Investigation of gas–solids flow characteristics in a conical fluidized bed dryer by pressure fluctuation and electrical capacitance tomography

It is important to investigate the gas–solids flow characteristics of fluidized bed drying processes to improve the operation efficiency and guarantee the product quality. This paper presents research into fluidized bed drying processes measured by high-frequency differential pressure fluctuation and electrical capacitance tomography (ECT). Power spectra analysis is combined with dynamic calibration for ECT to reveal the complex gas–solids flow behavior. Bubble characteristics are visualized by cross-sectional and quasi-3D ECT images. In addition, results by discrete wavelet transform analysis are given and compared with the analysis results of previous sections. It has been found that bubbles would coalesce in different ways under different operation conditions, and discrete wavelet transform sub-signals of ECT measurements are sensitive to particle moisture. This work reveals the complex hydrodynamic behavior in the fluidized bed dryer and provides valuable information for process control.     

Mixing–segregation phenomena of binary system in a fluidized bed

A study on mixing–segregation phenomena in a gas fluidized bed of binary density system was performed by analysis of the residence time distribution and mixing degree. The effect of particle mixing on the residence time distribution and solid mixing was studied in a binary particle system with different densities. Residence time distribution curve and mean residence time of each particle were measured according to the flotsam particle size, mixing ratio and gas velocity in a gas fluidized bed (0.109 m I.D., 1.8 m height). The characteristics of residence time distribution and the deviation of mean residence time of each particle are consistent with previous mixing index based on the axial concentration of jetsam. From this study, mixing index of binary particle system with different densities should be considered by not only axial concentration distribution of jetsam particle but also characteristics of residence time distribution. This result suggests that the solid movement by fluidization gas is more important than solid axial dispersion.     

Treatment of oilfield wastewater using algal–bacterial fluidized bed reactor

This study was carried out to investigate the ability of an artificial microalgal–bacterial consortium to remediate an oilfield wastewater using laboratory-scale fluidized bed reactors (FBRs) under lighting. The consortium consists of an oil-degrading bacterial community and a microalgae, Scenedesmus obliquus GH2. The microbes were immobilized onto Ca-alginate beads and filled to the FBRs. The influence of hydraulic retention time (HRT) on the removal of chemical oxygen demand (COD), total nitrogen, NH₃-N and oil concentration was investigated over 40 operational days. The synergistic relationship in the algal–bacterial microcosm was clearly demonstrated, since for the four parameters tested, the highest removal was recorded in the system inoculated with both algae and bacteria. In the algal–bacterial FBR, at 16 h HRT, the average effluent concentrations of COD, NH₃-N and oil were 92, 24 and 4.8 mg/L, respectively, corresponding to removal efficiencies of 70.8%, 61.2% and 84.2%, respectively. The effluent could meet the grade 2 as required by the national discharge standard of China. However, the effluent quality of the bacterial- or algal-only case could not satisfy the grade 3 discharge standard at 16 h HRT. The algal–bacterial biomass exhibits lowest effluent microtoxicity and highest dehydrogenase activity in comparison with bacterial-only and algal-only cases. This study reveals that the consortium containing dual microbial species has potential for microbial remediation of oilfield wastewater.     

The onset velocity of a liquid–solid circulating fluidized bed

The critical transition velocity, U_cr, previously defined by Liang et al. [W.-G. Liang, S.-L. Zhang, J.-X. Zhu, Y. Jin, Z.-Q. Yu and Z.-W. Wang, Flow characteristics of the liquid–solid circulating fluidized bed, Powder Technol., 90 (1997) 95–102.] to demarcate the liquid–solid conventional and circulating fluidization regimes, was found to vary with the total solids inventory and the solids feeding system. In this work, an onset velocity for circulating fluidization regime, U_cf, is proposed to give the lowest U_cr value and to provide a convenient demarcation velocity that is independent of system geometry. This liquid velocity is obtained by measuring the time required to empty all particles in a batch operated fluidized bed under different liquid velocities. This method can be used for a wide range of particles and involves less influence of the operating conditions such as the solids inventory and the solids feeding system. Compared to the critical transition velocity, this newly defined onset velocity is a more intrinsic parameter, only dependent on the liquid and particle properties. Based on the experimental results obtained in this work and other published results, the influence of particle properties and equipment setup on the onset velocity is also discussed.     

Hydrodynamics of an inverse liquid–solid circulating conventional fluidized bed

An inverse liquid–solid circulating conventional fluidized bed (I-CCFB) is realized by injecting particles from the top of a conventional liquid–solid fluidized bed (0.076 m ID and 5.4 m height) that is operated in a newly developed circulating conventional fluidization regime located between the conventional and circulating fluidization regimes. The I-CCFB can achieve a higher solids holdup compared to both conventional and circulating liquid–solid fluidized beds. A new parameter, the bed intensification factor, is defined to quantify the increased solids holdup observed with external solids circulation. The Richardson–Zaki equation is shown to be applicable to the I-CCFB regime and can be used to correlate the slip velocity and solids holdup, both of which increase with the solids circulation rate. A new flow regime map is presented, including the I-CCFB and a variety of other liquid–solid fluidized beds.     

Development of a CH4 dehydroaromatization–catalyst regeneration fluidized bed system

A pilot-scale methane dehydroaromatization–H₂ regeneration fluidized bed system (MDARS) was developed. In the MDARS, the catalyst circulation between a fluidized bed reactor and a fluidized bed regenerator with the help of a catalyst feeder allowed methane dehydroaromatization (MDA) and H₂ regeneration to be carried out simultaneously, which is good for maintaining a stable MDA catalytic activity. A fixed bed reactor (FB) and a single fluidized bed reactor (SFB) were also used for a comparative study. The experimental results showed that the catalytic activity in the MDARS was more stable than that in the FB and SFB reactors. The effects of some parameters of MDARS on the CH₄ conversion and product selectivity were investigated. To verify the feasibility and reliability of the MDARS, an eight-hour long-term test was carried out, which demonstrated that the operation of the MDARS was stable and that the catalytic activity remained stable throughout the entire experimental period.     

Discrete particle simulation of solids motion in a gas–solid fluidized bed

The solids motion in a gas–solid fluidized bed was investigated via discrete particle simulation. The motion of individual particles in a uniform particle system and a binary particle system was monitored by the solution of the Newton's second law of motion. The force acting on each particle consists of the contact force between particles and the force exerted by the surrounding fluid. The contact force is modeled by using the analogy of spring, dash-pot and friction slider. The flow field of gas was predicted by the Navier–Stokes equation. The solids distribution is non-uniform in the bed, which is very diluted near the center but high near the wall. It was also found that there is a single solids circulation cell in the fluidized bed with ascending at the center and descending near the wall. This finding agrees with the experimental results obtained by Moslemian. The effects of the operating conditions, such as superficial gas velocity, particle size, and column size on the solids movement, were investigated. In the fluidized bed containing uniform particles better solids mixing was found in the larger bed containing smaller size particles and operated at higher superficial gas velocity. In the system containing binary particles, it was shown that under suitable conditions the particles in a fluidized bed could be made mixable or non-mixable depending on the ratios of particle sizes and densities. Better mixing of binary particles was found in the system containing particles with less different densities and closer sizes. These results were found to follow the mixing and segregation criteria obtained experimentally by Tanaka et al.     

Coarse grid simulation of bed expansion characteristics of industrial-scale gas–solid bubbling fluidized beds

Two-fluid modeling of the hydrodynamics of industrial-scale gas-fluidized beds proves a long-standing challenge for both engineers and scientists. In this study, we suggest a simple method to modify currently available drag correlations to allow for the effect of unresolved sub-grid scale structures, by assuming that the particles inside each computational cell are presented in the form of a two-phase structure. This method would thus make it possible to simulate the hydrodynamics of industrial-scale bubbling fluidized beds of Geldart B and D particles with a coarse computational mesh. It is shown that with the proposed modification of the drag force correlation, the experimentally measured bed expansion characteristics of industrial-scale bubbling fluidized beds can be reasonably predicted at acceptable computational cost. Also the simulation result for the macroscopic solid circulation pattern is in qualitative agreement with the experimental data.     

Euler multiphase-CFD simulation on a bubble-driven gas–liquid–solid fluidized bed

An integrated flow model was developed to simulate the fluidization hydrodynamics in a new bubble-driven gas–liquid–solid fluidized bed using the computational fluid dynamic (CFD) method. The results showed that axial solids holdup is affected by grid size, bubble diameter, and the interphase drag models used in the simulation. Good agreements with experimental data could be obtained by adopting the following parameters: 5 mm grid, 1.2 mm bubble diameter, the Tomiyama gas–liquid model, the Schiller–Naumann liquid–solid model, and the Gidaspow gas–solid model. At full fluidization state, an internal circulation of particles flowing upward near the wall and downward in the centre is observed, which is in the opposite direction compared with the traditional core-annular flow structure in a gas–solid fluidized bed. The simulated results are very sensitive to bubble diameters. Using smaller bubble diameters would lead to excessive liquid bed expansions and more solid accumulated at the bottom due to a bigger gas–liquid drag force, while bigger bubble diameters would result in a higher solid bed height caused by a smaller gas–solid drag force. Considering the actual bubble distribution, population balance model (PBM) is employed to characterize the coalescence and break up of bubbles. The calculated bubble diameters grow up from 2–4 mm at the bottom to 5–10 mm at the upper section of the bed, which are comparable to those observed in experiments. The simulation results could provide valuable information for the design and optimization of this new type of fluidized system.     

Fabrication of core-shell structured TiC–Fe composite powders by fluidized bed chemical vapor deposition

Fluidized bed chemical vapor deposition (FBCVD) was an effective way of preparing the core-shell structured TiC–Fe composite powders by employing FeCl₃ as a precursor. Fully covered TiC–Fe composite powders with the controllable Fe contents were readily achievable. An excellent interfacial bonding was formed between the TiC and the deposited Fe coating. The defluidization caused by the directional growth and self-nucleation-aggregation of the deposited Fe particles was the major barrier to depositing high-Fe-content composite powders. But it could be prevented by controlling the gas partial pressure of precursor and further eliminating the directional growth mode and self-nucleation behavior of Fe atoms. The optimal deposition temperature was 600°C and the partial pressure was about 20 kPa, corresponding to the gasification temperature of 275°C.     

Three-dimensional numerical modelling of interactions between a gas–liquid jet and a fluidized bed

This paper presents the development of a novel mathematical model that describes spray injection and spreading into a fluidized bed of solid particles. The model also includes the gas–liquid flow through the nozzle followed by the gas-assisted atomization. An Eulerian approach that is independent of the nature of the continuous phase is adopted for all phases, which are gas (or bubbles), liquid (or droplets), and solid particles that may be covered with a liquid layer. Variation in sizes of bubbles and droplets is represented by the particle number density approach that takes into account both break-up and coalescence. The atomization is considered as a catastrophic phase inversion triggered by a critical local volume fraction. New relationships were obtained for liquid spreading due to wet particle collisions and for heat conduction between a solid particle and a surrounding liquid layer. The model is applied to simulate liquid injection into the fluidized bed for conditions that were previously experimentally studied and published. The comparison reveals a reasonable agreement in prediction of the cumulative liquid distribution for two experimental cases. In addition, we evaluated a jet penetration distance with the model to compare it with the one measured in another set of experiments. This comparison also yields a good qualitative agreement. Finally, we evaluated the influence of the fluidization velocity on liquid distribution in the bed.     

Non-intrusive determination of bubble size in a gas–solid fluidized bed: An evaluation

Bubble sizes measured in a column of diameter 290 mm with FCC particles utilizing both an intrusive optical probe and non-intrusive pressure analysis are compared. The pressure signals were decoupled by differential pressure analysis and incoherence analysis. It is shown that pressure fluctuations induced by jetting/bubble formation can be effectively filtered out by differential pressure and incoherence analysis. The differential pressure signals measured across a vertical interval less than half the maximum bubble size unreasonably damps the power spectral density intensity, leading to underestimation of bubble size and overestimation of mean frequency. In the present work, the incoherence analysis tends to estimate greater bubble size than differential pressure analysis. Bubble chord lengths are overestimated by optical probe signals because small bubbles are not detected. Bubble sizes calculated by the equation of Horio and Nonaka (1987) agree reasonably well with that estimated by incoherence analysis at relatively high superficial gas velocities.     

Effect of the operation parameters on the Fischer–Tropsch synthesis in fluidized bed reactors

For the Fischer–Tropsch synthesis (FTS), this paper presents a numerical investigation in a 3D fluidized bed reactor. The effect of the operation parameters such as bed temperature, superficial gas velocities, particle size and bed heights is discussed. A 3D-CFD model coupled with FTS chemical kinetics was set up. The computational results are compared with experimental data in terms of the components production rates, etc. The analysis shows that the bed heights, the bed temperature, the superficial gas velocities and particle sizes affect the C_5 + selectivity and the reaction rates. Product yields are dependent on the operating conditions especially the temperature.     

Modelling and numerical simulation of liquid–solid circulating fluidized bed system for protein purification

A novel liquid–solid circulating fluidized bed (LSCFB) was modelled for protein recovery from the feed broth. A typical LSCFB system consists of downer and riser, integrating two different operations simultaneously. A general purpose, extensible, and dynamic model was written based on the tanks-in-series framework. The model allowed adjusting the degree of backmixing in each phase for both columns. The model was validated with previously published data on extraction of bovine serum albumin (BSA) as model protein. Detailed dynamic analysis was performed on the protein recovery operation. The interaction between the riser and downer were captured. Parametric studies on protein recovery in LSCFB system were carried out using the validated model to better understand the system behaviour. Simulation results have shown that both production rate and overall recovery increased with solids circulation rate, superficial liquid velocity in the downer and riser, and feed solution concentration. The model was flexible and could use various forms of ion exchange kinetics and could simulate different hydrodynamic behaviours. It was useful to gain insight into protein recovery processes. The general nature of the model made it useful to study other protein recovery operations for plant and animal proteins. It could also be useful for further multi-objective optimization studies to optimize the LSCFB system.     

Minimum superficial fluid velocity in a gas–solid swirled fluidized bed

A swirl flow is achieved in a bed of solids by passing air through multiple fluid inlets, which are tangentially located at the base of a flat-based circular column. The minimum superficial velocities needed to achieve swirling of the bed are measured experimentally under varied conditions. An empirical correlation for the minimum swirl velocity has been proposed. The results indicate that a stable swirling regime operation of the bed is possible. There exists an upper limit of static bed depth beyond which stable swirling of entire bed is not possible. The minimum swirl velocities are found to be 1.2–1.3 times the minimum fluidization velocities predicted for conventional fluidized beds.     

Onset of an innovative gasless fluidized bed—comparative study on the fluidization of fine powders in a rotating drum and a traditional fluidized bed

Geldart group C powders were found to be fluidized in rotating drums without requiring any external fluidizing gas. As a result, a rotating drum was proposed as a new gasless fluidized bed in contrast to a traditional fluidized bed, leading to a considerable amount of energy savings. In addition, the fluidization qualities of a series of Geldart group C powders were found to be further improved with the assistance of drum rotation because of the shearing movement among particles that eliminates channeling and cracks and possibly also breaks agglomerates. There is potential for the new gasless fluidized bed to replace some traditional fluidized beds where the fluidizing gas is not used as a reactant.In the gasless fluidized bed, a boundary layer of compacted powder adjacent to the drum wall was observed. The powder in this layer is carried up to the freeboard and then falls back to the powder bed, forming a powder circulation in the drum. The circulating powder leads to a circulation of internal gas in the drum, which essentially acts as fluidizing gas to realize the fluidization of Geldart C powders in the drum. In contrast to the fluidization of Geldart C powders, Geldart groups B and D powders show cascading and cataracting motions instead in the rotating drum due to their requirement of higher fluidization gas velocities. Geldart group A powders experience a transition of powder behavior between Geldart group B–D powders and C powders.     

A fundamental CFD study of the gas–solid flow field in fluidized bed polymerization reactors

A Eulerian–Eulerian model incorporating the kinetic theory of granular flow was applied to describe the gas–solid two-phase flow in fluidized bed polymerization reactors. The model parameters were examined, and the model was validated by comparing the simulation result with the classical calculated data. The effects of distributor shape, solid particle size, operational gas velocity and feed manner on the flow behavior in the reactor were also investigated numerically. The results show that with the increase of solid particle diameter, the bubble numbers decrease and the bubble size increases, resulting in a smaller bed expansion ratio. Bed expansion ratio increases with increasing the gas inlet velocity. Moreover, the final fluidized qualities are almost the same for the plane distributor case and the triangle distributor case. There exists a tempestuous wiggle from side to side in the bed at the continuous feed manner, which could not be obtained at a batch feed manner.     

Development and verification of coarse-grain CFD-DEM for nonspherical particles in a gas–solid fluidized bed

Computational fluid dynamics coupled with discrete element method (CFD-DEM) has been widely used to understand the complicated fundamentals inside gas–solid fluidized beds. To realize large-scale simulations, CFD-DEM integrated with coarse-grain model (CG CFD-DEM) provides a feasible solution, and has led to a recent upsurge of interest. However, when dealing with large-scale simulations involving irregular-shaped particles such as biomass particles featuring elongated shapes, current CG models cannot function as normal because they are all developed for spherical particles. To address this issue, a CG CFD-DEM for nonspherical particles is proposed in this study, and the morphology of particles is characterized by the super-ellipsoid model. The effectiveness and accuracy of CG CFD-DEM for nonspherical particles are comprehensively evaluated by comparing the hydrodynamic behaviors with the results predicted by traditional CFD-DEM in a gas–solid fluidized bed. It is demonstrated that the proposed model can accurately model gas–solid flow containing nonspherical particles, merely the particle dynamics are somewhat lost due to the scaleup of particle size. Finally, the calculation efficiency of CG CFD-DEM is assessed, and the results show that CG CFD-DEM can largely reduce computational costs mainly by improving the calculation efficiency of DEM. In general, the proposed CG CFD-DEM for nonspherical particles strikes a good balance between efficiency and accuracy, and has shown its prospect as a high-efficiency alternative to traditional CFD-DEM for engineering applications involving nonspherical particles.     

Self-tuning control of continuous fluidized bed drying of baker’s yeast pellets

Abstract

Uncertainty in the kinetics of the second drying period represents the major challenge in continuous drying of particulate solids which can be overcome by adaptive feedback control. In this contribution, self-tuning control comprising online-state estimation and adaptive PI-control is considered to guarantee desired product moisture. Advantages of the approach over conventional PI-control are discussed. The performance is shown for fluidized bed drying of baker’s yeast. It is shown that the self-tuning approach is superior to nonadaptive approaches in case of parametric uncertainty even at non-nominal steady-states.     

Investigation on gas–solid flow regimes in a novel multistage fluidized bed

Gas–solid flow regime in a novel multistage circulating fluidized bed is investigated in this study. Pressure fluctuations are first sampled from gas–solid flow systems and then are analyzed through frequency and time–frequency domain methods including power spectrum and Hilbert–Huang transform. According to the flow characteristics obtained from pressure fluctuations, it is found that the gas–solid motions in the multistage circulating fluidized bed exhibit two dominant motion peaks in low and high frequencies. Moreover, gas-cluster motions become intensive for the multistage circulating fluidized bed in comparison with the fast bed. Unlike the traditional methods, the fuzzy C-means clustering method is introduced to objectively identify flow regime in the multistage circulating fluidized bed on the basis of the flow characteristics extracted from bubbling, turbulent, fast, and multistage fluidized beds. The identification accuracy of fuzzy C-means clustering method is first verified. The identification results show that the flow regime in the multistage circulating fluidized bed is in the scope of fast flow regime under examined conditions. Moreover, the results indicate that the consistency of flow regime between two enlarged sections exists. In addition, the transition onset of fast flow regime in the multistage circulating fluidized bed is higher than that in the fast bed.