Circulating fluidized bed (CFB) risers using Group A particles have traditionally, mostly, been considered to operate in the fast fluidization regime, which consists of a core-annulus flow profile with solids refluxing in the annulus layer. High gas and solids flow riser studies, however, suggest the existence of additional types of flow behaviours. Therefore, more studies are still needed to help clear uncertainties of local solids flow patterns in CFB risers of Group A particles, especially at gas and solids flow rates at or near those in commercial units. In this study, riser density and local solids flux profiles were measured in 0.3 m diameter risers of three CFB units at gas velocities of 9-16 m/s and solids fluxes of up to 700 kg/s center dot m(2). A variety of radial solids flux profiles were obtained, including a parabolic profile with a peak flux at about the radial centre, a nearly flat profile across the riser cross-section and an inverted parabola with peak upwards flux near the wall. At high gas velocity and solids flux, risers have no solids downflow at the wall. Multiple fluidization regimes were found to exist in the riser. The bottom dense part of the riser was in the dense suspension upflow regime, and the dilute upper part was in the dilute pneumatic transport regime. With commercial fluid catalytic cracking (FCC) risers operating at nearly similar conditions as tested here, it is likely that they also have one or both fluidization regimes and not the traditional fast fluidization regime. The data in this study fitted well on the Kim et al. fluidization regime map.

This paper primarily discusses the development of fluidization in North America in the 20th century. Although the history described is primarily in North America, it cannot be completely divorced from fluidization de-velopments in other parts of the world -such as the early fluidization research in Germany culminating in the first commercial fluidized-bed process, etc. Therefore, the paper primarily chronicles the development of fluidization from the early 1920s at the U.S. Bureau of Mines to the early 2000s.The paper describes how Fluid Catalytic Cracking and World War II combined to jump-start fluidization in North America in the 1940s. In addition, the growth of fluidization development is described by decade until the end of the century. The contribution of universities to the fluidization community in the United States and Canada are also explored. Some historical controversies regarding the choking velocity in pneumatic conveying, the location of the fast fluidization regime relative to the choking velocity and jets in fluidized beds are also discussed.Inevitably in such a paper it is impossible to recognize all who have contributed to this history. The authors would like to apologize for any unintended omissions to those who have made these significant contributions.

The standpipe has become a standard element of most circulation loops. In the past, standpipes have been thought of as a device to only allow solids to transfer from a low to a higher-pressure region. However, it also has another function in a solids circulation loop. It is also the device that adjusts its operation to balance the pressure around the solids circulation loop. An overflow standpipe adjusts the pressure drop across it by raising or lowering the solids level in the standpipe. However, the operation of the underflow standpipe is more complex. This self-adjusting function can cause underflow standpipe pressure drops to be negative, which seems counter-intuitive. How an underflow standpipe operating with a negative pressure build can occur in a circulation loop is described below using two commercial unit examples as illustrations of this phenomenon.

This study was targeted at understanding the local flow and flooding behaviors of fluidized bed strippers. Bed density, pressure fluctuations and bubble void fraction were measured in a 0.6 m diameter stripper for disk and donut, as well as grating internals using FCC catalyst particles. The grating stripper had a higher bed density and operated smoothly without flooding over a wider range of gas and solids flows than the disk and donut stripper. Flooding in the disk and donut stripper was a sudden rather than a grad-ual occurrence when a limiting solids flux was exceeded. Increasing fines composition and adding holes to the disk and donut trays mitigated flooding to some extent. The gratings stripper had flattened para-bolic radial bubble void fraction profiles whereas the disk and donut stripper had characteristic M -shape profiles. The results here are expected to be valuable for the operation and design of gas-solids flu-idized bed strippers.(c) 2022 Published by Elsevier Ltd.

Fluid catalytic cracking (FCC) risers operate at solids circulation fluxes of 400 to 800 kg/s-m and superficial gas velocities as high as 15 to 25 m/s. However, although extensive CFB riser studies have been conducted, most of the reported data are for risers operating at relatively low gas velocities (< 10 m/s) and modest solids circulation rates (< 200 kg/s-m). There is a lack of hydrodynamics data for conditions similar or close to those of commercial FCC risers. This paper discusses total pressure drop, apparent density and local solids flux measurements obtained from three 0.3-m-diameter risers 15, 22 and 24 m in height using FCC catalyst particles. The risers were operated at superficial gas velocities of 12 to 16 m/s and solids fluxes of about 70 to 700 kg/s-m. At low solids circulation fluxes the apparent density decreased exponentially from the bottom to the top of the riser. A dense lower region started to form as the solids flux was increased at constant gas velocity. The height of the dense region increased to nearly occupying half of one of the risers’ height at the highest solids flux. A variety of radial solids mass flux profiles were found in the risers depending on the superficial gas velocity and net solids mass flux. These included parabolic profiles with highest fluxes in the core region, flatter profiles, inverted parabolic profiles with the highest values near the riser walls as well as profiles with the highest solids flux near one wall and the lowest at the opposite wall. In contrast to parabolic solids flux profiles found in small diameter CFB risers, the solids mass flux profiles in the 30-cm-diameter risers tested here were relatively more flat. And, except for very few cases attributed to entrance effects, the net solids flow direction at all radial locations was found to be upward for the conditions used in this study. This would suggest that the widely reported upflow core and downflow annulus in low gas and solids flow risers are not representative of what takes place in commercial FCC risers.

Cyclones are an integral part of nearly all fluidized bed processes, and especially so for circulating fluidized bed (CFB) systems. Cyclones are extremely important to the successful operation of nearly all CFB processes. The two most important CFB processes fluidized catalytic cracking (FCC) and circulating fluidized bed combustion (CFBC) operate with different particle sizes and at different operating conditions. Therefore, the cyclone designs for each of these major CFB processes are also different. Cyclones for CFBC units are generally very large (8 to 10 m in diameter) and typically have only one stage. Cyclones for FCC units are much smaller (of the order of 1.2 to 2 m in diameter) and are designed for a minimum of 2 and as many as 4 stages in series. The average particle size flowing around FCC units is only about 70 microns, while the particles circulating in CFBC units are typically 150 to 200 microns. In this paper, how the differences in cyclone operation and design affect CFB system operation is described and discussed.

Many of the probes used to understand hydrodynamics in circulating fluidized bed risers intrude into the environment they are measuring, although assumptions are typically asserted that the intrusive probes do not affect the data collected. This could be a poor assumption in some cases and conditions. We found that intrusive fiber‐optic probe measurements consistently mis‐predicted the solids concentration compared to the nonintrusive pressure drop measurements outside the fully developed flow region of a riser containing fluid catalytic cracking catalyst or glass bead particles. The discrepancy was sensitive to superficial gas velocity, solid circulation rate, probe position, and flow direction. Barracuda VR™ computational fluid dynamics simulations confirmed this, and indicated that particle momentum was lost at the leading edge of the probe and particles were spilling over to the probe tip. Accordingly, new probe designs were proposed to mitigate the intrusiveness of a fiber‐optic probe for more accurate characterization. © 2017 American Institute of Chemical Engineers AIChE J , 63: 5361–5374, 2017

Fluidized bed design and scale-up depends strongly on particle characteristics such as size, shape, and for Geldart Group A particles, the level of fines (particles smaller than 44 microns). However, recent research has shown that particle clustering has a significant effect on fluidized bed hydrodynamics which impacts how these units should be designed and scaled up. This is especially true with the estimation of the solids entrainment rate and the cyclone collection efficiency. The amount of fines, particle shape and surface morphology play a role on the level of particle clustering in a fluidized bed. The fine particles are an excellent conduit for moving charge as electrons or ions which appear to be the dominant mechanism of electrostatics for Geldart Group A material in a bubbling fluidized bed. This electrostatic force trades off with particle momentum relaxation and rotational to translation momentum transfer with regard to forming a particle cluster. The issue is the quantification of this effect so more precise calculations can be made with particle entrainment rates and cyclone collection efficiency. Preliminary work on particle shear in a packed and fluidized beds, suggest that particle clustering can be measured and may provide a quantifiable metric for the level of particle clustering.

Deep gas fluidized beds of low-fines fluid catalytic cracking (FCC) catalyst particles can have severe gas maldistribution due to gas bypassing. Tests were conducted in a 0.6-m-diameter unit using 3.2% and 4% fines less than 44μm FCC catalyst particles to determine the influence of system pressure on gas bypassing in a fluidized bed of 3.66m static bed height. The freeboard pressure was varied up to 207kPag (30psig). Differential pressure fluctuations were measured at four locations around the column, bubble void fraction was measured at two opposite locations close to the column wall, and radial bubble void fraction profiles were measured at axial elevations of 0.9 and 1.52m. At no or low pressures, gas bypassing was present in the bed. With gas bypassing, differential pressure fluctuation intensities were significantly different around the column, significantly higher bubble void fractions were measured close to the inner wall on one side of the column than on the opposite side, and the radial bubble void fraction profiles were not symmetrical about the column axis. Increasing the system pressure weakened the intensity of gas bypassing. Gas bypassing disappeared at a freeboard pressure of about 100 to 140kPag (15 to 20psig).

Fluidized bed design and scale-up depends strongly on particle characteristics such as size, shape, and for Geldart Group A particles, the level of fines (particles smaller than 44 microns). However, recent research has shown that particle clustering has a significant effect on fluidized bed hydrodynamics which impacts how these units should be designed and scaled up. This is especially true with the estimation of the solids entrainment rate and the cyclone collection efficiency. The amount of fines, particle shape and surface morphology play a role on the level of particle clustering in a fluidized bed. The fine particles are an excellent conduit for moving charge as electrons or ions which appear to be the dominant mechanism of electrostatics for Geldart Group A material in a bubbling fluidized bed. This electrostatic force trades off with particle momentum relaxation and rotational to translation momentum transfer with regard to forming a particle cluster. The issue is the quantification of this effect so more precise calculations can be made with particle entrainment rates and cyclone collection efficiency. Preliminary work on particle shear in a packed and fluidized beds, suggest that particle clustering can be measured and may provide a quantifiable metric for the level of particle clustering.

The National Energy Technology Laboratory (NETL) worked with Particulate Solids Research Inc. (PSRI) to conduct the third CFD Challenge Problem in granular fluid flow to evaluate the progress and state of the art in simulating gas solids flow in a circulating fluidized bed. Both Group A and B particles were tested at several gas velocities and solids circulation rates. For both particle groups pressures and particle velocities were measured within the riser. For the Group B cases local radial solids fluxes and high speed pressure fluctuations were measured. Model predictions were compared against these experimental results and vetted in the workshop at the Circulating Fluid Bed X. The modelers were given detailed descriptions of the experimental facilities as well as physical property and small scale fluidization data on the different bed materials tested. Two general types of modeling simulations were submitted: Eulerian–Eulerian and Eulerian–Lagrangian. Both types of model had successes and failures indicating that good results are strongly influenced by resources such as available time, computational facilities, and experience level of the modeler. By comparing the predicted behavior the strengths and weaknesses associated with the different modeling approaches were identified and shortcomings could be targeted for future development and improvements.

Particle flows of high particle concentration are important in many fields, including chemical processing, pharmaceutical processing, energy conversion and powder transport. Circulating fluidized beds (CFB) are widely employed in industry because they enhance reaction rates and heat transfer through rapid mixing of particles at high particle concentrations and high particle flow rates. However, despite decades of research and industrial application, the real time behavior of particle flow fields in CFB's is still not well understood. One of the reasons is that experimental data is difficult to acquire in such harsh, opaque environments. In this study, a new high speed particle imaging velocimetry (high speed PIV) technology, developed by the USDOE National Energy Technology Laboratory (NETL), is applied to observe and measure the real time behavior of individual particle motion inside the risers of CFB's. High speed PIV data acquired in three pilot scale CFB units at two laboratories: two CFB's with 0.305 m diameter risers and one CFB with a 0.2 m diameter riser. The high speed PIV system records high speed videos of particle motion with excellent spatial and temporal clarity. The high speed videos are analyzed to measure the concentration and the two-dimensional motion (velocity and trajectory) of individual particles. Data sample rates for velocity vectors are in the range of 0.1 to 3 million vectors per second thereby providing full resolution of the temporal domain of particle velocity. To see and measure particle motion inside the CFB risers at high particle concentrations, a custom borescope was inserted into the risers. The CFB risers were operated over a wide range of industrially relevant conditions: superficial gas velocities from 6.5 to 18.3 m/s with solid fluxes from 20 to 400 kg/m(2)/s. The particles used in the CFBs included fluid cracking catalyst (FCC) with a mean diameter of 70 mu m, high density polyethylene (HDPE) with a mean diameter of 750 mu m, and glass beads with mean diameters of 170 and 650 mu m.High speed videos and high speed PIV data enabled careful study of the real time behavior of gas-particle flow fields in CFB risers. In all of the CFBs of this study, one or more "jets" of high speed gas were observed at any time in the CFB risers. The jets move around the riser and appear to wander from one location against the riser wall to another. The jets have width range of 1/10 to 1/2 of the riser diameter. When a jet moves away from an area, the void is immediately filled with large clusters of particles. The clusters have sizes up to several riser diameters and contain significant percentages of the total particle flow. Clusters reduce mixing and interaction of particles with the transport gas, and therefore may inhibit reaction rates. Shearing of the clusters by high speed jets gives rise to cluster shapes that are either undulating or in the form of long, thin vertical strands which are often called streamers. The well known core-annulus concentration profile does not exist in real time, but rather is a long time averaged phenomenon. The data and insight from this work should be valuable for design and operation of risers, and for development of computational fluid dynamic (CFD) models of riser flow fields. Published by Elsevier B.V.