Hydrodynamic Investigation On Magnetized Fluidized Beds With Geldart-B Particles

The typical gas-fluidization behavior of Geldart-B particles is that bubbles rise in the bed immediately after the superficial gas velocity (Ug) exceeds the minimum fluidization velocity (Umfo)-The excess gas (Ug-Umf0) does not disperse uniformly among particles; instead it passes quickly through the bed in the form of bubbles. The formation of gas bubbles deteriorates the fluidization quality and the gas-solid mass transfer.The magnetic field is often used to improve the fluidization quality of Geldart-B magnetizable particles as well as their admixture with Geldart-B nonmagnetizable particles, thus creating the magnetized fluidized bed (MFB). Compared with other techniques of improving the fluidization quality, the magnetic field has the primary advantage of not being affected by the harsh condition inside the fluidized bed reactor since it can work from the outside.Thus far, the research on the MFB with Geldart-B particles mainly focused on the famous magnetic stabilization flow regime ((Umf<Ug<Umb), which could be easily created under’ Magnetization FIRST’ operation mode. By contrast, little attention was paid to the magnetized fluidization flow regime (Ug>Umb). Besides, the research on’Magnetization LAST’ operation mode is far from sufficient:as for the MFB with purely Geldart-B magnetizable particles (termed magnetizable bed here), the flow regime transition under this operation mode remains a controversy; as for the MFB with admixtures of Geldart-B magnetizable and nonmagnetizable particles (termed admixture bed here), the flow regime transition is still unknown.Aiming at these shortcomings, this thesis presents a detailed study on the gas-solid flow hydrodynamics in the magnetized fluidization flow regime. Besides, the flow regime transition of the MFB under the two operation modes is carefully studied and compared. For the admixture bed. the particle segregation behavior is quantitatively investigated from both the experimental and mathematical perspectives. The principal results and major conclusions are as follows:(1) The traditional magnetized fluidization flow regime could be further divided into two distinguishable sub-regimes. For the magnetizable bed, the two sub-regimes are channel-bubbling and magnetized-bubbling; for the admixture bed, the two sub-regimes are segregation-bubbling and magnetized-bubbling. In the channel-bubbling and segregation-bubbling sub-regimes, due to the appearance of channels or segregation the magnetic field could hardly improve the fluidization quality. Only in the magnetized-bubbling sub-regime could the magnetic field effectively reduce the bubble size and improve the fluidization quality.(2) The axial magnetic field has a much stronger reduction effect on the bubble width than on the bubble length, causing the bubble shape to change from nearly spherical in the absence of a magnetic field to elliptical in the presence of a magnetic field. The mechanism of the bubble size reduction is different in the two types of magnetized fluidized beds. In the magnetizable bed the bubble size reduction arises mainly from that the magnetic field significantly enhances the cohesive force between magnetizable particles and increases the surface tension of the solid phase (a pseudo-fluid phase). According to the knowledge from phase interfacial physical chemistry, this would eventually make the bubble formation and growth up (i.e., the area increase of the phase interface) become more difficult. On the other hand, in the admixture bed the bubble size reduction results mainly from that the magnetic chains form a floating internal inside the bed which could effectively break up large gas bubbles as well as hinder the coalescence of small bubbles.(3) Under’Magnetization LAST’ operation mode, both the magnetizable and admixture beds exhibit four distinguishable flow regimes. For the magnetizable bed the four regimes are fixed, magnetized-bubbling, channel-bubbling, and magnetically condensed; for the admixture bed the four regimes are fixed, magnetized-bubbling, partial segregation-bubbling, and complete segregation-bubbling. Note particularly that under "Magnetization LAST’operation mode the magnetic stabilization flow regime could not be obtained.(4) Comparison of the flow regime transition between the two operation modes indicates that there exist four operation zones for the MFB:Ⅰ,Ⅱ,Ⅲ, and Ⅳ. It is found that in operation zone Ⅱ, the bed state depends not only on the values of H (magnetic field intensity) and Ug but also on their application sequence (i.e., operation mode). Such dependence in essence is the dependence of the bed state on the path taken in order to achieve it. The reason why in operation zone Ⅱ the bed state is path-dependent lies in that in this zone the MFB could have two different equilibrium states and different paths will probably lead to the two different equilibrium states, respectively. Experimental study demonstrates that the famous magnetic stabilization state is a metastable equilibrium state, which could not recover after a slightly larger perturbation is introduced. In operation zone Ⅱ, the reason why there is a metastable equilibrium state of magnetic stabilization aside from the stable equilibrium state lies in the difficulty in forming a new phase under ’Magnetization FIRST’ operation mode.(5) In the admixture bed particle segregation would often occur:the magnetizable particles play the role of jetsam while the nonmagnetizable particles work as flotsam. Under the action of the magnetic field the magnetizable particles exist in the form of magnetic chains. As a result, it is the magnetic chains rather than the original magnetizable particles that co-fluidize with the nonmagnetizable particles. Under’Magnetization LAST’ operation mode, the reason for the occurrence of particle segregation lies in the size growth of magnetic chains with increasing H. With the increase of the first introduced Ug, both the magnetic field intensities at which particle segregation begins (Hms) and completes (Hte) increase. A higher Ug means a stronger driving force for the mixing of the magnetic chains and nonmagnetizable particles, and therefore the magnetic chains need to grow even larger to sink to the bottom bed. Accordingly, the magnetic field intensity (Ums) needed to arouse the segregation is larger.(6) A preliminary mathematical model is established to calculate the particle segregation in the admixture bed. The calculated Hms-Ug relationship shows a good agreement with the experimental result, indicating that the magnetized segregation model established here could be used to predict the segregation behavior in the admixture bed and further to provide a theoretical direction on how to avoid or utilize such magnetized segregation.