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.     

INFLUENCE OF THE PARTICLE SIZE AND SUPERFICIAL GAS VELOCITY ON THE SUBLIMATION OF PURE SUBSTANCES IN FLUIDIZED BEDS OF DIFFERENT SIZES

In the present study the sublimation of large solid carbon dioxide particles inside fluidized beds of fine particles is investigated. A model which takes the surface area of the sublimable particles into account is used to describe the sublimation kinetics. Based on this model, the results of different experiments, namely single particle experiments using a precision scale, batch experiments in a laboratory-scale fluidized bed and continuous experiments in a larger circulating fluidized bed are compared. The main focus of the study is to evaluate the influences of the particle size, of the inert bed material, of the bed temperature and of the superficial gas velocity, respectively.     

INFLUENCE OF THE PARTICLE SIZE AND SUPERFICIAL GAS VELOCITY ON THE SUBLIMATION OF PURE SUBSTANCES IN FLUIDIZED BEDS OF DIFFERENT SIZES

Abstract

In the present study the sublimation of large solid carbon dioxide particles inside fluidized beds of fine particles is investigated. A model which takes the surface area of the sublimable particles into account is used to describe the sublimation kinetics. Based on this model, the results of different experiments, namely single particle experiments using a precision scale, batch experiments in a laboratory-scale fluidized bed and continuous experiments in a larger circulating fluidized bed are compared. The main focus of the study is to evaluate the influences of the particle size, of the inert bed material, of the bed temperature and of the superficial gas velocity, respectively.     

气固固循环流化床密相床层压力脉动研究

在不同实验条件下研究了气固固循环流化床密相床层的压力脉动。压力脉动的情况表明：细颗粒的循环及循环量对粗颗粒的流体力学行为有很大影响。气速越低,影响程度越大。     

DSMC-LES方法研究循环流化床内颗粒团聚物的流动特性

采用直接模拟Monte Carlo方法法DSMC）模拟颗粒间的碰撞,采用考虑颗粒脉动流动对气相湍流流动影响的大涡模拟（LES）研究气相湍流.单颗粒运动满足牛顿第二定律,颗粒相和气相相间作用的双向耦合由牛顿第三定律确定.数值模拟垂直管内气固两相上升流动,对管内气相速度和颗粒相速度、浓度以及聚团流动进行分析.研究平均单个颗粒团聚物的存在时间、颗粒团聚物的时间份额和颗粒团聚物的生成频率分布特性,模拟结果与文献的实验结果基本吻合.     

Axial pressure profile in circulating fluidized beds

Design and operation of a circulating fluidized bed requires the knowledge of fluid mechanics. According to heat and mass transfer as well as chemical reactions, the effect of the set superficial gas velocity on the axial pressure profile is of particular interest. The axial pressure profile was measured for a variety of solids, as a function of the superficial gas velocity, in a cylindrical circulating fluidized bed with an inner diameter of 0.19 m and an overall height of 11.5 m. Depending on the solids content and superficial gas velocity, two or one sections can be observed in the plant where the pressure gradient is constant. A pressure profile with one pressure gradient exists only at high gas velocities, so long as the acceleration pressure drop immediately above the gas distributor is negligible. Comparison of measured pressure drops in circulating fluidized beds with those measured in vertical pneumatic conveying led to a state diagram for vertical gas-solid flows. The operation behaviour of different types of circulating fluidized bed plants can be explained with the aid of this diagram.     

提升管与气-固环流床层耦合反应器的流体力学特性及流动模型

针对催化汽油辅助反应器改质降烯烃技术,在一套提升管与气-固环流床层耦合反应器大型冷模实验装置上,研究了上部环流床层的流体力学特性。结果表明,在环流床层与提升管耦合操作的情况下,床层内颗粒环流存在两种推动力,分别为静压差推动力和颗粒喷射推动力;环隙与导流筒之间的整体平均固含率差随导流筒表观气速增加而增加,随颗粒外循环强度增加而降低;颗粒环流速度随导流筒表观气速和颗粒外循环强度增加而增加。通过对环流床层进行动量衡算,建立了提升管与环流床层耦合流动的数学模型,模型平均相对误差在15.95％以内。     

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.     

大型循环流化床锅炉整体动态模型的构成及其求解方法

介绍了构成大型商用循环流化床锅炉整体模型的燃烧系统、汽水系统以及烟风系统模型的建立及综合方法 ,并提出了一种求解整体模型的“双桢速”算法 .采用这种整体模型的构成及综合方法能够有效地求解具有复杂物理化学特性和结构的大型复杂系统模型     

Temperature fluctuations and heat transfer in a circulating fluidized bed

Characteristics of temperature fluctuations and heat transfer coefficient have been investigated in the riser of a circulating fluidized bed (0.102 m ID and 4.0 m in height). Effects of gas velocity and solid circulation rate on the temperature fluctuations, suspension density and heat transfer coefficient between the immersed heater and the bed have been considered in the riser. To analyze the characteristics of temperature fluctuations at the wall of the riser, the phase space portrait and Kolmogorov entropy of the fluctuations have been obtained, and the relation between the temperature fluctuations and the heat transfer coefficient has been examined. It has been found that the heat transfer system becomes more complicated and irregular with decreasing gas velocity and increasing solid circulation rate or suspension density in the riser. The heat transfer coefficient and Kolmogorov entropy of the temperature fluctuations have decreased with increasing the superficial gas velocity, while they have increased with increasing the solid circulation rate or suspension density in the bed. The heat transfer coefficient has been well correlated in terms of the Kolmogorov entropy, suspension density as well as operating variables in the riser. This paper is dedicated to Professor Dong Sup Doh on the occasion of his retirement from Korea University.     

Transition from Low Velocity to High Velocity in a Three Phase Fluidized Bed

The onset liquid velocity demarcating the conventional and the circulating fluidization regimes of three‐phase fluidized beds was determined by measuring the time required to empty all particles in a batch fluidized bed at various liquid and gas velocities. Experiments were performed in a gas‐liquid‐solid circulating fluidized bed of 2.7 m in height using glass beads of 0.508 mm in diameter as solid phase and air and tap water as the fluidizing gas and liquid, respectively. The results show that gas velocity is a strong factor on the onset liquid velocity. Higher gas velocity yields a lower onset liquid velocity. It is also demonstrated that the onset liquid velocity has the same value as particle terminal velocity in a gas‐liquid mixture. Within the gas‐liquid‐solid circulating fluidization regime, the solids circulation rate is increased with the total liquid velocity and the auxiliary liquid velocity.     

Motion of a large object in a bubbling fluidized bed with a rotating distributor

This work studies the effect of a low-frequency rotating distributor on the motion of a large object immersed in a bubbling fluidized bed. The object size and density differ from those of the inert solids that conform the bed. Examples of objects moving in a bubbling fluidized bed include passive particles, catalysts and reactants. The rotation modifies the bed dynamics in the surroundings of the distributor and affects the motion of the object within the bed.A set of experiments was carried out in a lab-scale cylindrical bed, equipped with a perforated plate distributor that can rotate at around 1 Hz, for different bed aspect ratios, gas velocities, and object characteristics. Sizes were far larger than that of the solids of the dense phase and densities ranged from half the bed density to values around it. The experiments were video recorded, capturing the surface of the bed from above.As have often been noted, objects might remain in stagnant regions near the distributor and be “lost” or precluded to circulate. This can be avoided in most practical cases forcing the distributor to rotate. Also, the effect of rotation on the circulation time of the objects is presented, showing a general reduction of large circulation times.     

Detection of singularities in the pressure fluctuations of circulating fluidized beds based on wavelet modulus maximum method

Pressure fluctuations are often selected to analyze the dynamic changes in the gas-solid fluidized bed. In this study, the wavelet modulus maximum method is applied to analyze the pressure fluctuations in a circulating fluidized bed. Continuous wavelet transform is put on the pressure signals, then, maxima lines are extracted and the Lipschitz exponent, α, of every maximum line is calculated using the non-linear least-squares method. The mean of the negative α is adopted to describe the hydrodynamic characteristics in the circulating fluidized bed. The value of the mean of the negative α increases with the superficial gas velocity and turns stable when the flow regime is fast fluidization. This method has been tested effective under various operation conditions.     

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.     

The characteristics of cluster in a high density circulating fluidized bed

The cluster images in a high density circulating fluidized bed (HDCFB) measured by a one-dimensional optical fiber image analysis system are studied. The experimental results show that the characteristics of cluster in the riser vary greatly at different operating conditions. The radial cluster size, the cluster interception time and the probability of cluster appearance increase with bed density and decrease with gas velocity. Based on the analysis of the cluster image at various operating conditions, correlations of the radial cluster size, the cluster interception time and the probability of cluster appearance in a HDCFB are proposed. It is found that they are related to local solids concentration and gas velocity.     

Comparative study of flow structures in a circulating-turbulent fluidized bed

This paper presented a comparison of flow structures in a newly designed circulating-turbulent fluidized bed (C-TFB), which has a high solids holdup and solids flux, with two commonly used fluidized beds reactors (circulating fluidized bed (CFB) and turbulent fluidized bed (TFB)). Experimental results included instantaneous solids concentration and particle velocity, and local solids flux. Results indicated that the flow behavior in the C-TFB possessed both similarities and differences with the TFB and high-density CFB. Many distinct advantages of the C-TFB flows were found.     

隔板式内循环流化床压力脉动信号递归分析

隔板式内循环流化床中流化态及颗粒循环特性对压力脉动信号特征具有重要影响,其作用机制尚未完全清楚。测量了隔板式内循环流化床在不同气速比条件下的压力脉动信号,通过时域及递归分析,获得了压力脉动信号的标准差、递归率、确定性及香农熵等特征参数。结果表明,随着表观气速比的增加,内循环中颗粒循环状态存在未循环、鼓泡循环、过渡循环和湍动循环四个阶段;通过压力脉动信号的标准差、递归图黑白结构占比和递归特征参数可识别这四种循环状态,递归特征参数在不同循环区域内显示出良好的线性关系,可用于识别隔板式内循环流化床系统的循环状态。     

Phase holdup and liquid dispersion in gas-liquid-solid circulating fluidized beds

Axial and radial profiles of gas and solids holdups have been studied in agas-liquid-solid circulating fluidized bed at 140mm i.d..Experimental results indicate that the axialand radial profiles of gas and solids holdups are more uniform than those in a conventionalfluidized bed.Axial and radial liquid dispersion coefficients in the gas-liquid-solid circulating fluidizedbed are investigated for the first time.It is found that axial and radial liquid dispersioncoefficients increases with increaes in gas velocity and solids holdup.The liquid velocity has littleinfluence on the axial liquid dispersion coefficient,but would adversely affect the redial liquiddispersion coefficient.It can be concluded that the gas-liquid-solid circulating fluidized bed hasadvantages such as better interphase contact and lower liquid dispersion along the axial directionover the expanded bed.     

A review of some parameters involved in fluidized bed bioreactors

Three-Phase fluidized bed bioreactors have advantages over conventional chemical reaction systems. There is a lack of agreement over most major operational conditions, and a wide range of design variables are open to question. A large body of recent work in the field has been reviewed, with a degree of historical comparison and discussion. It has been found that aspects of fluidized bed biofilm reactors of vital importance include: choice of solid media, gas and liquid loadings, bacterial type and reactor mechanical design. A large proportion of the work in the field of three-phase fluidization is non-biologically specific, or not tested on a bacterially inoculated system. The majority of three-phase fluidized bed bioreactor work is in the field of water treatment. Although this work has highlighted the potential for use of bio-fluidized beds for this application, there are still specific problems hindering the large scale industrial acceptance of three-phase fluidized bed bioreactors.     

Flow regimes and critical velocity in a circulating fluidized bed

A laboratory scale circulating fluidized bed, 50 mm i.d. 4.97 m high, has been operated with different solid inventories in the downcomer. The operating conditions cover a wide range of superficial gas velocities and solid circulation rates. A critical gas velocity U_cr is defined as the gas velocity beyond which the interface between the dense bed and the dilute bed cannot be observed in the circulating fluidized bed at any solid circulation rate. Three different fluidization regimes exist at gas velocities lower than U_cr; they are: the dilute transport bed, the dense bed and the bed with an interface between the bottom dense bed and the upper dilute freeboard. An additional fluidization regime exists at gas velocities greater than U_cr where no interface can be found at any solid circulation rate. U_cr increases with increasing solid inventory. The height of the interface is significantly affected by the solid inventory, and is also greatly affected by the design of the pneumatic valve. The whole bed becomes a dense bed after the interface extends to the exit region of the bed.