Kamugasha, Dick; (1999) Design and development of an on-line sedimentation analyser. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis describes the design and development of a multiple vibrating reed analyser for the routine on-line procurement of sedimentation kinetic data in two phase media. Briefly, each reed system comprises a stainless steel rod pinned at an intermediate point along its length. One end is exposed to the settling suspension whilst the other is driven into transverse vibration at resonance using an alternating current electromagnet. The principle of operation of the device relies on the fact that the resonance frequency of a stiff reed performing simple harmonic motion in a fluid medium is directly related to the fluid bulk density. In the case of a settling solid/liquid suspension, or a two-phase liquid dispersion containing a heavier phase, the fluid bulk density and hence the hydrodynamic head decreases with time as the heavier phase settles. Sedimentation kinetic data including settling velocities and flux profiles are in turn obtained by continuous monitoring of the resonance frequencies of a number of reeds positioned at set levels along a settling tank. The feasibility of operation of the analyser has been successfully verified in conjunction with a variety of model and industrially relevant systems. The former include mono and polydisperse glass ballotini / water mixtures with solids particle size and concentration ranges 55 - 200 μ and 1.75 - 2.81 % v/v respectively. Measured settling velocities are in excellent accord (ca. ± 0.1 %) with those obtained from direct visual observation of suspension-clear liquid interfaces. The industrially relevant systems include kaolin/water suspensions with solids concentrations as high as 20 % v/v and oil / water emulsions with light phase concentrations in the range 30 - 50 % v/v. The performance of a number of empirical models including those proposed by Richardson and Zaki (1954), Garside and Al-Dibouni (1977), Bamea and Mizrahi (1973), Reed and Anderson (1980), Batchelor (1972) and Happel (1958) has been evaluated by comparing predicted settling velocities with those obtained using the analyser. We find that the model proposed by Richardson and Zaki produces the best agreement (ca ± 1.3 %) whilst Happel's model (1958) produces the worst results (ca ± 24 %). For polydisperse systems we observe that measured interface velocities correspond to Stokes (1851) particle sizes which are in reasonable agreement (ca. ± 6 %) with the experimentally determined smallest significant particle size in the sample distribution. Also, as the initial solids concentration increases, the Stokes diameter decreases indicating a greater tendency for particles to segregate resulting in more diffuse interfaces. Therefore, the behaviour of such systems is characterised by differential rather than hindered settling. By measuring the time delay between the onset of sedimentation and that resulting in a change in reed resonance frequencies at various locations along the settling tank we have been able to obtain estimates of propagation wave velocities marking the transition between steady and unsteady state behaviour. For the systems tested we find that the wave propagation velocity is, in general, independent of solids concentration but increases with the mean particle size.

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