Sedimentation of particles

Modes of Operation There is a close analogy between sedimentation of particles or macromolecules in a gravitational field and their elec trophoretic movement in an electric field. Both types of separation have proved valuable not only for analysis of colloids but also for preparative work, at least in the laboratoiy. Electrophoresis is applicable also for separating mixtures of simple cations or anions in certain cases in which other separating methods are ineffectual. 

The presence of a gas in the suspension results in an increase of the stirrer speed required to establish the state of complete suspension. The propeller usually requires a higher speed than the turbine. Furthermore, a critical volume gas flow exists above which drastic sedimentation of particles occurs. Hence, homogenisation of the suspension requires an increase of the rotational speed and/or a decrease of the gas flow rate. The hydrodynamics of suspensions with a solid fraction exceeding 0.25-0.3 becomes very complex because such suspensions behave like non-Newtonian liquids. This produces problems in the scale-up of operations. Hydrodynamics, gas hold-up, mass-transfer coefficients, etc. have been widely studied and many correlations can be found in literature (see e.g. Shah, 1991). 

Sedimentation of particles follows the principle outlined above [Eq. (1)] in which particles in the Stokes regime of flow have attained terminal settling velocity. In the airways this phenomenon occurs under the influence of gravity. The angle of inclination, t /, of the tube of radius R, on which particles might impact, must be considered in any theoretical assessment of sedimentation [14,19]. Landahl s expression for the probability, S, of deposition by sedimentation took the form ... 

Information on particle size may be obtained from the sedimentation of particles in dilute suspensions. The use of pipette techniques can be rather tedious and care is required to ensure that measurements are sufficiently precise. Instruments such as X-ray or photo-sedimentometers serve to automate this method in a non-intrusive manner. The attenuation of a narrow collimated beam of radiation passing horizontally through a sample of suspension is related to the mass of solid material in the path of the beam. This attenuation can be monitored at a fixed height in the suspension, or can be monitored as the beam is raised at a known rate. This latter procedure serves to reduce the time required to obtain sufficient data from which the particle size distribution may be calculated. This technique is limited to the analysis of particles whose settling behaviour follows Stokes law, as discussed in Section 3.3.4, and to conditions where any diffusive motion of particles is negligible. 

SMITH, T. N. Trans. Inst. Chem. Eng. 45 (1967) T311. The differential sedimentation of particles of various species. 

Another criterion for selection of a flow rate is based on considerations of the extent of sedimentation of particles of various sizes under flow conditions. This relation is developed by Wasp,... 

In The Netherlands clay is supplied by rivers and the sea. Clay is deposited due the sedimentation of particles. The sedimentation rate depends on the mass of a particle. Consequently bigger particles already sink to the bottom in the lower reaches of a river. The smallest particles reach the sea. Sea clay is usually fat as a result. Older clay layers are often covered by sand. 

The salinity-dependent variations of POC measured in the rivers Elbe and Ems also show decrease with increasing salinity (Fig. 14) (see also Lohse and Michaelis, 1983). However, it is difficult to differentiate between the effect of simple dilution and sedimentation of particles. 

Deposition other than in rain is termed dry deposition, and this includes sedimentation of particles, molecular and Brownian diffusion to surfaces, impaction on roughness elements and deposition under electrical or thermophoretic forces. The velocity of deposition is defined... 

Sedimentation of particles contributes significantly to deposition when the diameter exceeds about 5 pm. Instead of (6.13), the flux is given by... 

The main cause of sedimentation of particles in estuarine environments is aggregation [49]. Later, when the floes settle out they lose their identity and become part of the bottom sediment [49]. Therefore, when analyzing sediments, it is not a simple task to relate the size distribution in the sediment to the sizes, transport mechanisms, and behaviour of the particles before they were deposited. These waters tend to exhibit quite broad size distribution curves, although, if samples are collected and deflocculated, even flatter, broader distribution curves result, having smaller mode... 

The sedimentation of particles in the water column represents an important mechanism for sediment transport and is most fundamentally controlled by gravitational setting. Gravitational forces, which act in a downward direction, are also opposed by the viscous resistance of water in an upward direction. The sinking speed for spherical, slow sinking, single particles is described by Stoke s law (Allen, 1985) ... 

An Improved disc centrifuge photosedimentometer (DCP) was developed for use in the determination of the particle size and size distribution of latices, pigments and other particulates. Separation is based on Stokes Law for the sedimentation of particles in a centrifugal force field and does not rely on the use of particle size calibrants or standards. The DCP Instrument provides accurate stable particle size analyses over a wide range of conditions while at the same time is rugged enough for heavy use in both a research and quality control environment. A stand-alone data collection, analysis and management system was developed both for routine quality control operation and for research use of the instrument. 

Size analysis is based on sedimentation of particles as a function of their size due to gravitational pull or by using a centrifugal force. Can measure particles in the size range 0.01-100(xm. 

Equilibrium centrifugation is carried out at slower rotation speeds than for sedimentation velocity experiments, and with smaller sample volumes. In these experiments, equilibrium is achieved between the sedimentation of particles towards the... 

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