Conventional packed bed reactors of low tube to particle diameter ratio suffer from poor heat transfer near the tube wall, and also from increased axial dispersion compared with wide beds. In this dissertation the potential of a new reactor design aimed at overcoming these deficiencies is investigated. The radially-stratified bed provides more particles in the wall region to support improved heat transfer and also to flatten the voidage profile in an attempt to reduce the axial dispersion. An experimental study on the effect of stratification on voidage profiles using an image analysis technique showed that both the voidage and velocity profiles could be flattened. A study into the effect of different packing arrangements on axial dispersion has shown that the best arrangement for flattening the voidage profile can lead to over-compensation, resulting in channelling through the core and an increase in dispersion. However, a packing arrangement consisting of a binary mixture of large and small particles near the wall with the core of the bed packed solely with the larger, was shown to exhibit dispersion characteristics no worse than monosized packing. This same packing arrangement was also found to support improved heat transfer. At Re<SUB>p ></SUB> 1100 the heat transfer coefficient appropriate to the one-dimensional plug flow model was shown to increase by ca. 15%. A novel analysis of the one-dimensional reactor model has shown that use of a stratified bed of the same voidage, heat transfer coefficient, tube diameter, and feed flowrate as a monosized bed, results in a pressure drop which is 57% of that across a conventional bed of the same bed length. A further advantage of the stratified bed is that, on average, the catalyst particles are nearer the wall than in the conventional monosized bed. The magnitude of this and the other advantages was assessed by simulation of a reaction performed at high Reynolds numbers, the partial oxidation of ethylene. For this purpose, a new, plausible but simple two-dimensional model of the packed bed reactor was devised. A stratified bed was predicted to then have a pressure drop only some 34% of that of a conventional bed for the same overall conversion. This further reduction of some 23% in pressure drop stems largely from the reduction in packed length necessary to achieve a stipulated conversion. Alternatively, for the same selectivity, a stratified bed could reduce the pressure drop to about half that of a conventional bed of the same voidage. The potential of this novel design has thus been demonstrated: the next stage is optimisation.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:599875 |
| Date | January 1987 |
| Creators | Halliday, K. |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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