The flow properties of fluidized solids

Experimental measurements of the non-Newtonian flow properties of a 200-μm-diam. sand fluidized at 1.5 – 3 U_mf in open channel flow are reported.Asymmetry in the velocity profile was found to change with change in the experimental conditions, but it is thought primarily to be a consequence of air maldistribution which is partly generated by the bed flow in the experimental rig.There was evidence of varying degrees of drag across the distributor, from conditions of slip giving rise to a semi-plug flow profile at higher fluidizing velocities and bed flow rates, to conditions where the drag exacted across the distributor at lower solids flow rates exceeds that at the vertical channel walls.Bed flow properties deteriorate as the bed depth increases. This is a consequence of the deleterious effect of bubble growth within the bed.Reasonable agreement was obtained between velocity profiles predicted following Wheeler and Wissler and those measured under conditions where the drag across the distributor and the vertical channel walls was of similar magnitude. Some estimate was made of the relative effects of power law index, channel aspect ratio and slip on the solids flow.     

Moving-bed solids flow between two fluidized beds

A simplified form of the macroscopic momentum balance for two-phase flow is applied to moving-bed solids flow in an inclined pipe connecting two fluidized beds. The resulting equation shows that the gravity force of the material in the pipe is counter-balanced by the pressure drop across the pipe and the wall friction. In order to account for the wall friction, a friction factor analogous to the Fanning friction factor in fluid flow is defined, and a friction factor correlation is established based on the data of Trees [11]. The friction factor is found to be a function of the solids flow rate and the pipe diameter.The flow curves of moving-bed solids were also constructed based on the data of Trees. The resulting straight lines show the power-law type flow behavior of the solids. And from the power law indices, it has been shown that the data fit well with the generalized friction factor correlation by Metzner and Reed [16]. It is also observed that data of different pipe diameters yield separate flow curves, which is unusual for time-independent fluids. However, the possibility that moving-bed solids may be time dependent can be ruled out because data of different pipe lengths do fall on a single line.     

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.     

Lateral mixing of solids in gas—solid fluidized beds with continuous flow of solids

Lateral mixing of solids in a gas—solid fluidized bed with continuous flow of solids can be adequately expressed by the dispersion model. An expression for estimating the lateral dispersion coefficient in such a bed is proposed.     

The influence of distributor structure on the solids distribution and flow development in circulating fluidized beds

The influence of distributor structure on solids distribution is studied in two riser circulating fluidized bed reactors with different distributor structures but similar diameters. Optic fibre probes were used for the measurement of local solids distribution. The axial and radial distribution of solids holdup in the riser with a multi‐tube distributor is more uniform than that with a multi‐orifice distributor. The radial profiles of particle velocity in the riser with the multi‐tube distributor are also more uniform than that with the multi‐orifice distributor. In the riser with the multi‐tube distributor, both gas and particles are distributed more uniformly across the section, so that the flow acceleration is much more uniform and faster. The flow development is much faster and the fully developed region is reached early for the riser with the multi‐tube distributor. The distributor design is an important factor for the design of circulating fluidized bed reactor.     

高颗粒通量循环流化床的构型作用

In order to achieve high solids circulation rate (G_s),an idea of coupling a moving bed to the bottom section of the riser of a circulating fluidized bed (CFB) was proposed and tested.The results from the preliminary study demonstrated that the solids circulation rate in the new-structure bed approached 370 kg·m^-2·s^-1 at superficial gas velocities around 10.5 m·s^-1 for sand particles with an average Sauter mean size of 378 μm.This study was devoted to further justifying the effects of the coupled moving bed by performing comparative studies in two CFBs with conventional configurations.It was shown that the pressure at the riser bottom and the realized solid circulation rate were only about 15 kPa and 230 kg·m^-2·s^-1 in the two conventionally configured CFBs,obviously lower than 25 kPa and 370 kg·m^-2·s^-1 in the moving bed coupled CFB.These verified that the coupled moving bed increased the force driving particles form the particle recycling side into the riser.The study further tested the effect of a few specially designed riser exit configurations,revealing that a smooth riser exit could facilitate solids circulation to increase the solids circulation rate.     

Detailed measurements of flow structure inside a dense gas–solids fluidized bed

Detailed local flow structures are investigated in bubbling and turbulent fluidized bed with FCC particles. The operating conditions ranges from 0.06 to 1.4 m/s. Extensive experiments are carried out using a newly developed optical fiber probe system, which can measure the solids concentration and velocity at multi-points. The results reveal that with increasing U_g, local solids concentrations go through three evolution stages, reflecting a gradual regime transition process. Under all operating conditions, upflowing and descending particles co-exist at all measuring locations. The upflowing particle velocity is strong function of both superficial gas velocity and spatial position. However, the descending particle velocity mainly depends on superficial gas velocity. The bed radial symmetry and the effects of static bed height on the local flow structures are also investigated.     

The distribution of solids between stages in baffled fluidized beds

The partition of solids between stages in baffled fluidized beds has been studied. The bed, 9 ft tall and 11 in I. D., was divided into two or three stages by horizontal perforated plate baffles having open areas ranging from 11.5 to 34%. The spacing between baffles was varied from 1.5 to 3.5 ft. Four fluidized solids were studied and the velocity of the fluidizing air was varied between 0.2 and 2.5 ft/sec. A correlation was developed which enabled the solids holdup in each stage to be predicted.     

多层筛板流化床料层高度均匀性研究

通过测定流化床料层压差,研究了多层流化床顶层进料均匀性、进气及排气方式对多层流化床流化料层高度均匀性的影响。结果表明,布料越均匀,上中二料层高度越接近,且在低气速下易于形成较良好的流化层,但随着气速增大,其影响逐渐减弱;在低气速下进气方式对中下二料层均匀性有较大影响,但随气速增大影响也减弱,底部进气方式更易达到较好的料层均匀性;流化床的排气方式对流化料层基本无影响。因此均匀的布料和较均匀的进气预分布有助于均化各料层高度,并拓宽多层流化床的操作弹性。     

Drying of solids in vacuum fluidized bed

A drying process in a fluidized bed under vacuum conditions is experimentally and theoretically studied. A fluidized bed operating in reduced pressure presents a possibility for a better quality of production, with safer process conditions. A mathematical model is developed for the reduced‐pressure drying process in a fluidized bed. Some parameters were analyzed by comparing experimental results with model predictions. The predictions from the model agreed quite closely with the experimental data. The results showed that a decrease in pressure reduces the interparticular resistance. A good agreement was also found applying the model to data, at atmospheric pressure, from other authors.     

Microstructural aspects of the flow behaviour in a liquid—solids circulating fluidized bed

The local instantaneous and time‐average suspension densities were determined in a 76 mm diameter by 3 m tall liquid‐solids circulating fluidized bed riser using a fibre‐optic probe. Attempts were made to qualify the microflow structure through statistical analysis of the local bed voidage fluctuations obtained under different operating conditions for the first time. The results show that local microflow structure is uniform in the axial direction but non‐uniform in the radial direction with more flow fluctuation near the wall than in the core of the column for a given axial position. The standard deviation and intermittency index tend to increase with increasing solids circulating rates. Comparing with the gas—solids CFB, the liquid—solids CFB shows much more homogeneous flow structure in both the axial and radial microscopic flow behaviours. The microflow behaviours in the conventional liquid—solids fluidization, liquid—solids circulating fluidization and dilute‐phase liquid transport regimes are also characterized by examining the probability distribution and the intermittency index of the solids holdup.     

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.     

流化床中气固均匀分布的失稳现象

流化床因其均匀且剧烈的气固相互作用保证了其优异的流动和传递性能,因而广泛应用于化学工业中。因此,构建定量计算气固均匀分布的失稳临界点既是重要的学术问题又具有工程意义。本文分别使用气相和固体颗粒相的质量分数表示气固分布状态;引入颗粒床层压力载荷（Φ _T）描述分布器输入的规则负熵和固体颗粒床层自身混沌熵产生之间相互作用;由于密相颗粒床层远离平衡态且具有强非线性耗散项,因此需基于普利高津最小超量熵增原理给出气固密相流在并联系统均布状态的失稳临界点（Φ _Tc）：分布器和固体颗粒床层总熵增在气固均布和气固非均布情况下相等;由于并联系统的对称性,可将N单元路径并联系统气固均布稳定性分析简化为判断单元路径压降二阶导数正负;在此基础上讨论了操作参数、固体颗粒性质和分布器结构参数对气固密相床层均布稳定性的影响。此外,通过气体示踪和压力脉动频谱分析在直径为300mm冷模实验验证了颗粒床层压力载荷（Φ _T）对密相气固均布稳定性的影响;同时应用该方法论计算了工业流化床反应器临界床层高度、临界表观气速以及分布器临界阻力系数,指导了操作工况的调整和分布器结构设计,对比分析了改造前后的反应情况。     

流化床内颗粒混合研究

分析了流化床内颗粒混合机理，综合了床内水平方向和垂直方向上颗粒混合，将床内颗粒混合过程分成两部分：一是向上运动的尾迹相和向下运动的乳化相之间的对流交换，二是乳化相内横向扩散。建立了二维的对流扩散模型，数值结果和实验数据吻合。     

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.