344 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS In studying liquid-liquid systems, it is usually desirable to take a given impeller type and run a complete series of D/T ratios observing the type of emulsions produced attempting to determine whether there is an optimum ratio of impeller size to tank size required. Using standard impeller types, it is then possible to relate the experimental conditions to the program. Scale-up on these operations involves the consideration of the flow to fluid shear required as well as the power level involved in the system. Usually of utmost importance is to determine in the experimentation whether the desired level of flow to fluid shear can be achieved with standard design and will be of proven performance. If extremely high fluid shear is required involving very elaborate rotor- stator combinations, then there are often problems on scaling up to large machines for high capacities. LiQum-Soi. m Solids Suspension There are two types of suspensions in liquid-solid operations. In one case, the primary job of the mixer is to produce a suspension of the par- ticle sizes that are present so that the particles may enter into a reaction, or so that they may produce a uniform mixture for succeeding process steps, or a uniform mixture for use in filling processes. If a suspension is to be produced to use as the feed for the filling process, it is essential to know the degree of uniformity existing in the tank at various times. If the tank is operated batchwise with continuous draw-off of the product for filling, the proper suspension at low liquid level is often a problem. This can be solved in a practical way by placing a small well in the bot- tom of the tank, about 12 inches in diameter and ten inches deep, with an eight-inch diameter impeller operating in the well. If the larger impeller used in the main part of the storage tank is placed very close to the tank bottom, it is normally possible to maintain an acceptable degree of suspension down to very low levels, almost down to one-quarter of the volume of the well. Figure 11 illustrates this condition. On some solids suspension applications it is not necessary to maintain uniformity of solids throughout the tank. It is merely necessary to make sure that there is a continual contact of liquids and solids. A description of the degree of uniformity desired is essential in order to insure the proper mixer selection. Solids Dispersion The other liquid-solid contacting operation is involved in a dispersion of solids in which it is desired to reduce the size of agglomerates or particles in the system. The main characteristic of a fluid mixing system is the production of fluid shear. If fluid shear is sufficiently high to break

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.