For many engineers and scientists in the chemical process industries, mixing is an essential unit operation. Often, this essential process conversion step between the raw materials and the final product uses a stirred tank. As important as this step can be, actual mixer performance often is poorly understood.
Everyone wants uniform mixing, but each process involves different fluid properties and operating variables that establish the mixing intensity needed to achieve uniformity. Fluid properties such as density and viscosity, impeller type, tank size and other characteristics all influence mixing intensity. By gaining an understanding of a mixer's process capabilities, you will be better able to apply existing equipment or design new equipment for a specific process. It also is important ," but difficult ," to know how to describe the required amount of mixing.
Several steps are key to developing a better understanding and to defining the mixing capabilities of a mixer. Some steps such as creating a scale sketch or drawing are simple. Other steps that determine mixing intensity require some knowledge and definition of the basic mixing processes. In addition, equipment design involves mechanical consideration of shaft strength and critical speed. Occasionally, the situation could be sufficiently unique to defy simple description.
Is that drawing accurate?
Some aspects of mixer description such as the dimensions of the mixer are almost intuitive. A common assumption is that all computer-aided design (CAD) drawings are to scale. Unfortunately, the drawings supplied by equipment manufacturers often are general-purpose drawings with specific numerical dimensions. Even when a drawing appears to show mixer impellers that are evenly spaced and the same size, the dimensions might reveal something entirely different.
One of the critical dimensions for any mixer is the distance between the lower impeller and the bottom of the tank. Because the mixer and tank often are purchased separately, communications about design changes to original dimensions might be overlooked.
Even if head dimensions and straight-side lengths are unchanged, changes to nozzle height or type can alter shaft-length requirements. In addition, mixer design includes mechanical considerations such as shaft strength and critical speed. Mixer shafts must be strong and stiff enough to avoid mechanical problems.
The most obvious and potentially disastrous problem is a mixer shaft that is too long to fit the tank. A less obvious problem is a shaft that is too short. A short shaft is easier to design mechanically, but it might mean the lower impeller is above or only slightly below the minimum liquid level for a batch process. In any case, a poorly positioned lower impeller can cause problems during tank filling or emptying operations.
The simple and effective solution to mixer and tank compatibility problems is a scale sketch or drawing. Fig. 1 shows an example of a scale drawing generated by CerebroMix Light II, a computer program from CerebroMix Inc., Campinas, Brazil.
Figure 1. Ensure Perfect Fit
A properly scaled drawing ensures that correct mixer shaft length and impeller sizes are chosen.
Visual observation often provides a clue to potential problems. You should draw the tank to scale, including nozzle-mounting dimensions. You then can draw the mixer inside the tank, with the impeller locations properly scaled. Finally, the liquid level needs to be shown so you can check for effective operation.
An often-overlooked consideration in liquid-level determination is the space occupied by the mixer, baffles and other tank internals. For mixers with common pitched-blade, straight-blade and hydrofoil impellers, designers generally can assume the internals occupy about 10 percent of the open tank volume. In other words, expect and plan for the liquid level to be about 10 percent higher than that calculated from the inside tank dimensions.
To adequately describe mixer size or characterize mixer capability, you must consider quantity, difficulty and mixing intensity.
The quantity of material to be mixed might seem to be an obvious characteristic because a large mixer will be required for a large quantity of material. Quantity can be expressed in terms of volume or mass. In any case, you must know the fluid density or specific gravity to determine mixer power requirements adequately; thus, volume and mass can be related.
The difficulty of a mixer application depends on the process characteristics. For fluid blending, viscosity is a natural difficulty parameter. The more viscous the fluid, the more difficult the mixing task. A large mixer will be required to mix a viscous fluid.
Particle-settling velocity and solids concentration are difficulty parameters for solids suspension. Gas flow rate is a difficulty parameter for gas dispersion. The difficulty parameters for other dispersion processes might involve combinations of density ratio, viscosity ratio, bonding characteristics, particle size and related factors.
The third mixing factor is mixing intensity. Unlike quantity and difficulty, which are associated with measurable characteristics, mixing intensity is an illusive characteristic. It often is described in general terms such as mild, medium or violent. However, some definitive quantification is needed.
Some years ago, an article describing liquid blending proposed a 1-to-10 scale for the quantification of mixing intensity.1 The article reduced the method to tabular form for typical applications.
With the huge increase in computing capabilities since then, programming those quantification methods is a logical extension to mixing analysis. The basic premise of the "scale of agitation" is that a value of 1 represents the minimum intensity necessary for complete motion of the liquid; a value of 10 represents the maximum practical intensity, although higher intensities might be used for extreme cases.