Sedimentation behavior, effect

Sedimentation tests are of value particularly for rapid evaluation of the effects of aging, flocculants, vibration, and any other variables that conceivably could affect a rate of filtration. The results may suggest what kinds of equipment to exclude from further consideration and what kind is likely to be worth investigating. For instance, if sedimentation is very rapid, vertical leaves are excluded, and top feed drums or horizontal belts are indicated or it may be indicated that the slurry should be preconcentrated in a thickener before going to filtration. If the settling is very slow, the use of filter aids may be required, etc. Figure 11.1 illustrates typical sedimentation behavior. Figure 11.2 summarizes an experimental routine. 

Even though one can obtain good agreement between the measured and calculated light intensity profiles for sedimentation equilibrium experiments, this method has drawbacks for the routine assessment of the performance of the optical system. There is not commercially available a standard solute with guaranteed purity and certified sedimentation behavior. Moreover each experiment requires a correction baseline obtained from the cells spun with both sectors filled with solvent. Since a time period of 8 hr or more is required to achieve equilibrium, it becomes a formidible task to investigate the effect of a number of parameters. 

Phelps, R. A. and J. R. Cann On the modification of conalbumin by acid. II. Effect of pH and salt concentration on the sedimentation behavior, viscosity, and osmotic pressure of conalbumin solutions. Arch. Biochem. Biophys. 61, 51 (1956). 

Cann, J. R. and Fink, N. H. (1983). Effect of microheterogeneity on the sedimentation behavior of self-associating proteins. Biophys. Chem. 17, 29-34. 

The determination of the main dimensions of classifying crystallizers is briefly discussed in the following. For a comprehensive design method, see References [7.1, 7.39, 7.51]. In a classifying crystallizer, such as the Oslo and Messo crystallizer, a separation effect is caused by the different sedimentation behavior of different grain sizes in the upflowing solution. 

There are other, more difficult to characterize effects that lead to significant deviations from the settling velocity predicted by eqn [7]. The most common such effect occurs when the particles are nonsphe-rical. Another example is for biological particles which interact with the medium via hydration, the extreme case being for osmotic particles which can result in drastic changes in particle density. Interparticle forces, e.g., charge or hydrophobic effects, may also impact sedimentation behavior. 

The effects of citrate and isocitrate on the sedimentation behavior of the enzyme have been verified by direct electron microscopic examination. Electron microscopy of the avian liver acetyl-CoA carboxylase in the assay reaction mixture without tricarboxylic acid activator, in which the enzyme exists as the inactive 13 to 15 S species, reveals small protomeric forms having minimum and maximum dimensions of 70 A and 130 A, respectively (Fig. 7 4, reference [176]). Addition of either citrate or isocitrate, which activates the enzyme and leads to the rapidly sedimenting 47 to 50 S species, causes instantaneous polymerization (within 10 seconds) of the protomeric form giving rise to filamentous structures 70 to 100 A in width and up to 0.5// in length (Fig. IB, reference [176]). 

Selected suspensions were studied in detail regarding sedimentation behavior (LUMiSizer, L.U.M. GmbH) and dynamic mobility of the primary particles (Zetasizer, Malvern Instruments) to get a detailed information about the effect of changed suspension formulation on suspension properties. 

We hope to have shown that the SD implementation can be used for hydrodynamics for any appropriate DEM simulation, be it restructuring aggregates or aggregate sedimentation behavior, but the fuUy-resolved CFD methods are always required for validating the simulations. The free-draining approximation should not be considered as a vahd comparison for colloidal aggregates in shear flow as it neglects many-particle effects [35]. 

In a practical sense, stability of a dispersion ofttimes is accompanied by a retarded separation of the phases. Unfortunately, a quantitative definition cannot be based on this rate of separation because of the overwhelming influence of density, viscosity, and thermal effects. In short, a kinetic criterion, such as sedimentation rate, is not as likely to portray stability as one based on thermodynamic considerations. In this latter category are sediment volumes, turbidity, consistency, and electrical behavior. 

An important consideration prior to sample collection is transportation and storage. Samples should be treated so as to retain the integrity of the sample from the moment of collection to the time of analysis. The physico-chemical characteristics of a sediment sample change during drying, with effects on the sorption-desorption behavior of chemicals. 

---