Flow properties of homogeneously aerated,expanded emulsion phase of fine powders (quasi-solid emulsion phase viscosity)

The viscosity of quasi-solid emulsion phase (μ_e) of the “homogeneously aerated, expanded emulsion phase” (HAE emulsion phase) of fine powders within the gas velocity range of minimum fluidization (U_mf) and minimum bubbling (U_mb), was obtained at ambient and elevated temperatures by measuring the strain retardation time (τ) of the HAE emulsion phase in this study.The deformation coefficient (Y) was obtained by our previously developed experimental method [Powder Technol., 81 (1994) 177].We confirmed experimentally that the HAE emulsion phase behaves exactly as quasi-elastic body, by observing the emulsion phase volume expansion and contraction within the velocity range of U_mf and U_mb. It is to be noted that U_mb>U_mf. The volume element of the unit HAE emulsion phase could be expanded or contracted by the force balance between the excessive gas drag force (above the gravitational force) and the inter-particle force of the particles of the emulsion phase. This experimental evidence enabled us to decide using the visco-elastic rheological model of the Voigt–Kelvin model body (VK model body) in terms of mathematics to analyze our experimental data. The rheological parameters, i.e., the elastic deformation coefficient (Y) and quasi-solid viscosity (μ_e) of our experiments, were obtained by using the HAE emulsion phase.Using these experimental data together with the VK model body, we developed our experimental model equation including the measured strain retardation time (τ) to obtain the “quasi-solid viscosity” of the HAE emulsion phase (μ_e). The physical meaning of this viscosity of the HAE emulsion phase developed in this study is quite different from the “apparent quasi-liquid viscosity” defined in aggregative fluidized beds, under the condition of U_mf=U_mb.The interesting experimental data correlations of comprehensive rheological parameters (μ_e, Y, σ_f) of the HAE emulsion were obtained for fine powders at ambient and elevated temperatures.     

A new approach to the identification of transitions in fluidized beds

Earlier work with silica sand has indicated that in a system where a bed of particles is aerated at increasing superficial velocities, the von Neumann ratio, T, based on the bed pressure drop, may be useful in identifying both minimum fluidizing velocity, U_mf, and minimum bubbling velocity, U_mb. Plots of T^− 1 against superficial velocity exhibited significant change at velocities consistent with those where bed height first changed (onset of fluidization), the packed bed pressure drop underwent a transition from a monotonic linear function of superficial velocity to a steady value (onset of fluidization) and standard deviation of bed pressure drop rapidly increased (onset of bubbling). However, the suggestion that T might be a valid indicator of U_mf was supported by data for a single material only. In this paper additional data is presented that supports the suggestion that T might be useful in measuring U_mf, and also provides additional evidence for the potential utility of T in determining U_mb. In principle, this would allow a single campaign of pressure measurements to be used to identify both U_mf and U_mb.     

Behaviour of gas-fluidized beds of fine powders part I. Homogeneous expansion

The minimum bubbling velocity has been correlated from the results of experiments on 48 gas/solid systems and is a function of the density and viscosity of the fluidizing gas, the mean sieve size of the powder and the fraction of fines less than 45 μm. An improved equation is presented to predict U_mb/U_mf; this parameter also correlates well with the maximum non-bubbling bed expansion ratio H_mb/H_mf which is used to calculate the maximum dense phase voidage ?_mb. An equation based on the Carman-Kozeny theory can be used to predict bed voidages between incipient fluidization and bubbling. An accurate value of particle density is essential in these equations and a simple comparative method has been used to determine particle densities of fine porous powders.     

Bubble behavior in corrugated‐wall bubbling fluidized beds—Experiments and CFD simulations

A new concept to harness bubble dynamics in bubbling fluidization of Geldart D particles was proposed. Various geometrical declinations of a cold‐prototype corrugated‐wall bubbling fluidized bed were compared at different flow rates (U_g) to conventional flat‐wall fluidized bed using high‐speed digital image analysis. Hydrodynamic studies were carried out to appraise the effect of triangular‐shaped wall corrugation on incipient fluidization, bubble coalescence (size and frequency), bubble rise velocity, and pressure drop. Bubble size and rise velocity in corrugated‐wall beds were appreciably lower, at given U_g/U_mb, than in flat‐wall beds with equal flow cross‐sectional areas and initial bed heights. The decrease (increase) in size (frequency) of bubbles during their rise was sustained by their periodic breakups while protruding through the necks between corrugated plates. Euler‐Euler transient full three‐dimensional computational fluid dynamic simulations helped shape an understanding of the impact of corrugation geometry on lowering the minimum bubbling fluidization and improving gas distribution. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Fluidization of fine powders with air in the particulate and the bubbling regions

Several fractions of a silica—alumina catalyst and an impregnated catalyst were fluidized with air to check published bubble point correlations and to determine the expansion of the dense phase in the bubbling region. The minimum bubbling velocities were fairly close to those reported by Geldart and others, but U_mb varied with only the 0.6 power of the particle size. The dense phase expansion was significant even at velocities much above U_mb and corresponded to relative velocities up to 2 – 3 times U_mf.     

Fluidization with hot compressed water in micro-reactors

In this paper the concept of micro-fluidized beds is introduced. A cylindrical quartz reactor with an internal diameter of only 1 mm is used for process conditions up to and 244 bar. In this way, fast, safe, and inherently cheap experimentation is provided. The process that prompted the present work on miniaturization is gasification of biomass and waste streams in hot compressed water (SCWG). Therefore, water is used as fluidizing agent. Properties of the micro-fluid bed such as the minimum fluidization velocity (U_mf), the minimum bubbling velocity (U_mb), bed expansion, and identification of the fluidization regime are investigated by visual inspection. It is shown that the micro-fluid bed requires a minimum of twelve particles per reactor diameter in order to mimic homogeneous fluidization at large scale. It is not possible to create bubbling fluidization in the cylindrical micro-fluid beds used. Instead, slugging fluidization is observed for aggregative conditions. Conical shaped micro-reactors are proposed for improved simulation of the bubbling regime. Measured values of U_mf and U_mb are compared with predictions of dedicated 2D and 3D discrete particle models (DPM) and (semi)-empirical relations. The agreement between the measurements and the model predictions is good and the model supports the concept and development of micro-fluid beds.     

Determination of minimum fluidization velocity by pressure fluctuation measurement

Using the standard deviation of pressure fluctuations to find the minimum fluidization velocity, U_mf, avoids the need to de-fluidize the bed so U_mf, can be found for operational bubbling fluidized beds without disrupting the process provided only that the superficial velocity may be altered and that the bed remains in the bubbling fluidized state. This investigation has concentrated on two distinct aspects of the pressure fluctuation method for U_mf determination: (1) the minimum number of pressure measurements required to obtain reliable estimates of standard deviation has been identified as about 10000 and (2) pressure fluctuation measurements in the plenum below the gas distributor are suitable for U_mf determination so the problems of pressure probe clogging and erosion by bed particles may be avoided.     

Characteristics of fluidisation behaviour in a pressurised bubbling fluidised bed

The experiments were carried out in a bench‐scale fluidised bed of 90 mm in diameter to determine the influence of pressure on fluidisation characteristics of Geldart A and B particles over the range of pressure 0.1–4.5 MPa. For Geldart B particles, the results indicate that minimum fluidisation velocity (u_mf) was found to decrease with pressure whilst bed voidage at u_mf was unaffected, and the bed expansion height increase with pressure at fixed value of gas velocity was observed for both Geldart B and A particles. For Geldart A particles, minimum bubbling velocity (u_mb) bed voidage at u_mb and dense phase voidage were found to increase obviously with pressure, but a slight influence of pressure on u_mf was observed. The prediction values of high‐pressure fluidisation characteristics from the references' correlations developed at pressure were in agreement with the experimental data. © 2012 Canadian Society for Chemical Engineering     

Development of a tribolelectric procedure for the measurement of mixing and drying in a vibrated fluidized bed

Fluidization of fine, pharmaceutical powders makes them easier to dry, coat and mix. Fine powders, however, are difficult to fluidize well with gas flow only. Vibration can often help achieve smooth fluidization at a lower gas flow. The objective of the present study was, thus, to develop reliable and quick experimental methods to characterize mixing and drying in vibrated fluidized beds of fine powders.Effective mixing is critical in many industrial applications and, in gas-solid fluidized beds, requires gas velocities greater than the minimum bubbling velocity (U_mb). There are a number of techniques available for determining U_mb. However, they often are impossible or impractical to use in an industrial application. A new measurement technique involving the use of triboelectric probes was developed. Signal characteristics obtained from sophisticated signal analysis were used to identify the minimum bubbling velocities. These predictions corresponded well with the values obtained from more traditional laboratory methods such as the bed pressure gradient.In a fluidized bed, particles hitting a metal probe will generate a small triboelectric current. Triboelectric probes are able to detect rapid changes in particle surface properties. Surface properties of the particles were modified by wetting the particles in a low shear mixer. This change was detected by triboelectric probes at various locations inserted throughout the bed. The water adsorbed on the particles will begin to evaporate when exposed to the gas stream and the surface properties of the particles will gradually return to their original dry state. The triboelectric probes were able to monitor this drying process. The effects of vibration amplitude on the mixing and drying rate of the bed were also determined.     

The effects of particle and gas properties on the fluidization of Geldart A particles

We report on 3D computer simulations based on the soft-sphere discrete particle model (DPM) of Geldart A particles in a 3D gas-fluidized bed. The effects of particle and gas properties on the fluidization behavior of Geldart A particles are studied, with focus on the predictions of U_mf and U_mb, which are compared with the classical empirical correlations due to Abrahamsen and Geldart [1980. Powder Technology 26, 35-46]. It is found that the predicted minimum fluidization velocities are consistent with the correlation given by Abrahamsen and Geldart for all cases that we studied. The overshoot of the pressure drop near the minimum fluidization point is shown to be influenced by both particle-wall friction and the interparticle van der Waals forces. A qualitative agreement between the correlation and the simulation data for U_mb has been found for different particle-wall friction coefficients, interparticle van der Waals forces, particle densities, particle sizes, and gas densities. For fine particles with a diameter , a deviation has been found between the U_mb from simulation and the correlation. This may be due to the fact that the interparticle van der Waals forces are not incorporated in the simulations, where it is expected that they play an important role in this size range. The simulation results obtained for different gas viscosities, however, display a different trend when compared with the correlation. We found that with an increasing gas shear viscosity the U_mb experiences a minimum point near , while in the correlation the minimum bubbling velocity decreases monotonously for increasing μ_g.     

Transition to turbulent fluidization in a binary solids fluidized bed

This paper presents a study on the transition velocity from bubbling to turbulent fluidization in a binary solids fluidized bed. Experiments were carried out with two kinds of binary solids mixtures with FCC as fine particles and silica sands as coarse particles. The onset velocity to turbulent fluidization, U_c, determined by the measurement of pressure fluctuations, was found to increase with increasing the fraction of coarse/heavy solids. By introducing an equivalent particle diameter and an equivalent particle density, the results obtained in this study can properly be described by a general correlation of U_c proposed by Cai and co-workers (1989) for mono-density particles with relatively narrow size distribution.     

Effective particle diameters for simulating fluidization of non‐spherical particles: CFD‐DEM models vs. MRI measurements

Computational fluid dynamics—discrete element method (CFD‐DEM) simulations were conducted and compared with magnetic resonance imaging (MRI) measurements (Boyce, Rice, and Ozel et al., Phys Rev Fluids. 2016;1(7):074201) of gas and particle motion in a three‐dimensional cylindrical bubbling fluidized bed. Experimental particles had a kidney‐bean‐like shape, while particles were simulated as being spherical; to account for non‐sphericity, “effective” diameters were introduced to calculate drag and void fraction, such that the void fraction at minimum fluidization (ε_mf) and the minimum fluidization velocity (U_mf) in the simulations matched experimental values. With the use of effective diameters, similar bubbling patterns were seen in experiments and simulations, and the simulation predictions matched measurements of average gas and particle velocity in bubbling and emulsion regions low in the bed. Simulations which did not employ effective diameters were found to produce vastly different bubbling patterns when different drag laws were used. Both MRI results and CFD‐DEM simulations agreed with classic analytical theory for gas flow and bubble motion in bubbling fluidized beds. © 2017 American Institute of Chemical Engineers AIChE J, 63: 2555–2568, 2017     

Identification of the regime boundaries in bubble columns based on the degree of randomness in the signals†

A new parameter (degree of randomness (DR)) was defined for the identification of the main transition velocities, U _trans. The new method reconstructs the time series into multiple state vectors, thus generating non-overlapping vector pairs and then compares the distance between them with a pre-selected cut-off length. The DR values were extracted from gauge and differential pressure fluctuations as well as x-ray tomographic scans. At every U _trans value, the DR index exhibited a well-pronounced local minimum. Three cylindrical bubble columns (BCs) with various diameters (0.1, 0.14, and 0.45 m in ID) and one rectangular BC (width = 0.2 m, depth = 0.04 m) were used. They were aerated by means of different perforated plate gas distributors. It was found that in the cylindrical BCs the disintegration of the bubbly flow regime took place always at U _trans = 0.04 m/s. In the case of the rectangular BC the first critical velocity appeared at U _trans = 0.012 m/s. The lower boundary of the churn-turbulent regime was identified at U _trans = 0.11 m/s in the smallest cylindrical BC and at about U _trans = 0.095 m/s in the other two cylindrical BCs. In the case of the rectangular BC, the second critical velocity was identified at U _trans = 0.039 m/s. The low U _trans in the rectangular BC imply that the hydrodynamic regimes are less stable in this particular column due to higher degree of liquid turbulence. The calculated DR values from the gauge pressure fluctuations successfully distinguished the upper boundary of the gas maldistribution and the first transition sub-regime.     

Effects of pressure and type of gas on particle-particle interaction and the consequences for gas—solid fluidization behaviour

The fluidization behaviour of cracking catalyst has been studied up to pressures of 15 bar with different fluidization gases (Ar, N₂, H₂). A number of parameters of both the homogeneous and heterogeneous fluidized bed has been examined experimentally.The experimental results reveal that the minimum fluidization velocity (U_mf) is independent of the pressure. The bubble point velocity (U_bp) and the maximum bed expansion (H_bp) at this velocity increase with increasing pressure. This also holds for the dense phase voidage (ε_d) and the dense phase gas velocity (U_d) in the bubbling bed. The bubble size decreases drastically with increasing pressure. However, the above-mentioned parameters are also strongly dependent on the type of fluidization gas used.The cohesion constant of the powder was measured, using a tilting bed technique. The results reveal that the cohesion constant increases with increasing pressure. Analysis of the results of adsorption measurements of the different gases to the solid reveals for the adsorption as well as for the cohesion and for the beu expansion the same pressure dependence.It is believed that the gas adsorption influences the cohesion between the particles and hence the elasticity modulus introduced by Rietema and Mutsers [1,2]. The increasing elasticity modulus with increasing pressure also explains the increasing bed expansion with pressure.     

Behaviour of gas-fluidized beds of fine powders part II. Voidage of the dense phase in bubbling beds

The collapse rate technique has been used to evaluate the average dense phase properties in vigorously bubbling beds of fine powders. The results of experiments on 13 air/solid systems are used to correlate the average dense phase voidage, $\text{?}$_D, in terms of the physical properties of the gas and powder, and our predictive equations fit literature data. $\text{?}$_D increases as the particle density and mean particle size decrease, and as the fraction of fine <45 μm, gas viscosity and gas density increase.The gas velocity, U_D, through the dense phase of a bubbling fluidized bed has been calculated from $\text{?}$_D assuming Darcy's law and can now be predicted. Since it is less than the minimum bubbling velocity, the improved performance of fluidized bed reactors when the gas/solid properties are changed so as to increase $\text{?}$_D can be attributed more to smaller bubbles splitting and coalescing frequently rather than to the small amount of extra gas passing through the dense phase.     

Prediction of bubble behaviour in fluidised beds based on solid motion and flow structure

It has been demonstrated that the non-intrusive positron emission particle tracking (PEPT) could be a potential technique for observing bubble flow pattern, measuring bubble size and rise velocity in bubbling fluidised beds according to the solid motion in bubble and its wake. The results indicate that the behaviour of air bubbles varies greatly with the bed materials and superficial gas velocity. Three types of bubbling patterns (namely A, B and C) have been reported in this study, in which the pattern C is observed when the polyethylene fluidised bed is operated at the superficial gas velocity (U − U_mf) of 0.25–0.5 m/s and the ratio of bed height to bed diameter is unity. After the comparison of the results measured by the PEPT technique with the values calculated by using a number of empirical correlations, two modified correlations are recommended to calculate the bubble size based on the PEPT data.     

Transition velocities and phase holdups at minimum fluidization in gas–liquid–solid systems

Hydrodynamic experiments were performed using a 127‐mm diameter column with 3.2‐mm porous alumina, 3.3‐mm polymer blend, 5.5‐mm polystyrene and 6.0‐mm glass spheres, with water, aqueous glycerol solution and silicone oil as liquids, and air as the gas. The voidage at minimum fluidization fell initially to a minimum, then rose gradually with increasing superficial gas velocity, and was lower for three‐phase systems than for corresponding two‐phase (liquid–solid) fluidized beds. The compaction appears to be due to agitation by gas bubbles near the minimum liquid fluidization condition. The gas holdups agree reasonably well with the correlation of Yang et al. (1993). Curves of minimum liquid fluidization velocity, U_lmf, vs. superficial gas velocity, U_g always show U_lmf decreasing as U_g increases, initially in a concave‐downward manner, but sometimes concave‐upward.     

Modified gas‐perturbed liquid model (GPLM) for minimum fluidization velocity in a two‐phase inverse fluidized bed reactor with Newtonian and non‐Newtonian systems

In the present investigation minimum fluidization velocity, U_mf, in a two‐phase inverse fluidized bed reactor is determined using low‐density polyethylene and polypropylene particles of different diameters (4,6 and 8 mm) by measuring pressure drop. In a glycerol system U_mf decreased gradually with increase in viscosity up to a value of 6.11 mPa s (60%) and on further increase there was a slight increase in U_mf. In the case of the glycerol system the U_mf was found to be higher when compared to water. In the non‐Newtonian system (carboxymethylcellulose), U_mf decreased with increase in concentration in the range of the present study. The U_mf was found to be lower when compared to water as liquid phase. The modified gas‐perturbed liquid model was used to predict the minimum fluidization liquid velocity (U_lmf) for Newtonian and non‐Newtonian systems. Copyright © 2006 Society of Chemical Industry     

The flow of solid particles in an aerated inclined channel

The dynamic behavior of the flow of solid particles in an inclined open channel with air flow introduced through a porous base plate was examined. The solid particle velocity distributions were measured in detail using an optical probe.Five types of flow patterns were found depending on the air velocity and the slope of the channel. They were: sliding flow, immature sliding flow, splashing flow, bubbling flow, and gliding flow. The first two could be observed when the air velocity was less than the minimum fluidization velocity, U_mf, for the particles. The last two were observed when it became higher than U_mf. Splashing flow occurred when the slope of the channel was steep.