Bed Aspect Ratio Research Articles

DEM study on the force chain evolution of biaxial compression of pebble bed

Li4SiO4 and Li2TiO3 are granular materials in the pebble bed of fusion reactors, and their physical parameters and mechanical properties directly affect the working status and structural design of the pebble bed. The force chain can be employed to intuitively describe the mechanical properties of the pebble bed in the particle aggregate. Based on the discrete element method, we conducted a numerical study on the evolution characteristics of the force chain of Li4SiO4 and Li2TiO3 pebble beds under biaxial compression and explored the friction coefficient and pebble bed effect of the aspect ratio on the force chain distribution. The results revealed that as the particle friction coefficient increased, the force chain tended to be isotropically distributed, indicating that the friction mechanism was more conducive to a uniform distribution of the force chain. During compression, the average coordination number of the pebble bed increased with the friction coefficient. The Li2TiO3 particles had larger gravity than the Li4SiO4 pebble bed therefore, the Li2TiO3 pebble bed accounted for more force chains in the vertical direction. When the aspect ratio of the pebble bed was less than 0.5 or greater than 2.5, the distribution of force chains exhibited strong anisotropy. Conversely, when the pebble bed aspect ratio ranged between 0.5 and 2.5, the distribution of force chains tended towards an isotropic trend. Moreover, the number of force chains in each direction varied with changes in the aspect ratio of the pebble bed, characterized by a high concentration at the edges and a lower concentration in the middle. The results can provide an in-depth understanding of the force chain distribution and evolution characteristics in the pebble bed and provide a theoretical basis for designing and analyzing tritium breeding pebble beds.

Bubbles and bed expansion in low pressure fluidization

Fluidization at low pressure has significant benefits over traditional fluidized beds for the fine chemical industry. Yet there is a lack of knowledge of bubbling and bed behaviour under the slip flow regime (low pressure), and this presents a hindrance to the optimization of this process. In this work, the fundamental aspects of gas bubbles and bed expansion are investigated for a wider range of operating pressure, flow rates and bed aspect ratio. Typically, rising individual bubbles undergo a series of breakups and coalesce as they rise in the bed. The lowering of pressure below 14,884 Pa was found to inhibit this behaviour indicating a qualitative indication of the impact of pressure on the bubble dynamics. The bubbles formed at low pressure were larger in average size, greater in number at the inlet and have a higher aspect ratio. All these factors were directly related to the expansion of gas due to the lowering of operating pressure. Under a controlled environment with single rising bubbles, the gas expansion was predictable using the ideal gas equation. However, where the pressure is low enough (≤ 24,231 Pa) the inhomogeneous fluidization impacted the predictability and trend of the bubbling zone and bed expansion. Despite this, the overall consumption of fluidizing mass was magnitudes lower to generate similar sizes of bubbles at atmospheric pressure. This has a direct impact on the usage of fluidizing gas while maintaining similar gas-solid mixing behaviour. Overall, the benefits of low pressure can be obtained at moderate pressure between 49,130 pa and 24,231 Pa at AR 3 with homogeneous fluidization.

Hydrodynamics of expanded bed adsorption studied through CFD-DEM

The hydrodynamics of the Expanded Bed Adsorption process is studied through simulations combining Computational Fluid Dynamics and the Discrete Element Method. A representative base case is defined, based on process design parameters commonly encountered in literature. Then, 19 other cases are defined, each representing a singular adjustment to the column design, material properties, or operating conditions. The parameters that are varied are the expansion factor, liquid viscosity, bed aspect ratio, mean particle density, width of the particle density distribution, width of the particle size distribution, column taper angle, and column alignment angle. The impact of each adjustment on the bed behaviour is discussed, using the local particle size distribution and solids dispersion coefficient as main indicators of bed stability. Optimal performance was found for an expansion factor of two to three, and the combination of particle size distribution and particle density distribution was found to greatly improve bed stability. The mixing process of the liquid and solid phases is concluded to be of highly complex nature, and cannot simply be predicted from the liquid flow velocity.

Thermo-economic optimization of a novel confined thermal energy storage system based on granular material

Concentrated solar power is a suitable technology for production of green electricity. However, to attain a uniform electricity production, concentrated solar power should be coupled with large Thermal Energy Storage (TES) systems. Among the different technologies of TES systems, storage of sensible heat in granular material is widely used due to its simple operation. These TES systems store energy as an increase of temperature of a large mass of small solid particles, through which a fluid circulates exchanging heat. TES systems are typically operated in a fixed bed regime, maximizing their exergy output, thus limiting the maximum allowable velocity of the fluid flow. In this work, a novel confined bed is proposed to mechanically prevent the motion of the solid particles conforming the TES system even for high fluid velocities, to guarantee that the exhaust temperature of the fluid is maximum during a discharge process. In this novel confined bed, a thermocline evolves from bottom to top of the system, separating the low and high temperature of the bed during the discharge process. An analytical model was applied to describe the evolution of the thermocline and the effect of the different operating parameters on the thermocline thickness.The effect of the thermocline thickness was combined with a thermo-economic analysis of a confined bed TES system proposed for a case of study. The new confined bed here proposed was optimized considering thermodynamics aspects, namely the fluid exergy increment in the bed, and economic factors, specifically the total investment cost of the TES system. The optimization resulted in low values of the fluid velocity, between 0.2 and 0.4 m/s, but still higher than the minimum fluidization velocity of sand particles of 750 μm, justifying the requirement of a confined bed, and low bed aspect ratios, between 0.25 and 0.9, to prevent excessively high fluid pressure drops. However, the bed aspect ratio increases significantly for higher granular material particle sizes, up to a ratio of bed height to diameter of 3 for a particle size of 10 mm and a TES demand time of 6 h.

Reliable identification of the first transition velocity in various bubble columns based on accurate sophisticated methods

In bubble columns (BCs), the reliable identification of the first transition velocity Utrans-1, marking the end of the homogeneous regime, is an essential prerequisite for the successful BC design since this parameter marks the onset of bubble coalescence. In this work, a new database with 13 experimental Utrans-1 values at various operating conditions is reported. They are identified precisely by different sophisticated methods, which are considered most suitable and powerful for the analysis of the specific experimental data used. The BCs have different column diameters (and bed aspect ratios) and they are equipped with different perforated plate (PP) gas distributors. For Utrans-1 identification, eight different time-dependent signals recorded in air-deionized water, air-tap water, nitrogen-tap water, nitrogen-ethanol (96%), air-therminol and air-benzonitrile systems have been analyzed. The hidden information in the time series has been quantified by reliable identification parameters such as degree of randomness of the signal, Kolmogorov, information and reconstruction entropies and degree of disorder of the signal. Their profiles as a function of superficial gas velocity have been used for flow regime identification.In the case of column diameters between 0.1 m and 0.46 m and bed aspect ratios beyond 5, a simple empirical rule-of-thumb has been established: when the open area (OA) of the PP gas distributors is ≤1.0%, the Utrans-1 value should occur at around 0.03 m/s. On the other hand, when the OA is greater than 1.0%, then the Utrans-1 value is identifiable at 0.04 m/s. It has been found that this new rule yields better predictions than the most popular empirical correlation of Reilly et al. (1994).

Experimental study on the effect of bed aspect ratio to the drying rate of chilli for swirling fluidized bed dryer

Swirling Fluidized Bed Dryer (FBD) has better performance because of the improvement of air flow with less pressure drop. However less study focus on the performance of swirling FBD especially at small-scale. Thus, the objective of this study is to investigate drying characteristics of green chillies at different ratio of chilli, Rchilli and bed aspect ratio, RBed by the swirling fluidized bed dryer. An electrical blower with 600 W of power was used as a main source to supply air at flow rate of 0.324 kg/s which equals to the velocity at the distributor of 8.25 m/s. The chamber has 400 mm height and 150 mm diameter with maximum capacity of 2 kg. The air distributor used has 67% inclination angle to provide air swirling inside the FBD. It was found that RBed increased from 0.8 to 1.6, the drying performance decreased in which the final moisture ratio was higher at 0.72-0.79 for RBed = 1.6 as compared to better moisture ratio at 0.18-0.24 for RBed = 0.8. The drying rate was at 0.225-0.275 %/min for RBed = 0.8, and 0.075-0.10 %/min for RBed = 1.6. When the Rchilli increased from 1:1 to 2:1, the drying performance showed slightly decreased. Moisture ratio also slightly increased from 0.18 to 0.24 for RBed = 0.8, whereas from 0.72 to 0.79 for RBed = 1.6. This finding suggest that the RBed has more significant effect than Rchilli .

Hydrodynamic and Heat Transfer Study of a Fluidized Bed by Discrete Particle Simulations

A numerical simulation study was carried out to study the combined thermal behavior and hydrodynamics of a pseudo-2D fluidized bed using a computational fluid dynamics–discrete element method (CFD-DEM). To mimic the effect of heterogeneous exothermic reactions, a constant heat source was implemented in the particle energy equation. The effects of superficial gas velocity, bed height and heat source distribution were analyzed with the aid of averaged volume fraction and temperature distributions and velocity profiles. It was found that both the gas superficial velocity and the bed aspect ratio have a profound influence on fluidization behavior and temperature distributions.

Thermal design and management towards high capacity CO2 adsorption systems

One of the most immediate technologies to mitigate the emission of carbon dioxide is carbon capture and storage. This study investigates different thermal design alternatives to increase the capacity of adsorption system given a desired cycle time. This is equivalent to investigating the reduction of the cooling and heating times during the adsorption and desorption processes, respectively. The thermal design investigation involves the variation of bed aspect ratio, the option of using water as a coolant, and augmenting the bed by fins with varying length and pitch. Mg-MOF-74 is used as adsorbent since it shows high CO2 uptake at flue gas conditions. An experimentally validated CFD model is implemented by using user-defined-function (UDF) linked to the ANSYS Fluent program. Results show that rising the bed aspect ratio from 2.8 to 41.6 improves the CO2 uptake by 13% and reduces the desorption period to the third. Additional improvement in the CO2 uptake (10.9–13.6%) can be obtained by cooling and heating the bed by water as a function of the bed aspect ratio. The improvement jumps to 22.2% if fins are used to augment the heat transfer rates in the bed. The optimal CO2 uptake enhancement (35.2%) is achieved by using finned annular-pipe (fin-pitch is 12.5 mm) as the adsorbent bed with water cooling. This design methodology can be adopted to other adsorption systems, including solar-driven adsorption chillers and solar-driven adsorption desalination systems.

Autothermal reactor design for catalytic partial oxidations

A comprehensive analysis of the impact of various design and operating variables on the boundary of the region of autothermal operation of type III is presented. It is shown that for a fixed adiabatic temperature rise and space time, the largest region of autothermal operation is obtained in the homogeneous limit (small particles or high cell density monoliths) when the bed scale heat Peclet number approaches zero, mass Peclet number approaches infinity, and with smaller bed aspect ratios (height to diameter). For competing multiple oxidation reactions, while thin (shallow) beds could lead to larger region of autothermal operation, better selectivity of intermediate product may be attained using very thin beds. The impact of heat loss on the region of autothermal operation is also analyzed. When inter and intra-phase heat and mass transfer gradients are significant so that particle (channel) level ignition could occur, autothermal operation is feasible for any bed level heat Peclet number with reactor operated in the mass transfer controlled regime. The results of the analysis are applied to two processes of interest, namely, oxidative coupling of methane (OCM) with adiabatic temperature rise in the range 600–1200 K and catalytic partial oxidations (CPO) with adiabatic temperature rise in the range 200–500 K. For OCM, some feasible autothermal reactor designs that are suitable for different catalyst activity levels are proposed. For CPO, an autothermal reactor design is compared favorably to the traditional cooled multi-tubular reactor.

Fluidization in small-scale gas-solid 3D-printed fluidized beds

Additive manufacturing could be used to facilitate the rapid fabrication and testing of small-scale fluidized beds for use in screening applications, such as adsorbent screening for carbon capture. In this work, experiments were performed in order to map the different flow regimes produced in small-scale (Dh = 3–15 mm) gas-solid fluidized beds that were fabricated using additive manufacturing using the stereolithography approach. Here, the effects of bed hydraulic diameter (Dh), static particle height (Hs), and particle type/size (Dp and ρp) were considered. Pressure drop data and high speed camera images were used to develop simple flow regime maps for these printed beds showing the operating windows for packed bed, bubbling, slugging and turbulence applicable to a wide range of bed size to particle diameter ratios (Dh/Dp = 20–200) and gas velocities (Ug = 1–400 mm/s) in both ‘2D’ and ‘3D’ bed aspect ratios. Fast Fourier transforms of the pressure drop signals were also used to study the evolution of bubbling/slugging behaviour as the gas velocity was increased by creating 2D colour maps of the frequency spectra. These allowed a new quantitative method to be proposed for the identification of slugging – the point where the dominant frequency in the power spectrum becomes constant as the gas velocity increases. It is concluded in this study that the rougher surfaces generated by additive manufacturing do not influence the fluidization characteristics nor modify the wall effects of small-scale beds. Macro-scale fluidization could also be achieved at smaller Dh/Dp ratios in these 3D printed beds compared to more conventional Plexiglas beds (Dh/Dp = 75 compared to Dh/Dp = 300).

Effect of particle size distribution on mixing and segregation in a gas-solid fluidized bed with binary system

The mixing and segregation characteristics were investigated in gas-solid fluidized beds with binary solid mixture. Bed materials were constituted with binary solids, having different size and density. To investigate the effect of particle size distribution on the mixing characteristics, two other binary solid mixtures were used, which have similar mean particle size and particle density, but their particle size distribution was different to each other. Bed column has ID=0.14 m and H=2.14m. Bed aspect ratio was 3.0. Bed materials were two sets: one of bed materials was the mixture of ilmenite (dp=153 mm, ρs=3,860 kg/m3) and coke (dp =582 mm, ρs =1,762 kg/m3), which has wide size distribution. The other bed materials was the mixture of ceramic beads (dp =122 mm, ρs =3,800 kg/m3) and plastic beads (dp =813 mm, ρs =1,500 kg/m3), which has narrow size distribution. Bed composition of ilmenite-coke mixture was determined to 0.7:0.3 by mass ratio. And, bed composition of ceramic beads-plastic beads was 0.75:0.25 by mass ratio. Axial bed pressure drop was measured according to gas velocity. Bed composition was measured according to axial bed height by sampling. Bed pressure drop of ilmenite-coke mixture was maximized above Uo=0.15 m/s, and fully fluidization was occurred. However, criterion of mixing to segregation was not found in the axial bed composition according to gas velocity. In the case of ceramicbeads-plastic beads mixture, bed pressure drop was maximized at Uo=0.05 m/s, and the criterion of mixing to segregation was found at the same gas velocity.

Bubble-induced particle mixing in a 2-D gas-solid fluidized bed with different bed aspect ratios: A CFD-DPM study

A CFD-DPM method has been employed to investigate the influence of initial particle height to bed width (i.e., bed aspect ratio) on the particles mixing trend in a gas-solid fluidized bed. Moreover, the effect of particle size and density on the particles mixing have been investigated. Particles density has no considerable effect on the mixing time, but mixing time increases by increasing the particle diameter. The simulation results show that the initial bed aspect ratio has no considerable influence on the lateral mixing index, but in the axial direction at reduced bed aspect ratios the particles mix very well. It was found that changing the bed regime toward the slug flow is the main reason for poor mixing indices at higher bed aspect ratios. A correlation is proposed for prediction of the mixing index in single bubble regime on the basis of some dimensionless groups.

Multicomponent and multi-dimensional modeling and simulation of adsorption-based carbon dioxide separation

Abstract A two-dimensional transient numerical study of the adsorption of CO2/N2 and CO2/H2 mixtures on activated carbon and MOF-177 in a fixed bed is presented. Because most of the commercial CFD codes are not capable of simulating adsorption processes in a straightforward fashion, we developed an additional code to solve for the different species transport including adsorption and diffusion and used Fluent to simulate the adsorption process, fluid flow, heat and, mass transfer. We simulated the adsorption process at high temperature-low pressure and low temperature-high pressure conditions as well as the pressure swing adsorption. For adsorption of CO2/N2 and CO2/H2 mixtures, Toth and Viral models are used calculate equilibrium isotherms. The breakthrough curves obtained from the simulations compare well with the experimental data. Moreover, the effects of aspect ratio and geometric shapes were studied. The results show that varying the bed aspect ratio from 7.77 to 2 has an insignificant effect on the adsorption capacity and performance.

Improvement on particulate mixing through inclined slotted swirling distributor in a fluidized bed: An experimental study

Previous studies show that excellent particulate mixing in a fluidized bed can reduce the operating cost during fluidization. Therefore, this paper investigates enhancement of particulate mixing in a fluidized bed by using novel inclined slotted swirling distributor. To reduce the cost of pumping power, small size, low pressure blower is used in the study. Moreover, Geldart group B bed materials with different bed aspect ratios and distributor designs viz., perforated plate, circular edged slots (90°) and novel swirling (45°) distributors are used. The novel distributor with 45° inclined slots was found to be effective at enhancing the circulation rate. Swirling flow pattern of the bed materials in a clock-wise direction is obvious in shallow bed, and two-layer transversal-lateral circulation motions are observed in deep bed. It can be concluded from the study that excellent particulate mixing as per rotated distributors is made possible by novel swirling-type distributor without the implementation of electric motor and mechanical rotation.

Prediction of equilibrium mixing state in binary particle spouted beds: Effects of solids density and diameter differences, gas velocity, and bed aspect ratio

The state of solids mixing can be considered as a key factor in accomplishment of high efficiency and a uniform product in gas–solid fluidized-beds operating with several different particle sizes. In the present work, coupled DEM–CFD approach was used to carefully investigate the mixing process of binary particles in flat-bottom spouted beds. In this regard, simultaneous effects of particle specifications, operating conditions, and bed dimensions on the equilibrium mixing state of solid mixture were evaluated in detail. Moreover, taking into account the contribution of various parameters, the obtained simulation results were used to draw a correlation for predicting the equilibrium mixing index of similar spouted beds. In addition, the upper limit of particle specification difference and the appropriate bed dimensions to achieve a specific mixing index value was determined. Furthermore, the performance capability of spouted and conventional gas–solid fluidized beds regarding solids mixing was compared in detail.

Stagnant regions estimation in fluidized beds from bed surface observation

A novel approach to estimate the size of stagnant regions in large-scale fluidized beds by means of experimental data obtained from images recorded on the bed surface was presented. For this purpose, the internal structure of an induced maldistributed pseudo-2D fluidized bed was first studied. Half of the total distributor area was covered to generate an induced stagnant region. The size and shape of this area was studied for several relative gas velocities and bed aspect ratios. The defluidized area was found to be almost independent of the bed aspect ratio, however, it was found to decrease for higher relative gas velocities. A solids recirculation zone was also found above the defluidized zone. The size of this zone increases with relative gas velocity, suggesting that it is strongly related to bubble motion. A correlation was developed to relate the visible bubble flow to the size of the defluidized zone. The results obtained in the 2D bed were extrapolated to a 3D cylindrical fluidized bed with a half-covered distributor plate to estimate the volume of the defluidized zone. The visible bubble flow in the 3D facility was estimated. Using the proposed correlation for the defluidized volume in the 2D bed was used to estimate the defluidized volume in the 3D bed. Finally, the calculated values of the defluidized volume were compared with the experimental values in the 3D facility, obtaining a maximum relative error of 20% in the estimations.

Multiresolution Analysis of a Drying Process in a Rotating-Distributor Fluidized Bed

ABSTRACTHumidity and temperature measurements, together with pressure fluctuation signals, were employed to analyze a drying process in a lab-scale bubbling fluidized bed. The experimental facility was equipped with a rotating distributor. The operational conditions reported in sludge, paste, and granular drying processes were reproduced using silica sand as wet material and as inert medium. Experiments were performed by changing the bed aspect ratio and the water-sand content in the bed operated at ambient conditions. Four drying periods were found when analyzing humidity and temperature signals. The multi-resolution approach of the pressure fluctuation signals showed the effect of the sample drop over the bed surface on the fluidization conditions, by relating the drying periods with the bed dynamics. The drying process can affect the low- and high-frequency details of the pressure signal when the bed state changes towards defluidization, or just the high-frequency detail if the bed was at the maldistributed regime. The drying conditions needed to use the pressure signals to control the drying process are defined by means of a statistical monitoring approach. A comparison between the static and the rotating distributor tests showed clear benefits for the operation with the rotating distributor for shallow and deep beds. When the distributor was rotating, the average improvement of the drying rate was 11%, whereas the drying time was reduced by 40% for the tests of the maldistributed and defluidized regimes.

Simultaneous catalytic reduction of N2O and NOx produced in a fluidized bed incineration of sewage sludge

AbstractFluidized bed incineration of sewage sludge could produce large amounts of N2O due to the relatively high nitrogen content of the sewage sludge and low incineration temperatures. In this study, N2O and NOx emissions released during the fluidized bed incineration of dried sewage sludge powder were investigated. Sewage sludge feeding rate was about 8∼10 kg/h, and bed aspect ratio was fixed to ca. 1.2. Superficial velocity was adjusted to ca. 1.9 m/s. The concentrations of N2O and NOx in the exhaust gas were greatly influenced by bed temperature in a range between 850 °C and 950 °C. In the simultaneous catalytic reduction of N2O and NOx on an Fe/BEA pellet catalyst with NH3, most of the N2O and NOx were removed when heated above the inlet reaction temperature 410 °C. Thus, simultaneous catalytic reduction was shown to be a promising N2O abatement technology for the fluidized bed incineration of sewage sludge. © 2015 Society of Chemical Industry and John Wiley & Sons, Ltd

Maldistribution detection in bubbling fluidized beds

A severe maldistribution problem was induced with a half-covered gas distributor plate. Videos of the bed surface were recorded and analyzed to study how the boundary between maldistribution and stable fluidization is affected by the gas velocity and the bed aspect ratio. It was observed that the visual inspection of the bed surface reports no information about the maldistribution at the bottom bed. Several cases of moderate maldistribution (i.e. different distributions of open orifices) were investigated by means of pressure fluctuation signal analysis to provide a criterion for maldistribution detection. The effect of the measurement position on maldistribution detection was also investigated. The attractor comparison test, the S-test, as well as the statistical process control of the standard deviation and wide band energy regions, were applied to test their capability of online detecting and monitoring gas maldistribution. The statistical process control methodology, based on standard deviation of pressure fluctuation signals, can be successfully applied to online detection of maldistribution when the pressure probe is located between 50% and 75% the total bed height. Finally, the rotation of the distributor was studied as a counteracting mechanism, showing good results overcoming maldistribution problems in fluidized beds.

CFD study on solids flow pattern and solids mixing characteristics in bubbling fluidized bed: Effect of fluidization velocity and bed aspect ratio

The effect of fluidization velocity and bed aspect ratio on solid flow behavior in bubbling beds was investigated quantitatively and qualitatively. Having used the two-fluid model based on the kinetic theory of granular flow, the set of governing equations was solved by applying finite volume method in two-dimensional Cartesian frame. In the bed with the aspect ratio of 0.8–1.5, Geldart's group B glass beads were fluidized by air at the fluidization number in the range of 1.5–3.5. The predicted solid flow pattern, and the axial velocity of emulsion phase were consistent with the experimental data presented by Laverman et al.. In the studied range, the simulation results demonstrated a strong dependency of solid flow pattern on fluidization velocity, while its dependency on bed aspect ratio was found to be marginal. Aimed at analyzing this solid flow behavior, the predicted solid flow patterns were attributed to the variation of solid mixing characteristics along the bed. Simulation results showed that the local and global mixing intensity increased along the bed from the distributor surface up to the position where two vertically-aligned vortices meet each other. Then, its value remained approximately constant up to the bubble bursting height, where a dramatic surge up to the bed surface occurs. Apart from this, an improvement in the solid mixing characteristics was observed by increasing the fluidization velocity leading to higher solid dispersion and diffusion coefficients, which come along with the top vortices elongation and bottom vortices shrinkage. Regarding the effect of bed aspect ratio, it was observed that the global mixing characteristics increase more significantly in comparison to the local ones for higher bed aspect ratios. Moreover, the simulation results showed that the predicted solid dispersion coefficient are in the range of 2.60×10−4m2/s to 6.18×10−3m2/s which is remarkably larger than the predicted solid diffusivity coefficients in the range of 1.31×10−4m2/s to 4.28×10−4m2/s. The dispersion coefficient changes more profoundly with the variation of superficial gas velocity.