A consolidated flow regime map of upward gas fluidization

A new flow regime map, resulting from more fundamental studies on the hydrodynamics and new flow regimes, is proposed in response to more practical reclassifications of the existing regimes with the development of upward gas-solids fluidization systems. The previously reported flow regime maps and flow structures of some widely used fluidized beds are carefully examined. To better reflect the industrial applications, the fast fluidization regime is reclassified as high-density and low-density circulating fluidization regimes. A consolidated flow regime map is then proposed where the flow regimes of upward fluidization expand to include new types of fluidized beds such as circulating turbulent fluidized bed and high-density circulating fluidized bed. The proposed flow regime map consists of six flow regimes: bubbling, turbulent, circulating turbulent, high-density circulating and low-density circulating fluidization, and pneumatic transport. The transitions between the regimes are discussed with new correlations proposed for fluid catalytic cracking type particles. Analysis on the dominating phase in the different types of fluidized beds reveals the dynamic changeover from solids phase continuous in conventional low-velocity batch/“fixed” fluidization operations to gas phase continuous in high-velocity continuous/“moving” fluidization operations and provides more insights to the transitions between the flow regimes for industrial design and practice.     

Investigation of Drying Geldart D and B Particles in Different Fluidization Regimes

Drying of nylon (Geldart D) and expanded polystyrene (Geldart B) particles in fixed and fluidized beds were studied experimentally and theoretically. Fluidized bed dryers are sometimes operated at velocities beyond bubbling fluidization to mitigate against de‐fluidization of surface wet particles. It was found that theoretical analysis using three different drying methods could predict the constant‐drying rate at such velocities and also across the entire fluidization regimes (fixed bed, bubbling, slugging and turbulent fluidization) as long as the bed remains completely fluidized. Results also showed that the theoretical predictions were accurate beyond previously reported velocity limits in a laboratory scale dryer. During bubbling fluidization, the cross flow factor method was used effectively to predict the influence of bubble phase on drying rates. In the falling‐rate period, it is demonstrated that the drying behaviour of nylon at different gas velocities can be characterised by a single normalized drying curve.     

Experimental Methods in Chemical Engineering: Particle Size Distribution by Laser Diffraction—PSD

Particle size is the top cited physical property researchers report in The Canadian Journal of Chemical Engineering and among the top properties in all science disciplines. ^[1] Techniques to measure particle size distribution (PSD) include physical operations like sieving and sedimentation, and spectroscopic techniques like laser diffraction image analysis based on optical and electron microscopy, and elecro‐zone instruments. Here we concentrate on laser diffraction analysis (LDA) and review its basic principles, operations, limitations, uncertainties, and mention how it compares to other techniques. LDA is an instantaneous, user‐friendly, convenient, and non‐destructive method to assess PSD of inorganic powders. It measures the scattering angle and intensity of light after it passes through diluted particle dispersions suspended in either a gas or liquid. The Mie theory is an exact solution to resolve the diffraction intensity of light caused by particles that applies to while the Fraunhoffer approximation applies only to particles greater than 20 m. The 95 % confidence interval of five measurements of 56 m and 0.1 m irregularly shaped polyhedrons was . Based on a bibliometric analysis of LDA of the top 10 000 cited articles in 2016 and 2017, the major research clusters are: particle measurement, powder behaviour, pharmacy, comminution, and adsorption. Future work will continue to introduce more laser sources, combine multiple technologies, implement mobile light sources (dynamic light scattering), and better define characterize irregularly shaped particles.     

Convective heat transfer coefficient for the side-wall in a fixed bed

Fixed beds are widely used in the chemical and process industry due to their relatively simple yet effective performance. Determining the radial heat transfer at the wall in a fixed bed is crucial to predict the performance of columns. Heat transfer parameters often need to be obtained experimentally. Various Nusselt ${\mathrm{Nu}}_w$ versus Reynolds ${Re}_p$ correlations in literature show considerable scatter and discrepancies. The tube-to-particle diameter ratio $\frac{D_{\mathrm{t}}}{D_{\mathrm{p}}}$ and boundary conditions on the particle surface have been understood to affect heat transfer near the wall by virtue of influence on the near-wall porosity and mixing. In this work, a fixed bed consisting of mono-disperse particles is generated via gravity-forced sedimentation modelling utilizing the discrete element method for a $\frac{D_{\mathrm{t}}}{D_{\mathrm{p}}}$ ratio of 3.3. The system is meshed and imported in a computational fluid dynamics (CFD) solver. Fluid inlet velocity is varied to get ${Re}_p\in \left[1,1500\right]$ corresponding to the laminar and turbulent flow regimes. The particles are treated as boundaries with Dirichlet, Neumann, and Robin boundary conditions applied for the closure of energy balance. Another set of simulations is run with particles modelled as solids with varying thermal conductivities ($k_s/k_f$). The heat flux and volume-averaged fluid temperature calculated during post-processing are used to determine the wall heat transfer coefficient and, subsequently, the wall Nu number. Fifteen ${\mathrm{Nu}}_w$ versus ${Re}_p$ correlations are compiled and analyzed. A new semi-empirical correlation for the wall Nusselt number has been developed for a fixed bed packed with monodisperse spheres for $\frac{D_t}{D_p}=3.3$ and results compared with data published in literature. Additionally, the impact of buoyancy effect on the wall Nusselt number has been studied.     

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   

The liquid-gas mass transfer coefficient in open channel flow is correlated to the turbulent kinetic energy at the interface

Mass transfer coefficients at the gas-liquid interface were investigated for different flow configuration systems, a stirred tank reactor and a gravity pipe. Computational fluid dynamics (CFD) simulations were performed for all tested experimental conditions. Since a poorly soluble gas (oxygen) was used, the overall mass transfer coefficient was clearly correlated to the hydrodynamic conditions in the liquid phase. However, a generic correlation between averaged interfacial liquid velocity and mass transfer coefficients was not found for both geometries. Finally, the averaged turbulent kinetic energy (TKE) at the interface is the most relevant parameter that was correlated to the mass transfer coefficient for both systems. The same relationship between oxygen mass transfer coefficient K _L,O2 and TKE () can be applied for the two geometries investigated.     

Nonintrusive characterization of fluidized bed hydrodynamics using vibration signature analysis

There are many techniques to characterize the hydrodynamics of fluidized beds, but new techniques are still needed for more reliable measurement. Bed vibrations were measured by an accelerometer in a gas–solid fluidized bed to characterize the hydrodynamics of the fluidized bed in a nonintrusive manner. Measurements were carried out at different superficial gas velocities and particle sizes. Pressure fluctuations were measured simultaneously. Vibration signals were processed using statistical analysis. For the sake of the evaluation, the vibration technique was used to calculate minimum fluidization velocity. It was shown that minimum fluidization velocity can be determined from the variation of standard deviation, skewness, and kurtosis of vibration signals against superficial gas velocity of the bed. Kurtosis was proved to be a new method of analyzing vibration signals. Results indicate that analyzing the vibration signals can be an effective nonintrusive technique to characterize the hydrodynamics of fluidized beds. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

液固和气液固微型流态化研究进展

在百年流态化的研究过程中,涉及到直径不同的流化床。但是,多以流化床的大型化为研究目标,对微型流化床及其本身特性的研究很少。作为专门处理固体颗粒的流态化单元过程,其装置的微型化将兼具微通道反应器和宏观流化床各自的优点,是流态化研究的重要方向。鉴于气固微型流化床已有全面的国内外进展综述,本文仅对液固和气液固微型流化床的国内外研究进展进行分析。结论性内容包括：液固微型流化床床径减小,壁面效应增强,最小流化液速实验值大于Ergun公式计算值;需对描述液固均匀膨胀流化规律的Richardson-Zaki方程加以修正。气液固微型流化床内存在4种典型流型：半流化、弹状流、分散鼓泡流和液体输送流;由于床径减小,出现半流化状态,依据压降表观液速关系曲线等无法确定最小流化液速;气液固微型流化床的反应性能得以有效提升;最后给出了进一步研究的方向,以期为后续研究提供参考。     

床形结构对气固流化床流化质量和气体返混特性的影响

用实验方法比较了一个二维床和一个大型三维床内FCC颗粒流化床在鼓泡域和湍动域内的流化质量和气体返混特性。实验结果表明,床形对A类颗粒气固流化床具有非常大的影响。二维床和三维床的流动和气固混合行为既具有相似性,如床膨胀随气速的变化趋势;也具有很大的差异性,既包括三维床流化质量差、轴向气体扩散系数大等量上的不同,又包括压力脉动、轴向气体扩散系数的变化趋势以及湾流模式等质上的不同。总之,在本研究中,二维床体现的是一种具有强烈壁效应的小型流化床的特征,而三维床则体现的是静床高度具有很大影响的大型流化床的特征。     

Experimental study on the effects of gas permeation through flat membranes on the hydrodynamics in membrane-assisted fluidized beds

Recently, many novel reactor concepts based on membrane fluidized bed reactors have been proposed. In this work, the effects of gas permeation through flat membranes on the hydrodynamics in a pseudo-2D membrane-assisted gas–solid fluidized bed have been investigated experimentally. A combination of the non-invasive techniques (Particle Image Velocimetry (PIV) and Digital Image Analysis (DIA)) was employed to simultaneously investigate solids phase and bubble phase properties in great detail. Counter-intuitively, addition of secondary gas via the membranes, that constituted the confining walls of a gas–solid suspension at conditions close to incipient fluidization, did not result in a larger, but in a smaller equivalent bubble diameter, while gas extraction on the other hand, resulted in a larger equivalent bubble diameter, although in this case the effect was less pronounced. This could be explained by changes in the larger scale particle circulation patterns due to gas extraction and addition via the membranes.     

Numerical study of the mixing characteristics in an air swirling fluidized bed with monodisperse and multi-sized particles

Powder mixers are used in many industries. In the present work, a new type of air swirling mixer was designed and optimized with eight horizontally arranged inlet pipes at the tangential inlet angle of 35°. The mixing of multi-sized spherical particles (2.0, 3.0, 4.0, and 5.0 mm) was numerically investigated in the air swirling mixer by coupled computational fluid dynamics–discrete element method. The numerical results showed that multi-sized particles achieved comparable mixing performance to monodisperse particles. The Lacey index for multi-sized particles increased initially, and then reached a maximum value at 0.824. The upward velocity of the particles, $v_{\mathrm{z}}$, increased initially, and then decreased to zero along the bed height. The maximum value of $v_{\mathrm{z}}$ occurred at a height of 40 mm. Particle radial velocity was larger near the wall than at the mixer tube centre area. The smallest particles aggregated in three layers. The collision number of the particles reached a maximum at bed height of 120 mm, which was consistent with the position of the maximum stress of the particles against the tube wall.     

流化床反应器过程强化技术

Fluidized beds enable good solids mixing, high rates of heat and mass transfer, and large throughputs, but there remain issues related to fluidization quality and scale-up. In this work I review modification techniques for fluidized beds from the perspective of the principles of process intensification (PI), that is, effective bubbling sup-pression and elutriation control. These techniques are further refined into (1) design factors, e.g. modifying the bed configuration, or the application of internal and external forces, and (2) operational factors, including altering the particle properties (e.g. size, density, surface area) and fluidizing gas properties (e.g. density, viscosity, or velocity). As far as two proposed PI principles are concerned, our review suggests that it ought to be possible to gain improve-ments of between 2 and 4 times over conventional fluidized bed designs by the application of these techniques.     

Effect of bed size on hydrodynamics in 3‐D gas–solid fluidized beds

It is well known that hydrodynamics observed in large scale gas–solid fluidized beds are different from those observed in smaller scale beds. In this article, an efficient two‐fluid model based on kinetic theory of granular flow is applied, with the goal to highlight and investigate hydrodynamics differences between three‐dimensional fluidized beds of diameter 0.10, 0.15, 0.30, 0.60, and 1.0 m, focusing on the bubble and solids flow characteristics in the bubbling regime. Results for the 0.30 m diameter bed are compared with experimental results from the literature. The bubble size evolution closely follows a correlation proposed by Werther for small beds, and a correlation proposed by Darton for sufficiently large beds. The bubble size increases as the bed diameter is increased from 0.10 to 0.30 m, and remains approximately constant for bed diameters from 0.30 to 1.0 m. Concurrently, an increase in bubble rise velocity is observed, with a much high bubble rise velocity in the largest bed of diameter 1.0 m due to gulf stream circulations. The dynamics in shallow and deep beds is predicted to be different, with marked differences in bubble size and solids circulation patterns. © 2015 American Institute of Chemical Engineers AIChE J, 61: 1492–1506, 2015     

Local time-averaged gas holdup in a fluidized bed with side air injection using X-ray computed tomography

Local time-averaged gas holdup in a 10.2 cm diameter fluidized bed is determined using X-ray computed tomography (CT) over a range of superficial gas velocities (U_g), side air injection flow rates (Q_side), and fluidized bed material. Without side air injection, only small variations in the local time-averaged gas holdup are observed for beds composed of glass beads, ground walnut shell, or ground corncob. With the introduction of side air injection, which simulates the immediate volatilization of biomass in a fluidized bed gasifier, a distinctive plume is observed along the reactor wall above the side injection port. The plume gradually expands toward the center of the bed as height increases; the expansion is found to increase with increasing Q_side. As U_g increases, fluidization becomes more uniform and the effect of the side air injection on the fluidization hydrodynamics is less pronounced. Additionally, increasing U_g increases overall gas holdup and bed expansion. Of the three bed materials examined, ground corncob fluidization is the least affected by side air injection and shows the highest overall gas holdup while glass bead fluidization is much more affected by side air injection and has the lowest overall gas holdup. This study demonstrates the usefulness of X-ray CT in noninvasively visualizing detailed internal features of fluidized beds. These results will be used in future studies to validate computational fluid dynamics (CFD) models of fluidized beds.     

气固脉冲流化床流体力学特性的研究

在φ７０ｍｍ的流化床内，采用聚氯乙烯、玻璃珠和不规则天然刚玉等Ｂ类、Ｄ类颗粒，测定了０￣５．０Ｈｚ脉冲频率下气固流化床的基本流体力学特性，探讨了影响床层流化特性的一些主要因素，并根据实验数据对脉冲流化床的临界流化速度和临界流化压降的无因次准数式进行了关联。     

A modified fuzzy clustering framework for catalyst activity monitoring in the selective catalytic reduction system

The catalyst activity monitoring in the selective catalytic reduction (SCR) system is of great importance for safety and economic operation in the power plant. To address the problem, a framework based on clustering considering time delay has been proposed. A compound parameter, $q$, is put forward in this paper as a strategy to remove the influences from gas volume (power output), inlet NO_x concentration, and outlet NO_x concentration to the ammonia amount. A modified entropy-based fuzzy clustering (EFC) method is proposed by a threshold varying model and then tested for its efficiency by four datasets from the University of California, Irvine (UCI) machine learning repository. With the maximum mutual information entropy coefficient (MIC) method for detecting time delay and the modified EFC method, process data from three working levels are handled for automatically obtained clustering centres. The proposed activity value, $\mu$, is then calculated based on 1440 process data before and after the catalyst replacement shown in boxplot figures. The results of the framework are analyzed to be in accordance with the real working conditions, with $\mu$ values and fluctuation ranges starting to fall near first from the 721st sample in the 24th box.     

Influence of weak electrostatic charges and secondary flows on pneumatic powder transport

The dynamics of the transported powder determines the functionality and safety of pneumatic conveying systems. The relation between the carrier gas flow, induced by the flown-through geometry, and the powder flow pattern is not clear yet for electrostatically charged particles. This paper highlights the influence of relatively minor cross-sectional secondary flows and electrostatic forces on the concentration and dynamics of the particles. To this end, direct numerical simulations (DNS) capture the interaction of the continuous and dispersed phases using a four-way coupled Eulerian–Lagrangian strategy. The transport of weakly charged particles in channel flows, where turbopheresis defines the particle concentration, is compared to duct flows, where additional cross-sectional vortices form. For both geometries, the Stokes number ($St=8,32$) and the electrical Stokes number (${\mathit{St}}_{\mathrm{el}}=\left[0,1,2,4\right]\times {10}^{-3}$) are varied, and the turbulent carrier flow was fixed to ${\mathit{Re}}_{\tau }=360$. The presented simulations demonstrate that secondary flows, for the same ${\mathit{Re}}_{\tau }$, $St$, and $S{t}_{\mathrm{el}}$, dampen the effect of particle charge. In a duct flow, vortical secondary flows enhance the cross-sectional particle mobility against the direction of electrostatic forces. Compared to a duct flow, in a channel, the wall-normal aerodynamic forces are weaker. Thus, electrical forces dominate their transport; the local particle concentration at the walls increases. Further, electrostatic charges cause a stronger correlation between the gas and particle velocities. In conclusion, despite being weak compared to the primary flow forces, secondary flow and electrostatic forces drive particle dynamics during pneumatic transport.     

Mixing dynamics in bubbling fluidized beds

Solids mixing affects thermal and concentration gradients in fluidized bed reactors and is, therefore, critical to their performance. Despite substantial effort over the past decades, understanding of solids mixing continues to be lacking because of technical limitations of diagnostics in large pilot and commercial‐scale reactors. This study is focused on investigating mixing dynamics and their dependence on operating conditions using computational fluid dynamics simulations. Toward this end, fine‐grid 3D simulations are conducted for the bubbling fluidization of three distinct Geldart B particles (1.15 mm LLDPE, 0.50 mm glass, and 0.29 mm alumina) at superficial gas velocities U/Umf = 2–4 in a pilot‐scale 50 cm diameter bed. The Two‐Fluid Model (TFM) is employed to describe the solids motion efficiently while bubbles are detected and tracked using MS3DATA. Detailed statistics of the flow‐field in and around bubbles are computed and used to describe bubble‐induced solids micromixing: solids upflow driven in the nose and wake regions while downflow along the bubble walls. Further, within these regions, the hydrodynamics are dependent only on particle and bubble characteristics, and relatively independent of the global operating conditions. Based on this finding, a predictive mechanistic, analytical model is developed which integrates bubble‐induced micromixing contributions over their size and spatial distributions to describe the gross solids circulation within the fluidized bed. Finally, it is shown that solids mixing is affected adversely in the presence of gas bypass, or throughflow, particularly in the fluidization of heavier particles. This is because of inefficient gas solids contacting as 30–50% of the superficial gas flow escapes with 2–3× shorter residence time through the bed. This is one of the first large‐scale studies where both the gas (bubble) and solids motion, and their interaction, are investigated in detail and the developed framework is useful for predicting solids mixing in large‐scale reactors as well as for analyzing mixing dynamics in complex reactive particulate systems. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4316–4328, 2017     

Experimental investigation of a rotating fluidized bed in a static geometry

Rotating fluidized beds in a static geometry are based on the new concept of injecting the fluidization gas tangentially in the fluidization chamber, via multiple gas inlet slots in its cylindrical outer wall. The tangential injection of the fluidization gas fluidizes the particles tangentially and induces a rotating motion, generating a centrifugal field. Radial fluidization of the particle bed is created by introducing a radially inwards motion of the fluidization gas, towards a centrally positioned chimney. Correctly balancing the centrifugal force and the radial gas-solid drag force requires an optimization of the fluidization chamber design for each given type of particles. Solids feeding and removal can be continuous, via one of the end plates of the fluidization chamber.The fluidization behavior of both large diameter, low density polymer particles and small diameter, higher density salt particles is investigated at different solids loadings in a 24 cm diameter, 13.5 cm long non-optimized fluidization chamber. Scale-up to a 36 cm diameter fluidization chamber is illustrated.Provided that the solids loading is sufficiently high, a stable rotating fluidized bed in a static geometry is obtained. This requires to minimize the solids losses via the chimney. With the polymer particles, a dense and uniform bed is observed, whereas with the salt particles a less dense and less uniform bubbling bed is observed. Solids losses via the chimney are much more pronounced with the salt than with the polymer particles.Slugging and channeling occur at too low solids loadings. The hydrostatic gas phase pressure profiles along the outer cylindrical wall of the fluidization chamber are a good indicator of the particle bed uniformity and of channeling and slugging. The fluidization gas flow rate has only a minor effect on the occurrence of channeling and slugging, the solids loading in the fluidization chamber being the determining factor for obtaining a stable and uniform rotating fluidized bed in a static geometry.     

Robust control of discrete minimum and non-minimum phase systems via data-driven virtual reference feedback tuning and IMC

In this paper, we develop a novel robust control approach for discrete minimum and non-minimum phase systems via a combined data-driven virtual reference feedback tuning ($\text{VRFT}$) and internal model control (IMC) scheme. The first step in the conventional $\text{VRFT}$ method controller design is the selection of the closed-loop reference model ($M\left(z\right)$), and $M\left(z\right)$ selection is still an open problem. The integration of the $\mathrm{IMC}$ scheme and the VRFT method provides the advantage of flexibility in controller design due to the incorporation of the $\mathrm{IMC}$ filter. As a result, the proposed design method begins with the selection of $M\left(z\right)$ and $\mathrm{IMC}$ filter. Unlike the standard $\text{VRFT}$ method, the proposed combined $\text{VRFT}$ and $\mathrm{IMC}$ design approach has the unique feature of taking into account a robustness property of dynamics, namely, maximum sensitivity ($M_{\mathrm{s}}$) as the design specification for the $M\left(z\right)$ and IMC filter selection. Moreover, the proposed approach includes a robustness specification that resolves the trade-off between performance and robustness in real-time controller design. Furthermore, the robustness guarantee with plant uncertainties and controller fragility is elucidated. The proposed approach is validated using numerical simulations and experimental validation through the temperature control process. Compared to conventional $\text{VRFT}$ controllers, experimental and simulation results show that the proposed controllers have less tracking error, minimize control effort, and improve robustness.