Bed Cross-section Research Articles

Digital image analysis of hydrodynamics two-dimensional bubbling fluidized beds

A new method of digital image analysis has been developed to study the hydrodynamics of two-dimensional bubbling fluidized beds with a digital video camera. The method comprises simultaneous of the size and velocity of gas bubbles, and the axial and radial distribution of bubble voidage. It provides a better estimation of the visible bubble flow than from local probe methods. Also a good estimation of the throughflow can be gotten, which is of great importance for combustor applications. Parallel to the approach of Darton et al. (Transactions of the Institution Chemical Engineering 55 (1977) 274) for three-dimensional fluidized beds, an equation for the bubble diameter of two-dimensional beds is developed, D b=0.89[(U 0−U mf) (h+3.0 A 0/t)] 2/3 g 1/3, where h is the bed height above air distributor; t is the thickness of the two-dimensional bed. For Group B particle and a given gas velocity U 0, if the bed height is sufficient, bubble diameter D b could be able to reach a maximum value at a certain height. The height is defined as the maximum bubble height h ∗ , beyond which bubbles do not grow further and become unstable and break up. The height h ∗ is a particle-size dependency, and can be expressed as h ∗=A(1+3 exp(−U 0/U mf))D t, where A=0.45 and D t is the bed diameter. The bubble rising velocity U b can be described by U b=Φ gD b (Φ=0.80−1.0) within the height h ∗ . Also, the bubble velocity is kept constant beyond the height h ∗ . The bubble density δ b is not uniform over the bed cross-section. It increases with particle size, and increases quite slowly for high fluidization velocities. The gas throughflow decreases much slowly along with the bed height. Beyond the height h ∗ , the gas throughflow is almost kept constant. Also, the throughflow is a significant part of the total gas flow, especially for high fluidization velocity, and increases almost linearly with the fluidization velocity.

Application of chaos analysis to pressure fluctuation data from a fluidized bed dryer containing pharmaceutical granule

The S-statistic, a statistical test between chaotic attractors for fluidized systems introduced by van Ommen et al. [J.R. van Ommen, Monitoring fluidized bed hydrodynamics, PhD thesis, Technical University of Delft (2001).; AIChe J., 46 (11) (2000), 2183] has been applied to pressure fluctuations collected in a conical fluidized bed of dry pharmaceutical granule as well as a fluidized bed of wet granule as it progresses to the dry state. In a dry bed, the S-statistic has been found sensitive to the particle size distribution (PSD). Changes in bed hydrodynamics arising from PSD have been found to be most easily resolved at low gas velocities, indicating segregation at superficial velocities less than 2 m/s. In a drying bed, two stable states have been identified. These states correspond to visual observations of the presence of a centralized core of bubbles in a bed of high moisture content and more uniform fluidization throughout the bed cross-section toward the end of the drying process. The effect of moisture has been found to dominate the hydrodynamic changes taking place within the bed in the drying process identified with the S-statistic. The hydrodynamic changes identified by the S-statistic are not discerned by frequency and amplitude analysis techniques. Response of the S-statistic to the hydrodynamic changes associated with drying indicates the potential application of this technique to the quantification of fluidized bed hydrodynamic behavior.

Experimental investigations on heat transfer from suspension to impact separators in the riser column of a circulating fluidized bed combustor

In the present work experiments are conducted to investigate the effect of operating parameters on heat transfer from bed to U-beam impact separators located in the top region of the riser column. The effect of suspension density and bed temperature on heat transfer from bed to the impact separators (test sections) are investigated. The experimental unit consists of a circulating fluidized bed riser column, which is 0.23 m×0.23 m in bed cross-section, 6.3 m in height with a return leg and back pass. The U-beam impact separators are located in the top region of the riser column. Furnace oil # 2 is burnt in the unit and the experimental investigations are conducted. Water is circulated through the U-beam impact separators. The presence of the impact separators in the top region of riser column helps in solids separation and also to absorb certain fraction of heat liberated in the furnace. The bed to U-beam impact separator heat transfer coefficient increases with suspension density due to increased particle concentration, which results in higher cluster and particle heat transfer. The heat transfer coefficient increases with bed temperature due to increased convection and radiation.

An investigation of liquid maldistribution in trickle beds

The influence of liquid maldistribution at the top of the packing on flow characteristics in packed beds of gas and liquid cocurrent downflow (trickle beds) is experimentally investigated. Particular attention is paid to the effect of gas and liquid flow rates on flow development. Tests are made in the trickling and pulsing flow regimes. A uniform, a half-blocked and a quarter-blocked liquid distributor is tested. Packings of various sizes and shapes are employed. Data are presented on pressure drop and liquid holdup as well as trickling to pulsing flow transition. Diagnosis of radial and axial liquid distribution is made by means of conductance probes. The effects of liquid foaming, bed pre-wetting, top-bed material, and blockage midway the bed on liquid distribution are also examined. Overall, liquid waves in the pulsing flow regime have a beneficial effect, promoting uniform liquid distribution in the bed cross section.

EFFECT OF PARTICLE SIZE DISTRIBUTION ON LOCAL VOIDAGE IN A BENCH-SCALE CONICAL FLUIDIZED BED DRYER

ABSTRACT The effect of particle size distribution (PSD) on local voidage has been investigated in a conical fluidized bed containing dried placebo pharmaceutical granule. For each of the five PSDs examined, the static bed height was varied between 0.12 and 0.17 m and the superficial gas velocity was varied between 0.05 and 0.75 m/s. The local voidage was measured using a twin-plane electrical capacitance tomography (ECT) system. A wide PSD containing 12 wt% solids with a diameter of 2 mm or larger resulted in two different types of gas flow: an annular gas flow up to a gas velocity of 0.50 m/s and a centrally concentrated gas flow above 0.50 m/s. The mixtures containing less coarse material exhibited a centrally concentrated gas flow surrounded by a dense phase at the walls of the bed over the entire range of gas velocities and bed heights examined. Consideration of previous work by other researchers suggests that the behavior of the wide PSD mixture is due to segregation at the lower velocities. The local voidage was sensitive to small changes in static bed height. For the wide PSD mixture at a fixed gas velocity, the gas tended to spread more uniformly over the bed cross-section as static bed height increased. The opposite was true of the other mixtures, i.e., the gas flow became more centralized with increasing bed height.

DEVELOPMENT AND APPLICATION OF AN EXPERT SYSTEM FOR EVALUATION OF SCOUR AND STREAM STABILITY

This paper describes a field-deployable decision support system that assists state highway department bridge inspectors in recognizing potential scour hazards and assessing the vulnerability of bridge sites in a cost effective manner. Scour and stream stability is an extremely important problem related to bridge safety as approximately 75% of all bridge failures are related to scour. The purpose of the expert system is to: (1) determine the state of scour risk at a bridge based upon site observations and site history and (2) catalog/store/retrieve information describing bridge site conditions. The system requests from the user a wide range of information about the bridge and bridge site, such as bank countermeasure presence and damage, location erosion, debris accumulation, and bed cross section profile and based upon an extensive rule base determines if abatement actions are necessary. This paper briefly describes what the system does and how the “knowledge” contained in the system is implemented. Major features of the system are described, including Stream Cross Sectional Analysis, Photographic Cataloging/Storage/ Retrieval, System Help, and a Logic Tracing Facility. The range of System Conclusions that the system produces is also briefly presented. Testing and evaluation of the system are also described as well as potential future modifications.

Local gas hold-up measurements in fluidized bed and slurry bubble column

Fiber optic probes can be used to evaluate flow properties in gas–solid and gas–liquid–solid systems. In this study, the local flow structure of bubbling flow was investigated in a slurry bubble column and in a fluidized bed. The bubble flow characteristics could, therefore, be determined along the bed cross section in various operating conditions. In both systems, the results were validated by independent measurements. Comparison of the radial profiles of bubble hold-up obtained in the slurry bubble column and in the gas–solid fluidized bed surprisingly showed quantitative agreement. Further investigations are required at different scales, but this result supports already proposed analogies between the two systems.

Effect of riser exit geometry on bed hydrodynamics and heat transfer in a circulating fluidized bed riser column

This paper reports the variation of suspension density along the riser column and the effect of riser exit geometry on bed hydrodynamics and heat transfer in the upper region of a circulating fluidized bed (CFB) riser column. The experiments are conducted in a CFB riser column which is 102 mm × 102 mm in bed cross-section (square), 5.25 m height, with a return leg of the same dimension. The unit is made up of interchangeable plexiglass columns. The superficial primary air velocity is varied between 4.2 and 6.4 m/s. The suspension density profile along the riser height is influenced by the exit geometry. With a 90° riser exit geometry, the suspension density profile in the upper region of the CFB riser column increases towards the riser exit. This particular trend has been observed for about 2 m length in the top region of the riser. The change in suspension density profile in the top region influences the variation of heat transfer coefficient. With a 90° riser exit geometry, the suspension density increases towards the riser exit, which in turn increases the heat transfer coefficient. The effect of riser exit geometry on hydrodynamics and heat transfer is significant for about 2 m length in the upper region of the riser column. Copyright © 2001 John Wiley & Sons, Ltd.

Prediction of heat transfer coefficient in the cyclone separator of a CFB

This paper deals with the study of heat transfer behaviour in the cyclone separator of a circulating fluidized bed (CFB) and how it is affected by relevant operating parameters. The experiments are conducted in a 140 mm diameter cyclone of a cold bed set-up of 102 mm×102 mm bed cross-section, 5.25 m height CFB. The cyclone separator is designed according to the high-efficiency Lapple design and made to accommodate two identical heat transfer probes. From the experimental results, the heat transfer coefficient is found to increase with increase in solid circulation rate, as well as gas superficial velocity. The effect of bed inventory and heat flux on heat transfer coefficient has also been investigated. An empirical equation has been developed to predict the heat transfer coefficient in the cyclone separator based on dimensional analysis. The experimental results are compared with the predicted results and a good agreement has been observed. There is an optimal distance from the entry of the cyclone where the local heat transfer coefficient is maximum. The collection efficiency of the cyclone separator has been measured for all the operating conditions. Copyright © 2000 John Wiley & Sons, Ltd.

Gas flow maldistribution in moving beds of monosized particles

This work is concerned with the structure of moving beds, in particular the steady-state dense phase downwards flow of monosized particles in a vertical cylindrical vessel. The structure of a moving bed is probed using gas flows through a `frozen’ bed, which is a moving bed which has been carefully stopped to preserve its structure. The gas flows are exploited in two ways, (i) by injecting a pulse of CO 2 tracer into a steady inlet flow of air, and measuring its concentration in the outlet air, thus measuring the gas residence time distribution (RTD), and (ii) by injecting a pulse of CO 2 at a particular position in the bed, and measuring its concentration at a position vertically above the injection point, thus measuring the gas velocity at a particular radial position in the bed; a radial traverse then yields a radial profile of tracer gas velocity. The concentration of the CO 2 tracer is measured using a mass spectrometer. Separate experiments using glass spheres of diameter 1, 3 and 5 mm in a bed of diameter 0.14 m show significant gas flow maldistribution, in both RTD and radial profile of tracer gas velocity. The level of maldistribution depends crucially on the frictional properties of the wall of the vessel. When the wall of the vessel is smooth, the RTD has the Gaussian shape characteristic of axially dispersed plug flow, and the radial profile of tracer gas velocity is flat; when the wall of the vessel is rough, the RTD is wide and asymmetric, and the gas flow maldistribution is shown directly in the radial profile of tracer gas velocity. Limited results are also presented for polypropylene spheres and cylinders, and steel spheres. The measured RTD, with maldistribution, is successfully described by modelling the gas flow using axially dispersed plug flow with no radial mixing but taking into account the measured gas velocity variation over a bed cross section, together with end zones each consisting of a well-mixed volume and a dead volume.

Effect of lateral and extended fins on heat transfer in a circulating fluidized bed

Experimental investigations are made in a hot circulating fluidized bed unit to study the effect of lateral, and lateral and extended fins together on heat transfer coefficient and on the rate of heat transfer. The circulating fluidized bed (CFB) unit consists of a riser column of 102 × 102 mm in bed cross-section (square), 5.25 m in height with the return leg of the same dimensions. Both liquid petroleum gas and coal are used as the fuel. A mathematical model is also proposed to predict the heat transfer coefficient to a water-wall test section with lateral and extended fins. The model results are compared with the present experimental data and also with those of published literature and a fair agreement has been observed.

TECHNIQUES TO EVALUATE THE EFFECTIVENESS OF FISH HABITAT RESTORATION WORKS IN STREAMS IMPACTED BY LOGGING ACTIVITIES

The Watershed Restoration Program (WRP) in British Columbia was initiated under the auspices of Forest Renewal B.C. to restore, protect and maintain fisheries, aquatic and forest resources adversely impacted by past forest-harvesting practices, by carrying out community-based remedial work on hillslopes and streams. Specific restoration components focus on impacted hillslopes, gullies, riparian areas, stream channels and fish habitat. Fish habitat restoration is typically carried out after an assessment of upslope risks (followed by rehabilitation) and instream conditions. Restoration techniques include restoring fish access, streambank stabilization, rehabilitating channelled reaches to provide fish holding pools and riffles, re-establishing instream cover and structure by addition of boulder clusters and large woody debris (LWD), restoring and creating off-channel habitat, and resupply of nutrients. After the restoration work is completed, research evaluation and operational monitoring are conducted to determine the overall success and ultimate benefit of the work. Research evaluation is not project-specific, but may involve evaluation of a specific technique (e.g., LWD placements), and synoptic studies and ’paired assessments’ of experimental control and treatment streams. Routine and project effectiveness monitoring is project-specific and is carried out after the instream work is completed and has passed post-construction inspection. Routine monitoring is a low intensity activity to determine functionality and condition of the restoration works. It is mainly a visual activity, and may include simple measurements or indicators (e.g., for structural stability), and can be undertaken on an annual basis or after each major storm. Project effectiveness monitoring is of higher intensity, and is conducted on a selected sub-set of projects where there are uncertainties of outcome or adaptive management benefits. This monitoring typically includes fish sampling and physical measurements: pool widths and depths, stream gradient, channel form, and channel substrate characteristics. Engineering level surveys (to tie bed and water surface elevations and cross sections to a specific benchmark) are frequently required for quantitative comparisons of restoration works (before, as-constructed and post-restoration). Where possible, project effectiveness monitoring should be related to published estimates of increases in fish abundance due to habitat improvements (bio-standards) established during the planning and prescriptive phase. Evaluation and monitoring of fish habitat restoration projects, both in the shorter and longer term, are essential to improve biological, technical and cost effectiveness of projects, and to identify and incorporate innovations. Evaluation techniques described in the paper were developed to evaluate activities to restore streams impacted by past logging practices, but the underlying principles apply to stream restoration wherever forest, urban or agriculture practices have resulted in stream and riparian impairment.

A Dinosaur Tracksite in an Early Liassic Tidal Flat in Northern Italy: Paleoenvironmental Reconstruction from Sedimentology and Geochemistry

Lateral and vertical sedimentological, ichnological, and geochemical characteristics of a tracksite at Lavini di Marco (Southern Alps, Northern Italy) allowed for the reconstruction of a semi-arid, coastal, life-sustaining environment for sauropods and theropods that lived in the western margin of tropical Pangaea at the Triassic-Jurassic transition. Isotopic analyses reveal that there was little influence of meteoric water in the diagenetic transformation of trampled sediments. Consequently, freshwater lenses were probably ephemeral, but could still sustain some vegetation and support animal life. The environment was dominated by the presence of marine waters in sediment pores. Intense evaporation caused partial dolomitization in the morphologically low lying parts of the site by modified marine waters. Dolomite precipitation favored the preservation, via early cementation, of dinosaur footprints by carbonates and, possibly, other soluble minerals. Early cementation in semiarid and arid settings could play a major role in the fossilization of dinosaur footprints on carbonate tidal flats. Dinosaur footprints provide unequivocal proof of subaerial exposure of the platform, which has important consequences for both cyclo- and sequence stratigraphic studies. Commonly, bed surfaces in Alpine outcrops are scarce and paleontologic studies have had to rely upon vertical outcrop faces. In this case, the recognition of dinoturbation structures becomes very important. At Lavini we had the opportunity to study the same footprints both on the bedding surface and in bed cross-section. Here we provide detailed descriptions of dinoturbation structures that should aid in the recognition of subaerial exposure surfaces in Mesozoic tidal flats. The preserved footprints at Lavini indicate that fossilization of ichnofaunas is possible in a variety of subenvironments within a tidal flat, in a general inter-supratidal setting characterized by alternating periods of seawater influx and dryness.

Mass transfer in packed beds at low Peclet numbers—wrong experiments or wrong interpretations?

Much research has been focused on mass transfer phenomena in packed beds. For Peclet numbers above 200, empirical relations have been derived that predict the value of the mass transfer coefficient as a function of the Reynolds number and the Schmidt number. These relations are more or less similar to the well-known relation that Ranz and Marshall derived for mass transfer around a single sphere in an infinite mediumSh = α + βReγScδ For packed beds of spherical particles an α-value of 3.89 can be calculated on basis of fundamental considerations. However, Sherwood numbers much lower than this minimum value have been observed at Peclet numbers below 100. Several explanations have been proposed for this apparent discrepancy, such as misinterpretation of the experimental results due to unjustified neglection of axial dispersion or wall channeling. In this work, a model that predicts the combined effects of axial dispersion and wall channeling has been developed. With this model, it is possible to explain the results obtained with undiluted beds in which all particles are active in the process of mass transfer. However, such an explanation is not possible for the results obtained with diluted beds in which not all particles are active. Therefore, in the case of diluted beds other reasons for the apparent drop in mass transfer rate must exist. In the present investigation, it is demonstrated that the drop again originates from misinterpretation of the experimental results. It is shown, both experimentally and theoretically, that low Sherwood numbers can be obtained when large differences exist between the local concentration, experienced by an active particle and the mixed cup concentration of the whole bed cross-section.

Axial and radial heat transfer studies in a circulating fluidized bed

The axial and radial variation of the heat transfer coefficient in a circulating fluidized bed riser column, and the effect of operating parameters thereon, are investigated. The experimental set-up consists of a riser column of 102 mm×102 mm in bed cross-section, 5·25 m in height with a return leg of the same dimensions. The unit is fabricated with plexiglass columns of 0·6 m in length which are interchangeable with one another. Two axial heat transfer test sections of 102 mm×102 mm in cross-section, 500 mm in height, and made of mild steel, are employed for the axial heat transfer study and one horizontal tube section of 22·5 mm OD made of mild steel is employed for the radial heat transfer study. The primary air velocity is varied between 4·21 and 7·30 m s−1. Local sand of mean size (dp) 248 μm is used as the bed material. One empirical model with the help of dimensional analysis has been proposed to predict the heat transfer coefficient to a bare horizontal tube in a CFB riser column and the model results are validated with the experimental data; good agreement has been observed. © 1997 John Wiley & Sons, Ltd.

Axial and radial heat transfer studies in a circulating fluidized bed

The axial and radial variation of the heat transfer coefficient in a circulating fluidized bed riser column, and the effect of operating parameters thereon, are investigated. The experimental set-up consists of a riser column of 102 mm×102 mm in bed cross-section, 5·25 m in height with a return leg of the same dimensions. The unit is fabricated with plexiglass columns of 0·6 m in length which are interchangeable with one another. Two axial heat transfer test sections of 102 mm×102 mm in cross-section, 500 mm in height, and made of mild steel, are employed for the axial heat transfer study and one horizontal tube section of 22·5 mm OD made of mild steel is employed for the radial heat transfer study. The primary air velocity is varied between 4·21 and 7·30 m s−1. Local sand of mean size (dp) 248 μm is used as the bed material. One empirical model with the help of dimensional analysis has been proposed to predict the heat transfer coefficient to a bare horizontal tube in a CFB riser column and the model results are validated with the experimental data; good agreement has been observed. © 1997 John Wiley & Sons, Ltd.

Effect of operating parameters on heat transfer in the lean phase region of a CFB riser column

The radial variation of the heat transfer coefficient across the bed width, including the effect of fins and fin shapes (surface roughness), on the heat transfer characteristics in the lean phase region of a circulating fluidized bed riser column are investigated. Three test sections (bare horizontal tube, V-fin tube and square fin tube) are employed for the investigation. The experimental unit consists of a riser column of 102×102 mm in bed cross-section, 5·25 m in height with a return leg of the same dimensions, and both made up of plexiglas columns. For the same operating conditions, the provision of fins results in a drop in heat transfer coefficient, but increases total heat transfer owing to increased surface area. The present experimental data are compared with published literature and good agreement has been observed. The experimental data also corresponds to the two-zone hydrodynamics (i.e. the core–annulus structure) of a circulating fluidized bed. © 1997 John Wiley & Sons, Ltd.

Dimension measurements of hydrodynamic attractors in circulating fluidized beds

Experimental measurements of the correlation dimension, Kolmogorov entropy, and Lyapunov exponent of circulating fluidized bed (CFB) chaotic attractors were obtained by recording differential pressure and γ-ray porosity time series along the height of a cold experimental CFB operating with 75 μm diameter fluid catalytic cracking (FCC) catalyst. The attractor dimension did not vary with respect to the type of measurement taken. Both differential pressure and localized γ-ray densitometry measurements showed the existence of a low order hydrodynamic attractor, whose dimension varied between 1.5 and 2.0 over the range of gas velocities and solids fluxes studied. Differential pressure measurements indicated that the attractor dimension decreased slightly in the lower section of the CFB and at higher solids fluxes. Localized radial γ-ray bed porosity measurements indicate that the attractor dimension did not significantly vary across the bed cross-section but did show a tendency to be slightly lower near the riser wall.

Modelling of particle mixing and segregation in the transverse plane of a rotary kiln

Thorough mixing of particles in the transverse plane of a rotary kiln or drier is essential to the uniform heating or cooling of the charge and ultimately, to the generation of a homogeneous product. However, differences in particle size and density result in a de-mixing process whereby smaller or denser particles segregate to form an inner core or kidney of segregated material which may never reach the bed surface to be exposed to freeboard temperatures. A model has been developed to predict the preferential movement of particles in the shearing active layer. This model determines the extent of fine particle segregation and is based on the principle of percolation in the active layer, whereby fines sift through the matrix of the bed to form the segregated core. Incorporating results of a granular flow model developed for this purpose, it has been possible to establish the dimensions of the segregated core as well as fines (jetsam) concentration in the rest of the bed cross-section. This result is necessary in assessing the effect of segregation on bed temperature nonuniformities in rotary kilns.

Flow regimes of the three‐phase circulating fluidized bed

AbstractGas‐solids circulating fluidized beds have been successfully used in catalytic cracking of heavy oil, coal combustion, and some metallurgical and physical processes (Grace, 1990). Gas‐liquid‐solids fluidized beds are operated mainly in conventional fluidization regimes without solids circulation or in the transport regime with low solids holdups (less than 5%) (Fan, 1989). A circulating/fast fluidization regime, however, has not been studied. A three‐phase circulating fluidized bed has several potential applications in biochemical and chemical processes. Three‐phase fluidized‐bed bioreactors generally use light and small particles (Berk et al., 1984). Circulating operation can promote solids mixing and increase product throughput per unit bed cross section, while high shear stress can promote biofilm renewal (Pirozzi et al., 1990). In three‐phase hydrotreating reactors, solids catalysts lose their activity due to the deposit of metal and coke on the surface. Circulating operation not only regenerates deactivated catalyst continuously using accompanying downcomers but also transfers heat to and from the reactor. This article discusses the flow regimes of the three‐phase circulating fluidized bed.