Characterization of powder flow: Static and dynamic testing

Many characterization techniques are available to determine the flow properties of powders; however, it is debated which method(s) are the most appropriate. In this study, sample fine powders with a medium particle size between 22 and 31 µm were characterized using a variety of techniques that tested powders under different stress states, ranging from static to dynamic. It was found that characterization techniques that were more dynamic such as fluidized bed expansion were best suited for predicting the fluidization performance while characterization techniques that were more static such as cohesion were better for predicting agglomeration. It was also found that results from static and dynamic characterization do not necessarily agree, where fine powders that showed good fluidization performance also displayed increased agglomeration, and vice versa. This suggests that flow properties are dependent upon the stress state and that no single technique is suitable for the full characterization of a powder. In other words, both static and dynamic characterization techniques must be employed to completely understand the flow properties of a powder and predict how it will behave under different process conditions.     

Effect of agglomerate properties on agglomerate stability in fluidized beds

Fluidized bed agglomeration is used to stabilize particulate mixtures and reduce dust emissions. This technology is applied to a variety of production processes for the pharmaceutical, chemical, fertilizer and food industries. In most of these applications, agglomerate stability is an essential criterion. Agglomerates and granules that do not conform to size and shape specifications may create problems in downstream processes, such as tableting, thus compromising process efficiency and product quality. When an agglomerate is formed in a fluidized bed, it can grow by incorporating other bed particles, split into smaller fragments, or be eroded by fluidized bed solids. The objective of the present study is to determine the critical agglomerate liquid content at which the rates of agglomerate growth and shrinkage are balanced when artificial agglomerates made from glass beads and water are introduced into a fluidized bed. This study examined the effects of agglomerate size, agglomerate density, liquid viscosity, binder concentration, and fluidizing gas velocity on the critical initial liquid content. This study found that small agglomerates and low density agglomerates displayed higher critical initial moisture contents. When the viscosity was increased by using sugar solutions, agglomerates were very stable and had very low critical initial moisture contents. The study also found that as the superficial gas velocity increased, the agglomerates started to fragment, rather than erode.     

Solids mixing in the riser of a circulating fluidized bed

Gas/solid and catalytic gas phase reactions in CFBs use different operating conditions, with a strict control of the solids residence time and limited back-mixing only essential in the latter applications. Since conversion proceeds with residence time, this residence time is an essential parameter in reactor modelling. To determine the residence time and its distribution (RTD), previous studies used either stimulus response or single tracer particle studies.The experiments of the present research were conducted at ambient conditions and combine both stimulus response and particle tracking measurements. Positron emission particle tracking (PEPT) continuously tracks individual radioactive tracer particles, thus yielding data on particle movement in “real time”, defining particle velocities and population density plots.Pulse tracer injection measurements of the RTD were performed in a 0.1 m I.D. riser. PEPT experiments were performed in a small ( I.D.) riser, using ¹⁸F-labelled sand and radish seed. The operating conditions varied from 1 to 10 m/s as superficial velocity, and 25- as solids circulation rate.Experimental results were compared with fittings from several models. Although the model evaluation shows that the residence time distribution (RTD) of the experiments shifts from near plug flow to perfect mixing (when the solids circulation rate decreases), none of the models fits the experimental results over the broad (U,G)-range.The particle slip velocity was found to be considerably below the theoretical value in core/annulus flow (due to cluster formation), but to be equal at high values of the solids circulation rate and superficial gas velocity.The transition from mixed to plug flow was further examined. At velocities near U_tr the CFB-regime is either not fully developed and/or mixing occurs even at high solids circulation rates. This indicates the necessity of working at U> approx. ( to have a stable solids circulation, irrespective of the need to operate in either mixed or plug flow mode. At velocities above this limit, plug flow is achieved when the solids circulation rate . Solids back-mixing occurs at lower G and the operating mode can be described by the core/annulus approach. The relative sizes of core and annulus, as well as the downward particle velocity in the annulus (∼U_t) are defined from PEPT measurements.Own and literature data were finally combined in a core/annulus vs. plug flow diagram. These limits of working conditions were developed from experiments at ambient conditions. Since commercial CFB reactors normally operate at a higher temperature and/or pressure, gas properties such as density and viscosity will be different and possibly influence the gas-solid flow and mixing. Further tests at higher temperatures and pressures are needed or scaling laws must be considered. At ambient conditions, reactors requiring pure plug flow must operate at and . If back-mixing is required, as in gas/solid reactors, operation at and is recommended.     

CFD simulation of hydrodynamics of gas–solid multiphase flow in downer reactors: revisited

In the present work, a k₁–ε₁–k₂–k₁₂ two-fluid model based on the kinetic theory of granular flow (KTGF) was employed to predict the flow behavior of gas and solids in downers, where the particles of small size as 70 μm in diameter apparently interact with the gas turbulence. The turbulence energy interaction between gas and solids was described by different k₁₂ transport equations, while the particle dissipation by the large-scale gas turbulent motion was taken into account through a drift velocity. Johnson–Jackson boundary condition was adopted to describe the influence of the wall on the hydrodynamics. The simulation results by current CFD model were compared with the experimental data and simulation results reported by Cheng et al. (1999. Chem. Eng. Sci. 54, 2019) and Zhang and Zhu (1999. Chem. Eng. Sci. 54, 5461). Good agreement was obtained based on the PDE-type k₁₂ transport equation. The results demonstrated that the proposed model could provide good physical understanding on the hydrodynamics of gas–solid multiphase flow in downers. Using the current model, the mechanism for formation and disappearance of the dense-ring flow structure and the scale-up characteristics of downers were discussed.     

Gas–liquid interfacial areas in three-phase fixed bed reactors

A simple model, based on the theory of liquid emulsions, is proposed to develop correlations suitable for estimating gas–liquid interfacial areas for the bubble flow regime in three-phase fixed bed reactors operated with gas and liquid flowing cocurrently upflow. The model is also employed for trickle-bed reactors for estimating the increase in gas–liquid interfacial areas when these reactors are operated under high pressure conditions. It is found that developed correlations account satisfactorily for most of the data of gas–liquid interfacial areas available in the literature for cocurrent upflow three-phase fixed bed reactors and for trickle-bed reactors operated at high pressure conditions.     

CFD simulation of dilute phase gas-solid riser reactors: Part I—a new solution method and flow model validation

A three-dimensional simulation of a dilute phase riser reactor (solid mass flux: ) is performed using a novel density based solution algorithm. The model equations consisting of continuity, momentum, energy and species balances for both phases, are formulated following the Eulerian-Eulerian approach. The kinetic theory of granular flow is applied. The gas phase turbulence is accounted for via a k-ε model. An extra transport equation describes the correlation between the gas and solid phase fluctuating motion. The solution algorithm allows a simultaneous integration of all the model equations in contrast to the sequential multi-loop solution in the conventional pressure based algorithms, used so far in riser simulations. The simulations show an unsteady behaviour of the flow, but a core-annulus flow pattern emerges on a time-averaged basis. The abrupt nature of the T type outlets causes a significant recirculation of gas and solid from the top of the riser. The flow near the outlets is highly non-symmetric and has a three-dimensional character. A significant decrease of the gas phase turbulence and particle granular temperature across the riser length is attributed to the presence of small particles, which is qualitatively consistent with the experimental data from literature.     

Comparison of Two‐Fluid and Discrete Particle Modeling of Dense Gas‐Particle Flows in Gas‐Fluidized Beds

Both two‐fluid models embedding the kinetic theory of granular flow for particulate phase stress (TFM) and discrete particle models (DPM) are widely used for the numerical simulation of gas fluidization. In this study, a detailed comparison between results obtained from both TFM and DPM is reported, including axial and radial solid concentration profiles, solids circulation patterns, pressure drop and its standard deviation and granular temperature. It was shown that good agreement can be obtained even in cases of low restitution coefficient, which suggests the possible applicability of kinetic theory of granular flow beyond its nominal range of validity and clearly indicates that the continuum treatment of the solids phase in TFM provides a good approximation of its discrete nature.     

From bubbling to turbulent fluidization: Advanced onset of regime transition in micro-fluidized beds

Membrane fluidized bed reactors have been proposed and demonstrated as an effective reactor concept for ultrapure hydrogen production with integrated carbon dioxide capture. Recent experimental studies have shown that the hydrogen permeation rate through the membranes and the mass transfer rate from the bubble phase to the emulsion phase are the two main limiting factors in this type of reactors. To this end, we propose the concept of a micro membrane fluidized bed reactor (MMFBR) as a possible method to remove those two limitations. The idea of the MMFBR is that a significantly larger membrane area per unit reactor volume can be accommodated, thereby removing the limitation of the hydrogen permeation rate through the membranes. Furthermore, we numerically show with discrete particle simulations that the onset of turbulent fluidization is advanced significantly in a MMFBR, which allows the bed to be operated at the turbulent fluidization regime at a relatively low gas velocity. This is quite beneficial, since it provides a gentler environment for the membranes, and indicates a significant attenuation or possible removal of mass transfer limitations due to the well-known excellent mass transfer characteristic of turbulent fluidization.     

Sedimentation behaviour study by three optical methods — granulometric and electrophoresis measurements, dispersion optical analyser

Taking into account the importance of characterisation of stability of dispersed systems in both fundamental research and industrial technology, three optical techniques, were used and compared to study sedimentation behaviours of mineral suspensions.The agglomeration and dispersion phenomena of mineral suspensions in water and ethanol media, has been studied by a granulometric and sedimentation approach (electrophoresis mobility, particle size distribution, clarifying speed, sediment formation).So, this experimental work has perfectly shown the necessity to achieve a complementary approach by these three optical methods to completely investigate the sedimentation behaviours.     

Dynamic evolution of PSD in continuous flow processes: A comparative study of fixed and moving grid numerical techniques

The present study provides a comprehensive investigation on the numerical solution of the dynamic population balance equation (PBE) in continuous flow processes. Specifically, continuous particulate processes undergoing particle aggregation and/or growth are examined. The dynamic PBE is numerically solved in both the continuous and its equivalent discrete form using the Galerkin on finite elements method (GFEM) and the moving grid technique (MGT) of Kumar and Ramkrishna [1997. Chemical Engineering Science 52, 4659-4679], respectively. Numerical simulations are carried out over a wide range of variation of particle aggregation and growth rates till the dynamic solution has reached its final steady-state value. The performance of the two numerical methods is assessed by a direct comparison of the calculated particle size distributions and/or their moments to available steady-state analytical solutions.     

Friction on a solid sphere exposed to gas–liquid and gas–liquid–solid flow in bubble column and fluidized bed reactors

Local velocity gradients on a solid spherical surface have been studied in a bubble column and in two- and three-phase fluidized beds, in order to clarify the influence of gas flow. The electrochemical method, measuring apparent local mass transfer coefficients, was verified and used to obtain the local velocity gradients, shear stresses and total frictional forces. The observed mass transfer rate was independent of liquid velocity, owing to a non-changing flow structure around the particles and not to averaging opposing effects. The identity in flow structure also held for three-phase fluidized beds up to a superficial gas velocity of 5 cm s^?1. The dramatic increase in velocity gradient on gas introduction was not a result of decreased homogenous density, but was caused by a change in the turbulent structure around a particle, leaving a larger portion of the total drag as frictional drag, thus improving the mass transfer characteristics of the bed. Use of velocity gradient measurements, including span of fluctuations and exposure time, to predict biomass growth and mechanical degradation in a reactor is also discussed.     

Application of fiber optic reflection probes to the measurement of local particle velocity and concentration in gas—solid flow

New, technically sophisticated measurement techniques must not be used blindly. In order to ensure accurate measurements, the limits of such devices must be well known. In this study a single-fiber optic reflection probe was thoroughly tested in two-phase gas—solid flows for the measurement of particle velocity and concentration. A novel, dynamic calibration procedure for particle concentration measurements has been proposed and tested in the downflow reactor section of a pilot-scale circulating fluidized bed. The results show that, as a particle concentration measuring device, the probe is very sensitive to electrostatic effects in the flow medium. The velocity measurements are relatively unaffected and were shown to be reproducible with errors of between 10 and 15% for particle velocities of up to 8 m/s.     

Three-dimensional CFD study of liquid–solid flow behaviors in tubular loop polymerization reactors: The effect of guide vane

A three-dimensional (3D) computational fluid dynamics (CFD) model, using an Eulerian–Eulerian two-fluid model incorporating the kinetic theory of granular flow, is adopted to describe the steady-state liquid–solid two-phase flow under conditions employed in a tubular loop propylene polymerization reactor composed of loop and axial flow pump. The model is validated by comparing its simulation result with the classical calculated data as well as a set of data collected from certain pilot plant in China. The entire flow behaviors and the effects of guide vane on them in the reactor are also investigated numerically. Especially, the whole field in the loop reactor with the guide vane is obtained via the above model. The results show that a guide vane weakens the turbulent intensity, reduces the component of the rotating velocity, and contributes to the uniform distribution of the particles in the reactor. The second flow phenomenon is successfully predicted in the loop reactor with the guide vane.     

Comparison of fluidized bed flow regimes for steam methane reforming in membrane reactors: A simulation study

An important decision in the design of fluidized bed reactors is which of several flow regimes to choose. Almost all fluidized bed reactor models are restricted to a single flow regime, making comparison difficult, especially near the regime boundaries. This paper examines the performance of fluidized bed methane reformers with three models—a simple equilibrium model and two kinetic distributed models, based on different assumptions of varying sophistication. Membranes are incorporated to improve reactor performance. Eighteen cases are simulated for different flow regimes and membrane configurations. Predictions for the fast fluidization and turbulent flow regimes show that the rate-controlling step is permeation through the membranes. Bubbling regime simulations predict somewhat less hydrogen production than for turbulent and fast fluidization, due to the effects of interphase crossflow and mass transfer. Overall reactor performance is predicted to be best under turbulent fluidization operation. Practical considerations also affect the advantages, shortcomings and ultimate choice of flow regime.     

Simulation of a continuous granular mixer: Effect of operating conditions on flow and mixing

Continuous particulate blending has received much attention in the recent decade as it provides a more economical and efficient alternative over batch blending for large scale manufacturing. The influence of operating conditions in continuous blenders is not well understood quantitatively, even though qualitative parallels can be drawn with batch blending for similar geometries. This work investigates the influence of fill level and impeller rotation rate (Froude number) in a horizontal bladed continuous blender using the discrete element method (DEM). Particle flow within the blender was found to be strongly dependent on the Froude number and fill level, with distinct fluidized and quasi-static regimes at smaller fills. The axial flow rates showed significant variation with Froude number and fill, and also showed considerable variation over the course of a shaft revolution. Favourable mixing was obtained at smaller impeller rotation rates for larger fills, but at larger impeller rotation rates for smaller fills.     

Gas–liquid flow in circular microchannel. Part I: Influence of liquid physical properties and channel diameter on flow patterns

This paper presents an experimental investigation on influence of liquid physical properties and channel diameter on gas–liquid flow patterns in horizontal circular microchannels with inner diameters of 302, 496 and 916 μm. Several liquids with different physical properties, i.e. water, ethanol, three sodium carboxymethyl cellulose (CMC) solutions (0.0464%, 0.1262%, 0.2446% CMC) and two sodium dodecyl sulfate (SDS) solutions (0.0608%, 0.2610% SDS) are chosen as working fluid and nitrogen as working gas. By using a high-speed photography system, flow patterns such as bubbly flow, slug and unstable slug flow, churn flow, slug-annular and annular flow are observed and identified on the flow regime maps. The results show that the liquid physical properties (viscosity and surface tension) and channel diameter affect the flow pattern transitions remarkably. Comparison with existing models in literature implies that these transitions cannot be well predicted. As a result, an effort is put into the proposition of a new empirical model taking the effects of channel size and liquid physical properties into account.     

Solids flow pattern in gas-flowing solids-fixed bed contactors. Part I: Experimental

An experimental investigation of the solids flow pattern in gas-flowing solids-fixed bed contactors is presented. The apparatus and procedures for determining the dynamic and static solids holdups, solids residence time distribution and the extent and rate of the exchange between particles in the static and dynamic solids holdup are described in detail.Experiments were performed in a bench scale system, containing a column (diameter ) packed with glass beads of 16 mm in diameter packed up to the height of 0.8 m. Tracer experiments with a step input in flowing solids phase were used for determining the residence time distribution and exchange between particles. Fine solids (spheres with mean diameter of ) of two different colors (all other properties being the same) were used in the tracer experiments to determine the residence time distribution and the exchange between static and dynamic solids holdup. In both types of experiments, the response curves have been obtained via color analysis of digital photos. All experiments have been repeated at different operating conditions, with a broad variation of solids mass flux and gas velocity, and reproducibility at set conditions was checked.The obtained experimental results are discussed and the observed important characteristics of the solids flow pattern are outlined. The effects of the solids flux and gas velocity on the solids flow pattern are presented and analyzed.     

Solids flow pattern in gas-flowing solids-fixed bed contactors. Part II: Mathematical modeling

In this paper a mathematical model for solids flow in gas-flowing solids-fixed bed contactors is developed. The presented four-parameter model assumes plug flow with axial dispersion in the dynamic, fast moving solids zone and the exchange of particles from this zone with the static zones. The complex dynamic behavior of the ‘static’ particles is described with a model of exchange between the dynamic and three static zones: (a) the ‘fast’ exchanging, (b) the ‘slow’ exchanging and (c) the ‘dead’ zone.The model parameters are optimized on the basis of two different tracer experiments: the step change response at the outlet—x_dyn, and the static holdup response—x_st. The model was tested for seven experimental series which correspond to different operating conditions (e.g., different superficial gas velocity and solids mass flux).The influence of the operating conditions on the model parameters is presented and discussed. Model reduction was also implemented in order to analyze the model simplifications and alternatives. Model sensitivity analysis was performed as well.     

Synthesis of diphosphate Mn2−xMgxP2O7 solid solutions with thortveitite structure: New pink ceramic dyes for the colouration of ceramic glazes

Solid solutions of Mn and Mg mixed diphosphates Mn_2-xMg_xP₂O₇ (0.2 < x < 1.5) have been prepared by calcination (1000 °C/4 h) of coprecipitate precursors and characterized for the first time (through thermal analysis, XRD, SEM/EDX and UV-vis-NIR techniques). Its colouring performance (CIE-L*a*b* parameters) within a ceramic glaze has been also analyzed. XRD and SEM/EDX results confirm the formation of a homogeneous and continuous β-Mn_2-xMg_xP₂O₇ solid solution having the thortveitite structure, isomorphous to β-Mn₂P₂O₇ and β-Mg₂P₂O₇. The diphosphates exhibit light brown or pinkish-white colours associated to Mn²⁺ ions in low-symmetry octahedral coordination. Interestingly, they develop intense purple (x < 1) or pink (1.0 ≤ x ≤ 1.5) colours once enamelled (5 wt.%) within a single-firing ceramic glaze, which are mainly associated to the presence of Mn³⁺ ions according to UV-vis-NIR spectra. Therefore, the Mn-Mg binary diphosphates could be used as new pink ceramic dyes for the colouration of conventional ceramic glazes.     

Combustion of methane lean mixtures in reverse flow reactors: Comparison between packed and structured catalyst beds

The scope of this work is to compare systematically the performance of particle beds and monolithic beds in catalytic reverse flow reactors used for combustion of lean methane/air mixtures, using alumina-supported palladium as catalyst. Different values of gas surface velocity (0.1–0.3 m/s), particle diameter (3–6 mm, for particle bed), cell density (200–400 cpsi, for structured bed) and catalyst/inert ratio (0.4–1) were used for the simulation of the combustion of 3500 ppm methane in both kinds of reverse flow reactor. An unsteady one-dimensional heterogeneous model has been developed and solved using a MATLAB code. The model, physical parameters and transport properties used had been experimentally validated in a previous work, operating with a particle bed reverse flow reactor. Results obtained indicate that the reverse flow reactor is more stable when the catalyst particle beds are use, although the difference with the monolith bed decreases as surface velocity increases. In contrast, pressure drops in the bed are higher for the particle bed.