The mass transfer coefficient in covered, right-cylindrical tanks full of liquid, turbulently agitated at various speeds by turbines with six flat blades, was measured by the rate of solution of suspended solids in water and in 45% sucrose solutions.Screened crystals in the following U. S. mesh sizes were used: boric acid: 18/20, 16/18, 16/20, 14/16, 12/14, 10/12, 8/10, 6/8; rock salt: 6/8, 4/6. Pellets were benzoic acid: 0.126 in. long by 0.218-in. diam.; salt: 0.565-in. diam. by 0.531-in. long (over rounded ends).Tanks were 6, 12, 18, and 30 in. Turbines were 2, 3, 4, 6, 9, and 12 in. in diameter, centrally located. Four full-length baffles 10% of the tank diameter wide were spaced at 90 deg. A few runs were made without baffles.The coefficient of mass transfer was found to be independent of particle size and Schmidt number (Ns, = 735 to 62,000) and could be correlated with turbine Reynolds number in each tank, with larger tanks yielding smaller coefficients at the same N R~. An empirical equation which fits all the data from the baffled tanks within about 4% (in the range 0.1 < k < 2) isIn ( A treatment of the data according to dimensionless groups provides another correlation:t is shown that for the systems used 1 / D is essentially proportional to Ns,".", and so the effect of diffusivity here is only apparent.A recent review of the available information for computing the stage efficiency of mixer-settler extractors (1 9 ) revealed the severe limitations of our knowledge of the mass transfer coefficients for the continuous phase which surrounds the dispersed particles in the mixer. For baffled vessels e TURBINE SHAFl SAMPLER (SEEFIG 3) \

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poises. The solute was tagged with the isotope sodium-22.The transient profile in the diaphragm, which is predicted by the solution of the appropriate equations, results in a flux that was monitored using a scintillation detector and an electronic scaler. The difhsivity of this high-viscosity system can be measured in a matter of hours with this technique. Figure A diagram of the experimental setup appears in general nature of the setup is a diaphragm with side from which liquid is drawn out, circulated through the counting chamber, and returned to the cell. The cell is a Pyrex fine frit with a porosity of 4 to 5p. The total volume of the circulation system including the upper reservoir is 250 CC. DESCRIPTION OF APPARATUSThe mixing apparatus, in conjunction with the circulation of the liquid, rapidly disperses the solute throughout the system as it enters the upper reservoir. The Paddle on the mixing rod, which turns at about 1 rev./sec., is placed as dose to the diaphragm as possible. This prevents a concentration buildup in a stagnant layer above the diaphragm.The counting chamber (about 20 cc.) is streamlined to prevent stagnant liquid pockets from being formed. The disk shape of the glass chamber results in a good geometric efficiency for a maximum counting rate. A lead shield houses the counting chamber to lower the background count and thus increase the sensitivity of tracer detection.A thermostatically controlled heater and a continuously operating fan maintained the constant-temperature Plexiglas box at 32 e 0.5"C. A desiccant was placed in the box, and all penetrations were sealed. The peristaltic pump is of the pulsating positive displacement type with capacities from 0.5 to 250 cc./min.A scintillation counter was used. This consisted of a 1.25-in. thallium-activated sodium iodide crystal, a photomultiplier tube, preamplifier, an amplifier, single-channel analyzer and a scaler. The analyzer threshold was set high enough to suppress noise yet low enough to record the gammas following the decay of sodium-22. A second, shielded scintillation unit was used to continually monitor the background while the main counter was PROPERTIES OF TRACER SOLUTIONThe selection of the sodium-22 isotope in the form of sodium chloride was based on its hard gamma emission, known difFusivity in water, ability to dissociate completely in glycerol at a concentration of 1.25 X loT4 normal, and a long half-life (2.7 yr.) . PROCEDUREFor standardization, the prepared aqueous solution is pumped into the lower reservoir until it contacts and rises into the diaphragm. Bubbles are removed from under the diaphragm through the side arm. AU tubing and the counting chamber in the circulation system are initially filled with distilled water. Water is then added to the upper reservoir to bring the total of the circulation system to 250 cc. Circulation and mixing are started immediately.The glycerine run is similar. When filling the lower reservoir, care must be used to get as little as possible of the glycerol solution through the diaphragm...