Solid-liquid mixing settling solids

The PAC activated slndge system is a modified activated sludge process. PAC is added to the aeration tank where it is mixed with the biological solids. The mixed liquor solids are settled and separated from the treated effluent in a gravity clarifier. Polyelectrolyte will normally be added prior to the clarification step to enhance solids-liquid separation. If phosphorus removal is necessary, alum is often added at this point. Even with polyelectrolyte addition, tertiary filtration is normally required to reduce the level of effluent suspended solids. The clarifier nnderflow solids are continuously returned to the aeration tank. A portion of the carbon-biomass mixture is wasted periodically to maintain the desired solids inventory in the system. 

Solid-liquid mixing involves the suspension, distribution, and the drawing down of solids by agitation. In addition to vessel geometiy, impeller variables include type, diameter, number, speed, and location. Process results include the desired level (quality) of suspension, such as just off-the-bottom, complete uniformity, or any intermediate condition. The slurry properties, density difference (solid/liquid), viscosity, and solids concentration all determine how difficult the task may be. As alternatives to stirred vessels, jets (see Section 9.10) can be used for light-duty suspension. Literature references deal mainly with settling solids as opposed to floating solids. We will try to address both conditions. 

The magnitude of the free settling velocity has proven useful in characterizing solid suspension problems into easy, moderate, or difficult categories (see Table 10-2). It is also used in solid-liquid mixing correlations, as described below. 

The procedure for phase separation follows the schematic in Figure 4-115 [32A]. To prepare the three test phases, a 1 9 ratio by volume of mud to seawater is mixed for 30 min. The pH is adjusted to that near seawater (pH = 7.8-9.0) by the addition of acetic acid. The slurry is allowed to settle for one hour. A portion of the supernatant is filtered through a 0.45- im filter. The filtrate is designated as the liquid phase. The remaining unfiltered supernatant of the slurry is the suspended particulate phase, while the solid phase is the settled solid material at the bottom of the mixing vessel. 

Tanks are used in innumerable ways in the chemical process industry, not only to store every conceivable liquid, vapor, or solid, but also in a number of processing applications. For example, as well as reactors, tanks have served as the vessels for various unit operations such as settling, mixing, crystallization (qv), phase separation, and heat exchange. Herein the main focus is on the use of tanks as liquid storage vessels. The principles outlined, however, can generally be applied to tanks in other applications as well as to other pressure-containing equipment. 

For suspension of free-settling particles, circulation of pseudoplastic slurries, and heat transfer or mixing of miscible liquids to obtain uniformity, a speed of 350 or 420 r/min should be stipulated. For dispersion of dry particles in liquids or for rapid initial mixing of liquid reactants in a vessel, an 1150- or 1750- r/min propeller should be used at a distance Df/4 above the vessel bottom. A second propeller can be added to the shaft at a depth Da below the liquid surface if the submergence of floating liquids or particulate solids is otherwise inadequate. Such propeller mixers are readily available up to 2.2 kW (3 hp) for off-center sloped-shaft mounting. 

The suspension of solids in a liquid is perhaps the most common application of stirred tanks. Usually, the solid particles are denser than the carr3ung fluid resulting in continuous settling of the particles towards the bottom of the tank. To avoid permanent settling of solids and to obtain a satisfactory mass transfer flux to the solid surface, mixing is provided to keep the solids in suspension. 

The concept of air as a colloid and the term aerosol for air containing an assembly of suspended particles were originally introduced by Schmauss and Wigand (1929). Colloids are inherently stable because fine particles are subject to Brownian motion and resist settling by sedimentation. The individual aerosol particles may be solid, liquid, or of a mixed variety, and all types are found in the atmosphere. Solid particles in the air are called dust. They are primarily formed by the erosion of minerals at the earth surface and enter the atmosphere by wind force. Sea spray from the ocean surface provides a prolific source of liquid droplets, which upon evaporation produce sea-salt crystals or a concentrated aqueous solution thereof. Solid and liquid particles also arise from the condensation of vapors when the vapor pressure exceeds the saturation point. For example, smoke from the open and often incomplete combustion of wood or agricultural refuse arises at least in part from the condensation of organic vapors. 

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