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The conversion of lignite under Chemical Looping Combustion conditions differs sharply from air-fired combustion as high volumetric gas concentrations of CO2 and steam prevail in the reactor and the oxygen for the fuel conversion is provided by a solid oxygen carrier. In this work a lab scale fluidized bed reactor (ID = 53 mm) was used to measure the conversion behavior of German Rhenish lignite under Chemical Looping Combustion conditions. The carrier used was 14 wt-% copper oxide on γ-alumina. Special focus was laid on the particle size of the char during gasification, hence 3–6 different size fractions, in the range of 150–1500 μm were converted during each test series, respectively. The gasification experiments showed that gasification of lignite in a fluidized bed of inert sand particles is slightly faster for steam compared to CO2 gasification. In the inert sand bed, chemical reaction limitation of the kinetics could be seen for all lignite size fractions below 850 μm diameter. Gasification rates were in the range of literature findings for fluidized bed gasification in inert sand beds. The inhibition effects of both CO and H2 were reproduced by applying CO and H2 as fluidization gases during gasification. Gasification experiments in a fluidized bed of a completely oxidized oxygen carrier showed for both CO2 and steam gasification an enhancement of the char conversion, even more for smaller particle sizes of the lignite. Simultaneous gasification with CO2 and steam showed only slight increases in the measured gasification rates compared to steam only gasification. The Shrinking Core Model was generally able to describe the experiments. A resistance approach was used to describe the strong mass transfer limitations seen in the reactive bed for larger char particles particularly at higher temperatures. Kinetic parameters were extracted for the simultaneous steam and CO2 gasification under in-situ Gasification Chemical Looping Combustion conditions.

As a result of the increasing transport cost of biomass and benefiting from the more efficient treatment of a compacted, dustless product, the use of pelletized biomass has gained interest over rec...

A novel flowsheet simulation environment is applied to simulate a system of interconnected fluidized bed reactors as they are used for the process of chemical looping combustion of solid fuels. Dynamic models of the main process equipment are used in order to capture dynamic behavior of hydrodynamics inside an actual system, which delivers the experimental validation data. The process itself is carried out in a pair of strongly coupled fluidized bed reactors. Furthermore, a cyclone is used for gas-solid separation and two loop seal siphons prevent gas leakages from one reactor to another. The experimental plant operated at Hamburg University of Technology, which is modeled here, comprises a circulating fluidized bed air reactor and a two-stage bubbling bed fuel reactor. Operation of the plant is carried out at ambient condition and so are the simulations. All the mentioned process units are connected into a flowsheet via material streams and the whole process is simulated for 750 s of runtime. Within this time, the fluidization velocity of the fuel and the air reactor are changed to mimic effects, which occur during actual operation. The simulation was closely accompanied by experiments at the experimental facility and the results of simulation and experiments were compared. The bed mass, solid circulation and pressure profiles were simulated with good agreement to experimental findings. Dynamic effects were captured by the simulations and the system's response to varying the fluidization speed, that is the change of bed masses, was predicted correctly. Time scales, in which these changes occur, were in the same range for experiments and simulations.

Due to the advantages of high energy density and convenient transportation, the use of pelletized biomass as fuel for power supply has gained interest over recent years. Chemical looping gasification (CLG) integrates the process of gasification and hot gas conditioning, with the objective to obtain syngas with a low tar amount. The work presents some experimental results using sawdust pellet with high-volatile and low-ash content as fuel, and a natural manganese-iron ore as oxygen carrier for CLG in a bubbling fluidized bed. Without the addition of external steam, the syngas, tar, and char collected at different temperatures (750–950 ℃) and oxygen carrier to biomass ratios (0.2–1.2) were investigated to determine the gasification performance of sawdust pellet. Three-phase product distributions during CLG process were identified under different operating parameters. The increase of reaction temperature enhances the gas production, with decreasing amount of liquid and solid. Also, it has significant influence on the main gases (CO, CO2, H2 and CH4) distributions in the syngas. Meanwhile, more Polycyclic-Aromatic Hydrocarbons (PAHs) are cracked to Mono-Aromatic Hydrocarbons (MAHs) and further decomposition into syngas, which is confirmed by further Gas Chromatography-Mass Spectrometer (GC–MS) analyses. The Mn/Fe-based oxygen carrier also exhibits a beneficial effect on catalytic cracking of PAHs into MAHs. As for the remaining char, a higher temperature is more sensitive to produce the char with more porous structure and brittle texture.

This review of industrial applications of fluidized-bed reactors is focused on the fluid catalytic cracking (FCC) process and the fluidized-bed combustion. Both processes have a tremendous economic importance. In the FCC process, the main challenges are the increase of runtime, the adaption of the product spectrum to regional requirements of the market, and the adaption to heavier feedstocks. In fluidized-bed combustion the main trends are towards larger units sizes, higher efficiency, and further reduced emissions while keeping the boiler as flexible as possible with regard to the mixture of fuels.

In the current paper a simulation tool based on previous work of Burgschweiger and Tsotsas [1] is implemented within the framework of the flowsheet simulation program SolidSim [4]. The implementation within the flowsheet simulation program allows the simulation of the drying process for many different liquids and solids. It can also simulate the drying of mixtures of different solids. The module computes the vapor loading and temperature of the out flowing gas and the moisture and temperature distribution of the solids at the outlet taking into account their residence time distribution, their particle size and their moisture distribution at the inlet. Furthermore the model has been extended by a simple approach to simulate the drying in an elongated fluidized bed dryer. After validation the model has been used to study the influence of residence time, particle size and inlet moisture distribution in addition to drying of mixture of different types of solid. For the modeling it is assumed, that there is no classification occurring within the bed. This assumption is commonly fulfilled as long as the particle size distribution is not too wide and the fluidization velocity is sufficiently high. In this case the mixing action of the bubbles is dominating the segregation tendencies.

This study investigated the applicability of computational particle fluid dynamics (CPFD) numerical schemes for simulating flows in circulating fluidized bed (CFB) risers. Gas–solid flows were simulated in CFB risers containing Geldart A particles with both low and high solid fluxes as well as in a CFB riser containing Geldart B particles for three flow conditions using CPFD. The results are compared to experimental data and previous two-fluid model (TFM) simulations. The time-averaged axial and radial distributions of the solid concentration show that the bottom-dense, upper-dilute and core-annulus heterogeneous structures were successfully captured by the CPFD calculations, but only qualitatively. The results differ from experimental data for Geldart A particles and high solid fluxes, although they were more accurate than two-fluid simulations with conventional drag models. Two-fluid modeling with the EMMS (energy minimization multi-scale) drag model gave more accurate results than the CPFD simulations. The results indicate that the drag force in the CPFD scheme is still overestimated, although the cumulative method used to compute drag force is more accurate than the proportional method in the two-fluid model. An EMMS drag model which takes into account the intrinsic heterogeneity in the CFB risers is needed for the CPFD scheme. The effect of the realistic particle size distribution was seen in the by CPFD results.

In the present work a simulation module is developed for the description of a bubbling fluidized bed reactor and for the riser reactor of a circulating fluidized bed which is suitable for use in the framework of a flowsheet simulation system. A population balance approach is used to determine the particle size distribution within the reactor in steady-state. The model allows to describe the individual fate of particles in terms of attrition (abrasion and shrinkage) and transport effects. The fluidized bed reactor model is one-dimensional. Models of fluid mechanics are combined with kinetic approaches for the description of the local reaction. The implementation within the framework of a simulation package for solids processes allows for the flexible coupling of process models, e.g. of the fluidized bed reactor with one or more cyclone stages for gas cleaning or with other fluidized beds in fluidized bed systems.

In part I of this paper a new fluidized bed reactor module was presented for the simulation of a bubbling fluidized bed and of the riser of a circulating fluidized bed with the flowsheet simulation tool SolidSim so that several process models can easily be coupled. In the present part II the newly developed fluidized reactor modules are connected to existing cyclone modules to simulate a complex coupled fluidized-bed process, the selective oxidation of n-butane to maleic anhydride in a riser–regenerator system [1] , [2] . The calculation results for a laboratory setup are found to be in good agreement with experimental data from the literature. The subsequent simulation of an industrial scale sized plant shows that a high solids recirculation rate is needed to achieve satisfying maleic anhydride yield. The usefulness of flowsheet simulation is demonstrated by further process simulation calculations with varied solids recovery systems.

Attrition of catalyst represents a significant economic penalty due to the necessity to make-up the losses on a continual basis. Designing fluidized bed systems to minimize attrition may sacrifice gas–solid contact efficiency that requires pressure to distribute gases in process vessels uniformly. In this paper, the attrition performance of DuPont's vanadium pyrophosphate oxide catalyst (VPP) used in their commercial Circulating Fluidized Bed (CFB) process to convert n-butane to maleic anhydride has been examined. The analysis is based on simulating the commercial conditions in the individual vessels combined with the physical characteristics of the catalyst determined in the laboratory. The comparison of the catalyst loss rate measured in the industrial plant and the simulation results indicates that the measured values are below the values predicted by the simulation. However, the relative change in the catalyst loss rate with operating time is well described. The simulation allows the quantification of the attrition sources in the process. The simulation results revealed that in the present case over 60% of the catalyst loss originates from attrition in the fluidized beds, whereas only 16% originates from attrition in the cyclones. The simulation is an appropriate tool to investigate the influence of parameter changes. Using the simulation the design parameters and operating conditions can be optimized to minimize the catalyst loss rate in the industrial plant.

Modern production processes in chemical, pharmaceutical and biological industries are characterized by complex process structures, which consist of different apparatuses and process steps. Modeling the entire process requires simulating all units altogether, while taking into account interconnections between them. Nevertheless, in the area of solids processing, there is nowadays an unfilled gap from the side of computer support of process modeling in allowing effective optimization and prediction of the behavior of the whole plant. This paper presents a tool for flowsheet simulation which allows the simulation of the stationary behavior of complex processes dealing with solids and its extension towards dynamic modeling. Also, a new simulation concept is proposed on the basis of the multiscale approach. On the macroscale, flowsheet simulation is performed with the help of the SolidSim system. Parameters for the macromodels in SolidSim are predicted by microscale simulation. The models for the two scales are then coupled by inter-scale communication laws. Application of the proposed modeling concept is shown by an example of fluidized bed granulation.

In the present work, the steady-state behavior of a Chemical Looping Combustion process of interconnected fluidized bed reactors is simulated. The simulations have been carried out in two different scales, 50 kWth and 100 MWth . Attrition model derived from small scale laboratory experiments has been employed for the prediction of the process behavior in terms of attrition and Oxygen Carrier loss. Information on Oxygen Carrier characteristics and reaction kinetics were taken from literature. Realistic circulation mass flows of Oxygen Carrier particles are obtained and Oxygen Carrier losses are quantified. The large scale process looses significantly more Oxygen Carrier than the small scale process based on the same amount of thermal energy produced. Incomplete conversion in the air reactor could be identified as a critical point. Another issue is the fuel gas bypassing the Oxygen Carrier particles through bubbles in the large scale process which leads to lowered fuel conversions. The simulations indicate that a similar performance of a pilot scale and a large scale process is not guaranteed due to the scale-up effect on fluid dynamics. Furthermore, the simulations allow an assessment of the influence of the quality of the solids recovery system on the Oxygen Carrier loss. The distribution of the losses between possible origins is investigated and different changes in the solids recovery system are discussed regarding their potential to decrease the Oxygen Carrier loss. For example, the addition of a second-stage cyclone after the air reactor of the large scale process reduces the Oxygen Carrier loss significantly.

Nowadays, software tools for the flowsheet simulation of industrial processes are commonly used for design, simulation, balancing, troubleshooting and optimization purposes. Most of the tools are applicable to fluid processes only and cannot be effectively used for processes which involve solids. In this contribution we want to present the conceptual design of a new system applicable for the dynamic flowsheet simulation of complex solids processes. This system is developed as an enhancement to the existing simulation program. The novel software is able to simulate the unsteady behaviour of complex circuits of granulation processes. The transient behaviour during the start-up and changing of the process or material parameters can also be examined. As flowsheet examples, a typical spray granulation process with different schemes consisting of fluidized bed granulators, screens, mill and splitters was used. The mathematical model of the fluidized bed granulator is described by a one-dimensional population balance equation and coupled with heat and mass transfer and simple fluid dynamics. Received simulation results have shown that the proposed concept of the dynamic flowsheet simulation of granulation processes can be used effectively and has the potential to be generalized for other types of solids processes.

A model is presented for the simulation of reactive gas–solids flows in large industrial reactors. Circulating fluidized bed (CFB) combustors with several thousands of cubic meters reaction volume are probably the largest reactors of this type. A semi-empirical modeling approach has been chosen to model the three-dimensional concentration distributions of gas and solids components and temperatures inside the combustion chamber of such boilers. Two industrial CFB boilers are investigated in detail: the 105 MWe Duisburg combustor in Germany and the 235 MWe Turow combustor in Poland. The semi-empirical model approach is described first. Then the model is used to show how the three-dimensional concentration and temperature fields are formed by the interaction of several local phenomena. Good agreement between simulation and measurements has been achieved.

This paper proposes a strategy for the global sensitivity analysis in the flowsheet simulation on solid processes which allows to examine and quantify the influences of given parameters on specific target criteria. The strategy is basically divided into two parts: The first is the effect analysis which provides the range of possible values of a target variable. This permits an estimation of the accuracy of a simulation with uncertain parameters. As the second part the influence analysis builds a hierarchy of the different uncertain parameters according to their impacts on a specific target criterion. This latter information can be used to decrease the efforts for parameter estimation in a given process simulation by focussing the efforts on the most influential parameters. The strategy for global sensitivity analysis suggested here is applied to a soil washing process where an oil-contaminated soil is to be cleaned such that both the amount of the contaminant concentrate to be disposed and the residual oil contamination of the cleaned soil are to be minimized.

The optimization of a novel wet centrifugal fluidized bed classifier is discussed in this paper. Experiments in a pilot-scale apparatus and CFD calculations for the pure liquid flow reveal that the Coriolis force induces a strong tangential flow relative to the rotating vessel. Based on this finding a discharge mechanism for the coarse fraction has been developed which consists of a simple nozzle and thus allows a continuous and stable operation of the classifier. The measured pressure drop characteristic of the coarse particle bed reveals the range of fluidizing velocities which are optimal for the performance of classification. With the optimized classifier sharp classifications with a strong reduction of the misclassification of fines were achieved in different experiments with quartz, limestone and glass beads. Down to cut sizes of 5 μm sharp classifications were achieved.

In the current work, a model of the fluid mechanics in the riser of a circulating fluidized bed (CFB) has been implemented using computational fluid dynamics (CFD). The model developed shall be used in future as the basis of 3D-reactor model for the simulation of large scale CFB combustors. The two-fluid model (TFM) approach is used to represent the fluid mechanics involved in the flow. The computational implementation is accomplished by the commercial software FLUENT. Different closure formulations are tested on a simplified geometry. Two different turbulence formulations, namely the swirl modified RNG k–ɛ model and the Realizable k–ɛ model, are tested in combination with two different approaches to solid phase turbulence, namely the dispersion and per phase approach. One focus of the current work is put on the study of different drag correlations. Besides the drag correlations by Syamlal et al. [Syamlal, M., Rogers, W., & O’Brien, T. J. (1993). MFIX documentation theory guide. Technical Report DOE/METC-94/1004, U.S. Department of Energy (DOE). Morgantown Energy Technology Center: Morgantown, WV] and Gidaspow [Gidaspow, D. (1994). Multiphase flow and fluidization. New York: Academic Press] the EMMS model has been used to determine the momentum exchange between the two phases. The resulting formulation is then used to simulate a 1-m × 0.3-m cold CFB setup and is validated by experimental results [Schlichtharle, P. (2000). Fluid dynamics and mixing of solids and gas in the bottom zone of circulating fluidized beds. Unpublished doctoral dissertation, Technische Universitaet Hamburg-Harburg, Shaker Verlag: Aachen].

The concept of simulation-supported measurement is suggested for the elucidation of processes occurring in the combustion chambers of large-scale circulating fluidized bed combustors where the desired information cannot be obtained by direct measurements. The concept is illustrated with the example of secondary air injection where the way the air is released, the penetration depth and the evenness of air distribution over the cross-sectional area of the combustion chamber are of interest. The measured information consists of lateral profiles of oxygen concentrations measured with gas sampling probes at two ports which were located 5 and 9 m, respectively, above the level of secondary air injection. The simulation is carried out on the basis of a 3D semi-empirical fluid-mechanical model of the circulating fluidized bed which is combined with models of gas and solids mixing, fuel distribution, devolatilization and combustion of char and volatiles. The combination of the simulation with the measurements yields a clear picture of the mechanism of secondary air injection, its penetration into the combustion chamber and its effect on the local combustion processes. The results confirm the usefulness of the concept of simulation-supported measurement for this application.

A dynamic simulation model of the particle population in a circulating fluidized bed combustor with external heat exchanger has been developed. It considers the fluid dynamic processes in the various parts of the system, as well as the particle attrition. To handle multiple solids types simultaneously and to fulfill the mass balances, some of the fluid dynamic sub-models taken from the literature were modified. The model allows to calculate the solids mass flows as well as the corresponding particle size distributions at any point inside the combustion system. The model has been applied to the combustion plant of Stadtwerke Neumunster in Germany, which operates on refuse-derived fuel. The particle balancing model provides new insights into the operating behavior of such a system. In particular, the calculation of the residence time of different particle classes in the system reveals a very broad distribution of size dependent average residence times, ranging from several minutes to a maximum of roughly 40 h. A size fraction exists between 100 and 300 μm with a maximum average residence time of about 40 h. The Preprint submitted to Elsevier Science 15 August 2008 simulation provides a means for examining possibilities to control the particle size distribution in the combustion system. It is shown how a recirculation of a fine ash fraction can be used to control the bed particle size distribution in the combustion chamber.

In the present work a method for the treatment of design specifications in the simulation of complex solids processes has been developed. Adding a design specification to the flowsheet of a given solids process means that the solid product should fulfill a certain requirement. For example, its Sauter mean diameter should be kept within a certain interval or the mass fraction of particles above a certain threshold should be kept below a certain limit. The method developed here is not only able to handle product specifications but also other stream properties of the process. These properties can be either distributed or concentrated. Even dependently distributed properties e.g. a particle size-dependent contamination can be handled. The treatment of design specifications has been implemented as an extension of the flowsheet simulation system SolidSim. Practical applications of the implemented extension are illustrated with three examples: (i) a simple process for the preparation of brown coal, (ii) a process for the treatment of gravel and sand and (iii) a complex process for the mechanical separation and dewatering of harbor sediments contaminated with heavy metals.

Ash management in fluidized bed combustion systems means keeping the particle size distribution of the bed inventory in a given range. A dynamic particle population balancing model was developed for this purpose. It was successfully applied to a refuse-derived fuel fired combustor and a coal-fired circulating fluidized bed combustor. Both were large-scale commercial units. The model uses the concept of the attrited ash particle size distribution which represents the particle size distribution of the attrited ash including the generated fines and replaces the consideration of the particle attrition in the model calculations. The model offers the possibility to gain additional information about the particle size distributions and the solids mass flows at any location of the fluidized bed system. In addition, the model provides information about the dynamic behavior of the plant and about mean residence times of particle size classes in the plant. Uncertainties about the ash formation characteristics of fuels make the management of the bed inventory a very important issue. In this context the population balancing model is used to predict the plant behavior under various operating conditions. The results of the calculations carried out give useful information about the possibilities to manage the ash inventory of such a plant. It could be shown that the recirculation of a fine fraction of the bottom drain solids is a very effective method to manage the particle size distribution of the bed inventory. The calculation results further reveal that the mean residence time of particles is strongly dependent on their size.

By combining the advantages of a high-pressure fluidized bed with the adjuvant properties of a supercritical fluid, namely RESS-process, solid proteins could be encapsulated by paraffin. Two different irregular shaped proteins, a model protein (bovine serum albumin, BSA) and a pharmaceutical protein (insulin) were investigated. Mixing the highly non-spherical protein particles with lactose allows its usage as bed material despite the unfavourable shape properties. The paraffin encapsulation of respective bed mixtures with BSA was successful with paraffin loadings up to 9.1 g/100 g bed and yields up to nearly 100%, depending on process parameters. During the encapsulation process, breakage of the bed material occurred, resulting in mean sauter diameters of even less than 35 μm. Dissolution tests pointed out, that 100% dissolution after more than 180 min could be achieved. Thus, the particle breakage induced by the nozzle jet during the coating process seems to support the encapsulation of fines. A principle application of the process for insulin encapsulation for a selected set of process parameters has been demonstrated.

Solids volume concentration and solids velocity were determined in an 8.5 m high circulating fluidized bed (CFB) riser with two types of bubble-cap distributors by applying a capacitance probe. In the bottom region of CFB, the solids volume concentration in the centre region is low, while solids concentration increases significantly towards the wall with the highest solids concentration at wall approaching the value at the packed bed. Furthermore, solids volume concentration at high-pressure drop of a bubble cap is lower than that at low-pressure drop of a bubble cap at all lateral positions. The pressure drop of the distributor has little influence on the profiles of solids volume concentrations and on the profiles of solids velocities in CFB upper dilute region. It has been found that the pressure drop of the distributor has little influence on the axial apparent solids concentration in upper dilute region and on the external circulation rate.

Many of the models used in the flowsheet simulation of solids processes are more or less empirical. In order the get reliable results from a flowsheet simulation it is often necessary to adjust model parameters to measured data. Because measured values are always erroneous to some degree in the first step a data reconciliation calculation has to be performed. Based on the reconciled data that fulfil the balance equations the adjustment of the model parameter can be done by use of numerical optimization methods. In the present work a method for data reconciliation as well as for the adjustment of model parameters has been developed with respect to solids processes. The developed methods have been implemented as extensions of the flowsheet simulation system SolidSim. The practical application of the developed extensions is illustrated by example of a process for the dressing of brown coal operated by RWE Power AG in Germany.

The flow pattern in the bed and in the freeboard of a liquid–solid fluidized bed operated in the centrifugal field was investigated. Centrifugal accelerations between 20 and 500 times the gravity were acting on the particles. Both the radius-dependent centrifugal acceleration and the Coriolis force were found to strongly influence the fluid dynamics. A CFD simulation of the liquid flow in absence of particles showed high tangential velocities which were two orders of magnitude higher than the radially oriented fluidizing velocity. Measurements with a 1 m diameter rotor confirmed the calculations and revealed that also the liquid–solid fluidized bed is moving in the tangential direction although with lower speed than the pure liquid. The bed expansion under these conditions can be described by a Richardson–Zaki type correlation. On a etudie le profil d'ecoulement dans le lit et dans l'espace libre d'un lit fluidise liquide–solide fonctionnant dans un champ centrifuge. Des accelerations centrifuges de 20 a 500 fois la gravite agissaient sur les particules. On a trouve que l'acceleration centrifuge dependante du rayon et la force de Coriolis ont une forte influence sur la dynamique des fluides. Une simulation CFD de l'ecoulement liquide en l'absence de particules montre des vitesses tangentielles elevees de deux ordres de grandeur superieures a la vitesse de fluidisation dans le sens radial. Des mesures avec un rotor de 1 m de diametre confirment les calculs et revelent egalement que le lit fluidise liquide–solide se deplace dans le sens tangentiel, quoique moins vite qu'avec le liquide pur. L'expansion de lit dans ces conditions peut etre decrite par une correlation de type Richardson–Zaki.

Die losungsmittelfreie Ausfallung wasserloslicher Substanzen und deren nachfolgende Verkapselung erlaubt es, ruckstandsfreie Verbindungen mit definierbarem Auflosungsverhalten herzustellen. Eine mit uberkritischem CO2 betriebene Hochdruckwirbelschicht bietet dabei eine Moglichkeit, auch temperaturempfindliche Substanzen wie Proteine zu behandeln. Im Beitrag wird die Proteinausfallung aus wassriger Losung auf biokompatible Tragerpartikel mit nachgeschalteter Paraffinbeschichtung beschrieben. Es zeigt sich, dass sich in einem Zweistufenprozess nahezu agglomerationsfreies beschichtetes Produkt herstellen lasst.

The performance of a catalytic fluidized-bed reactor is strongly dependent on the properties of the catalyst of which the particle-size distribution is one. The main influences on the particle-size distribution are attrition of the catalyst particles, and the classifying effect of the solids recovery system. In a fluidized-bed reactor a particle will be subjected to attrition due to different mechanisms in different parts of the system, namely attrition by gas jets near the bottom of the fluidized bed, bubble-induced attrition in the fluidized bed itself, and attrition during the passage through a cyclone. All these different attrition mechanisms are considered in this work by different mathematical models. It is known that a fresh catalyst is much more fragile and exhibits a much higher attrition rate at the beginning of exposure to mechanical stress than under steady-state conditions. Depending on the mechanism the particles need different times or in the case of attrition in a cyclone a certain number of passages to reach a constant value of the attrition rate. In the fluidized-bed system a particle will during its aging experience all the different attrition mechanisms. In order to summarize the effect of these stresses on the particle within the different parts of the fluidized-bed system, the concept of the “stress history” has been developed, which allows a uniform treatment of the different attrition mechanisms. This concept has been implemented into an existing population balance model. Experiments with a FCC-catalyst in a fluidized/bed-cyclone circulation system are well described by this model. © 2007 American Institute of Chemical Engineers AIChE J, 2007

In the present work a particle population balance model for a circulating fluidized bed combustor (CFBC) is developed, that allows the description of the fate of individual particles in terms of attrition and transport effects. Operating conditions and the actual particle size distribution (PSD) are considered in the description of the shrinking of particles by attrition. Furthermore, a submodel has been developed, that takes into account the fact, that fresh particles have a higher attrition rate than particles which stayed for some time in the system and which therefore have been subjected to some stress already. The presented population balance model is able to describe the dynamic adjustment of the PSD in a given system. It has been validated with experimental data from a pilot-plant combustor, i.e. a CFBC with an inner diameter of 0.1 m and a height of 15 m. A comparison between experimental and calculated data shows the applicability of the model. In a second step the model has been applied to predict the effect of a load change and a change of the fuel, respectively, on the PSD inside the boiler and thus on the operating characteristics

The existing literature data on the saturation carrying capacity G(S)* for vertical concurrent gas-particle flow are limited to Archimedes numbers Ar below 1000. By experimental measurement and dimensionless analysis the present article extended Ar to 2500 to demonstrate how G(S)* varies with gas velocity under these high Ar values. The obtained result revealed that G(S)* is subject to the same kind of correlation with as velocity irrespective of Ar. The degree that G(S)* depends on gas velocity, however, generally decreases with raising Ar and it tends to be a constant when Ar becomes higher than 530. This shows in fact that the flows with heavy/large particles or dense/viscous gas possess a low increase in particle-carrying capability with increasing gas velocity until a steady specific capability per m/s is reached at Ar of 530. The article also reanalyzed the influence of column diameter on G(S)*, demonstrating that the column diameter influence exists only in small columns and for flows with heavy/large particles. The influence is negligible in commercial conveying columns and differs according to Ar in those small columns employed in laboratory studies. All of these findings resulted in an upgraded correlation for G(S)*, which manifested not only wide applicability to flows in differently sized columns and with Ar of up to 2500 but also better accuracy in the mentioned flow conditions when comparing with several other literature correlations. (c) 2006 Elsevier Ltd. All rights reserved.

One of the standard tasks in modern chemical industry is solid–liquid separation. While numerous equipments are commercially available to the engineer for separation with cut sizes in the range of several tens of micrometers the number of capable devices is significantly reduced with decreasing cut size. For very small cut sizes in the range of 1–10 μm none of the established commercially available devices allows sufficient classification with reasonable throughputs and separation sharpness. The present work presents the simulation and optimization of a previously developed classifier that allows sharp separations of suspensions at industrial-scale throughputs with cut sizes in the micrometer range. The optimization was carried out by experimental screening of the influences of main parameters as well as numerical calculation of the flow profile with the CFD program CFX. With the knowledge of the flow profile it is possible to determine and minimize the sources of misclassified particles. The paper explains the results of numerical simulations and the derived optimization as well as its implementation into the design of the prototype and results of initial tests.

Aluminum coatings were created onto glass beads by chemical vapor deposition in a fluidized bed reactor at different temperatures. Two different routes were examined. First, tri-isobutyl-aluminum (TIBA) vapor was enriched in nitrogen and thermally decomposed inside the fluidized bed to deposit elemental aluminum. On the other hand, liquid injection of TIBA via a two-fluid nozzle directly into the fluidized bed was tested. To ensure homogeneous coating on the bed material, the fluidizing conditions necessary to avoid agglomeration were investigated for a broad range of temperatures. Also, different glass types and pretreatments of the substrate surface were investigated to elucidate the influence of the surface chemistry on the growth and morphology of the layers deposited.

A comprehensive experimental study has been carried out to investigate the fluidization behavior with supercritical carbon dioxide at pressures up to 30 MPa for various solids which behave as Geldart A and B powders, respectively, under ambient conditions. From pressure drop measurements minimum fluidizing velocities and bed voidages were determined. Capacitance probe measurements yielded suspension voidages, bubble sizes and velocities. It was found that Ergun's equation is also applicable to the flow of a supercritical fluid through a fixed bed and that the Wen and Yu relationship can be used for the determination of the minimum fluidizing velocity. Correlations for homogeneous bed expansion, minimum bubbling velocity and the visible bubble flow were derived.

A three-dimensional model of a circulating fluidized bed gasifier was developed, which uses continuous radial profiles of velocities and solids hold-up with regard to the description of fluid mechanics. A complex reaction network of sewage sludge gasification is included in the model. In the simulation calculations the influence of the axial location and the number of feeding points was examined for gasifiers of different scales. It was found that due to the very fast decomposition of the volatiles and the high volatile content in the sewage sludge, lateral mixing of the gas around the feeding port is not complete, and plumes with high pyrolysis gas concentrations are formed. Better mixing was found to be achieved by a larger number of fuel feeding ports which are recommended in the case of high-volatile fuels and are shown to be necessary for larger bed widths.

In the present work, a particle population balance for a circulating fluidized bed combustor (CFBC) is developed that allows a description of the fate of individual particles in terms of attrition and transport effects. In the modeling of abrasion and shrinkage of particles, we consider besides operating conditions and particle size distribution also the residence time of particles in the system. The model takes account of the fact that fresh particles have a higher attrition rate than particles that have stayed for some time already in the system. The model aims at the description of the dynamic adjustment of the particle size distribution in a given system. The model is validated with experimental data from a pilot plant scale combustor, i.e., a CFBC with an inner diameter of 0.1 m and height of 15 m. A comparison between experimental and calculated data shows the suitability of the model. The model gives furthermore the opportunity for studying the effects of changes in operating conditions on the steady state solids particle size distribution and solids mass fluxes. The results of modeling a load change of the CFBC are presented and discussed in this work.

Flow maldistribution was investigated in an 8.5 m high circulating fluidized bed with a rectangular cross section of 1 × 0.3 m using a 14-bubble-cap distributor and a 33-bubble-cap distributor. Experimental results indicate that the bubble caps in the center region have a large flow rate and the bubble caps at the wall region have a small flow rate. The extent of the flow maldistribution is quantitatively characterized by the ratio of the maximum flow rate through a bubble cap to the average flow rate through all bubble caps (maldistribution number, Vmax/Vav). A correlation is proposed for predicting flow rate distribution, maldistribution number, in the bubble cap distributor by the ratio of distributor pressure drop to riser pressure drop (pressure-drop ratio, ΔPd/ΔPr). © 2005 American Institute of Chemical Engineers AIChE J, 2005

A population balance model has been formulated to simulate the time-dependent behavior of the catalyst inventory of a fluidized-bed reactor/regenerator loop. The reactor is operated in the circulating fluidized bed regime while the regenerator operates in the bubbling bed mode. Both reactor and regenerator are equipped with cyclones for solids recovery. The model considers attrition from various sources, i.e., from grid jets, bubble action in the fluidized bed and inside the cyclones. The simulation calculations demonstrate the strong interaction between attrition and efficiency of the solids recovery. A high efficiency of the solids recovery and at the same time a low rate of attrition requires a special design and arrangement of the cyclones. The time-dependent simulations reveal that it takes a considerable time for the system to reach a steady state. The catalyst loss rate from the system stabilizes after roughly 1 week but the Sauter diameter of the catalyst inventory in the loop needs about 3 weeks before a steady state is reached under the conditions investigated.

A modeling approach is presented that is able to handle the most important aspects of industrial fluidized-bed reactors. A particular focus is to describe the relationship between catalyst attrition, solids recovery in the reactor system, and chemical performance of the fluidized-bed reactor. The competing influences of attrition of the catalyst particles and efficiency of the solids recovery lead to the etablishment of a catalyst particle size distribution in the bed inventory, which, in turn, influences via the hydrodynamic characteristics of the fluidized bed and, thereby, the performance of the chemical reactor. The usefulness of this approach is illustrated with model calculations for a fictituous first-order reaction in which the fluidized bed is equipped with different solids recovery systems including one single-stage cyclone, several cyclones in parallel, and two- and three-stage cyclone systems. Model calculations illustrate the importance of a high efficiency of solids recovery to keep the fine...

Solids volume concentration was determined in an 8.5-m-high circulating fluidized bed (CFB) riser with two types of bubble cap distributors by using capacitance probes. In the bottom region of the CFB, the lateral solids volume concentration in the center region is low with slight change, whereas lateral solids volume concentration increases significantly toward the wall with the highest solids concentration at wall. Furthermore, lateral solids volume concentration at high-pressure drop of a bubble cap is lower than that at low-pressure drop of a bubble cap. Horizontal solids volume concentration in the center region remains flat and rises toward the wall. However, horizontal solids volume concentration at entrance region ranges from 0.43 to 0.5 at low superficial gas velocity. The pressure drop of a distributor has little influence on lateral profiles of solids volume concentrations in CFB upper dilute region. It has been found that the pressure drop of the distributor has little influence on the axial a...

Co-combustion of sewage sludge together with coal or wood has been investigated in two circulating fluidized bed (CFB) plants, a laboratory scale plant and a pilot scale 12MWth CFB boiler, in both of which the gas residence times are comparable to those in commercial plant. The investigation focuses on emissions of harmful gases from co-combustion compared to mono-combustion in CFB and the influence of air supply. The result shows that co-combustion can be carried out in CFB plant designed for the base fuel without exceeding EU or German emission limits for sludge energy fractions of less than 25%, except for the chlorine emission that may have to be reduced by flue gas treatment. Although sewage sludge contains large quantities of nitrogen and sulfur, the beneficial properties of CFB lead to considerable reduction of nitrogen oxides, and only a few percent of the nitrogen was effectively converted to NO or N2O. Sulfur can be captured by conventional limestone addition, but for wood as a base fuel this method is not as efficient as for coal.

Various particles with sizes between 100 and 200 μm were encapsulated with waxes commonly used in technical coating applications. For this, a homogeneous mixture of molten paraffin and supercritical carbon dioxide was prepared in an autoclave and injected into the high-pressure fluidized bed through a nozzle from the bottom. Due to the different conditions in the mixing autoclave and the fluidized bed, the paraffin precipitated in the vicinity of the nozzle and adhered to the solid particles. A complete, thin, uniform, and solvent-free coating was produced. The use of two paraffins with different alkane compositions resulted in dissimilar spreads on glass beads due to their different glass transition temperatures. A smaller pressure drop across the nozzle led to more uniform and even coatings. Glass beads, ceramic spheres, potassium chloride, and lactose showed similar coating results, whereas different morphologies were observed with a plastic material, characterized by a rougher surface and a lower surface energy. The high quality of the coating was confirmed by standard dissolution tests with coated potassium chloride crystals and lactose agglomerates.

Jetting fluidized transition velocity (ujtv) is investigated in a 300 mm ×50 mm 2D jetting fluidized bed with two nozzles for eight materials, including four binary mixtures. The experiments show that the peak in the curve major frequency versus jetting gas velocity corresponds to ujtv. The ujtv increases with increasing nozzle distance, particle diameter, but decreases with increasing static bed height and nozzle diameter. A correlation is proposed for prediction of ujtv.

An experimental investigation of the gas and solids flow in a pilot-scale circulating fluidized bed (CFB) cold model with two different abrupt exit configurations (L-shape and extended top) has been carried out. Measurements of axial pressure profiles, high-speed video images of the flow phenomena at the wall as well as local optical probe measurements inside the exit zone are presented. Contrary to published results obtained in bench-scale CFB risers the axial profiles of the apparent solids volume concentration obtained by pressure measurements showed no indication of an increased solids hold-up in the vicinity of the exit, which confirms the conclusion by Pugsley et al. (Can. J. Chem. Eng. 75 (1997) 1001) that this is a scale effect. The local probe measurements showed the well-known core–annulus flow structure prevailing until the riser top. In the vicinity of the exit this flow structure is superimposed by a strong horizontal velocity component directed to the exit duct. In comparison to the conventional L-shaped abrupt exit the extended top does not increase the solids inventory in the riser.

As an initial step for predicting the energy conversion efficiency of circulating fluidized bed (CFB) combustors and for future emission modeling a model is presented which estimates the char holdup and the spatial distribution of the char particle size distribution inside a CFB combustion chamber based on the initial fuel size distribution and the operating conditions of the combustor. For the prediction of the three-dimensional flow structure the model employs semiempirical correlations drawn from measurements. The submodel describing the fate of a single particle entering the combustion chamber is divided into three steps, namely, devolatilization, primary fragmentation and char combustion, which are treated independently. The model was validated against measurements conducted at the CFB combustor of Chalmers Tekniska Hogskola (CTH) in Goteborg/Sweden. Subsequent model calculations for different fuels are demonstrating the influence of the fuel characteristics on the char holdup.

Dynamic behaviors were investigated using ashes of three sizes in an 82-mm-i.d. stainless steel fluidized bed at temperatures up to 1000 °C. Pressure fluctuation signals were analyzed using power spectral density function (PSDF) analysis, chaos analysis, and wavelet analysis. The major frequency of the pressure fluctuation signals is in the range from 20 to 80 Hz in the fixed state, whereas the major frequency ranges from 1 to 9 Hz in the fluidized state. The correlation dimension and Kolmogorov entropy are zero in the fixed state, and then they increase with increasing fluidization number in the transition region. Finally, these two exponents vary little with changing fluidization gas velocity in the fully fluidized state. Through a wavelet transform, the fluctuating pressure signals of the bed can be decomposed into approximations and details at different resolutions. The number of peaks in the five-detail signal represents the bubble formation frequency, which is in agreement with the major frequency o...

Solids mixing in the bottom zone of a circulating fluidized bed was investigated by means of solid carbon dioxide as a tracer in combination with gas concentration and temperature measurements. The experiments were performed in a cold model CFB riser with a rectangular cross-section of 0.3×1.0 m and a height of 8.5 m at operating conditions which are typical of CFB combustors (u=3 m/s, Gs=20 kg/m2 s, dp=150 μm). The measurements show a high extent of mixing in the vertical direction, whereas horizontal mixing in the bottom zone was found to be limited. If the tracer was injected into the solids return line, the observed horizontal temperature gradients were significantly lower, which is due to additional convection caused by externally circulating solids returned into the riser just above the bottom zone. A one-dimensional convection–dispersion model was used to calculate the temperature distribution within the bottom zone. The horizontal dispersion coefficient was found to be much higher in comparison to dispersion coefficients measured in bubbling fluidized beds as well as in the upper dilute zone of circulating fluidized beds.

Incineration is one of the major methods for the disposal of sewage sludge. Currently, several plants are incinerating mechanically dewatered (wet) sludge (20–40 wt.% d.m.) or semi-dried sewage sludge (30–55 wt.% d.m.), although some plants burn dry sludge (with more than 80 wt.% d.m.). Whereas significant information is available on NOx and N2O emissions characteristics of wet and dry sludge, not much has been reported on semi-dried sludge. This paper presents some of the results obtained from the combustion of semi-dried sludge in a semi-pilot scale fluidised bed combustor (150 mm in diameter and 9 m high) together with some measurements from a large-scale FBC incineration plant (7 m2 bed area, 9 m high and a capacity of 3 t/h dry sludge). The investigations have shown that semi-dried sludge exhibit emission characteristics which are similar to those of wet sludge. NOx decreases slightly whereas N2O remains more or less the same with increase in oxygen concentrations. Just like wet sludge, staged combustion was not effective for the reduction of NOx and N2O. However, increasing the freeboard temperature led to rapid reduction of N2O and some NOx reduction was achieved using flue gas recycling technique. Comparison shows that the results from the test rig were more or less similar to those obtained from the large-scale plants.