Particle agglomeration and control of gas-solid fluidized bed reactor with liquid bridge and solid bridge coupling actions
Introduction
Particle agglomeration is an important phenomenon in gas-solid fluidized beds. It extensively occurs in various industrial processes such as drying and granulation, combustion, as well as gas-phase olefin polymerization. It appears that causes of particle agglomerations are so different, for example, as in the steps of particle agglomeration formation in fluidized beds granulators [1], [2], [3], [4]. Firstly, liquid bridge interaction via binder causes liquid coating on particles or formation of small agglomerations. Secondly, the liquid solvent evaporate and then particles are bonded together by solute solid bridge. In this way, particle growth and granulation can be finally achieved. In fluidized bed combustors, particle agglomerations are caused by interactions between bed particles and ash content formed at a lower melting temperature [5], [6], [7]. In polyolefin fluidized bed reactors, the condensed mode with liquid addition can effectively enhance heat transfer capability and thus it is extensively used in industrial reactors [8], [9], [10]. However, the added liquid and relatively high temperatures in above-mentioned processes will severely alter bed hydrodynamics, such as liquid bridge and solid bridge interactions among particles, and further affect heat and mass transport behaviors of the fluidized bed. From an in-depth analysis, the common essential cause of particle agglomerations can be ascribed to continuous increases in inter-particle forces induced by liquid bridge, solid bridge or electrostatic effects. In summary, the inter-particle forces play an essential role in agglomeration formation and evolution, which needs to be clearly studied.
So far, a large number of researches have been focused on particle growth, agglomeration characteristics and fluidization behaviors in liquid-containing gas-solid fluidized bed reactors (LCGSFBRs) [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]. In early studies, Seville et al. [17] reported the addition of a non-volatile liquid to Geldart’s B type particles makes the fluidization behavior change from A type to C type, and then McLaughlin et al. [15] concluded inter-particle forces severely affected the fluidization characteristics. Subsequently, Iveson et al. [3], [21], [22] conducted systematic experiments and analyses on wet granulation process and then proposed a particle growth regime map based on particle collision deformability and liquid saturation, which can be used to distinguish various particle growth regimes under certain operating conditions. Later, Weber and Briens et al. [14], [15], [23], [24], [25], [26] found liquid properties, particle properties, and operating parameters exerted great impacts on agglomeration stability and growth mechanism. Against an industrial background of olefin polymerization, Zhou et al. [20], [27], [28] systematically studied the fluidization characteristics of LCGSFBRs by establishing a multi-scale characterization method based on multiple measurement techniques (acoustic[29], pressure and camera) and various analytical means. In this way, the investigated the changes in particle agglomerations, gas bubbles, and overall regimes; moreover, they [27] then constructed a multiple temperature zone fluidized bed reactor (MTZFBR) by liquid spraying scheme according to the olefin polymerization process. In recent years, liquid bridge force module has been gradually introduced into CFD-DEM simulation and theoretical analysis. Mikami et al. [16] developed a DEM simulation code based on soft-sphere interaction at particle collision while considering the inter-particle cohesive forces in the numerical simulation, and then successfully obtained the agglomeration fluidization behavior of wet particles. After that, substantial progresses have been made in modeling fluidization behaviors and agglomeration characteristics of liquid-bridge cohesive particles [11], [30], [31], [32], [33].
Meanwhile, studies on solid bridge force action have also received extensive attention from both academic and industrial circles. As we know, the solid bridge force is different from liquid bridge, van der Waals forces and electrostatic forces, since it is induced by sintering, a time-variant process in Arrhenius relationship with temperature [30]. The solid bridge is formed in the mutual contacting zones by diffusion, cohesion and other mechanisms. Tardos et al. [31], [32] believed that the breaking forces of agglomerations come from gas bubble actions, and according to the relation between agglomeration strength and breaking forces, they built a theoretical model to describe the relation between the unsteady states of fluidization and the agglomerations formed by high temperature sintering. Yates et al. [33] reviewed the experimental and theoretical studies in high temperature gas-solid fluidization processes at the early stage, and mainly discussed the temperature effects on critical gas fluidization velocity, gas bubble, voidage and other key parameters. Cui et al. [34] discovered that hydrodynamics of FCC reactor varied considerably with bed temperature and proposed a modified two-phase model integrating the effects of temperature and those of superficial gas velocity on hydrodynamics. Lettieri et al. [35] studied the temperature effects on fluidization behaviors of FCC catalysts with the bed collapse method. Bouffard et al. [36], [37], after adding a PEA/PMMA copolymer layer onto the surface of glass particles in a spheronizer, measured the inter-particle forces among coated particles under different temperatures and found inter-particle cohesive forces can be controlled by changing temperature and coating thickness. Similarly, Shabanian et al. [38], [39] employed sugar particles with a PEA/PMMA coating to study the effects of inter-particle forces on hydrodynamic behaviors and found that the increases in inter-particle forces caused variations in particle Geldart types and increases in minimum fluidization velocity and critical turbulent velocity. According to the above researches, changes in temperature show essential impacts on the relation between hydrodynamic forces and inter-particle forces.
When analyzing the former researches as listed above, we find that most of them focus on the measurement and analysis of particle agglomeration with a single liquid or solid bridge inter-particle force. The studies have reached a common conclusion that increases in inter-particle forces led to significant changes in particle fluidization characteristics and the transition of particle behavior type from non-cohesive Geldart B type to Geldart A or even cohesive Geldart C type. By comparison, the agglomeration process with liquid bridge and solid bridge inter-particle forces coexisting shows more complicated characteristics and thus needs further studies. On one hand, liquid is added into reactors as a necessary component to intensify mass-transfer and heat-transfer capabilities. On the other hand, under high temperatures, solid bridge force and liquid bridge force conditions are interacting with each other, and thus reactions as well as transport phenomena in LCGSFBR will become more and more complicated. The inter-particle forces for agglomerations are so strong in such complex conditions that they become the main causes of severe irreversible agglomerations in industrial processes. When the mass-transfer and heat-transfer conditions severely deteriorate or the undesirable heat-transfer state lasts for too long, the irreversible agglomerations will occur frequently, affecting the stable operation of the fluidized bed and eventually leading to blockage of pipelines and unscheduled shutdown of plants. Therefore, studies of particle agglomerations with multiple inter-particle forces coupling are of great theoretical significance and industrial values. Moreover, studies of particle fluidization and agglomeration process under such complex conditions are far from adequate [28].
In recent years, most researchers investigated particle cohesion and agglomeration due to thermal effects by using external heating methods, such as indirect air heating and wall heating [40], [41], [42]. Nevertheless, the external heating methods just keep fluidized particles in the condition of a constant temperature or a varied temperature with gradual changes. It is impossible to simulate a real exothermic reaction environment in fluidized beds. In other words, the thermal environment where heat accumulates rapidly in the reaction zones or in the reactive particles can not be created. In order to overcome this difficulty, a gas-solid fluidized bed simulation setup based on electromagnetic induction heating effects was proposed and applied in this work. The growth rate of bed temperature can be controlled by regulating the heating power so that we can simulate particle fluidization and agglomeration behaviors in the exothermic reaction environments within the fluidized bed as real as possible. Moreover, combining the liquid bridge action, this work also conducted experiments and force balance analysis of wet particles in the fluidized beds by considering effects of both liquid bridge and solid bridge actions. To be specific, the effects of liquid content, heating power and related operating parameters on liquid bridge force, solid bridge force and particle agglomeration characteristics are systematically studied to reveal how coupling actions of liquid bridge and solid bridge forces can affect particle agglomeration characteristics.
Section snippets
Theoretical analysis
Fig. 1 shows the force balance analysis of two particles with liquid bridge and solid bridge during fluidization. In this condition, a single particle are mainly under actions of four kinds of forces, i.e. drag force, Fd, gravity force, Fg, liquid bridge force, Fliq and solid bridge force, Fsol. It can be obtained from the force balance analysis that Fd will prevent particles from agglomerating while Fg may either promote agglomeration formation and growth or prevent agglomeration, which
Experimental setup and methods
The industrial background of this work is condensed mode operation of olefin polymerization gas-phase fluidized bed reactor. The polymerization heat is transferred from catalyst active sites inside polyethylene particles to particle surfaces and then is removed by fluidization gas. The reaction heat of the particles transfers from the inside to outside by heat conduction. External heating method is difficult to simulate the real heat transfer conditions. Thus, a novel electro-magnetic heating
Effects on liquid bridge force
The relationship between static bridge force of wax/graphite composite particles and liquid content is shown in Fig. S5. It can be obtained from the figure that the inter-particle force (Fliq, s) increases with the liquid content of the composite particles.
Effects on solid bridge force
On one hand, the addition of liquid into fluidized bed causes decrease in bed temperature because of liquid temperature increase and its following evaporation, leading to the increase of surface viscosity which is adverse to solid bridge
Conclusions
This work has proposed an experimental apparatus combining electro-magnetic induction heating system with fluidized bed reactor to simulate real particle heat reaction release and heat transfer conditions for industrial polymerization fluidized bed reactors. We have investigated effects of liquid content on agglomeration behaviors of wax/graphite composite particles with different fluidization gas temperatures, gas velocities and heating powers. Moreover, this work performs a force balance
Acknowledgments
The work was supported by National Natural Science Foundation of China (21506181, 91434205, 21525627), Outstanding Youth Science Foundation of Zhejiang Province (LR14B060001), Natural Science Foundation of Hunan Province (2016JJ3113), Specialized Research Fund for the Doctoral Program of Higher Education (20130101110063) and Hunan 2011 Collaborative Innovation Center of Chemical Engineering & Technology with Environmental Benignity and Effective Resource Utilization.
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