Wet Fluidized Bed Research Articles

Carbon dioxide desorption behavior of potassium carbonate supported on gamma-alumina solid sorbent in wet fluidized bed under steam atmosphere

Since the carbon dioxide (CO2) capture using solid sorbent is a reversible reaction, the solid sorbent can be regenerated by the desorption process. Therefore, the desorption process is one of the key important processes for the CO2 capture system. Traditionally, most of the literature studies focus on the desorption of solid sorbent under an N2 atmosphere. However, the desorption process of the solid sorbent is inappropriate in the real system because the system will need another process to separate CO2 and nitrogen (N2) after the desorption process. This study focused on the CO2 desorption of potassium carbonate supported on gamma-alumina (K2CO3/γ-Al2O3) in a wet fluidized bed under a steam atmosphere by using the multiphase computational fluid dynamics (CFD) simulation. The effects of water thickness and dry restitution coefficient on CO2 desorption rate were investigated to provide a realistic particle collision behavior and to explore their effects on CO2 desorption phenomena. Moreover, the effect of steam velocity on the hydrodynamic behaviors of fluidization which on CO2 desorption rate was studied. The simulated results demonstrated that all the parameters, water thickness, dry restitution coefficient, and steam velocity had significantly affected system hydrodynamics and CO2 desorption rate in the wet fluidization desorption process. Furthermore, the effect of desorption temperature on CO2 desorption rate was evaluated for finding the appropriate temperature for CO2 desorption process of K2CO3/γ-Al2O3. The results showed that the appropriate desorption temperature for CO2 desorption under steam atmosphere was the temperature over 150 °C.

Review of CFD-DEM Modeling of Wet Fluidized Bed Granulation and Coating Processes

Wet fluidized bed granulation and coating processes have been widely used in the pharmaceutical and food industries. The complex gas–solid flow coupled with heat and mass transfer in such processes made it hard to form complete control over the apparatuses. To serve better design, scaling-up, and optimization of granulators and coaters, the underlying micro-scale mechanisms must be clarified. Computational fluid dynamics coupled with the discrete element method (CFD-DEM) provides a useful tool to study in-depth the gas-solid hydrodynamics of the granulation and coating processes. This review firstly introduced the fundamental theory of CFD-DEM from governing equations, force calculation, and coupling schemes. Then the application of CFD-DEM in simulating wet fluidized bed granulation and coating was presented. Specifically, the research focus and the role of CFD-DEM in resolving issues were discussed. Finally, the outlook on the development of CFD-DEM in the context of granulation and coating was given.

Bubble behaviour investigation in a wet fluidized bed using digital image analysis

AbstractDue to the presence of liquid bridge forces, wet particles reveal totally different fluidization behaviours than dry fluidized beds. This paper studies the bubble dynamics of wet Geldart‐D particles in a wet 2D fluidized bed. A digital image analysis method is adopted to identify bubbles and extract bubble properties based on MATLAB software. During fluidization, the bubble feature analysis of optimized binary images captures the bubble size variation, bubble fraction, and bubble shape. The results show that the increasing liquid saturation promotes the gas holding capacity of the emulsion phase, leading to the decreasing bubble fraction. When the particle size grows, the stability of the emulsion phase is promoted and hence the average bubble diameter increases. As the liquid saturation increases, the enhancing liquid bridge forces limit the bubble growth and promote the bubble breakage, which contributes to the small bubbles. With respect to the wet particles, of which the diameter is 1 mm and the liquid saturation is over 15%, slugging is likely to be observed, resulting in the increasing equivalent bubble diameter. The growing liquid saturation increases the wide aspect ratio, decreases the shape factor, and disperses the dependence between the two parameters. This indicates that small and irregular bubbles are likely to be seen in wet fluidized beds.

Couscous manufacture in fluidized bed by wet agglomeration of wheat semolina

Agglomeration is a key unitary operation in a number of industries (pharmaceutical, chemical, food, civil engineering). The agglomeration process controls the structural characteristics and final properties of agglomerates. One of the activities involving food agglomeration is the production of couscous by agglomeration of durum wheat semolina. Couscous is considered the most important traditional dish among the Maghreb people. The industrial process of couscous production includes various stages, the most important of which is wet granulation of semolina, which contributes to the quality of the final product. The agglomeration of cereal powders from different origins (durum wheat, maize and barley) has been performed in a variety of equipment such as high shear mixers, drum mills and fluidized beds. However, the agglomeration of semolina in fluidized beds has had very limited study. The purpose of this research is, therefore, to study couscous production using durum wheat semolina in a fluidized bed equipped with a spray nozzle. The fluidized bed has the advantage of generating strong particle movement and intense mixing to increase the size of the granules evenly throughout the mass used.The efficiency of this process is determined by the couscous yield defined as the mass ratio of couscous to raw material. The results showed that couscous can be produced from semolina by wet fluidized bed agglomeration with a specific effect of fluidification air flow, liquid flow, bed temperature and spray liquid properties on the couscous quality (size, brittleness and morphology) as well as on yield. The latter rose by 60% when the water containing flour was sprayed. Furthermore, the results of this study showed that granules size changes directly with the liquid flow rate, while temperature and air flow have an opposite effect. It was also found that changing binder components have an effect on the quality of the agglomeration of the product. Key words: Couscous, agglomeration, fluidization, semolina, friability, granulation, atomization, size enlargement, drying

Two-fluid modeling of a wet spouted fluidized bed with wet restitution coefficient model

Hydrodynamics of wet fluidized beds is significantly affected by the liquid bridge force between solid particles. In this study, two-fluid modeling of wet fluidizations with wet restitution coefficient model was realized in Ansys Fluent® through its user-defined function portal. Wet restitution coefficient was calculated using the sub-model proposed by Davis et al. (Elastohydrodynamic rebound of spheres from coated surfaces, J. Fluid Mech., 2002, 468, 107–109.), which depends on liquid viscosity, liquid layer thickness, and impacting velocity. A spouted bed with central air nozzle at the bottom was chosen as the simulation system. The simulation was first validated against available experimental and CFD-DEM results. Then, effects of the liquid viscosity and liquid layer thickness on fluidization hydrodynamics and distributions of relevant quantities were investigated. The simulation results demonstrate that the effect of liquid viscosity is significant while the influence of liquid layer thickness can be ignored in the current study. Finally, some thoughts on future studies for two-fluid modeling of wet fluidizations with wet restitution coefficient model were discussed.

Two-Fluid-Model-Based Full Physics Simulations of Mixing in Noncohesive Wet Fluidized Beds

Mixing in a noncohesive wet fluidized bed was studied using the two-fluid model (TFM) and a zero-dimensional (0D) approach. The employed TFM was extended to simulate droplet deposition on the parti...

Mechanistic model to simulate wet agglomerate breakage in a gas-solid fluidized bed

A mathematical model that describes wet agglomerate breakage in a gas-solid fluidized bed was developed, utilizing the energy approach to assess agglomerate stability and the Kinetic Theory of Granular Flows (KTGF) to describe collisions between bed primary particles and agglomerates. Wet agglomerates are held together by liquid bridges that resist breakage due to capillary and viscous forces. The probability of breakage depends on the balance between the collisional energy and the energy required to break the necessary number of liquid bridges. The model considers two types of breakage: abrasion, characterized by individual primary particles being stripped from an agglomerate, and fragmentation, where an agglomerate breaks into two pieces, each containing multiple primary particles. Simulations were performed using the Eulerian-Eulerian method with either a monodispersed or a polydispersed approach to describe the size distribution of agglomerates during breakage. Comparison with available experimental data revealed good qualitative agreement between the predicted and measured abrasion and fragmentation rates, with some quantitative discrepancies that can be attributed to model limitations and experimental uncertainties. The effects of superficial gas velocity and liquid viscosity on agglomerate breakage are investigated in detail. The model can serve as a valuable tool to study breakage of wet agglomerates, and it can be extended to include agglomerate formation for a more comprehensive description of hydrodynamics of wet fluidized beds.

Experimental study on mixing behaviors of wet particles in a bubbling fluidized bed

Mixing behavior of binary particles with different densities in a wet fluidized bed has been experimentally investigated in this work. Some important hydrodynamic characteristics during mixing, such as the minimum fluidization velocity, flow pattern, and flotsam particle distribution are measured. The differences between dry and wet particles are systematically compared. It is found that the minimum fluidization velocity Umf of wet particles is higher than that of dry particles due to the liquid bridge force between particles. Umf reaches its maximum when the liquid saturation S = 0.1, and then decreases, and does not change much when the liquid saturation in the region of S = 0.2–0.3. Liquid addition can bring two opposite effects on the mixing of a binary mixture of particles with different densities. First, the intensity of bubbling in the wet fluidized bed is lower than that in the dry fluidized bed at the same operation gas velocity Uf, which will weaken the mixing. Secondly, the liquid bridge forces between particles can restrain the density segregation, which is beneficial for mixing. A simple method is developed based on the analysis of forces on flotsam particles to evaluate the effect of liquid bridge force on mixing. A segregation index Se, defined as Se = Flift/Fresis, where Flift contains the drag force Fd and buoyancy force Fb, and Fresis contains the gravitation force Fg and liquid bridge force Flb, is used to indicate the segregation potential. The calculated results show that Se > 1 in the dry fluidized bed and Se < 1 in the wet fluidized bed, and dry particles are more inclined to segregate than wet particles.

The effect of liquid bridge model details on the dynamics of wet fluidized beds

Wet fluidized beds of particles in small periodic domains are simulated using the CFD‐DEM approach. A liquid bridge is formed upon particle‐particle collisions, which then ruptures when the particle separation exceeds a critical distance. The simulations take into account both surface tension and viscous forces due to the liquid bridge. We perform a series of simulations based on different liquid bridge formation models: (1) the static bridge model of Shi and McCarthy, (2) a simple static version of the model of Wu et al., as well as (3) the full dynamic bridge model of Wu et al. We systematically compare the differences caused by different liquid bridge formation models, as well as their sensitivity to system parameters. Finally, we provide recommendations for which systems a dynamic liquid bridge model must be used, and for which application this appears to be less important. © 2017 American Institute of Chemical Engineers AIChE J, 64: 437–456, 2018

Development and study of measurement methods for bogging in a fluidized bed

In the Fluid Coking™ process, heavy oil is contacted with hot fluidized coke particles. If the local concentration of liquid is too high, fluidization is poor, a condition commonly known as bogging. The objective of this study is to identify practical methods for early bogging detection, simulating heavy oil at coker temperatures with a lighter oil at room temperature and determining the impact of the oil concentration on bubble properties, on fluctuations of the bed pressure gradient, and on the transmission of sound through the bed. Bogging, as determined from changes in bubble properties, occurred when the oil mass fraction was increased from 0.25 to 0.275wt%.A Kolmogorov-Smirnov test of the wavelet coefficients of pressure fluctuations, optimised with a genetic algorithm, can detect early bogging more effectively than other methods using pressure fluctuations. A major advantage of this method is that its results are not affected by moderate variations in fluidization gas velocity. The success of this new bogging detection method is explained by studying the transmission of sound of different frequencies through dry and wet fluidized beds, which could also be used to detect bogging: bogging sharply increases the speed of sound from below 30m/s to well above 40m/s. A theoretical model confirmed that the changes in bubble properties caused by bogging affect the transmission of sound through the fluidized bed. The geometry of the gas bubbles and their distance from the wall were measured with capacitance sensors to understand how bubble properties affect sound transmission. The propagation of sound with bubbles of various geometries was simulated with Comsol.

Simulating wet gas–solid fluidized beds using coarse-grid CFD-DEM

Simulations of wet fluidized beds of particles in small periodic domains have been carried out using a CFD-DEM approach. A liquid bridge forms upon particle–particle collision, which then ruptures when the particle separation exceeds a critical distance. The simulations take into account only the surface tension force of attraction due to the liquid bridge. Increasing the strength of cohesion leads to larger agglomerates, and correspondingly, higher gas velocities are required to fully support the particles. The slip velocity results from the simulations have been correlated in terms of a Bond number characterizing the strength of cohesion, volume of liquid in the bridge, and particle volume fraction. The CFD-DEM results are systematically coarse-grained to expose the dependence of the filtered drag coefficient on Eulerian filter size, surface tension forces, liquid loading, and solids loading in wet gas–solid fluidized beds.

Industrial-scale demonstration of a new sorbent reactivation technology for fluidized bed combustors

To minimize the disposal of highly reactive spent sorbent from a fluidized bed combustor, a new method for reactivation has been developed. The method consists of grinding the spent ash in a rotary mill, hydrating the ash with an excess of water, and mixing the wet ground ash with dry solids to absorb the excess water. The mixing process eliminates the formation of a concrete-like product that normally results as wet fluidized bed combustor ash ages. Pilot-scale combustion trials proved to be successful, and the process was scaled up using a 35 MWt utility boiler at Purdue University. The test lasted for 3 days and resulted in net reduction of limestone sorbent use of 18%. The results generated in this work have been used to develop an economic evaluation for a 165 MWe circulating fluidized bed (CFB) boiler, which projects significant savings due to reduction of limestone supply and ash disposal costs. The evaluation also suggests that the process is cost competitive with other processes, albeit that those processes have not been demonstrated at industrial scale. Furthermore, it also has the potential to make a small net reduction in CO 2 emissions, due to reduced limestone usage.

Correlation between loose density and compactibility of granules prepared by various granulation methods

The objectives of this study were to prepare the lactose granules by various granulation methods using polyethylene glycol 6000 (PEG 6000) as a binder and to evaluate the effects of granulation methods on the compressibility and compactibility of granules in tabletting. Lactose was granulated by seven granulation methods — four wet granulations including wet massing granulation, wet high-speed mixer granulation, wet fluidized bed granulation and wet tumbling fluidized bed granulation; and three melt granulations including melt high-speed mixer granulation, melt fluidized bed granulation and melt tumbling fluidized bed granulation. The loose density, angle of repose, granule size distribution, mean diameter of granules, and the tensile strength and porosity of tablets were evaluated. The compactibilities of granules were varied by the granulation methods. However, the difference in compactibility of granules could not be explained due to the difference in compressibility, since there was no difference in Heckel plots due to granulation methods. Among their granule properties, the loose density of granules seemed to have a correlation with the tablet strength regardless of the granulation methods.

Particle granulation in fluidized bed

A mechanism of particle granulation in a wet fluidized bed is proposed on the basis of experimental data, and test calculations are performed.