An experimental investigation of vertical coaxial flows of densely loaded gas-solid streams

Circulating fluidized-bed catalytic reactor designs have become more complex over time in a continuous effort to improve yields and productivity. The next generation of designs may include staging the introduction of fresh catalyst for very fast multistep reactions. Such a staged catalyst design would most likely provide a zone in the reactor where coaxial streams of catalyst-laden gas merge and mix. This two-phase fluid dynamics problem has seen essentially no treatment in the literature.; Ducted coaxial flows of homogeneous gas streams exhibit interesting recirculation patterns when the inner-to-outer stream velocity ratio is either large enough or small enough. The existence of these recirculation patterns became evident during the development of ducted fan jet engines and other aerospace devices. It is very interesting to determine if similar reciculation patterns exist in ducted gas-catalyst two-phase coaxial flows. Significant recirculation patterns in such a reactor system would be detrimental to the design objectives by causing catalyst and gas backmixing and excessive catalyst holdup.; In this investigation, the City College Fast Fluidized Bed Facility was modified to provide ducted coaxial flow of gas-catalyst streams. Two catalyst streams from a single standpipe are separated and treated independently before merging into a single riser flow. Each of the separated streams can be controlled independently to some extent by controlling the acceleration gas to each stream. In this equipment, the inner-to-outer velocity ratio can be controlled in the range from 0.7 to 3.0.; There is no direct method to date to provide a visual observation of the gas-catalyst two-phase flow inside a high-density fluidized bed. Therefore, a number of instrumental methods were used in these measurements to provide a picture of the flow structures of this ducted two-phase coaxial flow. The experimental results show consistently that recirculation patterns containing solids occur in these flows in a manner very similar to the patterns found in the homogeneous flows. The recirculation is above the mouth of the inner tube when the inner-to-outer velocity ratio is 0.7 and is against the wall at a higher elevation when the inner-to-outer velocity ratio is 3.