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  • Author: Wenqi Zhong x
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Experimental investigations on the mixing behavior of a spherical particle in a spout-fluid bed with a cross section of 0.1 m×0.03 m and height of 0.5 m were carried out. The image processing technology and bed-frozen method were employed to measure the axial and radial concentration of the tracer at different times. Four various initial conditions (top, side, bottom and middle loading) were used to examine the influence of the tracer location on the mixing. In addition, the underlying mixing mechanisms were preliminarily discussed in terms of the evolution of the mixing index. It is shown that the initial tracer location affects the initial mixing rate but not the final mixing degree. The top and bottom loading cases characterised by dominant axial mixing perform significantly better than the side loading case characterised by dominant radial mixing. Due to the axial rapid convection and radial intense diffusion, the mixing speed for a middle loading case is much faster compared to other cases. The results also demonstrate that the convection caused by the circulation and diffusion among the circulation rolls are two main mixing mechanisms in a spout-fluid bed.

Fluidization, mixing and segregation of a biomass-sand mixture in a 3D gas-fluidized bed have been investigated by means of visual observation, pressure fluctuation analysis and the bed-frozen method. Three types of mixtures are considered, in which biomass is a thin long stalk, and sand belongs to the Geldart B category. Experiments are carried out in a segmented fluidized bed equipped with multiple pressure transducers. Three initial packing conditions and two experiment procedures are used. The fluidization velocity varies to cover a wide range. Results show that in the local fluidization region, the mixing and segregation patterns are sensitive to the initial packing condition. In the case of a fully segregated state with biomass at the bottom, the bed inversion can be significantly observed due to the great segregation tendency of biomass. Further analyses indicate that the mixing ratio exerts a subtle influence on the competition between mixing and segregation by disturbing the coalescence and break-up of the bubble. In addition, the pressure fluctuation signal proves to be helpful in understanding the dynamic features of the phenomenology.

Detailed knowledge of the fluidization mechanics of biomass itself, or of mixtures of biomass and inert particles is critical to successful design and operation of fluidized bed involving biomass. In this study, the fluidization behaviors of biomass alone and sand-biomass mixture were experimentally and theoretically studied. A segmented fluidized bed equipped with multiple pressure transducers was used as the experimental apparatus. The cotton stalk with a shape of stalk was employed as biomass material, and three different sizes of sand were utilized as inert particle. By mixing cotton stalk and sand, three binary systems were obtained, in which only the weight percentage of biomass particle in each mixture is different. The fluidized beds experienced the following states, depending on gas superficial velocity and initial mixture fraction: fixed, bubbling fluidization and turbulent fluidization. It is found that the additive of sand has positive influence on the fluidization of biomass and the initial arrangement of bed has influence on the development of pressure drop with the gas velocity. The developed equation for predicting the minimum fluidization velocity (Umf) of binary mixture quite satisfactorily predicts the Umf values of mixture with low biomass concentration. In addition, the correlation expressed in terms of Reynolds and Archimedes numbers to predict transition velocity (Uc) is proposed. Calculated Uc is in excellent agreement with the experimental data.

Steady-state tracer gas measurements were carried out to study the gas mixing behaviors in a spout-fluid bed with a cross section of 0.3 m x 0.03 m and height of 2 m. Two different tracer gases were simultaneously injected, one was injected into the spouting gas flow and the other was injected into the fluidizing gas flow. Radial tracer gas concentrations at various bed elevations under different flow patterns were measured. The mechanism of gas mixing was discussed based on the racer gas concentrations and the flow patterns recorded by a high-resolution digital CCD camera. It was found that gas mixing in spout-fluid beds was due to both convection and dispersion. A three-region mixing model was developed to describe the gas mixing in the spout-fluid bed. The spout jet region and the boundary region were modeled with a mass transfer model; the annular region was modeled with a dispersion model. Effects of spouting gas and fluidizing gas flow rate on the gas exchange between the spout jet and the annular dense region, and the gas dispersion in the annular dense region were examined with flow patterns. The results showed that increase in spouting gas velocity and fluidizing gas flow rate could both promote the gas mixing in spout-fluid beds. The gas-solid flow pattern transited from internal jet to spouting to spout-fluidizing, and the gases were better mixed. But the gases became poorly mixed when the flow pattern transited from stable flow to instable flow.

Bed segregation is inevitable in biomass-sand fluidized beds because of density and size differences, but segregation pattern can be significantly different under various operational conditions. In the present work, the combination of image processing technology and pressure fluctuations analysis is used to investigate the fluidization and segregation. Experiments have been carried out in a 3D fluidized bed. The biomass material studied is cotton stalk. Two groups of binary systems are considered, obtained by mixing sand with biomass particles of different sizes and/or concentrations. In addition, the bed-frozen method is also adopted to examine the segregation degree. It is found that the transition velocity from bubbling to turbulent fluidization regime decreases with the increase of biomass size and concentration. The results also show that the biomass concentration has little influence on the segregation degree, and the biomass size significantly affects the rate of segregation, which increases with the size. The pressure gradient and standard deviation of pressure fluctuation are demonstrated to be useful for indicating the bed segregation, which are validated by the result by the bed-frozen method.

Abstract

Mixing efficiency is one of the most significant factors, affecting both performance and scale-up of a gas-solid reactor system. This paper presents an experimental investigation on the particle mixing in a multiple spouted bed. Image processing technique was used to extract the real-time information concerning the distribution of particle components (bed materials and tracer particles). A more accurate definition of the tracer concentration was developed to calculate the mixing index. According to the visual observation and image analysis, the mixing mechanism was revealed and the mixing rate was evaluated. Based on these results, the effects of operation parameters on the mixing rate were discussed in terms of the flow patterns. It is found that the detection of the pixel distribution of each component in RGB images is not affected by the interference of air void, thus maintaining good measurement accuracy. Convective transportation controls the particle mixing in the internal jet and spout, while shear dominants the particle mixing in the dense moving region. Global mixing takes place only when the path from one spout cell to the other is open. This path can be formed either by the bubbles or particle circulation flows. The mixing rate is linked to the bubble motion and particle circulation. Provided that there are interactions between the spout cells, any parameters promoting the bubble motion and circulation can increase the mixing rate. Finally, a mixing pattern diagram was constructed to establish the connection between the flow structure and mixing intensity.

Abstract

A three-dimensional Eulerian multiphase based computational model was developed to simulate the black liquor gasification processes in a fluidized bed gasifier (FBG) at low temperature. The standard k-e model and kinetic theory of granular model were used to simulate the gas phase and solid phase, respectively. Black liquor pyrolysis, homogeneous reactions and heterogeneous reactions were taken into account in chemical model. The reaction rates of homogeneous and heterogeneous reaction were determined by Arrhenius–Eddy dissipation reaction rate and kinetic reaction rate. Simulations were carried out at four different operating conditions, i.e. reactor temperature was kept at 550 degree centigrade or 600 degree centigrade, and nitrogen or air was used as fluidizing medium. The calculated results were in well agreement with the experiment used as calibration. Base on the simulation, gas-sold flow patterns and gas species molar fraction distributions were obtained, the relationship of gas composition profiles with the temperature and the fluidizing media were discussed.

A high-flux circulating fluidized bed coal gasifier cold model which consists of a vertical riser (0.06m-I.D.×5m-high), two downcomers (0.04m-I.D.×3.5m-high and 0.1m-I.D.×3m-high), an inertial separator, a cyclone and two solid feeding devices were established. Geldart group B particles with mean diameters of 140 ?m and densities of 2700 kg/m3 were used as bed materials. Flow behaviors were investigated with the solid mass flux ranges from 108 to 395 kg/m2 and the superficial gas velocity ranges from 7.6 to 10.2 m/s. The pressure drop, apparent solids holdups, average slip velocity and solids-to-air mass flow ratio under different operating conditions were obtained. The results showed that the riser total pressure drop increased sharply with bed height in the low elevation but slowly in the high elevation, since the solids holdup was higher in the low region than that in the high region. The solids holdup increased with the increasing of solids mass flux while it decreased with increasing superficial gas velocity. A dense suspension upflow flow (DSU) structure was found only existing in the low elevation while the rest upper region was still in the dilute phase, and the length of DSU flow structure increased with solids mass flux. The average slip velocity was found to be the strong function of apparent solids holdup; increasing apparent solids holdup leads to the increase of slip velocity. The riser total pressure drop and apparent solids holdup increase with the solids-to-air mass flow ratio.

Abstract

The translational and rotational motions of biomass particle in a spout-fluid bed have been investigated. Two kinds of cylindrical biomass particles with different densities were used as bed solids, which were obtained from the jujube tree. To describe the translational and rotational characteristics, virtual particle concentration and rotation angle were introduced. The former obtained by microwave heating-infrared thermal imaging technique was compared with the actual particle concentration determined by box-counting method. Based on the temperature distribution, the translational position and rotation angle of multiple tracers were distinguished. The results show that the heating characteristic of tracer depends on the tracer material. Compared to the spherical particles, non-spherical biomass particle is more likely to cause dead zone in the bed. In the flow regime of fluidizing, the flow characteristic of biomass particle relies on the jet gas velocity. The high resistance resulting from the interparticle cohesion forces hinders the rise of bubbles and weakens the radial dispersion of particle. In the fountain, homogeneous biomass particle rotates uniformly, while non-homogeneous biomass particle exhibits non-uniform rotation. In the annulus, the rotation characteristic of biomass particle depends on the structure of fountain and particle trajectory in the annulus.

Abstract

Particle dispersion in a spout-fluid bed was investigated using time series of particle trajectory by evaluation of dispersion coefficient based on Einstein’s expression. A series of experiments were performed in a lab-scale spout-fluid bed using microwave heating and infrared thermal imaging (MH-ITI) technique for recording the position of a tracer. The influence of gas velocity on the particle dispersion in different flow regimes has been taken into account. The results show that the particle dispersion behavior depends on the flow regime. In the flow regime of internal jet, the increase of gas velocity improves the axial dispersion. In the case of jet in fluidized bed with bubbling, increasing fluidizing gas velocity promotes the radial dispersion at the lower part of bed and the axial dispersion at the upper part of bed. In the flow regime of jet in fluidized bed with slugging, the effect of fluidizing gas is related to the slug pattern. The result also indicates that the transition of flow regime can be evaluated from the particle trajectory.