FLUID MIXING OF COSMETIC FORMULATIONS 345 down the agglomerates or particles, then it is possible to carry out the required degree of dispersion in the system. Again, one of the key factors in studying dispersion processes is to determine whether standard turbines or propellers can be used to produce the required degree of dispersion. If they can, they offer the most economi- cal and practical means of achieving the required process result and can be scaled up to almost any size system. If it is determined that very special speeds or impellets are required, then scale-up characteristics are sometimes a problem. In general, research work has shown that at the optimum flow to fluid shear ratio, the process results obtained by all mixing impellets are essen- tially the same. There may be a wide difference in appearance and D/T ratio of a given impeller type compared to another for the same process restlit, but this reflects the difference in mechanical characteristics of the various impeller types. The key factor is to then pick out an impeller combination which gives the most practical mixer for the full size instal- lation. DRAW-OFF Figure 11.--Illustrating one method for maintaining uniformity of draw-off to a very low liquid level. Small well is placed in the bottom of the mixing tank to insure uniform- ity of the last portion. EXPERIMENTAL EQUIPMENT AIR T I--Ds-- • Figu re 12.--Details of mixing vessel, experi- mental equipment.

346 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Laboratory experiments should always be carried out with a range of impeller size to tank size ratios to determine how critical this' factor is. G^s-L•Qtr•r) CONTACTING Gas-liquid contacting is normally involved with absorption or stripping operations. In absorption, it is desired to absorb one or more of the components of the gas stream into the liquid phase. The gas may either remain dissolved in the liquid phase or may enter into a subsequent reaction with either a liquid reactant or a solid reactant. In stripping operations, an inert gas is passed through the tank and removes a com- ponent present in the liquid phase. In either of these operations, labo- ratory studies offer a very rigorous technique for scaling up to large size systems. By measuring the absorption rate as a function of mixer speed for three or four different speeds at a giv. en gas rate, and at one of these speeds for three or four different gas rates, it is normally possible to predict the ab- sorption characteristics of a full size system. These systems may be scaled up to batch or continuous operations or to multistage operations. By the use of standard impeller types in these studies, it is possible to accurately design the full size unit. In gas-liquid systems, mixing has a large effect on the absorption effi- ciency and it is desirable to accurately study the process characteristics of the system. Gas is normally admitted to the tank through an open pipe or sparge ring placed beneath the impeller (Fig. 12). Drawing gas down from the surface can work very well on small size units up to two or three feet in diameter. It does not work effectively in large diameter systems, unless gas is drawn down as a supplement to gas being introduced beneath the impeller. Laboratory mixers which draw gas down from the surface can be used ro give an indication of the process results, but they do not give the data needed for full scale design. BLENDING PROCESSES Blending of several miscible fluids is one of the most usual process requirements. Depending upon the tank size being used, different types of equipment are often encountered. The most effective flow pattern for blending is normally a baffled pattern, although in a very particular condi- tion of fluid viscosity and tank shape, unbafFled operation can be used. In vertical cylindrical tanks, blending may be accomplished by either top-entering or side-entering mixers. In tanks up to 100 gallons and fluids of 4000 centipoises or lower, portable or permanently mounted propeller type mixers are suitable (Fig. 1). These may be mounted in an angular off-center position to achieve the desired flow pattern. The power level