An experimental study of droplet stability and separation performance in dewatering hydrocyclones

Sinker, Alastair Brenton (1995) An experimental study of droplet stability and separation performance in dewatering hydrocyclones. University of Southampton, Doctoral Thesis.

Abstract

This thesis describes research undertaken to further the current understanding of flowfield and dispersed phase behaviour of a dewatering hydrocyclone in an effort to aid its economic viability by improving its phase separation performance. The dewatering efficiency of this device has been shown to be highly dependent on both the dynamic stability (imparted by the dispersed phase drop size distribution and physical & operational parameters) and the interfacial stability (imparted by third part surface active agents) of the unresolved emulsion. The investigation has thus focused on firstly the fundamental behaviour of the dispersed phase during its passage through a Southampton designed 35mm dewatering hydrocyclone and its influence on the phase separation performance achieved and secondly the viability of preconditioning the feed dispersion with a demulsifier to heighten the resolution efficiency.

An additive free virgin diesel (gas oil) at ambient temperatures has been primarily used as the test medium. Emphasis has been placed on experimental procedures and instrumentation, particularly in the area of drop sizing where an on-line image based system and a semi on-line laser diffraction based technique have been developed which both provide reliable drop sizing data in flows where the inlet dispersed phase concentration ≤ 10%. The influence of operational parameters such as inlet flowrate, inlet dispersed phase concentration, inlet drop size distribution and split ratio (in particular) on the separation performance are also studied.

The results of the extensive experimental programme have been used as the basis to develop a conceptual model of this dispersed phase behaviour and areas within the cyclonic flowfield have been identified where either (or both) the opposing processes of droplet coalescence and turbulent shear induced droplet break up operate. The Weber Number analysis has been developed in an attempt to characterise these processes. An improvement of 30% in the dewatering efficiency was noted when only 10 ppm of a non-ionic demulsifier was added to the feed stream but the results did display a marked sensitivity to changes in operational parameters.

This investigation has uncovered some surprising results; the nature of the observed outlet drop size distributions was in complete contrast to previous thinking, severe limitations in the use of the bottle test as a predictive tool have been highlighted and the current understanding of the mechanisms which control the behaviour of the dispersed phase has been significantly extended. This has allowed recommendations for geometric and operational improvements.

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