Study On The Motion Behavior Of A Large Dense Object Immersed In A Fluidized Bed

Simultaneous treatment of dissimilar materials, that differ in their constitutive properties, namely density, size or shape etc. is normally encountered in a fairly large number of industrial applications of gas-solid fluidization technology. Usually, objects with high density and large size are mainly in the dense zone of a bed and may be captured in the stagnant zones over the air distributor, having paramount influence on the momentum, energy and mass transfer of the bed, as well as the stable operation of the bed. Therefore, in this thesis, the motion behaviors of dense large objects in the dense zone of a fluidized bed were studied by experiment measurement, mechanical analysis and numerical simulation. The results will enrich the multi-components fluidization theory and provide a reference for design, operation and optimization of the fluidized bed reactor including dense solids with large size difference.An innovative method combing electrical capacitance tomography(ECT) technique, tracing concept and image processing technique to trace a large object’s motion trajectory in the dense zone of a fluidization bed is proposed. For the bed configuration, particle properties and operating conditions studied, good tracing result can be got when the object size is not smaller than one tenths of measured section and the dimensionless fluidization velocity /crv v is not larger than 4.2. The maximum location error is about one fifths of the object size.The influence of the object properties, superficial gas velocity and the inclined angle of air distributor on the residence time distribution(RTD) and motion trajectory of a large dense object in an uneven air distribution bed was investigated by the above ECT tracing technique. Results show that, there exist trajectory repeatability, discontinuity and residence time randomness, which are caused by the friction force with the air distributor and the fluctuation of the surrounding gas-solid flow. With the decrease of the gas velocity, the mean residence time(MRT) of an object increases with a trend of slow at first then getting fast. When gas velocity keeps decreasing, the object only arrives at the lower side of the air distributor at a certain probability, until eventually stagnates over the air distributor. There exists a turning gas velocity for each object, lower than which MRT increases sharply, and a critical arriving gas velocity, low than which MRT diverges to infinity. As object’s size and density increase, MRT decreases initially and increases subsequently with a turning point. Based on probability and statistics concept, the motion state of the large objects in a fluidized bed can be clarified into four cases: ○1 moving over the air distributor, but may be involved into fluidization(leaving distributor) sometimes; ○2 continuously rolling/sliding on the distributor; ○3 discontinuously rolling/sliding on the distributor; ○4 stagnated over the distributor and keeping motionless. Meanwhile, the influence of the experimental variables on the evolution of states was analyzed and characterized. In addition, based on dimensionless analysis, the empirical correlations of MRT of the objects with experimental variables in state ○2 and state ○3 were established. The correlations have a favorable prediction capability, with the most of the relative errors within ±30%.Considering the effect of the inclined air distributor, the mechanical balance equation of a spherical object in the dense zone of a fluidized was established based on two-phase flow theory. Then further analysis on the effect of experimental variables on objects’ residence times was taken and the theoretical mechanical conditions required for the evolution of the four motion states were given.Finally,the characteristics of the forces acted on the object from the surrounding gas-solid flow in a fluidized bed were studied by Euler-Euler multiphase fluid simulation, and the dynamical fitting equations of the instantaneous forces were proposed. Combine with mechanical balance equation and take into account the randomness pulsation of the gas-solid flow, the mechanical model of a large object moving in the fluidized bed was established. Several numerical experiments were conducted according to the model, and the RTD results calculated were compared with the experimental results. Results show that the main mechanisms controlling the motion of large dense objects have been covered by the mechanical model, thus it can describe and reflect the motion characteristic of the objects.