Until now, the onset velocity of circulating fluidization in liquid-solid fluidized beds has been defined by the turning point of the time required to empty a bed of particles as a function of the superfcial liquid ve...Until now, the onset velocity of circulating fluidization in liquid-solid fluidized beds has been defined by the turning point of the time required to empty a bed of particles as a function of the superfcial liquid velocity, and is reported to be only dependent on the liquid and particle properties. This study presents a new approach to calculate the onset velocity using CFD-DEM simulation of the particle residence time distribution （RTD）. The onset velocity is identified from the intersection of the fitted lines of the particle mean residence time as a function of superficial liquid velocity. Our results are in reasonable agreement with experimental data. The simulation indicates that the onset velocity is infuenced by the density and size of particles and weakly affected by riser height and diameter, A power-law function is proposed to correlate the mean particle residence time with the superficial liquid velocity. The collisional parameters have a minor effect on the mean residence time of particles and the onset velocity, but influence the particle RTD, showing some humps and trailing. The particle RTD is found to be related to the particle trajectories, which may indicate the complex flow structure and underlying mechanisms of the particle RTD.展开更多
Solid descending and residence properties in COREX shaft furnace with a center gas supply device (CGSD) were studied using the discrete element method. Solid flow pattern, residence time distribution, and local solid ...Solid descending and residence properties in COREX shaft furnace with a center gas supply device (CGSD) were studied using the discrete element method. Solid flow pattern, residence time distribution, and local solid residence time (SRT) were analyzed. Moreover, the effects of the rotation speed of screws, the radius, and height of CGSD were investigated. The results show that the solid flow is dominantly a plug flow in the shaft furnace with the CGSD. In the upper region of the shaft furnace for gas-solid reduction, uniform distribution of local SRT is observed, which is preferable to match the even gas distribution expected due to the CGSD and slots. For different rotation speeds of the screws, the distribution of solid flow patterns is similar. The averaged residence time, dispersed plug volume fraction, and dead volume fraction decrease with increasing the rotation speed of the screws. The radius of the CGSD affects the solid residence in the shaft furnace. Local SRT above the CGSD increases with the increase in the radius of CGSD, possibly leading to the formation of more agglomerates. Hence, the radius of CGSD should be chosen carefully. The height of CGSD has a minor effect on solid residence in the shaft furnace.展开更多
Based on the axial bed depth model of straight rotary kiln proposed by prior researchers,the equation of mean residence time of solids in carbon rotary kiln with variable diameter and turn-down rig was developed.Cold ...Based on the axial bed depth model of straight rotary kiln proposed by prior researchers,the equation of mean residence time of solids in carbon rotary kiln with variable diameter and turn-down rig was developed.Cold experiments were carried out on a laboratory rotary kiln under ambient conditions.The comparison between calculated results and experiment data showed that calculated results of the residence time of solids agreed well with measurements and the prediction error varied in a small range of-15%—+12%.No fitting parameter was necessary.The mass flow rate of solids increased and the mean residence time decreased with increasing kiln rotation speed and kiln slope.The mass flow rate of solids increased and the mean residence time might possibly decrease or increase with increasing dam height at the feed end.展开更多
A gas-solids fast separator was studied for its potential application in the process of coal pyrolysis to acetylene in plasma.The CFD method was employed to simulate the flow behavior in the gas-solids fast separator ...A gas-solids fast separator was studied for its potential application in the process of coal pyrolysis to acetylene in plasma.The CFD method was employed to simulate the flow behavior in the gas-solids fast separator based on the realizable k-ε turbulence model and the discrete particle model(DPM).The separation efficiency and residence time of gas phase in the gas-solids fast separator could be calculated.The numerical simulations were validated by the experimental results at a low velocity of the inlet gas(e.g.,4 m·s-1).With the increase of gas velocity at the inlet,the separation efficiency was increased,and the residence time of gas phase was reduced accordingly.The separation efficiency approached 100% when particle diameter was larger than 20 μm.When inlet velocity was 100 m·s-1,the mean residence time of gas phase was about 35 ms.To be noted,the performance of the gas-solids fast separator could be improved,for example by shortening the length of the separator,with a reduced residence time of gas phase at ～20 ms.It is expected that the gas-solids fast separator can meet the stringent demand of the coal pyrolysis to acetylene process for the milliseconds reaction,quench and separation.展开更多
基金long term support from the National Natural Science Foundation of China(Grant Nos.21222603 and 91434121)the Ministry of Science and Technology of China(Grant No.2013BAC12B01)the Strategic Priority Research Program of the Chinese Academy of Sciences(Grant No.XDA07080301)
文摘Until now, the onset velocity of circulating fluidization in liquid-solid fluidized beds has been defined by the turning point of the time required to empty a bed of particles as a function of the superfcial liquid velocity, and is reported to be only dependent on the liquid and particle properties. This study presents a new approach to calculate the onset velocity using CFD-DEM simulation of the particle residence time distribution （RTD）. The onset velocity is identified from the intersection of the fitted lines of the particle mean residence time as a function of superficial liquid velocity. Our results are in reasonable agreement with experimental data. The simulation indicates that the onset velocity is infuenced by the density and size of particles and weakly affected by riser height and diameter, A power-law function is proposed to correlate the mean particle residence time with the superficial liquid velocity. The collisional parameters have a minor effect on the mean residence time of particles and the onset velocity, but influence the particle RTD, showing some humps and trailing. The particle RTD is found to be related to the particle trajectories, which may indicate the complex flow structure and underlying mechanisms of the particle RTD.
基金The authors would like to thank the National Natural Science Foundation of China (Grant No. 51804027)China Postdoctoral Science Foundation (Grant No. 2017M610769)Fundamental Research Funds for the Central Universities (Grant No. FRF-TP-17-036A1) for their financial supports.
文摘Solid descending and residence properties in COREX shaft furnace with a center gas supply device (CGSD) were studied using the discrete element method. Solid flow pattern, residence time distribution, and local solid residence time (SRT) were analyzed. Moreover, the effects of the rotation speed of screws, the radius, and height of CGSD were investigated. The results show that the solid flow is dominantly a plug flow in the shaft furnace with the CGSD. In the upper region of the shaft furnace for gas-solid reduction, uniform distribution of local SRT is observed, which is preferable to match the even gas distribution expected due to the CGSD and slots. For different rotation speeds of the screws, the distribution of solid flow patterns is similar. The averaged residence time, dispersed plug volume fraction, and dead volume fraction decrease with increasing the rotation speed of the screws. The radius of the CGSD affects the solid residence in the shaft furnace. Local SRT above the CGSD increases with the increase in the radius of CGSD, possibly leading to the formation of more agglomerates. Hence, the radius of CGSD should be chosen carefully. The height of CGSD has a minor effect on solid residence in the shaft furnace.
文摘Based on the axial bed depth model of straight rotary kiln proposed by prior researchers,the equation of mean residence time of solids in carbon rotary kiln with variable diameter and turn-down rig was developed.Cold experiments were carried out on a laboratory rotary kiln under ambient conditions.The comparison between calculated results and experiment data showed that calculated results of the residence time of solids agreed well with measurements and the prediction error varied in a small range of-15%—+12%.No fitting parameter was necessary.The mass flow rate of solids increased and the mean residence time decreased with increasing kiln rotation speed and kiln slope.The mass flow rate of solids increased and the mean residence time might possibly decrease or increase with increasing dam height at the feed end.
文摘A gas-solids fast separator was studied for its potential application in the process of coal pyrolysis to acetylene in plasma.The CFD method was employed to simulate the flow behavior in the gas-solids fast separator based on the realizable k-ε turbulence model and the discrete particle model(DPM).The separation efficiency and residence time of gas phase in the gas-solids fast separator could be calculated.The numerical simulations were validated by the experimental results at a low velocity of the inlet gas(e.g.,4 m·s-1).With the increase of gas velocity at the inlet,the separation efficiency was increased,and the residence time of gas phase was reduced accordingly.The separation efficiency approached 100% when particle diameter was larger than 20 μm.When inlet velocity was 100 m·s-1,the mean residence time of gas phase was about 35 ms.To be noted,the performance of the gas-solids fast separator could be improved,for example by shortening the length of the separator,with a reduced residence time of gas phase at ～20 ms.It is expected that the gas-solids fast separator can meet the stringent demand of the coal pyrolysis to acetylene process for the milliseconds reaction,quench and separation.
