Centrifugal sedimentation, particle sizing

To ensure that only the finer fraction of the sediment slurry was processed, a shipboard centrifugal cone separator was connected to the slurry transfer hose to remove the coarse-sediment fraction. The cone separator was a 101.6-mm diameter, urethane-coated centrifugal cone (Demco 275). Under a normal operational pressure of 221 kPa the cone separator is capable of delivering 57.0 L/min of sediment slurry, whose sediment particle size ranged from 2 to 32 jitm as measured on the particle data Model 111 analyzer. 

Sedimentation (qv) techniques, whether based on gravitational forces or centrifugation, derive the particle size from the measured travel rates of particles in a Hquid. Before the particle analysis is carried out, the sample is usually dispersed in a medium to break down granules, agglomerates, and aggregates. The dispersion process might involve a simple stirring of the powder into a Hquid, but the use of an ultrasonic dispersion is preferred. 

Particle Size Distribution. Almost every feed slurry is a mixture of fine and coarse particles. Performance depends on the frequency of distribution of particle size ia the feed. Figure 5 shows that whereas all of the coarse particles having a diameter greater than some are separated, fewer of the very fine particles are, at any given feed rate. The size distribution frequency of particles ia feed and centrate for a fine and coarse feed are quite different. More coarse particles separate out than fine ones. Classification of soHds by size is often done by centrifugal sedimentation. 

Fig. 10. Centrifugal sedimentation field-flow fractionation equipment deposits particles along the circumference of the disk by size. The fluid enters and...

Glassification. Classification (2,12,26,28) or elutriation processes separate particles by the differences in how they settle in a Hquid or moving gas stream. Classification can be used to eliminate fine or coarse particles, or to produce a narrow particle size distribution powder. Classification by sedimentation iavolves particle settling in a Hquid for a predetermined time to achieve the desired particle size and size distribution or cut. Below - 10 fim, where interparticle forces can be significant, gravitational-induced separation becomes inefficient, and cyclone and centrifugation techniques must be used. Classification also separates particles by density and shape. Raw material separation by differential sedimentation is commonly used in mineral processiag. 

Two-phase suspension systems produce beaded products with broader particle-size distribution (e.g., 1-50 /rm). The microspherical particles usually need to be classified repeatedly to reduce the particle-size distribution in order to improve the resolution and efficiency in the separation for use in chromatography. The actual classification process depends on the size range involved, the nature of the beaded product, and its intended applications. Relatively large (>50 /rm) and mechanically stable particles can be sieved easily in the dry state, whereas small particles are processed more conveniently in the wet state. For very fine particles (<20 /rm), classification is accomplished by wet sedimentation, countflow setting, countflow centrifugation, or air classification. 

These operations may sometimes be better kno Ti as mist entrainment, decantation, dust collection, filtration, centrifugation, sedimentation, screening, classification, scrubbing, etc. They often involve handling relatively large quantities of one phase in order to collect or separate the other. Therefore the size of the equipment may become very large. For the sake of space and cost it is important that the equipment be specified and rated to Operate as efficiently as possible [9]. This subject will be limited here to the removal or separation of liquid or solid particles from a vapor or gas carrier stream (1. and 3. above) or separation of solid particles from a liquid (item 4j. Reference [56] is a helpful review. 

Gravitational settlement is allowed to proceed for 4 to 10 minutes, according to the particle-size range of the sample. The sedimentation tube is then centrifuged to reduce the time required for the smaller particles to reach the bottom. By measuring the volume of particles accumulated as a function of time, the equivalent spherical size distribution of the sample may be computed from formulae based upon Stokes law. In addition to the specially designed sedi-... 

Sedimentation analyses must be carried out at concentrations which are sufficiently low for interactive effects between particles to be negligible so that their terminal falling velocities can be taken as equal to those of isolated particles. Careful temperature control (preferably to 0.1 deg K) is necessary to suppress convection currents. The lower limit of particle size is set by the increasing importance of Brownian motion for progressively smaller particles. It is possible however, to replace gravitational forces by centrifugal forces and this reduces the lower size limit to about 0.05 p,m. 

Particle size distributions of natural sediments and soils are undoubtedly continuous and do not drop to zero abundance in the region of typical centrifugation or filtration capabilities. Additionally, there is some evidence to indicate that dissolved and particulate organic carbon in natural waters are in dynamic equilibrium, causing new particles or newly dissolved molecules to be formed when others are removed. Experiments with soil columns have shown that natural soils can release large quantities of DOC into percolating fluids [109]. 

The mass median diameter (MMD) is the most common descriptor of primary particle size and may be determined by sieving or centrifugal sedimentation. The volume median diameter, as determined by laser diffraction, may be used as an approximation of MMD, provided that the particle density is known and does not vary with size, and that the particle shape is near spherical. The MMD of a powder can be used as a predictor of aerodynamic diameter by Eq. (1),... 

The dramatic changes in haze particle size seen with alterations in protein-to-polyphenol ratio in a model system, would, if this also occurs in real beverages, have profound effects on both sedimentation (e.g., cold maturation in a tank or centrifugation) and filtration operations. 

Collection of particles is based on filtration, gravitational and centrifugal sedimentation, inertial impaction and impingement, diffusion, interception, or electrostatic or thermal precipitation (e.g., see Spurny, 1986, Chapter 3). The choice of method depends on a number of parameters such as the composition and size of the particles, the purpose of the sample, and acceptable sampling rates. Table 11.10 summarizes some of the commonly used methods and the size ranges over which they are effective. 

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