Experimental and numerical investigations on the hydrodynamics of gas-solid fluidized bed with an inclined agitator

Highlights

• Inclined agitation is utilized to improve the gas-solid fluidization quality.

• Hydrodynamics in fluidized bed are investigated experimentally and numerically.

• Inclined agitation has a positive effect on the decreases of bubble size.

• Inclined agitation enhances the gross circulation of solid particles.

• Agitation generates higher collision frequency and more activity of particles.

Abstract

The effect of inclined agitation on the fluidization quality of a gas-solid fluidized bed is experimentally and numerically investigated. The pressure characteristics are characterized by the standard deviation of pressure drop and power spectra. The particle velocity, solid volume fraction and granular temperature distributions are investigated by numerical modeling. It is found that inclined agitation enhances the gross circulation of solid particles and enlarges the affect region of the agitator, improving the gas-solid fluidization quality. The increase in the agitation speed leads to higher frequency of collisions and more activity of particles, resulting in decreases of bubble sizes and a more homogeneous solid volume fraction distribution. Wide-blade agitator can also improve the fluidization performance.

Introduction

The fluidized bed reactors (FBRs) have been widely used in the petroleum, chemical, energy, environmental protection, pharmaceutical, food processing and metallurgical fields because of the simple structures and high mass and heat transfer rates [1,2]. For the processes related to the cohesive particles or nonuniform fluidization, such as granulation, drying, crystallization and even ironmaking [[3], [4], [5], [6], [7], [8]], mechanical agitation is usually employed to break the bubbles by the intensive shear force and improve the gas-solid fluidization quality.

Since the 1950s, the phenomenon that mechanical stirring can reduce the pressure fluctuation in fluidized beds has attracted wide attention [9,10]. The influencing factors involved can be mainly divided into three categories: 1) the geometry of fluidized bed, such as the structure of gas distribution plate, the type and size of agitators, the shape and angle of blade, etc. 2) operating conditions, for example the agitation speed and the superficial gas velocity. 3) physical properties of solid particles, for instance the density and size distribution of particles. Changing these parameters will result in the changes of hydrodynamics including velocity distributions, volume fraction distributions and pressure characteristics, which will affect the fluidization quality [1,[11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]].

The pressure fluctuation in the fluidized bed consists of local bubble induced fluctuations, global bed oscillations and others. Once generated, pressure waves propagate in a fluidized bed like compressible waves, with little attenuation, resulting in local pressure fluctuations [14]. Under the action of mechanical stirring, the bubbles break up and even disappear, presenting uniform fluidization [15]. Zhang et al. [19] found that agitation and gas turbulence jointly determine the fluidized quality. The area of the blade also has a great influence on the pressure fluctuations. Wang et al. [22] found that the gas-solid two-phase flow structure along the bed elevation is controlled by three mechanisms: the gas distributor at the bottom, the mechanical agitation in the middle, and the free movement of the bubbles at the top. The pressure fluctuations in the fluidization zone can be significantly reduced by using agitators, so as to improve the fluidization quality. An interesting phenomenon was discovered by Dong et al. [18]. Mechanical agitation promotes bubble growth at low fluidizing gas velocity but reduces the bubble sizes with increasing gas velocities, which is caused by the combined movement of vertical and horizontal blades for the arch agitator.

To acquire more detailed information on the agitated fluidized bed, computational fluid dynamics (CFD) has been employed to predict the complex gas-solid two-phase flow [2,[22], [23], [24], [25], [26], [27], [28], [29], [30]]. Mechanical agitation prevents agglomeration of fine particles in a fluidized bed and thus decreases the effective particle diameter and increases the void fraction [25]. Gu et al. [31] revealed that the increase in agitation speed can reduce the standard deviation of pressure fluctuation in bed and reduce the bubble diameter, transforming bubbling fluidization to dispersed fluidization. Actually, with the increment in agitation speed for radial-flow impeller, the radial velocity of solid particles increase, which reduces the pressure fluctuations [2]. The solid mixing in the fluidized bed is controlled by the gross circulation and internal circulation [29]. The shear force generated by the agitator can reduce the bubble size and internal circulations, enhancing the fluidization performance.

Vertical agitation is applied to the fluidized bed motioned above. To improve the fluidization quality, the agitators are usually large in diameter and high in height. A new fluidized bed with an inclined agitator is proposed in this paper. The inclined agitator is installed on the fluidized bed in order to enhance the effect of the agitator and improve the fluidization quality.

In this work, the influence of vertical and inclined agitations on the fluidization behavior is investigated experimentally and numerically. The Euler dual fluid model, RNG k-ε turbulence model and modified drag model are chosen to simulate gas-solid two-phase flow in the fluidized bed. The pressure fluctuations, power spectra, particle velocity distribution and solid volume fraction distribution for different agitation speeds in the fluidized bed with an inclined agitation are investigated in the paper. The work in this paper gives a new opinion for the design of gas-solid fluidized bed.