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Various soil washing processes have found commercial application in West Germany for some years now. The requisite plant and purification efficiencies achieved have been described in numerous publications [1–14]. Up until now very few studies have been made of the fundamentals of washing processes, so that the ultimate choice of a process for a specific soil clean-up situation or the adaptation of a process to fulfil a given task can only be made on an empirical basis.

Chinese/German reporters take notes for each session: new ideas, significant achievements, open questions, matters of dispute, areas of cooperation Reporters give a short summary of their impressions of the session (5 min) in the Final Discussion All participants add their opinions in the Final Discussion A summary of the conclusions of the Workshop based on contributions by the session reporters is to be published in “China PARTICUOLOGY” Overall purpose of the workshop

Mathematical modelling of fluidized bed reactors. Among the many fluidized bed models to be found in the literature, the two-phase model originally proposed by May has proved most suitable for accommodation of recent advances in flow mechanics: this model resolves the gas/solids fluidized bed into a bubble phase and a suspension phase surrounding the bubbles. Its limitation to slow reactions is a disadvantage. On the basis of the analogy between fluidized beds and gas/liquid systems, a general two-phase model that is valid for fast reactions has therefore been developed and its validity is confirmed by comparison with the experimental results obtained by other authors. The model describes mass transfer across the phase interface with the aid of the film theory known from gas/liquid reactor technology, and the reaction occurring in the suspension phase as a pseudo-homogeneous reaction. Since the dependence of the performance of fluidized bed reactors upon geometry is accounted for, the model can also be used for scale-up calculations. Its use is illustrated with the aid of design diagrams.

Determination of optimum operating conditions of industrial fluidized bed reactors. Fluidized bed technology has long been successfully used in numerous industrial processes. Nevertheless, the design and optimization of fluidized bed reactors still give rise to considerable difficulties owing to the complex nature of the gas/solids flow. Advances in the mathematical description of fluidized bed reactors have now been made by formulation of a model describing the catalytic reaction in a gas/solids fluidized bed in terms of absorption of reactants from the bubble phase with subsequent pseudohomogeneous reaction in the catalyst suspension surrounding the bubbles. Empirical relations are reported for mass transfer, specific transfer area, and bubble gas hold-up, taking into account the influence of fluidized-bed geometry which is of great importance in scale-up. The state of development reached and the practical use of the model is demonstrated by evaluation of various reactions taken from the literature. Any desired reaction mechanism can be incorporated into the model. It is shown for the example of the selectivity behaviour of a system of consecutive reactions that a detailed knowledge of the reaction kinetics is a necessary condition for the design and operational optimization of industrial fluidized bed reactors.

Die Masstabsvergroserung hat sich in der Vergangenheit oftmals als wesentliches Hindernis bei der Entwicklung neuer Wirbelschichtverfahren erwiesen, indem Reaktoren, die auf der Grundlage von Messungen an kleinen Laborapparaturen ausgelegt wurden, im Betriebsmasstab nicht den erwarteten Umsatz brachten. Die vorliegende Arbeit fuhrt das Masstabsvergroserungsproblem der Wirbelschichttechnik auf charakteristische Veranderungen der Stromungsmechanik beim Wechsel der Reaktorgeometrie zuruck. Ausgehend von Gasverweilzeitverteilungs- und Umsatzmessungen anderer Autoren an Wirbelschichten im Durchmesserbereich zwischen 5 und 300 cm mit Hohen bis zu 10 m sowie anhand von Messungen lokaler stromungsmechanischer Eigenschaften an Wirbelschichten mit Durchmessern zwischen 10 und 100 cm wird eine Masstabsvergroserungsfunktion abgeleitet, die eine einheitliche Darstellung des Stoffaustausches zwischen Blasen- und Suspensionsphase fur Wirbelschichtsysteme von extrem unterschiedlichen Abmessungen liefert und eine physikalisch begrundete Interpretation der Einflusse von Gefasdurchmesser, Wirbelschichthohe und Einbauten auf das Betriebsverhalten eines Wirbelschichtreaktors ermoglicht.

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.

Mathematical modeling of fluidized bed reactors. In the past fluidized bed reactor modeling has focused on heterogeneously catalyzed gas-phase reactions. Recently, however, greatly increased activity is obvious in the field of fluidized bed combustion where attempts are made to describe coal combustion in the fluidized bed and emission of sulphur dioxide and nitric oxides from FBC units, with the ultimate aim of modeling the operating behavior of industrial fluidized bed combustors. The present paper surveys the state of the art in fluid bed reactor modeling. The influence of the fluid-dynamic state of fluidization which is governed by gas and solids properties, fluidizing velocity, and bed geometry, as well as the influence of different zones within the reactor and of design details, respectively, are discussed on the basis of several models described in the literature.

It is demonstrated that the convective solids transport occurring in large diameter gas fluidized beds can be predicted quantitatively on the basis of measured properties of the bubble phase. Based on the fundamental findings of Rowe and co-workers [5], who have shown the solids mixing in gas fluidized beds for particle diameters greater than 100 μm to be caused solely by the action of rising bubbles, an equation has been derive from extensive measurements of the bubble development in a 1 m diam. fluidized bed of quartz sand which relates the convective solids mass flow due to solids transport in the bubble wakes to easily determinable parameters. The predictions of this relationship are found to bein good agreement with direct measurements of the convective solids transport carried out by Schmalfeld [21] on a pilot scale in a semicylindrical bed of 0.8 m diam.

It is the interrelation between mixing of solid particles and gas—solid reactions in fluidized beds which is of particular significance for the design and operation of fluidized bed reactors for non-catalytic gas—solid reactions. A model for the calculation of the distribution of a solid reactant over the cross-sectional area of the fluidized bed under conditions of gas—solid reactions is developed. The general tendencies are illustrated with the examples of coal gasification and coal combustion, respectively. Detailed calculations have been carried out for the combustion of coal in an atmospheric fluidized bed combustor. As reactions, the combustion of char and volatiles and the release and capture of sulfur dioxide are taken into account. The steady state size distribution of the coal particles in the combustor and an effective reaction rate constant based on this size distribution are evaluated. The influence of the mechanisms of attrition and elutriation as well as that of the Sherwood number on this rate constant are shown. The performance of the model is demonstrated by an application of the 20-MW pilot plant of Tennessee Valley Authority to the geometry and the operating conditions. On this plant detailed measurements of oxygen, carbon dioxide and sulfur dioxide profiles above the bed surface have been carried out. A good agreement between these measurements and the predictions of the model is obtained.

Presented at the 1st World Congress on Particle Technology held in Nuremberg (F.R.G.) April, 1986 A novel experimental technique for the investigation of solid particle mixing in gas-solid systems is suggested which is suited for on-line measurements under steady-state conditions. The method is based on subliming tracer particles. Its applicability is demonstrated by an investigation of the distribution of feed particles in gas-fluidized beds. Carbon dioxide snow particles have been used as the tracer material. The sublimation is an endothermal process where both the heat effect and the formation of a gaseous product may be used for the detection of the tracer particles. One method is to measure the local bed temperature, which will be lowered in the presence of tracer material, due to the low temperature of the carbon dioxide snow particles. Another method consists in measuring the concentration of the gaseous sublimation product carbon dioxide. While the former method is more suited for qualitative measurements of the spatial tracer distribution inside the bed, the latter method yields quantitative information about the local tracer concentration. Measurements have been performed in a pilot-scale fluidized bed with a cross-sectional area of 2.0 m × 0.3 m with bed heights up to 1.0 m. The solids mixing has been investigated by unsteady-state as well as steady-state measurements. The sublimation technique turned out to be a valuable tool in the investigation of the mixing process, which finally led to a new design correlation for the prediction of solids mixing in gas-fluidized beds.

A fluidized bed reactor model is presented which describes the catalytic reaction occurring in a fluid bed as an absorption from the bubble phase with a subsequent pseudohomogeneous reaction in the catalyst suspension phase surrounding the bubbles. Empirical correlations are given for interphase mass transfer, specific interfacial area and bubble gas holdup which all take the influence of reactor geometry into account. The practical use of the model is demonstrated by an interpretation of other authors' measurements including the oxidation of ammonia, the hydrogenation of ethylene and the decomposition of nitrous oxide.

Fluidized bed technology: recent developments for power station engineering and pollution abatement. The principle of fluidization has been successfully applied to numerous areas of chemical engineering. Combustion of coal in a fluidized bed consisting of particles of ash and limestone or dolomite permits low-pollution utilization of coals having lower heating values or high sulphur contents without any need for removing sulphur from the exhaust gases. The present state of the art and world-wide development work on fluidized combustion are considered, as are the future prospects of this new energy technology.

In Analogie zur Korngrosenmestechnik wurde eine Methode zur Bestimmung der Grosenverteilung derjenigen Blasen, die im zeitlichen Verlauf nacheinander an einem Ort im Innern einer Gas/Feststoff-Wirbelschicht existieren, entwickelt, bei dem zur Meswerterfassung eine stromungsgunstig geformte, miniaturisierte, kapazitive Sonde verwendet wird. An Wirbelschichten von 20 und 100 cm Durchmesser durchgefuhrte Experimente zeigen, das sich mit diesem Verfahren sehr genau die lokalen Blasengrosenverteilungen messen lassen. Die gemessenen Verteilungen lassen sich durch logarithmische Normalverteilungen approximieren.

Fiber optic probes were used to measure local instantaneous solids concentrations and local solids velocities inside a 0.4 m diameter bed. The velocity information was obtained from cross-correlation calculations. A detailed signal analysis yielded information about the local distributions of solids concentrations as well as of the local solids velocities. The measurements lead to a detailed picture of the flow structure of the circulating fluidized bed which consists essentially of two phases, i.e. a dense phase and a lean phase. A model is suggested which interrelates solids concentrations, solids velocities, and gas velocities in the individual phases on the basis of the measurements presented here. Integral values of the net gas velocity and net solids mass flow obtained by averaging the local values over the bed's cross-sectional area are in good agreement with the fluidizing velocity and the solids mass flow introduced into the bed.

The disposal of dredged sediments has become an ecological problem in Hamburg as well as in other seaports since these sediments have turned out to be contaminated with heavy metals (Table 1). The traditional solution of using the dredged sludge for filling up low marshland is no longer accepted by the public. The port authorities are therefore forced to look for alternative solutions to this problem, the dimension of which is obvious from the quantities which have to be dredged annually. In the fairways and harbour basins of the port of Hamburg about 2.5 million m3 of mud and sand have to be dredged per year (Christiansen et al. 1982) and the corresponding figure for Rotterdam is of the order of 20 million m3. Facing this problem Hamburg’s port authorities have initiated a research program, the Hamburg Dredged Material Research Program, in the scope of which the reasons and origin of sedimentation and the contaminations in sediments as well as alternative methods for treatment and deposit of harbour mud are being investigated (Christiansen et al. 1982).

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.

Circulating fluidized bed combustors (CFBC) of industrial scale have sometinIes diameters of more than 10m and heights exceeding 40 m. Depending on the intensity of the reactions and the location of the insertion of reactants into the bed, temperature effects can not be neglected. A simulation tool for the investigation of these large-scale systems including sub-models for fluid dynamics, dispersion, reactions, mass balances and an enthalpy balance has been implemented. The simulation results provide three-dimensional distributions of temperatures inside the combustion chamber. The simulation results have been validated with results of local measurements in a 235 MWe CFBC of Elektrownia Turow in Poland.

A direct consequence of the complex fluid mechanics of the circulating fluidized bed (CFB) is its complex mixing behavior. For both, solids and gas mixing, different scales of the mixing processes may be distinguished, namely the micro, meso and macro scales. While the micro scale is connected to the molecular scale and is mostly described by transfer coefficients, the meso and macro scales, are directly related to the flow pattern inside the CFB. On the meso-scale mixing effects are associated with the formation, disintegration and movement of clusters and strands. Macro-scale mixing is introduced via the existence of large-scale flow patterns, e.g. the core annulus structure. The present paper intends to give an overview on the mixing in a circulating fluidized bed and on its effects on the performance of CFB reactors. Special attention will be given to the interdependence between the reaction rate and the requirements for mixing. The combustion process in a CFB will mainly be used to exemplify this interdependence.

Optimisation de la combustion du charbon en lit fluidise afin de reduire des emissions de NO x et CO pour repondre aux exigences de nouvelles normes de qualite d'air