FLUID MIXING OF COSMETIC FORMULATIONS 351 Tube size has an effect on the mixer side coeffi- cient. h ,x 1/"¾/• (6) This indicates that the tube should be as small as practical considering the pressure drop, ease of cleaning and fabrication. Large diameter high flow impellers are most effective in heat transfer processes. Heat transfer is controlled by the pumping capa- city of the impeller. CONTINUOUS FLOW PROCESSES Continuous flow processing has characteris- tics that are often desirable in cosmetic manu- facture. There are two extremes in equipment for continuous flow processing. To obtain effective mixing, it is necessary to use a flow pattern and fluid regime that usually insures "perfect mixing" in each section of the con- tinuous flow apparatus. This means that each element of fluid is in each zone for various lengths of time. Probability methods have been used to predict the amount of material remaining in the system for various lengths of time. This is covered by MacMullin and Weber (1). In a batch system, each particle is in for the same length of time so that the product of a continuous system of "perfect mixing" may not be similar to batch products if residence time considerations are essential. The other extreme of equipment is a long tubular reactor which approaches "plug flow." This means that mixing is severely restricted and is limited only to mixing across a cross section area of the tube. The particles are in the system for the same length of time as they would be in a batch system, but the mixing characteristics are much inferior to that pro- duced in batch mixing apparatus. Figure 16.--Lightnin CM-Contactor, continuous multi- stage contactor.

352 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS .. ,. •:.•: ..... ß : . :. ,". 4 :•,• • .:' - '.•' • • ß : . ß , :,. •: ..z.: :• '• -•- .:• . • :• ,•.,. : , .•:. •":,.:.:•,., -., • .. • .. ' ' "- .•.: ':: ., • - ...½.:... , .. ... •. .. . • .• .• •:' •. 5" ' . . . :::.•: : , :. ...•. -- •. ,:- : , ß . ".. ' .-... -.: ........... .---.. ......... . .... . : ....... .: : : Figure 17.--Close-up of Lightnin CMContactor• continuous multistage contactor. } roma consideration of the two types of mixing equipment extremes, it can be extended to two types of chemical processing extremes. If' the process involves a chemical concentration, then the only difference be- tween the two types of continuous processing apparatus and batch runs is the volume and residence time required to achieve the desired process result. The product from either continuous system will be the same as those from a batch system. If the process involves residence time as a factor, which is typified by a polymerization in which the molecular size is a function of the residence time, then the application of the continuous system must be carefully scrutinized. The product from either the multistage mixer column, or a tubular reactor will, in general, be different from a batch product.