Bubbles In Bed Research Articles

Capability of the TFM Approach to Predict Fluidization of Cohesive Powders.

The fluidization behavior of cohesive particles was investigated using an Euler–Euler approach. To do so, a two-fluid model (TFM) platform was developed to account for the cohesivity of particles. Specifically, the kinetic theory of granular flow (KTGF) was modified based on the solid rheology developed by Gu et al. J. Fluid Mech.2019. The results of our simulations demonstrated that the modified TFM approach can successfully predict the formation of particle agglomerates and clusters in the fluidized bed, induced by the negative (tensile-dominant) pressure. The formation of such granules and clusters highly depended on the particle Bond number and the tensile pressure prefactor. To evaluate fluidization regimes, a set of simulations was conducted for a wide range of particle cohesivity (e.g., Bond number and tensile pressure prefactor) at two different fluidization numbers of 2 and 5. Our simulation results reveal the formation of four different regimes of fluidization for cohesive particles: (i) bubbling, (ii) bubbling–clustering, (iii) bubble-less fluidization, and (iv) stagnant bed. Comprehensive analysis of the shear-to-yield ratio reveals that the observed regime map is attributed to the competition between the shear stress and yield stress acting on the particles. The obtained regime map can be extended to incorporate the effect of dimensionless velocity and dimensionless diameter as a comprehensive fluidization chart for cohesive particles. Such fluidization charts can facilitate the design of fluidized beds by predicting the conditions under which the formation of particle agglomeration and clustering is likely in fluidized beds.

Three-dimensional simulation of coal catalytic hydrogasification in a pressurized bubbling fluidized bed

Coal catalytic hydrogasification (CCHG) is an efficient approach to produce CH4 with high yield and high thermal efficiency. A numerical modelling of CCHG in a pressurized bubbling fluidized bed has been conducted to track the whole reaction process by using the multiphase particle-in-cell (MP-PIC) algorithm during the residence time of particles. The gas–solid flow dynamic properties, such as bubble size and bed expansion height, were validated against with empirical correlations. After incorporating the CCHG kinetics with MP-PIC, the simulated bed temperature, gaseous compositions, and CH4 formation behavior were verified with the experimental data. The trajectory and physiochemical properties of char particles (particle diameter, temperature, heat transfer coefficient and drag function) during the whole residence time were explored comprehensively. The results showed that a high temperature region appeared in the dense bed located 20–30 mm above the plate distributor initially, which led to the profound fluidization performance and reactivity of char particles with diameter range of 138–249 μm. With the CCHG proceeding, particle diameter decreased gradually, while the heat transfer coefficient and drag force were strengthened. A uniform bed temperature was generated afterwards resulted by the enhanced gas-particle heat transfer. Whereas, when the particle diameter decreased to less than 46 μm, char particles were blown out of the dense bed, resulting in the low reactivity and difficulty of separation for the practical application. The numerical results in this work shed light on further scaling up and design of pressurized fluidized bed for coal catalytic hydrogasification.

Effect of Solid Particles on the Gas Hold Up in the Fluidized Bed Columns: Experimental and Mathematical Studies

The present research investigated the effect of solid properties on the gas holdup of the fluidization bed bubble columns (FBCS). All experiments were performed in the constant clear tap water of 80 cm height. The range of solid particle diameters was 0.7 – 2 mm with two different densities of 1075 and 1200 kg/m3, superficial air velocities 4 – 7 cm/s. It was observed that there are proportional relationships between superficial gas velocity and particle diameter with the gas holdup. While an inverse relationship between solid concentration and particle density with the gas holdup. Mathematical and statistical analysis was also used as a powerful way to represent the gas hold up as a function of different operating conditions.

Gasification of N-rich fibreboard in an air-blown fluidized bed reactor: A study on the fate of tars, NH3 and HCN during oxidative mild Hot Gas Filtration

Wood wastes, such as Medium Density Fibreboard (MDF), are interesting low-cost fuels for gasifiers. The present study examines the effect of oxidative Hot Gas Filtration (HGF) on gasification indicators and pollutants (tars, NH3 and HCN) of producer gas. Pellets of N-rich MDF were gasified in an air-blow bubbling bed reactor at 800 °C coupled with a downstream HGF unit equipped with multilayer metallic filters and operated at mild temperature (400–600 °C). The fate of pollutants was assessed by complementary analytical techniques (HPLC, UV-fluorescence, GC/MS-FID and wet chemical methods for N-compounds). The deposit of the filter was also sampled after gasification trials and characterised by SEM-EDX.For the reference tests, conducted without the HGF unit, tar concentration was 6.5 g/Nm3 (dry gas, benzene-free) while, 43% of Fuel Bounded Nitrogen (FBN) was converted to NH3 and 4% to HCN. Oxidative HGF tests showed that secondary reactions inside the HGF unit undergo in homogeneous (gas-gas) and heterogeneous (gas-solid) phase. At a relatively high secondary air flow rate brought in the HGF unit, the severe oxidation of the dust cake particles led to irreversible melting, aggregation and sintering phenomena of ash deposits on the surface of the filter, as well as, to a drop of 49% in the tar yield. Overall, this work demonstrates that the addition of secondary air inside the HGF unit should be carefully tailored in order to minimise the exothermic oxidation of dust cake particles and, to promote long-term operation of the filter.

Numerical Simulation of Fluidized Bed Gasifier Coupled with Solid Oxide Fuel Cell Fed with Solid Carbon

A model of a fluidized bed coupled with direct carbon solid oxide fuel cell (SOFC) is developed to explore the effect of coupling between fluidized bed and solid oxide fuel cell. Three gas–solid flow regimes are involved including fixed bed, delayed bubbling bed and bubbling bed. The anode reaction of SOFC is treated as the coupling processes of Boudouard gasification of carbon and electrochemical oxidation of CO. The effects of inlet velocity of the fluidizing agent CO2, carbon activity, channel width and coupling extent on the system performance are investigated. The results show that the inlet velocity of CO2 can promote the gasification rate in the anode, but too high velocities may lower CO molar fraction. The gasification rate generally increases with the increase of the channel width and carbon activity. The overlapping area between the anode surface and the initial carbon bed, gas–solid regime and carbon activity have a significant influence on the gasification rate and the maximum current density the system can support. Overall, the mass transport in the anode is dramatically enhanced by the expansion of the carbon bed, back-mixing, solid mixing and gas mixing, especially for the delayed bubbling bed and bubbling bed. This indicates that the adopted coupling method is feasible to improve the anode performance of direct carbon solid oxide fuel cell.

Process intensification in binary adsorption of VOC-water vapor on zeolite in a countercurrent fluidized bed adsorber

In this study, fast ideal adsorbed solution theory was coupled with a two-phase bubbling bed approach to describe the adsorption of volatile organic compounds (VOCs) on zeolite in the presence of water vapor in a multistage fluidized bed adsorber. The binary adsorption of VOC-water vapor was predicted using their single component adsorption isotherms. The model was verified by experimental data obtained at a wide range of operating conditions before being used for studying the process intensification. Validation tests revealed that the model could accurately predict the experimental removal efficiencies (R2 = 0.94), as well as VOC concentration profiles inside the bed (R2 = 0.98). The intensification simulations showed that increasing the adsorbent feed rate is effective when there is a need for more adsorption sites (e.g. at high inlet concentrations), and is quite ineffective when the adsorption process is limited by low solid-gas contact time (e.g. high air flow rates). Increasing the adsorbent feed rate can also diminish the interference of water vapor in adsorption of VOC (even at RH as high as 75%). Reducing air flow rate at constant VOC load is always effective specially when there are enough adsorption sites available (e.g. high adsorbent feed rate, and low VOC loads and RHs). Similarly, increasing the number of stages can effectively improve the fluidized bed performance at high adsorbent feed rates and low VOC inlet concentrations and RHs. Using 3 adsorbers of 2 stages instead of 1 adsorber of 6 stages can improve the removal efficiency up to 34.5% in the range of operating conditions simulated. While having the same high weir height along the bed yields better performance than having the same low weir height, an optimized arrangement of weir heights in a descending order from the top to the bottom of the bed would maximize the removal efficiency.

Comparative CFD modeling of a bubbling bed using a Eulerian–Eulerian two-fluid model (TFM) and a Eulerian-Lagrangian dense discrete phase model (DDPM)

Eulerian–Eulerian and Eulerian-Lagrangian numerical approaches are both widely used to investigate hydrodynamic behavior in dense gas-solid fluidized bed reactors, yet there has been a lack of comparative investigations involving the two. Therefore, the present work compares the numerical performance of a two-fluid model (TFM) and a dense discrete phase model (DDPM) in describing the hydrodynamic behavior of a pilot-scale bubbling bed reactor. The effects of several different parameters – drag force, grid size, fluid time-step, time-averaging interval, particle-wall specularity coefficient, particle-particle restitution coefficient, particle-wall reflection coefficient, and numbers of parcels (the latter two parameters only for the DDPM) – are investigated and compared for the two approaches using two-dimensional (2D) simulations. The numerical results for both models indicate that sub-grid drag correction based on the energy minimization and multiscale (EMMS) theory is the most essential modeling parameter to account for the multiscale structures (i.e., bubbles and void spaces) and to resolve the axial and radial solid distributions. Both the TFM and DDPM, coupled with the EMMS/bubbling drag, predict similar hydrodynamics behavior; however, the better accuracy in terms of axial and radial solids concentration profiles is achieved from the TFM approach. Further, our grid size analysis results indicate that the DDPM generates a better grid-independent solution than the TFM. This important advantage of the DDPM over TFM makes it a suitable candidate for large-scale industrial applications by employing it on much coarser grids. Meanwhile, the results from the time-step, time-averaging interval, specularity coefficient, restitution coefficient, reflection coefficient, and numbers of parcels represent a minor effect on the overall hydrodynamics behavior of the pilot-scale bubbling bed reactor.

Preliminary study for the suitability of eucalyptus chips and coal for combustion in a fluidized bed reactor

The combustion with fluidized bed reactors has as main advantages the best energy utilization of combustible materials and a lower generation of pollutants. The fluidization success depends on the characteristics of the particles that compose the bed. This research aimed to perform a preliminary evaluation and characterization of the energy potential and the fluidization curves in fluidized beds formed by binary mixtures of eucalyptus chips + sand and mineral coal + sand. We tested: 1) physical characterization of solid fuels; 2) chemical characterization of combustible materials; 3) thermogravimetric analysis of fuels; 4) determination of the fluidization curves and minimum fluidization velocity for a polydisperse bed. We observed 19.15 MJ kg-1 of lower calorific value for eucalyptus chips and 10.1 MJ kg-1 for coal. The increase in biomass percentage in mixture caused a pressure drop in bed, indicating the formation of preferred paths and a necessity to increase fluid velocity. The fluidization of coal and Eucalyptus chips can be viable in a bubbling fluid bed process, motivating future theoretical and experimental studies involving the application of this methodology in the development of clean and sustainable technologies.

Prediction of particle circulation rate in an internally circulating fluidized bed with a central draft tube

Controlling the internal particle circulation rate quantitatively is of great importance to the operation of the internally circulating fluidized bed with a central draft tube used in the biomass gasification field. In this work, the effects of the operating parameters on the internal particle circulation rate were investigated on a laboratory experimental apparatus. The results indicated that the internal particle circulation rate increased with the increasing air velocity in the central draft tube, and gently increased with the air velocity in the annular bubbling bed. The internal particle circulation rate increased with the increasing initial static bed height in the annular bubbling bed and decreased with the increasing particle size. The correlation formula was obtained through the regression of experimental data. Furthermore, a dynamic prediction model was established based on the theory of fluid mechanics, and the matching degree between the predicted values and experimental data was larger than 80%.

Reduced-order model for CaO carbonation kinetics measured using micro-fluidized bed thermogravimetric analysis

A reduced-order model was developed to describe the CaO carbonation kinetics by simplifying the detailed one-dimensional rate-equation-based grain model. The reduced-order model was derived using the Thiele modulus method and is an analytical model, which can be easily used in the reactor scale model or computational fluid dynamics (CFD) simulation. In the reduced-order model, physical/chemical steps occurring during the carbonation process were considered, including external diffusion of gas around the particle, intraparticle diffusion, surface reaction, product islands growth and product layer diffusion. This model can be used to investigate the complex kinetic behaviors such as reaction controlled steps, kinetics transition from chemical reaction to product layer diffusion, and the effect of particle structure change on intraparticle diffusion, such as pore plugging phenomenon. The prediction accuracy of reduced-order model was verified by the detailed one-dimensional model. In order to demonstrate the application of the reduced-order model for reactor scale analysis, the CaO carbonation kinetics under different temperatures (600–750 °C), CO2 concentrations (25–75 vol%) and particle sizes (0.2–3 mm) were measured using a novel micro-fluidized bed thermogravimetric analysis (MFB-TGA) method, which is based on real-time mass measurement of CaO sample in a fluidizing state. The reduced-order model was implemented into a K-L two-phase bubbling bed reactor model to analyze the experimental data of a micro-fluidized bed reactor. Results indicated that the two-stage behavior of carbonation kinetics can be described successfully using the model, and the effect of particle structure change on pore plugging and intraparticle diffusion can be well predicted and explained. The carbonation is first-order reaction when the CO2 partial pressure is within 75% (1 bar). The effect of particle size on carbonation kinetics shows three-zone characteristics of reaction/diffusion combined controlling mechanism, and was discussed in detail. The reduced-order model provides a promising application for describing multiscale mechanisms (surface, particle and reactor) of carbonation kinetics.

TISSUE ENGINEERING BIOREACTORS: POTENTIAL APPLICATIONS AND SCALE UP STRATEGY

Tissue engineering bioreactors have been used in order to achieve production of artificial tissue, increasing cell proliferation capacity and yield and/or in vitro tissue/disease modelling. Although it is still discussing how to obtain functional and vascular tissue with these bioreactors, preclinical and clinical studies are ongoing. Tissue engineering bioreactors have been used as lab-scale bioreactors until now. Crucial potential application areas can be created by increasing the production capacity and bioprocess efficiency of these bioreactors. In this review, recent bioreactor technologies such as spinner flask, rotating wall/bed, hollow fiber membrane, perfusion and mechanical stimuli bioreactors are briefly presented in terms of their potential applications in medical field especially in the scope of scale-up approaches such as bubble column, stirred tank, membrane, air lift, fluidized packed and bed bioreactors.
 Peer Review History: 
 Received 1 May 2020; Revised 11 June; Accepted 6 July, Available online 15 July 2020
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 Name: Dr. Sally A. El-Zahaby
 Affiliation: Pharos University in Alexandria, Egypt
 E-mail: sally.elzahaby@yahoo.com
 
 Name: DR Viney Chawla
 Affiliation: University Institute of Pharmaceutical Sciences
 Baba Farid University of Health Sciences
 Sadiq Road, Faridkot-Punjab 151203
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On the Thermal Regime of the Protective Granular Bed of a Gas Distributor

A numerical solution of a two-temperature problem on the thermal state of an aerated granular bed, constituting the thermal protection of the gas distributor of a high-temperature bubbling bed, is carried out. An equation is obtained for the coefficient of the efficiency of cooling the gas distributor. It is shown that on the side of the bubbling bed the surface temperature of the granular bed is independent of its thermal conductivity and thickness.

Characterization of bubble behaviors in a dense phase pulsed gas–solid fluidized bed for dry coal processing

Pulsed gas–solid fluidized beds can effectively separate fine coal, and bubbles play an important role in creating suitable separation conditions. The present study performed statistical and image analyses of the evolution of bubbles in a two-dimensional pulsed gas–solid fluidized bed using a high-speed dynamic camera. The effects of apparent gas velocity, pulsation frequency and particle size on bubble characteristics and bed expansion were analyzed. The results indicate that, when a fluctuation frequency is added, the expansion height of the bed increases, the effect of attachment to the bed wall decreases, the leading diameter and rising velocity of the bubbles both decrease and the degree of bubble deformation increases. These trends are also more obvious for fine particles. These findings demonstrate that a high density pulsed gas–solid fluidized bed can effectively combine gases and solids to produce a uniform, stable mixture. The bubble diameter and rising velocity were also simulated in the present work, and the relationship between the two was established using a fitting model with an error within 5%. This model provides an effective means of predicting bubble velocity as well as studying the distribution of the bubble phase and improving the stability of the bed density.

Desulfurization using limestone during sludge incineration in a fluidized bed furnace: Increased risk of particulate matter and heavy metal emissions

Incineration of sludge can be an effective method to minimise waste whilst producing useful heat. However, incineration can cause secondary pollution issues due to the emission of SO2, therefore a set of experiments of sludge incineration in a bubble bed furnace were conducted with limestone addition to study desulfurization of sludge incineration flue gas. As expected, over 93% emission of SO2 was reduced with limestone addition, and that of CO and NOx were increased and decreased respectively when the fuel feeding rate raised. The distribution of fly ash was also increased by raising the fuel feeding rate due to increasing fragmentation of the ash. However, distributions of PM2.5 and heavy metals in submicron particles have dramatically increased with limestone desulfurization. The mechanism was revealed by SEM and EDS statistical analysis, indicating that the reaction between aluminosilicate and calcium made particles agglomerate and eutectic mixtures form, these larger ash particles were found to divide between collection as cyclone ash and fragmentation into finer particles that bypassed the cyclone. Those fine particles provided more surface area for heavy metal condensation. Furthermore, it was found that the reaction mechanism for semi-volatile metals involved them being released from the sludge and forming PM1 particles due to the vaporization-condensation mechanism, leading to higher emission of PM1 and distribution of heavy metals in PM1. Thus, it should be considered that there may actually be higher emission risks of PM and heavy metal emissions when aiming to desulfurize a flue gas using Ca-based minerals in certain circumstances.

Simulations of sorbent regeneration in a circulating fluidized bed system for sorption enhanced steam reforming with dolomite

In this work, the sorption enhanced steam reforming (SESR) method was developed for improved hydrogen (H2) production, and the drawbacks of conventional steam reforming processes on H2 yield and purity were overcome. However, the SESR process is discontinuous and requires regeneration after sorbent saturation with CO2. The circulating fluidized bed reactor (CFBR) system has previously been proposed for continuous H2 production, with both reforming and sorbent regeneration occurring simultaneously. The main aim of this work was to determine the feasibility and performance of SESR with a proper design and conditions in conjunction with the CFBR system. The reforming riser and bubbling bed regenerator are studied separately but related to each other. Two-dimensional transient models using the Euler‒Euler approach and kinetic theory of granular flow were used for fluid dynamic simulations combined with the decarbonation kinetics of dolomite, to investigate a conceptual regenerator system and determine its key conditions. A mixture of the Ni-based catalyst and dolomite from the risers was injected with a flux of 200kg/(m2s) and a catalyst to sorbent ratio of 2.54kg/kg. A double-stage bubbling bed regenerator system was designed with 1.2m width, 0.8m bed height, a gas inlet velocity of 0.2m/s and solid preheating at 950°C. The used dolomite was regenerated with an assumed CaO conversion of 3%; the almost fresh dolomite was then released with good mixing of the catalyst and sorbent.

Research on gas bubble formation using CFD during gas kick

Gas bubble formation behavior in the wellbore has influence on wellbore temperature and pressure by varying bubble movement characteristics. For now, most researches on gas bubble migration along the wellbore bases on bubble bed gas-liquid two-phase flow models in chemical industry, which is far away from the reality of the situation of wellbore during drilling process. Bubble channel is perpendicular to gravity and buoyancy during drilling, but it is parallel to gravity and buoyancy in the chemical industry. Based on computational fluid mechanics, gas bubble formation which channel perpendicular to buoyancy and gravity is simulated using CFD, the process of gas inflow, gas bubble formation and departure is analyzed. The flow field changes around bubbles and influential factors of gas bubble formation are also studied in this paper. Simulations indicate that the shape of the bubble during gas kick has always maintained asymmetric over development process. Formation pore radius and fluid density have significant effect on departure radius and time of gas bubbles, while the influence of fluid viscosity is slight. The main contribution here is that gas bubbles formation characteristics in bottom hole are revealed based on bubbles accumulation rules of bottom hole.

Combustion Performance of Sludge From a Wastewater Treatment Plant in Fluidized Bed. Factorial Modeling and Optimization of Emissions

The combustion performance of a bio-solid waste from a wastewater treatment plant was studied in terms of efficiency and gaseous emissions, by carrying out experiments in a bubbling fluid bed unit and was compared with that of an agricultural waste. The effects of excess air and fuel feeding rate were investigated and a factorial design and modelling was developed in order to provide optimal values of process variables for minimizing emissions. The results showed that sewage sludge fuel burned at a much lower temperature than the woody fuel. CO emissions were low, SO2 emissions from the agricultural waste negligible, while those of sewage sludge exceeded legislation limits when excess air was below 50%. An increase in excess air from 30 to 50%, or of fuel feeding rate from 0.48 to 0.6kg/h, resulted in higher NOx emissions from the woody fuel, exceeding emission guidelines. Concerning SO2 and NOx emissions from sewage sludge fuel, the effect of feeding rate was the same, however that of excess air was opposite, reducing their levels in flue gases. Combustion efficiency for both fuels was high, between 98.5 and 99.6%. Factorial design analysis and modeling indicated that for the woody fuel the optimum emission performance was accomplished when the feeding rate was 0.48 kg/h and the excess air 30%. For the sewage sludge fuel the optimum combustion and emission performance was achieved when the feeding rate was 0.72 kg/h and the excess air 50%.

Precise identification of the end of the gas maldistribution in bubble columns equipped with perforated plate gas distributors

The gas maldistribution in bubble columns (BCs) is detrimental to their performance. In the literature hitherto, there is no any systematic studies about this practical problem. In this work, for the first time the upper boundary of the gas maldistribution has been identified based on different entropies (total and maximum information as well as Shannon) extracted from various time series. In addition, three other parameters (maximum number of visits in a region, number of crossings of the mean and modified average absolute deviation) have been introduced for successful identification of the range of gas maldistribution. Different BCs equipped with various types of perforated plate (PP) gas distributors have been used. Both shallow and deep bubble beds have been studied. Water and organic liquids have been investigated.It has been found that when the hole diameter (of the PP gas distributor) is between Ø 0.5 × 10−3 m and Ø 1.32 × 10−3 m, the gas maldistribution is stable up to superficial gas velocity Ug of 0.02 m/s. In the case of nitrogen-ethanol system, the transition velocity has occurred at much lower Ug (0.0062 m/s). Only when the hole diameter is as large as Ø 4.0 × 10−3 m, then the upper boundary of the gas maldistribution shifts to somewhat higher Ug value (0.034 m/s).

Analyzing dominant particle-flow structures inside a bubbling fluidized bed

The particle phase of a gas-particle flow inside a bubbling fluidized bed is characterized by strong unsteady flow patterns and intense meso-scale fluctuations that give rise to an intense mixing rate. As such, it is important to gain a deeper insight into how particle-flow structures and the associated fluctuating velocity field contribute to the overall bed dynamics. To this end, advanced post-processing methodologies, i.e., the Proper Orthogonal Decomposition (POD) and the swirling strength criterion, are applied to the particle flow fields predicted by a “two-fluid model” of a cylindrical bubbling bed to identify and analyze the dominant spatio-temporal patterns of the particle phase. Three-dimensional POD results indicate that the dominant particle fluctuating velocity patterns are principally aligned in the axial direction, corresponding to particle mixing by the bubble wakes, with significant laterally directed fluctuating velocity vectors at the bed surface, corresponding to mixing caused by the bubbles bursting. The particle velocity gradient tensor is decomposed based on the swirling strength criterion and reveals formation of extended and flat structures that may be considered as a characteristic feature of the particle vortical motions in bubbling beds.

Investigating the flow structures in semi-cylindrical bubbling fluidized bed using pressure fluctuation signals

Hydrodynamic characteristics of a gas-solid semi-cylindrical fluidized bed was experimentally investigated and compared with that of a cylindrical bed by analysis of pressure fluctuations. Pressure fluctuations were analyzed in time and frequency domains using standard deviation, power spectral density function and discrete wavelet transform methods. Experiments were carried out in two semi-cylindrical and cylindrical fluidized beds of 14 cm in diameter each, operating in the bubbling fluidization regime at ambient pressure and temperature. Both beds were filled with glass beads of various sizes (120, 290 and 450 µm). The superficial gas velocity was varied in the range of 0.2–0.8 m/s. Results showed that although the minimum fluidization velocity is influenced by the particle size, it is not affected by the geometry of the bed. It was shown that the hydrodynamics of both beds are very similar and the difference is negligible. Number of large bubbles is slightly larger in the semi-cylindrical bed as compared with the cylindrical bed. Also, increase in the particle size and superficial gas velocity result in a greater difference between the number of large bubbles in both beds and the number of large bubbles in the semi-cylindrical bed increases slightly faster than in the cylindrical bed.