Solids Holdup Research Articles

Influence of high-speed jet on the particle distributions in downer

The large-scale cold model experiments are carried out to investigate the influence of high-speed jets on the distribution of particles in downer. The cases of both downward jets (co-current contact) and upward jets (counter-current contact) are studied. Generally, it is found that the solid holdups in the initial mixing zone of feed with particles are higher under the influence of counter-current jets, which could form the local “thickening” region in downer. To make a quantitative comparison, the radial non-uniformity index of solid holdups RNI(εs) in the jet mixing zone is calculated. Results show that the solid holdups in the counter-current jet mixing zone are more uniformly distributed in radial and the axial jet influence zone is shorter. By changing the jet gas velocity, its effects on the particle distributions are analyzed. Finally, based on experimental data, the distributions of solid holdups in co-current and counter-current jet influence zones of downer are formulated into formulas.

Three-phase CFD-DEM study on the hydrodynamics of a riser system with liquid injection

Riser reactors are frequently applied in the process industry for highly important catalytic processes. In this work, a computational fluid dynamics- discrete element method (CFD-DEM) for a riser with liquid injection was developed. This model treats the gas as a continuous phase whereas the liquid droplets and catalyst particles are treated as discrete elements. The presence of droplets causes particles to be wet, which is taken into account via coverage models of the particles. In addition, the liquid on the particles will change the collision behavior, which is included using an energy balance approach. This CFD-DEM study is conducted simulating a lab-scale pseudo-2D riser. Based on the comparison, the assumption of fully covered particles results in a significant increase in the solids holdup and clustering behavior of the riser which, especially at the low liquid flow rate, results in an over-prediction. The partial coverage simulations show that only half of the particles are covered, which has a major effect on the collisions.

Bubble characteristics in a novel microbubble gas-liquid-solid fluidized bed

The bubble characteristics are critical in gas-liquid-solid fluidized beds and microbubbles significantly enhance mass transfer in multiphase systems. In this work, a novel concept of microbubble gas-liquid-solid fluidized bed is proposed. Telecentric camera is employed to measure and analyze the microbubble characteristics including bubble size and gas holdup in this three-phase fluidized bed. Additionally, solid holdups in the fluidized bed are also examined. The results demonstrate that the bubble size adheres a lognormal distribution and is significantly influenced by superficial gas velocity. Bubble diameter exhibit a relatively uniform distribution in the radial direction. Bubbles exceeding than 1 mm substantially affect local gas holdup, and the microbubble flow promotes a more uniform distribution of local gas holdup in radial position. The average gas holdup deviation is less than 15 % compared to overall gas holdup obtained from pressure drop. Although microbubbles are more abundant, larger bubbles (> 1 mm) contribute more to solid particle fluidization. This paper introduces a methodology for assessing smaller-sized microbubbles in three-phase flow. The hydrodynamic analysis of the microbubble gas-liquid-solid fluidized bed establishes a foundational framework for enhancing gas-liquid mass transfer in fluidized bed.

Oily wastewater treatment using a CNT sorbent in a three-phase circulating fluidized bed

Fine emulsified heavy-oil droplets in wastewater can be easily dispersed in aquatic system, making it challenging to remove them using sorbents. Carbon nanotube (CNT) sorbents were prepared using a m-cresol solvent to remove emulsified heavy oil in wastewater. The bead-shaped and density-controlled sorbents (diameter: 5.22 mm; density: 1169 kg/m3) facilitated efficient contact with emulsified oil and were suitable for use in a three-phase circulating fluidized bed (TPCFB). The sorbents feature a sorption capacity of 2.4 g oil/g sorbent which remained consistent for at least 10 cycles. The hydrodynamic characteristics of the TPCFB reactor (120 × 80 mm; height: 800 mm) were determined and it was found that the circulation of both liquid and solid within the TPCFB was driven by the density difference between the riser and downcomer (Δρ). The solid holdups in the downcomer (εs,d) were almost constant with gas velocity and were proportional to the amount of sorbent inventory. The optimal region for emulsified oil removal was determined to be the downcomer, where turbulent flow was generated that facilitated close contact between the particles and the liquid. The solid circulation rate (Gs) increased to a constant value with Δρ, with the maximum Gs being proportional to the sorbent inventory. The optimal conditions for wastewater treatment were determined to be a maximum εs,d of 0.03 with a Gs value above 10.0 kg/m2s and a sorbent inventory of 90 g. The oil removal rate reached a maximum of 94.8 % at Gs = 12.2 kg/m2s. Used CNT sorbent exhibited unique oil sorption characteristics of CNTs, featuring an even distribution of sulfur on the surface and a weight loss arising from loss of oil at 350–500°C.

A multivariable submersible sensor for monitoring in real-time industrial flotation cells

This paper describes the development and validation of a submersible sensing technology for industrial flotation cells. The sensor provides simultaneous real-time measurements of gas and solid holdups, as well as the density and viscosity of the slurry in the pulp zone of flotation machines. The submersible device comprises a gas exclusion cell, which resembles an inverted truncated cone assembled to a single-straight tube Coriolis meter. When the sensor device is vertically immersed in the aerated slurry, a continuous bubble-free flow of slurry is induced through the device, which allows the Coriolis meter to provide measurements of the slurry flow, density, and viscosity. The magnitude of the induced flow is a primary function of the sensor’s geometry and gas holdup. Applying the Bernoulli equation reveals that gas holdup can be expressed as the square of the induced flow velocity. A discharge coefficient parameter is introduced to compensate for variations in the velocity profile and flow losses. This coefficient is a function of the fluid Reynolds number, calculated in real-time based on the Coriolis measurements. The sensing technology was assessed in a pilot column operated in a water–air system and industrial forced-air and self-aspirated large flotation cells. Results demonstrate the sensor’s capabilities to provide reliable, accurate, real-time, long-term continuous gas dispersion and solid suspension-related variables, which could be used for supervision, control, and optimization applications.

Research on the generalization issue of the heterogeneous QC-EMMS drag model for gas-solid fluidization

In the circulating fluidized bed (CFB), the non-uniform mesoscale structure formed by the particle clustering effect causes a substantial decrease in the gas-solid drag. Since the degree of particle clustering varies with operating conditions, an accurate drag model needs to be universal in different heterogeneous flow conditions. In this study, the relationships between the Ψ factor in clusters' solid holdup model that characterizes the flow non-uniformity and the operating parameters of CFB (including slip velocity Reynolds number, Re*, bed-averaged solid volume fraction, εs,bed, and solid mass circulation rate, Gs) are established. It improves the adaptability of the QC-EMMS drag model under different working conditions. In previous research, we established two types of models that related Ψ factor to Re* and εs,bed respectively. However, there exists a problem of high dispersion of points, indicating that the selected parameters cannot fully describe the flow non-uniformity. Therefore, Gs is reintroduced to modify the two types of models. The results show that the prediction accuracy of the modified models is improved and the relative error is <10%, indicating that the non-uniform factor Ψ has a strong correlation with Gs. In addition, the quantitative relation between Re*, εs,bed, and Gs is derived from modified models, and the trend of relation is highly consistent with the fluidization diagram proposed by Yerushalmi J., which verifies the accuracy of modified models. Finally, numerical simulation of typical CFB cases proves the adaptability of the modified models in wide operating conditions of fluidization.

Experimental Investigation of Liquid Holdup in a Co-Current Gas–Liquid Upflow Moving Packed Bed Reactor with Porous Catalyst Using Gamma-Ray Densitometry

This study explores the dynamics of liquid holdup in a lab-scale co-current two-phase upflow moving packed bed reactor, specifically examining how superficial gas velocity influences the line average external liquid holdup at a fixed superficial liquid velocity. Utilizing gamma-ray densitometry (GRD) for precise measurements, this research extends to determining line average internal porosity within catalyst particles. Conducted with an air–water system within a bed packed with 3 mm porous particles, the study presents a novel methodology using Beer–Lambert’s law to calculate liquid, gas, and solid holdups and catalyst porosity that is equivalent to the internal liquid holdup that fills the catalyst pores. Findings reveal a decrease in liquid holdup corresponding with increased superficial gas velocity across axial and radial locations, with a notable transition from bubbly to pulse flow regime at a critical velocity of 3.8 cm/sec. Additionally, the lower sections of the packed bed exhibited higher external liquid holdup compared to the middle sections at varied gas velocities. The liquid holdup distribution appeared uniform at lower flow rates, whereas higher flow rates favored the middle sections.

Investigating choking phenomena in CFB risers under different operating parameters

Choking phenomena in circulating fluidized bed (CFB) risers pose a significant challenge to industrial operations. To tackle this concern, multiphase particle-in-cell (MP-PIC) simulations are employed to investigate the impact of varying solids flux, bed height, and riser size on solids transport in CFB risers. The research sheds light on the complex interplay of operating parameters in CFB systems. The transition of solids holdup profiles from exponential to S-shaped curves with increasing solids inventory is observed, accurately predicting the choking point. By adjusting geometric dimensions, the choking point distance from the outlet at a fixed length-to-diameter ratio can be efficiently shifted. The findings provide a deeper understanding of saturation carrying capacity (SCC) in CFB risers, which can be leveraged to optimize industrial performance. This research is instrumental in mitigating choking effects and enhancing the design and operation of CFB systems.

Effect of operating conditions on the performance of riser reactor for oxidative coupling of methane

Oxidative coupling of methane (OCM) is characterized by high temperature and strong heat released. A riser reactor is designed for the OCM process to improve gas-solid contacting and heat transfer. This work illustrates the effect of hydrodynamic conditions and feed gas composition on the performance of an OCM riser reactor. The findings suggest that oxidative reactions predominantly occur within the acceleration zone and developed flow zone. Furthermore, at the low superficial gas velocity accompanied by a high solid circulating rate, the C2 yield initially increases in the acceleration zone, followed by a decrease in the developed flow zone and deceleration zone. The optimal operating conditions are a specific region corresponding to the solids holdup ranges from 0.015 to 0.02. Moreover, the C2 yield can be elevated using an appropriate feed mixture.

Study on flow characteristics and phase holdup in a slurry bubble column coupled with mild agitation

A pilot-scale experimental setup was constructed to investigate the effect of mild agitation on the bubble characteristics and phase holdup in a slurry bubble column. Mild agitation positively impacts the axial uniform distribution of solid holdup, though it shows insignificant influence on the radial distribution. In homogenous regime, mild agitation promotes the coalescence of bubbles, and the effect becomes stronger with increasing agitator speed. The mild agitation contributes to a decrease in bubble size in heterogeneous flow regime. Mild agitation presents a significant effect on the gas holdup by adjusting the bubble size and bubble motion trajectory. The modification was introduced to predict the gas holdup considering the effects of mild agitation, a necessary consideration for applications requiring mild agitation. This adapted model predicts gas holdup with a maximum error of 12.9%.

Revealing instantaneous gas-particle segregation in bubbling and turbulent fluidized beds through fiber optic sensing

Bubbling and turbulent fluidized beds (BFB and TFB) are widely used in the chemical and energy industries, owing to their enhanced heat transfer and improved operation flexibility. In BFB and TFB, the gas-particle suspension is featured with a dilute phase (bubbles) and a dense phase. The gas exchanges between the bubbles and the dense phase often through an intermediate region, further enhancing the gas-particle contact. However, few comprehensive studies have been conducted on the intermediate region and the dense phase. In this work, the instantaneous solids holdup was obtained in BFB and TFB through optical fiber sensing. Subsequently, through the use of statistical signal analysis, solids holdup thresholds were proposed to discriminate between the bubbles, the intermediate region (named as ‘mid-phase’) and the dense phase. Finally, gas-particle segregation details were further revealed through the quantification of the different phases to advance the development of future modelling, design and scale-up processes.

Insights into the gas-solid hydrodynamics and thermochemical characteristics in a pilot-scale chemical looping combustion unit using MP-PIC

Chemical looping combustion (CLC) emerges as an efficient pathway for CO2 capture and storage, yet complex in-furnace phenomena are still far from being understood. Accordingly, the multiphase reactive flow during the CLC process in a pilot-scale dual circulation fluidized bed (DCFB) is numerically investigated by the multi-phase particle-in-cell method considering thermochemical sub-models. After model validations, the thermochemical behaviors and the impact of operating parameters on CLC performance are explored, followed by unveiling the underlying mechanisms of heat and mass transfer characteristics. The pressure gradient is explicitly correlated with the solid holdup in two reactors. Gas-solid flow in the fuel reactor (FR) is more heterogeneous than that in the air reactor (AR) due to complex reactions and large amounts of gas/particle exchanges with other parts of the DCFB, and the whole CLC unit shows good circulating and heat transfer performance. Higher temperatures improve fuel conversion and increase CO2 yield. Elevating the air/fuel ratio (ψ) significantly enhances CLC performance at ψ ≤ 1.0 but it insignificantly affects the CLC performance at ψ > 1.0. A higher total inlet flow rate leads to suppressed fuel conversion and a subsequent decline in CO2 yield. The particle heat transfer coefficient (HTC) in the AR is approximately 450 W/(m2·K), significantly higher than that in the FR of about 300 W/(m2·K).

Experimental study of dynamic characteristics of solid holdup fluctuations in a gas-solid cyclone reactor

Local transient solid holdup signals were measured by using an optical fiber probe at different axial and radial positions in the gas-solid cyclone reactor. The chaotic time series analysis of transient solid holdup was adopted to characterize the dynamic characteristics. The maximum and minimum solid holdup indicates that the high variability of gas-solid interaction. With the increase of the gas / solid ratio, the frequency of solid holdup fluctuation increased while the mean value of amplitude decreased; besides, the level of detail in the power spectrum fluctuations increased. In the diffusion chamber, the overall Kolmogorov entropy (K entropy) and correlation dimension (Dc) were higher indicating the strong chaos characteristic of gas-solid flow. In the agglomeration chamber, higher solid holdup initially reduced gas-solid interaction, decreasing flow randomness, but a subsequent increase led to irregular particle collisions and an upturn of K entropy and Dc. In the annulus chamber, K entropy and Dc were lower around the inner wall and outer wall compared to the middle region. In the separation chamber, K entropy and Dc tended to increase and then decrease as the radial position increased. Finally, the relationship between K entropy, Dc, and time-averaged solid holdup was analyzed.

Pressure fluctuations in a fluidized bed of binary particles with significant differences in particle size

Pressure fluctuation signals are investigated in a binary particle fluidized bed in which two types of particles have similar physical properties but significant differences in particle size (90 μm Vs. 1800 μm). The relationship of the measured pressure drop to gas velocity exhibits a distinct ‘‘step’’ profile, indicating that the incipient fluidization of binary particle bed can be characterized by different stages, i.e. the fixed bed, the fine particle fluidization, the coarse particle partial fluidization and the binary particle complete fluidization. And then the statistical method and scale decomposition method are used to quantify pressure fluctuation amplitude and frequency in each stage. The result shows that when binary particles are completely fluidized, the bubble size and frequency in binary particle systems are larger than that in sole fine particle systems. Additionally, binary particle systems need higher gas velocity to achieve turbulent fluidization. Furthermore, the local solids holdup measured with optical fiber probes is also employed to verify the result of pressure fluctuation analysis.

Parametric study of solid holdup in upper dilute region of fast fluidized beds based on improved separate-phase-coexistence model

Parametric study of solid holdup in upper dilute region of fast fluidized beds based on improved separate-phase-coexistence model

Characteristics of local flow structure with different particle properties in a gas-solid diameter-varying fluidized bed

The effects of particle property on gas-solid flow in the diameter-varying riser are experimentally examined using the optical fiber probe in this research. Local flow features are obtained by signal analysis method under the operating conditions of 3–7 m/s and 8–63 kg/m2s. The results show that solid holdup distribution of glass beads is wider than that of catalyst particle particles in the wall region but is narrow in the center region. Solid holdup fluctuations tend to decrease with the increase of particle density under low-concentration conditions but increase at high-concentration conditions. The results from power spectrum density (PSD) show that the increase in particle density strengthens the periodic flow features in the enlarged section. The motion intensity decreases at the particle level but increases at the cluster level for high-density particles. Additionally, as particle density rises, the solid concentration discrepancies between the straight section and the enlarged section widen. In contrast to the upper straight section, the effects of particle density on gas-solid flow are minimal in the enlarged section.

Experimental investigation of the behavior of non-spherical particles in a small-scale gas-solid fluidized bed

The present work examined the effect of particle shape on the fluidization behavior at different inlet superficial gas velocities. The experiments are performed in a laboratory-scale 3D circular fluidized bed column of 1 m height and 0.067 m inner diameter with Geldart D particles of four different shapes. The effect of particle shape on pressure drop, bed expansion, and solids holdup are analyzed. Non-spherical particles have lower minimum fluidization velocities and higher bed expansion than spherical particles. The shape of particles significantly impacts the solids holdup, and it has been observed that spherical particles exhibit a higher solids holdup at the same superficial velocity. The time series analysis of the pressure signals is investigated by the frequency domain method using the Fast Fourier Transform (FFT). The FFT is implemented in Matlab R2021b to obtain the Power Spectral Density (PSD). The analysis of the PSD pattern shows the flow regime transition with the change in the particle shape. PSD pattern has observed a broad range of frequencies between 1 and 5 Hz. The dominant frequency of around 3 Hz corresponds to the bubbles or slugs that pass the bed.

A numerical study of fluidization characteristics in transition from aggregative to particulate flow

A comparative study on the fluidization characteristics from aggregative to particulate fluidization with increasing fluid density and viscosity is carried out using the two-fluid models (TFM), to study the vast transitional states in between. Two representative transitional fluidization states are observed as the fluidized state changes progressively from aggregative to particulate fluidization. As the discrimination number (Dn) adopted to delineate the transition decreases, the increasing number of local vortexes promotes more internal particle circulation, and a reduction in the bubble phase and the dense phase fractions but an increase in solids holdup in the intermediate phase. As a result, the intermediate phase become more interconnected and solids movement are enhanced. The interaction between the solids and the fluid phases is also increased and the intensity of particle-particle interaction is decreased with decreasing Dn. Fluid density has more significant effect on the above changes than fluid viscosity.

A finite‐discrete element model for simulating collision and fragmentation of nanoparticle agglomerates

AbstractThe fragmentation/adhesion behavior of nanoparticle agglomerate collision, which is challenging to model, is a crucial factor affecting fluidization. In this study, a discrete‐finite element method (FDEM) based cohesive crack model is developed to simulate normal collisions between a complex agglomerate (around 1 mm) and a wall. In the FDEM model, the complex agglomerate is built from primary agglomerates (around a few micrometers), whose adhesive force and Young's modulus are measured by an atomic force microscope (AFM). The simulation results including fragmentation morphology and plastic deformation agree well with the collision experiments. Finally, the sensitivity of the model is tested, including adhesive force, solid holdup, and Young's modulus. Fragment distribution is found to fit well with a two‐parameter Weibull function and damage ratio is found to have a polynomial relation with a modified Weber number. The model output shows a potential to be utilized in macroscopic model.

Numerical study of aeration flow angle on powder flow and thermal behaviors in fluidized bed particle solar receivers

The particle flow and thermal characteristics in fluidized bed particle solar receivers with different aeration flow angles were numerically investigated by employing the Eulerian-Eulerian two-fluid model. The filtered drag correlation was utilized to characterize the interaction between SiC particles of Geldart-A classification and gas. The model underwent validation using experimental data from the literature, confirming that the simulated solid holdup and local heat transfer rate agreed well with the experimental measurements. Simulation results show that the particle volume fraction was higher along the walls in the bottom section. In both +45° and + 60°, the particles returned to the aeration flow region. The aeration had a relatively small impact on the particle distribution at the center, whereas the other two regions (x / D = 0.5 and 0.95) exhibited a significant decrease in particle volume fraction at the height of aeration injection. For different aeration injection structures, the equilibrium temperature rise reached approximately 51% of the difference between the wall and inlet temperatures. The temperature rise of particles in FBPSR was attributed to the direct heat transfer from the heating wall to particles and he return of hot particles.