不同充气条件下密相输运床返料系统气固流动数值模拟

采用计算颗粒流体力学对密相输运床返料系统内的气固流动行为进行了数值模拟,分析了曳力模型和颗粒最大堆积浓度等参数对模拟结果的影响,确定了合适的模型参数。通过对比3组工况的模拟结果,获得了与实验结果基本一致的立管压力分布和固体循环流率随充气条件的变化规律,并分析了立管内压力梯度分布、气体流动方向、颗粒浓度分布等。结果表明立管充气口处压力梯度绝对值为局部最大值;当立管充气口气量为零时,会使充气口上方一段距离的压力梯度绝对值较小;充气量增大到一定值时会在充气口附近形成明显的气泡。当缺少立管高位充气时,会导致立管下部区域形成大的压力梯度,增加颗粒下落阻力。充气松动颗粒的作用仅对充气口附近区域有一定影响,更大的作用是在立管内形成均匀的压力梯度分布,使立管内气固流动状态保持上下一致。在制定充气方案时,应根据固体循环流率确定立管压降,补充合适气体量以维持气体下行速度均衡,使得各段的平均压力梯度相同。     

The characteristics of particle flow in the overflow and underflow standpipe of fluidized beds

Characteristics of particle flow in the standpipes of a 10 cm I.D.×120 cm high fluidized bed were investigated. The standpipes used in this experiment were vertical overflow and vertical underflow standpipes. Sand particles and polyethylene powders were employed as the bed materials. The effects of standpipe diameter, gas velocity and particle properties on the solid flow rate were determined. The experimental results showed that the flow behaviors of solids through the overflow and underflow standpipes are different with variations of operating conditions. For both standpipes, the mass flow rate of solids was strongly dependent on the standpipe diameter. For the overflow standpipe, the increase of gas velocity increased the solids flow rate. But for the underflow standpipe it decreased the solid flow rate. From the measured pressure drops, solid fractions in the standpipes were determined by the momentum balance. The obtained experimental data of solids mass flow rate were well correlated with the pertinent dimensionless groups for underflow as well as overflow standpipes.     

非对称结构气动分配阀调节特性冷态实验

气动分配阀是在循环流化床双回路系统中使用的返料与分流装置,其返料与分流的调节特性直接影响循环流化床双回路系统的运行。通过冷态实验,研究了气动分配阀的各控制风流量以及来料流率对调节特性的影响,结果表明:气动分配阀的调节区域可以划分为截止区、调节区、近饱和区、饱和区,这些区域的调节特性不同;在调节区,通过调节气动分配阀的控制风流量,可改变双侧返料量的比例;在低于单侧最大返料流率时,各侧返料流率随来料流率的增加而线性增加。     

影响FCC再生立管输送催化剂影响因素的分析

再生立管是FCC装置再生器和提升管反应器之间再生催化剂循环的输送管,其操作复杂性在于立管内催化剂的流态受多种因素影响。本文中在1.0Mt/a FCC装置上,通过测量立管改造前后不同操作条件时的轴向压力分布,考察再生立管输送催化剂的影响因素。生产运行结果表明,影响立管操作的主要因素包括催化剂密度和平均粒径、立管几何结构、滑阀安装位置、松动风性质和流量等;选用低密度催化剂和高黏度流化介质可以减小气泡尺寸,维持反应温度稳定;松动风流量应根据立管推动力、滑阀压降和反应温度及时调整,避免填充流。另外,立管结构和滑阀的安装位置对立管推动力影响较大,分析结果可供立管设计和装置改造参考。     

气固循环流化床负压差下料立管的压力脉动特性

通过对气固循环流化床负压差下料立管的压力测量,分析了立管下料过程的压脉动特性。负压差下立管内的颗粒由立管上部的低压端流向下部的高压端的过程中,对所夹带的气体产生压缩导致了立管内气固两相流动的不稳定性,表现为立管压力的脉动以及颗粒下行速度的波动和密度分布的不均匀。压力脉动的幅度随负压差的增加而增大。     

Intermixing of fluidizing gas streams in a compartmented circulating fluidized bed

The intermixing of fluidizing gas between the compartments of a compartmented circulating gas fluidized bed fitted with a V-valve and riser combination has been experimentally studied. The intermixing of fluidizing gas that flows co-current (cross flow) as well as counter-current (back flow) to the circulating solids has been investigated. At low aerations, gas cross flow increased proportionally with the increase in solid circulation rate. But at high aerations, gas cross flow increased with aeration even when solid circulation decreased. Studies on back flow of gas revealed some interesting observations. At high fluidizing bed height on upstream side back flow of V-valve, gas has been negligible. But at low upstream bed heights and low aeration in V-valve and riser back flow of V-valve, gas has been substantial.     

Downflow of solids in standpipes

General two-phase theory is applied to the flow of gas-solid mixtures in standpipes. It is shown that at a fixed flow rate both fluidized bed flow and packed bed flow can coexist in the same standpipe. Matsen's analysis of standpipe flow is extended and a general method for determining flow pattern and gas entrainment rate in standpipe flow is described. Data from a commercial standpipe in a fluid bed catalytic cracking unit are analysed. The analysis shows that fluidized bed flow occurs in the standpipe in support of Matsen's work on standpipe flow in another fluid bed catalytic cracking system.Flow instability, in standpipe is discussed in the light of this work and a mechanism of flow instability based on transition in flow pattern is postulated.     

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.     

含内构件的循环流化床的动态压力特性

针对含内构件的循环流化床,以石英砂为物料,使用动态压力传感器测量了含内构件的流化床中气固两相流的动态压力,分析了床内的瞬时压力特性. 结果表明,在进出口总压降中,文丘里压降最大,占主床压降的60%以上. 表观气速和固体颗粒循环流率共同影响循环流化床内的压力特性. 压力瞬时波动功率谱分析表明,压力波动对应一个主频,表观气速越小、颗粒循环流率越大时,压力波动越大,且循环流化床底部压力波动比上部大. 加入内构件能有效引导气流,使流动更均匀.     

COMPREHENSIVE STUDY OF PRESSURE-DROP BALANCE AND PERFORMANCE OF AN L-VALVE IN A CIRCULATING FLUIDIZED BED

Pressure-drop balance and L-valve performance in a circulating fluidized bed (CFB) were investigated for variation in gas superficial velocity (U_g), secondary air fraction (SA), total solids inventory (I_s), and external aeration flow rate (Q_A), at ambient temperature. The study shows that the solids discharge rate of an L-valve (G_s) changes with the system control variables. Apparently, a wider range of G_s is achieved at increasing U_g, decreasing SA, increasing I_s, and increasing Q_A. The pressure-drop analysis indicates that these characteristics are directly governed by the loop pressure balance. Standpipe bottom pressure is found to be an important indicator of the operating status of the riser, standpipe, and L-valve. The experimental results confirm that an L-valve can be operated effectively by regulating the standpipe bottom pressure. The knowledge obtained from this study is essential for maintaining the performance of a CFB system for combustion operation, as well as for operating circulating fluidized beds on a practical scale.     

Solid Recycle Characteristics of Loop‐seals in a Circulating Fluidized Bed

Solid recycle characteristics through a conventional and a newly developed loop‐seal (0.08 m i.d.) system are determined in a circulating fluidized bed of FCC or silica sand particles. In the loop‐seal developed here, gas was injected downward tangentially to the wall of the loop‐seal to increase solids mass flux with stable flow. For conventional loop‐seal, solids mass fluxes increase linearly with increasing aeration rate but it reaches a maximum value. At the same aeration rate with different aeration locations (0.1 – 0.6 m) in a conventional loop‐seal, a maximum solids mass flux is seen at a height to diameter ratio of 2.5. For the newly developed loop‐seal, mass fluxes of FCC and sand particles are higher and more stable than in conventional loop‐seal at the same aeration rate. The solid mass fluxes obtained have been correlated with the aeration rate and Archimedes number.     

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.     

负压差立管插板阀调节颗粒质量流率的实验分析

在大型循环流化床装置上,通过改变负压差立管上的插板阀的开度,测定不同颗粒质量流率条件下插板阀上下的流态和脉动压力,以此分析插板阀对下料过程颗粒质量流率的调控机制。实验结果表明,对于立管的下料过程插板阀的开度存在一个临界开度,将插板阀的开度范围划分为非可控制区和可控制区。当插板阀的开度大于临界开度时,处于非可控制区,不能进行颗粒质量流率的调节,此时插板阀上下的流态一致,脉动压力曲线相似;当插板阀的开度小于临界开度时,处于可控制区,可以进行颗粒质量流率的调节,此时插板阀上的流态是移动床,阀下的流态是雨状下落流,上下的脉动压力曲线不同。     

Drying Kinetics in the Riser of Circulating Fluidized Bed with Internals

Solid particles were dried in the riser of circulating fluidized bed with internals to study the drying kinetics. Experiments were conducted in a circulating fluidized bed, having perforated plates as internals covering wide range in the operating parameters. The effects of various operating parameters, i.e., initial moisture content, temperature, and flow rate, of the heating medium and solid circulation rate on the rate of drying have been critically examined. It has been observed from the present investigation that the presence of internals enhances the solids holdup in the riser of circulating fluidized bed. The drying efficiency of a circulating fluidized bed with internals has been compared with the drying performance of a circulating fluidized bed without internals under the same operating conditions.     

Drying Kinetics in the Riser of Circulating Fluidized Bed with Internals

Solid particles were dried in the riser of circulating fluidized bed with internals to study the drying kinetics. Experiments were conducted in a circulating fluidized bed, having perforated plates as internals covering wide range in the operating parameters. The effects of various operating parameters, i.e., initial moisture content, temperature, and flow rate, of the heating medium and solid circulation rate on the rate of drying have been critically examined. It has been observed from the present investigation that the presence of internals enhances the solids holdup in the riser of circulating fluidized bed. The drying efficiency of a circulating fluidized bed with internals has been compared with the drying performance of a circulating fluidized bed without internals under the same operating conditions.     

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.     

Introduction to papers presented at the 8th annual fine particle society conference,Chicago, August, 1976

Industrial standpipes for gas—solid recirculation systems are often installed with slide valves at their lower end for control of solid flow and pressure balance. By combining the equations for gas—solid dense phase flow with equations for flow of fluidized solids through an orifice, a set of equations is presented for standpipe design. The equations permit quantitative design of a standpipe and can be used to predict the effects of terminal pressure, slide valve opening position and aeration rate on gas and solid circulation rates, flow pattern and pressure profile in standpipes.     

Comparisons of particle cluster diameter and concentration in circulating fluidized bed riser and downer using computational fluid dynamics simulation

The information of particle cluster dynamics is necessary for improving the performance of a circulating fluidized bed system. The main objective of this study is to compare the particle cluster diameters and concentrations from computational fluid dynamics simulation results between circulating fluidized bed riser and downer. The calculation methodologies are based on the concept of kinetic theory of granular flow and statistics. The mathematical model was verified by using the experimental dataset from literature and used for computing the particle cluster dynamics. In the circulating fluidized bed riser and downer, a dense and dilute core-annulus flow structures were obtained, respectively. The particle cluster in the circulating fluidized bed riser possessed more heterogeneity movements than that in the circulating fluidized bed downer. This can be explained by the system flow direction. About the particle cluster dynamics, the particle cluster diameters and concentrations in the circulating fluidized bed riser were higher than the ones in the downer. The calculated values were comparable to the empirical correlations. This confirms the validity of the calculation methodologies. Particle cluster dynamics and its example application inside circulating fluidized bed riser and downer were also discussed.     

Investigation on gas–solid flow regimes in a novel multistage fluidized bed

Gas–solid flow regime in a novel multistage circulating fluidized bed is investigated in this study. Pressure fluctuations are first sampled from gas–solid flow systems and then are analyzed through frequency and time–frequency domain methods including power spectrum and Hilbert–Huang transform. According to the flow characteristics obtained from pressure fluctuations, it is found that the gas–solid motions in the multistage circulating fluidized bed exhibit two dominant motion peaks in low and high frequencies. Moreover, gas-cluster motions become intensive for the multistage circulating fluidized bed in comparison with the fast bed. Unlike the traditional methods, the fuzzy C-means clustering method is introduced to objectively identify flow regime in the multistage circulating fluidized bed on the basis of the flow characteristics extracted from bubbling, turbulent, fast, and multistage fluidized beds. The identification accuracy of fuzzy C-means clustering method is first verified. The identification results show that the flow regime in the multistage circulating fluidized bed is in the scope of fast flow regime under examined conditions. Moreover, the results indicate that the consistency of flow regime between two enlarged sections exists. In addition, the transition onset of fast flow regime in the multistage circulating fluidized bed is higher than that in the fast bed.     

Solid concentration and velocity distributions in an annulus turbulent fluidized bed

Solid concentration and particle velocity distributions in the transition section of a?200 mm turbulent fluidized bed (TFB) and a?200 mm annulus turbulent fluidized bed (A-TFB) with a?50 mm central standpipe were mea-sured using a PV6D optical probe. It is concluded that in turbulent regime, the axial distribution of solid concen-tration in A-TFB was similar to that in TFB, but the former had a shorter transition section. The axial solid concentration distribution, probability density, and power spectral distributions revealed that the standpipe hin-dered the turbulence of gas–solid two-phase flow at a low superficial gas velocity. Consequently, the bottom flow of A-TFB approached the bubbling fluidization pattern. By contrast, the standpipe facilitated the turbulence at a high superficial gas velocity, thus making the bottom flow of A-TFB approach the fast fluidization pattern. Both the particle velocity and solid concentration distribution presented a unimodal distribution in A-TFB and TFB. However, the standpipe at a high gas velocity and in the transition or dilute phase section significantly affected the radial distribution of flow parameters, presenting a bimodal distribution with particle concentration higher near the internal and external wal s and in downward flow. Conversely, particle concentration in the middle an-nulus area was lower, and particles flowed upward. This result indicated that the standpipe destroyed the core-annular structure of TFB in the transition and dilute phase sections at a high gas velocity and also improved the particle distribution of TFB. In conclusion, the standpipe improved the fluidization quality and flow homogeneity at high gas velocity and in the transition or dilute phase section, but caused opposite phenomena at low gas ve-locity and in the dense-phase section.