Averaged suspended solids density

It is common to describe the axial distribution of solids in the CFB by a graph of radially averaged suspended solids density versus height, commonly called the "density profile". It is generally found by measurement of the distribution of pressure along the riser and recognition that the suspended solids constitute the major contributor to the pressure drop. Assuming this to... 

Fig. 9-8 Histogram of dissolved solids of samples from the Orinoco and Amazon River basins and corresponding denudation rates for morpho-tectonic regions in the humid tropics of South America (Stal-lard, 1985). The approximate denudation scale is calculated as the product of dissolved solids concentrations, mean armual runoff (1 m/yr), and a correction factor to account for large ratios of suspended load in rivers that drain mountain belts and for the greater than average annual precipitation in the lowlands close to the equator. The correction factor was treated as a linear function of dissolved solids and ranged from 2 for the most dilute rivers (dissolved solids less than lOmg/L) to 4 for the most concentrated rivers (dissolved solids more than 1000 mg/L). Bedrock density is assumed to be 2.65 g/cm. (Reproduced with permission from R. F. Stallard (1988). Weathering and erosion in the humid tropics. In A. Lerman and M. Meybeck, Physical and Chemical Weathering in Geochemical Cycles," pp. 225-246, Kluwer Academic Publishers, Dordrecht, The Netherlands.)...

In a DAF thickening process, air is added at pressures in excess of atmospheric pressure (2.1 to 4.9 kg/cm1 2 3 4 30 to 70psig) either to the incoming sludge stream or to a separate liquid stream. When the pressure is reduced and turbulence is created, air in excess of that required for saturation at atmospheric pressure leaves the solution as very small bubbles of 50 to 100 pm in diameter. The bubbles adhere to the suspended particles or become enmeshed in the solids matrix. As the average density of the solids-air aggregate is less than that of water, the agglomerate floats to the surface. The floated solids build to a depth of several inches at the water surface. Skimmers continuously remove the float.58... 

Example. Consider a sample of solid particles for which the average particle size is unknown. The particle density is known (2 g/cm3 at 20 °C) and a sedimentation column (see Figure 2.10) is filled with the solids suspended in water. Measurements indicate a terminal velocity of 0.013 cm/minute. What is the average particle diameter ... 

Sykes and Gomezplata13 determined the liquid-solid mass-transfer coefficient for 0.32-cm-diameter spherical particles suspended in stirred aqueous iodine solutions. The particle density was within 5 percent of the liquid density. The effects of impeller speed (200 through 600 rev min- ), Schmidt number (770 through 11,300), and impeller type (fan-disk turbine, propeller, and 45° paddle and turbine) on the mass-transfer coefficient were examined. The data were correlated with an average deviation of 8 percent by the following expression ... 

It is possible to extend the procedure developed above to certain multiphase applications in which the power requirement has to be estimated. In the case of low-viscosity liquid/liquid systems, as encountered in solvent extraction, and for coarse solids suspended in low-viscosity liquids at low concentrations, the operation is likely to be carried out in the turbulent region. The single-phase power curves can be used in such instances with the mean density being used in both the power number and Reynolds number. However, such an approach must not be used for gas/liquid systems where predictions based on average density values can lead to gross over-estimates of the power requirement. This is considered in detail in Chapter IS. 

It is possible to remove small particles using dispersed or dissolved gas flotation devices. These units are primarily used for removing suspended hydrocarbons from water. Gas is normally dispersed into the water or released from a solution in the water, forming bubbles approximately 30-120 pm in diameter. The bubbles form on the surfaces of the suspended particles, creating particles whose average density is less than that of water. These rise to the surface and are mechanically skimmed. In the feed stream, chemicals called "float aids" are normally added to the flotation unit to aid in coagulation of solids and attachment of gas bubbles to the solids. The optimum concentration and chemical formulation of float aids are normally determined from batch tests in small-scale plastic flotation models on-site. Because of the difficulty of predicting particle removal efficiency with this method, it is not normally used to remove solids from water in production facilities. 

---