This invention relates to a reversible mixing impeller, and more particularly to such an impeller which is designed to perform two different mixing functions, depending upon the direction of rotation.
The mixing processes, such as used in the chemical industry, often place conflicting demands upon the design of the mixing impeller. In the operation of a mechanically agitated batch reactor, an example of an agitation schedule might be described by the following typical steps:
a. The reactor or vessel is filled with a low viscosity liquid which is then heated to a reaction temperature.
b. Gas is introduced from a sparger. Mass transfer of the gas dispersed by the impeller is followed by a fast exothermic reaction in the liquid phase.
c. Temperature control is maintained by boiling off and refluxing some of the liquid while gas addition and dispersion by a mixer impeller continues.
d. The solid product of the reaction is precipitated and kept in suspension by the mixing impeller, while the particle size distribution is developed.
e. Suspension viscosity rises, and it becomes non-newtonian.
f. Finally, the reactor vessel is emptied. A relatively uniform concentration of the suspension is maintained as the level falls within the vessel.
At another time, the same reactor vessel or mixing tank may be used for a different product, thereby placing a different set of demands on the mixing impeller. Therefore, it is not surprising that the selection of an impeller for any given batch operation is frequently a matter of compromise. When considering a preferred design for any single stage of the program, as set forth in the above example, one might select a disc turbine impeller or a profiled impeller or one with a large swept volume. Obviously, one impeller alone is unlikely to meet optimally the particular mixing or blending requirements, since one impeller may have high efficiency for producing an axial flow, while another may have high efficiency in transferring energy from the impeller into a radial flow.
Traditional mixing impeller blades are symmetrical about some definable axis, and the power numbers of such impellers do not depend significantly on their direction of rotation. This implies that a fraction of the total kinetic energy of the outflow, associated with small scale turbulent fluctuations, is the same whichever way the impeller is rotated.
Complex impeller forms with specific advantages have also been developed. These designs have provided profiled, cambered hydrofoils with low drag coefficients, and produce large scale convective flows, usually axial, with a minimum of turbulence. Such modern agitator impellers are often asymmetric, designed to operate in a particular direction of rotation, and users must take care to ensure that they are mounted and operated properly.
It is believed that very little has been done to design an impeller which is reasonably efficient for the intended purpose, in either of two directions of rotation for the purpose of providing significantly different mixing effects with significantly different power numbers, depending on the direction of rotation. A single example is shown in U.S. Pat. No. 4,305,673 issued Dec. 15, 1991. In that impeller, primarily radial outflow is caused in either direction of rotation, the principal difference being that of the power number ratio between clockwise to counterclockwise rotations. Thus the impeller is described as simply drawing less power in one of two directions of rotation. Only one blade form is disclosed, Hurt lacks any concept of converting between primarily axial convective flow and primarily radial high turbulence and high shear flow when the rotation is reversed.