Agitation of liquid systems requiring a high shear characteristic

Abstract

The systematic study of agitation, as a chemical engineering operation, has been undertaken in most of the categories possible, but certainly not in the high shear section of the technology. Even the usual very empirical studies are lacking. The reasons typically offered for neglecting this area are lack of adequate theoretical background, limited scope of application, plethora of variables involved, and extreme diversity of equipment types available. The state of the art has improved little since Scoville (12), in 1895, reported that "five minutes of very rapid trituration will accomplish more in emulsifying an oil or balsam than an hour of slow trituration."High shear agitation as discussed here refers to the classes of application known as emulsification, dispersion, homogenization, etc. It is the processing area that lies between the agitation intensity produced by conventional impellers operated at high power per unit volume and the shear forces generated by homogenizers or colloid mills. It is a relatively narrow range, at present served by a variety of high speed impellers, most of them being modified disks or cones or the rotor-stator type.The term high shear may be defhed in several ways. First to be considered should naturally be a theoretical one. Discussions of the fundamental hydrodynamics such as those by Calderbank (3) and Hinze (7) do much to help understand the forces controlling dispersion processes. But the mechanism of shear involved in fluid agitation has not yet been satisfactorily established in terms that can be applied to practice. Although theories in favor of cavitation, velocity change, direction change, and mechanical forces have been offered, none has been verified.A better understanding of the process is obtained from consideration of the impeller action. The shear/flow characteristics of various standard mixing impellers in the conventional range of use have been evaluated by a number of investigators (1, l o ) , and the general concept of shear is established as a function of impeller geometry, system geometry, and power-speed relationships. It is mathematically improper to say, as is often done, that a high shear impeller invests the majority of the total energy via shear and a minority in flow. But it can be stated that performance and power are adjusted to maximize the head term (WD') and reduce the flow ( N D'), and that is done by means of a relatively small D / T ratio, a high speed, and a small opposed blade area.SpecSc literature on high shear agitation of liquid systems is predominately qualitative in nature. Case history publications have been universally testimonial in nature and reflect the proprietary designs involved. Patents abound but add nothing of technical value. A good general treatment of the subject is found in the book by Clayton ( 5 ) . Several investigators (6, 9) reported quantitative agitation data on emulsification problems, and the papers by Brothman ( 2 ) and Miller and Rushton (8) discuss possible factors controlling shear. All of these sources recog...

Oil dispersion with a turbine mixer

“…gives that ultimate diameter. This was demonstrated by Fondy and Bates (1963). The data for Batch 9 are plotted this way in Figure 2.…”

“…Both functions require energy from the impeller. Fondy (1963) has shown the ultimate diameter of emulsion produced is controlled solely by tip speed of the impeller with a given emulsion at constant temperature. Most of the energy consumption, on the other hand, seems to go into moving (pumping) the fluid and in gross turbulence.…”

Chemical and Process Engineering

“…One of these was the low shear, high flow efficiency, HE3 impeller, 𝐷 = 0.063 m, recommended for efficient blending. The other two were the ultra high shear ChemShear impellers, Style 2, CS 2, and Style 4, CS 4, each of which are recommended for emulsification processes where very small drops are required (Fondy and Bates, 1963). These impellers are shown diagrammatically in Figure 1 and futher details of the Chemshear impellers are given in Table 1.…”

Hydrodynamic Performance of a Ring‐Style High‐Shear Impeller in Newtonian and Shear‐Thinning Fluids

“…Fondy and Bates [7] compared the performance of two HSIs, operating in the transitional flow regime, with respect to the marine propeller and the flat-blade turbine. They reported that for the same power demand smaller mean particle diameters were obtained with HSIs.…”