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
SAFE DESIGN OF COOLED TUBULAR REACTORS FOR EXOTHERMIC, MULTIPLE REACTIONS; PARALLEL REACTIONS-I
Previously reported design criteria for cooled tubular reactors are based on the prevention of reactor temperature runaway and were developed for single reactions only. In this paper it is argued that such criteria should be based on the reactor selectivity, from which eventually a maximum allowable temperature can be derived. To this end and for the pseudo-homogeneous, one dimensional model of acooled tubular reactor in which two parallel, irreversible first order exothermic reactions are carried out, two criteria are develooed for the safe desien and oneration of the reactor. The criteria enable us tochoose tube diameters and operating conditions, which are sare in view of the derived selectivity and of possible runaway as well. The method outlined can be used in the initial design stage and requires kinetic information on both the desired and the undesired reaction.
New Alumosilicate Fillers Based on Sedimentary Rocks for Asphalt Concrete
Asphalt concrete is the most common material for highway and motorway construction. The quality of asphalt is determined, to a large extent, by properties of asphalt binder. Fillers, which are mineral powders from carbonate rocks and aggregates fines, such as limestone and dolomite, are often used in the composition of bitumen mastics affecting the performance of asphalt. This article explores the feasibility of using the fines of aluminosilicate sedimentary rocks as fillers. These materials are composed of clay minerals, which change their properties upon the contact with water. Normally, the use of such fillers is restricted because of poor water resistance and swelling of asphalt concrete. In order to improve the performance of these fillers, the thermal modification at moderate temperatures of 500–600 C has been proposed. Such treatment provides sufficient structural stability of obtained materials and results in the reduction of water absorption of asphalt, improved water resistance (up to 2.5 times) and also, in reduced swelling (up to 9 times). It has been demonstrated that improvement in the filler performance can be achieved by a heat treatment. Such treatment induces changes in the mineral composition and converts the structure of clay minerals into the frame structure of zeolite, as confirmed by X-ray diffraction and infrared spectroscopy. Due to thermal treatment, there is a change in the acid-base properties of the surface of the filler, which is reflected in the profiles of the main adsorption centers. As a result, due to chemisorption, the modified aluminosilicate fillers are able to interact with bitumen. The application of new filler materials in asphalt concrete enables to enhance the performance.