Investigation of circulating fluidized bed riser and downer reactor performance for catalytic ozone decomposition.

摘要

Catalytic ozone decomposition reaction was employed to study the characteristics of the gas-solid circulating fluidized bed (CFB) riser and downer reactors. Optical fiber probes and an ultraviolet (UV) ozone analyzer were used to obtain a complete mapping of local solids holdup, particle velocity, and ozone concentration profiles at different axial and radial positions in a 76 mm i.d., 10.2m high riser and a 76 mm i.d., 5.8 m high downer reactors. The superficial gas velocity (Ug) and the solids circulation rate (Gs) were 2∼5 m/s and 50-100 kg/(m2·s), respectively. Based on the spatial distributions of catalyst particles and gas reactant in the riser and the downer reactors, hydrodynamics and reactor performance were related. Further, comparisons between the riser and the downer reactors were made.;Solids flow developed much slower in the riser than in the downer. Negative particle velocity was observed in the near-wall region for nearly the entire height ofthe riser. The average particle velocity for the entire riser was 0.8∼0.96 times higher than the superficial gas velocity, and increased with increasing superficial gas velocity and decreasing solids circulation rate. However, in the downer the average particle velocity was 1.13∼2.13 times higher the superficial gas velocity, and increased with both superficial gas velocity and solids circulation rate.;Axial ozone concentration profiles were significantly deviating from plug-flow behavior. Very strong interrelation was observed between the spatial distributions of solids and gas reactant. Higher local solids holdups will give lower ozone concentrations. Higher solids holdup in the riser led to higher conversion than that in the downer for the same operating condition. Most conversion occurred in the entrance region or flow developing zone of the riser and downer reactors. Radial gradients of the reactant concentration increased with the height in the riser, whereas in the downer maximum gradients occurred in the mid-section of the reactor column. For both the riser and the downer reactors, the radial gradients of the reactant concentration decreased with superficial gas velocity and solids circulation rate.;Contact efficiency was introduced to account for the incomplete contacting between gas and solid phases. Overall contact efficiency in the downer was higher than that in the riser, and increased with increasing superficial gas velocity and decreasing solids circulation rate. At different heights of the riser and downer reactors, gas-solid contact efficiency was high in the flow developing zone and decreased with height in the fully developed flow zone. Strong turbulence in the entrance region was attributed for high contact efficiency in this region.;Under the above operating conditions, axial solids holdup profiles in the riser and the downer could be approximated by an exponential decay function: the solids holdup was high in the flow developing region and gradually decreased downstream in the fully developed region. The radial gradients of the solids holdup profiles in the riser were much higher than those in the downer, showing that the downer had much more uniform solids distribution. The average solids holdup for the entire riser reactor was about 1.5 times higher than the predicted value from Gs/(rhopU g). However, for the downer reactor this ratio dropped to 0.45∼0.98 which increased with increasing superficial gas velocity and decreasing solids circulation rate.;Rotational asymmetry of reactant concentrations was observed in the CFB riser reactor, which was mainly an effect of the solids entrance structure. The asymmetrical reactant distribution extended to some height less than the length of the flow developing zone, and then disappeared. This asymmetry decreased with superficial gas velocity and increased with solids circulation rate. In the solids entrance region, multiple angular measurements were suggested to provide a better representation of the true reactant concentration.;Keywords: Circulating fluidized bed riser/downer, hydrodynamics, reactor performance, catalytic ozone decomposition, solids holdup, particle velocity, solids flux, gas-solid contact efficiency, rotational asymmetry.