Solid mixing

In the experimental measurement of the solids mixing process, the radioactive particles were released from the top center of the cylindrical bed column into the bed free surface under each operating condition. The detector outputs were sampled at 50-ms intervals. A sample result of the average number of counts at each detector level was plotted versus time in Fig. 9.21 for the 500-pm glass particles at u0 = 54.8 cm/s (Moslemian, 1987). 

In the experiment, the sampling of the detectors was initiated slightly sooner than the release of the radioactive particles to ensure the recording of the events near zero time. As the particles were released, the outputs of the detectors positioned above the free surface (Levels 1 and 2 shown in Fig. 9.5) reached their maximum within a small fraction of second because of the passage of the falling tracer particles. The steady-state averaged count rate did not overlap for the four levels because of the variation in density distribution seen by detectors at different levels. From the curves in Fig. 9.21, three types of information could be obtained. The asymptotic steady-state values of the count rates were measures of the mean density distributions of the radioactive particles in the bed. The time required to reach these asymptotic values was an indication of the mixing rate of the bed particles. The shapes of the curves yielded information on the manner in 

FIGURE 9.21 Mixing of 10 g of 500-pm radioactive (Na24) glass particles in a bed of500-pm glass particles at u = 54.8 cm/s 

In Fig. 9.21, each experimental curve showed an overshoot before settling into its asymptotic value. This overshoot was a consequence of the large-scale solids recirculation in contrast to smaller scale diffusion. The overshoot was characteristic at lower velocities and it disappeared at greater velocities. The mixing behavior is similar for particles of differing size. 

FIGURE 9.22 Numerical and experimental mixing results for 500-pm glass particles at un = 54.8 cm/s 

FERNANDO J. MUZZIO, ALBERT ALEXANDER, CHRIS GOODRIDGE, ELIZABETH SHEN, and TROY SHINBROT 

KONANUR MANJUNATH, SHRIKANT DHODAPKAR, and KARL JACOB 

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AIChE Standard Testing Procedure for Solids Mixing Equipment, American Institute of Chemical Engineers, New York. 

Fan, Chen and Lai, Recent Developments in Solids Mixing, Powder Technol. 61,255-287 (1990). 

Properties Affecting Solids Mixing Wide differences among properties such as particle-size distribution, density, shape, and surface characteristics (such as elec trostatic charge) may m e blending very difficult. In fact, the properties of the ingredients dominate the mixing operation. The most commonly observed characteristics of solids are as follows ... 

A standard testing procedure for solids-mixing equipment is available (Ref. 1). This contains details and references pertaining to sampling from solids mixtures for both batch and continuous mixing. 

Types of Solids-Mixing Machines There are several types of solids-mixing machines. In some machines the container moves. In others a device rotates within a stationaiy container. In some cases, a combination of rotating container and rotating internal device is used. 

The process steps listed in Table I9-I can sometimes be used to promote mixing. However, they are primarily for funcI ions other than solids mixing. (Note precautions for pneumatic conveying and vibrating in Table I9-I.)... 

Uniformity of Mixture The proper type of mixer shoiJd be chosen to assure the desired degree of batch homogeneity. This cannot be compromised for other conveniences. Information is given under Types of Solids-Mixing Machines about the special abilities of various lands of machines to blend different types of materials. 

Increase solids mixing. Improve powder flowahihty of feed. Increase agitation intensity (e.g., impeller speed, fluidization gas velocity, or rotation speed). 

Suspended solids are often the process objective that requires a specific degree of uniformity. Five guides to better liquid/solid mixing are ... 

There is approximately a 22% deviation between the experimental and the distribution mean residenee time. However, the main purpose was to use the information from the RTD eurve to improve the reaetor operation. The results of the RTD provided vital information eoneern-ing the effeets of operating eonditions and struetural designs on solid-mixing patterns in fluidized systems. The perfeet mixing funetion was generated by e , where 6 = t/f. Figure 8-19 shows plots of these funetions against dimensionless residenee time 6. 

Liquid-gas-solids mixing 275 Liquid-liquid extraction, mass transfer 599 Liquid metals, heat transfer 523 meters 269... 

Solids-liquid-gas mixing 275 Solids-liquid mixing 275 Solids—solids mixing 275 Sonic velocity 150, 156,158, 189 Sorel effect, thermal diffusion 589 Spalding, D. B, 393,562 Sparrow, E. M. 465, 564 Specific energy, open channel flow 98... 

Solid mixed ionic-electronic conductors (MIECs) exhibit both ionic and electronic (electron-hole) conductivity. Naturally, in any material there are in principle nonzero electronic and ionic conductivities (a i, a,). It is customary to limit the use of the term MIEC to those materials in which a, and 0, 1 do not differ by more than two orders of magnitude. It is also customary to use the term MIEC if a, and Ogi are not too low (o, a i 10 S/cm). Obviously, there are no strict rules. There are processes where the minority carriers play an important role despite the fact that 0,70 1 exceeds those limits and a, aj,i< 10 S/cm. In MIECs, ion transport normally occurs via interstitial sites or by hopping into a vacant site or a more complex combination based on interstitial and vacant sites, and electronic (electron/hole) conductivity occurs via delocalized states in the conduction/valence band or via localized states by a thermally assisted hopping mechanism. With respect to their properties, MIECs have found wide applications in solid oxide fuel cells, batteries, smart windows, selective membranes, sensors, catalysis, and so on. 

A selection chart for solids mixing equipment is given by Jones (1985). 

The topical homochirality problem is presently being investigated in several research laboratories across the world. One new object of study is systems with eutectic mixtures. The addition of chiral dicarboxylic acids that co-crystallise with chiral amino acids to aqueous mixtures of d- and L-amino acids allows tuning of the eutectic composition of the amino acids in several cases, these systems yield new eutectic compositions of 98% ee or higher. Thus, solid mixed crystals with a ratio... 

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