SAFE DESIGN AND OPERATION OF FLUIDIZED-BED REACTORS : CHOICE BETWEEN REACTOR MODELS
For three different catalytic fluidized bed reactor models, two models presented by Werther and a model: presented by van Deemter, the region of safe and unique operation for a chosen reaction system was investigated. Three reaction systems were used: the oxidation of benzene to maleic anhydride, the oxidation of naphthalene to phthalic anhydride, and the oxidation of ethylene to ethylene oxide. Predictions of the optimal yietd, the operating temperature and the conversion were also subjects ofour study. It appeared that for reactions carried out in a Auidired bed operating under conditions of good fluidization all models predicted the same region of safe and unique operation. For a well-designed fluid&d bed only the constraint of uniqueness is affected by the reactor model chosen. Predictions of the yield, conversion and operating temperature appeared to fit slightly less well. But still a good indication can be obtained from any of the models since the deviation in the results was less then a few percent for all three reaction systems. The strongest deviations between the models occurs in the region of gas loads only slightly higher than the minimum fluidization velocity. As the heat transfer characteristics are bad at low gas loads this region is unsuitable for highly exothermic reactions where large amounts of heat have to be removed by the coolant. In the region of good heat transfer with gas loads at least several times higher than the minimum the three mod& predict the same resulk+. Fur this rtzabon wc finally rccommt-nd tht: use of simple models. INTRODUCTION Over the last two decades many studies have been published dealing with the modeIling of fluidized-bed reactors. The aim of these models is to describe the operation of a fluidized-bed reactor in terms of operating and design parameters. For an extensive discussion of the various models we refer to van Swaaij (1985). Recently studies have been presented that investigate the difference in results obtained from different reactor models. Johnson et al. (1987) studied the production of phthalic anhydride from naphthalene. Three reactor models were investigated; a model presented by Kunii and Levenspiel, a reactor model developed by Kato and Wen, and a model presented by Grace. They verified the models studying a reactor of an industrial scale. For the phthalic anhydride reactor they concluded that all three models predicted the conversion and product distribution equally well. The influences of the grid jets and the freeboard appeared to be negligible. Jaffrtis et al. (1984) studied the agreement between two reactor models-the model of Orcutt, Davidson and Pigford and the model of Kato and Wen-for the oxidation of benzene to maleic anhydride in a small-scale laboratory unit. They concluded that both models predicted the same conversion but only the model of Kato and Wen calculated the correct product distribution. A reactor model should lead to results such as yields, heat production and stability characteristics of the reactor. Reactor models can be used in two ways: either to describe the performance of an existing ‘Author to whom correspondence should be addressed. reactor-so that the operator is able to control his process in a more elaborated sense--or to design a reactor, in which case the model should be able to predict scale up effects as well. Westerink and Westerterp (1990) presented a method to combine both uses into one method to design and operate a fluidized-bed reactor. In their approach a plot is made of the slope of the heat withdrawal rate line-as known from the theory of tank reactors-vs the residence time in the reactor. In this plot all constraints to the design and operation can be shown and result in a region of possible operating parameters for which operation is safe, and an a priori required yield or selectivity can be obtained. In this region we can select a set of design parameters. The other way round is to have a set of design and operating parameters as obtained from an existing plant, that results in a point of operation in the plot. In that case the possible region of operation can be influenced by altering the lines that account for the constraints. To obtain the operating region in a fluidized-bed reactor Westerink and Westerterp used the simple model of van Deemter (1961) for a system of two consecutive or parallel reactions. They used correlations presented by van Swaaij and Zuiderweg (1972) to calculate the mass transfer coefficient, and of Werther (1978) to calculate the bubble hold-up. The use of these correlations is not compulsory: the region of operation derived with their approach can be obtained from any model and works for extensive networks of multiple reactions as well. Of course, the predicted region of safe operation will be affected by the model that is chosen. To study this effect we will investigate the predictions of three models for the region of safe operation, and use three
Ermittlung optimaler Betriebseinstellungen für technische Wirbelschichtreaktoren
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.
CFD simulation of hydrodynamics and heat transfer in gas phase ethylene polymerization reactors
Abstract The hydrodynamics and temperature of a two-dimensional gas–solid fluidized bed of gas phase olefin polymerization reactor had been studied. A two-fluid Eularian Computational Fluid Dynamics (CFD) model with closure relationships according to the kinetic theory of granular flow has been applied in order to simulate the gas–solid flow. Fluidization regime and gas–solid flow pattern were investigated using three different drag models. Model predictions of bed pressure drop were compared with corresponding experimental data reported in the literature to validate the model. The predicted values were in reasonable agreement with the experimental data. The temperature behavior of fluidized bed with various drag models was investigated. The temperature gradient in the primary section of the bed was much larger than the gradient in other sections and the effect of all drag models on temperature gradient along the bed was approximately similar.