GRAPHICAL SOLUTION OF MULTISTAGE FLUIDIZED BED HEAT EXCHANGER

The design of multistage fluidized beds for heat exchange necessitates the solution of the mass and energy balance equations combined with the equilibrium relations for each stage. This paper presents a novel way of applying well known methodology to a different technology, one where it has not been widely applied.

In the present work a McCabe-Thiele type of graphical approach is presented for both counter-current and cross-current contacting multiple fluidized beds. The necessary equations for a multistage calciner are developed and the application of the proposed method is demonstrated. Generalized fluidized bed efficiency for counter-current and cross-current multiple fluidized bed is presented.     

MATHEMATICAL MODELING OF FLUIDIZED BED HEAT REGENERATORS

Based on the two-phase theory of fluidization, a fairly rigorous model is developed to describe the dynamic behavior of fluidized-bed heat regenerators. This model takes into account the major hydrodynamic aspects of bubbling fluid beds. Based on the assumption that the gas leaves the bed in thermal equilibrium with the solids, a rather simplified model is presented. Exact analytical solutions are obtained for both models. The predictions of the more rigorous model are found to be in good agreement with many experimental observations. On the other hand, the simplified model gives satisfactory results only when the value of the parameter m₁is relatively large. Using the rigorous model, it has been shown that the bed thermal efficiency can be improved by reducing the time of operation, increasing the mass of solids or decreasing the bed aspect ratio. Also it has been found that when the gas flow-rate is increased, the amount of heat transferred to the solids displays a non-monotonic behavior and passes through a maximum.     

HEAT TRANSFER BETWEEN HORIZONTAL TUBES AND FREEBOARD REGION OF A LOW-RANK COAL FLUIDIZED BED COMBUSTOR

Experiments are conducted to measure the heat transfer rates to air-cooled horizontal tubes of diameter 38.1mm in the freeboard region of a pilot-scale fluidized-bed combustor burning North Dakota lignite- Silica sand with diameter ranging from 883 to 973 μim is used as bed material. The tests are carried out without limestone addition and ash recycle, at average bed temperatures ranging from 698 to 1117K, freeboard temperatures of 700 to 1139 K and superficial velocities of 1.74 to 1.86 m/s. Among the existing correlations, those proposed by George and Grace (1982), and Byam et al. (1981) are found to predict the experimental results quite well and their root-mean-square deviations are found to be 15.8 and 18.4 percent respectively. @KEYWORDS: Heat transfer, Fluidized bed Coal combustor     

EFFECT OF OPERATING VARIABLES ON BED TO WALL HEAT TRANSFER IN A CIRCULATING FLUIDIZED BED

Heat transfer coefficients were measured for 227 and 87 micron size particles of silica sand in a 102 mm dia. and 5.5 m tall circulating fluidized bed made of plexiglass. Effects of solid circulation rate, fluidization velocity, bed density and particle sizes were studied. The heat transfer coefficients were calculated from the temperatures measured along the axis of a 25 mm dia 100 mm long cylindrical heat flux meter.     

A THEORETICAL STUDY OF THE LIMITING HEAT TRANSFER COEFFICIENT IN FLUIDIZED BEDS

A new model is presented to describe the heat transfer process occurring in fluidized beds under the limiting condition of very short contact times between the heat transfer surface and the emulsion phase. Unlike other work in this area the proposed model assumes that the particles are in constant motion close to the surface throughout the heat transfer process. The effect of the non-continuum gas film assumption (the Smoluchowski effect) when particles are very close to the surface is found to be of secondary importance when compared with the movement of particles close to the surface. Expressions for both the instantaneous and time averaged heat transfer coefficients for individual and a spatial distribution of parlicles are presented.     

脱硅中液固循环流化床清洁传热

考察了铝土矿熟料溶出粗液脱硅加热过程中的结垢行为,采用有机-无机复合材质的惰性固体颗粒研究了循环流化床对脱硅加热过程中硅渣结垢的清除及防止性能.结果表明：硅渣结垢曲线为具有诱导期的渐近式曲线,硅渣结垢机理为结晶结垢和微颗粒沉积结垢混合机理;循环流化床不仅能有效防止硅渣结垢的形成,而且能完全清除已有的硅渣垢层,硅渣结垢的清除速率随操作流速及固体颗粒浓度的增大而增大;惰性固体颗粒的引入不影响硅渣结垢的机理.根据液固流化床的防垢机理建立了结垢模型,其预报值与实验值吻合较好.     

SIMULATION OF VARIABLE FLOW FLUIDIZED BED HEAT REGENERATORS

The Niederlinski Index (NI) has been used only as a stability condition to date. This paper expands upon the use of the NI and shows that, in addition to the traditional function of the sign of the NI, the size of the NI is, in fact, an overall interaction measure itself, and that it provides important insights into Interaction Measurement (IM) and Variable Pairing (VP) issues. The use of the NI is thus extended to propose a new more comprehensive steady state VP criterion. This criterion is shown to avoid ambiguities present in other VP rules and IMs. Theoretical justification for the new pairing rule is provided with supporting examples.     

复合生物流化床流动特性研究

本文对具有导流简流化床与填料固定床相结合的复合生物流化床的床层流体力学特性、混合特性及传质特性进行了研究。复合床的流型可分为 PPBM,PFBM,PCBM,其中 PCBM 最适于流化操作,通过对不同固体颗粒的实验,对床层压降及摩擦因数、床层平均气含率、气泡下潜深度、液体的循环速率、床层混合特性及气-液传质系数进行了讨论和关联,并得出了相应的计算模型。     

DRYING OF GRANULAR PARTICLES IN A MULTISTAGE INCLINED FLUIDIZED BED WITH MECHANICAL VIBRATION

A mathematical model for the drying rate of granular particles in a multistage inclined fluidized bed(IFB) is presented from the standpoint of simultaneous heat and mass transfer, with taking the effect of mechanical vibration added vertically into consideration.

Steady-state distributions for the temperatures and concentrations of the particles and the heating gas, and for the moisture content of the particles are numerically calculated based on the present model. The calculated results show fairly good agreement with the experimental data, which were obtained from the drying experiments of brick particles in a three-stage IFB using comparatively low temperature air(40-60°C) as the heating gas.

It has been found within the range of the experimental conditions employed that, the mechanical vibration added vertically enhances the over-all drying rate of the particles and its effect can be considered equivalent to an increase in the air velocity.     

快速流化床内探头设置方向对传热系数测量结果的影响

本文在一高8m,直径186mm 的快速流化床中采用小型电加热探头测定了床层与换热面之间的局部传热系数,实验中通过将探头向上和向下设置考察了探头设置方向对局部传热系数的影响,实验表明探头在床中的设置方向不同将对传热系数的测定值产生一定的影响:另外在同一床层浓度下,操作气速和固体循环速率的变化将对传热系数产生直接的影响     

HYDRODYNAMICS AND HEAT TRANSFER OF LIQUID FLUIDIZED BED SYSTEMS

After some genera] remarks about liquid/solid fluidization, various aspects of liquid fluidization hydrodynamics and heat transfer have been analyzed on a coherent basis. For each of these cases, the state-of-the-art has been reported, together with some indication of areas that deserve further attention. Major emphasis is placed on the hydrodynamic behavior of fluidized beds, heat transfer mechanisms from surface-to-bed and on the effect of various parameters on the type of fluidization and heat transfer coefficient. This review covers the various correlations developed over the years for the prediction of bed voidage, heat transfer coefficient and optimum conditions of liquid/solid fluidized beds.     

FLOW BOILING HEAT TRANSFER IN A NEW VAPOR-LIQUID-SOLID THREE-PHASE CIRCULATING FLUIDIZED BED EVAPORATOR

In order to enhance heat transfer and solve the fouling problems in boiling processes, a boiling system was designed by adding solid particles to the boiling liquid. In this paper, both theoretical analyses and experimental studies on the flow boiling heat transfer in a vapor-liquid-solid three-phase circulating fluidized bed evaporator were carried out. Based on the analysis of the heat transfer characters of this three-phase flow boiling, a mathematical model for predicting the flow boiling heat transfer coefficients of the vapor-liquid-solid three-phase circulating fluidized bed evaporator was developed. The experiments show that, in the presence of solid particles the flow boiling heat transfer is enhanced and is about 2 times that of the vapor-liquid two-phase one. The predicted results of the flow boiling heat transfer coefficients agreed well with the experimental data.     

离心流化床中强制对流换热的实验研究

对离心流化床干燥器中气体与被干燥颗粒物料之间的强制对流换热进行了实验研究,获得了各主要运行参数对气固换热系数的影响规律,并利用场协同原理分析了对流换热强化的机理. 实验证明,在一定转速范围内,在气流速度方向和热流方向（温度梯度方向）一致时,换热的准则关联式具有Nu＝CRePr的形式. 获得了满足Nu～RePr呈直线关系的Pe(Pe=RePr)数变化范围和临界Pe数,当Pe数大于临界值后,离心流化床中对流换热强度随Pe数增加而增大的趋势会明显减缓并偏离线性区.     

循环床非流化回料管内竖管传热的强化

非流化移动床内颗粒的流动为柱塞流，颗粒间无相对运动，竖管传热主要靠颗粒间导热。由于导热热阻大，在管壁周围的颗粒吸热后不易传出，而在其周围形成一个热边界层，阻止了传热。利用在竖管上加上的扰流圈，使近壁处颗粒发生扰动，而使热颗粒传出，冷颗粒得以进入管壁周围，从而起到强化传热的作用。强化后的平均传热系数可增加２０％左右。平均传热系数值可达２２０Ｗ／（ｍ￣２·Ｋ）。     

流化床连续聚合反应器出料状态分析

研究了粗颗粒树脂加压流态化时气固混和物自由出料过程的物理模型，建立了出料气含率与流化床结构参数、操作条件之间的关系。计算表明出料气含率的理论预测值和测量值一致。运用此结论进一步计算了流化床乙烯聚合反应生产聚乙烯工业过程中乙烯的原料单耗值，为工业生产指标的分析计算提供了理论依据     

喷液流化床中传热特性

引言流化床反应器由于其特有的众多优点,尤其是传热效果好、床层温度均匀,而在工业上得到广泛的应用．然而随着工业的发展,生产能力的提高,对流化床的换热能力提出了更高的要求．于是出现了许多强化流化床传热的方法,例如乙烯氧氯化反应中的内置换热管法;乙炔制醋酸乙烯中的外加夹套法;催化裂化反应中的粒子循环法．在80年代初期出现了一种流化床带液操作的新方法,它在气相法聚乙烯生产中的应用（其操作工艺被简称为冷凝工艺）具有远大的前景．该工艺是在一般的气相法聚乙烯流化床反应工艺的基础上,在流程中添加易挥发的液体,使反…     

STOCHASTIC BEHAVIOR OF FLUIDIZED PARTICLES IN A LIQUID-SOLID FLUIDIZED BED

In a liquid-solid fluidized bed, the apparently irregular or stochastic behavior of particles gives rise to various flow regimes depending on parameters such as the particle size, liquid flow rate, static bed height and axial position in the bed. It is highly plausible that this irregular behavior manifests itself as pressure fluctuations; thus, the effects of these parameters on the particle behavior or the particle flow regime were investigated through measurement and spectral analysis of the pressure fluctuations. The results indicate that the amplitude of pressure fluctuations exhibits a maximum and that the decay constant in the autocorrelation function attains its minimum at the intermediate liquid flow rate where the particle flow regime undergoes transition from the cluster circulation to the individual quasi random motion. The model composed of the periodic and stochastic components of pressure fluctuations, is in good accord with the experimental results in terms of both the autocorrelation and power spectral density functions.     

EFFECT OF HIGH VOIDAGE ON MASS TRANSFER COEFFICIENT IN A FLUIDIZED BED

Experiments were carried out to determine the gas-solid mass transfer coefficient for the air-naphthalene system under fast-fluidized conditions. The data were obtained under conditions of high voidage (as high as 98%) and high gas velocities (0·5 m/s-2·2 m/s). The results indicate that the correlation for the mass-transfer coefficient in a gas-solid fluidized bed under low voidage conditions established by previous investigators can be extended to the fast fluidization regime. Significantly lower mass transfer coefficients were, however, obtained under high voidage compared to low voidage conditions.     

用于二甲醚催化裂解的下行式循环流化床

从天然气或煤制合成气等非石油资源（经由二甲醚）制取低碳烯烃的工艺中，二甲醚催化裂解制烯烃是决定性的步骤．采用新型的并流下行循环流化床（CDCFB），能较好地解决反应热的导出，以维持反应温度的均衡，保持反应的高转化率与高选择性，并可使反应和再生操作能连续平稳地进行本文提出了一种用于实施二甲醚催化裂解反应的CDCFB结构形式，并用实验考察了整套装置的可操作性及各种操作与结构参数对催化裂解工艺的适应性，测定了反应再生系统的流体力学性能，为热态试验装置提供了设计依据