Elsevier

Chemical Engineering Science

Volume 195, 23 February 2019, Pages 730-736
Chemical Engineering Science

Hydrodynamics of a bubble-driven liquid-solid fluidized bed

https://doi.org/10.1016/j.ces.2018.10.018Get rights and content

Highlights

  • A new bubble-driven liquid-solid fluidized bed has been proposed.

  • The loose velocity Uloose and initial fluidization velocity Uin were defined.

  • Three flow regimes were distinguished by Uloose and Uin.

  • Two flow mechanisms for particles fluidization were proposed.

Abstract

A new fluidization system, the bubble-driven liquid-solid fluidized bed, adopting particles with a slightly higher density than the fluid, has been proposed. During the experiment process, air was introduced into the bottom of the column as driving force, and superficial gas velocity ranged from 0 to 28 mm/s and liquid was continuous phase but maintained at the stationary state. The variation of pressure drops in the bottom section near the gas distributor was measured to identify the demarcation of flow regimes and determine the loose velocity (Uloose) and initial fluidization velocity (Uin). It was found that Uloose and Uin both increase with the initial hydrostatic height but are independent of the initial liquid height. Moreover, solids holdup was analyzed using a photographic technique while gas holdup was calculated from the pressure drop variation and solids holdup. A non-uniform axial solids distribution exists for all gas velocities. Additionally, two fluidization mechanisms were proposed based on the experimental observations.

Introduction

Gas-liquid-solid fluidized beds have been commonly used in industries involving chemical, petrochemical, electrochemical and biochemical processes because of its superior mass, heat and momentum transfer efficiencies between the phases (Fan, 1989). The traditional gas-liquid-solid fluidized beds usually employ particles much heavier than the fluid, thus requiring the liquid flow to support the solids. The dynamics in such systems, including the conditions for fluidization, the transition of flow regimes and the distribution of phases, have drawn great attention in the last decades (Muroyama and Fan, 1985, Yang et al., 2007, Jena et al., 2008, Pjontek and Macchi, 2014).

Generally, the start of particle fluidization is characterized by the minimum fluidization velocity, Umf, which could be inferred from the pressure drop variation through the whole bed (Lee and AL-Dabbagh, 1978). As the liquid flow is the main driving force for heavy particles, Umf usually refers to the minimum liquid fluidization velocity, Ulmf (Zhang et al., 1995, Felipe and Rocha, 2007, Briens et al., 1997). Further transition of flow regimes beyond minimum fluidization is normally evaluated according to the different flow characteristics, such as bubble properties and particle movements by changing the liquid flowrate at a fixed gas velocity (Matsuura and Fan, 1984, Chen et al., 1994, Zhang et al., 1997, Yang et al., 2007). Also, Zheng et al. (1999) reported a clear boundary between the bottom dense region and the top freeboard region in the traditional fluidization regime and a fairly uniform distribution in the circulating fluidization regime in the liquid-solid fluidized bed.

Although numerous studies focused on the fluidization behaviors by changing the liquid flowrate, only a few literature focused on the variation of flow patterns at different gas velocities, but none with liquid as the stationary state. Meanwhile, gas could be the only driving force to lift the particles with density close to the fluid. Such a fluidized bed which uses bubbles as the only driving force and adopts particles slightly heavier than the fluid are defined here as a bubble-driven liquid-solid fluidized bed. Such bubble driven fluidized beds may find many industrial applications in biochemical, food and environmental processes as well as wastewater treatment.

Compared to three phase airlift reactors (Chisti and Moo-Young, 1987) which could also have no net flow of liquid, the driving force in a bubble driven fluidized bed is the up-flow of bubbles but is the density difference in an airlift reactor. Meanwhile, no fluid circulation in the bubble driven system could prevent the possibility of particle entrainment at high loadings.

Recently, Sun, 2017, Huang, 2017 proposed fluidization systems only using bubbles to fluidize solid particles slightly lighter than the fluid. In contrast, for the slightly heavier particles, most studies concentrated on the comparison between their flow and flow of much heavier particles using liquid as the driving force (Zheng et al., 1999, Cui and Fan, 2004, Briens and Ellis, 2005). These results confirmed that the flow characteristics and the regime transitions can be significantly affected by the particle density. However, no systematic study has been reported for a system employing slightly heavier particles with liquid as the stationary state, although Epstein, 1981, Muroyama and Fan, 1985 mentioned fluidized beds could be realized in such conditions.

In this study, the hydrodynamics in a bubble-driven liquid-solid fluidized bed were studied. Pressure drop measurement and visualization methods were employed to define the transition of flow regimes and phase holdups. Furthermore, some basic fluidization parameters were applied to characterize the flow behaviors. Phase distributions in the column were investigated and the flow mechanisms of particles were proposed.

Section snippets

Experimental apparatus

The experimental apparatus for the bubble-driven fluidized bed system is shown in Fig. 1. A cylindrical plastic glass column with an inner diameter of 0.889 m and a height of 1.8 m was used. Air was introduced into the bottom of column through an annular porous quartz gas distributor with outer and inner diameters being 4.6 cm and 2.8 cm, respectively, as shown in Fig. 1. This distributor generates bubbles with a diameter of approximately 1 mm. The number of bubbles increases with the

Experimental observation

The fluidization in the bubble-driven system was controlled by the gas flowrate. For the fluidization of P1, by introducing a small amount of gas into the bed, some particles near the packed bed surface flowed upward to the top of the whole bed. With increasing gas velocity, a packed bed, a dense phase and a dilute phase could be observed from the bottom up, although the boundaries among them were hard to distinguish. This is different from the distinct boundary between the dense phase and

Conclusions

Experiments were conducted in a new bubble-driven liquid-solid fluidized bed with bubbles being the only driving force. Pressure drop measurements as well as visualization methods have been used to investigate the flow characteristics in the column. Some key findings are as follows:

  • (1)

    The loose velocity Uloose and initial fluidization velocity Uin could be defined by the valley and peak value of the pressure drop variations in the bottom section near the gas distributor. Three flow regimes, i.e.,

Acknowledgements

The research is partially supported by State Key Laboratory of Heavy Oil Processing (SKLOP20163001), Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology (BM2012110), National Natural Science Foundation of China (21306014) and Natural Science and Engineering Council of Canada (NSERC). Y. Liu also thanks the foundation of Jiangsu Government Scholarship for Overseas Studies.

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