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Highlights • 3D fully resolved pore-scale simulations of multiphase flow using a VOF-IBM method. • Validation test cases for multiphase porous media flows with wettability effects. • Water flooding simulations through regular single and random multi-pore structures. • Effect of capillary number, contact angle and viscosity ratio on the mobility of flow. Abstract In this paper we quantify the effect of capillary number (Ca) , contact angle (θ) and viscosity ratio (M) on the mobility of multiphase flow through porous media. The focus is mainly on oil-water flows through porous rocks observed during the water flooding process. Simulations are performed using a finite volume method employing a staggered grid formulation. Interactions between fluids and complex solid boundaries are resolved by a direct forcing, implicit and sharp interface immersed boundary method (IBM). The fluid-fluid interface is tracked by a mass conservative sharp interface volume of fluid (VOF) method. IBM and VOF are coupled by imposing the contact angle as a boundary condition at the three phase contact line. Our methodology has been verified/validated for several test cases including multiphase Poiseuille flow in a channel, a viscous finger in a channel and mesh convergence of the contact force. Two types of porous structures are considered: (i) a repeated single pore and (ii) a random multi-pore arrangement. Temporal evolution of phase pressure difference and oil saturation have been studied as viscous fingers penetrate the pores. We observed that the residual oil saturation for different capillary numbers shows exactly the opposite trend for the single and multi pore arrangement. The residual oil saturation for multi-pore shows a well defined linear trend with log Ca , θ and log M. [ABSTRACT FROM AUTHOR]

A novel nonintrusive technique is presented to investigate hydrodynamic and thermal behavior of gas-solid spout-fluidized beds with liquid injection, by simultaneously capturing visual and infrared images. Experiments were performed in a pseudo-2D bed with draft plates filled with glass or γ-alumina particles to investigate the effect of liquid injection and particle properties on the flow characteristics. For the glass particles under dry and wet conditions, time-averaged particle velocities show similar quasi-steady-state behavior. However, under wet conditions, lower particle velocities were observed in both spout and annulus as compared with the dry system. Whereas, γ-alumina particles do not show considerable variation in the particle velocities under dry and wet conditions and fluidize well at higher liquid injection rates. Additionally, for the glass particles, the particle temperature significantly decreases as compared to the γ-alumina particles. © 2015 American Institute of Chemical Engineers AIChE J, 61: 1146-1159, 2015 [ABSTRACT FROM AUTHOR]

Particle–particle (P–P) and particle–fluid (P–F) interactions are two fundamental phenomena in dense particulate flows which interact mutually to compose a variety of flow structures. Using discrete particle simulation and energy budget analysis, we quantitatively explore the effect of competition between these interactions on flow patterns in dense gas-fluidized beds. A diagram, picturing flow pattern formation and its evolution, along with the altering roles of P–P collision and P–F interaction in their competition, is presented and detailed energy analysis is used to explore how P–P collision and P–F interaction drives flow pattern formation and transition. It is shown that the flow structures in various flow regimes, ranging from the fixed to the turbulent regime, can be reproduced from these simulations. Systems with strong collisional dissipation but weak gas–particle interaction display a distinct emulsion–bubble two-phase flow structure. On the contrary, systems with strong gas–particle interaction but less pronounced collisional dissipation produce uniform structures, as often observed in the uniform regime near the incipient fluidization point. If these two interactions are equally important, the system features complex flow patterns resembling those displayed in the turbulent fluidization regime. Energy analysis demonstrates that the competition between P–P collision and P–F interaction determines flow structure formation and its evolution. The flow regime transition is actually the macro-scale expression of the altering of degree of dominance of P–P and P–F interactions. [Copyright &y& Elsevier]

Abstract: The formation and evolution of flow structures in dense gas-fluidized beds with ideal collisional particles (elastic and frictionless) are investigated numerically by employing the discrete particle method, with special focus on the effect of gas–particle interaction. It is clarified that heterogeneous flow structures still exist in systems with elastic and frictionless particle collisions, even though the particles in the emulsion phase are more loosely packed. The flow structures in various flow regimes, including uniform, bubbling and turbulent fluidization, can be reproduced from these simulations. Systems with strong particle fluctuating motion but weak gas suspension display an emulsion-bubble structure. On the contrary, systems with weak particle fluctuation motion tend to produce uniform structures. If these two interactions are equally important, the system features complex flow patterns resembling those displayed in the turbulent fluidization. Energy analysis has revealed that the fluctuating motion of particles corresponds to the existence of heterogeneity in ideal collisional systems. The flow regime transition is actually the macro-scale expression of the altering degree of dominance of particle–particle and particle–fluid interactions. Particularly, it is also found that non-linear drag has the “phase separation” function by accelerating particles in the dense phase and decelerating particles in the dilute phase producing the fluctuating motion and thereby triggering non-homogeneous flow structure formation. [Copyright &y& Elsevier]

In the present study a method is presented for simulations of droplet collisions in three spatial dimensions under non-isothermal conditions using a Diffuse Interface Model, which is based on the non-isothermal Navier–Stokes–Korteweg equations combined with the Van der Waals equation of state. This particular Diffuse Interface Model can only be used for a single component fluid and the temperature needs to be close to the critical point of the fluid to prevent the interfacial thickness from being too small compared to the size of the computational domain. Results are produced with a numerical method based on the finite-volume approach in combination with a Total Variation Diminishing time-integration scheme. Simulations of droplet collisions show the existence of multiple collision regimes for which the energy transfer and dissipation processes during the collisions are studied in detail. [ABSTRACT FROM AUTHOR]

To accurately simulate industrial sized bubble columns, the accurate prediction of the turbulent structures is important. Currently used RANS and LES models cannot capture the dynamics of the bubble columns or require a high grid resolution, respectively. In this work the Detached Eddy Simulation (DES) was used to combine the advantages of RANS and LES approaches. The DES method was based on Spalart–Allmaras, k − ϵ and k − ω SST turbulence models and was used to simulate gas–liquid flow for a bubble column with a square cross-section. The results are compared with experimental data and Large Eddy Simulation (LES) results, based on the Vreman and Smagorinsky subgrid-scale models. Profiles of turbulence kinetic energy and average axial liquid and gas velocities are compared at three heights of the column: near the sparger, in the middle and at the top of the column. The obtained results are in a very good agreement with the experimental data and LES simulations, proving ability of DES to capture highly dynamic flow motion and accurately predict main liquid characteristics in gas–liquid systems. [ABSTRACT FROM AUTHOR]

A full-field hydrodynamic study under riser flow conditions is performed, by using a combined Particle Image Velocimetry (PIV) and Digital Image Analysis (DIA) technique. The employment of a temporal histogram-based method (THM) enables an accurate measurement of the solids volume fraction over the full field of a pseudo-2D riser unit. The full visual access to the riser section enables a complete characterization of the complex transient particulate phase flow patterns. Full-field riser hydrodynamics are quantified at different operating conditions defined by the superficial gas velocity. Under these conditions, a cluster detection method is utilized to characterize the heterogeneity of the riser flow. [ABSTRACT FROM AUTHOR]

Direct numerical simulations are conducted to characterize the fluid–particle heat transfer coefficient in fixed random arrays of non-spherical particles. The objective of this study is to examine the applicability of well-known heat transfer correlations, that are proposed for spherical particles, to systems with non-spherical particles. In this study the spherocylinders are used to pack the beds and the non-isothermal flows are simulated by employing the Immersed Boundary Method (IBM). The simulations are performed for different solids volume fractions and particle sizes and low to moderate Reynolds numbers. Using the detailed heat flow pattern, the average heat transfer coefficient is calculated for the different operating conditions. The numerical results show that the heat-transfer correlation of spherical particles can be applied to all test beds of spherocylinders by choosing a proper effective diameter. Our results reveal that the diameter of a spherocylinder is the proper effective diameter for characterizing particle–fluid heat transfer. [ABSTRACT FROM AUTHOR]

Abstract: Present work describes the development of a non-intrusive image analysis technique to measure the bubble size distribution in bubbly flows. Generally image analysis methods for bubble characterization in bubbly flows are suitable at low void fractions. These methods depend mainly on the appropriate thresholding/conversion of the image into a binary image, dividing liquid and bubble objects. Clusters of overlapping bubbles are discarded using the criteria of a shape factor, thus leaving only solitary bubbles within an image. When the void fraction is low, the amount of discarded bubble data is very small compared with the obtained solitary bubble data. For bubbly flows with larger void fractions, the discarded data of overlapping/clustering bubbles is considerable. In this work the overlapping clusters are passed through a watershedding algorithm in order to segment the groups of bubble object areas into individual bubbles. The developed image algorithm is verified with synthetic bubble images and applied for an actual system involving bubbly flow in a pseudo-2D bubble column. [Copyright &y& Elsevier]

Abstract: Coarse grid simulations of Geldart A particles in bubbling fluidized beds using standard two-fluid (TF) models, where the constitutive laws are based on the homogeneity of sub-grid scale structures, have been demonstrated to be unsuccessful due to the existence of significant sub-grid scale heterogeneous structures. However, a definite consensus on the fundamental origin of the failure is still lacking. Some claim that the existence of significant inter-particle cohesive forces results in the sub-grid scale heterogeneous structures which takes the form of agglomerates or clusters; others claim that the inter-particle cohesive forces are not the dominant factor, and that the poor performance of TF models is primarily due to the fact that the grid sizes and the time step used in previous studies are too coarse to fully resolve the bubbling structure. To this end, a short overview is firstly presented to discuss the role of scale resolution and inter-particle cohesive force in two-fluid modeling of Geldart A particles; We then qualitatively explain, using a state-of-the-art discrete particle method, why the methods based on the existence of aggregates or clusters work quite well, although the sub-grid scale structure is not properly identified, that is, the sub-grid scale structure takes the form of a bubble-emulsion phase. [Copyright &y& Elsevier]

Abstract: An earlier developed Discrete Element Model (DEM) for simulating dense gas–solid two-phase flow has been used to study the formation of bubbles at a single orifice. A systematic theoretical study was conducted to study the effect of particle size and injection rate on the bubble growth process for Geldart type D particles. Theoretical results obtained for pseudo-2D beds were compared with predictions from approximate models reported in the literature, the Two-Fluid Model (TFM) and laboratory experiments. The Caram and Hsu model, an approximate model, gives a fairly good description of the trend of bubble size and the superficial gas velocities with time when the change in bubble boundary (defined by the gas voidage) with injection rate is taken into account. The DEM bubble size was found to be sensitive to the value of the gas void fraction defining the contour of the bubble boundary. The bubble behavior is influenced by minimum fluidization velocity, as defined by gas and particle properties, and the injection rate. Higher injection rates give larger bubbles and a small change in leakage rate. The detachment time for the DEM bubbles seem independent of particle size and injection rate when the bed width is small. Predictions from the DEM were consistent with the TFM. [Copyright &y& Elsevier]

Abstract: In recent years CFD has proven itself as a valuable tool for gaining insight in flow phenomena in general and complex multiphase flows arising in process equipment in particular. However for (dispersed) multiphase flows, the reliability of the outcome of these computations depends in a sensitive way on the correctness of the representation of the phase interactions (for instance due to drag and lift forces) which leads to the well-known and difficult closure problem. In this paper we report results of direct numerical simulations supplemented with dedicated experiments to obtain quantitative data for the representation of the lift force. This force is known to be responsible for the segregation of small and large (deformed) bubbles in bubbly flows through pipes and bubble columns. Both numerical simulations using an improved front tracking (FT) model and experiments under well-defined conditions have been performed for air bubbles rising through water/glycerine mixtures, where the bubble diameter, liquid viscosity and linear shear rate were varied. The numerical simulations show a good agreement with the correlation presented by for spherical bubbles at sufficiently high Reynolds numbers. For large deformed bubbles a good agreement with the correlation by was found over a wide range of liquid viscosities, although the computed lift force was always slightly lower. Therefore a new correlation has been proposed, which combines a fit of the numerical data for deformed bubbles with the correlation by for small bubbles. Finally, it was shown that the shear rate has no significant influence on the drag and lift coefficient. An experimental set-up (similar to the one used by Tomiyama) was constructed using a running belt submerged in a liquid, consisting of a glycerine–water mixture of varying viscosity. PIV measurements have been used to calibrate the linear shear field and to obtain the flow profile around the bubbles. Contrary to the numerical simulations, the experimental data show a very strong influence of the shear rate on the lift force coefficient. This may be attributed to the rigid behaviour of the contaminated bubble surface, which changes the shear stress at the bubble interface. [Copyright &y& Elsevier]

Abstract: Fluidized beds employing fine powders are finding increased application in the chemical and petrochemical industry because of their excellent mass and heat transfer characteristics. However, in fluidized bed chemical reactors axial gas back mixing can strongly decrease the conversion and selectivity. By insertion of membranes in fluidized beds large improvements in conversion and selectivity can be achieved, firstly by optimizing axial concentration profiles via distributive feeding of one of the reactants or selective withdrawal of one of the products, and secondly, by decreasing the effective axial dispersion via compartmentalization of the fluidized bed. Moreover, insertion of membrane bundles in a suitable configuration impedes bubble growth, thereby reducing reactant by-pass via rapidly rising large bubbles. In this work the influence of the presence and configuration of membrane bundles and the effect of gas addition via the membranes on the effective axial dispersion was studied experimentally. Steady state concentration profiles were measured where a CO2-tracer was injected at different locations through a probe (point injection) or via the membranes (line injection) into a square fluidized bed containing glass particles (75–, 2550kg/m3) fluidized with nitrogen distributed via a porous plate. Different bed configurations, viz. without internals, with vertical or horizontal membrane bundles were investigated and the effects of overall fluidization velocity and gas flow ratio of gas fed through the membrane bundles and the porous plate distributor were studied. Experimental results revealed that the insertion of vertical and horizontal membrane bundles decreases the effective axial dispersion considerably compared to a bed without internals. The point injection experiments indicated the importance of a non-uniform lateral emulsion phase velocity profile. The line injection experiments clearly pointed out the importance of bubble-to-emulsion phase mass transfer limitations. Gas addition through the membrane bundles decreases the effective axial gas dispersion enormously by almost annihilating the solids down flow along the walls and by decreasing the average bubble size and bubble fraction. [Copyright &y& Elsevier]

Hydrogen is emerging as a future alternative for mobile and stationary energy carriers in addition to its use in chemical and petrochemical applications. A novel multifunctional reactor concept has been developed for the production of ultrapure hydrogen from light hydrocarbons such as methane for online use in downstream polymer electrolyte membrane fuel cells. A high degree of process intensification can be achieved by integrating perm-selective hydrogen membranes for selective hydrogen removal to shift the methane steam reforming and water–gas-shift equilibriums in the favourable direction and perm-selective oxygen membranes for selective oxygen addition to supply the required reaction energy via partial oxidation of part of the methane feed and enable pure CO2 capture without costly post-treatment. This can be achieved in a proposed novel multifunctional bi-membrane bi-section fluidised bed reactor [Patil, C.S., van Sint Annaland, M., Kuipers, J.A.M., 2005. Design of a novel autothermal membrane assisted fluidized bed reactor for the production of ultrapure hydrogen from methane. Industrial and Engineering Chemistry Research 44, 9502–9512]. In this paper, an experimental proof of principle for the steam reforming/water–gas-shift section of the proposed novel fluidised bed membrane reactor is presented. A fluidised bed membrane reactor for steam reforming of methane/water–gas-shift on a commercial noble metal-based catalyst has been designed and constructed using 10 H2 perm-selective Pd membranes for a fuel cell power output in the range of 50–100W. It has been experimentally demonstrated that by the insertion of the membranes in the fluidised bed, the thermodynamic equilibrium constraints can indeed be overcome, i.e., increased CH4 conversion, decreased CO selectivity and higher product yield (H2 produced/CH4 reacted). [Copyright &y& Elsevier]

Abstract: Numerical simulations of the bubbly flow in two square cross-sectioned bubble columns were conducted with the commercial CFD package CFX-4.4. The effect of the model constant used in the sub-grid scale (SGS) model, , as well as the interfacial closures for the drag, lift and virtual mass forces were investigated. Furthermore, the performance of three models [Pfleger, D., Becker, S., 2001. Modeling and simulation of the dynamic flow behavior in a bubble column. Chemical Engineering Science, 56, 1737–1747; Sato, Y., Sekoguchi, K.,1975. Liquid velocity distribution in two-phase bubble flow. International Journal of Multiphase Flow 2, 79–95; Troshko, A.A., Hassan, Y.A., 2001. A two-equation turbulence model of turbulent bubbly flows. International Journal of Multiphase Flow 27, 1965–2000] to account for the bubble-induced turbulence in the – model was assessed. All simulation results were compared with experimental data for the mean and fluctuating liquid and gas velocities. It is shown that the simulation results with and agree well with the measurements. When is increased, the effective viscosity increases and subsequently the bubble plume becomes less dynamic. All three bubble-induced turbulence models could produce good solutions for the time-averaged velocity. The models of Troshko and Hassan and Pfleger and Becker reproduce the dynamics of the bubbly flow in a more accurate way than the model of Sato and Sekoguchi. Based on the comparison of the results obtained for two columns with different aspect ratio ( and ), it was found that the model of Pfleger and Becker performs better than the model of Troshko and Hassan, while the model of Sato and Sekoguchi performs the worst. It was observed that the interfacial closure model proposed by Tomiyama [2004. Drag, lift and virtual mass forces acting on a single bubble. Third International Symposium on Two-Phase Flow Modeling and Experimentation, Pisa, Italy, 22–24 September] performs better for the taller column. With the drag coefficient proposed by Tomiyama, the predicted slip velocity agrees well with the experimental data in both columns. The virtual mass force has a small influence on the investigated bubbly flow characteristics. However, the lift force strongly influences the bubble plume dynamics and consequently determines the shape of the vertical velocity profile. In a taller column, the lift coefficient following from the model of Tomiyama produces the best results. [Copyright &y& Elsevier]

Abstract: To describe the hydrodynamic phenomena prevailing in large industrial scale fluidized beds continuum models are required. The flow in these systems depends strongly on particle–particle interaction and gas–particle interaction. For this reason, proper closure relations for these two interactions are vital for reliable predictions on the basis of continuum models. Gas–particle interaction can be studied with the use of the lattice Boltzmann model (LBM), while the particle–particle interaction can suitably be studied with a discrete particle model. In this work it is shown that the discrete particle model, utilizing a LBM based drag model, has the capability to generate insight and eventually closure relations in processes such as mixing, segregation and homogeneous fluidization. [Copyright &y& Elsevier]

Abstract: In this paper a hybrid model is presented for the numerical simulation of gas–liquid–solid flows using a combined front tracking (FT) and discrete particle (DP) approach applied for, respectively, dispersed gas bubbles and solid particles present in the continuous liquid phase. The hard sphere DP model, originally developed by Hoomans et al. [1996. Chemical Engineering Science 51, 99–108] for dense gas–solid systems, has been extended to account for all additional forces acting on particles suspended in a viscous liquid and has been combined with the FT model presented recently by Deen et al. [2004a. CD-ROM Proceedings of Fifth International Conference on Multiphase Flow, ICMF’04, Yokohama, Japan, May 30–June 4, 2004; 2004b. Chemical Engineering Science 59, 1853–1861] for complex free surface flows. In this paper, the physical foundation of the combined FT-DP model will be presented together with illustrative computational results highlighting the capabilities of this hybrid model. The effect of bubble-induced particle mixing has been studied focusing on the effect of the volumetric particle concentration. In addition the retarding effect (drag modification) on the bubble rise velocity due to the presence of the suspended solid particles has been quantified. [Copyright &y& Elsevier]

Abstract: A 3D discrete bubble model is adopted to investigate complex behavior involving hydrodynamics, mass transfer and chemical reactions in a gas–liquid bubble column reactor. In this model a continuum description is adopted for the liquid phase and additionally each individual bubble is tracked in a Lagrangian framework, while accounting for bubble–bubble and bubble–wall interactions via an encounter model. The mass transfer rate is calculated for each individual bubble using a surface renewal model accounting for the instantaneous and local properties of the liquid phase in its vicinity. The distributions in space of chemical species residing in the liquid phase are computed from the coupled species balances considering the mass transfer from bubbles and reactions between the species. The model has been applied to simulate chemisorption of CO2 bubbles in NaOH solutions. Our results show that apart from hydrodynamics behavior, the model is able to predict the bubble size distribution as well as temporal and spatial variations of each chemical species involved. [Copyright &y& Elsevier]

Abstract: In this work, mass transfer at the shell side of an in-line hollow fiber array subjected to cross-flow is simulated by applying the domain decomposition method combined with orthogonal grid generation. Two-dimensional Navier–Stokes equations written in stream function–vorticity variables, were separately solved along with a species conservation equation for different arrays. The main factors influencing the concentration fields, local mass transfer rates and global mass transfer rates in the laminar flow of –200 and –300 with pitch to tube diameter ratios of 1.45, 1.50, 1.75, 1.85 and 2.00 are discussed in detail. Mass transfer correlations obtained from the numerical simulations show good agreement with typical empirical correlations proposed earlier. [Copyright &y& Elsevier]

Abstract: Correct prediction of spontaneous bubble formation in freely bubbling gas–solid fluidized beds using Eulerian models, strongly depends on the description of the internal momentum transfer in the particulate phase. In this part, the comparison of the simple classical model, describing the solid phase pressure only as a function of a solid porosity by an empirical correlation and assuming the solid phase viscosity constant, which is referred to as the constant viscosity model (CVM), with the more fundamental model based on the kinetic theory of granular flow (KTGF), in which the solid phase properties are described in much more detail in terms of instantaneous binary particle–particle interactions, has been extended for freely bubbling fluidized beds. The performance of the KTGF and the CVM in predicting the hydrodynamics of freely bubbling fluidized beds has been compared with experimental data and correlations taken from the literature. In freely bubbling fluidized beds at relatively low gas velocities, bubble formation is initiated by inelastic particle–particle interactions. When accounting for the dissipation of granular energy by particle collisions, the KTGF predicts much larger bubbles with a much sharper interface in comparison to the CVM. The average bubble size distribution predicted by the KTGF showed better agreement with correlations as well as experimental data from the literature. Although both models showed an increase in the predicted average bubble size with increasing superficial gas velocities, the discrepancy in the predicted bubble size becomes smaller, indicating the growing importance of the gas particle interactions in the bubble formation process at higher gas velocities. The rise velocity predicted by the KTGF and the CVM is approximately the same and in good agreement with correlations available in the literature. Since the KTGF predicts somewhat larger bubbles, also the predicted visible bubble flow is higher in comparison to the CVM. In very dense regions in the fluidized bed the KTGF based on instantaneous binary collisions needs to be extended for additional frictional stresses in addition to the kinetic and collisional transport mechanisms. The extra frictional stresses were implemented via a relatively simple semi-empirical closure model and proved to have a significant influence on the predicted bubble size, rise velocity and visible bubble flow rate, where the model predictions strongly depend on the empirical constants. To further enhance the performance of the KTGF to describe the hydrodynamics of freely bubbling beds a more fundamental description of the frictional stresses on the particle level should be incorporated. [Copyright &y& Elsevier]

Abstract: In the Eulerian approach to model gas–solid fluidized beds closures are required for the internal momentum transfer in the particulate phase. Firstly, two closure models, one semi-empirical model assuming a constant viscosity of the solid phase (CVM) and a second model based on the kinetic theory of granular flow (KTGF), have been compared in this part in their performance to describe bubble formation at a single orifice and the time-averaged porosity profiles in the bed using experimental data obtained for a pseudo two-dimensional fluidized bed operated with a jet in the center. Numerical simulations have shown that bubble growth at a nozzle with a jet is mainly determined by the drag experienced by the gas percolating through the compaction region around the bubble interface, which is not much influenced by particle–particle interactions, so that the KTGF and CVM give very similar predictions. However, this KTGF model does not account for the long term and multi particle–particle contacts (frictional stresses) and under-predicts the solid phase viscosity at the wall as well as around the bubble and therefore over-predicts the bed expansion. Therefore, in the later part of the paper, the bubble growth at a single orifice and the time-averaged porosity distribution in the bed predicted by the KTGF model with and without frictional stresses are compared with experimental data. The model predictions by the KTGF are improved significantly by the incorporation of frictional stresses, which are however strongly influenced by the empirical parameters in this model. In Part II the comparison of the CVM and KTGF with experimental results is extended to freely bubbling fluidized beds. [Copyright &y& Elsevier]

Abstract: Dense gas–particle flows are encountered in a variety of industrially important processes for large scale production of fuels, fertilizers and base chemicals. The scale-up of these processes is often problematic, which can be related to the intrinsic complexities of these flows which are unfortunately not yet fully understood despite significant efforts made in both academic and industrial research laboratories. In dense gas–particle flows both (effective) fluid–particle and (dissipative) particle–particle interactions need to be accounted for because these phenomena, to a large extent, govern the prevailing flow phenomena, i.e. the formation and evolution of heterogeneous structures. These structures have significant impact on the quality of the gas–solid contact and as a direct consequence thereof strongly affect the performance of the process. Due to the inherent complexity of dense gas-particles flows, we have adopted a multi-scale modeling approach in which both fluid–particle and particle–particle interactions can be properly accounted for. The idea is essentially that fundamental models, taking into account the relevant details of fluid–particle (lattice Boltzmann model (LBM)) and particle–particle (discrete particle model (DPM)) interactions, are used to develop closure laws to feed continuum models which can be used to compute the flow structures on a much larger (industrial) scale. Our multi-scale approach (see Fig. 1) involves the LBM, the DPM, the continuum model based on the kinetic theory of granular flow, and the discrete bubble model. In this paper we give an overview of the multi-scale modeling strategy, accompanied by illustrative computational results for bubble formation. In addition, areas which need substantial further attention will be highlighted. [Copyright &y& Elsevier]

A critical comparison of a hard-sphere discrete particle model, a two-fluid model with kinetic theory closure equations and experiments performed in a pseudo-two-dimensional gas-fluidised bed is made. Bubble patterns, time-averaged particle distributions and bed expansion dynamics measured with a nonintrusive digital image analysis technique are compared to simulation results obtained at three different fluidisation velocities. For both CFD models, the simulated flow fields and granular temperature profiles are compared. The effects of grid refinement, particle–wall interaction, long-term particle contacts, particle rotation and gas–particle drag are studied. The mechanical energy balance for the suspended particles is introduced, and the energy household for both CFD models is compared. The most critical comparison between experiments and model results is given by analysis of the bed expansion dynamics. Though both models predict the right fluidisation regime and trends in bubble sizes and bed expansion, the predicted bed expansion dynamics differ significantly from the experimental results. Alternative gas–particle drag models result in significantly different bed dynamics, but the gap between model and experimental results cannot be closed. In comparison with the experimental results, the discrete particle model gives superior resemblance. The main difference between both CFD models is caused by the neglect of particle rotation in the kinetic theory closure equations embedded in the two-fluid model. Energy balance analysis demonstrates that over 80% of the total energy is dissipated by sliding friction. Introduction of an effective restitution coefficient that incorporates the additional dissipation due to frictional interactions significantly improves the agreement between both models. [Copyright &y& Elsevier]

This paper reports on a numerical study of fluidization behavior of Geldart A particles by use of a 2D soft-sphere discrete particle model (DPM). Some typical features, including the homogeneous expansion, gross particle circulation in the absence of bubbles, and fast bubbles, can be clearly displayed if the interparticle van der Waals forces are relatively weak. An anisotropy of the velocity fluctuation of particles is found in both the homogeneous fluidization regime and the bubbling regime. The homogeneous fluidization is shown to represent a transition phase resulting from the competition of three kinds of basic interactions: the fluid–particle interaction, the particle–particle collisions (and particle–wall collisions) and the interparticle van der Waals forces. In the bubbling regime, however, the effect of the interparticle van der Waals forces vanishes and the fluid–particle interaction becomes the dominant factor determining the fluidization behavior of Geldart A particles. This is also evidenced by the comparisons of the particulate pressure with other theoretical and experimental results. [Copyright &y& Elsevier]

In this paper, an efficient ghost-cell based immersed boundary method is introduced to perform direct numerical simulation (DNS) of mass transfer problems in particulate flows. The fluid-solid coupling is achieved by implicit incorporation of the boundary conditions into the discretized momentum and species conservation equations of the fluid phase. Taking the advantage of a second order quadratic interpolation scheme utilized in the reconstruction procedures, the unique feature of this ghost-cell based immersed boundary method is its capability to handle mixed boundary conditions at the exact position of the particle surface as encountered in systems with interplay between surface reactions and diffusion. A fixed Eulerian grid is used to solve the conservation equations for the entire computational domain. Following a detailed verification of the method in the limiting case of unsteady molecular diffusion without convection, we apply our method to study fluid-particle mass transfer for flow around a single sphere and a dense stationary array consisting of hundreds of spheres over a range of Damköhler numbers. [ABSTRACT FROM AUTHOR]

As a result of highly exothermic reactions during gas-phase olefin polymerization in fluidized bed reactors, difficulties with respect to the heat management play an important role in the optimization of these reactors. To obtain a better understanding of the particle temperature distribution in fluidized beds, a high speed infrared (IR) camera and a visual camera have been coupled to capture the hydrodynamic and thermal behavior of a pseudo-2D fluidized bed. The experimental data were subsequently used to validate an in-house developed computational fluid dynamics and discrete element model (CFD-DEM). In order to mimic the heat effect due to the exothermic polymerization reaction, a model system was used. In this model system, heat is released in zeolite 13X particles (1.8–2.0 mm, Geldart D type) due to the adsorption of CO 2 . All key aspects of the adsorption process (kinetics, equilibrium and heat effect) were studied separately using Thermogravimetric Analysis (TGA) and Simultaneous Thermal Analysis (STA), and subsequently fluidized bed experiments were conducted, by feeding gas mixtures of CO 2 and N 2 with different CO 2 concentrations to the bed, where the total heat of liberation could be controlled. The combined infrared/visual camera technique generated detailed information on the thermal behavior of the bed. Furthermore, the comparison of the spatial and temporal distributions of the particle temperature measured in the fluidized bed with the simulation results of CFD-DEM provides qualitative and quantitative validation of the CFD-DEM, in particular concerning the thermal aspects. [ABSTRACT FROM AUTHOR]

Abstract: In this paper a novel simulation technique is presented to perform Direct Numerical Simulation (DNS) of fluid flow and heat transfer in dense fluid–particle systems. The unique feature of our fluid–solid coupling technique is the direct (i.e., implicit) incorporation of the boundary condition (with a second-order method) at the surface of the particles at the level of the discrete momentum and thermal energy equations of the fluid. Contrary to lattice Boltzmann or other commonly used immersed boundary implementations, our method does not require using any effective diameter. A fixed (Eulerian) grid is utilized to solve the Navier–Stokes equations for the entire computational domain. Dissipative particle–particle and/or particle-wall collisions are accounted via a hard sphere discrete particle approach using a three-parameter particle–particle interaction model accounting for normal and tangential restitution and tangential friction. Following the detailed verification of the method several dense multi-particle systems are studied in detail involving stationary arrays of particles and fluidized particles. [Copyright &y& Elsevier]

Abstract: Recently the importance of spouted bed technology has significantly increased in the context of drying processes as well as granulation, agglomeration or coating processes. Within this work the fluid dynamics of a novel spouted bed plant with two adjustable gas inlets is investigated. By analysis of gas phase pressure fluctuation spectra by means of fast Fourier transformation algorithm and quantitative particle image velocimetry (PIV) measurements, different operation ranges will be identified and depicted graphically. A stable spouting range will be identified if a uniform and circulating particle motion without occurrence of dead zones is observable resulting in equal gas phase pressure fluctuations. Particle velocity vector maps as well as root mean square (rms) velocity plots provided by the PIV measurements confirm the results of the determination of the stable spouting range by analysis of the gas phase pressure fluctuations. [Copyright &y& Elsevier]

Abstract: In this contribution the technical feasibility of the reverse flow catalytic membrane reactor (RFCMR) concept with porous membranes for energy efficient syngas production is investigated. In earlier work an experimental proof of principle was already provided [Smit, J., Bekink, G.J., van Sint Annaland, M., Kuipers, J.A.M., 2005a. A reverse flow catalytic membrane reactor for the production of syngas: an experimental study. International Journal of Chemical Reactor Engineering 3 (A12)], but compensatory heating was required and problems related to the mechanical strength of the powder-based YsZ catalyst and the steel filter were reported. Therefore, in Part 1 the performance of a catalyst with improved mechanical strength and porous membranes with excellent temperature resistance was tested in an isothermal membrane reactor. For this purpose a novel sealing technique was developed that could withstand sufficiently high pressure differences and temperatures. Very high syngas selectivities close to the thermodynamic equilibrium could be achieved for a considerable period of time without any increase in pressure drop and without any decrease in syngas selectivity. Using the catalyst, several experiments were performed in a RFCMR demonstration unit and the influence of different operating conditions and design parameters on the reactor behaviour was investigated. It is shown that very high syngas selectivities (up to 95%) can be achieved with a maximal on-stream time of 12h, without using any compensatory heating and despite inevitable radial heat losses. In Part 2 a reactor model is discussed that can well describe the experimental results presented in this part. [Copyright &y& Elsevier]

Abstract: A detailed reactor model has been developed to describe the experimental results that were obtained with the reverse flow catalytic membrane reactor (RFCMR) demonstration unit, presented in Part 1. The reactor model is based on a model that was used previously [Smit, J., van Sint 2005b. Annaland, M., Kuipers, J.A.M., 2005b. Feasibility study of a reverse flow catalytic membrane reactor with porous membranes for the production of syngas. Chemical Engineering Science 60(24), 6971–6982] to demonstrate the conceptual feasibility of the RFCMR concept at industrially relevant conditions. To accurately capture the radial heat losses and heat buffering effects in the RFCMR demonstration unit, the existing model has been extended with a detailed description of the two-dimensional heat transport in the insulation layer. By simulating a number of experimental cases discussed in Part 1, it is shown that the radial heat losses and therefore also the axial temperature profiles in the RFCMR demonstration unit can be very well described, without using any fitting parameters. Furthermore, it is shown that also the syngas composition can be predicted reasonably well by simply assuming local thermodynamic equilibrium. The very good agreement between the experimental results and the predictions of the reactor model clearly support the results of the conceptual feasibility study. [Copyright &y& Elsevier]

Abstract: In order to model the complex hydrodynamic phenomena prevailing in industrial scale gas–solid bubbling fluidised bed reactors and especially the macro-scale emulsion phase circulation patterns induced by bubble–bubble interactions and bubble coalescence, a discrete bubble model (DBM) has been developed. In the DBM, the (larger) bubbles are modelled as discrete elements and are tracked individually during their rise through the emulsion phase, which is considered as a continuum. The DBM, originally developed for the description of gas–liquid flows, has been adapted to cope with bubbles with a diameter larger than the size of an Eulerian cell, which is required in view of the large bubble size distribution at higher gas flow rates. Moreover, a new drag model for a single bubble rising in a fluidised bed derived from empirical correlations has been implemented, as well as a simple model to account for bubble coalescence and break-up. The strong advantage of the DBM compared to other models previously reported in the literature for the description of large-scale fluidised beds is that it fully accounts for the two-way coupling between the bubbles and the emulsion phase, which enables direct computation of the emulsion phase velocity profiles. Comparison of the results of simulations ignoring bubble coalescence and simulations taking bubble coalescence properly into account demonstrated the significant effect of bubble coalescence on the large-scale circulation patterns prevailing in bubbling fluidised beds. The simulation results for the lateral profiles of the visible bubble flow rate have been compared qualitatively with experimental results reported by Werther [1974. Influence of the bed diameter on the hydrodynamics of gas fluidized beds. A.I.Ch.E. Symposium Series 70(141), 53–62]. The effect of the superficial gas velocity on the velocity and porosity profiles has been studied. In general, it can be concluded that the DBM is able to capture the salient features of the hydrodynamics of bubbling fluidised beds. However, further research is required to improve the closure equations for the bubble behaviour, bubble–bubble interactions and bubble coalescence and break-up to enable a complete quantitative description. [Copyright &y& Elsevier]

Abstract: Closures for the drag and virtual mass forces acting on a single air bubble rising in initially quiescent pure water have been numerically investigated using direct numerical simulation techniques. A 3D Front Tracking model was used and the results were compared with simulation results obtained with a 2D Volume of Fluid model to assess the influence of the third dimension. In the simulations realistic values were taken for the physical properties, i.e., a density ratio of 800. The computed time-averaged terminal rise velocity and mean aspect ratio for individual air bubbles ranging in equivalent diameter from 1 to 10mm rising in pure water compare well with available experimental data. [Copyright &y& Elsevier]

Abstract: In this paper the concept of micro-fluidized beds is introduced. A cylindrical quartz reactor with an internal diameter of only 1mm is used for process conditions up to and 244bar. In this way, fast, safe, and inherently cheap experimentation is provided. The process that prompted the present work on miniaturization is gasification of biomass and waste streams in hot compressed water (SCWG). Therefore, water is used as fluidizing agent. Properties of the micro-fluid bed such as the minimum fluidization velocity (), the minimum bubbling velocity (), bed expansion, and identification of the fluidization regime are investigated by visual inspection. It is shown that the micro-fluid bed requires a minimum of twelve particles per reactor diameter in order to mimic homogeneous fluidization at large scale. It is not possible to create bubbling fluidization in the cylindrical micro-fluid beds used. Instead, slugging fluidization is observed for aggregative conditions. Conical shaped micro-reactors are proposed for improved simulation of the bubbling regime. Measured values of and are compared with predictions of dedicated 2D and 3D discrete particle models (DPM) and (semi)-empirical relations. The agreement between the measurements and the model predictions is good and the model supports the concept and development of micro-fluid beds. [Copyright &y& Elsevier]