The performance of binary particles mixing and gas-solids contacting,which is considered qualitatively to have a significant influence on the heat transfer in internal heated circulating fluidized beds,is carefully in...The performance of binary particles mixing and gas-solids contacting,which is considered qualitatively to have a significant influence on the heat transfer in internal heated circulating fluidized beds,is carefully investigated by means of a numerical approach in the newly developed high solids-flux downer lignite pyrolyzer(φ0.1 m×6.5 m).Since binary particles are used in this system,a reasonably validated 3 D,transient,multi-fluid model,in which three heat transfer modes relating to the convection,conduction and radiation are considered,is adopted to simulate the flow behavior,temperature profiles as well as volatile contents.The simulation results showed that the solids stream impinges the left wall surface initially and turns towards the right wall in the further downward direction and then shrinks during this process resulting in that the solids concentrate a little more at the central region.In the further downward section of the downer,the particle flow disperses near the right wall and develops uniformly.Meanwhile,the coal phase is slowly heated in the downer and it is found that most of the heat absorbed by the coal is from the convection heat transfer mode.To explore the heat transfer mechanism more quantitatively,two indexes(mixing index and contacting index)are proposed,and it is found that the mixing index initially increased fast and later remained at a relatively flat state.For the contact index,it shows a trend with a first rising and then falling,finally rising continuously.Also,it is found that the convection heat transfer is closely correlated to the contacting status of gas-coal which indicates that the improving of the gas-coal contacting efficiency should be an effective way to strengthen the coal particle heating process.展开更多
This paper presents an experimental study on the flow patterns of FCC particles in a 140 mm ID Circulating Fluidized Bed with concurrent upflow and downflow gas-solid suspension. Based on the distribution of local par...This paper presents an experimental study on the flow patterns of FCC particles in a 140 mm ID Circulating Fluidized Bed with concurrent upflow and downflow gas-solid suspension. Based on the distribution of local particle velocity and particle concentration measured by a Fiber-Optical Probe Laser Doppler Velocimeter and a Fiber Optical Probe System respectively, the different flow patterns of local particls concentration, local particle velocity, local particle fluctuating velocity and sectionally average particle velocity in concurrent upflow and downflow gas-solid system have been investigated. It is found that the particle flow in the concurrent downflow is much more uniform radially than that in the concurrent upflow riser. The investigation of flow patterns in different flow systems is of significance to the development of a new gas-solid reactor.展开更多
The coking observation and particle flow behaviour in both thermal plasma and cold plexiglas downers were investigated in a binary particle system formed by injecting coarse inert particles (carrying coke away and sco...The coking observation and particle flow behaviour in both thermal plasma and cold plexiglas downers were investigated in a binary particle system formed by injecting coarse inert particles (carrying coke away and scouring wall) and fine coal powders into the downer reactor. The results demonstrate that this scheme is a rational selection to prevent coking on downer walls and improve particle velocity distribution along the radial direction. When injected coarse particles mixed with fine powders in downers, the fluctuation of local particle velocity in the radial direction becomes smaller and two peaks in the radial distribution of local particle velocity occur due to the improved dispersing character and flow structure, which are beneficial to the thermo-plasma coal cracking reaction and coking prevention.展开更多
Fluid catalytic cracking(FCC)technologies of downer reactors,which have reached the demonstration or commercial scale,are systematically discussed,i.e.,millisecond catalytic cracking,fluidization lab of Tsinghua Unive...Fluid catalytic cracking(FCC)technologies of downer reactors,which have reached the demonstration or commercial scale,are systematically discussed,i.e.,millisecond catalytic cracking,fluidization lab of Tsinghua University,and high-severity FCC.Moreover,aiming to promote industrial application,the fundamental studies are comprehensively described,particularly focusing on high-density downer reactors,clusters,and up-scaling.Furthermore,from the perspective of industrial application,some research directions toward further developments are suggested.展开更多
The hydrodynamic behavior in a high-density downer reactor was studied. A two-fluid model based on the kinetic theory of granular flow with a k-ε turbulent model was developed to simulate the flow behavior in the sys...The hydrodynamic behavior in a high-density downer reactor was studied. A two-fluid model based on the kinetic theory of granular flow with a k-ε turbulent model was developed to simulate the flow behavior in the system. This simulation achieved an averaged solid fraction in the bed as high as 18% in this operating regime. The flow development in high-density downer consists of 3 regions, which are first acceleration, second acceleration, and fully developed regions. In the fully developed region, the lateral distribution of the solid volume fraction is low and almost uniform in the center region with a high density peak near the wall region. Gas and solid velocities gradually increase toward the wall and form a peak near the wall region. In addition, the solid volume fraction, gas and solid velocities increase with solid circulation rate.展开更多
The performance of a novel conical-type downer pyrolyzer is carefully evaluated via numerical simulation.The study explicitly accounts for mass transfer effects by using a multi-scale mass transfer model.To achieve si...The performance of a novel conical-type downer pyrolyzer is carefully evaluated via numerical simulation.The study explicitly accounts for mass transfer effects by using a multi-scale mass transfer model.To achieve simultaneous high precision and computational efficiency,an enhanced strategy for calculating the multi-scale mass transfer coefficient in heterogeneous phase reaction systems is proposed by treating mass transfer and reaction as independent processes.This strategy is coupled with a discrete distributed activation energy model formulated in the Arrhenius framework.A comprehensive analysis is performed to investigate the axial distributions of key parameters,including the average concentration of solid reactants(X_(s)),the volatile concentration on particle surfaces(X_(sf)),and the volatile concentration in the bulk gas phase(X_(f))under varying pyrolysis temperatures,carrier gas velocities,and solid mass fluxes.The findings reveal that Xs and Xf exhibit intuitive,monotonic trends,while Xsf demonstrates a more complex behavior,increasing due to ongoing reactions yet decreasing with mass transfer proceeding.The simulation results verify the advantages of the conical-type downer pyrolyzer,which can achieve significantly higher volatile concentrations than conventional designs.展开更多
For the high-temperature and short-contact time gas-solid reaction process,riser and downer are considered appropriate reactors.To realize an intensive and complete mixing of reactants with catalysts,the feed raw is a...For the high-temperature and short-contact time gas-solid reaction process,riser and downer are considered appropriate reactors.To realize an intensive and complete mixing of reactants with catalysts,the feed raw is always introduced in the form of high-speed jets.In this study,in order to investigate the mixing effects of different types of high-speed jets in riser and downer,traceable ozone is injected with the high-speed feed jets to react with catalyst particles.By detecting the decomposition of ozone,the gas-solid mixing and reaction in riser and downer under the influence of both co-current and counter-current injections are analyzed.The relative ozone concentration is used to reflect the location reaction extent and its radial nonuniformity index is proposed to compare the results in riser and downer.It is found that the jet influence zone in downer provides a relatively better environment for the mixing of feed jets with catalysts.In the riser,introduction of counter-current injections could improve the uniformity of gas-solid mixing in the initial contact region of feed with catalysts.展开更多
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 di...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.展开更多
Based on the EMMS model, the local slip velocity between gas and solid is systematically analyzed and a theoretical correlation of local slip velocity with local voidage for a downer is derived as follows:U_s(r)/U_t=D...Based on the EMMS model, the local slip velocity between gas and solid is systematically analyzed and a theoretical correlation of local slip velocity with local voidage for a downer is derived as follows:U_s(r)/U_t=D^(8/7)(1-ε_mt)^(-2/7)[(1-ε(r)/(ε(r))]^(8/7)]ε(r)(47/14)((ε(r)-ε_(mt))/ε(r)) Using this correlation, the local gas-solid slip velocity in a downer is calculated. The calculated results are well consistent with experimental data. In addition, the variation of the local slip velocity with its corresponding solid holdup is also dis-cussed.展开更多
In this paper, a model for fast coal pyrolysis in a cocurrent downer reactor is developed, in which both hydrodynamics and coal pyrolysis kinetics are simultaneously considered. The results of simulations based on thi...In this paper, a model for fast coal pyrolysis in a cocurrent downer reactor is developed, in which both hydrodynamics and coal pyrolysis kinetics are simultaneously considered. The results of simulations based on this model display reasonable agreement with experimental data obtained using Huolinhe coal as the feedstock, and this model is therefore suitable for predicting the fast pyrolysis of specific coal types. A series of simulations of fast coal pyrolysis in a cocurrent downer demonstrated that coal devolatilization is almost complete in the inlet region within a time span of 0.4 s, and that higher temperatures improve the pyrolysis efficiency. However, the yield of liquid products is decreased with increasing pyrolysis temperatures, especially above 670 ℃, because of additional cracking of the liquids.展开更多
Pyrolysis technology has received increasing attention in recent years due to its great potential in the field of lowrank coal clean and efficient conversion.Since pyrolysis reaction is very fast and prone to overreac...Pyrolysis technology has received increasing attention in recent years due to its great potential in the field of lowrank coal clean and efficient conversion.Since pyrolysis reaction is very fast and prone to overreaction,the downer-type reactor is considered as a pyrolyzer due to its unique plug flow reactor characteristics.However,the low solids holdup,which is not beneficial for the fast heat transfer,limits its industrial application.Thus,how to realize high-density operation is crucial to the successful application of the downer reactor.Herein,the definition and strategies of high-density operation in the downer were introduced at first.And then,considering the increasing influence of computational fluid dynamics(CFD)in the fluidization industry,the state-of-the-art progress in downer simulation was reviewed,in which the newly developed drag models for downers were carefully discussed and compared.Also,to help prediction of the pyrolysis behaviors,the widely used pyrolysis kinetic models were systematically summarized.Combined with the potential of the downer in the field of coal pyrolysis,the relevant research progress of hot-state simulation of the downer pyrolyzer were introduced and analyzed.Finally,the suggestions on how to carry out follow-up work were given.It is expected that this review could give a better understanding for designing and optimizing downer pyrolyzer.展开更多
A mathematical model has been developed for the simulation of gas-particle flow and fluid catalytic cracking in downer reactors. The model takes into account both cracking reaction and flow behavior through a four-lum...A mathematical model has been developed for the simulation of gas-particle flow and fluid catalytic cracking in downer reactors. The model takes into account both cracking reaction and flow behavior through a four-lump reaction kinetics coupled with two-phase turbulent flow. The prediction results show that the relatively large change of gas velocity affects directly the axial distribution of solids velocity and void fraction, which significantly interact with the chemical reaction. Furthermore, model simulations are carried out to determine the effects of such parameters on product yields, as bed diameter, reaction temperature and the ratio of catalyst to oil, which are helpful for optimizing the yields of desired products. The model equations are coded and solved on CFX4.4.展开更多
Numerical simulation of fully developed hydrodynamics of a riser and a downer was carried out using an Eulerian-Lagrangian model, where the particles are modeled by the discrete element method （DEM） and the gas by t...Numerical simulation of fully developed hydrodynamics of a riser and a downer was carried out using an Eulerian-Lagrangian model, where the particles are modeled by the discrete element method （DEM） and the gas by the Navier-Stokes equations. Periodic flow domain with two side walls was adopted to simulate the fully developed dynamics in a 2D channel of 10 cm in width. All the simulations were carried out under the same superficial gas velocity and solids holdup in the domain, starting with a homogenous state for both gas and solids, and followed by the evolution of the dynamics to the heterogeneous state with distinct clustering in the riser and the downer. In the riser, particle clusters move slowly, tending to suspend along the wall or to flow downwards, which causes wide residence time distribution of the particles. In the downer, clusters still exist, but they have faster velocities than the discrete particles. Loosely collected particles in the clusters move in the same direction as the bulk flow, resulting in plug flow in the downer. The residence time distribution （RTD） of solids was computed by tracking the displacements of all particles in the flow direction. The results show a rather wide RTD for the solids in the riser hut a sharp peak RTD in the downer, much in agreement with the experimental findings in the literature. The ensemble average of transient dynamics also shows reasonable profiles of solids volume fraction and solids velocity, and their dependence on particle density.展开更多
Cocurrent gas-solid downer reactors have many applications in industry because they possess the tech- nological advantages of a lower pressure drop, shorter residence time, and less solid backmixing when compared with...Cocurrent gas-solid downer reactors have many applications in industry because they possess the tech- nological 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.展开更多
Typically,heating or high-temperature treatment has been used to regenerate solid sorbent.In this study,the depressurized regeneration using a circulating fluidized bed downer was proposed and the significance of its ...Typically,heating or high-temperature treatment has been used to regenerate solid sorbent.In this study,the depressurized regeneration using a circulating fluidized bed downer was proposed and the significance of its operating parameters was identified.Two-dimensional computational fluid dynamics were employed to systematically investigate the effects of operating parameters on carbon dioxide depressurized regeneration with potassium carbonate solid sorbent particles.The simulated model was based on a laboratory scale circulating fluidized bed downer.The chemical equilibrium model for predicting the highest outlet carbon dioxide mass fraction was then used.A central composite design was employed to identify the main,quadratic,and interaction effects of operating parameters to the regeneration process.The operating parameters consisted of the outlet system pressure,inlet gas velocity,and inlet solid circulation rate,while the response variable was the released outlet carbon dioxide mass fraction.Among the multiple operating parameters,there were two main operating parameters and their combinations,namely the inlet gas velocity,outlet system pressure,square of inlet gas velocity,and interaction between inlet gas velocity and outlet system pressure,which had great impacts on the regeneration.All the main,quadratic,and interaction effects were explained.Then,the optimal operating conditions were obtained through the response surface method.展开更多
The structural optimization of baffle internals for fast pyrolysis of coal with particulate mixing and heat transfer in a downer reactor using the discrete element method(DEM)has been investigated in this research.The...The structural optimization of baffle internals for fast pyrolysis of coal with particulate mixing and heat transfer in a downer reactor using the discrete element method(DEM)has been investigated in this research.The pyrolysis terminal temperature at the exit of the downer reactor is not only decided by the volume-feeding-rate ratio of the coal to the sand,but also is affected by the inner structural design of the baffle internals in the downer reactor.As presented in the previous publication of the author,the inhibition from the baffle internals in a downer reactor can improve the particulate-mixing degree and heat carrier,and increase the mean residence time of the coal and heat-carrier particles in the downer reactor.The structure of the baffle internals in the downer reactor mentioned in this research can be optimized by the independently developed 3D soft-sphere model of the DEM programme of a 40-mm baffle length,a 30°baffle-slope angle and at least four baffles designed in the downer reactor,which is beneficial for the process design of coal pyrolysis with a heat carrier in the downer reactor.展开更多
基金supported by the National Natural Science Foundation of China(U1710101)。
文摘The performance of binary particles mixing and gas-solids contacting,which is considered qualitatively to have a significant influence on the heat transfer in internal heated circulating fluidized beds,is carefully investigated by means of a numerical approach in the newly developed high solids-flux downer lignite pyrolyzer(φ0.1 m×6.5 m).Since binary particles are used in this system,a reasonably validated 3 D,transient,multi-fluid model,in which three heat transfer modes relating to the convection,conduction and radiation are considered,is adopted to simulate the flow behavior,temperature profiles as well as volatile contents.The simulation results showed that the solids stream impinges the left wall surface initially and turns towards the right wall in the further downward direction and then shrinks during this process resulting in that the solids concentrate a little more at the central region.In the further downward section of the downer,the particle flow disperses near the right wall and develops uniformly.Meanwhile,the coal phase is slowly heated in the downer and it is found that most of the heat absorbed by the coal is from the convection heat transfer mode.To explore the heat transfer mechanism more quantitatively,two indexes(mixing index and contacting index)are proposed,and it is found that the mixing index initially increased fast and later remained at a relatively flat state.For the contact index,it shows a trend with a first rising and then falling,finally rising continuously.Also,it is found that the convection heat transfer is closely correlated to the contacting status of gas-coal which indicates that the improving of the gas-coal contacting efficiency should be an effective way to strengthen the coal particle heating process.
文摘This paper presents an experimental study on the flow patterns of FCC particles in a 140 mm ID Circulating Fluidized Bed with concurrent upflow and downflow gas-solid suspension. Based on the distribution of local particle velocity and particle concentration measured by a Fiber-Optical Probe Laser Doppler Velocimeter and a Fiber Optical Probe System respectively, the different flow patterns of local particls concentration, local particle velocity, local particle fluctuating velocity and sectionally average particle velocity in concurrent upflow and downflow gas-solid system have been investigated. It is found that the particle flow in the concurrent downflow is much more uniform radially than that in the concurrent upflow riser. The investigation of flow patterns in different flow systems is of significance to the development of a new gas-solid reactor.
文摘The coking observation and particle flow behaviour in both thermal plasma and cold plexiglas downers were investigated in a binary particle system formed by injecting coarse inert particles (carrying coke away and scouring wall) and fine coal powders into the downer reactor. The results demonstrate that this scheme is a rational selection to prevent coking on downer walls and improve particle velocity distribution along the radial direction. When injected coarse particles mixed with fine powders in downers, the fluctuation of local particle velocity in the radial direction becomes smaller and two peaks in the radial distribution of local particle velocity occur due to the improved dispersing character and flow structure, which are beneficial to the thermo-plasma coal cracking reaction and coking prevention.
基金the funding of the project by SINOPEC (No. 120009)
文摘Fluid catalytic cracking(FCC)technologies of downer reactors,which have reached the demonstration or commercial scale,are systematically discussed,i.e.,millisecond catalytic cracking,fluidization lab of Tsinghua University,and high-severity FCC.Moreover,aiming to promote industrial application,the fundamental studies are comprehensively described,particularly focusing on high-density downer reactors,clusters,and up-scaling.Furthermore,from the perspective of industrial application,some research directions toward further developments are suggested.
文摘The hydrodynamic behavior in a high-density downer reactor was studied. A two-fluid model based on the kinetic theory of granular flow with a k-ε turbulent model was developed to simulate the flow behavior in the system. This simulation achieved an averaged solid fraction in the bed as high as 18% in this operating regime. The flow development in high-density downer consists of 3 regions, which are first acceleration, second acceleration, and fully developed regions. In the fully developed region, the lateral distribution of the solid volume fraction is low and almost uniform in the center region with a high density peak near the wall region. Gas and solid velocities gradually increase toward the wall and form a peak near the wall region. In addition, the solid volume fraction, gas and solid velocities increase with solid circulation rate.
基金supported by the National Natural Science Foundation of China(grant Nos.22108262,22378285)China Scholarship Council(grant Nos.202308140163,202306930019),Fundamental Research Program of Shanxi Province(grant No.20210302124600)+1 种基金Research Project Supported by Shanxi Scholarship Council of China(grant No.2022-138)Fund Program for the Scientific Activities of Selected Returned Overseas Professionals in Shanxi Province(grant No.20220014).
文摘The performance of a novel conical-type downer pyrolyzer is carefully evaluated via numerical simulation.The study explicitly accounts for mass transfer effects by using a multi-scale mass transfer model.To achieve simultaneous high precision and computational efficiency,an enhanced strategy for calculating the multi-scale mass transfer coefficient in heterogeneous phase reaction systems is proposed by treating mass transfer and reaction as independent processes.This strategy is coupled with a discrete distributed activation energy model formulated in the Arrhenius framework.A comprehensive analysis is performed to investigate the axial distributions of key parameters,including the average concentration of solid reactants(X_(s)),the volatile concentration on particle surfaces(X_(sf)),and the volatile concentration in the bulk gas phase(X_(f))under varying pyrolysis temperatures,carrier gas velocities,and solid mass fluxes.The findings reveal that Xs and Xf exhibit intuitive,monotonic trends,while Xsf demonstrates a more complex behavior,increasing due to ongoing reactions yet decreasing with mass transfer proceeding.The simulation results verify the advantages of the conical-type downer pyrolyzer,which can achieve significantly higher volatile concentrations than conventional designs.
基金the National Key Research and Development Program of China(grant No.2022YFA1506200)the open foundation of State Key Laboratory of Chemical Engineering(grant No.SKL-ChE-23B02).
文摘For the high-temperature and short-contact time gas-solid reaction process,riser and downer are considered appropriate reactors.To realize an intensive and complete mixing of reactants with catalysts,the feed raw is always introduced in the form of high-speed jets.In this study,in order to investigate the mixing effects of different types of high-speed jets in riser and downer,traceable ozone is injected with the high-speed feed jets to react with catalyst particles.By detecting the decomposition of ozone,the gas-solid mixing and reaction in riser and downer under the influence of both co-current and counter-current injections are analyzed.The relative ozone concentration is used to reflect the location reaction extent and its radial nonuniformity index is proposed to compare the results in riser and downer.It is found that the jet influence zone in downer provides a relatively better environment for the mixing of feed jets with catalysts.In the riser,introduction of counter-current injections could improve the uniformity of gas-solid mixing in the initial contact region of feed with catalysts.
基金the Strategic Priority Research Pro-gram of the Chinese Academy of Sciences(No.XDA07080400)the Informationization Program of the Chinese Academy ofSciences(No.XXH13506-301)。
文摘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.
基金supports from the National Program of Basic Research(No.G1999022103)Key Project of the National Natural Science Foundation of China(No.29936090)are gratefully acknowledged.
文摘Based on the EMMS model, the local slip velocity between gas and solid is systematically analyzed and a theoretical correlation of local slip velocity with local voidage for a downer is derived as follows:U_s(r)/U_t=D^(8/7)(1-ε_mt)^(-2/7)[(1-ε(r)/(ε(r))]^(8/7)]ε(r)(47/14)((ε(r)-ε_(mt))/ε(r)) Using this correlation, the local gas-solid slip velocity in a downer is calculated. The calculated results are well consistent with experimental data. In addition, the variation of the local slip velocity with its corresponding solid holdup is also dis-cussed.
基金financially supported by the International Science & Technology Cooperation Program of MOST(No.2011DFA61360)the Strategic Priority Research Program of the Chinese Academy of Sciences(No.XDA07080400)the National Nature Science Foundation of China(No.20703047)
文摘In this paper, a model for fast coal pyrolysis in a cocurrent downer reactor is developed, in which both hydrodynamics and coal pyrolysis kinetics are simultaneously considered. The results of simulations based on this model display reasonable agreement with experimental data obtained using Huolinhe coal as the feedstock, and this model is therefore suitable for predicting the fast pyrolysis of specific coal types. A series of simulations of fast coal pyrolysis in a cocurrent downer demonstrated that coal devolatilization is almost complete in the inlet region within a time span of 0.4 s, and that higher temperatures improve the pyrolysis efficiency. However, the yield of liquid products is decreased with increasing pyrolysis temperatures, especially above 670 ℃, because of additional cracking of the liquids.
基金the National Natural Science Foundation of China(No.U1710101,22108262,21908135)Shanxi Province Science Foundation for Youths(20210302124600,201901D211435)Shanxi Province Foundation for Returness(2019-20),China.
文摘Pyrolysis technology has received increasing attention in recent years due to its great potential in the field of lowrank coal clean and efficient conversion.Since pyrolysis reaction is very fast and prone to overreaction,the downer-type reactor is considered as a pyrolyzer due to its unique plug flow reactor characteristics.However,the low solids holdup,which is not beneficial for the fast heat transfer,limits its industrial application.Thus,how to realize high-density operation is crucial to the successful application of the downer reactor.Herein,the definition and strategies of high-density operation in the downer were introduced at first.And then,considering the increasing influence of computational fluid dynamics(CFD)in the fluidization industry,the state-of-the-art progress in downer simulation was reviewed,in which the newly developed drag models for downers were carefully discussed and compared.Also,to help prediction of the pyrolysis behaviors,the widely used pyrolysis kinetic models were systematically summarized.Combined with the potential of the downer in the field of coal pyrolysis,the relevant research progress of hot-state simulation of the downer pyrolyzer were introduced and analyzed.Finally,the suggestions on how to carry out follow-up work were given.It is expected that this review could give a better understanding for designing and optimizing downer pyrolyzer.
基金the Natura Science Foundation of China under contract number:20176024
文摘A mathematical model has been developed for the simulation of gas-particle flow and fluid catalytic cracking in downer reactors. The model takes into account both cracking reaction and flow behavior through a four-lump reaction kinetics coupled with two-phase turbulent flow. The prediction results show that the relatively large change of gas velocity affects directly the axial distribution of solids velocity and void fraction, which significantly interact with the chemical reaction. Furthermore, model simulations are carried out to determine the effects of such parameters on product yields, as bed diameter, reaction temperature and the ratio of catalyst to oil, which are helpful for optimizing the yields of desired products. The model equations are coded and solved on CFX4.4.
基金supported by the National Natural Science Foundation of China (NSFC) under Grant Nos. 20306012 and 20806045the National Foundation of PR China for Authors of Excellent Doctoral Dissertations (No. 200245)Specialized Research Fund for Doctoral Program of Higher Education (No.20050003028)
文摘Numerical simulation of fully developed hydrodynamics of a riser and a downer was carried out using an Eulerian-Lagrangian model, where the particles are modeled by the discrete element method （DEM） and the gas by the Navier-Stokes equations. Periodic flow domain with two side walls was adopted to simulate the fully developed dynamics in a 2D channel of 10 cm in width. All the simulations were carried out under the same superficial gas velocity and solids holdup in the domain, starting with a homogenous state for both gas and solids, and followed by the evolution of the dynamics to the heterogeneous state with distinct clustering in the riser and the downer. In the riser, particle clusters move slowly, tending to suspend along the wall or to flow downwards, which causes wide residence time distribution of the particles. In the downer, clusters still exist, but they have faster velocities than the discrete particles. Loosely collected particles in the clusters move in the same direction as the bulk flow, resulting in plug flow in the downer. The residence time distribution （RTD） of solids was computed by tracking the displacements of all particles in the flow direction. The results show a rather wide RTD for the solids in the riser hut a sharp peak RTD in the downer, much in agreement with the experimental findings in the literature. The ensemble average of transient dynamics also shows reasonable profiles of solids volume fraction and solids velocity, and their dependence on particle density.
基金We appreciate financial support from the Strategic Prior- ity Research Program of the Chinese Academy of Sciences （No. XDA07080400） and the Natural Science Foundation of China （Nos. 21376244 and 91334107）.
文摘Cocurrent gas-solid downer reactors have many applications in industry because they possess the tech- nological 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.
基金financially supported by the Scholarship from the Graduate School,Chulalongkorn University to commemorate the 72nd anniversary of his Majesty King Bhumibol Aduladejthe Postdoctoral Fellowship,the National Research Council of Thailand/Chulalongkorn University(Mid-Career Research Grant:NRCT5RSA63001-24)the Ratchadaphiseksomphot Endowment Fund(CU-GR_62_34_23_11)of Chulalongkorn University。
文摘Typically,heating or high-temperature treatment has been used to regenerate solid sorbent.In this study,the depressurized regeneration using a circulating fluidized bed downer was proposed and the significance of its operating parameters was identified.Two-dimensional computational fluid dynamics were employed to systematically investigate the effects of operating parameters on carbon dioxide depressurized regeneration with potassium carbonate solid sorbent particles.The simulated model was based on a laboratory scale circulating fluidized bed downer.The chemical equilibrium model for predicting the highest outlet carbon dioxide mass fraction was then used.A central composite design was employed to identify the main,quadratic,and interaction effects of operating parameters to the regeneration process.The operating parameters consisted of the outlet system pressure,inlet gas velocity,and inlet solid circulation rate,while the response variable was the released outlet carbon dioxide mass fraction.Among the multiple operating parameters,there were two main operating parameters and their combinations,namely the inlet gas velocity,outlet system pressure,square of inlet gas velocity,and interaction between inlet gas velocity and outlet system pressure,which had great impacts on the regeneration.All the main,quadratic,and interaction effects were explained.Then,the optimal operating conditions were obtained through the response surface method.
文摘The structural optimization of baffle internals for fast pyrolysis of coal with particulate mixing and heat transfer in a downer reactor using the discrete element method(DEM)has been investigated in this research.The pyrolysis terminal temperature at the exit of the downer reactor is not only decided by the volume-feeding-rate ratio of the coal to the sand,but also is affected by the inner structural design of the baffle internals in the downer reactor.As presented in the previous publication of the author,the inhibition from the baffle internals in a downer reactor can improve the particulate-mixing degree and heat carrier,and increase the mean residence time of the coal and heat-carrier particles in the downer reactor.The structure of the baffle internals in the downer reactor mentioned in this research can be optimized by the independently developed 3D soft-sphere model of the DEM programme of a 40-mm baffle length,a 30°baffle-slope angle and at least four baffles designed in the downer reactor,which is beneficial for the process design of coal pyrolysis with a heat carrier in the downer reactor.
