Density-based separation is a common requirement in many processes – e.g. water treatment, mine waste processing, or hydrocarbon extraction. Letting gravity do the work by using settling vessels is common, especially in large-scale applications. In a settling vessel, heavy components sink to the bottom and lighter components rise to the top; the material of interest is pulled from the vessel at the appropriate height.
The residence time required in a settling vessel depends on the difference in density of the materials, the viscosity of the liquid phase, and the vessel size. Bigger vessels have greater capacity, but the larger the vessel, the greater the footprint and the longer the residence time required to achieve a given separation.
One way of improving the performance of gravitational settling is to use an inclined plate settler. An inclined plate settler consists of a series of inclined channels made of plates mounted in a vessel. Material is fed into the channels between the plates and settles. However, the distance the material must settle corresponds to the size of the gap between the plates rather than the size of the vessel. This is because of the Boycott effect.
The Boycott effect was first reported by Arthur Edwin Boycott in 1920. He noticed that red blood cells settle much faster in an inclined tube rather than in a vertical one. This is because settling in a geometry with inclined surfaces produces a convective flow that transports components along the surfaces much faster than their settling velocities alone. As a result, material can be separated after settling to the surface of the inclined channel, where it can be carried away by the Boycott effect, rather than having to settle the entire height of the vessel.
#density #gravityseparation #fluiddynamics #fluids #settling
