The Experts below are selected from a list of 16398 Experts worldwide ranked by ideXlab platform
Jesse Zhu - One of the best experts on this subject based on the ideXlab platform.
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Simulation of chemical reaction process in gas-particle CFB Downers by anisotropic turbulent mass transfer model
Chemical Engineering Research and Design, 2018Co-Authors: Xigang Yuan, Yuanyuan Shao, Jesse ZhuAbstract:Abstract A recently developed model, called the Reynolds mass flux (RMF) model is adopted for the simulation of multiphase flow involving chemical reaction in gas-particle CFB Downers. The newly deduced Reynolds mass flux equation is utilized for closing the turbulent mass transfer equation, thereby realizing the simulation of anisotropic turbulent mass transfer. The simulations are validated with experimental data in literatures. The results indicate that the turbulent mass diffusion in Downers would show significant anisotropic character.
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radial solids flow structure in high flux gas solids circulating fluidized bed Downers
Powder Technology, 2016Co-Authors: Jesse Zhu, Chengxiu Wang, Jinsen Gao, Xingying Lan, Shahzad BarghiAbstract:Abstract High flux gas-solids circulating fluidized bed downer reactors have unique characteristics especially suitable for very fast reactions. Detailed hydrodynamics of such reactors are experimentally studied using FCC particles at various superficial gas velocities (1–7 m/s) under high flux conditions up to 700 kg/m 2 s for the first time. Results show that although the radial distribution of solids holdup is somewhat less uniform under very high flux conditions in the Downers, it is still much more uniform compared to riser reactors. Radial profiles of solids holdup, particle velocity and solids flux are significantly affected by the operating conditions. Particle velocity distribution is characterized by a relatively flat core and an annulus, where the particle velocity slightly increases towards the wall under low flux. In very high flux downer reactors, the shape of the local particle velocity becomes parabolic. It is also found that relationships between local solids holdup and particle velocity are different in the Downers compared to the risers due to their different flow characteristics. Compared to the riser reactor, downer has a self-adjusting mechanism to perform with the nearly “ideal” plug flow nature.
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Performance evaluation of high density riser and downer: Experimental study using ozone decomposition
Chemical Engineering Journal, 2015Co-Authors: Chengxiu Wang, Jesse Zhu, Shahzad BarghiAbstract:Abstract Reactor performance of high density circulating fluidized bed (CFB) riser and downer is studied with superficial gas velocities of 3–9 m/s and solids circulation rates of 100–700 kg/m2 s using ozone decomposition reaction. Results show that the reactant conversion in the riser and downer is closely related to the hydrodynamics, with solids holdup being the most influential parameter on ozone decomposition. Both axial and radial distribution profiles of the ozone concentration and the solids holdup in the downer are more uniform than that in the riser. Overall ozone conversion increases with the increase of solids circulation rate and/or the decrease of superficial gas velocity in both reactors. Compared to the riser reactor where the overall conversion is even less than the reactant conversion in stirred-tank reactor, overall conversion in the downer is less than but very close to that in ideal plug flow reactor. Contact efficiency is higher in the downer than in the riser indicating that gas–solids contacting in the downer is better due to the uniform solids flow structure and the reactant concentration.
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Solids concentration in the fully developed region of circulating fluidized bed Downers
Powder Technology, 2008Co-Authors: Hui Zhang, Jesse ZhuAbstract:Abstract To investigate solids concentration in the fully developed region of co-current downward gas–solid flow, actual solids concentrations were measured in a circulating fluidized bed (CFB) downer with 9.3 m in height and 0.1 m in diameter using a fiber optical probe. The results obtained from this work and in the literature show that the average solids concentration in the fully developed region of the CFB Downers is not only a function of the corresponding terminal solids concentration, but the operating conditions and particle properties also have influences on the average solids concentration in the fully developed region of the CFB Downers. Particle diameter and density affect the solids concentrations differently under different operating conditions. Downer diameters almost have no influence on the solids concentrations. By taking into account the effects of operating conditions, particle properties and downer diameters, an empirical correlation to predict the solids concentrations in the fully developed region of CFB Downers is proposed. The predictions of the correlation are in good agreement with the experimental data of this work and in the literature.
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friction between gas solid suspension and circulating fluidized bed Downers
2007Co-Authors: Hui Zhang, Jesse ZhuAbstract:Friction between co-current downflow gas-solid suspension and the column wall was investigated. A new model to predict pressure drops due to friction between the gassolid suspension in the fully developed section and the downer wall was developed. The results show that the friction between the gas-solids suspension and the downer wall causes a significant deviation of the apparent solids concentrations from the actual ones, especially for those operating conditions with higher superficial gas velocities and solids circulation rates. When the superficial gas velocity is greater than 8 m/s, the actual solids concentrations in the fully developed region of the downer can be up to 2~3 times of the apparent values. After the frictional pressure drop is considered, the predicted actual solids concentrations by the proposed model agree well with the experimental values.
Chengxiu Wang - One of the best experts on this subject based on the ideXlab platform.
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3D CPFD Simulation of Circulating Fluidized Bed Downer and Riser: Comparisons of Flow Structure and Solids Back-Mixing Behavior
Processes, 2020Co-Authors: Yancong Liu, Xiaogang Shi, Chengxiu Wang, Gao Jinsen, Lan XingyingAbstract:The difference of gas-solids flow between a circulating fluidized bed (CFB) downer and riser was compared by computational particle fluid dynamics (CPFD) approach. The comparison was conducted under the same operating conditions. Simulation results demonstrated that the downer showed much more uniform solids holdup and solids velocity distribution compared with the riser. The radial non-uniformity index of the solids holdup in the riser was over 10 times than that in the downer. In addition, small clusters tended to be present in the whole downer, large clusters tended to be present near the wall in riser. It was found that the different cluster behavior is important in determining the different flow behaviors of solids in the downer and riser. While the particle residence time increased evenly along the downward direction in the downer, particles with both shorter and longer residence time were predicted in the whole riser. The nearly vertical cumulative residence time distribution (RTD) curve in the downer further demonstrated that the solids back-mixing in the downer is limited while that in the riser is severe. Solids turbulence in the downer was much weaker compared with the riser, while the large clusters formation near the wall in the riser would hinder solids transportation ability.
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3D CPFD simulation of gas-solids flow in the high-density downer with FCC particles
Powder Technology, 2020Co-Authors: Xiaogang Shi, Yancong Liu, Chengxiu Wang, Jinsen Gao, Xingying LanAbstract:Abstract The gas-solids flow in high-density Downers with FCC particles was simulated by Computational Particle Fluid Dynamics (CPFD) approach. A drag model was proposed based on the equivalent diameter of cluster with the analysis of force balance on particle cluster. A global sensitivity analysis method was used to analyze the influencing factors of cluster size. The drag model was incorporated into the three-dimensional (3D) CPFD approach to simulate the gas-solids flow in two different Downers operating at various conditions. 3D CPFD simulation provided detailed trajectories and distributions of solids. Good agreements of the predicted solids holdup and velocity with experimental data were achieved at both low and high-density conditions, which verified the rationality and applicability of the present drag model considering the characteristics of particle cluster in the downer. Finally, the flow and clustering behavior in the high-density downer were adequately analyzed and compared with that in low-density downer.
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radial solids flow structure in high flux gas solids circulating fluidized bed Downers
Powder Technology, 2016Co-Authors: Jesse Zhu, Chengxiu Wang, Jinsen Gao, Xingying Lan, Shahzad BarghiAbstract:Abstract High flux gas-solids circulating fluidized bed downer reactors have unique characteristics especially suitable for very fast reactions. Detailed hydrodynamics of such reactors are experimentally studied using FCC particles at various superficial gas velocities (1–7 m/s) under high flux conditions up to 700 kg/m 2 s for the first time. Results show that although the radial distribution of solids holdup is somewhat less uniform under very high flux conditions in the Downers, it is still much more uniform compared to riser reactors. Radial profiles of solids holdup, particle velocity and solids flux are significantly affected by the operating conditions. Particle velocity distribution is characterized by a relatively flat core and an annulus, where the particle velocity slightly increases towards the wall under low flux. In very high flux downer reactors, the shape of the local particle velocity becomes parabolic. It is also found that relationships between local solids holdup and particle velocity are different in the Downers compared to the risers due to their different flow characteristics. Compared to the riser reactor, downer has a self-adjusting mechanism to perform with the nearly “ideal” plug flow nature.
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Performance evaluation of high density riser and downer: Experimental study using ozone decomposition
Chemical Engineering Journal, 2015Co-Authors: Chengxiu Wang, Jesse Zhu, Shahzad BarghiAbstract:Abstract Reactor performance of high density circulating fluidized bed (CFB) riser and downer is studied with superficial gas velocities of 3–9 m/s and solids circulation rates of 100–700 kg/m2 s using ozone decomposition reaction. Results show that the reactant conversion in the riser and downer is closely related to the hydrodynamics, with solids holdup being the most influential parameter on ozone decomposition. Both axial and radial distribution profiles of the ozone concentration and the solids holdup in the downer are more uniform than that in the riser. Overall ozone conversion increases with the increase of solids circulation rate and/or the decrease of superficial gas velocity in both reactors. Compared to the riser reactor where the overall conversion is even less than the reactant conversion in stirred-tank reactor, overall conversion in the downer is less than but very close to that in ideal plug flow reactor. Contact efficiency is higher in the downer than in the riser indicating that gas–solids contacting in the downer is better due to the uniform solids flow structure and the reactant concentration.
Hui Zhao - One of the best experts on this subject based on the ideXlab platform.
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modeling the axial hydrodynamics of gas solid counter current Downers
Particuology, 2020Co-Authors: Juanbo Liu, Xinhua Liu, Zhixin Zhang, Hui ZhaoAbstract:Abstract Gas–solid counter-current downer reactors, in which particles move downward in an upward gas flow, can achieve high solid concentration for high heat and/or mass transfer rates. However, the particles may reverse their direction or even be carried out of the reactor as the gas flow rate increases. This is closely related to “flooding” in counter-current flows. The energy minimization multiscale (EMMS) model well describes multiscale heterogeneity in gas–solid cocurrent upward flows. It is further developed to simulate gas–solid counter-current downward flows because similar heterogeneity can also be found in Downers. The model characterizes well the axial hydrodynamics and predicts an inflexional voidage variation with superficial gas velocity in the fully developed region. This is supported by a simulation based on computational fluid dynamics and the discrete element method. The flooding predicted by the model agrees better with experiment than previous models.
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Modeling the hydrodynamics of cocurrent gas-solid Downers according to energy-minimization multi-scale theory
Particuology, 2016Co-Authors: Zhixin Zhang, Xinhua Liu, Hui ZhaoAbstract:Abstract Cocurrent gas–solid downer reactors have many applications in industry because they possess the technological advantages of a lower pressure drop, shorter residence time, and less solid backmixing when compared with traditional circulating fluidized bed risers. By introducing the concept of particle clusters explicitly, a one-dimensional model with consideration of the interphase interactions between the fluid and particles at both microscale and mesoscale is formulated for concurrent downward gas–solid flow according to energy-minimization multi-scale (EMMS) theory. A unified stability condition is proposed for the differently developed sections of gas–solid flow according to the principle of the compromise in competition between dominant mechanisms. By optimizing the number density of particle clusters with respect to the stability condition, the formulated model can be numerically solved without introducing cluster-specific empirical correlations. The EMMS-based model predicts well the axial hydrodynamics of cocurrent gas–solid Downers and is expected to have a wider range of applications than the existing cluster-based models.
Hui Zhang - One of the best experts on this subject based on the ideXlab platform.
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Mechanism analysis of the solids holdup variations in downer reactors based on volumetric flux
Chemical Engineering Science, 2019Co-Authors: Wenhao Lian, Hui Zhang, Chihiro Fushimi, Atsushi Tsutsumi, Xueer Pan, Shuang Zheng, Wei Zhang, Xiaogang Hao, Wei Huang, Guoqing GuanAbstract:Abstract Solids volumetric flux (Vs), which is expected to replace the solids mass flux (Gs) as the key factor influencing solids holdup in circulating fluidized beds, was proposed to investigate the solids holdup variation and predict the extreme operation conditions in the Downers. In this work, by using more than 400 sets of experimental data from different literature in Response Surface Methodology (RSM) analysis, it is found that the overestimation of particle density effect on the solids holdup can be avoided by converting the Gs into Vs. Further studies indicated that the difference of solids holdup (Δes) resulted from the different types of particles increased with the increase in Vs as well as the decrease in superficial gas velocity (Ug). Based on the normalized fitting results, the effects of different factors on the solids holdup were characterized. It is concluded that the effect extent was in the order of Vs > Ug >> particle size (dp) > particle density (ρp), which provided a guidance to control the solids holdup to satisfy the requirements in the downer by adjustment of various parameters, that is, Vs and Ug can adjust solids holdup roughly whereas dp and ρp can adjust solids holdup finely. Meanwhile, it is found that the upper limit of solids volumetric flux (Vs,max) can be increased by the increase of Ug, dp and/or ρp. In addition, it is considered that using Vs to replace Gs to define high-density operation in the downer could be more suitable in this study.
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Solids concentration in the fully developed region of circulating fluidized bed Downers
Powder Technology, 2008Co-Authors: Hui Zhang, Jesse ZhuAbstract:Abstract To investigate solids concentration in the fully developed region of co-current downward gas–solid flow, actual solids concentrations were measured in a circulating fluidized bed (CFB) downer with 9.3 m in height and 0.1 m in diameter using a fiber optical probe. The results obtained from this work and in the literature show that the average solids concentration in the fully developed region of the CFB Downers is not only a function of the corresponding terminal solids concentration, but the operating conditions and particle properties also have influences on the average solids concentration in the fully developed region of the CFB Downers. Particle diameter and density affect the solids concentrations differently under different operating conditions. Downer diameters almost have no influence on the solids concentrations. By taking into account the effects of operating conditions, particle properties and downer diameters, an empirical correlation to predict the solids concentrations in the fully developed region of CFB Downers is proposed. The predictions of the correlation are in good agreement with the experimental data of this work and in the literature.
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friction between gas solid suspension and circulating fluidized bed Downers
2007Co-Authors: Hui Zhang, Jesse ZhuAbstract:Friction between co-current downflow gas-solid suspension and the column wall was investigated. A new model to predict pressure drops due to friction between the gassolid suspension in the fully developed section and the downer wall was developed. The results show that the friction between the gas-solids suspension and the downer wall causes a significant deviation of the apparent solids concentrations from the actual ones, especially for those operating conditions with higher superficial gas velocities and solids circulation rates. When the superficial gas velocity is greater than 8 m/s, the actual solids concentrations in the fully developed region of the downer can be up to 2~3 times of the apparent values. After the frictional pressure drop is considered, the predicted actual solids concentrations by the proposed model agree well with the experimental values.
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Gas‐solids flow behavior: CFB riser vs. downer
AIChE Journal, 2001Co-Authors: Hui Zhang, Weixing Huang, Jesse ZhuAbstract:Comparisons are made in a circulating fluidized-bed riser/downer system between a 15.1 m high, 0.10 m ID riser and a 9.3 m high, 0.10 m ID downer, based on the measurements of the radial distributions of the local solids holdups and local particle velocities along the two columns. Although the core-annulus flow structures exist in both the riser and downer, the radial flow structure in the downer differs largely from that in the riser. The radial distributions of solids holdup and particle velocity in the downer are much more uniform than those in the riser, thus ensuring the low back mixing and the narrow particle residence time distribution in the downer. The axial flow structure in the downer is also more uniform than that in the riser. Due to the high particle acceleration and the high particle velocity in the downer, the overall solids holdup is significantly lower than that in the riser. The microflow structure in the downer, characterized by the low intermittency indices, is also more uniform than that in the riser. These key properties of the downer make it a very promising candidate for industrial applications where short reaction times and high product selectivity are required.
Xingying Lan - One of the best experts on this subject based on the ideXlab platform.
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3D CPFD simulation of gas-solids flow in the high-density downer with FCC particles
Powder Technology, 2020Co-Authors: Xiaogang Shi, Yancong Liu, Chengxiu Wang, Jinsen Gao, Xingying LanAbstract:Abstract The gas-solids flow in high-density Downers with FCC particles was simulated by Computational Particle Fluid Dynamics (CPFD) approach. A drag model was proposed based on the equivalent diameter of cluster with the analysis of force balance on particle cluster. A global sensitivity analysis method was used to analyze the influencing factors of cluster size. The drag model was incorporated into the three-dimensional (3D) CPFD approach to simulate the gas-solids flow in two different Downers operating at various conditions. 3D CPFD simulation provided detailed trajectories and distributions of solids. Good agreements of the predicted solids holdup and velocity with experimental data were achieved at both low and high-density conditions, which verified the rationality and applicability of the present drag model considering the characteristics of particle cluster in the downer. Finally, the flow and clustering behavior in the high-density downer were adequately analyzed and compared with that in low-density downer.
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radial solids flow structure in high flux gas solids circulating fluidized bed Downers
Powder Technology, 2016Co-Authors: Jesse Zhu, Chengxiu Wang, Jinsen Gao, Xingying Lan, Shahzad BarghiAbstract:Abstract High flux gas-solids circulating fluidized bed downer reactors have unique characteristics especially suitable for very fast reactions. Detailed hydrodynamics of such reactors are experimentally studied using FCC particles at various superficial gas velocities (1–7 m/s) under high flux conditions up to 700 kg/m 2 s for the first time. Results show that although the radial distribution of solids holdup is somewhat less uniform under very high flux conditions in the Downers, it is still much more uniform compared to riser reactors. Radial profiles of solids holdup, particle velocity and solids flux are significantly affected by the operating conditions. Particle velocity distribution is characterized by a relatively flat core and an annulus, where the particle velocity slightly increases towards the wall under low flux. In very high flux downer reactors, the shape of the local particle velocity becomes parabolic. It is also found that relationships between local solids holdup and particle velocity are different in the Downers compared to the risers due to their different flow characteristics. Compared to the riser reactor, downer has a self-adjusting mechanism to perform with the nearly “ideal” plug flow nature.
