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
Fluidized‐Bed reactor model verification on a reactor of industrial scale
Previous published comparisons between fluidized-bed reactor models and experimental data have almost exclusively been in laboratory and pilot-plant scale equipment. This paper compares data obtained in an industrial phthalic anhydride reactor of 2. 13 m dia. using naphthalene as the feedstock with three models, the Kato and Wen bubble-assemblage model, the Kunii and Levenspiel three-phase bubbling-bed model, and Grace's two-phase bubbling-bed model. For the conditions of operation (U = 0.43 m/s, dp = 53 μm, H = 7.9 m, T = 636 K), all three models give similar predictions and each gives a good overall prediction of the conversion and selectivity if the reaction kinetics are based on the early study of DeMaria et al. (1961). Hydrodynamic parameters needed for the models are calculated from equations available in the literature. Grid and freeboard effects appear to play relatively minor roles in determining the overall conversion and yields for the reactor in question.
Bubble chains in large diameter gas fluidized beds
Abstract Using statistically based measuring methods for the determination of local bubble size distributions and local average bubble shapes in gas fluidized beds, bubble characteristics have been measured in a fluidized bed column of 1 m diameter where quartz sand (minimum fluidizing velocity 0.0135 m/sec) was fluidized with air at velocities ranging from 0.05 to 0.30 m/sec. The results present experimental evidence that bubbles within large diameter fluidized beds do not rise completely randomly distributed in space but rather in the form of bubble chains which is in agreement with industrial operating experience in large scale fluid bed systems. Since the formation of bubble chains considerably reduces the residence time of the bubble gas this finding is of significance for the performance of fluidized bed reactors. The influence of the operating parameters on the extent of the bubble chain formation has been investigated and possible consequences of these results are discussed.