Size distribution gravitational sedimentation

For an estimate of the lower size limit, the displacement of the smallest particle by Brownian diffusion should be at least ten times smaller than its settling distance [10]. Other criteria could be selected since the error is both a function of the size and the spread of the distribution. It is reasonable however, for the sake of simplicity, to stipulate that if more than 10% of the distribution is smaller than the lower size limit, gravitational sedimentation should not be used. 

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. 

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-... 

Gravitational sedimentation causes a change in the particle size distribution anywhere in and below the cloud compared with the size distribution at stabilization time. Thus, to reconstruct the size distribution at stabilization time, corrections must be applied to the size distributions measured in the samples. These corrections were calculated by assuming Stokesian settling modified by a drag slip correction. It was assumed further that at stabilization time the cloud was axially symmetric and consisted of spherical particles. Wind and diffusion effects were neglected. 

BS 3406, British Standard Method for determinig Particle Size Distribution is a comprehensive standard that includes methods for both incremental and cumulative methods of particle size deterination. Part 1, 1984, covers Recommendations for Gravitational Liquid Sedimentation Mehods for Powders and Suspensions. Part 5, 1985 covers... 

ISO/WD 13317-1 Determination of Particle Size Distribution by Gravitational Liquid Sedimentation Methods- Part I (1996) General Principles and Guidelines, 360... 

The size of the droplets in an emulsion has a strong influence on many of its physicochemical and sensory properties, e.g., shelf life, appearance, texture, and flavor (1,2, 4). For example, the stability of an emulsion to gravitational separation or droplet aggregation can be greatly improved by decreasing the droplet size. This is because the velocity of sedimentation is proportional to the square of the droplet size. The size of the droplets in an emulsion is largely determined by the emulsifier type and concentration, the physicochemical properties of the component phases, and the homogenization conditions (4). A food manufacturer normally specifies a preestablished desirable droplet size distribution for a particular product. If the product does not meet this specification, it typically must be reprocessed or even discarded. 

Sedimentation Techniques. Other techniques that effect a physical separation include gravitational or centrifugal sedimentation, in which particles or emulsion droplets are separated on the basis of size and density. The separation that occurs can be quantified by monitoring X-ray or light absorbance as a function of position. Stokes law then can be used to determine the particle size distribution from the absorbance data as a function of the sedimentation time (73, 74). 

Sedimentation. The fine solids particle size distribution can be determined by sedimentation of the fine solids in a suspension. Table III presents a classification of the methods used in sedimentation (gravitational or centrifugal). 

In gravitational sedimentation the panicle concentration in the measurement zone remains equal to the initial concentration until the largest particle present in the suspension has settled through the zone. The concentration of particles in the measurement zone at time t represents panicles whose size is less than or equal to the Stokes diameter d< u>kes- rhe Stokes diameter is inversely proportional to the square root of time as shown by Equation 34-13. Therefore, a plot of concentration versus f/5j,okes represents the undersize distribution. 

BS 3406, Part 2 Methods for determination of particle size distribution — Recommendations for gravitational liquid sedimentation methods for powders and suspensions , 1984. 

The parameters for differential-sedimentation coagulation and settling have the same grouping of constants but different units because differential sedimentation is second order in the particle size distribution from Equation 2, while gravitational settling is first order in the size distribution as seen in Equation 1. All parameters were chosen to be independent of particle size, assuming further that particle density is also independent of particle size. 

Here, we treat the case of PSD analysis of particulate systems of micron-size range (i.e., with a size distribution extending above 1 xm). Since 1994, in our laboratories, this topic has been dealt with by means of a low-costsubset of sedimentation FFF (SdFFF), the gravitational field-flow fractionation (GrFFF) technique. GrFFF had aheady... 

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