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  • 1
    ISSN: 0930-7516
    Keywords: Chemistry ; Industrial Chemistry and Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: This contribution presents a chemical reactor model of a circulating fluidized bed (CFB) which takes into account adsorption effects and provides a description of the vertical profiles of gas concentration in the riser. The oxidation of NO is used as an example of a heterogeneous catalytic reaction. Mass transfer between the gas phase and solid particles in the CFB was investigated at various temperatures and solids mass flow rates. The measurements show that the mass transfer coefficient decreases with increasing solids mass flux and that the CFB resembles a bubbling fluidized bed as regards gas-solid contacting.
    Additional Material: 14 Ill.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 0930-7516
    Keywords: Chemistry ; Industrial Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: In the present paper, models are developed for the description of coalescence and splitting of bubbles in gas fluidized beds. The distinction between slow and fast bubbles, which originates from the Davidson model of gas flow in and around a rising bubble, was found to be decisive for the modelling of coalescence processes. On the basis of the respective models, it is shown that a change in bed temperature does influence the mechanisms of both bubble coalescence and splitting. In cases of both slow and fast bubbles undergoing splitting, the theory predicts a decrease of bubble size with temperature whereas in the case of a non-splitting system with fast bubbles, typically encountered with Geldart type B particles, no significant temperature effect on bubble size is expected. The theoretical predictions are shown to be in agreement with own measurements as well as with the results of other workers.
    Additional Material: 15 Ill.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Berkeley, Calif. : Berkeley Electronic Press (now: De Gruyter)
    ISSN: 1542-6580
    Source: Berkeley Electronic Press Academic Journals
    Topics: Process Engineering, Biotechnology, Nutrition Technology
    Notes: The fluidized bed reactor has a lot of advantages: excellent gas-solid contacting, no hot spots even with highly exothermal reactions, good gas-to-particle and bed-to-wall heat transfer and the ease of solids handling which is particularly important if the catalyst is quickly ageing. However, the list of disadvantages is as long: broad residence time distribution of the gas due to dispersion and gas-bypass in the form of bubbles, broad residence time distribution of solids due to intense mixing, erosion of bed internals and the attrition of the catalyst particles. A particular disadvantage of the fluidized bed reactor is its difficult scale-up. The historical experience with the FCC process is that in the early 40's of the last century this process was successfully scaled up from a 5 cm dia. pilot-scale unit to a 4.5 m dia. bed in the production unit. On the other hand, around 1950 the scale up of the Fischer-Tropsch synthesis in the fluidized bed failed completely. Modern process design should be able to avoid such disasters by making use of modeling and simulation tools. However, a modeling tool which is really helpful in planning and designing of an industrial fluidized bed reactor has to fulfill a lot of requirements. It should be able to describe the influence of the several changes which are typical for the scale-up process, for example enlargement of bed diameter, bed height and fluidizing velocity, changes of gas distributor design, introduction of in-bed heat exchanger tubes and baffles. In the present work a modelling approach is presented which 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 establishment of a catalyst particle size distribution (PSD) in the bed inventory which in turn influences via the hydrodynamic characteristics of the fluidized bed the performance of the chemical reactor. The usefulness of this approach is illustrated with model calculations for a fictituous first order reaction where the fluidized bed is equipped with different solids recovery systems including one single stage cyclone, several cyclones in parallel, two- and three-stage cyclone systems, respectively. Model calculations illustrate the significance of a high efficiency of the solids recovery in order to keep the fines in the system which is decisive for a high performance of the reactor. The calculations reveal that it may take months until a quasi steady state of the bed particle size distribution is obtained.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Berkeley, Calif. : Berkeley Electronic Press (now: De Gruyter)
    ISSN: 1542-6580
    Source: Berkeley Electronic Press Academic Journals
    Topics: Process Engineering, Biotechnology, Nutrition Technology
    Notes: In a circulating fluidized bed (CFB) combustor the reacting solids are locally fed into the combustion chamber. These reactants have to be dispersed across the reactor's cross-sectional area. Since the rate of mixing is limited this leads to a mal-distribution of the reactants and to locally varying reaction conditions. In order to describe the influence of mixing a three-dimensional model of the combustion chamber is suggested. The model is divided into three sub-topics. First, the flow structure in terms of local gas and solids velocities and solids volume concentrations is described. Second, mixing of the solids and the gas phase is quantified by defining dispersion coefficients, and finally the combustion process itself, i.e. the reaction kinetics, is modelled. The model was validated against data from measurements in the large-scale combustor of Chalmers University of Technology in Göteborg/Sweden. Insufficient fuel mixing generated mal-distributions of locally released volatiles, which were the basis for the uneven reactants distribution at steady-state. In the case of two-stage operation, the injected secondary air did not reach immediately the reactor's center but was slowly mixed with the main gas flow. The concentration gradients hardly vanish before the exit of the combustion chamber.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Berkeley, Calif. : Berkeley Electronic Press (now: De Gruyter)
    ISSN: 1542-6580
    Source: Berkeley Electronic Press Academic Journals
    Topics: Process Engineering, Biotechnology, Nutrition Technology
    Notes: A three-dimensional model has been developed to describe the injection of liquid reactants into fluidized bed reactors operating in the bubbling fluidized bed regime. The model considers the processes of liquid transport and evaporation in the vicinity of the point of injection. The underlying idea, which is supported by previous measurements, is that the particles in the dense suspension phase are wetted by the liquid or gas-liquid spray. The wetted particles are subsequently dried while they are following the gross solids circulation within the bed. The model considers the flow structure of the bubbling fluidized bed and the solids mixing with the aid of a hybrid model which combines semi-empirical models for bubble growth by coalescence and for bubble splitting with a CFD approach for the continuous emulsion phase surrounding the bubbles. Submodels for heat and mass transfer are used to describe the temperature and concentration fields in the vicinity of the injection nozzle and the drying process of the wetted particles with the resulting release of the vaporized injection liquid. The model was validated separately against flow structure measurements, solids tracer measurements and experiments with the injection of water and ethanol, respectively, into beds of FCC particles.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Berkeley, Calif. : Berkeley Electronic Press (now: De Gruyter)
    ISSN: 1934-2659
    Source: Berkeley Electronic Press Academic Journals
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Proper plant control and operation has to be based on measurements of process data at different locations within the process. The design of the instrumentation network is therefore an important step in the design phase of a given process. An advanced instrumentation network design should not only allow process control on a minimum data basis but should enable the plant operator to gain more insights into the functionality and capabilities of his process. For this purpose in-process balancing based on a suitably configured instrumentation network is necessary. To make full use of the measurements the data has to be processed by data reconciliation methods. In the present work an earlier approach by Chmielewski et al. (2002) has been applied to solids processes. This paper shows how the selected approach can be adopted to the measurement and processing of distributed variables, e.g. particle size and densities. An optimal instrumentation with regard to investment cost can be found that ensures that all necessary data for the reconciliation problem can be collected. The application to solids processes is illustrated with the examples of a simple screening-crushing process and a hydrocyclone cascade.
    Type of Medium: Electronic Resource
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  • 7
    ISSN: 1542-6580
    Source: Berkeley Electronic Press Academic Journals
    Topics: Process Engineering, Biotechnology, Nutrition Technology
    Notes: For better understanding the process of particle coating by chemical vapor deposition (CVD) in the fluidized bed, the simulation of the deposition process was combined with a discrete particle model (DPM). Based on the experimental results of the thermal decomposition of tri-isobutyl-aluminum (TIBA) to produce aluminum onto glass beads, mechanisms on the micro-scale were investigated by single particle tracking. Zones of excessive growth as well as zones of insufficient mixing were identified. In particular, the take-up of aluminum was traced for selected particles that exhibited a large mass of deposited aluminum what in turn provides insight into the homogeneity and quantity of the coating throughout the bed material.
    Type of Medium: Electronic Resource
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  • 8
    ISSN: 0934-0866
    Keywords: Chemistry ; Industrial Chemistry and Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Process Engineering, Biotechnology, Nutrition Technology
    Notes: The working principle of the single-fibre reflection (SFR) probe is that light emitted by a laser diode is guided into the measuring volume by the same fibre which receives the proportion of light reflected by the particles in the vicinity of the probe tip and transmits it back to a photosensitive element. In contrast to other configurations of fibre optical probes, the SFR probe is characterized by an unambiguous calibration graph over the entire range of solid volume concentration values. SFR probes have been successfully applied to different kinds of multiphase flow systems, e.g. fluidized beds, pneumatic conveying lines, elutriators and thickeners.A particular question for the interpretation of measurements has always been the effective size of the measuring volume, which is mainly determined by the solid volume concentration. In this paper a simplified mathematical model of the signal generation by backscattering of the emitted light at the particle surfaces is given. The theory takes into account the average optical properties of the solids and their particle size distributions.The particle properties are determined on the basis of this model, which finally delivers the shape, size and depth of the effective measuring volume. For particle sizes between 30 and 120 μm the depth of the measuring volume of a 600-μm fibre probe is between 0.2 mm for solid concentrations near the fixed-bed state and approximately 4 mm for solid volume concentrations as low as 0.1 vol.-%.
    Additional Material: 11 Ill.
    Type of Medium: Electronic Resource
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  • 9
    ISSN: 0009-286X
    Keywords: Chemistry ; Industrial Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: In Analogie zur Korngrößenmeßtechnik wurde eine Methode zur Bestimmung der Größenverteilung derjenigen Blasen, die im zeitlichen Verlauf nacheinander an einem Ort im Innern einer Gas/Feststoff-Wirbelschicht existieren, entwickelt, bei dem zur Meßwerterfassung eine strömungsgünstig geformte, miniaturisierte, kapazitive Sonde verwendet wird. An Wirbelschichten von 20 und 100 cm Durchmesser durchgeführte Experimente zeigen, daß sich mit diesem Verfahren sehr genau die lokalen Blasengrößenverteilungen messen lassen. Die gemessenen Verteilungen lassen sich durch logarithmische Normalverteilungen approximieren.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
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  • 10
    ISSN: 0009-286X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Wirbelschichtverfahren werden seit langem in einer Vielzahl technischer Prozesse erfolgreich eingesetzt. Trotzdem bereitet die Auslegung von Wirbelschichten angesichts der komplexen Natur dieser Gas/Feststoff-Strömung noch beträchtliche Schwierigkeiten. Zwar ist eine überschlägige strömungsmechanische Auslegung auf der Grundlage eines Wirbelschicht-Zustandsdiagramms möglich und für einige Anwendungen - insbesondere unter den physikalischen Verfahren und nichtkatalytischen Reaktionen - auch ausreichend. Eine genauere Auslegung erfordert aber zusätzlich eine Berücksichtigung strömungsmechanischer Detailvorgänge - z. B. Blasenbildung und -wachstum, Gasverteilung durch den Anströmboden und Bewegung des Feststoffs im Inneren der Wirbelschicht - sowie eine Kenntnis der verfahrenstechnischen Möglichkeiten zu ihrer Beeinflussung. Der Stand des Wissens auf dem Gebiet der Strömungsmechanik von Gas/Feststoff-Wirbelschichten wird dargestellt. Anhand von Beispielen wird die grundlegende Bedeutung der Strömungsmechanik für das Wärme- und Stoffaustauschverhalten von Wirbelschichten verdeutlicht.
    Additional Material: 21 Ill.
    Type of Medium: Electronic Resource
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