Pressure Drop and Gas Holdup Studies in a Spout-Fluid Bed

Spout-fluid beds are used for a variety of processes involving particulate solids. They are employed where the particle agglomeration, dead zones, and sticking of particles to the vessel are the common problems in conventional spouted beds. Applications involved are granulation, coating, drying, combustion, and gasification. In this study, experimental studies have been carried out in a cylindrical Perspex column (0.094 m internal diameter and 1.217 m height) using glass beads and air. The effects of initial bed loading, spout velocity, and background (fluidization) velocity on pressure drop and gas holdup have been investigated. It is found that the minimum spout-fluidizing velocity increases with increase in initial bed loading. The pressure drop and gas holdup increase with increasing bed loading. In spout-fluid bed condition, at a constant spout velocity, as the background gas velocity increases, the gas holdup increases, and it is found to be high for smaller bed loading and is low for larger bed loading at higher velocities. The fountain height increases as spouting velocity increases and it decreases with initial bed loading. The total velocity required to fluidize the particles in spout fluidization is lower in comparison to spouted beds and fluidized beds.     

Mixing characteristics of binary mixtures in a spout-fluid bed

Mixing characteristics of binary mixtures in a flat-bottom cylindrical spout-fluid bed using glass beads and air are reported in this work. Experiments were carried out to investigate the mixing characteristics for binary mixtures at three flow conditions, i.e., only spouting, only fluidization and spout-fluidization. The experiments were performed at different gas velocities, diameter ratios of binary mixtures and three different bed arrangements. Mixing index was determined for fluidized bed and static bed conditions. It was found that, in all cases, lowest-diameter ratio mixture gave good mixing index values. For all flow conditions, mixing index for large–small bed arrangement was increasing with time, whereas for small–large bed arrangement, the mixing index deteriorated with time. However, in both cases, the mixing index reached almost a constant value. For well-mixed bed arrangement and spout-fluidization flow condition, segregation and re-mixing were observed.     

Detecting Sudden Changes in Fluidization by Wall Vibration

Vibrations of fluidized bed walls reflect the nonlinear characteristics of bed hydrodynamics in gas–solid fluidized beds. Experiments were carried out in a lab-scale, two-dimensional fluidized bed operated at ambient conditions for three particle sizes, various gas velocities, three aspect ratios, and different probe locations. The S-statistic method, which is, in fact, the comparison of attractors of two dynamic signals in the state space, was used to determine de-fluidization condition in the bed. Different scenarios were tested to evaluate whether this method is able to detect changes in the hydrodynamics fluidized bed based on the bed vibrations, including change in the bed mass, particle size, and gas velocity. The results showed that this method is capable of detecting the de-fluidization state in the bed as a result of changes in gas velocity, particle size, and bed mass. However, an important factor is the location of probe, which can dramatically affect the capability of this method for detecting the de-fluidization state.     

A review on pulsed flow in gas-solid fluidized beds and spouted beds: Recent work and future outlook

This paper reviews the application of pulsed flow in fluidized and spouted beds, widely used in various industries. A number of pulsing studies have been performed to improve the performance of these beds, enhance mixing and promote homogeneity. One effective way to increase the efficiency is to pulse the incoming flow, removing inactive or dead zones, thereby preventing agglomeration and settling. Although numerous studies have been carried out on conventional beds, little has been written on pulsed beds, in spite of their proven advantages. The role of pulsations in hydrodynamics, mixing, segregation, heat transfer, drying, and agglomeration are among the topics addressed. Future needs are identified and projected.     

Particle scale study on heat transfer of gas–solid spout fluidized bed with hot gas injection

ABSTRACT

In this paper, the heat transfer characteristics of a 2D gas–solid spout fluidized bed with a hot gas jet are investigated using computational fluid dynamics-discrete element method. The initial temperature of the background gas and particles in the spouted bed was set to 300?K. The particle temperature distribution after injection of 500?K gas from the bottom, center of the bed, is presented. The simulation results indicate well heat transfer behavior in the bed. Then, statistical analysis is conducted to investigate the influence of inlet gas velocity and particle thermal conductivity on the heat transfer at particle scale in detail. The results indicate that the particle mean temperature and convective heat transfer coefficient (HTC) linearly increase with the increase in inlet gas velocity, while the conductive HTC and the uniformity of particle temperature distribution are dominated by the particle thermal conductivity. The conductive and convective heat transfer play different roles in the spout fluidized bed. These results should be useful for the further research in such flow pattern and the optimization of operating such spouted fluidized 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.     

Fluidization behavior of glass beads under different vibration modules

The expansion of free bubbling gas fluidized beds has been investigated experimentally in a two-dimensional perspex-walled bed. Glass beads were fluidized with dried air at varying gas velocities, while the bed was vibrated at different frequencies, amplitudes and directions to study their effects on the fluidization quality. The experimental results showed that the particle flow pattern depends on the vibration direction, especially at superficial gas velocities less than the minimum fluidization velocity U_mf. The effect of horizontal vibration on fluidization behavior of glass beads exists at superficial gas velocities less than U_mf, while the effect of vertical vibration on fluidization behavior still exists even at higher superficial gas velocities than U_mf.     

Two-dimensional CFD simulation of chemical reactions in tapered-in and tapered-out fluidized bed reactors

This article presents a simulation study of tapered-in and tapered-out fluidized bed reactors to investigate the influences of apex angle on the fractional conversion and the pressure drop of the fluidized beds in the presence of two types of chemical reaction with gas volume increase and reduction. The 2D behavior of tapered-in and -out fluidized beds was also compared with a columnar one from fractional conversion and bed pressure drop point of views. To validate the simulation results, the numerical predictions for the expansion ratio and the pressure drop of a tapered fluidized bed were compared with experimental data and good agreement was observed. The obtained simulation results clearly indicate that an apt apex angle exists in tapered-in reactors in which the fractional conversion reaches a maximum value; while the variations of the apex angle slightly affect the fractional conversion in tapered-out fluidized beds. Increasing the residence time of the gas phase in the upper section of tapered-in beds has positive influences on the fractional conversion, while a further decrease in the gas phase velocity in the tapered-in reactors has a negative effect on the fractional conversion. Moreover, higher bed pressure drop was observed in tapered-in reactors than that in the columnar and tapered-out ones.     

Bed expansion and fluctuation in cylindrical gas solid fluidized beds with stirred promoters

A profound and fundamental understanding of bed dynamics such as bed expansion ratio and bed fluctuation ratio of irregular particles of binary mixtures has been made in a cylindrical fluidized column for gas solid systems, resulting in a predictive model for fluidized beds. In the present work attempt has been made to study the effect of various system parameters (viz. rotational speed of the promoter, initial static bed height, superficial velocity of the fluidizing medium, particle size and density) on the bed dynamics through experimentation. The correlations for the bed dynamics have been developed on the basis of dimensional analysis. It was observed that the calculated values of bed dynamics agree well with the experimental values in most of the cases.     

Solid Flow Transition in Liquid and Three-Phase Fluidized Beds

Flow transition of solids in liquid and three phase fluidized beds of Newtonian and non-Newtonian fluids have been studied in a 15.2 cm-ID pyrex glass column. The relation between the fluid flow rate and the bed porosity in three phase fluidized beds have been determined in terms of effective volumetric flux of fluid phases from the modification of the Richardson and Zaki's equation. The modified particle Reynolds Number exhibited its maximum value with the variation of bed porosity in liquid and three phase fluidized beds. The drag coefficient changed its slope apparently at the bed porosity where the maximum value of the modified particle Reynolds number could be attained. At the flow transition condition, the continuity wave velocity, energy dissipation rate, and the continuity shock wave velocity found to have their maximum values. Also, the immersed heater-to-bed and wall-to-bed heat transfer coefficients, wall-to-bed mass transfer coefficient, liquid radial mixing coefficient and solid particle diffusivity in the literature data were found to have maximum values at the transition condition of liquid and three phase fluidized beds.     

Non-Newtonian Fluids in Spout-Fluid Beds: Mass Transfer Coefficient at Low Reynolds Numbers

Following on from the work of Anabtawi et al. (2003), this study examined how the volumetric liquid-phase mass transfer coefficient, k_La, of oxygen in air in three-phase spout-fluid beds was affected by varying the system parameters of bed height, bed diameter, gas velocity, and liquid velocity. The liquid used was 0.1% CMC solution, displaying a pseudo-plastic rheology, with 1.75 mm glass spheres as packing. The values of the Sherwood number were lower than in previous studies (Anabtawi et al., 2003), in the range 9,000-186,000. Gas velocity had a similar effect on k_La as in a bubble column, with results also giving good agreement with previous work on two-phase and three-phase spouted bed systems. The correlation obtained for the effect of liquid velocity on k_La compared well with that of Schumpe et al. (1989). An increase in the height of packing increased k_La to the power of 0.319, with an increase in column diameter also causing an increase in k_La, which is in agreement with the results of Akita and Yoshida (1973).     

Solid Flow Transition in Liquid and Three-Phase Fluidized Beds

ABSTRACT

Flow transition of solids in liquid and three phase fluidized beds of Newtonian and non-Newtonian fluids have been studied in a 15.2 cm-ID pyrex glass column. The relation between the fluid flow rate and the bed porosity in three phase fluidized beds have been determined in terms of effective volumetric flux of fluid phases from the modification of the Richardson and Zaki's equation. The modified particle Reynolds Number exhibited its maximum value with the variation of bed porosity in liquid and three phase fluidized beds. The drag coefficient changed its slope apparently at the bed porosity where the maximum value of the modified particle Reynolds number could be attained. At the flow transition condition, the continuity wave velocity, energy dissipation rate, and the continuity shock wave velocity found to have their maximum values. Also, the immersed heater-to-bed and wall-to-bed heat transfer coefficients, wall-to-bed mass transfer coefficient, liquid radial mixing coefficient and solid particle diffusivity in the literature data were found to have maximum values at the transition condition of liquid and three phase fluidized beds.     

Numerical simulation of gas-solid flow in a novel spouted bed: Influence of row number of longitudinal vortex generators

The flow characteristics in a novel cylindrical spouted bed with spherical longitudinal vortex generators is numerically investigated by two-fluid model (TFM) with kinetic theory for granular flow, the longitudinal vortex technology is adopted in the spouted bed so as to strengthen the particles radial mixing between spout and annulus zones, the row number effect (1–3 rows) of longitudinal vortex generators (LVGs) on gas–solid flow behavior in three dimensional spouted beds was numerically simulated. The CFD results show that, longitudinal vortices can effectively increase particle volume fraction near annulus zone in the spouted bed, the maximum increase of particle volume fraction near annulus region is 183％, and the pressure drop in spouted beds increases with increasing of LVGs’ row number. There exists an optimal row number (equal 2) of LVGs, at witch the radial velocity of particle phase reaches maximum in the limited spouted bed space, the value of turbulent kinetic energy of gas phase in spouted bed can be significantly promoted by longitudinal vortex, espeically in the spout zone and near the annulus region. Also, the enhancement effect of multi-row LVGs on turbulent kinetic energy of gas phase decreases when the cross section height of spouted beds increases.     

A U-SHAPED FIBER OPTIC PROBE TO STUDY THREE-PHASE FLUIDIZED BEDS

A novel U-shaped fiber optic recently developed for three-phase fluidized beds was applied in the present study for bubble characterization in a cylindrical bed. The static pressure profile along the fluidized bed was measured by a data acquisition system constituted by a pressure transducer, a scani-valve and a microprocessor. Air, water and 335µm glass beads were used as gas, liquid and solid phases respectively. Liquid was evenly distributed by a perforated plate and air was introduced above the distributing plate through four injectors. Single core silica fibers were used to guide the helium-neon laser beams into the fluidized bed. Five U-probes, whose design is based on the difference of refraction indices between the gas and the liquid phases, were used for bubble detection. The detecting probes were located on a measuring window at 53.3 cm from the grid. The design of the measuring window allowed the U-probes to be slid into the fluidized bed at different radial positions. Bubble characteristics such as axial bubble length and bubble velocity were investigated. The influence of fluidization conditions on the hold-ups of gas, solid and liquid was also studied.     

A U-SHAPED FIBER OPTIC PROBE TO STUDY THREE-PHASE FLUIDIZED BEDS

A novel U-shaped fiber optic recently developed for three-phase fluidized beds was applied in the present study for bubble characterization in a cylindrical bed. The static pressure profile along the fluidized bed was measured by a data acquisition system constituted by a pressure transducer, a scani-valve and a microprocessor. Air, water and 335µm glass beads were used as gas, liquid and solid phases respectively. Liquid was evenly distributed by a perforated plate and air was introduced above the distributing plate through four injectors. Single core silica fibers were used to guide the helium-neon laser beams into the fluidized bed. Five U-probes, whose design is based on the difference of refraction indices between the gas and the liquid phases, were used for bubble detection. The detecting probes were located on a measuring window at 53.3 cm from the grid. The design of the measuring window allowed the U-probes to be slid into the fluidized bed at different radial positions. Bubble characteristics such as axial bubble length and bubble velocity were investigated. The influence of fluidization conditions on the hold-ups of gas, solid and liquid was also studied.     

Particle exchange in horizontal two-compartment fluidized beds – CFD simulation and experimental validation

A series of ideally mixed fluidized beds is used to obtain a narrow product size distribution in industrial-scale applications. Thereby, the fluidized bed compartments or stages are divided from each other by weirs with defined openings (slots) for particle exchange. The knowledge of particle exchange streams and residence times in different compartments, respectively, is crucial for the process design and optimization of such processes. This particle exchange behavior between different compartments in 3D-fluidized bed systems cannot be easily evaluated by experiments.Hence, a CFD multiphase model is used here and applied to a horizontal two-compartment batch fluidized bed apparatus to obtain particle exchange streams as a function of process parameters, e.g. slot height and bed mass. Good comparison of numerical and experimental results shows that this model can reasonably predict the particle exchange in two-compartment fluidized bed processes.On the basis of this work, the validated and predictive multiphase model can be applied to continuous horizontal multi-compartment fluidized beds as a future perspective to identify best-case process parameters, e.g. optimal slot height.     

Numerical study on the effect of longitudinal vortex generator on semi-dry desulfurization process in 3D spouted beds

A longitudinal vortex generator was introduced into spouted beds to improve the heat and mass transfer rates in a semi-dry desulfurization process. The flue gas desulfurization processes in a 3D conventional spouted bed (CSB) and a spouted bed with a longitudinal vortex (LVSB) were simulated and verified experimentally. The simulation results show that the generator can strengthen the gas-particle-slurry contact in the spouting and annular areas. The average interphase slip velocities of the gas-particle and gas–liquid in the LVSB were 26.6 and 26.5 % higher than that in the CSB. The generator also increased the liquid temperature and interphase mass transfer rates in the spouted beds. The simulated desulfurization efficiencies of the CSB and LVSB were 77.75 and 80.85 %, respectively, which indicate the addition of a generator is beneficial for promoting the semi-dry desulfurization process.     

Comparative analysis of numerically derived drag models for development of bed expansion ratio correlation in a bubbling fluidized bed

For designing and operating fluidized bed reactors, bed expansion ratio is one of the most important parameters. In this research, a bubbling fluidized bed is simulated using three-dimensional Eulerian-Eulerian method that is incorporated with kinetic theory of granular flow (KTGF) to calculate the pressure drop, gas volume fraction (GVF) and bed expansion ratio. Grid optimization is firstly conducted to achieve suitable solution for further simulations. Subsequently, different numerically derived drag models are employed to investigate the effect of these models on gas-solid flow dynamics. Afterwards, the fluidized bed is simulated at different gas superficial velocities employing two different drag models respectively. Simulation results have been comprehensively validated against experimental data. Finally, an expression for bed expansion ratio has been formulated and compared with the empirical correlation. The proposed correlation holds reasonably well with various experimental values. This work provides a scalable way to aid in designing and operating process reactors.     

Entrainment of Heterogeneous Particles from Gas-Fluidized Bed

The entrainment of heterogeneous particles in a gas-fluidized bed for particle mixtures of categories A and B according to the Geldart classification was conducted in this study. The experiments were carried out in an acrylic column of 0.092 m diameter. The distributors were perforated plates of up to 5.9% free area fraction. Measurements of pressure gradients were made using 24 pressure taps. Glass beads with particle diameters from 60 to 400 μm were used. The mixtures were described using the Rosin-Rammler-Bennet granulometric distribution model. The transport disengaging height (TDH) heights were obtained using the Geldart methodology. A 2⁵ experimental design was applied relating the dispersion index, mean diameter, solid mass, superficial gas velocity, and free area fraction of the distributor to obtain the TDH heights. Slugging and transition for turbulence regimes characterized the behaviors of fluidized beds. The results showed that TDH heights for heterogeneous particles were dependent on the solid mass and superficial gas velocity.     

Experimental and modeling studies on mixing behavior of binary mixtures in a spout-fluid bed

Experimental and modeling studies have been performed to determine mixing characteristics of binary mixtures in a spout-fluid bed. Spherical glass beads of diameters (3.075, 1.7, 1.2, and 0.75?mm) and air as fluidizing medium have been used in the study. Effect of various system parameters, namely, initial static bed height, gas velocity, diameter ratio, mixture composition, and sampling time on mixing of binary particles has been experimentally investigated. A dimensionless correlation has been developed for mixing index. Mixing behavior has been modeled using artificial neural networks (ANNs). Training of ANN was performed using the Levenberg–Marquardt (LM) backpropagation algorithm to predict the mixing index. The predictions of the ANN were found to be in good agreement with the experimental results and predictions from developed correlations.