Bed Regime Research Articles

Comparison of macroscopic flow properties obtained by three methods of operation in a liquid–solid circulating fluidized bed

Experiments were conducted in a liquid–solid circulating fluidized bed (LSCFB) to study the flow regimes, operational instability, critical transitional velocity to circulating fluidized bed (CFB) regime, solids holdup and solids circulation rate by three experimental methods. The results indicate that the operational instabilities such as arch formation, liquid–solid separator blockage and solids return pipe blockage were observed in two of these methods at large primary and auxiliary liquid velocities. The critical transitional velocity that demarcates the expanded bed from CFB regime was observed to be different by these three methods. Each of these methods has its own merits and demerits in terms of operating range of CFB regime, solids holdup, solids circulation rate and application.

PEPT and discrete particle simulation study of spout‐fluid bed regimes

AbstractThe results of a combined experimental and simulation study on the flow regimes that can be encountered during spout‐fluid bed operation are reported. A regime map for a three‐dimensional (3D) spout‐fluid bed was composed, employing spectral analysis of pressure drop fluctuations and fast video recordings. In addition, 3D Euler–Lagrangian computations were performed to assess the capability of the model to reproduce the experimentally observed flow regimes. Spectral analysis of pressure drop fluctuations revealed that for most investigated regimes the model is able to predict the appropriate regime. The frequency at which the largest power is found is overpredicted by the model in most cases. The remaining differences between the simulated and the experimentally observed bed behavior are most probably related to the representation of the effective fluid–particle interaction in the model, which relies on local spatial homogeneity. The simulation results were compared with velocity maps determined from particle trajectories acquired using positron emission particle tracking. The model accurately reproduces measured particle velocities, including their root mean square, for all investigated conditions and is therefore able to capture the details of the particle flow in various flow regimes. © 2008 American Institute of Chemical Engineers AIChE J, 2008

Fluidization regimes in different spouted bed apparatus constructions

In this study, prediction of transition from fixed bed to spouting bed are carried out and different spouting regimes are characterized by analysis of pressure fluctuation signals. Pressure signals are measured in three different constructions of spouted bed apparatuses, operated under ambient conditions and under different spouting regimes. The presented spouted beds are developed for handling of non-spherical particles and poly dispersed and finely dispersed systems. Several flow regimes are found to exist as: fixed bed, channel formation, bubbling formation, stable spouting and slugging bed regimes. Different methods, based on time domain analysis, frequency domain analysis and phase space analysis have been applied to identify the flow regimes. Stable spouting regime is characterized by a low amplitude (respectively, low standard deviation) of pressure fluctuations, maximum in the dominant frequency. Time-series analysis of pressure fluctuations in spouted beds, using techniques from deterministic chaos theory, has revealed that a spouted bed is a chaotic system that can be characterized with a low-dimensional strange attractor.

Fluid bed gas RTD: Effect of fines and internals

The potential of fluidized beds for conducting intrinsic kinetics studies of fast and highly exothermic reactions was investigated via gas residence time (RTD) experiments in the fixed and bubbling bed regime with vanadyl pyrophosphate catalyst (VPO) containing 10%, 20%, 30% and 40% fines (catalyst fraction with d p < 44 μm). In order to characterize the divergence from plug flow, the RTD data was compared with two simple hydrodynamic models: axial dispersion and n-CSTRs in series. Results show that the simple models characterized well the gas phase hydrodynamics with the deviation from plug flow increasing with gas velocity. At velocities higher than minimum bubbling, the RTD curve changes from an ideal Gaussian shape and develops a “shoulder” or “tail”, which represents a deviation from ideal flow. It was found that fines had no effect upon the axial dispersion and perforated plate internals can decrease axial gas dispersion for the experimental conditions used. The results suggest that fluidized bed may be an alternative to fixed bed reactors for kinetic studies at low velocities and that internals may reduce the deviations from plug flow in the bubbling regime. Moreover results show that a sudden decrease in Peclet number ( Pe) characterizes the initiation of the bubbling regime. Based on this, a novel method for the experimental measurement of U mb is proposed.

Characterization of spouted bed regimes using pressure fluctuation signals

This work compares time, frequency and phase space analyses of pressure measurements in different spouted beds. The experiments were carried out in different constructions of spouted bed apparatuses, operated under ambient conditions and under different spouting regimes. Spouted beds are used when the conventional fluidized beds fail to achieve a homogeneous and stable flow regime as, for example, in the case of non-spherical particles and in poly dispersed and finely dispersed systems. Different fluidization regimes in spouted beds have been characterized by the analysis of pressure fluctuation signals. Several flow regimes are found to exist as: fixed bed, channel formation, bubbling formation, stable spouting and slugging bed regimes. Analyses of standard deviation and chaotic time series on pressure fluctuation signals are conducted to determine the transition gas velocities. A treatment technique using the Fast Fourier Transformation of measured pressure fluctuations was developed to create plots describing the bed behaviour evolution from fixed to slugging bed. At the beginning of stable spouting the amplitude of pressure fluctuations is uniform and small.

Sorption-enhanced steam reforming of methane in a fluidized bed reactor with dolomite as [formula omitted]-acceptor

An experimental investigation was conducted in which carbon dioxide was captured in order to shift the steam reforming equilibrium for the production of hydrogen. An atmospheric-pressure bubbling fluidized bed reactor (BFBR) of diameter 100 mm was operated cyclically and batchwise, alternating between reforming/carbonation conditions and higher-temperature calcination conditions to regenerate the sorbent. Equilibrium H 2 -concentration of > 98 % on a dry basis was reached at 600 ∘ C and 1.013 × 10 5 Pa , with dolomite as the CO 2 -acceptor. The hydrogen concentration remained at 98– 99 vol % (dry basis) after four reforming/calcination cycles. The total production time decreased with an increasing number of cycles due to loss of CO 2 -uptake capacity of the dolomite, but the reaction rate seemed unaffected. Variation of the superficial gas velocity within the bubbling bed regime showed that the overall reaction rate was sufficiently fast to reach equilibrium, making bubbling bed reactors attractive for this process.

Effect of particle size distribution on pressure drop and concentration profile in pipeline flow of highly concentrated slurry

The experiments were conducted in 54.9 mm diameter horizontal pipe on two sizes of glass beads of which mean diameter and geometric standard deviation are 440 μm & 1.2 and 125 μm & 1.15, respectively, and a mixture of the two sizes in equal fraction by mass. Flow velocity was up to 5 m/s and overall concentration up to 50% by volume for each velocity. Pressure drop and concentration profiles were measured. The profiles were obtained traversing isokinetic sampling probes in the horizontal, 45° inclined and vertical planes including the pipe axis. Slurry samples of the mixture collected in the vertical plane were analyzed for concentration profiles of each particle batch constituting the mixture. It was found that the pressure drop is decreased for the mixture at high concentrations except 5 m/s and a distinct change of concentration profiles was observed for 440 μm particles indicating a sliding bed regime, while the profiles in the horizontal plane remains almost constant irrespective of flow velocity, overall concentration and slurry type.

Flow regimes in a spout–fluid bed: A combined experimental and simulation study

Spout–fluid beds find a widespread application in the process industry for efficient contacting of large particles with a gas. However, detailed understanding of the complex behavior of these systems is lacking, which leads to significant scale-up problems in industry. In this paper we report results of a combined experimental and simulation study on the various regimes, which can be encountered during spout–fluid bed operation. A regime map for a 3D spout–fluid bed was composed employing spectral analysis of pressure drop fluctuations and fast video recordings. In addition 3D Euler–Lagrange computations were performed to assess the capability of the model to reproduce the experimentally observed flow regimes. The influence of the drag closure on the model results was assessed and the influence of the computational grid was studied using a new method for the implementation of the two-way coupling, which is proposed in this paper. For most regimes our model is able to predict the appropriate regime. The frequency, at which the largest power is found, is overpredicted by the model. Contrary to the experimental observations, our model did not predict any large slugs in the slugging bed regime. The remaining differences between the simulated and experimentally observed bed behavior is most likely related to the representation of the effective fluid–particle interaction in our model, which relies on local spatial homogeneity.

Severn Barrage, UK—environmental reappraisal

The biological and chemical regimes of the Severn Estuary are severely stressed by its physical regime. Estuarine flora and fauna naturally adapt to habitat conditions. The Severn is dynamic and turbid to such a degree, however, as to put it beyond the tolerance of most species. The foreshores have an exceptionally low carrying capacity for shorebirds and provide poor feeding and nursery areas for fish. Parts of the subtidal bed regime are barren and few support any live organisms. The turbid water excludes sunlight and suppresses dissolved oxygen. The ecosystem is limited to the most hardy and immature estuarine organisms and these are only present as a result of continual recruitment from the Bristol Channel. Furthermore, combined sea level rise, increased storminess and foreshore erosion are sustaining the downward spiral in ecosystem biodiversity. This severe suppression is natural; it is not induced by anthropogenic contaminants. Should a tidal power barrage be built, mean water level and shelter would increase and current strengths diminish. Large-scale reductions in suspended load and greater bed stability would encourage colonisation. Mixed substrates would develop, leading to biodiverse and abundant invertebrate and vertebrate communities. It is unusual for a major engineering project to result in the large-scale invasion of a suppressed ecosystem by organisms; this is an inevitable consequence of this project.

The effect of bed roughness on scalar fluctuations in turbulent boundary layers

The effect of bed roughness on an iso-kinetic chemical plume released into the logarithmic region of the bed boundary layer of an open channel flow is examined. The bed roughness is varied systematically to generate flows in the smooth (painted polyethylene sheets), transitional rough (2.5-mm and 11.5-mm gravel beds), and fully rough (21.0-mm gravel bed) regimes. Concentration fields are measured in horizontal planes using the planar laser-induced fluorescence (PLIF) technique. The time-averaged concentration follows a self-similar Gaussian transverse profile for each bed roughness. The plume width grows as a power law function of the streamwise distance (i.e., \(\sigma \propto x^n ,\) with n≈0.75 near the source and n≈0.5 farther downstream). As the bed roughness increases, the variance of the fluctuations is smaller and decreases faster. Near the plume source, the transverse profiles of the standard deviation of the scalar fluctuations are self-similar and Gaussian. For the two rougher beds, the transverse distribution of the standard deviation transitions downstream to a bi-modal profile. The probability density functions (PDFs) of the concentration fluctuations near the plume source are highly skewed distributions. Increased bed roughness accelerates the approach of the PDF towards a Gaussian distribution.

3-D Modeling of Liquid Injection into Fluidized Beds

A three-dimensional model has been developed to describe the injection of liquid reactants into fluidized bed reactors operating in the bubbling fluidized bed regime. The model considers the processes of liquid transport and evaporation in the vicinity of the point of injection. The underlying idea, which is supported by previous measurements, is that the particles in the dense suspension phase are wetted by the liquid or gas-liquid spray. The wetted particles are subsequently dried while they are following the gross solids circulation within the bed. The model considers the flow structure of the bubbling fluidized bed and the solids mixing with the aid of a hybrid model which combines semi-empirical models for bubble growth by coalescence and for bubble splitting with a CFD approach for the continuous emulsion phase surrounding the bubbles. Submodels for heat and mass transfer are used to describe the temperature and concentration fields in the vicinity of the injection nozzle and the drying process of the wetted particles with the resulting release of the vaporized injection liquid. The model was validated separately against flow structure measurements, solids tracer measurements and experiments with the injection of water and ethanol, respectively, into beds of FCC particles.

A population balance model of the particle inventory in a fluidized-bed reactor/regenerator system

A population balance model has been formulated to simulate the time-dependent behavior of the catalyst inventory of a fluidized-bed reactor/regenerator loop. The reactor is operated in the circulating fluidized bed regime while the regenerator operates in the bubbling bed mode. Both reactor and regenerator are equipped with cyclones for solids recovery. The model considers attrition from various sources, i.e., from grid jets, bubble action in the fluidized bed and inside the cyclones. The simulation calculations demonstrate the strong interaction between attrition and efficiency of the solids recovery. A high efficiency of the solids recovery and at the same time a low rate of attrition requires a special design and arrangement of the cyclones. The time-dependent simulations reveal that it takes a considerable time for the system to reach a steady state. The catalyst loss rate from the system stabilizes after roughly 1 week but the Sauter diameter of the catalyst inventory in the loop needs about 3 weeks before a steady state is reached under the conditions investigated.

Characterization and Selection of Materials for Air Biofiltration in Fluidized Beds

A methodology is described for the selection of the best material to employ in a fluidized biofilter applied to volatil organic compounds (VOC) treatment. Two different supports were considered: one natural, scrap wood, and one synthetic, polyurethane foam. In a first part, the main objective was to establish the fluidizability of the selected solids. Particles of increasing moisture content were tested in rigs equipped with different air distribution technologies. Maps of flow regimes were drawn by varying air velocity and bed height. In the second part, the sorption capacities of the selected materials were evaluated at different humidity levels for a soluble organic compound (ethanol) and an insoluble one (toluene). Their ability for supporting viable microorganisms was then measured. Fine particles with 45% moisture content fluidized with the tuyere distributor in channeling and slugging regimes, whereas the conical distributor generated a spouted bed regime. Coarse wood particles with 45% moisture commonly used in fixed bed biofilters were considered as less suitable due to limitations borne by their fluidization regimes. Finally polyurethane foam filled with Agar gel could not endure the mechanical stress to which particles are subjected in fluidized beds. It was shown that a 45% moisture content was optimum to ensure transfer for both soluble and hydrophobic compounds. Considering the biological criterion, it appeared that a 45% humidity level was sufficient to ensure biomass growth and biodegradation of pollutants on fine scrap wood particles.

Mutual Information Functions of Differential Pressure Fluctuations in Spouted Beds

The objective of this paper is to determine whether the flow regimes in spouted beds can be recognized by the mutual information function of pressure fluctuation signals. Differential pressure fluctuation time series were obtained in a gas spouted bed of 0.12-m i.d. at different axial and radial positions, with glass beads and silica gel particles. It was found that the packed bed has a low level and fast decay rate of mutual information, suggesting a quite random signal. Unstable spouting exhibits a high level of mutual information and strong periodic motion. While for a stable spouting regime, the level of mutual information and the decay rate around time zero are between those for the packed bed and unstable spouting regimes. The mutual information function for signals measured on the wall has higher predictabilities compared to those measured inside the bed. The determination of optimum time delay by the aid of the first minimum of the mutual information function was discussed. It has been found that ...

Conditions for formation of massive turbiditic sandstones by primary depositional processes

It can be shown from bedform stability diagrams, that each turbidity current, regardless of whether its density is low or high, or whether its history is simple (e.g., surging flow) or complex (e.g., sustained flow), should pass through a final waning phase in which flow conditions are prone to bedform development. Yet, many thick deep-marine sandstones of supposed turbidity-current origin are totally devoid of tractional structures, such as plane-parallel lamination and ripple cross-lamination. TDURE, a mathematical model for the calculation of duration of sediment deposition from high-density turbidity currents (HDTCs), is used to show that the absence of Bouma T b and T c divisions cannot be explained by rapid temporal flow deceleration only. The model calculations imply that discernible tractional structures are formed, even at residence times within the upper-stage plane bed and ripple stability regimes of several seconds and several tens of seconds, respectively. Therefore, other explanations have to be sought for the absence of lamination in turbiditic massive sandstones. These include incompatible grain size, post-depositional removal or obstruction of lamination by bioturbation and liquefaction, and erosion of laminated divisions followed by rapid burial.

Statistical and frequency analysis of pressure fluctuations in spouted beds

Differential pressure fluctuation time series were obtained in two gas spouted beds of 80 and 120 mm I. D. at different axial and radial positions. Statistical and frequency analyses of the differential pressure fluctuation signals were employed to recognize and characterize the flow regimes in spouted bed, i.e. packed bed, stable spouting and unstable spouting. Spouting behavior in deeper spouted beds was found to be significantly different from that in shallow spouted beds. The pressure fluctuations in shallow beds were quite random, while those in deeper beds showed strong periodic behavior. When the operation mode in beds changed from the packed bed regime to the stable spouting regime in a shallow bed, the standard deviation increased sharply. Skewness was found to be independent of flow regimes. Power spectrum densities of different flow regimes have different characteristics and thus can be used to identify the flow regimes. The influence of sampling locations was also discussed.

MODELING OF TOTAL SEDIMENT LOAD TRANSPORT IN ALLUVIAL RIVERS

Study of total sediment load transport is an important consideration of the fluvial process in many nowaday rivers. In order to improve the prediction of sediment transport and morphological behavior of the channel bed, a mathematical model taking bed load, suspended load and wash load into account has been developed. In contrast to previous researches, wash load has been especially accentuated from the perspective of cohesive sediment transport. For a better simulation of the interrelationship between different sediment loads and the river bed, cohesive and non-cohesive regimes were distinguished for the bed depending on the critical clay content. Two cases of with and without wash load have been performed using a 1D numerical modeling approach. The comparison between the computational results and field data exhibited its advantages over the conventional models.

Hydrodynamics and mass transfer in a lab-scale three-phase internal loop airlift

The present paper addresses gas–liquid–solids hydrodynamic regimes that establish in a lab-scale internal circulation airlift operated at low superficial gas velocity batch-wise with respect to liquid and solid phases. Silica sand and glass beads have been used as bed solids at hold-up ranging between 4 and 12% by volume. Transition conditions between fixed bed, fluidized bed and solids circulation regimes have been assessed by means of analysis of pressure data, both in the time- and frequency-domains. Gas–liquid mass transfer coefficient and liquid circulation velocity have been measured in the fluidized bed and solids circulation regimes.

Experimental Study of a Buoyant Particle Dispersion in Pipe Flow

An experimental investigation of a solid–liquid dispersion pipe flow was conducted using a pulsed ultrasonic Doppler velocimeter. The solid phase was composed of buoyant particles dispersed in water. This particular two-phase flow was used to mimic the behaviour of ice–water flow. Four diagnostics were used to study the complex nature of the two-phase flow mixtures: mean velocity profiles, space-time diagrams, probability density functions (PDF) and cumulative probability distribution functions (CPDF) of velocities. The results allow detection of the different flow patterns with moving bed, sliding and saltation regimes. Visualizations of the flow supplement the velocity measurements.

Measurements of near‐bed intra‐wave sediment entrainment above vortex ripples

In general, descriptions of suspended sediment transport beneath surface waves are based on the turbulent diffusion concept. However, it is recognised that this approach is questionable for the suspension of sediment when the seabed is rippled. In this case, at least if the ripples are sufficiently steep, the entrainment process is likely to be well organised, and associated with vortex formation and shedding from the ripples. To investigate the entrainment process above ripples, a study was carried out in a large‐scale wave flume facility. Utilising acoustic techniques, visualisations of the intra‐wave sediment entrainment above vortex ripples have been generated. The observations provide a detailed description of entrainment, which is interpreted here in relation to the process of vortex formation and shedding. It is anticipated that such measurements will contribute to the development of improved physical process models of sediment transport in the rippled bed regime.