:266 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS If data obtained on bench scale are to be used for scale-up, a six-inch tank size is usually a minimum. The impellets should be scale models of impellets to be used in the production size equipment. Baffles similar to those to be used on the production equipment should be employed in the bench scale. For pilot plant work, it is recommended that either of the two basic types of impellets be used for the pilot scale operation. Propeller type equipment is used up to 3 horsepower. If a preliminary analysis of the full scale operation indicates that it could be carried out with equipment of this size, then it is desirable to run the pilot plants under these conditions. If, on the other hand, it looks like more than 3 horsepower will be required on the full size basis, turbine type equipment should be used in the pilot plant. Runs should be made at three different impeller speeds. From looking at the slope of process result versus speed for these three runs, we can often tell the controlling mechanism of the process. The l/Fay in Pl/hich 1/arious Ratios 1/ary in Scale-Up Table 3 illustrates the way in which different ratios vary if other ratios are held constant. For example, if we maintain equal Reynolds numbers, which means equal ratios of inertia force to viscous force, then the total power per unit volume is very low, and equal Reynolds numbers are not usually the basis for mixing scale-up. On the other hand, if we maintain equal power per unit volume, then speed is lower, Reynolds number is higher, the maximum shear rate is very high, but the pumping capacity per unit volume is much lower than it was in the original tank. T.•m,v 3.--PV. OPEP. TIES OF A F•.rm MIXER ON ScAt E-UP Pilot Scale Property 5 Gallons Plant Scale 625 Gallons P 1.0 125 3125 25 0.2 P/Vol. 1.0 1.0 25 0.2 0.0016 N 1.0 0.34 1.0 0.2 0.04 D 1.0 5.0 5.0 5.0 5.0 Q 1.0 42.5 125 25 5.0 Q/Vol. 1.0 0.34 1.0 0.2 0.04 ND 1.0 1.7 5.0 1.0 0.2 ND2p 1.0 8.5 25.0 5.0 1.0 It is threefore essential to pick out some of the ratios that are important. When dealing with viscous materials, the maximum shear rates affect the characteristics of these systems, while minimum shear rates affect the ability to move throughout the tank. With pseudo-plastic materials, it may be desirable to go to large impeller size to tank size ratios, which imply much lower speeds tl•an were used on

SCAI,E-UP CONSIDERATIONS IN COSMETIC MANUFACTURE 267 the pilot plant experiments. In general, if viscous or other psuedo-plastic materials are used, gear drive portable mixers are commonly used up to 3 horsepower. If slower speeds are required, slow speed turbine type equip- ment is utilized. Above 3 horsepower, turbine units are normally used. SOLID SUSPENSION In giving process requirements for suspension operations, it is desirable to distinguish between the characteristics of the solids, whether they have a finite settling velocity or not. It is also desirable to distinguish between a suspension of solids and a requirement for breaking up agglomerates of solids. If the solids are difficult to wet out, this should also be noted. In the discussion below, three separate categories are treated: 1. Suspension of particles of essentially zero settling velocity. 2. Suspension of solids with a finite settling velocity. 3. Dispersion of solids and agglomerates. Essentially Zero Settling Felocity In these processes, the major requirement is to produce a uniform dis- persion throughout the entire medium. For batch operation, it is usually a matter of determining the time required to obtain a uniform suspension. [f the solids are difficult to pull in from the surface, an impeller is often placed near the surface to aid in this operation. However, this also induces air into the system. Once uniformity is achieved, there is no further ad- vantage in keeping the mixer on. Propeller type mixers come in sizes up to 3 horsepower and can handle viscosities up to 5000 cps. in tanks up to 500 gallons, and tanks up to 500(3 gallons in volume with waterAike fluids. Normally gear drive mixers at speeds from. 280 to 420 r.p.m. are used in these operations. Tank propor- tions of liquid depth equal to tank diameter are used, but other proportions may be used if required. If the power requirement is above 3 horsepower, turbine units are nor- mally used. These are equipped with either fan turbines or fiat blade tur- bines. Finite Settling Felocities In this case, solids are picked up in suspension rapidly but may settle out if the mixer is turned off. The degree of uniformity produced is a func- tion of the power applied to the system, but if the settling velocities of any of the particles are above 1 f.p.m., there may be gradients in concentration throughout the tank unless relatively high power levels are used. If the settling veolicities are below 1 f.p.m., the solids will be essentially uniform throughout the entire tank. As in the case above, gear drive propeller units are commonly used in