Superficial Gas Velocity Research Articles

Effects of superficial gas velocity on the performance of an air-lift internal circulation partial nitrification-anammox granular sludge reactor

The airlift internal circulation reactor for partial nitrification-anammox (PNA-ALR) has the advantages of a small footprint, high mass transfer efficiency, and the ease of formation of granular sludge, thus making it an effective biological treatment for ammonia-containing wastewater. Although superficial gas velocity (SGV) is an essential parameter for PNA-ALR, it is unclear how the magnitude of SGV impacts nitrogen removal performance. In this study, the nitrogen removal efficiencies of five PNA-ALRs with different SGV were measured during feeding with synthetic municipal wastewater. At an optimal SGV of 2.35 cm s−1, the PNA-ALR consistently maintained the total inorganic nitrogen (TIN) removal efficiency at 76.31% and the effluent TIN concentration was less than 10 mg L−1. By increasing or decreasing the SGV, the nitrogen removal efficiency decreased to a range between 30% and 50%. At lower SGV, the dead space in the PNA-ALR was increased by 21.15%, and the feast/famine ratio of sludge increased to greater than 0.5, which caused a disruption in the structure, and a large loss of, granular sludge. Computational fluid dynamics (CFD) simulations showed operation at a higher SGV, resulting in excessive shear stress of 3.25 N m−2 being generated from bubble rupture in the degassing section. Fluorescent staining determined a decrease of 26.5% in viable bacteria. These results have improved our understanding of the effects of SGV on a PNA-ALR during mainstream wastewater treatment.

Hydrodynamics and gas-liquid mass transfer in an oscillatory flow reactor: Influence of liquid properties

Liquid properties, such as, surface tension and viscosity are important parameters as they control gas-liquid mass transfer in bioprocesses. An oscillatory flow reactor provided with smooth periodic constrictions (OFR-SPC) was considered to evaluate its potential for mass transfer performance in non-pure gas-liquid systems. The effect of surface tension and viscosity on the volumetric (kLa) and liquid-side mass transfer coefficients (kL), interfacial area, (a), gas holdup (εG) and bubbles’ dynamics were investigated under different operational conditions (oscillation amplitude (x0) and frequency (f) and superficial gas velocity (ug)). Two liquid phases, ethanol and sucrose aqueous solutions covering a range of surface tension and viscosity values were used. For the bubble size distribution (BSD) measurements an image analysis technique was used. A Design of Experiment (DoE) methodology was implemented in this work to establish the relation of x0, f, ug, surface tension and viscosity with kLa. According to the results, changes in the liquid properties and operational conditions showed marked effects on bubble’s size and mass transfer. However, surface tension and viscosity had no significant influence on εG, contrary to the reported for common contactors, where εG increased in the presence of ethanol and decreased at moderate/high viscosities. Moreover, it was found that increasing the oscillatory movement notably improved kL, and kLa (2–6-fold), either in ethanol or sucrose solutions, compared to common reactors, even with moderate power consumption (∼105 W m−3). This improvement resulted from the bubbles’ breakage, which originates bubbles with small and approximately the same size (homogeneous regime) enhancing a, instead, lower oscillations resulted in large bubbles (heterogeneous regime). The results demonstrate that the OFR-SPC can ensure outstanding mass transfer rates, with potential and feasibility for use in gas-liquid bioprocesses, where mass transfer and liquid properties are important.

Experimental and numerical study on characteristics and mechanism of particles attrition in fluidized bed

Particle attrition is commonly existed in gas-solid two-phase flow process. A novel CFD-DEM particle attrition model is developed, a series of experiments/simulation studies are performed to quantitatively investigate the attrition mechanism of particles in fluidized beds. The particle attrition process could generally be divided into unstable attrition stage and stable attrition stage, while the results show that unsteady attrition stage should be subdivided into high-speed attrition phase and low-speed attrition phase. Both wear and fragmentation become more intense with increasing height-diameter ratio and superficial gas velocity. The wear ratio decreases with increasing particle size, while the fragmentation ratio firstly decreases and then increases. Particle attrition mechanism also presents spatial ununiformity, particles reaching the bottom are worn most significantly, while the proportion of fragmentation based on this study for the sidewall areas, the bottom area and the center area is about 69%, 22% and 9% respectively.

The fusion of deep learning and acoustic emission response methods for identifying solid particles in annular multiphase flows

Annular flows carrying sand are common flow patterns in high-production gas-bearing wells. The real-time monitoring of sand particle information in the annular flows of wellheads is critical for efficient commercial production. In this study, an experiment was designed to monitor sand production in annular multiphase flows, and methods were proposed to identify sand using empirical mode decomposition (EMD), the Hilbert–Huang transform (HHT), statistical analysis, and deep learning methods. Corresponding sand migration behaviours near pipe walls were observed by acoustic emission (AE); the behaviours included sand carried by the gas core (IMF1), forward liquid film (IMF2) and reverse liquid film (IMF3). Furthermore, relationships between the AE response and gas superficial velocity (14–18 m/s), liquid superficial velocity (0.00366–0.01351 m/s), and mean particle size (150–380 μm) were proposed, and the AE responses of different sand migration patterns were verified. Finally, CNN, LSTM, and CNN-LSTM deep learning models were constructed to identify particle sizes based on the optimized sand-carrying information. The accuracy of the CNN-LSTM model was 6.44% and 18.9% higher than that of the CNN model and the LSTM model, respectively, which significantly improved the accuracy of particle size identification in annular particle flows. Therefore, this research provides an efficient method for the intelligent identification of sand in multiphase annular flows.

Slug flow characteristics of air-liquid two-phase flow in horizontal pipes over a wide range of liquid viscosities

A large number of studies have been conducted to understand the slug flow characteristics of air-liquid two-phase flow in horizontal pipes and to capture parametric trends of critical flow parameters associated with slug flow, such as the liquid slug length, elongated bubble length, and elongated bubble velocity. Based on these findings, many attempts have been made to develop empirical correlations to predict the slug flow parameters. However, the validity of these correlations is limited to a specific working fluid and narrow ranges of geometrical or operating conditions, thereby reducing their effectiveness as useful general predictive tools. This limitation has created a need to develop empirical correlations that offer high accuracy over a broad application range, i.e., a wide range of liquid viscosities, superficial liquid and gas velocities, and pipe diameters. In this study, two-phase slug flow experiments were performed to examine the hydrodynamic features of slug flow under two different pipe diameters (2 and 4 cm) and two different liquid temperatures (25 and 45°C). In addition, a new consolidated database consisting of 1563 slug flow data points was amassed from 16 sources, including the present experimental data, covering a wide range of operating conditions and fluid properties. In particular, this study includes various liquids with a wide range of viscosities, such as heavy oil, light oil, and water. The parametric study of air-liquid slug flow at various viscosity levels revealed that as the mixture velocity increases, the liquid slug length tends to decrease when the liquid viscosity is high and increase when the liquid viscosity is low. On the other hand, the elongated bubble length and elongated bubble velocity increase with increasing superficial gas velocity, regardless of the liquid viscosity. Based on these findings, three empirical correlations were developed to predict the liquid slug length, elongated bubble length, and elongated bubble velocity. These correlations provide excellent predictive capability against the consolidated database, with overall MAE values of 18.1%, 20.6%, and 9%, respectively.

Modeling and experimental investigation for development of Combined Electrolysis and Catalytic Exchange process for hydrogen isotope separation

A process based on the Combined Electrolysis and Catalytic Exchange (CECE) has been developed for separation of hydrogen isotopes. Indigenously developed Platinum supported on Carbon Aerogel based hydrophobic catalyst was used for hydrogen-water isotope exchange reaction combined with hydrophilic packing in a catalytic exchange column. The exchange column has been studied in detail for various operating conditions in this study. The intrinsic kinetics for the catalytic reaction was determined through spinning basket reactor experiments, and activation energy was estimated to be 29.7 kJ/mol. The overall mass transfer co-efficient in a mixture of hydrophilic packing and hydrophobic catalyst packed bed was determined experimentally in a pilot scale counter current trickle bed reactor to be in the range of 0.15–0.25 s-1 for the proposed range of operating conditions. Individual mass transfer resistances for vapour-liquid mass transfer, external gas mass transfer and gas-solid mass transfer were estimated using widely accepted empirical correlations. Under benchmark operating conditions, the Damkohler number for the exchange column was found to be between 0.10 and 0.16, indicating a significant contribution of surface reaction to overall mass transfer resistance. The vapour liquid mass transfer was found to be fast compared to the gas-vapour reaction. Therefore, increasing hydrophobic catalyst proportion relative to hydrophilic packing was found to increase the overall mass transfer. A steady state process model has been developed to predict the performance of overall exchange process using the experimentally determined parameters. Validation of the process model was carried out using the experimental data generated on the pilot scale trickle bed reactor. Optimum operating conditions of temperature, hydrophobic catalyst to hydrophilic packing ratio, gas and liquid superficial velocities were determined.

Gas Holdup and Flow Regimes in a Thin‐Gap Bubble Column with Newtonian and Non‐Newtonian Liquid Phase

AbstractThe influence of confinement and rheological properties of the liquid phase on the gas holdup and flow regimes in a 2 mm gap bubble column intended as an intensified photobioreactor at high cell concentration is investigated. The results indicate that at a given superficial gas velocity (UGsup), the gas holdup is higher with viscous or non‐Newtonian (nN) fluids than in water and also in a thin‐gap column than in an unconfined one. This behavior increases when confinement ratio increases. Regime transitions occur at lower UGsup for nN fluids compared to water and for a thin‐gap column compared to classical columns. This information constitutes a first step toward the identification of optimal operating conditions for efficient operation of microalgae culture.

An experimental study on interfacial parameters in a narrow rectangular channel under different flow regimes

This study experimented with air-water up-flow in a 65 mm × 2 mm narrow rectangular duct. Forty-two flow conditions were acquired with the ⟨jg⟩: 0.47–8.58 m/s, the ⟨jf⟩: 0.1–1.5 m/s, involving the void fraction: 2.75%–85.7%. The high-speed camera was located at z/DH = 154.6, 309.2, and 463.8. For calculating the void fraction, interfacial area concentration (IAC), and bubble number density (BND) under various cases, an image processing scheme was proposed combining the non-overlapping characteristic of bubbles in the narrow gap. There are good consistencies between the void fraction of all conditions, the IAC of bubbly flow and the models, respectively. Group-1 and Group-2 parameters' evolution laws were discussed under the mechanisms of breakup, coalescence and pressure drop. Gas phase expansion was constantly influencing the parameters. The Group-1 bubbles’ substantial coalescence will decrease the total void fraction under the experimental cases located at the transition line of bubbly to slug or churn flow. Furthermore, the change of IAC influenced by the bubble breakup and coalescence was more remarkable than that of the void fraction, which the BND showed. Moreover, the change of IAC with the gas or liquid rate was not invariable under different superficial gas or liquid velocity, which was concerned with the transition mechanism of flow patterns. The database obtained will be helpful for the verification of the current interfacial area transport equation (IATE), especially for a higher void fraction.

Effect of louver baffle on the stability and separation performance of the gas-solid separation fluidized bed

Bed density stability is often the dominant factor restricting the efficiency of coal beneficiation in gas-solid separation fluidized beds (GSSFB). Here, in order to improve the fluidization quality, a louver baffle was introduced into a conventional GSSFB, converting it into a so-called louver baffle separation fluidized bed (LBSFB). The influence of the louver baffle on the fluidization characteristics and coal separation efficiency of the GSSFB were investigated using a combination of experiments and numerical simulations. The minimum fluidized gas velocity, pressure drop, and cross section density of the bed were measured when baffle was placed at two different positions (H=40 mm and H=80 mm). Meanwhile, the evolution of bubble generation and particle flow were simulated when the baffle was placed at H= 40 mm and the operating gas velocity was 7.52 cm/s. The results showed that the use of a single guide baffle could effectively break up the bubbles in the bed, inhibit bubble growth and particle backmixing, and restore the uniform distribution of the updraft. When the louver baffle was installed at a height of H = 40 mm and the superficial gas velocity was 7.52 cm/s, the standard deviation of radial density fluctuation in the bed was reduced to 0.028 g/cm3, which is more than 35% lower than that of the conventional GSSFB, and that of the axial density fluctuation was reduced to 0.146 g/cm3. Under these conditions, the separation of 6–13 mm raw coal in a LBSFB resulted in a clean coal yield of 69.97% and a probable error, E, value of 0.082 g/cm3

Analysis of dry reforming of methane under different fluidization regimes using a multiphase particle-in-cell approach

In the present study, the dry reforming of methane (DRM) has been simulated in fluidized-bed reactors using the multiphase particle-in-cell model. The model was meticulously built to investigate the effect of a wide range of superficial gas velocities covering particulate, aggregative, and lean-phase flow regimes on bed hydrodynamics, conversion, and yields of product gases. Constant values for catalyst loading, CH4:CO2:N2 ratio (1:1:1.3), and catalyst and gas properties were maintained in all simulations. The simulation results obtained are in good agreement with the experimental data reported in the literature. The results show that under different gas velocities, conversion is relatively indiscernible in the particulate regime. In contrast, for the inhomogeneous phases, the turbulent-fluidized bed had the best reactor performance with high CH4 and CO2 conversion rates, good CO + H2 productivity, and high CO/H2 molar ratio. This is due to the vigorous turbulent flow and relatively high gas–solid contact. Due to gas bypassing and backmixing triggered by bubbling, the bubbling-fluidized bed generally had the worst performance and below that of the fast-fluidized bed. The present study demonstrates that the performance of DRM reactions in fluidized-bed reactors is strongly related to the hydrodynamics. Moreover, it shows the significance of gas velocity on DRM conversion, yield, and overall reactor performance.

Bubbles in biomass fluidized bed gasifiers: A phenomenological probabilistic predictive model

Bubble properties in fluidized beds including size, velocity, and path, vary randomly. This article proposes a new PPPM (Phenomenological Probabilistic Predictive Model) l, that allows one to establish a vrb-BAC prediction band, to describe bubble data, obtained in a lab scale biomass-sand fluidized bed, by using CREC Optiprobes. The PPPM, represents a significant improvement from an earlier Probabilistic Predictive Model (PPM) reported by Torres and de Lasa in 2021, with the new PPPM being based on a balance of forces including gravity, drag, buoyancy, and minimum fluidization excess velocity. The resulting PPPM bubble rise velocity (vrb) is obtained as a function of the bubble axial chord (BAC) and is specifically evaluated by studying bubbles in a high-density sand (above 2500 kg/m3) fluidized bed. The PPPM leads to a behavioral band showing a normal bubble frequency distribution that becomes increasingly skewed as the gas superficial velocity increases. The proposed PPPM is shown to be applicable to single bubbles in a sand bed at incipient fluidization and to multiple bubbles in a biomass-sand bubbling bed. The PPPM is validated in the present study, by comparing it against 46,000 bubbles, under different gas superficial velocities and biomass loadings. By using the PPPM, it is shown 91% of all bubbles studied fall within the PPPM prediction band. Given the demonstrated ability of the PPPM, it is anticipated that this model will significantly contribute to the development of large-scale multiphase biomass gasifier models.

Drift-flux model for upward dispersed two-phase flows in vertical medium-to-large round tubes

The void fraction prediction is critical in nuclear safety analyses. Available one-dimensional computational codes heavily depend on the two-fluid model. The interfacial drag force in the momentum equation of the two-fluid model is currently formulated using the drift-flux model. Thus, the drift-flux model is critical for advanced two-phase flow analysis computational codes. The drift-flux model is characterized by two essential parameters: the distribution parameter and the drift velocity. The distribution parameter expresses the covariance of the area-averaged product of a void fraction and mixture volumetric flux. Flow channel geometry is a significant factor affecting the distribution parameter. The drift velocity expresses the relative velocity between gas and liquid phases. A bubble shape regime affects the drift velocity. The bubble shape regime is susceptible to flow regime dependence on the bubble Reynolds number. Bubbles in a spherical or distorted particle regime (group-1 bubbles) dominate in bubbly flow at a low void fraction, whereas bubbles in a slug or cap bubble regime (group-2 bubbles) play a critical role in beyond-bubbly flow. Whether slug bubbles or cap bubbles appear depends on the channel size. Thus, flow channel size significantly affects the drift velocity. The drift velocity dynamically changes along the two-phase flow, transforming from bubbly to beyond-bubbly flows.The present study first formulates the evolution process of the drift velocity using the two-bubble-group approach. A two-group-based formulation is simplified to model the drift velocity behavior at the bubbly-to-beyond-bubbly flow transition. A scheme is proposed to explicitly calculate the distribution parameter and drift velocity using operating parameters. A drift-flux correlation with the new drift velocity model is evaluated by 750 data collected from 8 different sources. Tested fluid systems are nitrogen-water, air-water, and steam-water systems. Flow channel diameter ranges from 0.0508 to 0.305 m. Test section height (L)-to-channel diameter (D) ratio L/D ranges from 9.41 to 130. Superficial gas velocity is changed from 0.0100 to 11.2 m/s, whereas superficial liquid velocity is changed from 0 (pool condition) to 2.6 m/s. Operating pressure covers from 0.1 to 4.6 MPa. A systematic evaluation demonstrates the validity of the newly developed drift velocity model for a wide range of test conditions. This model is simple enough to be implemented into existing one-dimensional computational codes without any substantial code architecture change.

Kinetic analysis on CO2 sequestration from flue gas through direct aqueous mineral carbonation of circulating fluidized bed combustion fly ash

In this work, the suitability of fly ash from 600 MW circulating fluidized bed boiler for CO2 sequestration through direct aqueous route was evaluated. The circulating fluidized bed combustion fly ash exhibited favorable carbon capture characteristics with a maximum CO2 sequestration capacity of 0.128 g CO2/g CFA and a corresponding carbonation efficiency of 78.17 %. A modified shrinking core model incorporated gaseous CO2 dissolution and time-dependent effective coefficient of internal diffusion was proposed to determine the gas–liquid-solid carbonation kinetics. The key kinetic parameters were assessed as a function of various carbonation conditions including reaction temperature, solid–liquid (S/L) ratio, energy input and superficial gas velocity. The mass diffusion through the liquid film near the gas–liquid interface was the rate-controlling step during the initial period of time, while diffusion through the expanded passivation layer constituted the controlling mechanism at the final stage. The model provides a more accurate representation of the gas–liquid-solid carbonation process and can be generalized to describe the carbonation of other solid wastes of interest.

CFD simulations to study bed characteristics in gas–Solid fluidized beds with binary mixtures of Geldart-B particles: A qualitative analysis

The bed dynamics of unary and binary fluidized beds play a key role in understanding the pressure drop and hence provides an opportunity for performance improvement of the beds. In the present work, characteristics of fluidized beds with binary mixtures of Geldart-B particles were investigated using CFD simulations. The phenomena of segregation and mixing using simulations were studied, both qualitatively and quantitatively, at a range of superficial gas velocities (0.3–0.6 m/s) and two different bed heights. The study was divided into two parts. In Part I, the current study, a qualitative analysis of flow patterns for seven different binary mixtures, is presented. The quantitative analysis, including particle and gas velocity profiles, particle volume fraction profiles, and correlations for minimum fluidization velocity and pressure drop, will be presented in Part II of this work. A mathematical model consisting of an Eulerian-Eulerian model with RNG k-ε model and KTGF model to capture the bubble dynamics was used. The standardized values of coefficients and plastic stresses have been used for all simulations. The CFD model was validated using experimental data from the literature. Qualitative predictions of volume fraction profiles of small-sized particles showed that, for mixtures within a range of 40%–60% Geldart-B type large particles, the bubble and solid particle dynamics were different from those of single particles of the superficial gas velocities considered. In contrast to the single particles in the given superficial gas velocity range that were in bubbling regime, the binary particles showed a transition from bubbling to slugging to turbulent regime, as demonstrated by qualitative analysis. A homogeneous regime was observed for lower superficial gas velocities for mixtures consisting of 0%–20% large particles.

Experiment and Modeling of Interfacial Area Concentration of Liquid Film in Upward Annular Two-Phase Flow

To accurately quantify the interfacial transfer terms in the two-fluid model, the reliable prediction of the interfacial area concentration (IAC) is crucial. The IAC in annular flow, especially the interface between the liquid film and gas core, is particularly important due to its relevance to critical heat flux and reactor operation safety. However, very few experimental and analytical studies have been performed that focus on the IAC of the liquid film in annular flow. In this work, the IAC of the liquid film is measured using a parallel-wire conductance probe for upward annular flow in a 25.4-mm one-dimensional pipe. A total of 25 flow conditions are measured with the range of superficial liquid velocity from 0.15 to 2.00 m/s and the range of superficial gas velocity from 10.0 to 29.6 m/s. The IAC radial profile is obtained from the liquid film time trace measured by the conductance probe, and the accuracy of this method is verified by flow visualization. The effects of the inlet gas and liquid flow rates on the characteristics of the IAC radial distribution as well as area-averaged IACs are analyzed. A new model is developed to predict the IAC radial distribution of the liquid film. The IAC profiles predicted by the model agree very well with the measured IAC profiles for typical annular flow conditions and have a reasonable agreement for the wispy annular flow conditions.

The effect of Alcanivorax borkumensis SK2, a hydrocarbon-metabolising organism, on gas holdup in a 4-phase bubble column bioprocess.

To design bioprocesses utilising hydrocarbon-metabolising organisms (HMO) as biocatalysts, the effect of the organism on the hydrodynamics of bubble column reactor (BCR), such as gas holdup, needs to be investigated. Therefore, this study investigates the first use of an HMO, Alcanivorax borkumensis SK2, as a solid phase in the operation and hydrodynamics of a BCR. The study investigated the gas holdup in 3-phase and 4-phase systems in a BCR under ranges of superficial gas velocities (UG) from 1 to 3cm/s, hydrocarbon (chain length C13-21) concentrations (HC) of 0, 5, and 10% v/v and microbial concentrations (MC) of 0, 0.35, 0.6g/l. The results indicated that UG was the most significant parameter, as gas holdup increases linearly with increasing UG from 1 to 3cm/s. Furthermore, the addition of hydrocarbons into the air-deionized water -SK2 system showed the highest increase in the gas holdup, particularly at high UG (above 2cm/s). The solids (yeast, cornflour, and SK2) phases had differing effects on gas holdup, potentially due to the difference in surface activity. In this work, SK2 addition caused a reduction in the fluid surface tension in the bioprocess which therefore resulted in an increase in the gas holdup in BCR. This work builds upon previous investigations in optimising the hydrodynamics for bubble column hydrocarbon bioprocesses for the application of alkane bioactivation.

CFD‐PBM simulation of the column flotation unit of FCMC: Importance of gas–liquid interphase forces models

AbstractThe cyclonic micro‐bubble flotation column (FCMC) is an efficient flotation device for the separation of fine minerals, but its mechanisms are rarely studied using computational fluid dynamics (CFD). This paper reports the air–water two‐phase computational fluid dynamics‐population balance model (CFD‐PBM) simulations for the column flotation unit of an FCMC. The shear stress transport (SST) k‐ω model with curvature correction (CC) is used to simulate turbulence effects. Then, the interphase forces models considering bubble size distribution are selected according to the experimental data in a bubble column, which is in analogy to the column flotation unit of the FCMC. Finally, the optimal combination of interphase forces models (i.e., the Ishii–Zuber drag force model, the Hosokawa–Frank wall lubrication force model, and the Lopez de Bertodano turbulent dispersion force model) is applied to simulate an FCMC with a superficial gas velocity of 0.0144 m/s. The results show that the CFD‐PBM simulation can achieve a relative error of 9.09% for gas volume fraction and −5.45% for bubble rising velocity, indicating the reliability of the selected combination of interphase forces models.

Study on Artificial Neural Network for Predicting Gas-Liquid Two-Phase Pressure Drop in Pipeline-Riser System

The pressure drop for air-water two-phase flow in pipeline systems with S-shaped and vertical risers at various inclinations (−1°, −2°, −4°, −5° and −7° from horizontal) was predicted using an artificial neural network (ANN). In the designing of the ANN model, the superficial velocity of gas and liquid as well as the inclination of the downcomer were used as input variables, while pressure drop values of two-phase flows were determined as the output. An ANN network with a hidden layer containing 14 neurons was developed based on a trial-and-error method. A sigmoid function was chosen as the transfer function for the hidden layer, while a linear function was used in the output layer. The Levenberg-Marquardt algorithm was used for the training of the model. A total of 415 experimental data points reported in the literature were collected and used for the creation of the networks. The statistical results showed that the proposed network is capable of calculating the experimental pressure drop dataset with low average absolute percent error (AAPE) of 3.35% and high determination coefficient (R2) of 0.995.

Hydrodynamics and mass transfer around active particles in dense gas-fluidized beds

The hydrodynamics of bubbling fluidized beds of Geldart B granular solids operated at ambient temperature and at 500 °C are investigated by uncooled capacitance probes at different gas superficial velocities. Statistical analysis of the time-series of the local bed voidage generates Probability Density Functions (PDF) that display multimodal patterns. A remarkable bimodal character is apparent in the part of the PDF that refers to the emulsion phase when the gas superficial velocity is increased. It is inferred that two phases with different porosity co-exist in the emulsion phase: a lower voidage LV-phase and a higher voidage HV-phase. The link between the LV- and HV-phases as assessed from PDF of capacitance signals and the actual flow patterns induced in the emulsion phase by the bubbles is open to scrutiny.Along a different path, the influence of the bed hydrodynamics and expansion patterns of the emulsion phase on the gas mass transfer around a freely moving coarse active particle is investigated. To this end, mass transfer-limited reaction experiments are performed, consisting in catalytic oxidation of carbon monoxide at 450 °C over Pt-loaded γ-alumina spheres immersed in the fluidized bed. The mass transfer coefficient around the freely moving active particles is measured and worked out to calculate the particle Sherwood (Sh) number. Results obtained from reaction experiments confirm the close relationship between mass transfer and expansion patterns of the emulsion phase, and the need for proper hydrodynamic characterization of the bed. A Frӧssling-type equation, Sh=2·∊e+K·(Ree/∊e)1/2·Sc1/3, well correlates the results. Sh increases with fluidization gas velocity and a best fitting is obtained using the bed voidage of the HV-region of the emulsion phase in the correlation.

Effects of operating parameters on the performance of moving bed filter

High-temperature gas purification technique is of great significance for environmental protection. Moving bed filtration is considered a promising technique. In this study, the moving granular bed filter with the gas-solid counter flow was proposed. The pressure drop and the collection efficiency were investigated with respect to the superficial gas velocity, the mass flow rate of the filtration granules and the dust inlet concentrations. In addition, the dust concentrations at different bed heights were measured with isokinetic sampling. The experimental results indicate that an increasing filter gas velocity or a decreasing mass flow rate of the granule leads to a decrease in dust collection efficiency. An empirical equation was given by combining the single particle filtration analysis with the measured dust concentration distribution along the vertical height of the filter to calculate the moving bed filtration efficiency.