Pulsating flow effect on the segregation of binary particles in a gas–solid fluidized bed

Axial mixing of fine particles in fluidized beds

A technique has been developed to give a quantitative measure of the contacting efficiency of liquid feed onto particles in a hot fluidized bed for petroleum applications. Copper naphthenate was used as a tracer to measure the coating of atomized bitumen feed onto solid coke particles in a fluid coking pilot plant. The pilot plant consisted of a reactor vessel, where bitumen was injected into the reactor as feed, and a regeneration vessel, where coke particles were heated and circulated back to the reactor. Atomized bitumen droplets were injected through a feed nozzle into the reactor section. A small concentration of copper naphthenate tracer, pre-diluted with bitumen, was injected upstream of the nozzle, following a step function with time. As the bitumen reacted, the copper formed solid deposits on the surface of the particles, effectively labeling the particles that had been contacted by the bitumen droplets. Frequent coke samples from the solids exiting and entering the reactor were taken during and after the injection of the tracer.Two types of measurements were conducted on the coke samples; bulk copper concentration in the samples, using Inductively Coupled Plasma (ICP), and copper concentration on individual coke particles using energy-dispersive X-ray (EDX) analysis. The data for bulk copper concentration confirmed that the solids were mixed rapidly in the reactor, approximating a Continuous Stirred Tank Reactor (CSTR). The EDX analysis of individual coke particles demonstrated, however, that a significant fraction of the coke particles within the samples were not labeled with copper. This result indicated that many of the particles in the fluidized bed were not contacted by bitumen in the reactor. Consequently, the tracer technique was used to compare the coating efficiency of bitumen on coke particles by operation of different nozzle designs.

Collison of sand particles in fluidized bed plays a very important role in determining hydrodynamics of fluidized bed. Effect of coefficient of restitution on hydrodynamics on 300 µm particles in gas-solid fluidized bed was investigated. Eulerian-Eulerian model was used for simulating hydrodynamics of fluidized bed. For investigation, different values of coefficient of restitution which correspond to different values of elasticity of particles were used. No bubbles were found for perfectly elastic collisions. With decrease in the value of coefficient of restitution from 1 to 0.8, bubble formation shows a positive effect.

A technique for determining devolatilization rates of large coal particles in a fluidized bed combustor

Experimental and CFD-DEM numerical evaluation of flow and heat transfer characteristics in mixed pulsed fluidized beds

Biomass energy has attracted considerable attention as a renewable and clean energy source that can be stored and transported effectively. A critical component in this context is the fluidized bed, which plays a vital role in biomass energy utilization. An in-depth understanding of the flow characteristics of non-spherical biomass particles within the fluidized bed is essential for improving energy efficiency. This study utilizes a combination of numerical simulations and experimental analysis to explore the flow patterns of biomass particles under eight distinct drag correlations. The investigation covers various flow regimes in the fluidized bed, concentrating on critical parameters such as flow evolution, gas-phase pressure distribution, particle phase velocity vectors, bed pressure drop, particle mixing characteristics, bed expansion height, and the energy-dynamic behavior of the particles. The results indicate that the Ergun resistance model closely matches the experimental data, particularly regarding the flow evolution of the particles. This model shows a more uniform mixing of the particles and greater stability in the magnitude and frequency of pressure fluctuations at the bed surface. This stability is reflected in the average pressure drop, highlighting the enhanced performance of the fluidized bed. Overall, this research sheds light on the intricate dynamics of non-spherical biomass particles in fluidized beds and provides valuable insights for optimizing particle design. By focusing on design enhancements, it is possible to achieve higher combustion efficiency, supporting the broader objective of sustainable energy utilization. In summary, this study emphasizes the significance of precise modeling and understanding of flow characteristics in the fluidized bed, which is crucial for advancing biomass energy technologies.

The overall objectives of this project are to develop experimental techniques for measuring the forces of fluidized particles, and to predict the solid-gas performance in fluidized beds by using data analysis system, and by elucidating the intrinsic mechanism of erosion and attrition phenomena in fluidized beds. The reduction of erosion and attrition rates is one of the critical engineering problems for the design and operation of fluidized bed combustors. Specifically, the objectives are to: (1) develop the experimental techniques to measure the forces of solid particles prevailing in fluidized beds: (2) measure and characterize the forces of solid particles in various types of fluidized beds with various configurations (conventional and spouted fluidized beds) and with different scales (10, 20, and 30cm) under various fluidization conditions (particle size, bed aspect ratio and gas velocity); (3) find and verify the mechanism of erosion rates of in-bed tubes and attrition rates of fluidized particles by forces of solid particles in fluidized beds. We developed three different kinds of measurement methods, i.e., fracture sensitive sensor, piezoelectric sensor and gas pressure fluctuation method. By using these methods the exact forces of solid particles, including the transient corporate in fluidized beds, were systematically measured. Simultaneously, the erosion rates of in-bed tubes and attrition rates of fluidized particles were measured. 69 figs., 9 tabs.

Mass transfer in fluidized beds is an important operation for separation processes. Two effects can be achieved by using fluidized beds in mass transfer processes increasing interface area and relative movement between the phases. These effects are both desirable because they lead to greater process rates. This paper presents an experimental investigation regarding mass transfer from the wall of a column to the fluid in a fluidized bed of inert spherical particles. The experiments were conducted in column 40 mm in diameter with spherical particles 0,8-3 mm in diameter and water as one fluidizing fluid. The method of dissolution of benzoic acid was used to provide very low mass flux. The average wall-to-fluid mass transfer coefficients were determined for two systems: single-phase fluid flow and a fluidized bed of inert particles The measurements encompassed a Reynolds number range from 100-4000 for single-phase flow and 600-4000 in fluidized beds. The mass transfer coefficients for both systems were calculated from weight loss of benzoic acid. The effects of superficial liquid velocity and particle diameter on the mass transfer coefficient were investigated. It was found that mass transfer was more intensive in the fluidized bed in comparison with single phase flow. The best conditions for mass transfer were reached at a minimum fluidization velocity, when the mass transfer coefficient had the greatest value. The experimental data were correlated in the form: jd = f(Re), where jd is the dimensionless mass transfer factor and Re the Reynolds number.

Gas–solid fluidized beds are complex particle systems, and the electrostatic behavior of particles in fluidized beds is even more complex, which is influenced by numerous factors such as particle properties and operating conditions. Current studies focus on the effect of a certain factor on particle charging without a global picture. Furthermore, there is no mathematical model that can describe the interaction of multiple factors on particle charging because it is difficult to build a model for such a complex system. Therefore, a new approach is needed. In this study, a model capable of accurately predicting the surface charge density of particles in monodispersed gas–solid fluidized beds within a certain range was developed based on the literature and experimental data through several machine learning methods including kernel ridge regression (KRR), support vector machine regression (SVR), and multilayer perceptron (MLP). SVR and MLP models gave the best results with R2 equal to 0.980 and 0.979, respectively. However, the sensitivity analysis showed that the MLP model was more reliable than the SVR model. In conclusion, the feasibility of using machine learning to analyze the charging behavior of particles in fluidized beds is demonstrated, and the proposed MLP model can serve as an accurate correlative tool for fast and effective estimation of particle surface charge density in gas–solid fluidized beds.

The state of the art of the theory of the process of combustion of single coal particles in fluidized beds is reviewed. The process of combustion of coal is divided into two stages: devolatilisation, and burning of the char. The devolatilisation and the subsequent combustion of the volatile matter takes an order of magnitude less time than that for burning of the char. Therefore, the burning of char dominates the characteristics and performance of the combustor. Generally, the combustion of char is found to be controlled by the mass transfer to and reaction on the external surface if the area of pores in the char is less than 0.3 × 106m2/m3. Existing information suggests that in a fluidized bed oxygen diffuses to the carbon surface and it enters into heterogeneous reaction with carbon producing mainly CO. The carbon monoxide burns to CO2 in a diffusion flame surrounding the char particle. The surface temperature of the char, which is always higher than the bed temperature, is found to increase with decreasing size of the char, but it may start decreasing again if the site of CO oxidation moves away from the char surface for very small char particles. The mass transfer rate to the char particles may be calculated by considering the radial variation of local voidages, but this effect is significant only in limited cases.

Comparison of hydrodynamics and operational characteristics of a multistage fluidized bed with high-density and low-density powders

A comprehensive mathematical model based on the discrete particle model and computational fluid dynamics was utilized to investigate mixing and segregation of particles in fluidized beds at high pressure. To quantify the extent of mixing in the bed, the Lacey mixing index was used. Simulations were carried out with different mass fractions of small particles at various pressures ranging from 1 to 64 bar and at various superficial gas velocities. The results showed that the bed transforms from a segregated state to a fully mixed condition when the operating pressure is increased. Vertical segregation of particles at low pressure was replaced by horizontal segregation of particles at high pressures in which small particles were collected mainly near the walls and large particles at the center of the bed. At the same pressure, the rate of segregation decreased with increasing the mass fraction of small particles.

Super-quadric CFD-DEM simulation of chip-like particles flow in a fluidized bed

Evaluating solids dispersion in fluidized beds of fine particles by gas backmixing experiments

Knowledge of both dense bed expansion and freeboard solids inventory are required for the determination of bed height in fluidized beds of fine particles, e.g., Fluidized Catalytic Cracking (FCC) catalysts. A more accurate estimation of the solids inventory in the freeboard is achieved based on a modified model for the freeboard particle concentration profile. Using the experimentally determined dense bed expansion and the modified freeboard model, a more practical method with improved accuracy is provided to determine the bed height both in laboratory and industrial fluidized beds of FCC particles. The bed height in a fluidized bed can exhibit different trends as the superficial gas velocity increases, depending on the different characteristics of the dense bed expansion and solids entrainment in the freeboard. The factors that influence the bed height are discussed, showing the complexity of bed height and demonstrating that it is not realistic to determine the bed height by a generalized model that can accurately predict the dense bed expansion and freeboard solids inventory simultaneously. Moreover, a method to determine the bed height, based on axial pressure fluctuation profiles, is proposed in this study for laboratory fluidized beds, which provides improved accuracy compared to observation alone or determining the turning points in the axial pressure profiles, especially in high‐velocity fluidized beds.