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The L-valve, as a solids flow control device, has been used extensively in fluidized beds and circulating fluidized beds for controlling the solids circulation rate. Knowledge on L-valves, however, is mostly limited to ambient flow conditions with Geldart A and B particles. Studies on the L-valve under high temperature conditions which are common in industrial applications are rare. This paper describes experimental studies that probe the characteristics of the L-valve operation under high temperature conditions using Geldart D particles. The relationships between the solids flow rate, aeration rate, and pressure drop are investigated and compared with those under ambient conditions using different groups of particles. The results indicate that the variation of the solids flow rate controlled by the L-valve aeration depends on the actual internal flow rate of the gas that flows through the L-valve rather than the external aeration flow rate of the gas that is introduced into the system. The actual aeration gas flow rate required for generating a given solids flow rate is the same so long as the location of the aeration tap is above the height-to-diameter ratio of 1.5 in the L-valve. Operating the L-valve with Geldart D particles needs more gas aeration compared to that with Geldart A and B particles for a given solids flow rate. However, under high temperature conditions, the solids flow for Geldart D particles in the L-valve behaves like those with much smaller particle sizes. For Geldart D particles, the aeration gas flow rate required under high temperature conditions is much less than that under ambient conditions. [ABSTRACT FROM AUTHOR]

Much of the fundamental research reported in the literature on gas−solid fluidization properties has been performed with large gas−solid fluidized beds. However, little is known regarding gas−solid fluidization in the mini- and microscale channel sizes ranging from 10−3to 10−2m and 10−5to 10−4m, respectively. The wall effects in the mini- and microchannels significantly affect the hydrodynamics of gas−solid fluidization. Such effects are examined experimentally in this study using FCC particles in six mini- and microchannels with sizes ranging from 700 μm to 5 mm. The data reveal a significant increase in the minimum fluidization and bubbling velocities as well as the wall friction in the mini- and microchannels compared to those in large fluidized beds. Additionally, the maximum stable bubble size increases with the superficial gas velocity and channel size. The round-nosed slug and the wall slug are observed in the channels. Correlations for predicting the fluidization regime transition in large fluidized beds are not adequate for predicting that in the mini- and microchannels. Also, differing from fluidization in a large bed, there is regime transition instability in that particulate fluidization is observed to form in the 700 μm and 1 mm channels through the bubbling/slugging transition as the gas velocity increases beyond that for the fixed bed. [ABSTRACT FROM AUTHOR]

Choking is commonly defined as a phenomenon where a sudden change in the solids holdup occurs in a gas-solid fluidization system. In this study, the electrical capacitance tomography (ECT) based on the neural network multicriteria optimization image reconstruction technique (NNMOIRT), developed by this research group, is used to probe the mechanism of choking formation by examining a real-time, quasi-3D cross-sectional flow structure of a circulating fluidized bed riser. The particles used are FCC catalysts. The ECT with NN-MOIRT reveals a double solids-ring flow structure and the presence of particle blobs at the center of the bed in the circulating fluidized bed (CFB) riser. This flow structure, when it is present at the entrance region of the riser, undergoes a distinct variation during the choking transition when the gas velocity is below the transport velocity Utr, but does not undergo a distinct variation when the gas velocity is above Utr This flow structure, when it is present at the upper region of the riser, however, does not vary distinctly at any gas velocity. Such flow structure variations are not appreciably affected by the solids feeding patterns with or without a gas or solids distributor, or by the gas humidity. For the low gas velocities (< Utr), the borescope measurement indicates that the solids concentration in the particle blob and the blob size all increase as the solids circulation rate increases. The disintegration of the enlarged blobs or blob jets and the collapse of the solids suspension characterize the mechanics of the initiation of the choking transition to the dense-phase fluidization regime attributed to solids suspension instability. [ABSTRACT FROM AUTHOR]