Particle sedimentation times

Since the latexes had fairly narrow particle size distributions, precise measurements of the sedimentation coefficients were obtained from the change in boundary position with time. The operating speed of the rotor was selected to give a sedimentation time of at least 20 minutes. 

The disc centrifuge photosedimentometer (DCP) is an instrument used to determine 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. Particles are assumed to be spherical. In the most common mode of operation the separation takes place in a cylindrical cavity of known dimensions and containing a known volume of fluid. A dilute suspension of the particles of Interest is injected onto the surface of the fluid while the disc is spinning and the particles sediment in the centrifugal force field radially outward past a detector system. The appearance time of the particle at the detector, t, in minutes is given by... 

Particle radii (R) are calculated from the sedimentation times (t) by means of the Stokes equation. For a spin fluid with density and viscosity gradients... 

The ability to make very precise measurements has allowed us to use the disk centrifuge to determine the thickness of adsorbed gelatin layers on silver bromide particles (4). When particles are coated with an adsorbed polymer layer, the sedimentation time reflects the size and density of the particle core as well as the thickness and density of the adsorbed layer. An apparent (incor-... 

The overall operation is comprised of several phases. In the first phase, the adsorbent material is expanded and equilibrated by applying an upward liquid flow to the column. To allow for sufficient contact time and efficient binding of the target molecule, the expansion should be three times that of the sedimented bed ( expansion ratio ), to a height of approximately 50 cm. A stable bed is formed when the adsorbent particles achieve equilibrium between particle sedimentation velocity and upward liquid flow velocity. In the second phase, the sample is applied to the expanded adsorbent. The crude, unclarified protein solution of intact or disrupted biomass is pumped upward on the column. In a well-defined process, the expanded adsorbent will remain stable and will not change its expansion ratio. However, if the... 

Figure 5.2 illustrates the effect of various stabilizers on the slurry sedimentation time after stopping US application in the determination of lead from lake sediments in particle sizes < 2 mm [26], The presence of nitric acid has the twofold effect of stabilizing slurries and increasing analyte leaching. The latter effect is a function of both the acid concentration and the nature of the target analyte [10]. 

In addition to these sedimentation properties, there is another operationally important parameter, the integral effect of the gravitational force on the partide, which determines the distance travelled by a particle over a given time interval. At a distance x from the centre of rotation the distance moved by particle in time t is given by the equation ... 

The time required for a particle to reach its terminal velocity is negligible, but excessively short sedimentation times should be avoided since concentration measurements fluctuate due to the initial agitation, up to a time of about 30 seconds. 

Three distinct methods are therefore available for calculating the mass distribution of particles in suspension for a series of sector-shaped tubes filled to a series of levels. First, the mass fraction of particles larger than a known diameter may be calculated from equation (8.77) and the distribution function determined from the slope of the cumulative mass deposited against S. Secondly, the distribution function may be calculated directly in terms of the first and second derivatives of the fraction sedimented with respect to the length of the column of suspension centrifuged by use of equation (8.77). Thirdly, from the sedimentation -time curve at a series of levels, the distribution functions may be calculated by the use of equation (8.77). 

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). 

Extended mouthpiece devices are also available (e.g., Azmacort , Aventis). With this device, the point of inspiration is removed further from the point of actuation, allowing greater evaporation time for the less volatile propellants, plus large particle sedimentation in the airstream, with a concomitant... 

Figure 2. Time series of the particle sedimentation rate in Lake Sempach. Fluxes are higher near the sediment surface in 86-m depth because of resuspension. The large maxima during summer correspond to algal blooms and subsequent biogenic calcite precipitation.

The difference schemes for particles sedimentation in centrifugal force field are shown in equation (8), (11), (12), and the direct difference format of equation (13). At a certain interval of time, the particlesconcentration and temperature and the solid fraction of every element are calculated, and then the condition under which the particles stop sedimenting is judged. The sedimentation of particles terminates when (a) the fraction of solid reach to 100% due to the solidification procedure, and (b) the volume fraction of reinforcement particles reach beyond a critical value 0.74 or 74%. ... 

Considerable discrepancies exist between the theory and practice of centrifugation. Complex variables not accounted for in Equations 18.10 and 18.11, such as concentration of the suspension, nature of the medium, and characteristics of the centrifuge, will affect the sedimentation properties of a mixed population of particles. Moreover, the frictional coefficient, f in the case of an asymmetrical molecule (e.g. a protein such as myocin) can be several times the frictional coefficient (i.e. f0) of a sphere. This results in particles sedimenting at a slower rate. Equation 18.10 can, therefore, be modified to give Equation 18.12 ... 

Fig. 7-28. Combined residence lifetimes of aerosol particles as a function of size. [Adapted from Jaenicke (1978c, 1980).] Important removal processes, active in various size ranges, are indicated. Coagulation and sedimentation time constants were calculated the time constant for wet removal is the residence time derived from 2,0Bi/210Pb and 222Rn/210Pb ratios (Martell and Moore, 1974). Curves 1 and 2 represent the background aerosol for rwel equal to 12 and 3 days, respectively. Curve 3 represents the continental aerosol with rwel = 6 days. The dashed line is calculated from a simple model for sedimentation equilibrium, as described in Section 7.6.3.

For monodisperse system the weight of settled particles as a function of time is given by a straight line (Fig. V-31), the slope of which is proportional to the concentration, c, of the dispersed particles (mass of particles per unit volume of dispersion), the particle sedimentation rate, u, the... 

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