Separation of dispersed suspension in rotating test tube

Woon Fong Leung

Research output: Journal article publicationJournal articleAcademic researchpeer-review

14 Citations (Scopus)

Abstract

An analytical model has been developed on separation, classification, and clarification of dispersed suspension with well-characterized particle size distribution using rotating tube centrifuge. The model is based on limiting trajectory analysis on individual particles of different sizes settling in the rotating tube under centrifugal acceleration with results expressed in a new dimensionless sedimentation Leung (Le) number, which corresponds with its counterpart for continuous-feed centrifuge. For sedimenting in a rotating tube the governing Le number depends on the effective viscosity of the suspension, the initial height of suspension, density difference between solid particles and suspension, and therefore operating a centrifuge centrifugal gravity, and the time duration that the suspension has been subject to centrifugation. Le is directly related to the cut size and at condition with small Le number leads to improved separation. In the first part of the model, centrifugal gravity G is assumed constant in the entire suspension domain wherein the effective radius for determining G is taken as the geometric mean of the smallest and largest radii of the suspension, and the sediment thickness is assumed negligible. The prediction from the model is compared with experimental measurements carried out in laboratory on four different suspensions. Laboratory measurements include (a) solids concentration, (b) particle size distribution, and (c) density, respectively, of feed and supernatant after subjecting test samples at different centrifugal gravities and time duration. Results of four laboratory test studies involving classification and separation of different suspension compared reasonably well with the model prediction. In two cases, the test results obtained from a small pilo t continuous-feed centrifuge were compared with that of the rotating tube based on the same test sample. In the second part, a variable-G model is also developed to account more accurately that the G-force increases linearly with radius across the suspension but also with negligible sediment thickness. The results confirmed that the constant-G model using the effective radius is reasonably accurate for engineering calculations. In the third part, the effect on separation due to sediment accumulating over time at the tube bottom has been investigated for the constant-G model. The accumulating sediment shortens the particle settling distance; on the other hand it also undermines particles settling with clarified liquid flowing radially inward (opposing sedimentation) from displacement of liquid in sediment formation. The improved model with finite sediment thickness compares slightly more favorably with experimental results under condition of high-solids capture and high-rate of sedimentation at small Le. Not only the rotating tube can be used to determine whether a given suspension is separable by centrifugation and also the pertinent parameters involved in scale-up associated with such laboratory testing, the present study addresses the feasibility of projecting results of process separation using continuous-feed centrifuges in absence of dynamic effects associated with complicated flow and sediment/cake solids transport. Pilot study using continuous-feed centrifuges is needed for demonstration and production scale-up and to further assess the impact based on any unforeseen complications.
Original languageEnglish
Pages (from-to)99-119
Number of pages21
JournalSeparation and Purification Technology
Volume38
Issue number2
DOIs
Publication statusPublished - 1 Aug 2004
Externally publishedYes

Keywords

  • Centrifuge
  • Clarification
  • Classification
  • Rotating tube
  • Sediment
  • Sedimentation

ASJC Scopus subject areas

  • Filtration and Separation

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