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The use of an axial dual-zone vortex chamber for spray drying applications is experimentally studied, focusing on water spray evaporation. In the hot zone of the vortex chamber, hot air (maximum 30...

Among the different thermo-chemical recycling routes for plastic waste valorization, gasification is one of the most promising, converting plastic waste into syngas (H2+CO) and energy in the presence of an oxygen-rich gas. Plastic waste gasification is associated with many different complexities due to the multi-scale nature of the process, the feedstock complexity (mixed polyolefins with different contaminations), intricate reaction mechanisms, plastic properties (melting behavior and molecular weight distribution), and complex transport phenomena in a multi-phase flow system. Hence, creating a reliable model calls for an extensive understanding of the phenomena at all scales, and more advanced modeling approaches than those applied today are required. Indeed, modeling of plastic waste gasification (PWG) is still in its infancy today. Our review paper shows that the thermophysical properties are rarely properly defined. Challenges in this regard together with possible methodologies to decently define these properties have been elaborated. The complexities regarding the kinetic modeling of gasification are numerous, compared to, e.g., plastic waste pyrolysis, or coal and biomass gasification, which are elaborated in this work along with the possible solutions to overcome them. Moreover, transport limitations and phase transformations, which affect the apparent kinetics of the process, are not usually considered, while it is demonstrated in this review that they are crucial in the robust prediction of the outcome. Hence, possible approaches in implementing available models to consider these limitations are suggested. Finally, the reactor-scale phenomena of PWG, which are more intricate than the similar processes—due to the presence of molten plastic—are usually simplified to the gas-solid systems, which can result in unreliable modeling frameworks. In this regard, an opportunity lies in the increased computational power that helps improve the model’s precision and allows us to include those complexities within the multi-scale PWG modeling. Using the more accurate modeling methodologies in combination with multi-scale modeling approaches will, in a decade, allow us to perform a rigorous optimization of the PWG process, improve existing and develop new gasifiers, and avoid fouling issues caused by tar.

Methane bi-reforming has gained interest recently for its potential of converting CH4 and CO2 into chemicals while avoiding catalyst deactivation by coke formation. Recycling CO2 produced can be considered, but introduces constraints on the allowable pressure drop over the reactor. The latter is particularly the case in low-pressure processes, such as those encountered in the steel industry. The paper reports on a multi-scale modeling approach for a structured catalytic reactor that offers lower pressure drop than classical pellets while improving heat transfer. The use of a thin catalytic coating furthermore improves the catalyst effectiveness. The coupled CFD-reaction model accounts for the details of the reaction mechanism and kinetics, intra-catalyst transport and the details of the flow pattern. Radiative heat transfer and thermal conduction in the reactor tube walls and in the reactor internals coated with catalyst are also taken into account. Several scale-bridging strategies have to be introduced and combined to allow a computationally tractable solution. This requires a variety of detailed experimental data, an aspect that is also discussed. The coupled CFD-reaction model was first validated using data from a bi-reforming pilot plant and then used to evaluate the performance of a structured reactor under typical commercial process conditions and to study optimization of the reactor design and potential increase in capacity.

A recent challenge in the modeling of particle flows is to build microstructure-informed drag models to overcome the average description of the fluid-particle force in the drag force correlations currently used in Euler-Lagrange and Euler-Euler models. To that end, we study through particle- resolved direct numerical simulations (PR-DNS) the flow past random assemblies of mono-dispersed spherical particles at three particle Reynolds numbers (10, 50, 100) and four solid volume fractions (0.10, 0.20, 0.30, 0.40). The present methodology is validated against theoretical and numerical results for the mean drag force in Stokes flows and finite Reynolds numbers flows. PR-DNS results are then used to characterize in details the statistics of the force distribution over the particle array, highlighting the substantial dispersion of the fluid force along the streamwise and transverse directions. The microstructure formed by the solid phase is described by means of a limited number of tensor quantities inspired from the fabric tensor used in granular media. Significant correlations are identified between the force experienced by a given particle immersed in a random array and a few key quantities that describe the anisotropy of its neighbourhood. A microstructure-based multi-linear model is proposed and validated against independent test cases. The model appears to perform best in the viscous and dense regimes. The addition of a stochastic contribution to the model allows to recover the correct level of force fluctuations at the cost of a lower correlation between the model and the data.

Structured catalytic reactors have the potential to combine reduced pressure drop and improved heat transfer compared to conventional pellets. In the present study, the pressure drop and heat transfer coefficient between the tube wall and the process gas were measured experimentally for annular structured ZoneFlow TM reactors of different design and for two commercial reference pellets. The ZoneFlow TM reactors differ by the design of the central rod that supports the near-wall annular casing. The experiments were carried out in a 1 m long reactor, at atmospheric pressure and with air flow rates from 70 to 330 Nm 3 /h. To measure the heat transfer coefficient, the furnace was set at a constant temperature that was varied in the range 100–500 °C and the axial profiles of the tube wall and gas temperatures were measured. Using the experimental data, correlations for the friction factor and the Nusselt number were derived and the introduced parameters estimated using non-linear regression. A correlation for the static contribution to heat transfer for the structured reactors was derived from 3D numerical simulations. The correlations were then used to evaluate the pressure drop–heat transfer advantage of the structured reactors compared to the reference pellets at typical industrial steam methane reforming conditions. The data show that ZoneFlow TM reactors can provide a roughly doubled heat transfer coefficient at comparable pressure drop than the tested reference pellets. Furthermore, modification of the central rod support structure was found an efficient way to balance the pressure drop–heat transfer advantage of ZoneFlow TM reactors using an identical design casing.

Vortex chambers allow the generation of rotating fluidized beds, offering high-G intensified gas-solid contact, gas-solids separation and solids-solids segregation. Focusing on binary particle mixtures and fixing the density and diameter of the heavy/large particles, transient batch CFD-coarse-grained DPM simulations were carried out with varying densities or sizes of the light/small particles to evaluate to what extent combining these three functionalities is possible within a vortex chamber of given design. Both the rate and quality of segregation were analyzed. Within a relatively wide density and size range, fast and efficient segregation takes place, with an inner and slower rotating bed of the lighter/small particles forming within the outer and faster rotating bed of the heavier/large particles. Simulations show that the contamination of the outer bed with lighter particles occurs more easily than contamination of the inner bed with heavier particles and increases with decreasing difference in size or density of the particles. Bubbling in the inner bed is observed with an inner bed of very low density or small particles. Porosity plots show that vortex chambers with a sufficient number of gas inlet slots have to be used to guarantee a uniform gas distribution and particle bed. Finally, the flexibility of particle segregation in vortex chambers with respect to the gas flow rate is demonstrated.

Rotating fluidized beds provide a unique opportunity to exploit fluidization under higher particle forces. The centripetal force in a rotating fluid bed is typically on the order of 10 times the force of gravity. Since the force keeping the particles in the unit is larger, the drag force can also be larger, allowing for higher gas and slip velocities. This operating regime provides intensified gas-solids contact through higher mass transfer, heat transfer, gas throughput, and bubble suppression. One application for using a rotating fluidized bed is in Chemical Looping Combustion (CLC). When solid fuels are used, oxygen carrier and ash are mixed in the process. In order to maintain high carbon capture efficiencies and recyclability of the oxygen carrier, the ash needs to be separated from the oxygen carrier. This separation can be done aerodynamically since the oxygen carrier is larger and heavier than the ash. It is theorized that rotating fluidized beds could improve both the gas-solid and solid-solid separation process efficiency and throughput as compared to conventional fluidized beds. A 43 cm diameter, 2.5 cm long vortex chamber has been designed and constructed to investigate the application to particle separation. A series of experiments have been performed to investigate the separation of different binary mixtures of solids. These experiments demonstrate the use of a rotating fluidized bed for high-G intensified particle separation that can be combined with high-G intensified gas-solids contact and gas-solids separation.

Coupled discrete particle method – computational fluid dynamics simulations are carried out to demonstrate the potential of combined high-G-intensified gas–solids contact, gas–solids separation and segregation in a rotating fluidized bed in a static vortex chamber. A case study with two distinct types of particles is focused on. When feeding solids using a standard solids inlet design, a dense and uniform rotating fluidized bed is formed, guaranteeing intense gas–solids contact. The presence of both types of particles near the chimney region reduces, however, the strength of the central vortex and is detrimental for separation and segregation. Optimization of the solids inlet design is required, as illustrated by stopping the solids feeding. High-G separation and segregation of the batch of particles is demonstrated, as the strength of the central vortex is restored. The flexibility with respect to the gas flow rate of the bed density and uniformity and of the gas–solids separation and segregation is demonstrated, a unique feature of vortex chamber generated rotating fluidized beds. With the particles considered in this case study, turbulent dispersion by large eddies in the gas phase is shown to have only a minor impact on the height of the inner bed of small/light particles.

The intensification of interfacial mass, heat, and momentum transfer makes vortex chambers potentially interesting for the efficient drying of paddy, allowing shorter drying times and/or more compact equipment. The presence of a shell introduces particular challenges. Intraparticle diffusion limitations are strong and may reduce the advantage from intensified interfacial mass and heat transfer and the efficiency of air usage. Furthermore, high shear and normal stresses in the fast rotating particle bed may cause damage to the paddy shell, posing problems for transport and storage. With these specific aspects in mind, the use of vortex chambers for paddy drying is experimentally evaluated.

High-G fluidization in vortex chambers allows intensifying the drying of granular materials. The modeling, simulation and scale-up of vortex chamber based biomass dryers are addressed. Non-stationary experiments, batch for the biomass, are carried out to complement the data on continuous woody biomass drying in vortex chambers available in the literature. The drying models differ in the way they do or do not account for interfacial mass and heat transfer limitations, a non-uniform distribution of the moisture in the biomass particles and intra-particle diffusion limitations. Discrimination between the different proposed drying models and estimation of the model parameter(s) follows from simulations of both the continuous and batch experiments and regression. The retained biomass drying model is then used to study scale-up of the technology, focusing on continuous operation. Two major issues are addressed: (i) the product uniformity and (ii) the air consumption and utilization. Different vortex chamber configurations are simulated and analyzed: a single chamber or different chambers operated in parallel or in series (compartmented), allowing introducing a CSTR-in-series type behavior for the particles, combined with air feeding in parallel or in series over the different chambers.

The intrinsic kinetics of Steam Methane Reforming (SMR) on a non-conventional nanostructured and strongly adherent Ni coating on a metal substrate was experimentally studied using an integral packed bed reactor. The coating was characterized by means of SEM, N2 adsorption/desorption, EDX, XRD and TPR. The reactor was designed and the operating conditions selected to guarantee negligible interfacial and intra-particle transport limitations, plug flow, isothermal operation and a sufficiently small pressure drop. Experiments were carried out at temperatures between 450 and 600 °C, space times between 0.033 and 0.1 mol/(gcat.s) and steam-to-carbon ratios of 2.87 to 5.53. Discrimination between potential reaction mechanisms and rate determining steps and estimation of the rate parameters and their confidence intervals followed from regression and statistical and physicochemical testing. Measurements confirmed that the water gas shift reaction reached equilibrium for each condition. A comparison with reported intrinsic kinetics for a conventional SMR catalyst was made and optimal catalyst coating thickness, accounting for intra-catalyst diffusion limitations was evaluated.

Gasification is the thermochemical conversion of solid fuel into the gas which contains mainly hydrogen, carbon monoxide, carbon dioxide, methane and nitrogen. In gasification, fluidized bed technology is widely used due to its various advantageous features which include high heat transfer, uniform and controllable temperature and favorable gas–solid contacting. Modeling and simulation of fluidized bed gasification is useful for optimizing the gasifier design and operation with minimal temporal and financial cost. The present work investigates the different modeling approaches applied to the fluidized bed gasification systems. These models are broadly classified as the equilibrium model and the rate based or kinetic model. On the other hand, depending on the description of the hydrodynamic of the bed, fluidized bed models may also be classified as the two-phase flow model, the Euler–Euler model and the Euler–Lagrange model. Mathematical formulation of each of the model mentioned above and their merits and demerits are discussed. Detail reviews of different model used by different researchers with major results obtained by them are presented while the special focus is given on Euler–Euler and Euler–Lagrange CFD models.

This review deals with gas–solid fluidized beds in vortex chambers. High-G fluidization can be achieved in a static geometry and allows significant process intensification. Thin, dense and more uniform particle beds can be obtained at high gas–solid slip velocities, intensifying interfacial transfer of mass, heat and momentum and reducing the gas–solid contact time. Existing fluidized bed processes can be carried out more efficiently and novel processing routes can be developed, e.g., involving cohesive particles or a dispersed liquid phase in relatively high concentrations. The first section of the review discusses the unique hydrodynamic characteristics of gas–solid fluidized beds in vortex chambers. The flow pattern, flexibility in the operating conditions and stability conditions are explained. The design of vortex chambers is dealt with in the second section and is critical for processing both larger and fine particles. The influence of the gas and solids in- and outlet design is focused on and insight is gained from recent theoretical, experimental and CFD studies. In the third section (potential) applications are discussed and process intensification and novel processing routes demonstrated. The fourth and last section presents extensions of the concept. Multi-zone operation and the integration of other technologies in vortex chambers are considered.

Vortex chambers allow high-G fluidization and are as such potentially interesting for processing cohesive particles. The first part of the paper deals with the formation of a rotating bed of cohesive particles in a vortex chamber. A careful vortex chamber design is required to generate a sufficiently strong high-G field and related sufficiently high particle residence time. Next, the low-temperature coating of the particles is addressed. A centrally positioned single-fluid pressure spray nozzle directed towards the bed is used. The influence of a dispersed coating solution on the bed stability and the segregation of uncoated and coated particles are studied. Agglomeration and the particle coating quality are analyzed by means of laser diffraction based particle size distribution measurements, SEM imaging and an active component release test. Because vortex chambers are extremely flexible with respect to the gas flow rate, the variation of the latter to control the agglomeration is focused on. The influence of the coating time and the coating solution injection pressure are also studied and the maximization of the coating efficiency discussed.

The performance of dual-zone structured catalytic reactors in natural gas steam reforming under typical commercial operating conditions is evaluated by means of detailed Computational Fluid Dynamics (CFD) simulations, including detailed reaction kinetics. A hybrid CFD model is developed, combining a porous medium type description of the reactor core zone and a detailed description of the reactor internals in the near-wall region. The porous medium model parameters are determined from specific pressure drop measurements in a wide mass flow rate range. Additional pressure drop measurements with the complete structure and with pellets serve for CFD model validation. The influence of the flow and catalyst distribution over the different reactor zones and the influence of the catalyst loading are then numerically studied. A comparison with a conventional packed bed reactor is made, and advantages of the dual-zone structured reactors with respect to pressure drop, heat transfer, and catalyst effectiveness are illustrated.

In atmospheric-pressure plasma reactors, the flow dynamics can be complex, determine the reactor performance and complicate scale-up. Coupling computational fluid dynamics to the calculation of the electric field and plasma chemistry is challenging because of the numerical stiffness introduced by the difference in time scale of the different phenomena involved. Focusing on low-temperature, atmospheric-pressure pure nitrogen plasma, a model and model reduction based solution strategy to deal with the numerical stiffness are presented and evaluated. The influence of the electric field on the flow dynamics and species concentration fields is first qualitatively studied by means of 2D simulations of a sharp-end plate-to-plane configuration. Next, a specific reactor prototype for low-temperature, atmospheric-pressure plasma-enhanced chemical vapor deposition for in-line surface treatments is simulated to illustrate the importance of accounting for the detailed flow dynamics.

Two design aspects of vortex chambers for the generation of gas-solids rotating fluidized beds are experimentally studied for different types of particles: the solids outlet(s) and the gas inlets. Efficient solids retention and minimal solids losses via the chimney are aimed at so that the gas and solids residence times can be controlled independently. The importance of a strong vortex in the central particle bed freeboard region is demonstrated. It is shown that separate, well-dimensioned and -positioned solids outlets prevent a significant presence of particles in the freeboard region, increasing the vortex strength in this region. This is found to be particularly important when fluidizing small/light particles. The ratio centrifugal force-to-radial gas-solid drag force that is generated by the gas injection is shown to also have an important impact. Theoretically it is shown that this ratio strongly depends on the particle characteristics and to what extent it can be increased by increasing the gas injection velocity, preferentially by reducing the gas inlet slot size and otherwise the number of gas inlet slots. Experiments with different vortex chambers and particles qualitatively confirm the theoretical expectations, but show that limitations are encountered. A very high gas injection velocity prevents efficient penetration of especially fine/light particles in the gas inlet jets which is detrimental for the transfer of tangential momentum between the gas and the particle bed. Slots smaller than the particle size are also shown to be inefficient, as they generate rotational motion of the particles around their own center of gravity.

The performance of a conventional fluidized bed and a rotating fluidized bed in a static geometry (RFB-SG) for the drying of woody biomass is experimentally studied. The performance with respect to the specific biomass drying rate, product uniformity, and air utilization is evaluated. RFB-SGs are shown to intensify the drying process by easily one order of magnitude, due to a combination of increased gas–solid slip velocities and improved particle bed density and uniformity. The cost in terms of increased and less efficient air utilization can be minimized by optimizing the RFB-SG drying chamber design.

This chapter deals with rotating fluidized beds in static vortex chambers. High-G operation allows operation in a range of conditions very different from those in conventional fluidized beds, e.g. achieving extremely short gas–solid contact times. High-G operation also allows intensifying interfacial transfer of mass, heat and momentum. The latter allows generating high-density fluidized beds at high gas–solid slip velocities and reducing bubbling, opening perspectives for significantly increasing the efficiency of fluidized bed devices. Intensified interfacial momentum transfer also allows fluidizing particles that cannot be properly fluidized in conventional fluidized beds, such as large Geldart D- and fine/cohesive Geldart C-type particles, opening perspectives for a variety of novel applications in the energy, food and feed and pharma industries. After an introduction to high-G fluidization, the hydrodynamic characteristics and design aspects of vortex chambers for the generation of rotating fluidized beds are discussed. Next, illustrations of (potential) applications taking advantage of the intensified interfacial mass and heat transfer and of the intensified reactions are presented. The chapter ends with extensions of the concept and an outlook.

The ZoneFlow reactor (Tribute Creations, LLC) is a tubular reactor with two types of internals: a core type and adjacent to the wall, a casing type, both covered with a thin layer of catalyst. It aims for higher energy efficiency and lower steam-to-carbon ratios in methane steam reforming by a judicious design of the relative position of the internals and their geometrical characteristics and a higher catalyst effectiveness. The performance of this novel structured catalytic reactor for synthesis gas production by methane steam reforming was simulated using a reactor model accounting in detail for the flow pattern by means of Computational Fluid Dynamics (CFD). A Reynolds Averaged Navier Stokes (RANS) approach was taken. Turbulence, heat transfer by convection and radiation, detailed reaction kinetics including coking, intraparticle diffusion limitations, and the compressibility of the gas phase were accounted for. Simulations were carried out to investigate the ZoneFlow reactor performance under typical commercial operating conditions and compare it with a conventional packed bed reactor using the same Ni catalyst.

Computational Fluid Dynamics is used to evaluate the use of a rotating fluidized bed in a static geometry for the catalytic cracking of gas oil. A Eulerian–Eulerian flow model is used in combination with the Kinetic Theory of Granular Flow. The catalytic cracking reactions are described by a 10-lump model. Catalyst deactivation by coke formation is included. To operate at low catalyst coke content, the catalyst residence time is small and the catalyst makes on average only a limited number of rotations in the reactor. Therefore,the catalyst bed cannot be considered well-mixed and a local distribution of the catalyst coke content is to be accounted for. The catalyst coke content distribution function has no pre-described functional form and is discretized. A continuity equation is then solved for each of the classes of catalyst with a given coke content. The impact of the strongly endothermic cracking reactions on the particle bed temperature uniformity is also studied.

Riser outlet effects induced by an L-outlet and by abrupt T-outlets with different extension heights and outlet surface areas are studied experimentally and computationally. Experiments are carried out in a cold flow riser. The cold flow riser has a diameter of 0.1 m and a height of 8.765 m and is operated in the dilute flow regime with a superficial gas velocity of 2.48-7.43 m/s and a solids flux of 3.0 kg/m²/s. Particle velocities are measured using Laser Doppler Anemometry (LDA). Vortex formation in the extension part of the riser is observed. The vortex circulates the solids along the wall opposite to the outlet, thus inducing a solids reflux. The flow pattern upstream the outlet is, however, hardly affected in the small diameter riser. The vortex position and length are affected by the extension height, but hardly by the outlet surface area and the superficial gas velocity. The use of an L-outlet significantly reduces the vortex formation. The experimental measurements are used to validate a 3D Eulerian–Eulerian and Kinetic Theory of Granular Flow (KTGF) based gas–solid flow model. In general, the calculated trends are qualitatively in agreement with the experimentally observed phenomena. The exact shape of the vortex is not always accurately predicted.

The concept and use of a rotating chimney for compressing rotating fluidized beds is theoretically and experimentally studied. A rotating chimney is shown to drastically increase the operating flexibility of rotating fluidized beds and to allow a significant improvement of the particle bed uniformity. In particular, the rate of solids losses via the chimney can be reduced and bubbling can be suppressed by compressing the rotating particle bed. (c) 2009 Elsevier B.V. All rights reserved.

The application of a rotating fluidized bed in a static geometry to fluid catalytic cracking is evaluated by means of computational fluid dynamics (CFD) simulations using an Eulenan-Eulenan model and the kinetic theory of granular flow The reactions arc described by a 10-lump model. First, the reaction kinetics is based on currently allowable clacking temperature and catalyst activity. Typical reactor dimensions required are presented, and an evaluation of the process intensification potential is made, based on a comparison with riser technology Next, the possibility of using a higher cracking temperature or a more active catalyst is evaluated

Gas-solid heat transfer in rotating fluidized beds in a static geometry is theoretically and numerically investigated. Computational fluid dynamics (CFD) simulations of the particle bed temperature response to a step change in the fluidization gas temperature are presented to illustrate the gas-solid heat transfer characteristics. A comparison with conventional fluidized beds is made. Rotating fluidized beds in a static geometry can operate at centrifugal forces multiple times gravity, allowing increased gas-solid slip velocities and resulting gas-solid heat transfer coefficients. The high ratio of the cylindrically shaped particle bed "width" to "height" allows a further increase of the specific fluidization gas flow rates. The higher specific fluidization gas flow rates and increased gas-solid slip velocities drastically increase the rate of gas-solid heat transfer in rotating fluidized beds in a static geometry. Furthermore, both the centrifugal force and the counteracting radial gas-solid drag force being influenced by the fluidization gas flow rate in a similar way. rotating fluidized beds in a static geometry offer extreme flexibility with respect to the fluidization gas flow rate and the related cooling or heating. Finally, the uniformity of the particle bed temperature is improved by the tangential fluidization and resulting rotational motion of the particle bed. (c) 2008 Elsevier Ltd. All rights reserved.

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. (C) 2008 Elsevier B.V. All rights reserved.

The new concept of a rotating fluidized bed in a static geometry opens perspectives for fluidized bed nuclear reactor technology and is experimentally and numerically investigated. With conventional fluidized bed technology, the maximum attainable power is rather limited and maximum at a certain fluidization gas flow rate. Using a rotating fluidized bed in a static geometry, the fluidization gas drives both the centrifugal force and the counteracting radial gas-solid drag force in a similar way. This allows operating the reactor at any chosen sufficiently high solids loading over a much wider fluidization gas flow rate range and in particular at much higher fluidization gas flow rates than with conventional fluidized bed reactor technology, offering increased flexibility with respect to cooling via the fluidization gas. Furthermore, the centrifugal force can be a multiple of earth gravity, allowing radial gas-solid slip velocities much higher than in conventional fluidized beds. The latter result in gas-solid heat transfer coefficients one or multiple orders of magnitude higher than in conventional fluidized beds. The combination of dense operation and high fluidization gas flow rates allows process intensification and a more compact reactor design.

To account for meso-scale phenomena in coarse grid simulations, correlation terms appearing in the filtered gas-solid flow equations have to be modeled. Possible approaches to describe filtered gas-solid momentum transfer are evaluated via a mixture speed of sound test. In contrast to the effective drag coefficient closure model for the filtered drag force, the generalized added mass closure model for the correlation between the solid volume fraction and the gas phase pressure gradient shows an acceptable behavior. Furthermore, the generalized added mass may become determining in describing filtered gas-solid momentum transfer when sufficiently coarse grids are used. 3D steady-state riser simulations demonstrate the impact of the generalized added mass based description of filtered gas-solid momentum transfer on the calculated gas-solid flow behavior, in particular on the gas-solid slip velocity profile. (c) 2007 Elsevier Ltd. All rights reserved.

The propagation speeds of linear waves in gas-solid suspensions depend strongly on the solids volume fraction and the wave frequency. The latter is due to gas-solid momentum transfer and allows a simple test on filtered gas-solid momentum transfer models. Such models may predict linear wave propagation speeds different from those obtained with the nonfiltered model at wave frequencies higher than the filter frequency, but not at wave frequencies lower than the filter frequency. For the filtered drag, an effective drag coefficient approach is shown to alter the linear wave propagation speeds in the entire wave frequency range, independent of the applied effective drag coefficient. Furthermore, as the effective drag coefficient decreases, the high frequency linear wave propagation speeds are gradually introduced at lower wave frequencies. For the filtered momentum transfer due to the correlation between the solids volume fraction and the gas phase pressure gradient, the behavior of an apparent added mass closure model and an apparent history force closure model are investigated. An apparent added mass introduces the filter frequency linear wave propagation speeds to frequencies higher than the filter frequency. The linear wave propagation speeds for wave frequencies lower than the filter frequency are, however, not altered. Furthermore, an apparent added mass introduces no intrinsic wave frequency dependence in the linear wave propagation speeds, in agreement with its source term in the non-filtered model. Hence, the frequency dependence of the linear wave propagation speeds at frequencies lower than the filter frequency is still to be provided by a drag type term. As such, the behavior introduced by an apparent added mass is acceptable for filtered models. Also, to a certain extent, an apparent added mass can restore the linear wave propagation speed behavior at wave frequencies lower than the filter frequency altered by an effective drag coefficient approach. The reformulation of the apparent added mass in terms of an apparent distribution of the filtered gas phase pressure gradient over the phases and an apparent (effective) drag force is investigated. An apparent history force introduces intrinsic wave frequency dependence in the linear wave propagation speeds and alters the latter from the low wave frequencies on. As such, the behavior introduced by an apparent history force is unacceptable for filtered models, (C) 2006 Elsevier Ltd. All rights reserved.

The new concept of a rotating fluidized bed in a static geometry (RFB-SG) is presented. The rotating motion of the particle bed and the tangential fluidization of the solids are obtained by the tangential injection of the fluidization gas via multiple gas inlet slots in the outer cylindrical wall of the fluidization chamber. The solids experience a radially outwards centrifugal force. The gas, on the other hand, is forced to move radially inwards towards a chimney with one or more outlet slots, creating a radially inwards gas-solid drag force and fluidizing the solids radially. The new fluidization concept is experimentally investigated and proven using one and the same non-optimized fluidization chamber design with either large diameter, low density polymer particles or small diameter, higher density Alumina particles. The fluidization chamber is operated in continuous mode, at different solids loadings and in the vertical and horizontal position. (c) 2007 American Institute of Chemical Engineers.

Inlet phenomena in a 0.1 m diameter cold-flow riser with a 35 ∘ inclined side inlet are studied experimentally using 3D-Laser Doppler Anemometry for solids fluxes of 0.5– 4.5 kg / m 2 / s and gas velocities of 5.3–7.4 m/s. In the vicinity of the solids inlet, radial gas-solids mixing is hindered and bypassing of the solids jet occurs, resulting in steep velocity gradients and off-centre maxima in the velocity field. The feeding conditions and the type of the solids affect the bottom operation and gas–solids mixing to a large extent: compared to FCC particles, silica particles extend the acceleration zone in the riser. Low gas flow rates and/or high solids feeding rates result in an increased penetration depth of the solids jet and in explicit bypass zones in the plane facing the inlet. High root mean square fluctuating particle velocities are observed at the solids jet boundaries. A non-aerated Y-inlet configuration causes vortex formation, inducing a small reflux into the upper dilute part of the standpipe. The influence of dilution of the inlet solids jet is also investigated using an “aerated” inlet configuration. Aerated inlets lead to better entrainment, improved radial mixing, less pronounced broader bypass zones and a firm reduction of the penetration depth. In the 0.1 m diameter riser, radial mixing quickly dissipates the non-uniformities introduced by the solids inlet. Reflection phenomena can, however, occur in the case of a non-aerated solids inlet.

Gasification is the thermochemical conversion of solid fuel into the gas which contains mainly hydrogen, carbon monoxide, carbon dioxide, methane and nitrogen. In gasification, fluidized bed technology is widely used due to its various advantageous features which include high heat transfer, uniform and controllable temperature and favorable gas–solid contacting. Modeling and simulation of fluidized bed gasification is useful for optimizing the gasifier design and operation with minimal temporal and financial cost. The present work investigates the different modeling approaches applied to the fluidized bed gasification systems. These models are broadly classified as the equilibrium model and the rate based or kinetic model. On the other hand, depending on the description of the hydrodynamic of the bed, fluidized bed models may also be classified as the two-phase flow model, the Euler–Euler model and the Euler–Lagrange model. Mathematical formulation of each of the model mentioned above and their merits and demerits are discussed. Detail reviews of different model used by different researchers with major results obtained by them are presented while the special focus is given on Euler–Euler and Euler–Lagrange CFD models.

Simultaneous solution algorithms for Eulerian-Eulerian gas-solid flow models are presented and their stability analyzed. The integration algorithms are based on dual-time stepping with fourth-order Runge-Kutta in pseudo-time. The domain is solved point or plane wise. The discretization of the inviscid terms is based on a low-Mach limit of the multi-phase preconditioned advection upstream splitting method (MP-AUSMP). The numerical stability of the simultaneous solution algorithms is analyzed in 2D with the Fourier method. Stability results are compared with the convergence behaviour of 3D riser simulations. The impact of the grid aspect ratio, preconditioning, artificial dissipation, and the treatment of the source terms is investigated. A particular advantage of the simultaneous solution algorithms is that they allow a fully implicit treatment of the source terms which are of crucial importance for the Eulerian-Eulerian gas-solid flow models and their solution. The numerical stability of the optimal simultaneous solution algorithm is analyzed for different solids volume fractions and gas-solid slip velocities. Furthermore, the effect of the grid resolution on the convergence behaviour and the simulation results is investigated. Finally, simulations of the bottom zone of a pilot-scale riser with a side solids inlet are experimentally validated.

The influence of the inlet configuration on the flow pattern in dilute circulating fluidized beds is investigated. The solid flow model used is based on the kinetic theory of granular flow (KTGF). Interactions between the fluctuating motion of the gas and the solid phase are accounted for. A line-implicit simultaneous solution algorithm based on dual-time stepping is used for the numerical integration. The discretization of the inviscid fluxes is based on a low-Mach reformulation of the multi-phase preconditioned advection upstream splitting method (MP-AUSMP). Simulation results for a side solids inlet configuration are qualitatively verified using 3D LDA data of a cold-flow circulating fluidized bed pilot unit. The pilot riser has a diameter of 0.1 m and a height of 8.765 m. The side inlet used for solids feeding is positioned 0.58 m above the gas inlet and makes a 35° angle with the riser axis. The cold-flow pilot unit is operated in the dilute regime with a superficial gas velocity of 5.31 m s−1 and a solids flux of 3 kg m−2 s−1. Experimental and simulation results show that bypassing of solids by the gas occurs, but that the bypassing effects are quickly dissipated by the viscous terms. The gas–solid flow model is capable of describing the experimentally observed mixing behaviour. In simulations of dilute industrial scale risers, the gas is fed over the entire bottom section, whereas two kinds of solids inlet configurations are used: (1) the solids are fed through a central inlet tube at the bottom of the riser; (2) the solids are fed through a side inlet at a certain distance from the bottom of the riser. The outlet is of the abrupt T-type. With the first inlet configuration, the radial mixing of the solids is seen to be hindered, due to bypassing of the gas via the outer ring of the reactor tube. The bypassing effects are not quickly dissipated by the viscous terms. The impact of the latter is less important in large than in small diameter risers. Bypassing of the gas also plays an important role with the second inlet configuration. At the height of the solids inlet, the gas is seen to flow preferentially aside of the solids inlet and at the opposite site of the riser. This results in vortex formation, responsible for the primary mixing of gas and solids. More downstream, the maximum solids flux is calculated at the side of the solids inlet. Radial mixing is seen to be related to the granular temperature and the gas phase turbulence. The dilute industrial scale riser simulations show that inlet and outlet configuration effects are not independent, but interact. This has an important impact on radial mixing. Inlet and outlet effects can oppose each other or cooperate, resulting in complex riser behaviour.

When coarse-grid calculations in two-phase flow are performed, mesoscale fluctuations, such as clusters, cannot be explicitly captured. Their impact on macroscale fluctuations, however, has to be taken into account by the introduction of an appropriate closure model. A new closure model to describe gas−solid drag is introduced. The effect of particle clustering on the interphase momentum-transfer coefficient is taken into account by introducing the concept of effective drag. Clustering results in a decreasing value of the interphase momentum-transfer coefficient because the contribution of particles in the cluster is considered to be negligible in dilute phase flow. For solids fractions greater than 1%, clustering phenomena become increasingly important, resulting in an important decrease of the interphase momentum-transfer coefficient. Calculation results for a riser reactor, both neglecting and including the effects of clustering on the gas−solid drag, are presented and validated with experimental data....

Simultaneous adsorption of SO 2 –NO x in a riser configuration is a novel route for flue gas cleaning. The riser operates at a low flux (2 kg m −2 s −1 ) of small diameter (d p =60 μm ) Na- γ -Al 2 O 3 sorbent particles. The reaction scheme is adopted from previous work (Ind. Eng. Chem. Res. 40 (2001) 119), without adjusting any of the kinetic parameters. The significant concentration gradient between the gas and solid phase mainly arises from the low solid fraction (typically 5×10 −4 ) in the riser. Enhancing the fluctuating kinetic motion of gas and solid phase increases the SO 2 adsorption, whereas the NO adsorption is decreased marginally. The solid recirculation in the top section of the riser, induced by the abrupt T outlets, significantly decreases the NO and NO 2 removal, while the SO 2 removal remains mostly unaffected. Therefore, it is desirable to avoid recirculation for a maximum NO x removal. A comparison of the 3D and a 1D model shows that higher SO 2 and NO removal efficiencies are predicted by the 3D model in the major part of the riser. However, these positive effects are largely neutralized by the negative effects of the outlet-induced recirculation, resulting in similar overall removal efficiencies calculated by the two models. Unlike the 1D model, the 3D simulation shows a considerable axial variation in the solid fraction and slip velocity. The 3D simulation also allows to calculate the effects of outlet geometry on the flow and reaction fields. The reactor efficiency can be improved by modifying the outlet configuration to minimize the recirculation.

Gas–solid flow in circulating fluidized beds is calculated using the Eulerian–Eulerian approach with the kinetic theory of granular flow. The usefulness of this approach and the necessity of performing 3D calculations are illustrated by calculating the exit effects of single abrupt outlet configurations of different outlet surface area.

Simultaneous adsorption of SO2–NOx is a new, promising process in treating industrial flue gases in a short-contact-time dilute phase riser with Na/γ-Al2O3 sorbent particles of 50–100 μm diameter at 370–430 K. A major part of the sorbent is recycled for maximum utilization of the sorbent capacity before regeneration. A steady-state model for the riser adsorber is developed by combining independently determined rate equations with a 1-D, two-phase hydrodynamic model of the riser. The simulation was validated with the operating data from a pilot-scale installation of 4.2 Nm3/s flue-gas capacity and solid fluxes between 1 and 3 kg·m−2·s−1. The riser operates in an extremely dilute phase with a solid volume fraction below 7.0 × 10−4 and a slip velocity that is constant for almost the entire riser length, indicating fully developed flow. The experimental observation that an increase in the SO2–NO ratio in the feed gas leads to a higher NO removal efficiency is explained as resulting from an increased concentration of the adsorbed SO2. The SO2 removal efficiency, however, may drop if free adsorption sites are not sufficiently available. Recycling of the sorbent has only a marginal effect on the NO removal, but helps to maximize the SO2 adsorption. For the first time, the simultaneous removal of SO2 above 95% and of NOx above 80% is reported using a dry, regenerable sorbent in a riser reactor.

The simultaneous adsorption of SO 2 and NO x on Na-γ-alumina was studied by means of step experiments in a fixed bed plug flow reactor at 387 K and atmospheric pressure. Typically the molar composition of the feed gas was 1.5% SO 2 , 1% O 2 , 4000 ppm NO, 500 ppm NO 2 , and Ar. First the adsorption behavior of the pure components was measured. SO 2 and NO 2 adsorb easily, whereas NO and O 2 do not adsorb. Moreover there is no influence of O 2 on the adsorption behavior of the pure components. NO and O 2 adsorption require the simultaneous presence of SO 2 , NO, and O 2 . The NO and O 2 adsorption rate is enhanced by an increasing SO 2 /NO ratio. The total amount of SO 2 adsorbed is not affected by the simultaneous adsorption of NO and O 2 . However, NO 2 adsorption increases the SO 2 adsorption capacity. In the presence of NO 2 most of the adsorbed NO x is released as NO.

A transient kinetic model has been developed for the simultaneous adsorption of SO2−NOx on Na/γ-Al2O3 sorbent, using step response experimental data from a fixed-bed microreactor. The reactor temperature ranges from 367 to 407 K, and the molar SO2/NO ratio from 2 to 8 with and without NO2. SO2 and NO2 readily adsorb on the sorbent surface, producing SO2* and NO2* species. However, NO and O2 only adsorb simultaneously and in the presence of sufficient SO2* on the surface. The proposed mechanism consists of chemisorption steps for SO2 and NO2, whereas NO and O2 are adsorbed simultaneously via an Eley−Rideal step involving a surface species derived from SO2*. The latter is followed by several consecutive steps involving more SO2* species as well as O2, leading to the formation of a complex with a stoichiometry of SO2/O2/NO of 10/5.5/1. The enhanced SO2 sorption capacity in the presence of NO2 is described adequately by considering that the adsorbed NO2* opens a new reaction path for SO2 adsorption. Both the ...

The process intensification potential of rotating- and circulating rotating fluidized beds in a static fluidization chamber when used for the low-temperature pyrolysis and gasification of biomass is numerically evaluated. The species continuity equations and energy balance equations are based on complete mixing for the particles within given zones of the particle bed and plug flow for the gas. The reaction mechanism accounts for pyrolysis of biomass and tar, gas-phase combustion, the water gas shift reaction, and combustion and gasification of char. A comparison with current circulating fluidized bed riser technology is made. The circulation of inert solid with a high heat capacity and the separation of the flue gas from the production gas are also studied.

In different types of reactors, gas and solid particles are brought into contact and gas-solid mass and heat transfer is to be optimized. This is for example the case with heterogeneous catalytic reactions, the porous solid particle providing the catalytic sites and the reactants having to transfer from the bulk flow to the solid surface from where they can diffuse into the pores of the catalyst [Froment et al., 2010]. Gas-solid heat transfer can, for example, be required to provide the heat for endothermic reactions taking place inside the solid catalyst. Intra-particle mass transfer limitations can be encountered as well, but this chapter will focus on interfacial mass and heat transfer. The overall rate of reaction is on the one hand determined by the intrinsic reaction rate, which depends on the catalyst used, and on the other hand by the rates of mass and heat transfer, which depends on the reactor configuration and operating conditions used. Hence, the optimal use of a catalyst requires a reactor in which conditions can be generated allowing sufficiently fast mass and heat transfer. This is not always possible and usually an optimization is carried out accounting for pressure drop and stability limitations. This chapter focuses on fluidized bed type reactors and the limitations of conventional fluidized beds will be explained in more detail in the next section. To gain some insight in where gas-solid mass and heat transfer limitations come from, consider the flux expression for one-dimensional diffusion of a component A over a film around the solid particle in which the resistance for fluid-to-particle interfacial mass and heat transfer is localized

The application of various transient techniques in heterogeneous catalysis (TAP, step-response experiments, SSITKA, TEOM), with the aim to determine reaction kinetics for design purposes, is presented for several cases. These cases, comprising catalytic cracking, diffusion in zeolites, simultaneous NO, and SO, removal, syngas production from methane by chemical looping and selective catalytic reduction of NOx, show that transient techniques can be well used for the purpose of rapid determination of the reaction kinetics without the laborious classical approach of steady-state kinetic measurements and without the need of high levels of sophistication to interpret and process the experimental data. In this respect transient kinetics deserve, next to fundamental catalysis studies, more frequent application in design Studies for industrially relevant reaction systems. Topics and challenges for further developments in transient studies are indicated. (c) 2008 Elsevier B.V. All rights reserved.