Mixing continued solids

At high ratios of fluidiziug velocity to minimum fluidizing velocity, tremendous solids circulation from top to bottom of the bed assures rapid mixing of the solids. For aU practical purposes, beds with L/D ratios of from 4 to 0.1 can be considered to be completely mixed continuous-reaction vessels insofar as the sohds are concerned. 

A reactor model based on solid particles in BMF may be used for situations in which there is deliberate mixing of the reacting system. An example is that of a fluid-solid system in a well-stirred tank (i.e., a CSTR)-usually referred to as a slurry reactor, since the fluid is normally a liquid (but may also include a gas phase) the system may be semibatch with respect to the solid phase, or may be continuous with respect to all phases (as considered here). Another example involves mixing of solid particles by virtue of the flow of fluid through them an important case is that of a fluidized bed, in which upward flow of fluid through the particles brings about a particular type of behavior. The treatment here is a crude approximation to this case the actual flow pattern and resulting performance in a fluidized bed are more complicated, and are dealt with further in Chapter 23. 

In the case of a system such as Fe-Ni, G is negative and this, combined with the ideal entropy, produces a smoothly changing curve with a single minimum (Fig. 6(a)). In this case a continuous solid solution is formed, i.e., Ni and Fe mix freely in the fc.c. lattice. However, for the case of a system such as Cu-Ag, G is quite strongly positive. In this case the addition of the ideal entropy produces a Gibbs energy curve with two minima, one at the Cu-rich end the other at the Ag-rich end (see Fig. 3.6(b) below). 

Manas-Zloczower, I. 1994. Dispersive mixing of solid additivesMining and Continuous Compounding of Polymefsadited by Mansa-Zloczower, I. and Tadmor, Z., Munich Carl Hanser Verlag, pp. 55-83. 

Multi-phase mixing is often seen in industries. In general, the distribution of not only the dispersed phase but also the continuous phase depends on the local position in the equipment in the case of a multi-phase operation such as gas-liquid mixing system, liquid-liquid mixing system, solid-liquid mixing system, and gas-liquid-solid mixing system. In order to evaluate the mixing state in such systems, both the dispersed phase and continuous phase should be considered. 

Monotectic mixtures arise when the individual components have similar melting points, molecular volumes and polymorphic forms. Figure 17.12(a) represents a possible phase diagram for monotectic mixtures. A typical monotectic solution occurs when SSS is mixed with SOS. Lutton (1955) determined that the a form was present and associated with limited solid solution formation, and contrasted with the a forms of other glyceride mixtures that formed continuous solid solutions (Rossell 1967). It was found, for this system, that tristearin incorporates about 50% of the SOS into a solid solution on the other hand, SOS incorporates very little SSS into a solid solution. 

Figure 2. Steady-state relationships between solids retention time, effluent waste concentration, and specific treatment efficiency for completely mixed, continuous flow biological treatment processes...

The polymer is first melted and then mixed with solid drug particles or liquid drugs. This mixture is suspended in an immiscible solvent and heated to 5°C above the melting point of the polymer under continuous stirring. The emulsion is then cooled below the melting point until the droplets solidify. 

Williams, J.C. Continuous mixing of solids—a review. Powder Technol. 1976, 15, 237-243. 

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. 

At an elevated temperature, iron and manganese tungstates form a continuous solid solution series. Due to the fact that the temperature during mineral crystallization was sometimes below the limit of miscibility, ferberite and huebnerite can also be found side by side and not as a mixed crystal, as would be the case at correspondingly high temperatures. 

The previous sections describe how mixing is accomplished in a liquid phase. However, many industrial processes carried out in stirred tank reactors involve mixing of solids, gases and other liquids in a continuous liquid phase. The presence of a second phase will affect both the power consumption and the flow pattern in the tank. In the sequel, the mixing phenomena caused by the presence of gas bubbles, liquid droplets and solid particles are discussed. 

A considerable increase in photodecomposition of a number of drug substances is reported under continuous mixing of solid samples during irradiation (Takacs and Reisch, 1986 Takacs et al., 1990). A solid-state photoreaction is influenced by the shape, size, and texture of the particles or drug device because these parameters will influence the absorption, reflection, and scattering properties of the sample. The ratio of absorbed to scattered radiation, 0, relates to the fraction of absorbed to reflected radiation (R) as given in Equation 16.2 (Carstensen, 1974) ... 

Formation of Mixed Crystals or Solid Solutions.—If the two single salts A and B do not crystallise out pure from solution, but form an unbroken series of mixed crystals (solid solutions), it is evident that an invariant system cannot be produced. The solubility curve will therefore be continuous from A to B, the liquid solutions of varying composition being in equilibrium with solid solutions also of varying composition. If, however, the series of mixed crystals is not continuous, there will be a break in the solubility curve at which two solid solutions of different composition will be in equilibrium with liquid solution. This, of course, will constitute an invariant system, and the point will correspond to the point C in Fig. 125. A full discussion of these systems would, however, lead us too far, and the above indication of the behaviour must suffice. ... 

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