Drying of Sodium Acetate in a Pulsed Fluid Bed Dryer

Sodium acetate crystals obtained from the reaction of acetic acid with sodium hydroxide are usually dried in rotary or fluidized beds. In this study, a batch pulsed fluid bed dryer with a 0.18 m² cross‐sectional area was used in an attempt to reduce energy consumption and increase productivity. Drying curves of sodium acetate were determined for different conditions: inlet air temperature of 65 and 80 °C and pulsation frequency of 0 rpm (conventional fluidized bed), 500 and 900 rpm (pulsed fluid bed). A 2² factorial design was used to analyze the results. The intermittent flow helped to break agglomerates and provided better contact between particles and the gas. Drying rates were higher under pulsed fluidization when compared to conventional fluidization. Conventional fluidized bed drying consumed 2.5 times more energy at 80 °C. The influence of temperature on the drying rate was also evident.     

压力脉动模型及其在脉冲流化床中的应用

考虑了流化床中分布板上形成的气泡不变形增长，最后以不同尺寸分布的气泡群形式通过床层，提出了能包含Ｄａｖｉｄｓｏｎ模型、Ｆａｎ模型在内的更一般化的流化床压力脉动模型。不但把气泡行为这个重要因素引进了压力脉动模型，而且成功地把压力模型应用于脉冲气流流化如何科学地运用脉冲气流来进一步改善流况提供了理论依据。     

Heat transfer in a pulsed bubbling fluidized bed

Surface-to-bed heat transfer and pressure measurements were carried out in a 0.17 m ID pulsed bubbling fluidized bed with glass bead and silica sand particles having mean diameters ranging from 37 μm to 700 μm to investigate the effects of flow pulsation on heat transfer and bed hydrodynamics. A solenoid valve was used to supply pulsed air to the bed at 1 to 10 Hz. The bed surface was found to oscillate with the frequency of pulsation, the oscillation's amplitude decreasing with frequency. The standard deviation of the bed pressure drop in the pulsed bed was found to be larger than that in the conventional bed due to the acceleration force imposed by pulsation. For both Geldart B and A particles, high frequency pulsation (7, 10 Hz) enhances the heat transfer compared to continuous flow, the enhancement diminishing with superficial gas velocity and particle size. For Geldart B particles, the effect of pulsation on heat transfer ceases around U_o/U_mf = 3.5, whereas 24% improvement in heat transfer coefficient was obtained for 60 μm glass bead particles (Group A) at superficial gas velocities as high as U_o/U_mf = 27. Furthermore, in the fixed bed (U_o/U_mf < 1) for Geldart B particles, 1 Hz pulsation was found to be very effective resulting in two- to three-fold increase in heat transfer coefficient compared to continuous flow at the same superficial gas velocity. The flow pulsation loses its effect on heat transfer with increasing static bed height, i.e., when H_bed/D > 0.85.     

A critical review of the complex pressure fluctuation phenomenon in gas-solids fluidized beds

The complex pressure fluctuation phenomenon in gas-solid fluidized beds is systematically examined in this paper based on a comprehensive review of the literature data. The local pressure fluctuations are composed of multiple sources, including local bubble induced fluctuations, global bed oscillations and propagating pressure waves originating in other locations (e.g. bed surface, distributor and windbox). The interaction and coupling among bubble motion, under-damped oscillations of fluidized particles and bed surface, propagating compressible pressure waves and flow pulsation in gas-solid fluidized beds creates the complexity of local pressure fluctuations, and is likely responsible for the formation of complex but unique flow patterns. A few attempts have been reported in the literature on examining the interaction between bed oscillations, plenum chamber air pulsation and propagating pressure waves in fluidized beds, showing some promises on predicting the local pressure fluctuations. Future work should be focused on predicting local and global pressure fluctuations and the formation of unique surface flow patterns by coupling different contributing mechanisms.     

THE EFFECTS OF DISTRIBUTOR DESIGN ON FLUIDIZED-BED HYDRODYNAMIC BEHAVIOR

should be addressed. The distributor was investigated for the purpose of design and scale up of large fluidized-bed combustors. Four orifice plates with different configurations were used to study the effect of distributor design on bubble formation and solid mixing. Experiments were carried out on a three-dimensional fluidized bed of 27.94 cm diameter and a two-dimensional bed with dimensions of 30.48cm ×1.27 cm. Motion pictures were used to study bubble formation and coalescence. Pressure profiles inside the three-dimensional bed were measured for several distributors to study bubble flow patterns, and tracer particles were used to study mixing patterns at various superficial velocities and particle sizes. The results show that the distributor plate with two-size orifices causes a non-uniform gas bubble flow inside the bed. This non-uniform gas bubble flow is associated with variations in local bed density and local voidage. Horizontal or radial solid circulation is also caused by this non-uniform gas bubble flow. The local bed density and voidage variations and the radial solid circulation cause the bubbles to move toward the area above the smaller orifices as the bubbles rise up and coalesce. This reduces the wall effect, and the bed is very uniformly fluidized when the two-size orifice plate with small holes in the center is employed.     

Measurements and numerical predictions of gas vortices formed by single bubble eruptions in the freeboard of a fluidised bed

Gas vortices generated in the freeboard of a bubbling fluidised bed have become the centre of increasingly more research due to the advances in experimental technology. The behaviour of gas flow in the freeboard of a bubbling fluidised bed is of interest for applications such as the gasification of coal where reactions of gas mixtures, as well as gas–particle heat and mass transfer take place. Knowledge of the hydrodynamics of the gas within the freeboard can be hard to characterise, especially the detailed behaviour of gases escaping from bubbles that erupt at the bed surface. In the present study, experiments were conducted on a rectangular three-dimensional gas–solid fluidised bed. The experiments used a particle imaging velocimetry (PIV) measurement technique to visualise and measure the gas flow within the freeboard after a single bubble eruption. A computational study was carried out using Eulerian–Eulerian, kinetic theory of granular flow approach with a quasi-static flow model and with LES used to account for gas turbulence. Results from a three dimensional simulation of the experimental fluidised bed were compared with experimental velocity profiles of gas flow in the freeboard of the gas–solid fluidised bed after a bubble eruption. The CFD simulations showed a qualitative agreement with the formation of the gas vortices as the bubble erupted. Consistent with experimental findings the CFD simulations showed the generation of a pair of vortices. However, the simulations were unable to demonstrate downward flow at the centre of the freeboard due to particles in free fall after a bubble eruption event was observed in the experiments. Velocity profiles from the CFD data are in reasonably good agreement with the characteristic trends observed in the experiments, whereas the CFD model was able to predict the gas vortices phenomena and the velocity magnitudes were over-predicted.     

Experimental investigation on dynamic behaviour of bubbles emerging from micro-capillary orifice in a flow channel

The flow phenomenon of liquid with bubbles is widespread in various industrial fields, which determines the mass transfer characteristics of the equipment. In this work, the dynamic behaviour of bubbles emerging from micro-capillary orifice in a flow channel was studied by a visualization experiment, while the effects of gas flow rate and liquid flow rate on these processes of bubble growth, departure, and inrush were explored. The experimental results showed that one bubble formation cycle can be divided into three stages: Waiting, departure, and inrush, as well as the dynamic behaviour of bubble emerging from micro-capillary orifice in a flow channel, were significantly affected by gas flow rate and liquid flow rate. At a higher gas flow rate, the growth time and the departure time were shorter, as well as the departure volume of the leading bubble and the inrush volume of the trailing bubble were smaller, while the transverse longitudinal ratio fluctuated more violently, and the swing amplitude of the bubble centroid was greater. With an increasing liquid flow rate, the growth time, the departure time, and the inrush time shortened, while the departure volume of the leading bubble decreased and the fluctuation of the bubble centroid weakened. These findings are conducive to improving the performance of the equipment by optimizing the design of the aerator to regulate the dynamic behaviour of bubbles.     

Drying of a Porous Material in a Pulsed Fluid Bed Dryer: The Influences of Temperature,Frequency of Pulsation,and Air Flow Rate

In this work, a four-section pulsed fluid bed apparatus with a 0.18 m² cross-section area was used to investigate the influence of pulsed-fluidization variables on the drying process of molecular sieves, a test material that was chosen because it presents an initial constant drying rate period. A two-level factorial design was developed to evaluate the influences of the inlet gas temperature—40 and 70°C—the frequency of pulsation—250 and 900 rpm—and the air flow rate—500 and 600 m³(STP)/h—on the drying rate. In addition, a comparison was made between the drying rates achieved with conventional and pulsed fluidization. Results showed that all the investigated variables affect the drying rate. Moreover, drying rates with conventional fluidization are considerably higher, which shows that one must expect a lower drying rate when pulsation is used in a drying process controlled by the external evaporation. Concerning fluid dynamics, this work also analyzed the influence of the frequency of pulsation on the pressure drop across the bed. The higher the frequency, the higher the pressure drop. That result can be explained by the reduction of channeling.     

Studies on Liquid—Gas and Three‐Phase Fluidized Beds with Pulsating Air Flows

The effects of air‐flow pulsation and water and air flowrates on the hydrodynamics of liquid—gas and three‐phase fluidized beds containing 3‐mm glass beads have been studied in a 90‐mm i.d. column. Under steady‐flow conditions, both types of bed contained a relatively large number of small bubbles. With a pulsing air flow, however, a smaller number of much larger bubbles or slugs were formed. This was attributed to different mechanisms of bubble formation at the distributor. Variations in phase holdup were explained in terms of the effects of the operating parameters on the bubble characteristics.     

Hydrodynamic behaviour of a draft tube gas-liquid-solid spouted bed

Experiments were conducted using various types of solid particles to investigate the hydrodynamic properties of a gas-liquid-solid spouted bed with a draft tube. The hydrodynamic properties under study include flow modes, pressure profile and pressure drop, bubble penetration depth, overall gas holdup, apparent liquid circulation rate and bubble size distribution. Three flow modes were classified: a packed bed mode, a fluidized bed mode and a circulated bed mode. It was found that the friction factor accounting for the friction loss in the bed varies linearly on a logarithmic scale with the Reynolds number defined based on the apparent liquid circulation rate. The bubble penetration depth in the annular region, overall gas holdup and apparent liquid circulation rate increase with an increase in gas or liquid velocity. At high gas flow conditions an optimal solids loading exists which yields a maximum apparent liquid circulation rate. A model was proposed to describe the liquid circulation behaviour in the draft tube three-phas spouted bed. The average bubble size in the draft tube region is higher than that in the annular region for both the dispersed bubble regime and the coalesced bubble regime in the draft tube region.     

Bubbling and Bed Expansion Behavior Under Vibration in a Gas‐Solid Fluidized Bed

The effect of vibration on the flow patterns and fluidization characteristics including the minimum fluidization velocity (u_mf), the void fraction (ϵ_mf) at u_mf and the bed expansion ratio were examined. The powders used were spherical glass beads and their diameters were 6, 20, 30, 60 and 100μm. For group A powders, the manner in which the vibration affects the bubble formation was examined from the bed expansion ratio and the index of n/4.65. The area of the homogeneous fluidization region was also observed. The homogeneous fluidization region was broadened at a certain vibration strength, where the value of n/4.65 was a minimum. The bubble formation was observed even for 20μm powder (group C), at large vibration strengths and at high gas velocities. Under such conditions, the bed expansion ratio increased suddenly due to bubble formation. The bubbles broke the irregular bed structure, including various properties of agglomerates. Although the channel breakage was dominant flow pattern for group C powders, the bubbles also played an important role in the improvement of the fluidization.     

气固流化床气泡发生频率的研究

在单孔二维气固流化床中(292mm×16mm)用高灵敏度电容探针研究气泡发生频率.以频谱分析仪分析气泡频率分布曲线.考察了一系列参数对气泡频率功率分布密度曲线的影响,其中包括颗粒直径(0.105—0.590mm),颗粒重度(590—2990kg/m~3),颗粒最小流化速度(0.0072—0.481m/s),床层初始高度(205—565mm),探针离孔口垂直距离,孔口气体流率(0.5—35×10~(-4)m~3/s)以及床层辅助流化气速(0—3倍最小流化速度)等.对于重度低的小颗粒流化床,单孔气泡发生频率符合Davidson和Harrison早先推导的模型.随着颗粒直径和重度的增大,实验数据与上述模型呈有规律的偏差.本文提出气体从形成中气泡的顶半球以最小流化速度值向乳浊相泄漏的模型,推导了气泡发生频率的基本方程.以本研究的泄漏模型,用数值计算方法在计算机上计算的气泡发生频率与实验数据相吻合.     

Distribution of the gas flow in fluidized beds with a slugging behaviour

Capacitance probe measurements of the visible bubble flow rate have been made in a pressurized fluidized bed burning coal. The bed, of 0.3 × 0.3 m cross-section, was operated at pressures between 1.0 and 2.0 MPa and at temperatures between 750 and 900°C. The fluidizing velocity was 0.95 m/s and the mean particle diameter was 0.9 mm. Based on the experimental results, a model of the gas distribution between the bubble phase and the particulate phase in fluidized beds with a slugging behaviour was developed. The model accounts for the lack of bubble flow obtained if the two-phase theory is employed. In order to verify the model, simultaneous measurements of the visible bubble flow rate and of the gas flow rate through the bubbles were carried out in a bed of similar geometry but operating at ambient conditions. In this bed the fludizing velocity was varied between 1.6 and 2.7 m/s and the mean particle diameter was 1.0 mm. The through-flow of gas was measured with the aid of pressure probes. Evaluation of the experimental results using the model showed that this gas through-flow in the bubble phase subsequently increases the superficial gas velocity in the particulate phase between the vertically aligned bubbles (slugs), and that this gas velocity in excess of the incipient fluidization velocity is responsible for the large deviation from the two-phase theory. The associated increase of the particulate phase voidage was calculated via the Ergun equation.     

脉冲气流对烯烃流化床气泡特性的影响

流化床具有良好的传热传质效率,因此广泛应用于煤粉燃烧、气力输送等场合。但是流化床在工作过程中会产生一定的恶性流动,脉冲的振动能量可以有效地提高流化床的传热系数,减少恶性流动现象发生。本文设计、搭建了烯烃流化床实验平台,通入混合脉冲气流。通过改变脉冲气流的相关参数,对静电信号进行功率谱密度函数分析。并基于静电信号估算气泡尺寸,从而获得流化床内气泡行为的变化。实验结果表明,脉冲气流的加入对气泡尺寸有一定的影响。随着脉冲频率的增加,气泡尺寸呈现先减小后增大的趋势,在脉冲频率为0.5Hz左右时气泡尺寸最小。脉冲气流加入后使得气泡尺寸减小,提高了颗粒流化效果,因此烯烃流化床内流化结束后的团聚颗粒质量明显降低。     

Effect of a dispersed immiscible liquid phase on the hydrodynamics of a bubble column and ebullated bed

Secondary undesired reactions in ebullated bed resid hydroprocessors can generate an additional dispersed liquid phase, referred as mesophase, which is denser and more viscous than the continuous liquid phase and affects the operation and transport phenomena of the fluidized bed. This study investigates the effect of a dispersed immiscible liquid phase on the overall phase holdups, bubble properties, and fluidization behavior in a bubble column and ebullated bed. The experimental system consisted of biodiesel as the continuous liquid phase, glycerol as the dispersed liquid phase, 1.3 mm diameter glass beads, and nitrogen. The addition of dispersed glycerol reduced the gas holdups in the bubble column for the studied gas and liquid superficial velocities. Dynamic gas disengagement profiles reveal a rise in the large bubble population and reductions to the small and micro bubble holdups when increasing the glycerol concentration. Liquid–liquid–solid bed expansions at various liquid flowrates confirm particle agglomeration in the presence of a more viscous dispersed liquid phase. Overall phase holdups in a gas–liquid–liquid–solid ebullated bed were obtained while varying the gas and liquid flowrates as well as the glycerol concentration. A coalesced bubble flow regime was observed in the bed region without glycerol whereas the addition of glycerol resulted in the dispersed bubble flow regime due to particle clustering and a greater apparent particle size. The resulting bubble flow regime increased the bed and freeboard region gas holdups due to enhanced bubble break-up. Observations of the fluidized bed behavior following the addition of the dispersed glycerol are also discussed.     

Hydrodynamics of the Pulsed Fluid Bed with Cyclically Relocated Gas Stream with Lengthwise Pulsation

The hydrodynamics studies of the pulsed fluid bed (PFB) with lengthwise pulsation are reported in the article. The pulsed fluid bed with lengthwise pulsation is a modification of the classical PFB. In the PFB dryer with lengthwise pulsation, the disk gas distributor cyclically relocates the gas stream to each pressure chamber situated under the grid, parallel to the lengthwise axis of the main chamber. Such structure of a dryer is more compact than the previous ones. The operation range, pressure drops, and bed structure of a new system with lengthwise pulsation have been shown.     

Hydrodynamics of the Pulsed Fluid Bed with Cyclically Relocated Gas Stream with Lengthwise Pulsation

The hydrodynamics studies of the pulsed fluid bed (PFB) with lengthwise pulsation are reported in the article. The pulsed fluid bed with lengthwise pulsation is a modification of the classical PFB. In the PFB dryer with lengthwise pulsation, the disk gas distributor cyclically relocates the gas stream to each pressure chamber situated under the grid, parallel to the lengthwise axis of the main chamber. Such structure of a dryer is more compact than the previous ones. The operation range, pressure drops, and bed structure of a new system with lengthwise pulsation have been shown.     

Effects of sparger height and orifice orientation on solids dispersion in a slurry bubble column

The effects of gas distributor height and the orientation of its orifices are investigated on solids dispersion and gas holdup profiles in a three-phase slurry bubble column. The height of the distributor was varied to cover locations from near column bottom to above the settled solids bed height. The orifice orientations were changed from upward facing to downwards facing directions. The measurements were conducted in a Plexiglas column of 0.15 m ID and 2.5 m height. The gas phase was oil-free compressed air while tap water was used as liquid phase. Glass beads with an average particle diameter of 35 μm and density of 2450 kg/m³ constituted the solid phase. The settled bed height was about 0.4 m which provided an average slurry concentration of about 15% (v/v) when all solids were dispersed. Both axial and column average phase holdups were measured. Effects of sparger location, gas jets formation and liquid circulation patterns on gas holdups and solids dispersion are analyzed. Empirical correlations are developed to relate sparger location to solids dispersion as a function of gas velocity. Optimum sparger height and orifice orientation is proposed based on the measurement of this study.