| Turbulent fluidization is a transition regime between bubbling fluidization and fast fluidization. Turbulent fluidized bed (TFB) has good performance on heat transfer and mass transfer with short gas-solid contact time and is suitable for gas-solid fast catalytic reactions. The typical industrial applications include:Methanol-to-Olefins (MTO), synthesis of acrylonitrile, FCC catalyst regeneration, and so on. With fast development of industrial catalyst technology and process intensification technology, the application realm of TFB is expanding. Since the development and scale-up of TFB reactor need the hydromechanics research as their basis, it’s necessary to start relevant investigation aiming at TFB characteristics.An important difference between TFB and bubbling/fast fluidized bed is the high proportion of transition section which located between dense phase section and dilute phase section. Since the catalyst hold up in the transition section is high, it plays an important role on TFB reactor performance. The axial and radial solid concentration distributions in transition section are highly non-homogeneous, thus it’s difficult to investigate the hydromechanics. Since the former researches paid less attention to the transition section, the elaboration of flow parameter distributions is quite insufficient. Also the flow measurement technology and computational fluid dynamic (CFD) model have some problems on reliability. In the light of all above, this paper focused on the hydromechanics of transition section in TFB. Both experimental and CFD simulation research were carried out. In the cold model experimental apparatus, solid concentration and particle velocity distributions in transition section with different operation conditions were measured. The distribution patterns were also simulated via CFD. The influence of standpipe on flow parameter distribution was investigated to get a deeper understanding of flow patterns in transition section of TFB and to figure out the possible method on process intensification and improvement. The content of this paper covers following aspects.1. Correction and data processing of optical probe signal in TFB. Based on the comparison of multiple measurement methods, optical probe was chosen to measure the solid concentration and particle velocity simultaneously. A downcomer was used to calibrate and correct the solid concentration. Meanwhile, a self-made device was used to calibrate and adjust the particle velocity. The calibration curve of solid concentration was obtained. The emitting light scattering of optical fibre was investigated, and then a velocity calibration method using filter threshold was developed. A new method based on cross-correlation theory was proposed to calculate time-averaged velocity of particles. Cross-correlation coefficient was introduced into the formula for time-averaged velocity as the second weighting factor. The time-averaged velocities calculated from different methods were compared. The solid volume flow rate based on different velocity calculating methods indicated that the new method is more reliable and is more proper for calculating time-averaged velocity of particles in turbulent regime.2. Solid concentration distribution in transition section of TFB. Solid concentration distribution and fluctuation parameters were measured in a fluidized bed with 100mm i.d. and 2200mm height using a PV6D optical fiber probe. The experiments showed two axial profiles of solids distribution in the transition section:exponential and S-shaped. The maximum gradients of both axial and radial solids concentration profiles were located in the transition section indicating that solids distribution in the transition section was much more non-uniform than that in the dilute-phase and dense-phase sections. The change of gas velocity and static bed height would cause a shift between exponential and S-shaped profiles, and significantly affect solids concentration in the dense region near the wall and at the bottom. Probability density distribution (PDD) of local solids concentration revealed that when static bed height was small, the location of maximum voidage would gradually move from core region to annular region with increasing gas velocity, resulting in a bimodal distribution of voidage. Standard deviation of local solids concentration indicated that the fluctuation in the center of bed decreased faster than that near the wall, which meant a quicker development of flow regime. A modified three-zone drag model was proposed by taking the effect of particle clustering into account for CFD simulation. The simulated solids concentration distribution agreed well with experimental data in the transition section except for the distributor zone.3. Particle velocity distribution in transition section of TFB. Particle velocity in a 95 mm ID and a 200 mm ID fluidized bed was measured with optical fiber probe in different operation conditions. Based on the measurement results, the turbulent fluidized bed was divided into 6 regions. Combining experimental results and simulation, the generation mechanisms of decelerating regions of top section and transition section in TFB were analysed. The results showed that the decelerating region on the top is caused by discharge section, which brought decreasing flow velocity and increasing solid concentration; meanwhile the decelerating region in the bottom| is caused by the developing gas velocity distribution and the decreasing solid concentration whose distribution tend to be flat along the axial height. Effects of fluidization section height, static bed height and gas velocity on decelerating regions were investigated. The decelerating region on the top declined with the decreasing fluidization section height, meanwhile the length of fully developed section and accelerating section shortened or even disappeared. The decelerating region on the bottom rose with the static bed height and descended with the gas velocity. The variation trend of decelerating regions was in accord with the one of transition section.4. Particle distribution characteristics in transition section of an annulus turbulent fluidized bed. Solid concentration and particle velocity distributions in the transition section of a 0200x7000 mm turbulent fluidized bed (TFB) and a same size annulus turbulent fluidized bed (A-TFB) with a 050 mm central standpipe were measured using PV6D optical probe. In turbulent regime, the axial distribution of cross-sectional average solid concentration in A-TFB with different superficial gas velocities and static bed heights was similar to that in TFB. However, the former had a shorter transition section than the latter. The measured axial solid concentration distribution, probability density, and power spectral distributions revealed that the standpipe hindered the turbulence of gas-solid two-phase flow at a low superficial gas velocity. Consequently, the bottom flow of A-TFB approached the bubbling fluidization pattern. By contrast, the standpipe facilitated the turbulence of gas-solid two-phase flow at a high superficial gas velocity, thus making the bottom flow of A-TFB approach the fast fluidization pattern. The solid concentration and particle velocity radial distributions indicated that the standpipe at a low gas velocity and in the bottom dense-phase section slightly affected these two distributions. Both of the parameters presented a pot-cover-like unimodal distribution in A-TFB and TFB. However, the standpipe at a high gas velocity and in the transition or dilute phase section significantly affected the radial distribution of the two flow parameters, presenting a tire-shape bimodal distribution. Thus, in the later situation, the standpipe destroyed the original core-annular structure and also improved the particle velocity and solid concentration distribution of TFB. |
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