Title page for ETD etd-05052008-095050


Document Type Master's Dissertation
Author Van der Westhuizen, Ina
Email ina.vanderwesthuizen@sasol.com
URN etd-05052008-095050
Document Title Trickle flow multiple hydrodynamic states : the effect of flow history, surface tension and transient upsets
Degree MEng (Chemical Engineering)
Department Chemical Engineering
Supervisor
Advisor Name Title
Me E du Toit Co-Supervisor
Prof W Nicol Supervisor
Keywords
  • transient upsets
  • surface tension
  • trickle bed
  • hysteresis
  • multiple hydrodynamic states
Date 2007-09-07
Availability unrestricted
Abstract
The existence of multiple hydrodynamic states (MHS) in trickle bed operation has been proved by hysteresis observed in flow loops, as well as variation between different prewetting modes. The most common theory presented as explanation for the existence of MHS, is the film vs. rivulet concept. Based on this concept, it was suspected that in-situ upsets might promote the formations of films, thereby providing a method through which the hydrodynamic states of the Dry and Levec modes can be manipulated to perform like the Kan Liquid and Super modes.

Large performance enhancements can be obtained by altering the prewetting procedure, even for systems with a low surface tension. For the water system, the gas liquid mass transfer coefficient of the Kan Liquid and Super modes could be as much as 6 times greater than that of the Dry mode. For the low surface tension system, the gas liquid mass transfer of the Kan Liquid and Super modes could be up to 8 times greater than that of the Dry mode. Through a thorough investigation of various types of transient upsets and manipulation strategies, it was confirmed that prewetting is indeed the only way by which drastic variation in hydrodynamic states may be obtained. None of the investigated upsets (hysteresis, periodic operation or surface tension doping) resulted in changes in the liquid morphology that could compare to the significant variation that was observed by varying the prewetting mode.

Two methods were identified by which the hydrodynamic gaps between the less uniform (Dry and Levec) modes and the more uniform modes (Kan Liquid and Super) could be bridged. The first is to reduce the Levec draining time, while the second method may be seen as an in-situ type of Kan Liquid prewetting. This type of prewetting was obtained during doping with a low surface tension liquid, at a flow rate associated with the high interaction regime for the low surface tension system.

Though the hysteresis cycles did not drastically alter the predominant flow type, interesting trends were observed, some of which raised doubt about the application of the films vs. rivulet concept. One mode in particular displayed behaviour which contributed to this doubt, namely the Kan Gas mode;

Gas liquid mass transfer on this mode decreased with an increase in liquid flow rate

Relatively low pressure drops on this mode corresponded to relatively high liquid holdup

It was the only mode that exhibited no hysteresis with gas flow variation, on any of the hydrodynamic parameters

The various trends and variations observed with the different types of upsets, leads to the conclusion that the concept of films vs. rivulets simply does not provide adequate explanation of the observed results. In general, two flow types may be distinguished. That which is caused by an initial increase in liquid flow rate as opposed to that which is caused by an initial increase in gas flow rate

An investigation to determine the behaviour of each of the investigated parameters near the transition boundaries on all the modes, as well as a repetition of this study with non-intrusive visual techniques is recommended.

University of Pretoria 2006

E825 AG

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