Evaluation of downer reactor performance by catalytic ozone decomposition

Radial distribution profiles of ozone concentrations were measured along an 8.50 m high and 0.09 m inside diameter gas/solid co-current down-flow circulating fluidized bed (downer) to characterize the reactor performance. Tests were conducted under a series of operating conditions at room temperature and near atmospheric pressure, with FCC particles as the bed material. Results show that the concentration distribution of the ozone tracer gas correlates well with the flow structure of the downer. There is quite a uniform radial distribution of ozone concentrations in the core region of all tested axial sections in the fully developed region of the downer, except for the near-wall region where there is a sharp decrease in ozone concentration. And there exists a relatively significant non-uniform distribution in the entrance acceleration region of the downer.     

Fractal characteristics of gas-solids flow in a circulating fluidized bed

A fractal approach is adopted to describe the dynamic behavior of a circulating fluidized bed. Two times series, differential pressure fluctuations along the riser height and solids momentum fluctuations along the radial direction, are measured and analyzed in terms of fractal dimensions. The influences of operating conditions and axial/radial positions on the fractal dimension are discussed. Attempts are also made to interpret the flow structure in the bed in terms of the fractal dimension. It is found that fractal analysis can provide a useful tool for understanding the characteristics of gas-solids flow in circulating fluidized beds.     

Solids concentration in the fully developed region of circulating fluidized bed downers

To investigate solids concentration in the fully developed region of co-current downward gas-solid flow, actual solids concentrations were measured in a circulating fluidized bed (CFB) downer with 9.3 m in height and 0.1 m in diameter using a fiber optical probe. The results obtained from this work and in the literature show that the average solids concentration in the fully developed region of the CFB downers is not only a function of the corresponding terminal solids concentration, but the operating conditions and particle properties also have influences on the average solids concentration in the fully developed region of the CFB downers. Particle diameter and density affect the solids concentrations differently under different operating conditions. Downer diameters almost have no influence on the solids concentrations. By taking into account the effects of operating conditions, particle properties and downer diameters, an empirical correlation to predict the solids concentrations in the fully developed region of CFB downers is proposed. The predictions of the correlation are in good agreement with the experimental data of this work and in the literature.     

Olive cake combustion in a circulating fluidized bed

In this study, a circulating fluidized bed of 125 mm diameter and 1800 mm height was used to find the combustion characteristics of olive cake (OC) produced in Turkey. A lignite coal that is most widely used in Turkey was also burned in the same combustor. The combustion experiments were carried out with various excess air ratios. The excess air ratio, λ, has been changed between 1.1 and 2.16. Temperature distribution along the bed was measured with thermocouples. On-line concentrations of O₂, SO₂, CO₂, CO, NO_x and total hydrocarbons were measured in the flue gas. Combustion efficiencies of OC and lignite coal are calculated, and the optimum conditions for operating parameters are discussed. The combustion efficiency of OC changes between 82.25 and 98.66% depending on the excess air ratio. There is a sharp decrease observed in the combustion losses due to hydrocarbons and CO as the excess air ratio increases. The minimum emissions are observed at λ=1.35. Combustion losses due to unburned carbon in the bed material do not exceed 1.4 wt% for OC and 1.85 wt% for coal. The combustion efficiency for coal changes between 82.25 and 98.66% for various excess air ratios used in the study. The ash analysis for OC is carried out to find the suitability of OC ash to be used as fertilizer. The ash does not contain any hazardous metal.     

Computation of system turbulences and dispersion coefficients in circulating fluidized bed downer using CFD simulation

In this study, the Eulerian computational fluid dynamics model with the kinetic theory of granular flow model was effectively used to compute the system turbulences and dispersion coefficients in a circulating fluidized bed (CFB) downer. In addition, the obtained model was used to simulate all the system velocities.     

Hydrodynamic similarity in circulating fluidized bed risers

Hydrodynamic similarity in the fully developed zone of co-current upward gas-solid two-phase flow systems under different operating conditions was investigated by measuring the axial profiles of pressure gradient, radial profiles of solid concentration and particle velocity in two circulating fluidized bed (CFB) risers of 15.1 and 10.5 m high, with FCC and sand particles, respectively. The experimental data obtained from this work and in the literature show that when the scaling parameter, G_s/(ρ_pU_g), is modified as , a detailed hydrodynamic similitude of the gas-solid flow in the fully developed zone of the risers under different operating conditions can be achieved. Furthermore, the experimental results from different gas-solid flow systems also show that as long as remains constant, there is the same solid concentration in the fully developed zone of different CFB risers with different particles. With the same , the local solid concentrations, the descending particle velocities, the cluster frequencies and the solid concentrations inside clusters in the fully developed zone of the risers all display the same axial and radial distribution, respectively. In other words, the empirical similarity parameter, , appears to have incorporated the effects of operating parameters (G_s and U_g), so that, the gas-solid flow in the fully developed zone of CFB risers under those different operating conditions but having the same shows similar micro- and macro-hydrodynamic characteristics. The study shows that the empirical similarity parameter, , is also independent of the upward gas-solid flow systems.     

Radiative heat transfer in circulating fluidized bed furnaces

Three methods of estimating the effective emissivity of a gas-particle suspension are compared and the radiative heat transfer coefficient of an isothermal suspension is defined. Heat flux measurements obtained from circulating fluidized bed combustors are examined. Radiation from a particle suspension with core temperature dominates the radiative heat transfer in the upper part of the furnace, where the particle density is low and no substantial particle boundary layers are formed. Over the lower parts of the heat transfer surfaces, where significant thermal and particle boundary layers are present, the radiative heat flux is dominated by emission from the relatively low temperature particle layer in the vicinity of the heat receiving surface.     

Numerical study of cluster formation in a gas–particle circulating fluidized bed

Simulations with two-way coupling are performed for two-dimensional gas–solid flow in a circulating fluidized bed with a total solids concentration of 3% in the riser. The motion of particles is treated by a Lagrangian approach, and particles are assumed to interact through binary, instantaneous, non-frontal, and inelastic collisions with friction. The model for the interstitial gas phase is based on the Navier–Stokes equations for two-phase flow with fluid turbulence calculated by using LES. Several porosity functions exist in the literature relating the drag force for a particle in a cloud to the drag force on an isolated particle. We have studied the influences of this porosity function, observing large differences in the local flow structure. The fluctuating gas–solid motion has been investigated showing a strong anisotropic flow behaviour, which is similar to experimental findings. The instabilities in these flows are strongly linked to the non-linear drag function due to the group effect of particles in a cloud. The collision parameters have been found to have an important influence on the cluster structures.     

Circulating fluidized bed hydrodynamics with air staging: an experimental study

The influence of secondary air injection (SA) on the hydrodynamics of circulating fluidized beds was studied in a 0.23-m ID riser. The secondary to primary air ratio, the vertical position, and the mode of injection (radial, tangential, and 45° entrance) are considered to be the key parameters of SA injection. It was found that the amount and location of SA have direct influence on the solids holdup and the segregation patterns in the riser. The SA divided the riser into two different flow zones: a dense turbulent zone below and a relatively dilute bed above the injection port. The mean solids velocity is found to be upwards with a greater magnitude in the center region. It is downwards with a smaller magnitude along the walls, suggesting core-annular flow structures for both above and below the SA injection region.     

Segregation and mixing effects in the riser of a circulating fluidized bed

Segregation and mixing effects of binary mixtures of particles having difference in sizes and densities were studied in 0.1016 m-diameter riser of a circulating fluidized bed at gas velocities between 2.01 and 4.681 m/s and solids circulation rate between 12.5 and 50 kg/m² s. Two groups of bed materials (three quartz sand-spent fcc catalyst mixtures with different initial mass % of sand and two coal-iron mixtures, one with almost same sizes but with different densities and the other having both different sizes and densities) were used. Using local axial mass % of heavier/coarser particles and their mean sizes the extent of segregation was evaluated. The influence of operating conditions like superficial gas velocity and solids circulation rate on segregation was examined and found that with their increase segregation effects generally tend to decrease and a uniform mixture conforming to initial composition of the mixture results. Using the data available in the literature and those of the present authors an empirical correlation to obtain the extent of segregation in CFBs has been proposed.     

Mean and fluctuating gas phase velocities inside a circulating fluidized bed

Gas phase velocities is an area in circulating fluidized beds (CFB) that has traditionally received little attention. The dynamics and motion of particles or clusters inside the bed has been the main focus of research. This is because particles dominate the fluid mechanics and heat transfer inside a CFB. However, gas phase motions also effect particle motion. Gas eddies or fluctuations can play an important role in transporting particles to and from the wall. They also help in providing a uniform temperature throughout the bed by promoting mixing. This paper deals with how particles effect the mean and fluctuating gas velocities throughout the cross-section of a riser.Gas velocities were measured inside a cold scale model CFB using a shielded hot wire anemometer. At the centerline, typical mean gas velocities were measured which were approximately twice the superficial gas velocity. These high velocities are likely caused by the negligible net gas upflow in the annulus region. The presence of many dense, downward flowing clusters in the annulus makes this a reasonable assumption.Previous work on gas phase turbulence in two phase flows has typically used either laser measurement techniques in very small diameter risers or in larger risers with very low particle concentration. The general results have shown that smaller particles, on the same order of magnitude as those typically used in CFB and FCC reactors, tend to damp out the gas phase fluctuations. This implies that gas phase motion behaves close to a laminar fashion. This present research measures gas phase fluctuations with typical particle concentrations inside a CFB (∼1-5%). The results indicate that at larger particle concentrations where clusters are formed, the gas phase fluctuations increase dramatically. This suggests that length scales based on cluster size, as opposed to particle size, should be used in estimating the increased levels of gas fluctuations caused by the solid phase. Hence, models which ignore the effect of clusters on the gas or which treat the gas phase as laminar like flow, yield a misleading picture of the flow dynamics inside a CFB riser.     

Modeling the hydrodynamics in a coupled high-density downer-to-riser reactor

A coupled high-density downer-to-riser (DtoR) reactor is proposed for the controlled reaction pathway in the fluid catalytic cracking (FCC) process with the desired products distribution, e.g., clean gasoline with less olefin content. Hydrodynamics in such a reactor coupling system is studied using a compressive model that considers the pressure balances around all the sub-units in the prototype. The continuity closure condition is used to determine the material balance of the solid particles flowing in the circulating fluidized bed system. The model predictions have good agreement with the experimental data in rather wide operating conditions, e.g., when the solids circulation rate goes to more than 400 kg/m² s. The effects of the solids inventory, the superficial gas velocity, the particle diameter and density, the inside diameter of risers, and the fractional opening of the control valve for the solids flow on the operation of the DtoR system, are investigated and discussed in detail. It is demonstrated that the model offers appropriate guidance for the design and the operation of the coupled circulating fluidized bed system.     

Sequential modular simulation of circulating fluidized bed reactors

Bypassing the mathematical complexity of equation-oriented approaches in predicting the performance of chemical reactors has recently stimulated a significant amount of interest. Among chemical reactors, circulating fluidized bed reactors (CFBRs) have secured an important role in a broad range of applications in energy sectors due to their advantages, including high fluid-solid contact efficiency, uniform temperature, and enhanced heat and mass transfer rates. Accordingly, modelling and predicting the performance of these reactors is of great importance. In this study, a sequence-based model was developed to predict the behaviour of CFBRs. Complex phenomena in CFBRs were mimicked by two sub-models, namely the hydrodynamics module, which addressed the physical changes, and the reaction kinetics module, which described the chemical evolution of species. The performance of the proposed model was validated with a library of catalytic ozone decomposition experimental data in CFBRs. This work introduces a new infrastructure for modelling CFBRs, which may be combined with the current process simulation software, such as Aspen Plus©, for advanced process modelling applications.     

Flow behaviors in the downer of a large-scale triple-bed combined circulating fluidized bed system with high solids mass fluxes

The flow behaviors in the downer of a large-scale triple-bed circulating fluidized bed (TBCFB) gasifier cold model, which is composed of a downer (Φ 0.1 m×6.5 m), a bubbling fluidized bed (BFB, 0.75×0.27×3.4 m³), a riser (Φ 0.1 m×16.6 m) and a gas-sealing bed (GSB, Φ 0.158 m×5 m), were investigated. Sand particles with a density of 2600 kg/m³ and an average particle size of 128 μm were used as bed materials. Solids mass fluxes were in the range 113–524 kg/m² s. Average solids holdup in the developed region of the downer increased with increasing solids mass flux. The gas seal between the riser and the downer had a large effect on the solids holdup distribution in the downer. Compared with the solids holdup in the riser, a relatively low solids holdup was formed in the downer even at high solids loadings. A pressure balance model was set up to predict the solids mass flux for this TBCFB system. It was found that the static bed height in the GSB had a great effect on the solids mass flux. The possibilities of achieving a high density solids holdup in a downer were discussed.     

Developments in the understanding of gas–solid contact efficiency in the circulating fluidized bed riser reactor:A review

In the last several decades, circulating fluidized bed reactors have been studied in many aspects including hydrodynamics, heat and mass transfer and gas–solid two phase contacting. However, despite the abundance of review papers on hydrodynamics, there is no summary paper on gas–solid contact efficiency to date, especially on high density circulating fluidized beds(CFBs). This paper gives an introduction to, and a review of the measurement of contact efficiency in circulating fluidized bed riser. Firstly, the popular testing method of contact efficiency including the method of heating transfer experiment and hot model reaction are discussed, then previous published papers are reviewed based on the discussed methods. Some key results of the experimental work are described and discussed. Gas–solid contact efficiency is affected by the operating conditions as well as the particle size distribution. The result of the contact efficiency shows that the CFB riser is far away from an ideal plug flow reactor due to the characteristics of hydrodynamics in the riser. Lacunae in the available literature have been delineated and recommendations have been made for further work.     

Flow behavior and regime transition in a high-density circulating fluidized bed riser

Flow behavior and flow regime transitions were determined in a circulating fluidized bed riser (0.203 m i.d. × 5.9 m high) of FCC particles (, ). A momentum probe was used to measure radial profiles of solids momentum flux at several heights and to distinguish between local net upward and downward flow. In the experimental range covered (; ), the fast fluidization flow regime was observed to coexist with dense suspension upflow (DSU). At a constant gas velocity, net downflow of solids near the wall disappeared towards the bottom of the riser with increasing solids mass flux, with dense suspension upflow achieved where there was no refluxing of solids near the riser wall on a time-average basis. The transition to DSU conditions could be distinguished by means of variations of net solids flow direction at the wall, annulus thickness approaching zero and flattening of the solids holdup versus G_s trend. A new flow regime map is proposed distinguishing the fast fluidization, DSU and dilute pneumatic transport flow regimes.     

Hydrodynamics and reactor performance evaluation of a high flux gas‐solids circulating fluidized bed downer: Experimental study

Reactor performance of a high flux circulating fluidized bed (CFB) downer is studied under superficial gas velocities of 3–7 m/s with solids circulation rate up to 300 kg/m²s using ozone decomposition reaction. Results show that the reactant conversion in the downer is closely related to the hydrodynamics, with solids holdup being the most influential parameter on ozone decomposition. High degree of conversion is achieved at the downer entrance region due to strong gas‐solids interaction as well as higher solids holdup and reactant concentration. Ozone conversion increases with the increase of solids circulation rate and/or the decrease of superficial gas velocity. Overall conversion in the CFB downer is less than but very close to that in an ideal plug flow reactor indicating a good reactor performance in the downer because of the nearly “ideal” hydrodynamics in downer reactors. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3412–3423, 2014     

Hydrodynamic modeling of a circulating fluidized bed

Hydrodynamics plays a crucial role in defining the performance of circulating fluidized beds (CFB). The numerical simulation of CFBs is very important in the prediction of its flow behavior. From this point of view, in the present study a dynamic two dimensional model is developed considering the hydrodynamic behavior of CFB. In the modeling, the CFB riser is analyzed in two regions: The bottom zone in turbulent fluidization regime is modeled in detail as two-phase flow which is subdivided into a solid-free bubble phase and a solid-laden emulsion phase. In the upper zone core-annulus solids flow structure is established. Simulation model takes into account the axial and radial distribution of voidage, velocity and pressure drop for gas and solid phase, and solids volume fraction and particle size distribution for solid phase. The model results are compared with and validated against atmospheric cold bed CFB units' experimental data given in the literature for axial and radial distribution of void fraction, solids volume fraction and particle velocity, total pressure drop along the bed height and radial solids flux. Ranges of experimental data used in comparisons are as follows: bed diameter from 0.05-0.418 m, bed height from 5-18 m, mean particle diameter from 67-520 μm, particle density from 1398 to 2620 kg/m³, mass fluxes from 21.3 to 300 kg/m²s and gas superficial velocities from 2.52-9.1 m/s.As a result of sensitivity analysis, the variation in mean particle diameter and superficial velocity, does affect the pressure especially in the core region and it does not affect considerably the pressure in the annulus region. Radial pressure profile is getting flatter in the core region as the mean particle diameter increases. Similar results can be obtained for lower superficial velocities. It has also been found that the contribution to the total pressure drop by gas and solids friction components is negligibly small when compared to the acceleration and solids hydrodynamic head components. At the bottom of the riser, in the core region the acceleration component of the pressure drop in total pressure drop changes from 0.65% to 0.28% from the riser center to the core-annulus interface, respectively; within the annulus region the acceleration component in total pressure drop changes from 0.22% to 0.11% radially from the core-annulus interface to the riser wall. On the other hand, the acceleration component weakens as it moves upwards in the riser decreasing to 1% in both regions at the top of the riser which is an important indicator of the fact that hydrodynamic head of solids is the most important factor in the total pressure drop.     

Annular wall layer thickness in circulating fluidized bed risers

The thickness of downward-flowing annular wall layers in circulating fluidized bed risers has been determined in the literature based on measured radial profiles of both local particle velocity and solids flux. The thickness of the wall layer is shown to be larger based on solids flux profiles than when based on particle velocity profiles, because fluctuations in local instantaneous particle velocity are correlated with fluctuations in local solids concentration. A new correlation is developed to predict the time-average thickness of the downflowing particle streamer layer based on solids flux measurements as a function of the cross-sectional average voidage. It successfully accounts for the variation of the wall layer thickness with axial location and solids circulation rate.     

Flow structure and thickness of annular downflow layer in a circulating fluidized bed riser

Flow structures were determined in a circulating fluidized bed (CFB) riser (0.203 m i.d.×5.9 m high) of FCC particles (d_p=70 μm, ρ_s=1700 kg/m³). A momentum probe was used to measure radial momentum flux profiles at several levels and to distinguish between upward and downward flow regions. Time-mean dynamic pressure (ΔP_m) decreases towards the wall in the range U_g=5-8 m/s, G_s=10-340 kg/m² s. The thickness of the annular downflow layer based on ΔP_m=0 reaches a maximum with increasing height. The annular downflow layer disappears locally with increasing solids mass flux (G_s) at a constant gas velocity, with achievement of the dense suspension upflow (DSU) regime. A new correlation is developed to predict the time-mean thickness of solids down-flowing layer based on solids mass flux and momentum flux. It successfully accounts for the variation of the annular layer thickness with height and G_s, and covers a wide G_s range right up to near the onset of the DSU regime.