Effect of static bed height in the upper fluidized bed on flow behavior in the lower riser section of a coupled reactor

To study olefin reduction by using an auxiliary reactor for FCC naphtha upgrading, a large-scale cold model of a riser-bed coupled to an upper fluidized bed was established. The effect of static bed height in the upper fluidized bed on particle flow behavior in the lower riser was investigated experimentally. A restriction index of solids holdup was used to evaluate quantitatively the restrictive effect of the upper fluidized bed. Experimental results show that, under the restrictive effect of the upper fluidized bed, the riser could be divided into three regions in the longitudinal direction: accelerating, fully developed and restriction. The axial distribution of solids holdup in the riser is characterized by large solids holdup in the top and bottom sections and small solids holdup in the middle section. Overall solids holdup increased with increasing static bed height in the upper fluidized bed, while particle velocity decreased. Such restrictive effect of the upper fluidized bed could extend from the middle and top sections to the whole riser volume when riser outlet resistance is increased, which increases with increasing static bed height in the upper fluidized bed. The upper bed exerts the strongest restriction on the area close to the riser outlet.     

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%.     

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%.     

Hydrodynamics in a new liquid–solid circulating conventional fluidized bed

A new type of liquid–solid fluidized bed, named circulating conventional fluidized bed (CCFB) which operates below particle terminal velocity was proposed and experimentally studied. The hydrodynamic behavior was systematically studied in a liquid–solid CCFB of 0.032 m I.D. and 4.5 m in height with five different types of particles. Liquid–solid fluidization with external particle circulation was experimentally realized below the particle terminal velocity. The axial distribution of local solids holdup was obtained and found to be fairly uniform in a wide range of liquid velocities and solids circulation rates. The average solids holdup is found to be significantly increased compared with conventional fluidization at similar conditions. The effect of particle properties and operating conditions on bed behavior was investigated as well. Results show that particles with higher terminal velocity have higher average solids holdup.     

Local solids flow structure in a countercurrent liquid-upward and solids-downward fluidized bed (CCLSFB) system

The local solids holdup and local particle velocity in a Countercurrent Liquid-upward and Solids-downward Fluidized Bed (CCLSFB) were investigated in details using optical fiber probes with two different models in a Plexiglas column of 1.5 m in height and 7.0 cm in inner diameter. A new flow regime map including fluidized bed, transition, and flooding regimes was established. The axial solids holdup distribution is almost uniform at low liquid velocity and/or solids flowrate and becomes less uniform with higher solids holdup at the top of the column after the operating liquid velocity is reaching the flooding velocity. The radial solids holdup profile is also nearly flat with a slightly lower solids holdup in the near-wall region at low liquid velocity and solids flowrate but becomes nonuniform as the operating liquid velocity approaches the flooding velocity. Two equations were also proposed to correlate radial local solids holdups. The descending particle velocity in CCLSFB increases with the decrease of the liquid velocity and the increase of the solids flowrate. A generally uniform particle velocity distribution was found in the axial direction, as well as in the radial direction except for a small decrease near the wall. These results on the local solids flow structure would provide basic information and theoretical supports for the design and industrial application of CCLSFB.     

Continuous segregation of binary heterogeneous solids in a fast-fluidized bed

Continuous segregation of binary heterogeneous solids (different density mixtures) is carried out in a gas–solid fluidized bed to study the effects of gas velocity, solids feed rate, feed composition and density difference of solids on the separation factor (recovery of flotsam at top outlet) and the quality of the product (purity of flotsam at top outlet) in a continuous fast-fluidized bed. The holdup of the bed material is obtained in each experimental run. It is observed that the separation factor decreases with increase in solids feed rate or density difference of solids, and increases with gas velocity or proportion of flotsam in the feed. The quality of the product decreases with increase in gas velocity or solids flow rate, and increases with feed composition or density difference of solids. The experimental results show that the separation factor and the quality of the product are more sensitive to gas velocity than to other operating parameters. Empirical correlations for predicting the separation factor and quality of the product are proposed based on the Richards model for individual flotsam mass fraction in the feed, and the predictions agree satisfactorily with the present experimental data.     

Flow dynamics and contact efficiency in a novel fast-turbulent fluidized bed with ring-feeder internals

The flow dynamics in a novel fast-turbulent fluidized bed (FTFB) with middle-upper expanding structure and two different ring-feeder internals (mixed and vortex ring-feeder) were studied to achieve a reduction in gasoline olefin production. Compared with a conventional circulating fluidized bed, the novel FTFB displayed unique characteristics and advantages. A higher solids holdup and more uniform solids holdup distribution existed in the diameter-expanding region, especially for the FTFB with vortex ring-feeder structure. A probability density distribution analysis indicated that the novel fluidized bed could reduce gas–solids segregation and enhance gas–solids interaction. A constant carbon dioxide tracer system was used to simulate the reactant gas distribution. The gas–solids contact efficiency was defined according to the solid dispersibility and the amount of gas covering the solid surface. Novel FTFB risers, especially those with vortex ring-feeders, have a much higher gas–solids contact efficiency than that of traditional risers.     

Hydrodynamics of gas-solid fluidization of a homogeneous ternary mixture in a conical bed： Prediction of bed expansion and bed fluctuation ratios

Hydrodynamic characteristics of fluidization in a conical or tapered bed differ from those in a columnar bed because the superficial velocity in the bed varies in the axial direction. Fixed and fluidized regions could coexist and sharp variations in pressure drop could occur, thereby giving rise to a noticeable pressure drop-flow rate hysteresis loop under incipient fluidization conditions. To explore these unique properties, several experiments were carried out using homogeneous, well-mixed, ternary mixtures with three dif- ferent particle sizes at varying composition in gas-solid conical fluidized beds with varying cone angles. The hydrodynamic characteristics determined include the minimum fluidization velocity, bed fluctuation, and bed expansion ratios. The dependence of these quantities on average particle diameter, mass fraction of the fines in the mixture, initial static bed height, and cone angle is discussed. Based on dimensional analysis and factorial design, correlations are developed using the system parameters, i.e. geometry of the bed （cone angle）, particle diameter, initial static bed height, density of the solid, and superficial velocity of the fluidizing medium. Experimental values of minimum fluidization velocity, bed fluctuation, and bed expansion ratios were found to agree well with the developed correlations.     

Experiments on the local heat transfer characteristics of a circulating fluidized bed

The role of particle diameter in the heat transfer of a gas–solid suspension to the walls of a circulating fluidized bed was studied for particles of uniform size. This work reports and analyzes new experimental results for the local bed to wall heat transfer coefficient, not including the radiation component, in a long active heat transfer surface length laboratory bed, which extend previous findings and clear up some divergences. The research included determining the effects of extension and location of the heat transfer surface, circulating solids mass flux and average suspension density. An experimental set-up was built, with a 72.5 mm internal diameter riser, 6.0 m high, composed of six double pipe heat exchangers, 0.93 m high, located one above the other. Five narrow sized diameter quartz sand particles − 179, 230, 385, 460 and 545 μm − were tested. Temperature was kept approximately constant at 423 K and the superficial gas velocity at 10.5 m/s. The major influence of suspension density on the wall heat transfer was confirmed, and contrary to other authors, a significant effect of particle size was found, which becomes more relevant for smaller particles and increasing suspension density. It was observed that the extension of the heat transfer surface area did not influence the heat transfer coefficient for lengths greater than 0.93 m.The heat transfer surface location did not show any effect, except for the exchanger at the botton of the riser. A simple correlation was proposed to calculate the heat transfer coefficient as a function of particle diameter and suspension density.     

Wake properties of a single gas bubble in three-dimensional liquid-solid fluidized bed

Experiments were performed to investigate the wake properties of a single gas bubble in a three-dimensional liquid-solid fluidized bed via a video camera moving at the same speed as the bubble. The solids holdup in the fluidized bed varied up to around 10%. The bubble size varied from 5 to 20 mm with corresponding bubble Reynolds numbers ranging from 1000 to 6500. The bubble was observed to have two types of wake configurations depending on the bubble size: the asymmetric/helical vortex wake for small bubbles and the symmetric wake for large bubbles. The bubble shape and relative rise velocity in the fluidized bed can be well-represented by correlations developed for single bubbles in liquid media, although the bubble shape in liquid-solid media is slightly more flattened compared to that in liquid media. The bubble rocking frequency was found to be independent of particle properties and to correspond in magnitude to the vortex shedding frequency in a two-dimensional liquid-solid fluidized bed. The average primary wake size in three dimensions is comparable to that in two dimensions.     

Continuous segregation of binary heterogeneous solids in fluidized beds

Continuous segregation of a binary mixture of heterogeneous (different density) solids is carried out in a gas–solid fluidized bed. We investigate how gas velocity, solids feed rate, flotsam feed composition, bottom discharge pipe diameter, and minimum fluidization velocity ratio of the flotsam to jetsam particles influence the solids holdup, separation factor, and product quality (flotsam purity at the top outlet). The results are interpreted in terms of solids holdup information. The results indicate that the separation factor decreases when the gas velocity, bottom discharge pipe diameter, flotsam feed composition, or the minimum fluidization velocity ratio increase, while the separation factor increases as the solids feed rate increases. The product quality decreases when the gas velocity, solids feed rate, or minimum fluidization velocity ratio increase, while the product quality increases as the bottom discharge pipe diameter or flotsam feed composition increase. Correlations for predicting the separation factor and product quality are proposed using a logistic model for individual flotsam feed compositions, which satisfactorily compares with the present experimental data.     

Evaluation of the effect of wall boundary conditions on numerical simulations of circulating fluidized beds

A computational fluid dynamics (CFD) modeling of the gas–solids two-phase flow in a circulating fluidized bed (CFB) riser is carried out. The Eularian–Eularian method with the kinetic theory of granular flow is used to solve the gas–solids two-phase flow in the CFB riser. The wall boundary condition of the riser is defined based on the Johnson and Jackson wall boundary theory (Johnson & Jackson, 1987) with specularity coefficient and particle–wall restitution coefficient. The numerical results show that these two coefficients in the wall boundary condition play a major role in the predicted solids lateral velocity, which affects the solid particle distribution in the CFB riser. And the effect of each of the two coefficients on the solids distribution also depends on the other one. The generality of the CFD model is further validated under different operating conditions of the CFB riser.     

Hydrodynamic behavior of an internally circulating fluidized bed with tubular gas distributors

To better understand the hydrodynamic behavior of an internally circulating fluidized bed, solids holdup in the down-comer （Eso）, solids circulation rate （Gs） and gas bypassing fraction （from down-comer to riser y~R, and from riser to down-comer yRD） were experimentally studied. The effects of gas velocities in the riser and in the down-comer （UR and UD）, orifice diameter in the draft tube （dor）, and draft tube height （HR） were investigated. Experimental results showed that increase of gas velocities led to increase in Gs and yDR, and slight decrease in yeD. Larger orifice diameter on the draft tube led to higher 8sD, Gs and yDR, but had insignificant influence on YRD. with increasing draft tube height, both Gs and YDR first increased and then decreased, while yRD first decreased and then increased. Proposed correlations for predicting the hydrodynamic parameters agreed reasonably well with experimental values.     

Some hydrodynamic aspects of 3-phase inverse fluidized bed

Hydrodynamics of 3-phase inverse fluidized bed is studied experimentally using low density particles for different liquid and gas velocities. The hydrodynamic characteristics studied include pressure drop, minimum liquid and gas fluidization velocities and phase holdups. The minimum liquid fluidization velocity determined using the bed pressure gradient, decreases with increase in gas velocity. The axial profiles of phase holdups shows that the liquid holdup increases along the bed height, whereas the solid holdup decreases down the bed. However, the gas holdup is almost uniform in the bed.     

Flow dynamics and contact efficiency in a novel fast-turbulent fluidized bed with ring-feeder internals

The flow dynamics in a novel fast-turbulent fluidized bed(FTFB) with middle-upper expanding structure and two different ring-feeder internals(mixed and vortex ring-feeder) were studied to achieve a reduction in gasoline olefin production.Compared with a conventional circulating fluidized bed,the novel FTFB displayed unique characteristics and advantages.A higher solids holdup and more uniform solids holdup distribution existed in the diameter-expanding region,especially for the FTFB with vortex ring-feeder structure.A probability density distribution analysis indicated that the novel fluidized bed could reduce gas-solids segregation and enhance gas-solids interaction.A constant carbon dioxide tracer system was used to simulate the reactant gas distribution.The gas-solids contact efficiency was defined according to the solid dispersibility and the amount of gas covering the solid surface.Novel FTFB risers,especially those with vortex ring-feeders,have a much higher gas-solids contact efficiency than that of traditional risers.     

Particle size distribution in CPFD modeling of gas–solid flows in a CFB riser

A computational particle fluid dynamics (CPFD) numerical method to model gas–solid flows in a circulating fluidized bed (CFB) riser was used to assess the effects of particle size distribution (PSD) on solids distribution and flow. We investigated a binary PSD and a polydisperse PSD case. Our simulations were compared with measured solids concentrations and velocity profiles from experiments, as well as with a published Eulerian-Eulerian simulation. Overall flow patterns were similar for both simulation cases, as confirmed by experimental measurements. However, our fine-mesh CPFD simulations failed to predict a dense bottom region in the riser, as seen in other numerical studies. Above this bottom region, distributions of particle volume fraction and particle vertical velocity were consistent with our experiments, and the simulated average particle diameter decreased as a power function with riser height. Interactions between particles and walls also were successfully modeled, with accurate predictions for the lateral profiles of particle vertical velocity. It was easy to implement PSD into the CPFD numerical model, and it required fewer computational resources compared with other models, especially when particles with a polydisperse PSD were present in the heterogeneous flow.     

3D full-loop simulation and experimental verification of gas-solid flow hydrodynamics in a dense circulating fluidized bed

Because of their advantages of high efficiency and low cost, numerical research methods for large-scale circulating fluidized bed （CFB） apparatus are gaining ever more importance. This article presents a numer- ical study of gas-solid flow dynamics using the Eulerian granular multiphase model with a drag coefficient correction based on the energy-minimization multi-scale （EMMS） model. A three-dimensional, full-loop, time-dependent simulation of the hydrodynamics of a dense CFB apparatus is performed. The process parameters （e.g., operating and initial conditions） are provided in accordance with the real experiment to enhance the accuracy of the simulation. The axial profiles of the averaged solid volume fractions and the solids flux at the outlet of the cyclone are in reasonable agreement with experimental data, thereby verifying the applicability of the mathematical and physical models. As a result, the streamline in the riser and standpipe as well as the solids distribution contours at the cross sections is analyzed. Computational fluid dynamics （CFD） serves as a basis for CFB modeling to help resolve certain issues long in dispute but difficult to address experimentally. The results of this study provide the basis of a general approach to describing dynamic simulations of gas-solid flows.     

Evaluation of the effect of wall boundary conditions on numerical simulations of circulating fluidized beds

A computational fluid dynamics （CFD） modeling of the gas-solids two-phase flow in a circulating fluidized bed （CFB） riser is carried out. The Eularian-Eularian method with the kinetic theory of granular flow is used to solve the gas-solids two-phase flow in the CFB riser. The wall boundary condition of the riser is defined based on the Johnson and Jackson wall boundary theory （Johnson ＆ Jackson, 1987） with specularity coefficient and particle-wall restitution coefficient.The numerical results show that these two coefficients in the wall boundary condition play a major role in the predicted solids lateral velocity, which affects the solid particle distribution in the CFB riser. And the effect of each of the two coefficients on the solids distribution also depends on the other one. The generality of the CFD model is further validated under different operatin~ conditions of the CFB riser.     

Circulating turbulent fluidization—A new fluidization regime or just a transitional phenomenon

While circulating fluidized bed （CFB） reactor has many advantages over the more conventional turbulent fluidized bed （TFB） reactor, it does at least have one significant shortcoming-the rather dilute solids volume concentration in CFB reactor gives rise to less ideal reaction intensity. On the other hand, while having higher reaction intensity, TFB reactor has one fatal drawback of particle back-mixing, making it not suitable for certain reactions such as catalytic reaction where the catalyst requires frequent regeneration. This paper describes some key issues in the development of a circulating turbulent fluidized bed （CTFB） reactor that combines the advantages of both TFB and CFB, that is, to have the high reaction intensity as in TFB but and also to have a suppressed solids back-mixing as in CFB due to a continuous net upflow of solids flux through the bed. Experimental results show enough evidence to suggest that a new fluidization regime is formed, the characteristics of which appears to be distinct from those observed in a regular TFB and from those in either the bottom or the upper sections of regular CFB and/or high-density CFB （HDCFB）. Fundamentally, the difference is that particle-particle interaction （collision） dominates the motion of particles in CTFB and TFB, while gas-particle interaction （drag force） is the key element that determines the two phase flow in CFB including HDCFB.     

Effects of various parameters on the attrition of bed material in a recirculating fluidized bed with a draft tube

We performed an experimental study to investigate the effects of various parameters on the attrition of bed material and its size distribution with increasing operation time in a recirculating fluidized bed (RCFB). The studied parameters included superficial velocity of fluidizing air, bed inventory, and spacing between the jet top and draft tube bottom (spacer height). The bed material was prepared from Indian Standard (IS) Grade I sand from sieves with a size range of 2.20–1.00 mm. Experiments were performed at ambient conditions, with the superficial air velocity ranging from 7.13–9.16 m/s, a bed inventory of 7–10 kg, spacing of 0.085 and 0.045 m between the jet top and draft tube bottom, and an operating time of 40 h. We investigated the influence of these parameters in terms of changes in the size distribution of particles, changes in the %-weight of particles of different size ranges, generation of particles with smaller diameters, the decrease of the downcomer bed height, variations in the coefficient of uniformity and coefficient of curvature, and material loss from entrainment of fines with increasing operation time. The mode of attrition was abrasion in all experiments. We found that with increasing operation time and other parameters (bed inventory, superficial air velocity, and spacer height) attrition of the bed material also increased. Generation and elutriation of fines were more pronounced at higher superficial air velocity, bed inventory, and spacer height.