Mechanism And Hydrodynamics In A High-Density Circulating Fluidized Bed

High-density circulating fluidized beds(HDCFB)have been widely applied in many fields,such as petrochemical engineering,energy conversion,environment protection due to efficient gas-solids contact and heat/mass transfer.However,there are few fundamental studies focused on HDCFB and the reason could be that it's difficult to achieve high-density operation in an experimental setup.Therefore,this work focuses on the mechanism by constructing a pilot HDCFB unit.The distributions of local solids holdup,particle velocities and clusters are systematically measured and analyzed to show the gas-solids flow structure throughout the whole riser.The results could guide the design and application of HDCFBs in industrial processes.An HDCFB unit with 18 m high was designed and set up in this work.The solids circulation rate(Gs)could reach up to 1800 kg/(m~2 s)which is more than those in the industrial processes.The effects of operating conditions on Gs were quantitatively investigated including the solids heights in the storage tank,opening degrees of the valve,superficial gas velocities and aeration rates in the bottom of the storage tank.Furthermore,a mathematical model was established based on the experimental results.This model not only predicts the solids circulation rate but also represents the pressure distribution around the loop of the circulating fluidized bed according to the device structures,particle properties and operating conditions.Therefore,the model and its results offer appropriate guidance for the design,optimization,and operation of an HDCFB system.Axial and radial distributions of solids holdup and particle velocities were systematically measured under Gs ranging from 100 to 1800 kg/(m~2 s).In the axial direction,the profiles show the exponential types with the cross-sectional average solids holdup of higher than 0.2.In the radial direction,the profiles are the parabolic curves at high-density operations rather than the“core-annular”structures under low-density operations.The development of gas-solids flow structure becomes slow with Gs increasing and the fully developed zone is observed at the position higher than 12 m at Gs>1400 kg/(m~2 s).Furthermore,when Gs increases,the standard deviations of solids holdup become higher,the peaks of probability density distribution become wider and the numbers of down-flowing particles become less and disappear.It indicates violent turbulence,efficient gas-solids contact and slight back-mixing which are all helpful for enhancing the reactor performance.Comparing the hydrodynamics in risers with different heights,lower risers wouldn't only limit the development of flow structure but also increase the solids holdup and decrease particle velocities at any positions in the entire riser,which suggests the necessity to research the hydrodynamics in a tall HDCFB riser.In comparison with other circulating fluidized bed reactors,the HDCFB with extremely large Gs(>1000 kg/(m~2 s))has many features,such as homogenous distributions,efficient gas-solids contact,slight back-mixing,large capacities and so on.A novel method for identifying particle clusters is established and the threshold has a definite physical meaning and varies with time based on instantaneous solids holdup.The clusters are characterized using this method.It's shown that the particles more easily aggregate into clusters with denser solids holdup,smaller sizes,and slower velocities at high-density operations.When Gs is higher than 1400 kg/(m~2 s),the cluster time fractions decrease dramatically and the clusters remain dense with slightly increasing sizes and velocities.It also indicates homogeneous gas-solids mixing,efficient contact and better performance for an HDCFB riser reactor.In a word,HDCFB is an efficient reactor with significant research meaning and wide application prospect.