lesson in reaction engineering. We examined the properties of catalytic cracking, its chemistry, kinetics, and thermodynamics, and their influence on design. Of the hundreds of complex reactions, catalytic and thermal, it is the conversion of hydrocarbon to coke and gas that dominates reactor and regenerator designs. The effects of hydrodynamics on cracking are examined through the concept of contact time distribution, and the discussion of the various reactor and regenerator types concludes with a summary of process control.

This study investigated solids mixing.of a group A fine powder in a 0.15 m I D expanded top fluid bed with a ferromagnetic tracer. The superficial gas velocity was raised from 0.075 to 1.1 m/s, causing the bed to go through bubbling, slugging, and turbulent fluidization regimes. A countercurrent flow model described the data well at low gas velocities. The bed assumed a more homogeneous appearance at higher gas velocities; a onedimensional axial dispersion model was used to correlate the data. Axial dispersion coefficients increased with gas velocity. The data agree well with literature data for low gas velocities. Most previous studies of solids mixing were limited to the bubbling and slugging regimes. The present work measured solids mixing by using ferromagnetically tagged tracer particles. Top FluidCommercial fluid-bed reactors using fine powders often operate in the turbulent fluidization regime when gas velocities exceed 0.3 m/s. This regime is characterized by rapid coalescence of bubbles, good gas-solid contact, and overall homogeneous appearance of the fluid bed. Bubbles are small, especially when a considerable fines fraction (less than 40 pm) is present.The reactor diameter, rather than bubble size, is the dominant scale for describing reactor fluid dynamics.Solids mixing data are usually correlated by the bubbling and countercurrent flow models. The present study was made to examine how well these models hold at higher gas velocities, when the bed assumes a more homogeneous appearance. The axial dispersion model was also reexamined.Solids mixing is an important phenomenon because it contributes to the isothermal appearance of gas-solid fluid-bed reactors. Solids and gas mixing are related phenomena. Both play an important role in gas-solid catalytic reactions. In some processes, backmixing of adsorbed products on catalyst particles can adversely affect process selectivity. Solids mixing depends on bed diameter, and this is important in fluid-bed scale-up. CONCLUSIONS AND SIGNIFICANCE1. The countercurrent flow model describes solids mixing well in the bubbling fluidization regime. It corresponds to the two-phase appearance of the fluid bed. The phase velocities required by the model can be easily measured by tracer response. Axial solids dispersion coefficient can be calculated from this model. The countercurrent flow model does not require information about bubble size (required by the bubbling bed model). This is advantageous, as bubble size varies with bed geometry and cannot be scaled-up to commercial-size beds.2. The countercurrent flow model does not describe solids mixing well in the turbulent fluidization regime. Phase velocities are hard to measure. The bed assumes a more homogeneous appearance: bubbles are smaller; they break up and coalesce rapidly. The turbulent appearance of the bed is enhanced for tall beds, and when the solids fines fraction is high (more than EXPERIMENTAL Review of Experimental Methods 20% less than 40 pm). 3.A onedimensional Taylor dispersion model fits the data well. ...

No abstract

Originating in the 1920' s and 1930's, two distinct fluidization arts have emerged, one for treating coarse solids and the other for fine powders. Fluidization research has tended to focus on bubbling beds of coarse solids, but designers of such beds for burning coal have learned to appreciate the importance of combustion of fine char particles in the freeboard. Designers of successful processes for powders have focused on bubble suppression. Since about 1980, combustion fluid beds of both types are challenging the conventional pulverized-coal boiler; they provide better means for controlling emissions from the combustion of high-sulfur fuels. Progress in the "bubbleless" fluidization of fine powders is increasing the fluid bed's competitiveness with the fixed-bed catalytic reactor. Efforts to advance the fluid bed for catalysis, besides increasing gas velocities beyond levels that most researchers have used in the past, must include systematic study of the level of fine particles smaller than 40 micrometers.