Demarcation of a new circulating turbulent fluidization regime

Transient flow behaviors in a novel circulating‐turbulent fluidized bed (C‐TFB) were investigated by a multifunctional optical fiber probe, that is capable of simultaneously measuring instantaneous local solids‐volume concentration, velocity and flux in gas‐solid two‐phase suspensions. Microflow behavior distinctions between the gas‐solid suspensions in a turbulent fluidized bed (TFB), conventional circulating fluidized bed (CFB), the bottom region of high‐density circulating fluidized bed (HDCFB), and the newly designed C‐TFB were also intensively studied. The experimental results show that particle‐particle interactions (collisions) dominate the motion of particles in the C‐TFB and TFB, totally different from the interaction mechanism between the gas and solid phases in the conventional CFB and the HDCFB, where the movements of particles are mainly controlled by the gas‐particle interactions (drag forces). In addition, turbulence intensity and frequency in the C‐TFB are significantly greater than those in the TFB at the same superficial gas velocity. As a result, the circulating‐turbulent fluidization is identified as a new flow regime, independent of turbulent fluidization, fast fluidization and dense suspension upflow. The gas‐solid flow in the C‐TFB has its inherent hydrodynamic characteristics, different from those in TFB, CFB and HDCFB reactors. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

气液固三相循环流态化

<正>由于三相流化床反应器在石油化工、湿法冶金、环境工程和煤的液化等工业领域得到越来越广泛的应用,近年来对传统的三相流化床进行了大量的基础研究并取得了很大进展。以往的研究大部分是针对低液速(u_l相似文献   

大颗粒三相环隙气升式环流反应器流体力学行为

研究了大颗粒体系气升式环流反应器的流体力学行为,考察了表观气速和颗粒质量分数对床层膨胀高度、循环液速和固含率分布的影响。实验结果表明,按颗粒的运动状态不同可以将反应器内的流动分为3个区域,即固定床区域、膨胀床区域和循环床区域,各流动区域内的流动行为存在显著差异。随着颗粒质量浓度的增大,起始流化气速和最小循环气速均显著增大。基于三相流化床的流化模型和环流反应器的特点建立了相应的数学模型,对大颗粒三相气升式环流反应器的起始流化气速和最小循环气速进行了预测,模型预测值与实验测量值吻合良好。     

气-液-固三相逆流化床动力学研究

在内径0.152 m,高2.5 m的气-液-固三相逆流化床中系统研究了动力学特性。获得了气体和液体速度及聚乙烯和聚丙烯颗粒密度对各相含率和最小液体流化速度的影响规律。研究发现随着气体速度的增加,液体最小流化速度降低;随着气体或液体速度增加,气体、液体和固体含率均增加。     

液固循环流化床的研究(Ⅰ)相含率及颗粒循环速率

在内径为140mm、高为3m的有机玻璃设备中对液固并流向上的循环流化床中两相流动特性进行了研究.相含率的研究表明,在循环流态化条件下,全床相含率轴向均匀分布;而径向具有显著的不均匀性,表现为中心区液相含率高,在r/R=0.7处液合率最低.颗粒循环速率的研究表明,在循环流态化区域,在一定的二次水流速下,颗粒循环速率不随总液速的改变而改变.由于在循环流态化下存在床层径向参数的不均匀性,因此床层的相合率与颗粒循环速率的关系与广义流态化方法预测的结果间存在差异,即在操作条件一定的情况下,床层内真实液合率比由广义流态化预测的低.     

Hydrodynamics of an inverse liquid–solid circulating conventional fluidized bed

An inverse liquid–solid circulating conventional fluidized bed (I-CCFB) is realized by injecting particles from the top of a conventional liquid–solid fluidized bed (0.076 m ID and 5.4 m height) that is operated in a newly developed circulating conventional fluidization regime located between the conventional and circulating fluidization regimes. The I-CCFB can achieve a higher solids holdup compared to both conventional and circulating liquid–solid fluidized beds. A new parameter, the bed intensification factor, is defined to quantify the increased solids holdup observed with external solids circulation. The Richardson–Zaki equation is shown to be applicable to the I-CCFB regime and can be used to correlate the slip velocity and solids holdup, both of which increase with the solids circulation rate. A new flow regime map is presented, including the I-CCFB and a variety of other liquid–solid fluidized beds.     

Hydrodynamic behaviour of a magneto airlift column in a transverse magnetic field

The hydrodynamics of a three‐phase airlift reactor of magnetic particles has been investigated in the presence of a transverse magnetic field. Experiments were carried out in two modes: applying the magnetic field to a static bed then increasing the field flow, and applying the magnetic field to a fluidized bed then increasing the magnetic field intensity. In magnetizing the first mode and parallel to the increasing gas superficial velocity, several bed regimes were observed, including: initial packed, stabilized, and fluidized beds. On the other hand, in magnetizing last mode and while increasing the magnetic field intensity, the fluidized bed changes from a fluidized to a stabilized to frozen bed. Bed expansion before the onset of fluidization increases as the magnetic field intensity increases. Minimum fluidization velocity was found to be strongly dependent on the magnetic field intensity and the minimum stabilization intensity was also strongly dependent on the gas velocity. The magnetic field intensity also affects the bed expansion hysteresis and the liquid circulation velocity. A photocell was used to measure the water circulation rate in the downcomer of the reactor.     

液固循环流化床的研究(Ⅱ)表观滑落速度及曳力系数

在本文(Ⅰ)报实验研究的基础上,进一步探讨了液固循环流化床中的两相流动特性.液固两相表观滑落速度的实测结果表明,实验结果与广义流态化方法的预测值间存在较大的差异.表观曳力系数的研究表明,实验回归的表观曳力系数与文献中的传统预测值间也存在较大的差异,进一步体现出由于状态参数的径向不均匀性带来的循环流态化与散式流态化间的差别.     

液固循环流化床的研究(Ⅱ)表观滑落速度及曳力系数

在本文(Ⅰ)报实验研究的基础上,进一步探讨了液固循环流化床中的两相流动特性.液固两相表观滑落速度的实测结果表明,实验结果与广义流态化方法的预测值间存在较大的差异.表观曳力系数的研究表明,实验回归的表观曳力系数与文献中的传统预测值间也存在较大的差异,进一步体现出由于状态参数的径向不均匀性带来的循环流态化与散式流态化间的差别.     

(Gas‐)liquid‐solid circulating fluidized beds and their potential applications to bioreactor engineering

Compared with conventional fluidized beds, circulating fluidized beds have many advantages including better interfacial contacting and reduced backmixing (Lim et al., 1995). While there are many reports on the gas—solid circulating fluidized systems, liquid—solid and gas—liquid—solid circulating fluidized bed systems have been scantily studied. However, extending current knowledge obtained in gas—solid systems to liquid—solids and gas—liquid—solid three‐phase systems is shown to open new horizons for applications of circulating fluidized bed technology and expected to lead to the development of highly efficient liquid—solid and gas—liquid—solid reactors, especially for the ever growing field of biotechnology. In order to fully appreciate the potential of those two types of liquid phase circulating fluidized beds, recent progress is reviewed in this article. Their potential applications to biochemical processes are also discussed.     

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.     

The onset velocity of a liquid–solid circulating fluidized bed

The critical transition velocity, U_cr, previously defined by Liang et al. [W.-G. Liang, S.-L. Zhang, J.-X. Zhu, Y. Jin, Z.-Q. Yu and Z.-W. Wang, Flow characteristics of the liquid–solid circulating fluidized bed, Powder Technol., 90 (1997) 95–102.] to demarcate the liquid–solid conventional and circulating fluidization regimes, was found to vary with the total solids inventory and the solids feeding system. In this work, an onset velocity for circulating fluidization regime, U_cf, is proposed to give the lowest U_cr value and to provide a convenient demarcation velocity that is independent of system geometry. This liquid velocity is obtained by measuring the time required to empty all particles in a batch operated fluidized bed under different liquid velocities. This method can be used for a wide range of particles and involves less influence of the operating conditions such as the solids inventory and the solids feeding system. Compared to the critical transition velocity, this newly defined onset velocity is a more intrinsic parameter, only dependent on the liquid and particle properties. Based on the experimental results obtained in this work and other published results, the influence of particle properties and equipment setup on the onset velocity is also discussed.     

三相流化床的流体力学特性研究

利用空气、水和玻璃珠颗粒为工作介质,对三相流化床流态化操作的流体力学特性进行了研究,主要研究了气体速度、液体速度、颗粒投加量与气含率的关系,以及气体速度与床层高度及气泡直径的关系。     

气-固微型流化床压降特性及最小流化速度的实验研究

在内径3～20 mm的4个气-固微型流化床中,分别考察了A类和B类两种类型颗粒的流化特性,同时研究了床几何结构、操作条件、物相性质等各因素对其最小流化速度的影响.结果 表明,气-固微型流化床中的床层压降特性与颗粒类型密切相关,不同的流动状态下两种类型颗粒的流动特性存在显著地差异.在固定床阶段,与B类颗粒相比,A类颗粒与...     

Phase hold-up and critical fluidization velocity in a three-phase inverse fluidized bed

We studied the hydrodynamic characteristics of a three-phase inverse fluidized bed made of a transparent acrylic column of 0.115 m inner diameter and 2 m heights. Air, water and polyethylene particles were used as the gas, liquid and solid phase, respectively. We used both hydrophobic low density polyethylene (LDPE) and hydrophilic LDPE as solid phase, and distilled water as liquid phase, and filtered air as gas phase. The LDPE was chemically treated by chlorosulfonic acid to change the surface property from hydrophobic to hydrophilic. We tried to solely investigate the effect of the surface hydrophilicity of polymeric particles on the phase holdup and the critical fluidization velocity of three-phase inverse fluidization. Thus, we measured the static pressure and eventually observed critical fluidization velocity. Critical fluidization velocity became smaller in case of using MDPE hydrophobic particles than LDPE hydrophilic particles. This was thought to be due to the retardation of rising bubbles near hydrophobic particles and, subsequently, the increase of gas hold-up.     

Phase distributions in a gas–liquid–solid circulating fluidized bed riser

The distributions of the three phases in gas–liquid–solid circulating fluidized beds (GLSCFB) were studied using a novel measurement technique that combines electrical resistance tomography (ERT) and optical fibre probe. The introduction of gas into a liquid–solid circulating fluidized bed (LSCFB), thus forming a GLSCFB, caused the increase of solids holdup due to the significantly decreased available buoyancy with the lower density of the gas, even with a somewhat increased liquid velocity due to the decreased liquid holdup giving space for the gas holdup. The gas passed through the riser in the form of bubbles, which tended to flow more through the central region of the riser, leading to more radial non‐uniformity in radial holdup of the phases. The gas velocity has the most significant effect on the gas phase holdup. While the gas velocity also has an obvious effect to the solids holdups, the liquid flow rate had a much more considerable effect on the phase holdups. The solids circulation rate also had a significant effect on the phase holdups, with increasing solids circulation rate causing much more increased solids holdup in the central region than close to the wall. A correlation was developed for the relative radial distributions of solids holdup in GLSCFB, as such radial profiles were found similar over a wide range of operating conditions, like those in a typical gas–solid circulating fluidized beds (GSCFB). Finally, the axial solids profiles in a GLSCFB was found to be much closer to those in an LSCFB which are very uniform, than those found in a GSCFB which are less uniform and sometime having a S shape. Water was used as the continuous and conductive phase, air was the gas phase and glass bead and lava rock particles were used as the solid and non‐conductive phase.     

Characteristics of a Pressurized Gas‐Solid Magnetically Fluidized Bed

The hydrodynamic, heat and mass transfer characteristics of a pressurized co‐current gas‐solid magnetically fluidized bed (MFB) were systematically investigated considering major influence factors, such as magnetic field strength, superficial gas velocity, and operating pressure. It was shown that this pressurized gas‐solid MFB has the advantages of a wider operation range of the superficial gas velocity under bubble‐free particulate fluidization, a larger bed voidage with smaller pressure drop across the bed, and larger heat transfer efficiency, compared with a conventional fluidized bed. Moreover, the minimum bubbling velocity, gas‐solid mass, and heat transfer coefficients were correlated at high accuracy within the investigated range of operating conditions.     

Macroscopic properties of liquid-solid circulating fluidized bed with viscous liquid medium

Experiments were conducted in a liquid-solid circulating fluidized bed to study the effect of liquid viscosity and solids inventory on pressure gradient, critical transitional liquid velocity, onset average solids holdup, axial solids holdup distribution, average solids holdup and solids circulation rate in circulating fluidization regime with riser operated in fixed inventory mode. The results indicate that critical transitional liquid velocity decreases with increase in liquid viscosity. The onset average solids holdup, on the other hand, increases with increase in either auxiliary liquid velocity or solids inventory. The variation of axial solids holdup distribution, average solids holdup and solids circulation rate with liquid viscosity when solid inventory was 0.15 m was dissimilar with either 0.25 m or 0.35 m solid inventory. Correlations were proposed for estimating the average solids holdup and are satisfactorily compared with experimental values.     

Prediction of the Behavior of a Liquid‐Fluidized Bed of Inert Particles Used for Separation by Density

A liquid‐fluidized bed of inert particles was used to separate a pure object from a mixture. One (binary solid‐liquid‐fluidized bed) or two (tertiary solid‐liquid‐fluidized bed) types of objects with relatively large‐sized particles were immersed in an inert‐particle bed, and the bed behavior was observed for different liquid velocities. The void fraction and apparent density of the inert‐particle suspension were predicted by considering the effect of the change in object position for different liquid velocities. The prediction method, which considers the change in the minimum fluidization velocity, accurately expressed the changes in the void fraction and the apparent density of the bed with the position of the objects in the bed. Using this method, the liquid velocity required to separate a certain kind of object from a mixture can be predicted.     

Clustering behaviour in gas–liquid–solid circulating fluidized beds with low solid holdups of resin particles

The flow in a gas–liquid–solid circulating fluidized bed is self‐organised and manifests itself with clustering of particles and bubbles. The clustering behaviour in the fluidized bed at low solid holdups of resin particles was experimentally investigated with a high‐speed image measurement and treatment technique of complementary metal oxide semiconductor to enhance the fundamental understanding on such a flow. Several new physical quantities were suggested to characterise such ordered flow structures. The main findings are as follows. The clusters of solid particles largely exist as doublets and triplets, the mixed groups of particles and bubbles mostly exist as one bubble carrying two to four particles. Increasing superficial liquid velocity, particle diameter or density weakens the aggregation degrees of both particle and mixed clusters in the riser and downer, except that the increase of superficial liquid velocity enhances the mixed clustering behaviour in the riser. The climbing of the auxiliary liquid velocity or liquid phase viscosity intensifies the aggregation behaviour, except that the increase of liquid phase viscosity reduces the mixed clustering degree in the riser. The influences of superficial gas velocity and surface tension of liquid phase on the clustering behaviour seem to be a little complex and the trends are not simply increasing or decreasing. The life cycle of solid particle clusters in the GLS riser is not sensitive to the operation conditions, being around 0.07 s. The mixed clusters' life cycle is more sensitive to the conditions and physical properties of phases, changing from 0.02 to 0.07 s.