Local Solids Flux Research Articles

Solids flux profiles of Geldart Group A particles in high‐velocity circulating fluidized bed risers

AbstractCirculating 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 · m2. 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.

Particle Velocity, Solids Hold-Up, and Solids Flux Distributions in Conical Spouted Beds Operating with Heavy Particles

Conical spouted beds operating with high-density particles have recently gained attention because of their potential use as nuclear fuel coaters for next-generation nuclear reactors. To design, scale-up, and manufacture these coaters, detailed investigation of local flow structure is of paramount importance. Therefore, in this study, local instantaneous particle velocity and solids hold-up and flux measurements were carried out in spouted beds having a wide range of cone angles (30°, 45°, 60°) using zirconia particles (dp = 0.5, 1 mm; ρp = 6050 kg/m3). Effects of axial height, particle diameter, conical angle, and static bed height on local flow behavior were investigated. Comparisons were also made with the results of low-density particle studies. It is shown that particle velocity decreases and solids hold-up and flux increase along the bed height in the spout. The solids circulation is augmented as particle diameter and conical angle are decreased and static bed height is increased.

Detailed hydrodynamics of high flux gas–solid flow in a circulating turbulent fluidized bed

Hydrodynamics of gas–solid flow were experimentally studied in depth in a circulating turbulent fluidized bed (CTFB) using FCC particles of 76μm at a wide range of air velocities (0.5–5.0m/s) and solids circulation rates from 0 to 420kg/m2s. A high density macro-flow structure, corresponding to homogeneous axial profiles with cross-sectional average solids holdup of higher than 0.25 and parabolic radial profiles of solids holdup from 0.15 to 0.50, was easily obtained throughout the entire CTFB. Using Moment Consistency Data Processing Method and Divided Phase Cross-Correlation Method, solids holdups of the dense and dilute phases and dense phase fraction, local net solids flux and apparent particle velocity were predicted from the solids holdup signals. The results demonstrated that CTFB was characterized by dilute dominating flow in the center and dense phase dominating flow in the annular region, different from dense phase dominating flow in bubbling fluidized bed and dilute phase dominating flow in fast fluidized bed. The results also displayed that the local net solids flux and the local apparent particle velocity were proportional to the measured solids circulation rates.

Prediction of Falling Solids Film Thickness near the Wallin Circulating Fluidized Bed Risers

The solid particles distribution in circulating fluidized beds (CFB) has been extensively studied because of its importance in the design of CFB boilers and reactors. The previous studies report that in the circulating fluidized beds, the solids near the wall move down ward as a film with axial variable thickness. Therefore, it is very important to predict solids film thickness near the wall to estimate the heat transfer between a solid suspension in the bed and wall surface. In the present paper, an expression for the local solids film thickness has been suggested from the data of local solids flux measurements by using U-tube probe. The data measured in two laboratory scale CFB units which consist of 0.0508m diameter & 3 m height and 0.1016m diameter & 6m height. Sand (182 µm and 2560 kg//m3) and FCC particles (99 µm and 1600 kg/ m3) are used as bed material. The operating gas velocity and solid circulation rates are in the range of 1-6m/s and 5-125kg/m2-s respectively for the material considered. The R2 values for the correlation developed are 0.577 for literature data, 0.3904 for small column (0.0508m ID & 4m Height) and 0.886 for large column (0.1016m ID & 6m height).

Gas‐solids flow structures in a novel circulating‐turbulent fluidized bed

AbstractIn this article a novel circulating‐turbulent fluidized bed (C‐TFB) featured by high solids holdup and high gross solids circulation has been introduced and tested. The purpose of the new design was to integrate conventional circulating and turbulent fluidized beds into a unique high‐density fluidization system for more efficient gas‐solid contact and significantly reduced solids backmixing. The hydrodynamic characteristics of the C‐TFB were analyzed in terms of differential pressure, solids concentration, particle velocity, and local solids flux distributions. An axial homogeneous flow structure was easily obtained with cross‐sectional average solids volume concentrations higher than 0.25 throughout the entire C‐TFB. At all measuring positions there was no net downflow of solids and a good gas‐solid mixing was observed. © 2008 American Institute of Chemical Engineers AIChE J, 2008

Comparative study of flow structures in a circulating-turbulent fluidized bed

This paper presented a comparison of flow structures in a newly designed circulating-turbulent fluidized bed (C-TFB), which has a high solids holdup and solids flux, with two commonly used fluidized beds reactors (circulating fluidized bed (CFB) and turbulent fluidized bed (TFB)). Experimental results included instantaneous solids concentration and particle velocity, and local solids flux. Results indicated that the flow behavior in the C-TFB possessed both similarities and differences with the TFB and high-density CFB. Many distinct advantages of the C-TFB flows were found.

Flow stability in a downer mixing chamber

Circulating fluidized-bed reactors operating in either riser or downer modes are used in many applications such as fluid catalytic cracking and fluid coking. In such applications, uniform contact of the liquid feed droplets and entrained particles is essential for minimizing heat and mass transfer resistances and achieving high yields of desirable products. This contact may be intensified in the mixing chamber of downer reactors. In order to achieve the best results, a mixing chamber should provide a stable liquid–solid dispersion. The objective of this study was to measure the stability of a liquid–solid flow in a downer equipped with a mixing chamber at its entrance. A novel technique was developed to determine the stability of the flow in the downer. Tribo-electric probes were used to obtain the local solids flowrates, and their signals were analyzed to determine the stability of the solids flow. A variety of different start-up conditions and injected liquids were tested to determine their effects on the stability of the system. This novel measurement technique has been proven to be a very reliable method of determining the stability. Although the system was non-chaotic, it was found that the system was not very stable locally, and was very sensitive to changes in start-up conditions. There were significant variations between replicate runs in the distribution of the local solids flux over the downer cross-section, which can be attributed to slight changes in start-up conditions. However, globally the system was found to be stable since, although shifted within the downer cross-section, similar structures were observed between replicate runs.

Scale-up effect of riser reactors (3) axial and radial solids flux distribution and flow development

The influence of riser diameter on the axial and radial solids flux and flow development is studied in three riser circulating fluidized bed reactors of different diameters (76, 100 and 203 mm i.d. risers). A suction probe was used for the direct measurement, while the calculated local solids flux data obtained from solids velocity and concentration measured by two separate fibre optic probes were used to compare. Two shapes were found for the radial profile of the solids flux, a parabolic shape and a flat core shape. The shapes obtained could be predicted, based on the operating conditions, using a new concept of the effective solids saturation carrying capacity. The radial profile of solids flux is less uniform in a larger riser than in a smaller riser. Flow development is slower with the increase of riser diameter. The operating conditions were found to affect the solids flux in each reactor in the same general fashion: increasing gas velocity decreased the amount of downflow solids in the risers; increases in the solids circulation rate caused more solids to flow downwards.

Novel multifunctional optical‐fiber probe: II. High‐density CFB measurements

AbstractThe novel parallel three‐fiber optical probe described in Part I of this article is applied to determine simultaneously local instantaneous solids volume concentration, velocity, and flux in multiphase suspensions. Radial distributions of local particle concentration, velocity, and flux and their fluctuations in a high‐density circulating fluidized bed (HDCFB) are presented. A strong correlation exists between fluctuations of local particle velocity and particle concentration. The optical probe is shown to provide a means of evaluating alternative measurement techniques that have commonly been employed in earlier studies.

Solids Fluxes in Consolidating Slurries

This study utilized a Computerized Axial Tomography Scanner (CATSCAN ) to measure the spatio-temporal evolutions of solids fraction in the gravitational sediments of the kaolin slurries and the UK ball clay slurries. Based on the obtained CATSCAN data the solids velocities in the consolidating sediment were evaluated. The local solids flux was noted not dependent solely on the local solids fraction as commonly assumed in the traditional solids fluxbased theories.

A preliminary study into the local solids fluxes in a downer reactor

A non-isokinetic sampling method was used to study the effects of gas velocity, solids circulation rate and axial and radial positions on the local solids flux in a gas–solids downer fluidized bed. The radial profiles of solids flux are highly dependent on the axial position. The local solids flux is also dependent on the overall solids circulation rate but not dependent on the gas velocity. The solids flux profiles in the downer were also found to be quite different from those reported in the riser.

A Comparison of Solids Fluxes in a Pair of Downer and Riser Reactors

A direct comparison on solids flux was enabled by measurements obtained in a pair of riser and downer circulating fluidized bed reactors, of the same diameter, using suction probes. The operating conditions and the axial position were found to affect the solids flux in each reactor in a different manner. The solids flux in the riser were affected to a large degree by the gas velocity, in contrast with the downer where no visible effect was detected from changes in the gas velocity. The axial position has an effect on the shape of the solids flux profiles in the downer, but only small effects were observed in the riser. On the other hand, increases in overall solids flux leads to the increase of local solids flux in both the downer and the riser.

Characterization of downflowing high velocity fluidized beds

AbstractA downer‐riser circulating high velocity fluidization apparatus was developed to study the fundamentals of downflowing gas‐solid particle mixtures. The acceleration and deceleration of solids due to the influences of the entrance and exit sections result in a relatively uniform axial solids distribution. Radial solid density profiles detected with an X‐ray imaging system in the downer show the existence of a core‐annulus flow with a dilute core surrounded by a denser wall region. Local solids flux profiles were obtained with an aspirating probe device and the solid velocity profile obtained from the two measured quantities. These confirm that the majority of solids segregates in a wall region that flows faster than the dilute core region. Thus, the shorter residence time in the high‐speed downer wall region is coupled with faster reaction rates due to the accompanying high concentration of catalyst, while the dilute core has slower reaction rates with longer residence time due to the lower catalyst concentration and flow velocity. This results in much more uniform reaction extent over the cross‐sectional area of the downer and, therefore, should improve the product selectivity.

Flow Characteristics of Coal Ash in a Circulating Fluidized Bed

Solids flow characteristics of coal ash are investigated with the measurement of the local solids flux, local solids concentration, and spatial distribution of particle size. Two sets of the solids sampling probe are used to quantify the local solids flux at different radial and axial locations. The macroscopic flow structure is presented by analyzing the radial profiles of solids flux and the local particle size distributions, while the mesoscale flow structure is illustrated on the basis of probability density function of instantaneous solids concentration, the intermittency index, and the cluster frequency. It is found that both macroscopic and mesoscopic structures of gas−solid flow of coal ash are different from those of commonly studied fluid catalytic cracking and sand particles. The differences include higher upward and downward solids fluxes in the lower dense region, significant particle segregation, and more uniform annular flow. Furthermore, a semiempirical model taking into consideration the ...

Annular wall layer thickness in circulating fluidized bed risers

AbstractThe thickness of downward‐flowing annular wall layers in circulating fluidized bed risers has been determined in the literature based on measured radial profiles of both local particle velocity and solids flux. The thickness of the wall layer is shown to be larger based on solids flux profiles than when based on particle velocity profiles, because fluctuations in local instantaneous particle velocity are correlated with fluctuations in local solids concentration. A new correlation is developed to predict the time‐average thickness of the downflowing particle streamer layer based on solids flux measurements as a function of the cross‐sectional average voidage. It successfully accounts for the variation of the wall layer thickness with axial location and solids circulation rate.

Influence of temperature on solids flux distribution in a CFB riser

This paper presents an investigation of the solids flux distribution in a 0.161 m ID circulating fluidized-bed riser, operating in temperature range 20–550°C. The superficial gas velocity and the imposed solids flux were varied from 2.5 to 5 m s −1 and from 2 to 60 kg m −2 s −1, respectively. For the operating conditions examined, the reduced solids flux, i.e. local solids flux divided by cross-sectional mean solids flux, was insensitive to changes in the imposed solids flux over a wide range at a given superficial gas velocity. As temperature increased the mean suspension density decreased, whilst the reduced solids flux profile became flatter and less parabolic in shape. Local fluctuations in solids flux were found to increase with increasing temperature. Analysis of results suggests that the effect of temperature on CFB hydrodynamics (e.g. the mean suspension density and the solids flux profiles) is mainly due to the change in viscosity of the fluidizing gas.

Characterising non-uniformities in gas-particle flow in the riser of a circulating fluidized bed

A sampling probe operating in a non-isokinetic manner was used to measure variations of local solids mass flux with radial, angular and axial position and time in the 152 mm i.d. riser of a circulating fluidized bed cold model. Time-averaged solids flux profiles exhibit a typically flattened parabolic shape with a region of downward flow adjacent to the riser wall. These profiles do not generally exhibit axial symmetry and a minimum of six radial profiles measurements are found to be necessary to describe the flow pattern adequately. These angular non-uniformities are reproducible, difficult to eliminate and although their form is probably system dependent, it is thought that such non-uniformities are likely to exist in all such gas-particle flow systems to a greater or lesser extent. Detail of the nature of the flowing suspension near to the riser wall has been delineated by analysis of variations in local solids flux with time measured using an adaptation of the same sampling probe.

Similar profiles of solid flux in circulating fluidized-bed risers

Results of detailed experimental measurements of solids mass flux profiles in circulating fluidized-bed risers of internal diameter 0.152 and 0.305 m are described. The ranges of superficial gas velocity and mean solids mass flux used were 3–5 m s −1 and 2–111 kg m −2 s −1, respectively, and the solids used had a surface-volume mean diameter of 74.9 μm and a particle density of 2456 kg m −3. The existence of the “similar profiles” regime first suggested by Monceaux and co-workers is confirmed. In this regime of gas—particle upflow radial profiles of reduced solids flux (local solids flux divided by cross-sectional mean solids flux) are insensitive to changes in imposed solids flux at a given superficial gas velocity. This regime is found to lie within the refluxing transport regime, characterized by a dilute, rapidly rising suspension in the core surrounded by a slowly falling denser suspension in the region adjacent to the walls. The onset of the similar-profiles regime is studied in detail and it is found that the mean solids flux, at which this regime begins, increases with superficial gas velocity. The authors' experimental findings are linked with those of other workers to produce a semi-empirical model for this regime capable of predicting profiles of solids flux and solids velocity. Comparison of predicted solids flux profiles with experimental data is very encouraging and indicates that, although the flow patterns in such vertical gas—particle systems are complex, their fluid-dynamic analysis may be relatively straightforward.