Fluidization Regime Research Articles

Systematic analysis of mixing and segregation patterns of binary mixtures in fluidised beds for multi-functional processes

Fluidized beds are increasingly used in renewable energy and chemical production due to their versatility in handling different solids for multi-functional industrial applications. The diversity in size and density of solid particles impacts fluidization, influencing mixing and segregation behaviours critical for optimizing chemical processes and reactor design. This study investigates the expansion and segregation behaviours of mixed Geldart group powders in binary systems, simulating polydispersed beds with different materials and catalysts. By applying a modified Cheung equation and an adapted Gibilaro-Rowe model, the study analyzes segregation behaviours of Geldart Group A and B materials at varying mixing rates and gas flow velocities. Results show a good match between experimental data and model predictions. Using novel non-invasive X-ray imaging, the study provides real-time analysis of mixing and segregation at different fluidization regimes and temperatures. These findings aid in designing and optimizing advanced thermochemical conversion technologies, enhancing process efficiency and resilience.

Experimental study of hydrodynamic characteristics in three-phase coarse particle fluidized flotation column

Understanding multiphase flow regimes and hydrodynamic characteristics in the three-phase coarse particle fluidized flotation column (TFC) is crucial in mineral processing. This study systematically investigated the influence of gas velocity (Ug) and water velocity (Uw) on the flow regimes in TFC, focusing on the identification and hydrodynamic characteristics of different flow regimes in the freeboard and bulk fluidized bed regions. Three distinct flow regimes (heterogeneous flow, pseudo-homogeneous flow and mono-homogeneous flow) and two transition water velocities (transitions from a heterogeneous flow regime to a pseudo-homogeneous flow (Ut1) and transitions from a pseudo-homogeneous flow to a mono-disperse homogeneous flow regime (Ut2)) were identified in the freeboard region based on the visual observation and bubble characteristic parameter acquisition system (BVW-2) monitoring the variations in the bubble size distribution. Visual observation combined with electrical resistance tomography (ERT) monitoring the solid concentration variations facilitated accurate determination of three flow regimes (fixed bed regime, agitated bed regime, and complete fluidization regime) and two transition points (minimum agitation water velocity (ULma) and minimum fluidization water velocity (ULmf)) in the bulk fluidized bed region. Furthermore, statistical and spectral methods were used to analyze pressure fluctuation signals and characterize fluidization behavior across different flow regimes, including the average pressure fluctuation (ΔP¯), the probability density function (PDF), normalized standard deviation (δ), kurtosis (K), skewness (S), autocorrelation function (ACF), power spectral density (PSD) and average cycle frequency (fc). The results indicated that it is the changes in dominant flow structures within the three-phase fluidized bed, such as variations in bubble behavior and particle movement under different Ug and Uw, that lead to differences in flow characteristics, thereby forming distinct flow regimes.

A Novel Liquid–Solid Fluidized Bed of Large-Scale Phase-Changing Sphere for Thermal Energy Storage

The storage of thermal energy has been hindered by the low heat-transfer rate of the solid phase of the phase-changing materiel. With water being the heat-transfer fluid as well as the liquid phase in the liquid–solid two-phase system, a novel type of fluidized bed is designed in this study. Numerous hollow spheres are fabricated with phase-changing materiel encapsulated. Adding the solid–liquid phase-change material capsules to the flowing fluid, the capsules are dispersed suspended in the carrier. The large spheres, 25 mm in present experiment, possess the merits of guaranteeing energy-storage density and tolerating internal interface chaotic motion. Both the fluidization status and phase-changing process are recorded by photography combined with image-processing technology. It is found that the large spheres, with density less than water, can be fluidized by the downward flowing fluid. As the flow rate increases, the expansion ratio of the solid phase increases and the regimes of incipient fluidization and bubbling fluidization can be observed. In comparison to the fixed bed, the oscillation of pressure drop across a fluidized bed is more severe, but the averaged value is less than the fixed bed. The melting and solidifying can be accelerated by 22.6% and 50%, respectively, thus proving the superiority of the fluidized bed in improving the heat-transfer rate while charging/discharging the thermal energy. Three types of basic movement of the spheres are shown to contribute to the enhanced phase-changing rate, which are shifting, colliding and rotating.

Managing Heat Transfer Intensity in a Fluidized Particle-in-Tube Solar Receiver

Particle flow structure and associated heat transfer coefficient are examined as a function of temperature in a single-tube fluidized bed solar receiver operating in upward particle flow mode. It is found that temperature has a strong effect on both fluidization regimes and wall-to-bed heat transfer coefficient that varies in the range 800-1200 W/(m2.K). Turbulent fluidization regime results in the most intense heat transfer between the irradiated wall and the fluidized particle.

Numerical Investigation into Gas-Solid Fluidized Bed Reactors for Activating the Iron-based Fischer-Tropsch Synthesis Catalyst

Abstract The present work conducted experimental and computational fluid dynamics simulation studies on the hydrodynamic characteristics of gas-solid fluidized bed reactors for activating the iron-based Fischer-Tropsch synthesis catalyst. In order to provide reliable guidance for the design and scale-up of the reactor, the present study applied experimental data from a pilot fluidized bed device to verify the accuracy of the drag force model used in the two-fluid model and further used the validated drag force model to simulate an industrial-scale fluidized bed activation reactor. It was found that the bubbling-EMMS drag model can accurately simulate the pilot-scale fluidized bed in the flow regimes ranging from bubbling to turbulent fluidization. Simulation studies on the industrial-scale fluidized bed activation reactor showed that the reactor was in a turbulent fluidization regime under the designed operating gas velocity, with high gas-solid dispersion efficiency, reasonable utilization of reactor space, and low gas-solid separation load.

Exploring the hydrodynamics of dense beds of Geldart B and D gas-fluidized particles through the analysis of capacitance probe signals

The hydrodynamics of Geldart B and D solids dense fluidized beds, of spherical/irregular morphology, have been characterized using capacitance probes at ambient temperature and 500 °C. A statistical approach applied to the time series of local bed voidage reveals, at specific experimental conditions, a characteristic bimodal distribution in the emulsion phase voidage: a more expanded, high-voidage, phase can be distinguished from a low-voidage phase with values close to minimum fluidization condition. Quantitative and qualitative differences among tested materials can be associated to different particles morphology. Emulsion phase voidages for the tested samples collapse on the same characteristic value once normalized with the voidages at incipient fluidization condition. The Richardson-Zaki equation proves to effectively correlate also in the bubbling fluidization regime the voidage and gas superficial velocity in the emulsion phase. The influence of hydrodynamics of fluidized bed dense phase on mass transfer in the emulsion phase is clearly highlighted.

A carbonator model for CO2 capture based on results from pilot tests. Part I: Hydrodynamics and reactor model

Carbonate looping (CaL) technology has the potential to efficiently capture CO2 from power plants and carbon-intensive industries such as cement production. Its feasibility has been demonstrated in numerous pilot campaigns, including tests validating advanced features such as indirectly heating the sorbent regenerator. However, some issues, such as the role of reactor hydrodynamics, need to be discussed more thoroughly to reliably scale up CaL plants for commercial operation. In this work, we present a novel carbonator model based on a systematic literature review and experimental data from pilot tests, including a rigorous analysis of carbonator fluidization regimes. We highlight that modeling assumptions commonly found in the literature can result in a significant overestimation of carbonator performance. Additionally, we offer guidelines for the appropriate selection of these assumptions. Our carbonator model provides an effective tool for the next scale-up step of CaL technology.

Machine learning analysis of pressure fluctuations in a gas-solid fluidized bed

Pressure fluctuations in a gas-solid fluidized bed involve much information of the dynamic system. To uncover the value and significance of the pressure fluctuations time series, two meaningful tasks, i.e., the fluidization regime classification and the future state prediction, are investigated using Machine Learning (ML) algorithms, including Back Propagation neural network (BP), Convolutional Neural Network (CNN), Long Short-Term Memory network (LSTM), Radial Basis Function network (RBF), Random Forest (RF) and Support Vector Machine (SVM). Findings indicate that the six ML methods rank their performance for the classification task from high to low as: BP ∼ RF > SVM > CNN ∼ LSTM ∼ RBF. For the one-step-ahead future state prediction, the accuracy goes from high to low as: BP ∼ RBF > CNN ∼ LSTM > SVM ∼ RF, whilst for the multistep-ahead prediction the LSTM approach shows the best performance. Additionally, the sampling frequency of pressure time series has significant effects on both tasks. Overall, this study demonstrates the capability of machine learning methods for the value analysis of nonlinear time series and the better operation of gas-solid fluidization systems.

Renewable methyl acetate production from dimethyl ether carbonylation in a fluidized bed reactor

The carbonylation of bio-based dimethyl ether (DME) using ferrierite offers a renewable approach to methyl acetate (MA) production with a potential reduction in carbon emissions. This work showcases the feasibility of employing fluidized-bed reactors in this process, leveraging benefits such as effective solid mixing, uniform temperature distribution, and facile catalyst purge and makeup. Through parametric studies, the impact of operating conditions on the reaction kinetics and reactor hydrodynamics was elucidated. Both physical and chemical catalyst deactivation were found to be dependent on the fluidization regime. Numerical models based on experimental results enabled the prediction of reaction yield and selectivity in the process simulation and optimization. Preliminary techno-economic analysis (TEA) and life cycle assessment (LCA) showed the potential for a 40% reduction in production costs and an 8% reduction in the global warming potential compared to the fossil-based benchmark. Overall, this study highlights the operation simplicity, cost competitiveness, and environmental benefit of MA production from DME carbonylation in a fluidized-bed reactor, providing insights for process scale-up and feedstock sourcing.

Pilot-scale investigation of high-temperature thermochemical energy storage based on the material system CaO/Ca(OH)2 in a bubbling fluidized bed

Utilizing fluidized bed technology for thermochemical energy storage with CaO/Ca(OH)2 is promising for temperatures from 400 °C to 600 °C in industrial-scale applications. Much progress has been made on laboratory scale to develop this technology in recent years. Continuously changing material properties throughout storage cycles, mainly due to particle breakage, are identified to be challenging for scale-up. Transferability to large-scale systems needs to be examined.Investigations on a pilot scale (di = 257 mm, h = 600 mm, steam for fluidization, 700 °C, 6 barg) are done. A temperature-induced (456 °C discharging and 586 °C charging) and a pressure-induced (1.5 barg discharging and 3.5 barg charging) cyclization is performed. The applicability of standard scaling rules (scaling factor of 17) is possible. Fluidization regimes and breakage rates are transferrable. A detailed experimental characterization of the storage materials fluidization properties that are crucial for scale-up is done using a fluidization test rig (di = 140 mm, h = 658 mm, nitrogen for fluidization, ambient temperature and pressure).

Influence of Temperature on Properties and Dynamics of Gas-Solid Flow in Fluidized-Particle Tubular Solar Receiver

A fluidized particle single-tube solar receiver has been tested for investigating the gas-particle characteristics that enable the best operating conditions in a commercial-scale plant. The principle of the solar receiver is to fluidize the particles in a vessel – the dispenser – in which the receiver tube is plunged. The particles are flowing upward in the tube, irradiated over 1-meter height, by applying an overpressure in the dispenser. Experiments with a concentrated solar flux varying between 188 and 358 kW/m² are carried out, and the particle mass flux varied from 0 to 72 kg/(m²s). The mean particles and external tube wall temperatures in the irradiated zone are heated from the ambient to respectively 700°C and 940°C. It is shown that the temperature rise leads to a decrease of the particle volume fraction. Furthermore, a self-regulation of the system is evidenced with a short transient regime. This characteristic is essential from the operational viewpoint. The thermal efficiency of the receiver increases with the particle flow rate, reaching between 60 and 75% above 30 kg/(m²s). Several fluidization regimes are identified thanks to pressure signal analyses, like slugging, turbulent and fast fluidization, showing that regimes transitions are strongly affected by the temperature.

Coupled kinetic and hydrodynamic model for a carbonator reactor of calcium looping process: Sulfur dioxide effect

The application of the calcium looping (CaL) process for CO2 capture has gained recognition as a highly efficient technology in mitigating greenhouse gases emission. In this study, by considering the characteristics of the adsorbent and the hydrodynamics of the fluidized bed in the fast fluidization regime, a fractal-like random pore kinetic model and a 1D hydrodynamic model were combined together. In order to validate the kinetic model, experimental carbonation data obtained from a lab-scale fluidized bed reactor under realistic operational condition and in the absence/presence of SO2 gas were used. The results demonstrated a satisfactory agreement between the model and experimental data, as indicated by the maximum absolute error of 1.23% and 1.78% in the absence and presence of SO2, respectively. Based on sensitivity analysis, the efficiency of the carbonator reactor was found to be predominantly affected by the solid inventory in the bed and the gas inlet velocity. Also, the results of this analysis indicate that in the presence of SO2, a higher make-up flow rate leads to a more pronounced enhancement in the efficiency of the carbonator reactor, as compared to the absence of SO2 in the reaction environment. Results of modelling indicated that the presence of SO2 necessitated a 36.5-fold increase in the minimum-required make-up flow to attain comparable efficiency obtained in the condition of the absence of SO2. Furthermore, the results of modelling as a function of number of cycles demonstrated a substantial decline in the efficiency of the carbonator reactor, which in the absence of SO2, the efficiency decreased from 68.13% to 18.81%, while in the presence of SO2, it decreased from 45.53% to 4.75% over 100 sequential cycles.

Experimental microwave assisted CO2 desorption of a solid sorbent in a fluidized bed reactor

In this study, a novel method has been applied to regenerate CO2 from a solid sorbent (Zeolite 13X) using a mono-mode microwave unit under fluidization conditions. Several parameters have been varied to investigate their effect on the desorption characteristics as well as on the energy consumption. While flow rate varied between 200 Nml/min to 1,000 Nml/min, regeneration temperature changed from 40 °C to 100 °C. Microwave initial forward power also varied from 15 W to 30 W.Volumetric heating of microwave led to desorption of CO2 from the sorbent effectively while fluidization enhanced the heat transfer within the sorbent by providing homogeneous heat transfer with lower desorption time and absorbed energy. The most dominant parameters on the desorption time and energy consumptions were found to be flow rate and regeneration temperature, respectively. Under different conditions, the energy consumptions to regenerate a kg CO2 varied between 2.84 MJ and 27.17 MJ. Even though the lowest energy consumption was obtained when the flow rate was 200 Nml/min at 40 °C; the time for a complete desorption found to be quite high with this flow rate. Therefore, a minimum energy consumption of 3.87 MJ/kg CO2 within a short period of regeneration time can be achieved under the fluidization regime with the flow rate of 1,000 Nml/min at 40 °C. While an increase in the flow rate also caused a slight increase in the energy consumption for low regeneration temperatures, it resulted in a significant decrease under higher regeneration temperatures. Furthermore, the main drawback of low flow rate conditions is that most of the energy was consumed to remove the last 10 % of CO2 from the sorbent. Therefore, fluidization plays a significant role on the removal of CO2, especially between the 90 % and 100 % regeneration percentages.This work is the first study in the literature that has investigated the effects of fluidization on the desorption characteristics of CO2 under mono-mode microwave conditions. It was proved that fluidization reduced the energy consumption of CO2 desorption with higher microwave efficiency.

Assessment of dynamic characteristics of fluidized beds via numerical simulations

Euler–Lagrange simulations coupled with the multiphase particle-in-cell (MP-PIC) approach for considering inter-particulate collisions have been performed to simulate a non-reacting fluidized bed at laboratory-scale. The objective of this work is to assess dynamic properties of the fluidized bed in terms of the specific kinetic energy of the bed material kS in J/kg and the bubble frequency fB in Hz, which represent suitable measures for the efficiency of the multiphase momentum exchange and the characteristic timescale of the fluidized bed system. The simulations have reproduced the bubbling fluidization regime observed in the experiments, and the calculated pressure drop Δp in Pa has shown a reasonably good agreement with measured data. While varying the bed inventory mS in kg and the superficial gas velocity uG in m/s, kS increases with uG due to the increased momentum of the gas flow, which leads to a reinforced gas-to-solid momentum transfer. In contrast, fB decreases with mS, which is attributed to the increased bed height hB in m at larger mS. An increased gas temperature TG from 20 to 500 °C has led to an increase in kS by approximately 50%, whereas Δp, hB, and fB are not sensitive to TG. This is due to the increased gas viscosity with TG, which results in an increased drag force exerted by the gas on the solid phase. While up-scaling the reactor to increase the bed inventory, bubble formation is enhanced significantly. This has led to an increased fB, whereas kS, hB, and Δp remain almost unchanged during the scale-up process. The results reveal that the general parameters such as hB and Δp are not sufficient for assessing the hydrodynamic behavior of a fluidized bed while varying the operating temperatures and up-scaling the reactor dimension. In these cases, the dynamic properties kS and fB can be used as more suitable parameters for characterizing the hydrodynamics of fluidized beds.

A Colourful Way to Unravel the Important Fluidization-Related Granule Size Effect on Semi-Continuous Drying.

Continuous twin screw wet granulation (TSWG) systems are possible pathways for oral solid dosage manufacturing in the pharmaceutical industry. TSWG requires a drying step after granulation before the tableting process. Typically, semi-continuous fluidized bed dryers (FBDs) are used for this purpose. At the same time, the pharmaceutical sector is interested in mathematical prediction models to save resources during the early drug product development (DPD) stage or to control manufacturing. Several authors have already developed prediction models for semi-continuous drying processes. However, these model structures reported systematic prediction offsets, which could be related to the incomplete implementation of fluidization and granule segregation phenomena. This study evaluates the complex fluidization behavior of wet granules in industrially relevant semi-continuous FBDs. A transparent perspex version of the dryer was used for the analysis of bed height, pressure drop, porosity, segregation, and spatial heating patterns at varying process settings. The investigated behaviors of the fluidizing bed will be helpful to derive phenomenological (sub)models for the detailed description of segregation in the semi-continuous fluidized bed system. In this study, it was found that semi-continuous FBDs are characterized by a change in fluidization regime from plug flow to a bubbling bed at the moment that the granule bed slumps. Secondly, the presence of size-based vertical segregation phenomena as well as spatial temperature differences were proven. The experimental results suggest that larger granules are dried under more intense drying conditions than smaller granules.

Copper-based chemical looping air separation process: Thermodynamics, kinetic modeling, and simulation of the fluidized beds

Chemical looping air separation is one of the potential processes for oxygen separation from air constituents using oxygen carriers with chemical looping oxygen uncoupling properties that are circulated between reduction and oxidation reactors. Copper-based oxygen carriers have been recognized as a suitable class of materials for the process due to their promising properties and low cost. In this study, copper-based material has been investigated for chemical looping air separation to identify the process parameters that affect the performance and oxygen generation. Firstly, thermodynamic analysis was performed to predict the suitable conditions for oxygen generation and oxygen carrier oxidation, and to model oxygen equilibrium partial pressure. Then, CuO (40 wt%)/ZrO2 was prepared by impregnation synthesis technique to examine the oxygen carrier kinetics using a thermogravimetric analyzer. Results showed that 1.5-dimensional Avrami-Erofeev model best fitted the reduction data with an activation energy of 230 ± 25.2 kJ/mol, whereas the oxidation data were fitted by 1.5 reaction order model with an activation energy of 211 ± 8.0 kJ/mol. The process was simulated using Aspen Plus software package, and the reactors were represented using Fluidbed units to account for both the kinetics and hydrodynamics, to identify the suitable fluidization regimes for specific operating conditions. Simulation results showed that the highest oxygen generation occurred while operating both reactors at bubbling fluidization regimes. Moreover, the temperature effects on the process performance and output were examined, and findings indicated that operating the reducer at 1000 °C and the oxidizer at 800 °C would give out the highest solids conversion and oxygen generation flowrate (23.26 ton/day) with specific energy consumption of 132.26 kWh/tonne of oxygen. However, operating the process at high temperatures would increase the energy penalty and the operating costs, and a more comprehensive technoeconomic study will be required to decide whether lower temperatures operation would be feasible.

Validation of mesoscience-based structural model for simulating gas–solid flows in circulating-turbulent fluidized beds

The circulating-turbulent fluidized bed (CTFB) is an important fluidization regime that can maintain a high solid concentration and high reaction intensity as in turbulent fluidized bed, and suppress solids back-mixing as in circulating fluidized bed. Computational fluid dynamics (CFD) studies on it are however very sparse. In this study, extensive three-dimensional CFD simulations of CTFB are carried out using the mesoscience-based structural model (Liu et al., 2023), where the gas-rich dilute phase and the particle-rich dense phase are defined as the two interpenetrating continua, instead of treating the gas and particles as the two interpenetrating continua as in classical two-fluid models. It is shown that the model is able to faithfully predict the hydrodynamics of CTFB, in terms of axial and radial solid concentration profiles and radial solid velocity profiles. This study not only provides a systematical CFD study on the hydrodynamics of gas–solid flow in CTFB but also offers validations of the mesoscience-based structural model via simulating the hydrodynamics of new fluidization regime (CTFB).

Heat transfer in a fluidized bed tubular solar receiver. On-sun experimental investigation

Using particles as heat transfer fluid in a solar receiver is an attractive way to increase the global efficiency of solar power plants. In the case of the particle-in-tube solar receiver concept, the fluidized particles circulate vertically in tubes thanks to a controlled pressure gradient and an additional air injection. Experiments are conducted with olivine particles in a one-tube mock-up at the 1 MW CNRS solar furnace (France). The tube of high aspect ratio (more than 3 m height over 48 mm internal diameter) is irradiated along a 1-m height with several solar flux configurations that correspond to realistic operation conditions of a solar thermal power plant. The novelties of this paper lie in extending the operating parameters compared to earlier studies and in linking the fluidization regimes to wall heat transfer. Slugging, turbulent fluidization and fast fluidization regimes are detected within the receiver tube. It is shown that the system can tolerate high solar flux densities, up to 800 kW/m2. The system proves to be highly controllable and flexible with respect to particle mass flux variation and transient operations. Particle temperature increase ranges from 100 to 650 °C depending on the particle mass flux. Maximum thermal efficiency and particle outlet temperature of respectively ∼ 60 % and 680 °C are obtained, proving that this technology can be combined with highly efficient thermodynamic cycles. Global wall-to-particle heat transfer coefficient varies strongly with particle mass flux. It reaches a quasi-plateau above 15 kg/(m2s) with a mean value of 1000 ± 200 W/(m2K) and peaks data up to 1500 W/(m2K). A dimensionless heat transfer index is derived to account for the effects of the particle mass flux on the experimental results. It highlights that both increasing the temperature and working in the turbulent fluidization regime result in the increase of the heat transfer between the tube walls and the particles.

Flow regimes and transitions in an ultra-high temperature gas-solid fluidized bed

Gas-solid fluidized bed reactors are widely used in industrial operations and thus have led to a significant amount of fundamental research over decades. For better design and operation of fluidized bed systems, it is essential to understand the fluidization regime and the related transition conditions. Dr. John R. Grace, a prominent figure and pioneer in fluidization science and technology, has made significant contributions to understanding fluidized bed systems, including fluidization regimes and their transitions. This study is a tribute to his extraordinary legacy, focusing on investigations of fluidization regimes and transitions in a laboratory-scale gas-fluidized bed operating from ambient to 1600 °C. The results indicate that the fluidization regime transitions follow three distinct pathways across different temperature ranges: below 300 °C, from 300 to 1400 °C, and above 1400 °C. These distinctive transitions arise as a result of the varying significances of hydrodynamic and interparticle forces in the fluidized bed of particles investigated.

Extending homogeneous fluidization flow regime of Geldart-A particles by exerting axial uniform and steady magnetic field

The homogeneous/particulate fluidization flow regime is particularly suitable for handling the various gas–solid contact processes encountered in the chemical and energy industry. This work aimed to extend such a regime of Geldart-A particles by exerting the axial uniform and steady magnetic field. Under the action of the magnetic field, the overall homogeneous fluidization regime of Geldart-A magnetizable particles became composed of two parts: inherent homogeneous fluidization and newly-created magnetic stabilization. Since the former remained almost unchanged whereas the latter became broader as the magnetic field intensity increased, the overall homogeneous fluidization regime could be extended remarkably. As for Geldart-A nonmagnetizable particles, certain amount of magnetizable particles had to be premixed to transmit the magnetic stabilization. Among others, the mere addition of magnetizable particles could broaden the homogeneous fluidization regime. The added content of magnetizable particles had an optimal value with smaller/lighter ones working better. The added magnetizable particles might raise the ratio between the interparticle force and the particle gravity. After the magnetic field was exerted, the homogeneous fluidization regime was further expanded due to the formation of magnetic stabilization flow regime. The more the added magnetizable particles, the better the magnetic performance and the broader the overall homogeneous fluidization regime. Smaller/lighter magnetizable particles were preferred to maximize the magnetic performance and extend the overall homogeneous fluidization regime. This phenomenon could be ascribed to that the added magnetizable particles themselves became more Geldart-A than -B type as their density or size decreased.