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Feed vaporization in the fluid catalytic cracking (FCC) process affects the yield of valuable products in risers and downers. Feedstock droplets sprayed into a fluid cracker undergo both homogeneous vaporization in the gas phase and heterogeneous vaporization as they collide with catalyst particles. This paper models both processes. Homogeneous vaporization, with both convection and radiation heat transfer, cannot completely vaporize oil droplets with diameters larger than 10 μm. Spraying droplets into a dilute cloud of catalyst particles forms a “tunnel” of hydrocarbon vapor that minimizes droplet−catalyst contact and vaporization. Alternately, spraying the droplets onto a dense jet of catalyst particles does not provide proper heat transfer and vaporization. Modeling indicated that the best way of ensuring fast vaporization of oil droplets in FCC risers and downers is to spray the droplets onto a jet of catalyst particles with a porosity ranging between 70 and 90%.

Fast circulating jluidized-bed reactors are used for many applications, both in riser and downer modes. Typical residence times are in the order of a few hundreds of milliseconds. A good understanding of initial gas-solids contact must be reached to realize the full potential of such reactors. Effective and stable gas-solids miring chambers were identified from the signals of simple and robust probes, which could be used in industrial and pilot plants. Criteria based on cross correlation or Hurst anabsis provided consistent results. Although this study used simplified momentum probes, the criteria it developed can be applied to the signal of any fast-response sensor which is affected by local hydrodynamics. Temperature, capacitance, ultrasonic, or y -ray absorption measurements could be used. Gas or liquid-liquid miring chambers could also be optimized with the same criteria.

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