Deviation from Ideal Behaviour

Eqns [13] and [14] showed the relative impact on settling velocity of the more important and controllable experimental parameters. However, there are other effects that are more difficult to characterize and which can result in significant deviations from the settling velocities predicted by these equations. The most common of these effects occurs when the particles are nonspherical, as these equations are derived from Stokes' equation assuming spherical particles. For nonspherical particles, eqns [13] and [14] may be modified with a correction where de is the diameter of a sphere whose volume equals that of the sedimenting particle (de/2 is the Stokes radius).

The net result of this modification is that non-spherical particles are predicted to sediment more slowly, which is a more accurate depiction of their real-world behaviour.

In addition to deviations from spherical-particle geometry, there are other effects that can lead to departure from predicted behaviour (nonideality) during sedimentation. For example, many biological particles interact with the medium via hydration, the extreme case being for those particles with osmotic properties, which can result in drastic changes in particle density and, in turn, sedimentation coefficients. Interparticle attractions, e.g. charge or hy-drophobic effects, may increase the effective viscosity of the medium. In more severe cases such attractions can lead to poor separations where the centrifugal energy is insufficient to disrupt the attractions between particles that are targeted for separation. This latter effect is aggravated by the fact that the larger or denser particle will lead as the particle pair migrates toward the rotor wall while the smaller or lighter attached particle follows in its wake, and therefore experiences less frictional drag. Particles may also concentrate locally to increase the effective medium density, or form aggregates that yield complicated sedimentation patterns. Because of such deviations from ideal behaviour, equivalent sedimentation coefficients, S*, defined as the sedimentation coefficient of an ideal spherical particle, are often reported for a given set of experimental conditions.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

Get My Free Ebook

Post a comment