Particle holdup in a liquid–solid circulating fluidized bed

An experiment was conducted to acquire a set of systematic data of particle holdup in risers of a liquid–solid circulating fluidized bed. In the experiment, two kinds of riser were provided, their inner diameter being 24 mm and 36 mm, respectively. Tested particles were of glass and ceramics, having a diameter range from 2.10 to 4.95 mm. Water at ambient conditions was used as the fluidizing liquid. Particle holdup was measured using a shut‐off method. Based on the experimental data, a correlation for predicting the particle holdup was derived, which could reproduce almost all experimental data with an accuracy of ±15%. The effect of the wall was not recognized within the experimental range, i.e., the diameter ratio of particle to riser is less than 0.2. The independent parameters affecting the flow characteristics of liquid–solid circulating fluidized beds were identified. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(3): 184–196, 2008; Published online in Wiley InterScience ( www. interscience.wiley.com ). DOI 10.1002/htj.20194     

Theoretical performance analysis of the multi‐stage gas–solid fluidized bed air preheater

The multi‐stage fluidized bed can be used to preheat the combustion air by recovering the waste heat from the exhaust gas from industrial furnaces. The dilute‐phase fluidized bed may be formed to exclude the excessive pressure drop across the multi‐stage fluidized bed. But, in this case, the solid particles do not reach to the thermal equilibrium due to relatively short residence time in each layer of fluidized bed. In this study, a theoretical analysis on the dilute phase multistage fluidized bed heat exchanger was performed. A parameter related to the degree of thermal equilibrium between gas and solid particles at the dilute‐phase fluidized beds was derived. Using this parameter, a relatively simple expression was obtained for the thermal efficiencies of the multi‐stage fluidized bed heat exchanger and air preheater. Copyright © 2001 John Wiley & Sons, Ltd.     

Numerical study of different gas–solid flow regimes effects on hydrodynamics and heat transfer performance of a fluidized bed reactor

In this study, a two‐?uid Eulerian–Eulerian model has been carried out applying the kinetic theory of granular flow (KTGF) to study the hydrodynamics and heat transfer behavior of a fluidized bed reactor simultaneously. The effects of different gas–solid flow regimes on the operating conditions and heat transfer rate between the hot air and two types of low and high‐density inert particles are investigated in a fluidized bed dryer. Different gas–solid flow regimes for wood and glass particles of groups A, B, and D of Geldart's classification are simulated to introduce the most optimal flow regime in terms of heat transfer rate and operating costs. The compromise between the heating rate, the height required for the reactor, and the ratio of the final mass to the initial mass of solid particles, which specifies the need for a cyclone separator showed that the bubbling regime of Geldart B powder for low‐density particles and the turbulent regime of Geldart D powder or bubbling regime of Geldart B powder for high‐density particles are the optimal operating conditions and flow regimes. Furthermore, it was concluded that the convective heat transfer is the dominant mechanism, which increases with increasing the air velocity and decreasing the particle diameter in each group.     

Eulerian simulations of bubble behaviour in a two‐dimensional gas–solid bubbling fluidized bed

In the present study, the CFD model is based on a two‐fluid model extended with the kinetic theory of granular flow. The simulation results of bubble diameter and bubble rise velocity are compared to the Darton equation and the Davidson model in a free bubbling fluidized bed. The predicted values are in reasonable agreement with the values from the Darton bubble size equation and the Davidson model for isolated bubbles. It is shown that the break‐up and direct wall interaction effects influence the dynamic bubble behavior in the free bubbling fluidized beds. Copyright © 2002 John Wiley & Sons, Ltd.     

Investigation of the bubble behaviour at the free surface of a large three-dimensional gas fluidized bed

Gas fluidization is generally associated with the formation of bubbles that critically influence the performance of fluidized bed processes (FBPs). Therefore, in the design, simulation and operation of FBPs, it is very essential to know the behaviour of the bubbles at the free surface. The size and growth of bubbles play an important role for determining properties such as bed expansion, solids entraiment, in-bed heat transfer and solid mixing. This paper presents a study on the behaviour of bubbles at the free surface of a large three dimensional gas-fluidized bed with square section of 61×61 cm². Measurements were carried out to determine the effects of bed height and excess air velocity on the bubble eruption diameter, frequency and bubble fraction. All experiments were performed at freely bubbling mode and the flow characteristics of bubbles were recorded by a video camera. Bed materials used were 593 μm raw perlite and 1233 μm sand falling within the categories of Geldarts Groups B and D, respectively. The fixed bed height ranged from about 8–18 cm for raw perlite and 9–26 cm for sand. The excess air velocity was varied between 0·5 and 1·75 cm s⁻¹ for raw perlite and 13 and 25 cm s⁻¹ for sand. Equations related to the bubble count, frequency, flow area shape factor and through-flow coefficient were given using a modified form of two-phase theory of fluidisation. Observations were made to validate the two-phase theory for two different particles. The flow area shape factor was in the range of 0·47–0·81 for raw perlite and 0·20 to 0·57 for sand, with mean values of 0·6 and 0·4, respectively. The through-flow coefficient was found to be between −0·68 and 2·82 for raw perlite and between 3·27 to 15·87 for sand, and was larger than predicted values of classical bubble models. © 1998 John Wiley & Sons, Ltd.     

Estimation of bed expansions in a freely-bubbling three-dimensional gas-fluidized bed

Knowledge of bed expansions is important in the design and operation of gas–solid fluidized beds. This paper presents a study on the estimation of expanded bed height in a large three-dimensional gas-fluidized bed with a square section of 0·61×0·61 m². All experiments were performed at the freely bubbling mode and the bed expansions were recorded by a video camera. Bed materials were used 593 μm raw perlite and 1233 μm sand falling within the categories of Geldart's Groups B and D, respectively. The bed height at minimum fluidization ranged from 0·0398 to 0·3176 m, while the excess air velocity from 0·034 m s⁻¹to 0·7453 m s⁻¹. Equations related to the bed expansion were given using a modified form of two-phase theory of fluidization. A correlation for the average bed expansion (void fraction) was also presented that has been derived from the principal form found successful in gas–liquid systems as follows: R=0·5482 d^−0·129_p(U_o−U_mf)^0·111 with an average deviation of less than 1%. The experimental findings were compared with previously reported results and were discussed in the light of available correlations. © 1998 John Wiley & Sons, Ltd.     

Pressure drop in liquid–solid circulating fluidized beds

An experiment was conducted to obtain data for pressure drop in risers of liquid–solid circulating fluidized beds, where two different risers (24 mm and 36 mm in diameter) were used. Tested particles were of glass and ceramics, and their diameter ranged from 2.10 to 4.95 mm. Water under ambient conditions was used as the fluidizing liquid. Pressure drop in the risers was measured using a reversed U‐tube manometer. Based on the experimental data, a calculation method was derived for predicting the pressure drop in the riser, which could reproduce almost all the data with an accuracy of ±20%. Furthermore, the effect of independent parameters was investigated on the pressure drop component ratios consisting of the total pressure drop. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20241     

Heat transfer characteristics of liquid–solid circulating fluidized beds

An experiment was conducted to obtain heat transfer data in liquid–solid circulating fluidized beds. In the experiment, two kinds of risers were provided, their inner diameter being 24 mm and 12 mm, respectively. Tested particles were of glass and ceramics, having a diameter range from 2.10 to 4.95 mm. Water at ambient conditions was used as the fluidizing liquid. The experimental data showed a trend where the heat transfer coefficient increases gradually with increasing liquid velocity approaching that for a liquid single‐phase flow (“heat transfer enhanced region”), and finally coincides with that for a liquid single‐phase flow (“liquid single‐phase heat transfer region”). The heat transfer coefficient in the heat transfer enhanced region was found to be a function of the slip velocity between liquid and particles. Based on the experimental data, a correlation was proposed for predicting the heat transfer coefficient in the entire region from the heat transfer enhanced region to the liquid single‐phase heat transfer region, which could reproduce the experimental data with an accuracy of ±15%. The proposed correlation agreed well with existing data. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(3): 127–137, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20200     

CFD modeling of hydrodynamic and heat transfer in fluidized bed reactors

In this study, a gas–solid fluidized bed reactor has been simulated applying CFD techniques in order to investigate hydrodynamic and heat transfer phenomena. Reactor model predictions were compared with corresponding experimental data reported in the literature to validate the model. The results indicate that considering two solid phases, particles with smaller diameters have lower volume fraction at the bottom of the bed and higher volume fraction at the top of the bed. In addition, it was revealed that bed expansion was larger when a bimodal particle mixture was applied compared with the case of mono-dispersed particles. Gas and solid phase temperature distributions in the reactor were also computed, considering the hydrodynamic of the fluidized bed and the heat generated by the solid particles. The results showed that gas temperature increases as it moves upward in the reactor due to the heat of polymerization reaction leading to the higher temperatures at the top of the bed.     

Flow characteristics of circulating gas–solid fluidized bed heat exchanger with multiple vertical pipes

Flow characteristics of a circulating gas–solid fluidized-bed heat exchanger with multiple vertical pipes were studied. The glass beads were circulated inside the vertical pipes of the heat exchanger with fluidizing air. The pressure drop and the circulation rate of solid particles were measured. In addition, one-dimensional velocity distribution of solid particles and the pressure distribution inside the vertical pipe were analysed. The prediction on the pressure drop with the circulation of solid particles was proved to be reasonably accurate by comparing with the measured results. © 1998 John Wiley & Sons, Ltd.     

Analysis of particle motion and heat transfer in circulating fluidized beds

In this study, the forces affecting the motion of particle clusters near the wall of a CFB were theoretically analysed. The motion trajectory and the contact time of clusters were determined from the proposed model for two cases, steel ball having density of 6980 kg m^?3 and sand having density of 2500 kg m^?3. Computational results showed that the construction and operational parameters such as the bed equivalent diameter, the gas velocity and the bed temperature have great influence on the contact time of clusters. Based on analysis of the contact time of clusters, a theoretical model was developed for predicting the particle–gas convection heat transfer coefficient. The results were compared with experiments and were a quite agreement with the measured data in the open literature which suggests that the theoretical analysis conducted in this work can very well describe the convection heat transfer in a CFB. Copyright © 2004 John Wiley & Sons, Ltd.     

Analogy between pressure drop and heat transfer in liquid–solid circulating fluidized beds

An analogy was found between the frictional pressure drop and the heat transfer in liquid–solid circulating fluidized beds. This investigation is based on the predicting correlations for the particle holdup, the heat transfer coefficient, and the pressure drop, which were all developed by the authors. When the heat transfer coefficients were expressed in terms of the modified j‐factor, then a close mutual relationship was observed between the modified j‐factor and the friction factor of the pressure drop due to liquid and particle flow. A correlation to express this mutual relationship was derived, which consists of the density ratio of particle to liquid and the non‐dimensional riser diameter. The heat transfer coefficient predicted from the derived correlation agreed well with the experimental data by the authors, and with existing data. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20246     

Characteristics of particle mixtures in a liquid‐fluidized bed

Behavior of particle mixtures was investigated in a liquid‐fluidized bed experimentally. In the experiment, two kinds of particles of different diameters, and of the same or different materials, were charged in a fluidization column and fluidized by water. Based on the observation of fluidization, a flow pattern map was proposed using the Archimedes number ratio and the terminal velocity ratio, which classifies the two patterns, that is, separation and homogeneous mix. Measured mean void fractions of particle mixtures agreed well with the values calculated from those for each particle independently. © 2001 Scripta Technica, Heat Trans Asian Res, 30(3): 175–184, 2001     

A CFD approach on simulation of hydrogen production from steam reforming of glycerol in a fluidized bed reactor

Hydrogen production from steam reforming of glycerol in a fluidized bed reactor has been simulated using a CFD method by an additional transport equation with a kinetic term. The Eulerian–Eulerian two-fluid approach was adopted to simulate hydrodynamics of fluidization, and chemical reactions were modelled by laminar finite-rate model. The bed expansion and pressure drop were predicted for different inlet gas velocities. The results showed that the flow system exhibited a more heterogeneous structure, and the core-annulus structure of gas–solid flow led to back-mixing and internal circulation behaviour, and thus gave a poor velocity distribution. This suggests the bed should be agitated to maintain satisfactory fluidizing conditions. Glycerol conversion and H₂ production were decreased with increasing inlet gas velocity. The increase in the value of steam to carbon molar ratio increases the conversion of glycerol and H₂ selectivity. H₂ concentrations in the bed were uneven and increased downstream and high concentrations of H₂ production were also found on walls. The model demonstrated a relationship between hydrodynamics and hydrogen production, implying that the residence time and steam to carbon molar ratio are important parameters. The CFD simulation will provide helpful data to design and operate a bench scale catalytic fluidized bed reactor.     

A study of solid circulation rate in a circulating fluidized bed

An experimental investigation under cold condition was made to study the effects of some operating/design parameters and non-mechanical L valve configuration on the solid circulation rate in a 4.5 m tall, 0.15 m diameter circulating fluidized bed with riser flow rate varying from 1400 litres/min to 2000 litres/min and bed inventory from 15 kg to 25 kg of sand of average sizes 200 μm, 400 μm and 500 μm. Solid circulation rate was estimated by measuring velocity of sand particle travelling through a vertical Perspex tube section at the bottom of the return leg. It was found to be in the range of 2.8 to 12.3kg/m²s, 0.07 to 9.1kg/m²s and 0.12 to 2.23kg/m²s for sand sizes of 200 μm, 400 μm and 500 μm, respectively for a horizontal L valve. Two mathematical correlations have been developed from the experimental results to predict solid circulation rate as a function of riser flow rate, aeration flow rate, total bed inventory and particle size used.     

35t_h沸腾锅炉的动态数学模型及其数字仿真

本文通过机理分析建立了DG35/39—7型锅炉的分段集中参数数学模型,并在数字计算机上进行了仿真,将得到的主要阶跃响应曲线与现场相同型号沸腾锅炉的试验结果进行了比较和分析。表明所建模型是正确的。仿真结果可用于这种型号沸腾锅炉自动控制系统的设计及参数整定时参考。     

An analysis for compressible flows in a packed bed with metathesis gas–solid reaction

Taking into consideration the mass exchange between gas and solid, and the density change of gas mixture due to reaction, a model is proposed for compressible flows in a packed bed with isothermal metathesis gas–solid reaction aA(g) + bB(s) = cC(g) + dD(s), When a ≠ c, the ratio c/a greatly affects the flows of gas mixture, but for a = c, this coupled model reduces to the non-coupled model. One-dimensional numerical solutions show that the velocity profiles obtained from coupled model greatly differ from that from the non-coupled model. The radius of pellets can change the velocity variation trend. The concentration profiles obtained from this coupled model also differ from that from the non-coupled model. A coupled model accounting for only mass exchange between gas and solid, but neglecting the density change of gas mixture due to reaction will produce an extra term, which results in a great deviation of the velocity and concentration from those from the more comprehensive coupled model.     

Heat transfer on tube bundles embedded horizontally in a liquid‐fluidized bed

Heat transfer on tube bundles embedded horizontally in a liquid‐fluidized bed was investigated experimentally. In the experiment, a total of 5 kinds of tube bundles in an equilateral triangular staggered arrangement, including a single tube, was used. Tested particles were of glass and ceramics, and their diameter range was from 2.1 to 6.0 mm. It was found that the distribution of local heat transfer coefficients around a tube depends not on the kind of particles, but on the tube pitch only, when a good fluidizing condition is maintained. Based on the experimental data, a new method was proposed to predict average heat transfer coefficient, which can be applicable for tube bundles having a tube pitch to diameter ratio of 1.2 to infinity (single tube). © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(2): 85–98, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20048     

Effect of pressure on thermal aspects in the riser column of a pressurized circulating fluidized bed

In the present paper the effect of pressure on bed‐to‐wall heat transfer in the riser column of a pressurized circulating fluidized bed (PCFB) unit is estimated through a modified mechanistic model. Gas–solid flow structure and average cross‐sectional solids concentration play a dominant role in better understanding of bed‐to‐wall heat transfer mechanism in the riser column of a PCFB. The effect of pressure on average solids concentration fraction ‘c’ in the riser column is analysed from the experimental investigations. The basic cluster renewal model of an atmospheric circulating fluidized bed has been modified to consider the effect of pressure on different model parameters such as cluster properties, gas layer thickness, cluster, particle, gas phase, radiation and bed‐to‐wall heat transfer coefficients, respectively. The cluster thermal conductivity increases with system pressure as well as with bed temperature due to higher cluster thermal properties. The increased operating pressure enhances the particle and dispersed phase heat transfer components. The bed‐to‐wall heat transfer coefficient increases with operating pressure, because of increased particle concentration. The predicted results from the model are compared with the experimentally measured values as well as with the published literature, and a good agreement has been observed. The bed‐to‐wall heat transfer coefficient variation along the riser height is also reported for different operating pressures. Copyright © 2005 John Wiley & Sons, Ltd.     

75 t/h电站循环流化床锅炉燃烧和污染物排放的数值模拟

以FLUENT软件为工具，运用数值模拟方法对75t/h电站循环流化床锅炉的炉内过程进行研究。计算和分析了炉内的温度分布，氧气、二氧化碳和一氧化碳的浓度分布，燃料颗粒的轨迹，氮氧化物(NOx)的排放。数值模拟的结果对循环流化床锅炉的设计和实际运行有一定的指导意义和参考价值。