Predicting the pressure drop across the solids flow rate control device of a circulating fluidized bed

The control of the solids circulation rate in circulating fluidized beds (CFB) can be obtained by means of a mechanical valve located at the bottom of the return leg. The valve acts by provoking a pressure drop that depends on the degree of the opening. The aim of this work is to develop a predictive model for the pressure drop in a butterfly valve used as a control device for the solids circulation rate. A model has been developed and validated against experimental data obtained from a 0.1 m id, 6 m high CFB using a group B powder. The equations proposed by, Jones and Davidson [D.R.M. Jones, J.F. Davidson, The flow of particles from a fluidised bed through orifices, Rheologica Acta 4 (1965) 180] and Cheng et. al. [L. Cheng, P. Basu, Solids circulation rate prediction in a pressurized loop seal, in: K. Chen (Ed.), Chemical Engineering Research and Design, vol. 76, 1998, p. 761] to predict the discharge rate of granular solid through orifices have been modified to account for the shape of the openings in the valve. A corrective parameter, which is based on the dimensionless hydraulic diameter of the valve opening, has been introduced. Very good agreement with the experimental data was obtained.     

Measurement of solid circulation rate in a circulating fluidized bed

A method using an optical mouse sensor was developed to monitor the moving velocity of a solid mass. Calibration was carried out using a rotating plate. Results clarified that the developed method is useful to monitor the velocity up to some limit (v < 0.3 m/s), which depends on the optical mouse sensor used. A solid circulation rate in a circulating fluidized bed (CFB) was measured using this method. Results obtained using this method show agreement with those of visual observations.     

Effect of solid circulation rate on coating efficiency and agglomeration in circulating fluidized bed type coater

Circulating fluidized bed was proposed to be used as a coater, and coating experiments of glass beads with silica powder were performed in a circulating fluidized bed. Glass beads and silica powder were chosen as model particles, because their shape was almost spherical. The respective effects of gas flow rates supplied from a distributor and from an air nozzle for solid circulation, feed rate of powder suspension and particle content in the bed on coating efficiency and agglomeration are mainly discussed. Coating efficiency in circulating fluidized bed coater was correlated well with solid circulation time rather than with gas flow rates or solid circulation rate, while the agglomeration among core particles was mainly governed by solid circulation rate.     

Continuous measurement of solids flow in a circulating fluidized bed

A modified impact probe for continuous measurement of solids circulation rate in a circulating fluidized bed has been developed based on a principle similar to a momentum probe. The response curves of solid flow from the probe have been characterized and calibrated in a test column (0.05 m-I.D.x0.80 m-high). The probe was validated in situ in the downcomer of a circulating fluidized bed (0.10 m-I.D.x4.80 m-high). The solid circulation rates obtained by the modified impact probe well agree with the measured solids circulation rate by the descent time method. Presented at the Int’/Sym. on Int’l Symp. on Chem. Eng. (Cheju, Feb. 8–10, 2001), dedicated to Prof. H. S. Chun on the occasion of his retirement from Korea University.     

The control of bed height and solids circulation rate in the standpipe of a cold flow circulating fluidized bed

Circulating fluidized beds (CFBs) are used widely in the chemical industry. Knowing or estimating the bed height in the standpipe and the solids circulation rate are essential for effective control of the system. This paper incorporates a 2-region model to calculate the bed height in the standpipe with a Kalman filter algorithm to estimate the solids circulation rate (SCR). Simulations of both the standpipe bed height and SCR were compared with experimental data and shown to give good agreement.

In addition, a neural network method was applied to model the entire cold flow CFB system and measured data sets were used to train the neurons of the network. Finally, a linear controller was applied to control both the bed height and solids circulation rate to desired set points. Simulations were performed for both positive and negative step inputs for both variables and satisfactory control was demonstrated using this controller in combination with the neutral network and Kalman estimator.     

循环床锅炉炉膛压降及轴向固体浓度的计算

本文提出了循环流化床锅炉炉膛的轴向压力及固体浓度分布的计算式。当给出表观气、循环固体流率以及颗粒性质时，可准确地预测炉膛压降与轴向固体浓度分布，为炉膛设计提供了计算依据。     

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.     

A device for measuring solids flowrate in a circulating fluidized bed

The on-line measurement of solids flowrate is important to numerous industrial processes. This paper considers a variation of impact-type solids flow meters suitable for use in numerous applications, including circulating fluidized beds (CFBs). The solids flowrate meter introduced herein is on-line, capable of operation in high temperature environments, and useful for a broad range of flowrates with good linearity, accuracy and fast response time. The flow meter works by measuring the torque that results on a hinged plate when falling solids impact the plate. A theoretical model of the device is developed and its results are compared to experimental data for the operation with various solids.     

Solids flow characteristics in loop-seal of a circulating fluidized bed

The hydrodynamics of solids (FCC) recycle in a loop-seal (0.08 m) at the bottom of the downcomer (0.08 m-I.D.x4.0 m-high) in a circulating fluidized bed (0.1 m-I.D.x 5.3 m-high) have been determined. Solid flow rate through the loop-seal increases linearly with increasing aeration rate. At the same aeration rate, the maximum solid flow rate can be obtained at a loop-seal height-to-diameter ratio of 2.5. The effects of solid inventory, solid circulation rate and gas velocity on pressure balance around the CFB have been determined. At a given gas velocity and solid circulation rate, pressure drops across the downcomer and loop-seal increase linearly with increasing solids inventory in the bed. At a constant solid inventory, pressure drops across the riser and the downcomer increase with increasing solid circulation rate but decrease with increasing gas velocity in the riser. The obtained solid flow rate has been correlated with pressure drop across the loop-seal.     

An analysis of pressure drop fluctuation in a circulating fluidized bed

The characteristics of pressure drop fluctuation in a 5.0 cm I.D.×250cm high circulating fluidized bed with fine polymer particles of PE and PVC were investigated. The measurements of time series of the pressure drop were carried out along the three different axial locations. To determine the effects of coarse particles and relative humidity of air on the flow behavior of polymer powders-air suspension in the riser, we employed deterministic chaos analysis of the Hurst exponent, correlation dimension and phase space trajectories as well as classical methods such as standard deviation, probability density function of pressure drop fluctuation. From a statistical and chaos analysis of pressure fluctuations, the upper dilute region was found to be much more homogenous flow compared to that in the bottom dense region at the same operating conditions. It was also found that the addition of coarse particles and higher humidity of air reduced the pressure fluctuations, thus enhancing flow stability in the riser. The analysis of pressure fluctuations by statistical and chaos theory gave qualitative and the quantitative information of flow behavior in the circulating fluidized bed.     

A model for the pressure balance of a low density circulating fluidized bed

The pressure balance along the solid circulation loop of a circulating fluidized bed equipped with a solid flux regulating device has been modelled and the influence of the pressure balance on the riser behaviour has been predicted.The solid circulation loop has been divided into many sections, where the pressure drop was calculated independently: riser, cyclone, standpipe, control device and return duct. A new theoretical model, that is able to predict the pressure losses in the return path of the solid from the standpipe to the riser, has been built. A new correlation for cyclone pressure loss with very high solid loads has been found on the basis of experimental data.The pressure loss in the riser has been calculated by imposing the closure of the pressure balance, ΣΔP = 0. Once the riser pressure drop had been calculated, the holdup distribution along the riser was obtained by imposing a particular shape of the profile, according to the different fluid-dynamics regimes (fast fluidization or pneumatic transport). In the first case, an exponential decay was imposed and the bottom holdup was adjusted to fit the total pressure drop, in the second case, the height of the dense zone was instead varied.The experimental data was used to develop the sub-models for the various loop sections have been obtained in a 100 mm i.d. riser, 6 m high, CFB. The solid was made of Geldart B group alumina particles. The tests were carried out with a gas velocity that ranged between 2 and 4 m/s and a solid flux that ranged between 20 and 170 kg/m²s. A good agreement was found between the model and experimental data.     

Dynamics of gas-particle flow in circulating fluidized beds

The flow of gas—particle mixtures in a circulating fluidized bed has been studied, probing the flow behavior under both stable and unstable operating conditions. A novel feature of our work is the use of electrical capacitance tomography to image particle distribution over the cross-section at one elevation in the standpipe. In addition, we have also obtained data on pressure profile and aeration rate in the standpipe, particle circulation rate in the circulating fluidized bed and riser gas flow rate under various operating conditions. Here, we report experimental results obtained for two different particles, both belonging to Geldart type A. At low aeration rates, a stable dense phase flow is obtained in the standpipe. At high aeration rates, the flow became unstable, manifesting low frequency oscillations in the flow characteristics. Our results suggest that, under conditions explored in the present study, this instability originates in the standpipe and that any attempt to model it should consider the interaction between the various components of the circulating fluidized bed system.     

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.     

Improvement of continuous solid circulation rate measurement in a cold flow circulating fluidized bed

A method is described to independently estimate the solids velocity and voidage in the moving bed portion of the NETL circulating fluidized bed (CFB). These quantities are used by a device that continuously measures the solids circulation rate. The device is based on the use of a rotating Spiral vane installed in the standpipe of a circulating fluid bed (CFB). Correlations were developed from transient experiments and steady state mass balance data to correct the solids velocity and solids fraction in the standpipe as a function of standpipe aeration rate. A set of statistically-designed experiments was used to establish the need for these corrections and to verify the accuracy of solid circulation rate measurements after correction. The differences between the original and corrected measurements were quantitatively compared.     

Flow phenomena in the exit zone of a circulating fluidized bed

An experimental investigation of the gas and solids flow in a pilot-scale circulating fluidized bed (CFB) cold model with two different abrupt exit configurations (L-shape and extended top) has been carried out. Measurements of axial pressure profiles, high-speed video images of the flow phenomena at the wall as well as local optical probe measurements inside the exit zone are presented. Contrary to published results obtained in bench-scale CFB risers the axial profiles of the apparent solids volume concentration obtained by pressure measurements showed no indication of an increased solids hold-up in the vicinity of the exit, which confirms the conclusion by Pugsley et al. (Can. J. Chem. Eng. 75 (1997) 1001) that this is a scale effect. The local probe measurements showed the well-known core–annulus flow structure prevailing until the riser top. In the vicinity of the exit this flow structure is superimposed by a strong horizontal velocity component directed to the exit duct. In comparison to the conventional L-shaped abrupt exit the extended top does not increase the solids inventory in the riser.     

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.     

Use of piezoelectric pressure transducers to determine local solids mass flux in the riser of a cold flow circulating fluidized bed

A method to determine local mass flux measurements within the riser of a circulating fluidized bed using the rate of impingement of particles on the surface of a piezoelectric pressure transducer is described. Statistically designed experiments with various solids circulation rates and riser gas velocities were conducted in the riser of a cold flow circulating fluidized bed to verify the accuracy of the method. Also, various techniques to relate the impingement rate to mass flux were employed. It is believed that this method delivers results in situations where more standard methods, such as isokinetic sampling, fail.     

循环流化床中颗粒振荡循环现象的实验研究

颗粒的循环特性是循环流化床研究的重点及热点之一,在建立颗粒循环时,颗粒循环由非稳态向稳态过渡。本文在循环流化床实验装置中,采用摄像法和压力检测法研究了不稳定颗粒循环条件下颗粒流动结构和各段压降的变化情况。首次发现了不稳定颗粒循环状态下的颗粒振荡循环现象及其具有的两个特征:下降段、提升段气固流动结构呈周期性变化;下降段压降p_d和提升段压降p_r周期性波动且p_d>p_r和p_dr交替出现。进一步对颗粒振荡循环过程进行受力分析,建立了颗粒振荡周期的计算公式。研究还发现在颗粒藏量一定的条件下,循环气流量或颗粒循环段阀门开度大于某一临界值时才能诱发颗粒振荡循环;颗粒振荡循环周期随着循环气流量的增大而增大,与阀门开度无关。研究结果为动态颗粒循环建立过程的研究奠定了基础,有助于快速地建立颗粒的稳定循环。     

基于气固两相流的双循环流化床提升管压降模型的预测和实验研究

为研究提升管颗粒循环流率对提升管压降的影响,搭建双循环流化床冷态实验系统,采用差压变送器进行提升管轴向区域压降的实验研究。基于提升管不同的颗粒速度计算方法,充分考虑加速区和充分发展区的不同压降机理,建立加速区、充分发展区和整个提升管压降模型,与实验结果比较发现：加速区颗粒速度采用滑移系数方法所得压降与实验值较吻合,在充分发展区进行压降计算时颗粒速度采用滑移速度等于终端速度计算所得结果较精确;在提升管压降计算时可综合考虑加速区和充分发展区适用的压降模型进行计算,可为实际生产运行中采用压差法进行提升管轴向颗粒浓度的分布提供一定参考,为提升管压降的在线监测提供指导。