Computational fluid dynamics-discrete element method simulation of stirred tank reactor for graphene production

Liquid phase exfoliation(LPE)process for graphene production is usually carried out in stirred tank reactor and the interactions between the solvent and the graphite particles are important as to improve the production efficiency.In this paper,these interactions were revealed by computational fluid dynamics–discrete element method(CFD-DEM)method.Based on simulation results,both liquid phase flow hydrodynamics and particle motion behavior have been analyzed,which gave the general information of the multiphase flow behavior inside the stirred tank reactor as to graphene production.By calculating the threshold at the beginning of graphite exfoliation process,the shear force from the slip velocity was determined as the active force.These results can support the optimization of the graphene production process.

Mixing effects of high-speed jets in gas-solid riser and downer reactors

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.

Hydrodynamics and OCM reaction performance in a bubbling fluidized bed reactor and a riser reactor

The reasonable reactor design is of great importance for increasing the C_(2) yield(C2H4 and C2H6)of the oxidative coupling of methane(OCM),and the OCM reactor should remove the heat released in reactions quickly and efficiently and minimize the consecutive reaction of ethylene to carbon oxides.The fluidized bed reactor is characterized by excellent heat transfer,superior mass transport,and large handling capacity,while fewer studies focused on large-scale fluidized bed reactors for the OCM reaction.Therefore,large cold-model experiments and computational fluid dynamics simulations were conducted to investigate hydrodynamics and the OCM reaction performance in a large-scale bubbling fluidized bed(BFB)and a large-scale riser.In the BFB reactor,consecutive reactions of ethylene are acute because of the strong gas back-mixing,high solids holdup,and non-uniform solids distribution.While the consecutive reactions of ethylene are negligible due to the plug flow structure and low solids holdup in the riser reactor.Further,both reactors can achieve isothermal operation for the OCM process.The C_(2) selectivity of 45.4% and C_(2) yield of 21.1% are obtained in the riser reactor,increasing by 20.3% and 5.8% individually than that in the BFB reactor.This study provides useful information and reference to the OCM reactor designandcommercialization.

Study on flow characteristics and phase holdup in a slurry bubble column coupled with mild agitation Author links open overlay panel

A pilot-scale experimental setup was constructed to investigate the effect of mild agitation on the bubble characteristics and phase holdup in a slurry bubble column.Mild agitation positively impacts the axial uniform distribution of solid holdup,though it shows insignificant influence on the radial distribution.In homogenous regime,mild agitation promotes the coalescence of bubbles,and the effect becomes stronger with increasing agitator speed.The mild agitation contributes to a decrease in bubble size in heterogeneous flow regime.Mild agitation presents a significant effect on the gas holdup by adjusting the bubble size and bubble motion trajectory.The modification was introduced to predict the gas holdup considering the effects of mild agitation,a necessary consideration for applications requiring mild agitation.This adapted model predicts gas holdup with a maximum error of 12.9%.

Optimal reaction conditions for pyridine synthesis in riser reactor

Pjridine has been generally synthesized by aldehydes and ammonia in a turbulent fluidized-bed reactor. In this paper, a novel riser reactor was proposed for pyridine synthesis. Experiment result showed that the yield of pyridine and 3-picoline decreased, but the selectivity of pyridine over 3-picoline increased compared to turbulent fluidized-bed reactor. Based on experimental data, a modified kinetic model was used for the determination of optimal operating condition for riser reactor. The optimal operating condition of riser reactor given by this modified model was as follows： The reaction temperature of 755 K, catalyst to feedstock ratio （CTFR） of 87, residence timeof3.8sandinitialacetaldehydesconcentrationof0.0029mol.L-1 （acetaldehydes to formaldehydes ratio by mole （ATFR） of 0.65 and ammonia to aldehydes ratio by mole （ATAR） of 0.9, water contention of 63wt% （formaldehyde solution））.

Modeling of pyridine synthesis process in a coupled fluidized bed reactor

To obtain the optimal operating conditions of a coupled reactor for pyridine synthesis,reactor modeling process is carried out in this paper.During the modeling process,the flow hydrodynamics,heat transfer behavior,inter-phase mass transfer behavior and reaction kinetics were taken into consideration consequently.Further,a regression program based on least square method was proposed to regress the model parameters.The prediction results agreed well with the experimental results with an average deviation of 5.9%.Finally,by setting suitable aim function,the optimal operating conditions of the coupled reactor for pyridine synthesis were determined.

基于人工神经网络的流化床流场演化模拟

计算流体动力学(CFD)是一种模拟流化床中复杂气固流动的常用方法,此方法计算效率较低,而人工神经网络(ANN)模型能克服这一缺点,实现高效计算。本工作结合CFD与人工神经网络,发展了一种快速获得流化床内流场演化的人工神经网络模型。该模型对颗粒浓度、气体压力和气固两相速度构建了不同的网络结构,以多相质点网格(MP-PIC)方法模拟流化床得到的结果作为数据集进行训练。验证结果表明,该人工神经网络模型成功实现了对流化床中的颗粒浓度、气体压力和气固两相速度的预测。在精度方面,三种网络结构模型均可准确预测一个时间步长的流场数据,在进行长时间流场预测时仍存在误差。在计算效率方面,人工神经网络模型的计算速度约为MP-PIC方法的13000倍。

Effect of operating conditions on the performance of riser reactor for oxidative coupling of methane Author links open overlay panel

Oxidative coupling of methane(OCM)is characterized by high temperature and strong heat released.A riser reactor is designed for the OCM process to improve gas-solid contacting and heat transfer.This work illustrates the effect of hydrodynamic conditions and feed gas composition on the performance of an OCM riser reactor.The findings suggest that oxidative reactions predominantly occur within the acceleration zone and developed flow zone.Furthermore,at the low superficial gas velocity accompanied by a high solid circulating rate,the C_(2)yield initially increases in the acceleration zone,followed by a decrease in the developed flow zone and deceleration zone.The optimal operating conditions are a specific region corresponding to the solids holdup ranges from 0.015 to 0.02.Moreover,the C_(2)yield can be elevated using an appropriate feed mixture.

Hydrodynamics in commercial-scale internally circulating fluidized beds with different central downcomer outlets

Standpipes,or downcomers,are commonly used in fluidized beds to transport particles.The outlet structure of the downcomer greatly affects the performance of flow from it and even overall reactor performance.In this study,the hydrodynamics in commercial-scale internally circulating fluidized beds(ICFBs)with central downcomers having different outlet structures was investigated using computational fluid dynamics simulations with an energy minimization multi-scale drag model.The predicted results closely agreed with experimental data.Results showed that in an ICFB with a downcomer outlet directly open to the bed(model A),nearly 12.7%to 5.4%of the gas in the draft tube bypasses into the downcomer.In the ICFB models B and C with a conic baffle below the downcomer,the gas bypass is significantly weakened or even eliminated when the diameter of the conic baffle is 1.1 times that of the downcomer(model C).In addition,the solids circulation mass flux in ICFBs increased by about 62.5%,from 126.8 kg/(m2 s)in model A to 206 kg/(m2 s)in model C.

Flow hydrodynamics in a gas–solid fluidized bed reactor with two reaction zones

A novel reactor with two reaction zones is proposed for pyridine synthesis.The flow hydrodynamics were investigated in experiments.Pressure taps and a PV-6D optical fiber were used to measure the pressure fluctuation and particle concentration,respectively.Hilbert–Huang analysis was adopted to distinguish the flow patterns through pressure fluctuation.Results show that the flow patterns are bubbling fluidization and fast fluidization in the regions below(reaction zone I)and above(reaction zone II)the nozzles,respectively.The radial distribution of the cluster time fraction was obtained through signal waves of the particle concentration.Analysis of the cluster time fraction revealed two radial distributions.In the region around the nozzles,the cluster time fraction ranged from 0 to 0.2 and concentrated at radial positions r/R=0–1,which resulted from unsymmetrical catalyst feeding.In reaction zone II,the cluster time fraction ranged from 0 to 0.2,and the radial distribution indicated a core–annulus structure.

Heat transfer characterization and improvement in an external catalyst cooler fluidized bed

Gas-solid fluidized beds have been widely used in heat transfer processes,and so there have been many studies focused on increasing heat transfer in such units.In the present work,a pilot scale cold mode experimental rig was constructed to assess the effects of hydrodynamics on bed-to-wall heat transfer and to investigate various means of enhancing heat transfer in a dense gas-solid fluidized bed with external solid circulation.The experimental results show that heat transfer in the dense region played a dominant role in total bed-to-wall heat transfer,accounting for more than 88%of the total heat transfer load.Heat transfer could be lowered as a result of solids bypass that occurred because of external solids circulation,but this effect was weakened by the radial mixing of particles.The heat transfer characteristics identified in this study indicate that a specially designed baffle can be used to enhance bed-to-wall heat transfer.After installing such baffles in the fluidized bed test structure,a 70%increase in the total heat transfer coefficient was obtained.

Influence of the pressure drop and the air lock on the solid flux in a cross-flow moving bed

The gas-solid flow pattern in a rectangular cross-flow moving bed is simulated by the multiphase particle-in-cell(MP-PIC)model with the Barracuda software.The computed results are verified by the experimental data.In the bed,the actual solid flux generally equals the solid flow rates in the solid feed and discharge tubes.However,these two flow rates are greatly influenced by the air lock and the pressure drop in the solid feed and discharge tubes,namely,the negative and positive pressure gradients,respectively,rather than the traditional opinion that they are merely controlled by the valve openings.The pressure drops in these tubes are calculated by the proposed“common pressure pool with multiple outlets”(CPPMO)and the“common pressure pool”(CPP)methods.It is found that the local gas resistance dominates the pressure drop in the solid discharge tubes,while the gas frictional resistance determines the pressure drop in the solid feed tube.In addition,when the solid flow rate nearly tends to zero in the solid feed tube,the air lock forms.A solid flux equation is then given by considering both the air lock and the pressure drop factors in the cross-flow moving bed.

Characteristics of clusters in the jet influence zone of riser

By analyzing solid holdup signals in the jet influence zone of risers,the characteristics of clusters were studied.The solid holdups inside clusters and their distribution ranges were calculated under the cases of both upward and downward feed jets.Moreover,the effects of the jet velocity on the cluster characteristics were analyzed.The solid holdups inside clusters have higher values and wider distribution ranges in the upward feed injection zone than in the downward zone,implying that the number of individual particles in a cluster is unstable under the influence of upward jets.For the case of downward jets,cluster formations in the jet influence zone of risers can more easily maintain stability,offering a better mixing environment for jets and particles.As the jet velocity increases,the solid holdups inside clusters in the riser wall of the upward injection zone increase accordingly,while the distributions of solid holdups inside clusters have no significant changes if the feed jets are downward.This phenomenon confirms that improved operational flexibility can be obtained if downward jets are mixed with the prelift gas-solid flow in risers.

逆流式移动床过滤器过滤性能的实验研究

通过大型冷模实验考察了气固逆流式移动床过滤器床层表观气速、再生气速对装备操作的影响。在无尘负荷条件下,装备压降随床层表观气速增大呈抛物线式上升的关系;能够用修正的Ergun公式进行预测。通过加尘实验发现,除尘效率与气体停留时间变化趋势相同,且与再生气速呈正相关。床层压降则表现出与床层气速呈正相关、与再生气速呈负相关的变化趋势。实验结果表明,该装备对于粉尘的捕集效率较高,在入口浓度为8.175 g/m^(3)、表观气速在0.122~0.305 m/s范围内,除尘效率始终大于98.0%,且对13μm以下的细微粉尘有较高的粒级效率,接近99.99%。

二元颗粒在气固流化床中的轴向分布特性

通过大型冷模实验测量了二元混合颗粒在流化床内沿轴向的压力分布,考察了混合颗粒截面平均浓度沿轴向的变化特点。通过分析床层压差分布的转折点,确定了密相区与稀相区的相交界面高度,根据实验结果给出了经验关联式。通过压力信号标准差分析了流化床内混合颗粒流化性能与表观气速和颗粒混合比例的关系。实验结果表明,平均颗粒浓度沿流化床轴向呈下降趋势,且在密相区中随表观气速增加而减小,在稀相区中随表观气速增大而增大。二元颗粒中大颗粒比例xl为0.685时,密相区总平均颗粒浓度存在最大值。密相区与稀相区相交界面高度随表观气速增大而提高。当0.225≤xl≤0.479和0.561≤ug≤1.122 m/s时,流化床内二元颗粒的流化性能和混合程度达到最佳。

Experimental study on filtration performance of the moving bed granular filter with axial flow

The filtration performance of the moving bed granular filter with axial flow (MBGF-AF) is investigated through a large cold experiment. The effect of different operation parameters on the filtration performance (collection efficiency, pressure drop) of the axial-flow moving bed filter is investigated in combination with the dust deposition effect and the mechanism of trapping dust by the capturing particles. The results show that the collection efficiency of MBGF-AF is enhanced by decreasing the superficial gas velocity, increasing the inlet dust concentration properly, or decreasing the moving velocity of the capturing particles. A model covering the above operation parameters is established to calculate the collection efficiency of the moving bed granular filter. It is used in a wide range of operating parameters for the MBGFs.

Investigation on gas-solids hydrodynamics and matching enhancement in feedstock injection zone of double-level nozzle riser

With the development of current energy economy,it is necessary to improve the product distribution of fluid catalytic cracking process,which is achieved by a riser reactor with double-level of nozzles.The new riser is constructed by adding a level of secondary nozzle 0.5 m below the main nozzle of traditional riser.This paper investigates the gas-solids flow and oil-catalyst matching feature based on the optical fiber and tracer technologies.According to the distribution of solids holdup,particle velocity and dimen-sionless jet concentration,the feedstock injection zone can be divided into the upstream flow control region,the main flow control region,and the secondary flow control region in the radial direction.The size of the regions is changed by the jet gas velocity and axial height.There is a poor match of secondary nozzle jet to particles below the main nozzle.The jet gas from secondary nozzles can improve the matching effect of oil-catalyst near the wall and reduce the probability of coking above the main nozzle.

Constraint strength and axial/radial particle velocity profiles for an integrated riser outlet

To study axial/radial profiles of particle velocity in the affected region of an integrated riser outlet, a cold model was developed for the integrated riser reactor combining the gas-solid distributor with the fluidized bed. Constraints, related to the gas-solid distributor and the upper fluidized bed, imposed on the particle flow in the riser outlet region, were investigated experimentally. The experimental results showed that with increasing superficial gas velocity, these constraints have strong influences on particle flow behavior, the particle circulation flux in the riser, and the height of the static bed material of the upper fluidized bed. When the constraints have greater prominence, the axial profile of the cross-sectionally averaged particle velocity in the outlet region initially increases and then decreases, the rate of decrease being proportional to the constraint strength. Along the radial direction of the outlet section, the region where the local particle velocity profile tends to decrease appears near the dimensionless radius r/R = 0.30 initially and then, with increasing constraint strength, gradually extends to the whole section from the inner wall. Based on the experimental data, an empirical model describing the constraint strength was established. The average relative error of the model is within 7.69%.

Characteristics of flow and liquid distribution in a gas-liquid vortex separator with multi spiral arms

Fischer-Tropsch(F-T)synthesis is an important route to achieve the clean fuel production.The perfor-mance of gas-liquid separation equipment involving in the progressive condensation and separation of light and heavy hydrocarbons in the oil-gas products has become a bottleneck restricting the smooth operation of the F-T process.In order to remove the bottleneck,a gas-liquid vortex separator with simple structure,low pressure drop and big separation capacity was designed to achieve the efficient separation between gas and droplets for a long period.The RSM(Reynolds Stress Model)and DPM(Discrete Phase Method)are employed to simulate the flow characteristics and liquid distribution in the separator.The results show that the separation efficiency is influenced by the flow field and liquid phase concentration in the annular zone.The transverse vortex at the top of spiral arm entrains the droplets with small diam-eter into the upper annular zone.The entrained droplets rotate upward at an angle of about 37.4°.The screw pitch between neighbor liquid threads is about 0.3 m.There is a top liquid ring in the top of annular zone,where the higher is the liquid phase concentration,the lower is the separation efficiency.It is found that by changing the operating condition and the annular zone height the vortex can be strengthened but not enlarged by the inlet velocity.The screw pitch is not affected by both inlet velocity and annular zone height.The liquid phase concentration in the top liquid ring decreases with both the increases of inlet velocity and annular zone height.The total pressure drop is almost not affected by the annular zone height but is obviously affected by the inlet velocity.When the height of annular zone is more than 940 mm,the separation efficiency is not changed.Therefore,the annular zone height of 940 mm is thought to be the most economical design.

Gas-solids separation model of a novel FCC riser terminator device: super short quick separator (SSQS)

The gas flow field and the separation efficiency of a novel fluid catalytic cracking (FCC) riser terminal device, named as Super Short Quick Separator (SSQS), were studied. On the basis of above investigations, a section-lateral-mixing separation model was proposed, which included both the effect of inertia and structure of gas outlet on particles capture. After final modification, the results predicted with this model could be in good agreement with the cold experimental data. According to this model, the separation efficiency of SSQS is mainly influenced by the difference between the arc radius and the center pipe radius as well as the magnitude of particle tangential velocity.