The pressure field around a two-dimensional gas bubble in a fluidized bed

Measurements of the entire pressure field around a two-dimensional bubble rising in an incipiently fluidized bed are reported. Above the center of pressure line, the field is Laplacian and reasonably well described by Davidson's theory. The pressure wake is closed. It extends for 12 bubble radii below the stagnation point and four radii on either side of the vertical axis of symmetry.     

Distribution of bubble properties in a gas-liquid-solid fluidized bed

The behavior of bubbles in a cocurrent gas-liquid-solid fluidized bed was investigated in a column of 76.2 mm ID in this study. The particles used were glass beads of 3 and 6 mm and a binary mixture of these particles. A novel dual electrical resistivity probe system was developed and utilized to obtain bubble properties including bubble size and rise velocity. The distributions of the bubble properties in the gas-liquid-solid fluidized bed were evaluated for three flow regimes: the dispersed bubble flow regime; the coalesced bubble flow regime; and the slug flow regime.     

On the mechanism of bubble formation in a fluidized bed

A mechanism of bubble formation in a nonhomogeneous fluidized bed, based on instability theory, is suggested. In this mechanism it is assumed that the “bubbles” are formed at the bottom of the bed by the growth of prominences appearing because of the instability to perturbations of the lower surface of the bed. A bubble breaks off when the volume of the prominence becomes sufficiently large so that the buoyant force equals the sum of the bed resistance opposing its growth and the inertial forces. An equation for the diameter of the departing bubble is established.     

Particle velocities in a gas fluidized bed

A force acts on the walls of a fluidized bed container due to the collisions of the particles with these walls. A method is described for measuring this force, here called the particle pressure to distinguish it from the gas pressure. Using a model similar to the elementary kinetic theory of gases, an equation has been derived for calculating the average particle velocity from this particle pressure. Comparison shows the calculated velocities to be in fair agreement with reported measurements made photographically and with scintillation counters.     

The bubble behavior in the gas-solids fluidized bed

The bubbling frequency in a gas-solids fluidized bed and its stochastic variation along the axial direction of the bed were determined by means of the capacitance method. The stochastic model proposed, more specifically a pure-death process, appears to be capable of describing the resultant data well.     

Heat transfer and bubble dynamics in a three-phase inverse fluidized bed

In this study, surface-to-bed heat transfer experiments were performed to gain insight on heat transfer and hydrodynamics in a three-phase inverse fluidized bed. Air, tap water or 0.5 wt.% aqueous ethanol, and polypropylene were, respectively, the gas, liquid and solid phases. The solid loading was varied from 0 to 30 vol.%, and the gas and liquid superficial velocities from 2 to 50 mm/s and 0 to 21 mm/s, respectively. Visual observations were associated with measured phase holdups and instantaneous heat transfer coefficients. Larger gas velocities lead to an increase in bubble size due to the transition to the coalesced bubble flow regime. The greater turbulence induced by the larger bubbles increases the average heat transfer coefficient. On the other hand, adding ethanol reduces the heat transfer coefficient. Solid concentrations up to ∼13 vol.% increase the average heat transfer coefficient whereas higher solid concentrations tend to lower it. The distribution of instantaneous heat transfer coefficient peak height is wider at higher gas and liquid velocities while the addition of a surfactant narrows it. Gas holdups and average heat transfer coefficients are both compared with existing correlations, which are then adjusted for a better fit.     

Solids entrainment in a large gas fluidized bed

Entrainment from a gas fluidized bed of cracking catalyst in a 2-ft diameter column equipped with continuous solids recycle has been studied. Radial profiles of entrainment rate and mean particle size were relatively flat. The entrainment rate decreased rapidly with increasing height above the bed and with decreasing fluidizing velocity. The existence of a transport disengaging height above which the entrainment rate and mean particle size become essentially constant was confirmed experimentally. This may be taken as the height above the bed surface at which the inverse Froude number becomes equal to 1000.     

Interphase mass transfer in a gas fluidized bed

Mass transfer rates have been measured for bubbles containing ozone injected into an air-fluidized two-dimensional bed of particles. The overall mass transfer coefficient, corrected for unsteady bubble growth effects and with entrance effects eliminated, lies between predictions of models which assume that throughflow and diffusion are both additive and rate-controlling and those models which assume that mass transfer is governed by diffusion at the cloud boundary. The experimental value is consistent with a model which bases mass transfer purely on the throughflow predicted by Murray. Failure to account for bubble growth leads to drastic overestimation of the mass transfer coefficient.     

Single bubble eruptions in gas fluidized beds

The motion of the solids thrown above the bed surface by single bubble eruptions was studied using a high speed video system. Data on the motion of the bubble and emulsion phases are presented as functions of bubble size, type of bed material, fluidizing velocity and bed depth. When plotted in dimensionless form, the trajectories of the tops of the bulge layer, bubble and wake can be condensed into three universal curves which represent the trajectories for these points for the entire range of bed materials, bubble sizes, fluidization velocities and bed depths tested. There appeared to be no significant effect of excess air, bubble size or bed depth on the dimensionless heights reached by the wake and bulge materials. In all cases the bulge material was ejected to a greater height above the bed than the wake.     

Prediction of bubble size in a gas fluidised bed

It is shown that existing data on bubble size (when this is not restricted by the vessel dimensions) can be described by

where (U — U_mf) is the excess gas flow, h is the height above the distributor and h₀, a measure of the initial bubble size, characterises the distributor. h₀ is effectively zero for a porous plate but may be more than a meter for large tuyeres.     

Axial dispersion of gas in a circulating fluidized bed

An axial dispersion of gas in a circulating fluidized bed was investigated in a fluidized bed of 4.0 cm I.D. and 279 cm in height. The axial dispersion coefficient of gas was determined by the stimulus-response method of trace gas of CO₂. The employed particles were 0.069 mm and 0.147 mm silica-sand. The results showed that axial dispersion coefficients were increased with gas velocity and solid circulation rates as well as suspension density. The experimentally determined axial dispersion coefficients in this study were in the range of 1.0-3.5 m²/s.     

Entrainment of coal particles in a gas fluidized bed

Entrainment from an air fluidized bed of coal particles in a 14.5 cm-ID column has been studied. The effects of gas velocity and mean particle size on the transport disengaging height(TDH) and entrainment rate have been determined The entrainment rate increased with the increase in the fluidizing air velocity but decreased with increase in the mean particle size and the column height above the fluidized bed The date of entrainment rate and the TDH have been correlated in terms of Froude and Archimedes numbers.     

Gas-Liquid mass transfer in a three-phase fluidized bed with floating bubble breakers

The effects of liquid and gas velocities, particle size and volume ratio of floating bubble breakers to solid particles (V_f/V_s) on both the volumetric mass transfer coefficient, k_la, and the gas-liquid interfacial area, a, have been determined in three-phase fluidized beds with floating bubble breakers. Beds having a volume ratio (V_f/V_s) of about 0.15 showed a maximum increase in both k_la and a of about 30% in comparison to that in the corresponding bed without floating bubble breakers. The volumetric mass transfer coefficient in three-phase fluidized beds with or without floating bubble breakers can be estimated from the surface renewal frequency of liquid microeddies and the particle size.     

An investigation on gas mixing in a fluidized bed

The radial dispersion of gas in a fluidized bed of 10 cm diameter, charged with resin beads, was investigated using a new strip staining technique with bromine as tracer. Paper strips containing KI, which were placed in tubes with capillary tips, provided stains which had previously been calibrated against air—bromine mixtures flowing under similar conditions in the bed. A similar trend of the concentration profiles related to the two-dimensional diffusion model was obtained.     

Simulation of a bubbling fluidized bed process for capturing CO2 from flue gas

We simulated a bubbling bed process capturing CO₂ from flue gas. It applied for a laboratory scale process to investigate effects of operating parameters on capture efficiency. The adsorber temperature had a stronger effect than the regenerator temperature. The effect of regenerator temperature was minor for high adsorber temperature. The effect of regenerator temperature decreased to level off for the temperature >250 °C. The capture efficiency was rather dominated by the adsorption reaction than the regeneration reaction. The effect of gas velocity was as appreciable as that of adsorber temperature. The capture efficiency increased with the solids circulation rate since it was ruled by the molar ratio of K to CO₂ for solids circulation smaller than the minimum required one (G _{s, min} ). However, it leveled off for solids circulation rate >G _{s, min} . As the ratio of adsorber solids inventory to the total solids inventory (x _w1) increased, the capture efficiency increased until x _w1=0.705, but decreased for x _w1>0.705 because the regeneration time decreased too small. It revealed that the regeneration reaction was faster than the adsorption reaction. Increase of total solids inventory is a good way to get further increase in capture efficiency.     

Two-area model for bubble distribution in a turbulent fluidized bed of fine particles

The flow behavior of gas and solid was investigated in FCC simulator of φ710×4000/φ870×11000mm.The axial and radial distributions were detected with matrix fiber-opticprobes.It was found that the distribution of bubble diameter in the turbulent region of the fluidizedbed of fine particles was different from the results reported for lab-scale experiments.Radially therewere three areas,i.e.,the central(r/R=0-0.4),the intermittent or stable(r/R=0.4-0.8)and thenear wall(r/R=0.8-1.0)areas respectively.It was noticed that bubbles were almost non-existing atthe near wall area.Hence,according to the coalescence and splitting theory of bubbles,a two-areamodel of bubble diameter distribution was proposed and a dimensionless parameter(γ_M)regarded asan index for'quality'of fluidization was deduced.     

高密度浓相流化床中气泡的兼并与分裂特性

利用先进的高速动态分析系统对二维床中气泡的行为进行了研究,通过对所拍摄图象的分析处理．得到了不同介质流化床内形成的气泡形状、大小、聚并及分裂的基本规律和特点．实验研究表明．气泡的兼并主要是两气泡问的合并、被合并气泡总是从气泡的尾涡区曳入气泡;气泡分裂主要发生在操作气速较大或大气泡中,是由于其顶部粒子流(或“剪切流”)的侵入造成的;操作气速较低,粒度、密度较大粒子形成的流化床更易于造成气泡的湮灭。