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Some of the most common unit operations being used in the chemical industry are based upon packed bed configurations. Multi-tubular catalytic reactors are a prominent example of such systems which are believed by many experts to be the heart of the chemical profession and frequently the most expensive units of the plant. They offer extensive surface area between phases of the matter in a compact physical manner, and also are relatively easy to construct and maintain. The successfuld esign of such systems depends on models, which should describe the processes occurring within the fixed bed and must also be able to accurately represent the intrinsic behaviour of the reactor quantitatively. It is therefore necessary to have good estimates of the associated transport parameters, namely the wall heat transfer coefficient, effective radial and axial thermal conductivity coefficients and radial and axial dispersion coefficients, in order to design efficiently. Unfortunately, the accuracy of the design data for the prediction of these parameters are in doubt. They are considered to be unreliable because of being based on poorly defined packed bed models in terms of structure of the packing. The non-uniformities of voidage in the packing matrix must be appreciated and hence be included in descriptive models of the physical system in order to establish reliable design data. As the roles of modelling and experimentation are complementary, a number of beds comprised of equilateral cylindrical particles were prepared to examine their global and local structural properties, with a view to identifying and characterising features which could be used for prediction purposes. The microstructural details within packed beds were studied by means of an image analyser, so that the local variations of voidage in angular, axial and radial directions could be portrayed. It is only on the basis of this kind of information that a well-defined description of the bed structure becomes accessible. This research activity has been concerned with the study of several factors influencing the packing structure such as entrance/exit and wall effects, reproducibility and scaling problems. Also succeeding the accurate data acquisition stage, a number of correlations have been developed for a wide scope of diameter ratios covering the industrial range. These correlations allow the mean and local voidage distributions to be predicted reliably. |
