Bubble chains in large diameter gas fluidized beds

Using statistically based measuring methods for the determination of local bubble size distributions and local average bubble shapes in gas fluidized beds, bubble characteristics have been measured in a fluidized bed column of 1 m diameter where quartz sand (minimum fluidizing velocity 0.0135 m/sec) was fluidized with air at velocities ranging from 0.05 to 0.30 m/sec. The results present experimental evidence that bubbles within large diameter fluidized beds do not rise completely randomly distributed in space but rather in the form of bubble chains which is in agreement with industrial operating experience in large scale fluid bed systems. Since the formation of bubble chains considerably reduces the residence time of the bubble gas this finding is of significance for the performance of fluidized bed reactors. The influence of the operating parameters on the extent of the bubble chain formation has been investigated and possible consequences of these results are discussed.     

The measurement of bubble flows in fluidized beds by electrical probe

Measurements of bubble velocities, dimensions and flow rates in a two-dimensional fluidized bed by a dual electrical capacitive probe are compared with measurements from cine photography; the cine photographs and electric measurements were taken at the same point in a fluidized bed and at the same time. It was found that both sets of measurements were in agreement only when the conditions of electrical measurement were arranged to exclude spurious signals, while still retaining sensitivity, and when the theory of measurement included the effects of bubble retardation and distortion, and allowed for the stochastic incidence between the bubble front and the probes.     

X-ray measurements of bubble hold-up in fluidized beds with and without vertical internals

1. Download : Download full-size image

     

Propagation characteristics of pressure disturbances originated by gas jets in fluidized beds

An experimental investigation has been carried out on velocities and amplitudes of pressure disturbances in fluidized beds made of 100–200 μm glass ballotini. Disturbances were originated by gas jetting in a 0.35 m i.d. fluidized bed. A fluidization tube 0.10 m i.d. has also been used. Different types of disturbances have been induced in the bed contained in this tube: injection of a freely rising bubble and of a captive bubble; injection of a bubble chain; and compression of the bed free surface. The dynamic wave character of the disturbances has been shown. Velocities and amplitudes of waves moving through the beds have been measured. In particular, wave velocities have been compared with theoretical results obtained by the application of “pseudo-homogeneous” and “separated phase flow” models.     

Wake properties of a single gas bubble in three-dimensional liquid-solid fluidized bed

Experiments were performed to investigate the wake properties of a single gas bubble in a three-dimensional liquid-solid fluidized bed via a video camera moving at the same speed as the bubble. The solids holdup in the fluidized bed varied up to around 10%. The bubble size varied from 5 to 20 mm with corresponding bubble Reynolds numbers ranging from 1000 to 6500. The bubble was observed to have two types of wake configurations depending on the bubble size: the asymmetric/helical vortex wake for small bubbles and the symmetric wake for large bubbles. The bubble shape and relative rise velocity in the fluidized bed can be well-represented by correlations developed for single bubbles in liquid media, although the bubble shape in liquid-solid media is slightly more flattened compared to that in liquid media. The bubble rocking frequency was found to be independent of particle properties and to correspond in magnitude to the vortex shedding frequency in a two-dimensional liquid-solid fluidized bed. The average primary wake size in three dimensions is comparable to that in two dimensions.     

Mass transfer between bubbles and the dense phase in gas fluidized beds

Mass transfer between a bubble and the dense phase in gas fluidized beds of Group A and Group B particles was proposed based on previous experimental results and literature data.The mass transfer coefficient between bubbles and the dense phase was determined by k_(be) = 0.21d_b.A theoretical analysis of the mass transfer coefficient between a bubble and the dense phase using diffusion equations showed that the mass transfer coefficient between a bubble and the dense phase is k_(be) ∝ε_(mf)(Du_b/d_b)~(1/2) in both three- and two-dimensional fluidized beds.An effective diffusion coefficient in gas fluidized beds was introduced and correlated with bubble size as De = 13.3d_b~(2.7)7 for Group A and Group B particles.The mass transfer coefficient k_(be) can then be expressed as k_(be) = 0.492ε_(mf)(u_bd_b~(1.7))~(1/2) for bubbles in a three-dimensional bed and k_(be) = 0.576ε_(mf)(u_bd_b~(1.7))~(1/2) for bubbles in a two-dimensional bed.     

Heat transfer and hydrodynamic studies on two- and three-phase fluidized beds of floating bubble breakers

Heat transfer characteristics in three-phase fluidized beds of floating bubble breakers have been studied in a 0.142 m I.D. x 2.0 m high Plexiglas column fitted with an axially mounted cylindrical heater.Effects of the liquid and gas velocities, the particle size, the volume ratio of floating bubble breaker to particles on phase holdup, the vertical bubble length, and the heat transfer coefficient have been determined.In the bubble-coalescing regime, the heat transfer coefficient in three-phase fluidized beds having the volume ratio V_f/V_s of 10–15% produced a maximum increase in heat transfer coefficient of about 20% in comparison to that in the bed without floating bubble breakers. Also, bubble length and gas-phase holdups exhibited their maximum and minimum values at a volume ratio of 10–15%. The heat transfer coefficient in three-phase fluidized beds of floating bubble breakers can be estimated from the surface renewal model with isotropic turbulence theory.Heat transfer coefficients expressed in terms of the Nusselt number have been correlated with the particle Reynolds number and the volume ratio of floating bubble breakers to particles.     

Segregation characteristics of irregular binaries in gas solid fluidized beds——An ANN-approach

Binary mixtures of irregular materials of different particle sizes and/or particle densities are fluidized in a 15-cm diameter column with a perforated plate distributor. An attempt has been made in this work to determine the segregation characteristics of jetsam particles for both the homogeneous and heterogeneous binary mixtures in terms of segregation distance by correlating it to the various system parameters, viz. initial static bed height, height of a layer of particles above the bottom grid, superficial gas velocity and average particle size and/or particle densities of the mixture through the dimensional analysis. Correlation on the basis of Artificial Neural Network approach has also been developed with the above system parameters thereby authenticating the development of correlation by the former approach. The calculated values of the segregation distance obtained for both the homogeneous and heterogeneous binary mixtures from both the types of fluidized beds (i. e. under the static bed condition and the fluidized bed condition) have also been compared with each other.     

Time-series analysis of pressure fluctuations in gas–solid fluidized beds – A review

This work reviews methods for time-series analysis for characterization of the dynamics of gas–solid fluidized beds from in-bed pressure measurements for different fluidization regimes. The paper covers analysis in time domain, frequency domain, and in state space. It is a follow-up and an update of a similar review paper written a decade ago. We use the same pressure time-series as used by Johnsson et al. (2000). The paper updates the previous review and includes additional methods for time-series analysis, which have been proposed to investigate dynamics of gas–solid fluidized beds. Results and underlying assumptions of the methods are discussed.     

Flow structure and bubble dynamics in supercritical water fluidized bed and gas fluidized bed: A comparative study

Supercritical water (SCW) fluidized bed is a new reactor concept for hydrogen production from biomass or coal gasification. In this paper, a comparative study on flow structure and bubble dynamics in a supercritical water fluidized bed and a gas fluidized bed was carried out using the discrete element method (DEM). The results show that supercritical water condition reduces the incipient fluidization velocity, changes regime transitions, i.e. a homogeneous fluidization was observed when the superficial velocity is in the range of the minimum fluidization velocity and minimum bubbling velocity even the solids behave as Geldart B powders in the gas fluidized bed. Bubbling fluidization in the supercritical water fluidized bed was formed after superficial velocity exceeds the minimum bubbling velocity, as in the gas fluidized bed. Bubble is one of the most important features in fluidized bed, which is also the emphasis in this paper. Bubble growth was effectively suppressed in the supercritical water fluidized bed, which resulted in a more uniform flow structure. By analyzing a large number of bubbles, bubble dynamic characteristics such as diameter distribution, frequency, rising path and so on, were obtained. It is found that bubble dynamic characteristics in the supercritical water fluidized bed differ a lot from that in the gas fluidized bed, and there is a better fluidization quality induced by the bubble dynamics in the supercritical water fluidized bed.     

The local structure of gas fluidized beds —I. A statistically based measuring system

A system is described for measuring the parameters characterizing the local state of fluidization in beds of arbitrary sizes. This system is based on a miniaturized capacitance probe shaped so as not to disturb the local state of fluidization. Based on a statistical analysis of the signal, the mean bubble pulse duration, the number of bubbles striking the probe per unit time and the local mean bubble rise velocity are measured. The latter is measured by using the cross-correlation technique. From these parameters, further characteristics of the local state of fluidization are derived, in particular the local mean pierced length of bubbles, the local bubble volume fraction and the local bubble gas flow.     

Bubble wake solids content in three-phase fluidized beds

It is shown that existing equations for predicting the holdups of wakes behind bubbles in three-phase fluidized beds are not entirely satisfactory. A new model is then developed whereby the wake is treated as the sphere-completing volume of a spherical cap bubble, due allowance being made for hydrodynamic interactions between bubbles. The generalized wake equations of Bhatia & Epstein (1974) are applied to compute the ratio of solids holdup in the wakes to that in the remaining liquid of the bed. Using experimental data from the literature, a rational equation is then generated for predicting this ratio from measured variables, and a mechanism for wake solids entrainment is proposed which is consistent with this equation.     

Continuous segregation of binary heterogeneous solids in fluidized beds

Continuous segregation of a binary mixture of heterogeneous (different density) solids is carried out in a gas–solid fluidized bed. We investigate how gas velocity, solids feed rate, flotsam feed composition, bottom discharge pipe diameter, and minimum fluidization velocity ratio of the flotsam to jetsam particles influence the solids holdup, separation factor, and product quality (flotsam purity at the top outlet). The results are interpreted in terms of solids holdup information. The results indicate that the separation factor decreases when the gas velocity, bottom discharge pipe diameter, flotsam feed composition, or the minimum fluidization velocity ratio increase, while the separation factor increases as the solids feed rate increases. The product quality decreases when the gas velocity, solids feed rate, or minimum fluidization velocity ratio increase, while the product quality increases as the bottom discharge pipe diameter or flotsam feed composition increase. Correlations for predicting the separation factor and product quality are proposed using a logistic model for individual flotsam feed compositions, which satisfactorily compares with the present experimental data.     

CFD simulation for reduction of pyrolusite in fluidized beds

In this study, a CFD model coupled with heterogeneous flow structure, mass transfer equations, and chemical reaction kinetics is established to forecast the pyrolusite reduction reaction behavior. Compared with the previous studies which ignore the volume change of solids phase, the influence of volume shrinkage on reaction and flow behavior is explored in this research. Volume shrinkage of pyrolusite is proved to be non-negligible in predicting the conversion rate. The negligence of volume shrinkage leads to the overestimation of conversion rate for its inaccurate estimation of surface area for reaction. Besides, the influence of volume shrinkage on the reaction is found smaller in the scaled-up reactor.     

Gas and particle motion in jets in fluidized beds

Entrainment of solid particles by gas jets discharged downwards through slotted nozzles into bubble-free beds of fluidized particles is considered. The gas flow in the jet is calculated for irrotational flow, using a correlation established previously for slot opening as a function of operating variables. The momentum boundary layer thickness and shear stress at the horizontal interface between jet and particles are then calculated by integral boundary layer analysis. The calculated shear stress distributions are consistent with measurements of the momentum of bed particles caused to saltate by the jet, and explain the dependence of particle movement on the various operating variables. The results provide a direct confirmation of a hypothesis due to Owen on the mechanism of saltation.