Sedimentation particle size distributions

Similar to EFDC, the SEDZLJ model simulates erosion of sediments into suspended load or bed load depending on the applied bed shear stress and the sediment particle size distribution at the surface of the bed. Fine-grained (i.e., cohesive) sediments are assumed to be transported entirely with the suspended load and therefore do not contribute to the bed load flux. For noncohesive sediments, the ratio of suspended load to total load transport (qs/qt) for eroding sediments is estimated using the following relationship, which is based on flume data collected by Guy etal.(1966) ... 

In particle-size measurement, gravity sedimentation at low soHds concentrations (<0.5% by vol) is used to determine particle-size distributions of equivalent Stokes diameters ia the range from 2 to 80 pm. Particle size is deduced from the height and time of fall usiag Stokes law, whereas the corresponding fractions are measured gravimetrically, by light, or by x-rays. Some commercial instmments measure particles coarser than 80 pm by sedimentation when Stokes law cannot be appHed. 

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. 

The particle size deterrnined by sedimentation techniques is an equivalent spherical diameter, also known as the equivalent settling diameter, defined as the diameter of a sphere of the same density as the irregularly shaped particle that exhibits an identical free-fall velocity. Thus it is an appropriate diameter upon which to base particle behavior in other fluid-flow situations. Variations in the particle size distribution can occur for nonspherical particles (43,44). The upper size limit for sedimentation methods is estabHshed by the value of the particle Reynolds number, given by equation 11 ... 

Aerosol Dynamics. Inclusion of a description of aerosol dynamics within air quaUty models is of primary importance because of the health effects associated with fine particles in the atmosphere, visibiUty deterioration, and the acid deposition problem. Aerosol dynamics differ markedly from gaseous pollutant dynamics in that particles come in a continuous distribution of sizes and can coagulate, evaporate, grow in size by condensation, be formed by nucleation, or be deposited by sedimentation. Furthermore, the species mass concentration alone does not fliUy characterize the aerosol. The particle size distribution, which changes as a function of time, and size-dependent composition determine the fate of particulate air pollutants and their... 

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. 

Mechanical analyses determine the particle-size distribution in a soil sample. The distribution of coarse particles is determined by sieving, and particles finer than a 200 or 270-mesh sieve and found by sedimentation. 

Before beginning a size determination, it is customary to look at the material, preferably under a microscope. This examination reveals the approx size range and distribution of the particles, and especially the shapes of the particles and the degree of aggregation. If microscopic examination reveals that the ratios between max and min diameters of individual particles do not exceed 4, and indirect technique for particle size distribution based on sedimentation or elutria-tion may be used. Sedimentation techniques for particle size determination were first used by Hall (Ref 2) in 1904, He showed that the rate of fall of individual particles in a fluid was directly related to the particle size by the hydrodynamic... 

SHARPLESMICROMEROGRAPH- A commercially available air sedimentation device introduced in 1953 (Ref 6) that provides rapid particle size distribution determinations for powdered materials by the application of Stokes law for particles falling thru a static column of gas... 

Turbidimetric methods for particle size analysis (Refs 10, 16 27) are used in conjunction with sedimentation procedures to determine the concn of particles at given distances below the surface at successive time intervals. From these data, and Stokes law, particle size distribution can be calculated... 

Thru a combination of sedimentation and transmission measurements, a particle size distribution can be found. Tranquil settling of a dispersion of non-uniform particles will result in a separation of particles according to size so that transmission measurements at known distances below the surface at selected time intervals, will, with Stokes law, give the concn of particles of known diameter. Thus, a size frequency distribution can be obtained... 

CAHN RG ELECTROBALANCE/SEDIMENTATION ACCESSORY. This app produces a continuous, visible chart record of the sediment weight collected on the balance pan. It has been evaluated for measuring particle size distributions of the primary expls Lead Styphnate, Lead Azide, Tetracene by Hutchinson (Ref 41). 

Liquid Sedimentation Techniques for Measuring the Particle-Size Distribution of Primary Explosives , PATR 4387 (1972) 42) Anon, Stan-... 

A dispersion of the sample is placed on top of a liq of greater density. The rate of sedimentation is detd by measuring the sediment vol at fixed time intervals. The results are converted to a size distribution by Stoke s Law Nitrogen Adsorption. The amt of N adsorbed on a sample is detd by carefully measuring the press change of a known vol of N exposed to a known wt of dry mat at constant temp. The info is used to detn the surface area which is converted to a particle size distribution Turbidometric Methods. The absorption of a beam of light passing thru a suspended sample in a suitable liq is measured as a function of time. 

ALL METHODS FOR PRESENTING DATA FROM THE MEASUREMENT OF PARTICLE SIZE DISTRIBUTIONS, WHETHER INSTRUMENTAL, SEIVING, SEDIMENTATION, OR PHOTOMETRIC METHODS, MEASURE FRACTIONS OF THE TOTAL PARTICLE DISTRIBUTION. IF THE METHOD IS SENSITIVE, THE FRACTION-SEGMENTS CAN BE SMALL, AND THE MEASURED PARTICLE DISTRIBUTION WILL BE CLOSE TO THE ACTUAL ONE. IF THE MEASUREMENT IS LESS SENSITIVE, THERE MAY BE SIGNIFICANT DEVIATIONS FROM THE CORRECT PSD. 

The size of a spherical particle is readily expressed in terms of its diameter. With asymmetrical particles, an equivalent spherical diameter is used to relate the size of the particle to the diameter of a perfect sphere having the same surface area (surface diameter, ds), the same volume (volume diameter, dv), or the same observed area in its most stable plane (projected diameter, dp) [46], The size may also be expressed using the Stokes diameter, dst, which describes an equivalent sphere undergoing sedimentation at the same rate as the sample particle. Obviously, the type of diameter reflects the method and equipment employed in determining the particle size. Since any collection of particles is usually polydisperse (as opposed to a monodisperse sample in which particles are fairly uniform in size), it is necessary to know not only the mean size of the particles, but also the particle size distribution. 

For parenteral products specific consideration needs to be included for tonicity adjustment, emulsion globule size, ease of resuspension and sedimentation rate, particle size and particle size distribution, viscosity and syringeability, and crystal form changes. Full consideration should be included of the proposed instructions for dilution or reconstitution of products and of compatibility with the proposed solvents or diluents. This should include a demonstration that the proposed storage temperature and extremes of concentration are suitable. 

In field studies on coagulation and sedimentation in lakes (Weilenmann et al., 1989), the particle size distribution at concentrations at several depths in the water column of Lake Zurich were measured (Fig. 7.15). 

Model simulations of particle volume concentrations in the summer as functions of the particle production flux in the epilimnion of Lake Zurich, adapted from Weilenmann, O Melia and Stumm (1989). Predictions are made for the epilimnion (A) and the hypolimnion (B). Simulations are made for input particle size distributions ranging from 0.3 to 30 pm described by a power law with an exponent of p. For p = 3, the particle size distribution of inputs peaks at the largest size, i.e., 30 pm. For p = 4, an equal mass or volume input of particles is in every logaritmic size interval. Two particle or aggregate densities (pp) are considered, and a colloidal stability factor (a) of 0.1 us used. The broken line in (A) denotes predicted particle concentrations in the epilimnion when particles are removed from the lake only in the river outflow. Shaded areas show input fluxes based on the collections of total suspendet solids in sediment traps and the composition of the collected solids. 

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. 

There are two important effects due to particle sedimentation which may affect the rate of filtration. First, if the sediment particles are all settling at approximately the same rate, as is frequently the case in a concentrated suspension in which the particle size distribution is not very wide, a more rapid build-up of particles will occur on an... 

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]. 

Sedimentation techniques are also utilized for particle size distribution for particles on the order of 0.1-50 pm. Capillary hydrodynamic chromatography (HDC) gives particle size distributions for particles of about 0.005-0.7 pm. 

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