Minimize Blending Time

Calculating the time actually needed can lead to economic and operational benefits.

By David S. Dickey, MixTech, Inc.

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Most industrial applications blend liquids for much longer than required — incurring unnecessary burdens on both energy budgets and equipment. Reducing blend time offers an opportunity for increased productivity and decreased costs. Knowing and understanding process requirements is the first step in estimating proper blend time for your application.

This article focuses on stirred-tank applications, typically involving center-mounted top-entering mixers. Most low-viscosity turbulent applications require baffles to prevent uncontrolled swirling because solid-body rotation of liquid doesn't create effective mixing. These systems have been studied well enough to provide some actual quantified guidance for predicting process blend times. Other mixers, such as angle-mounted portable mixers, when appropriately positioned quickly blend low-viscosity batches, usually in less than a minute or two [1].

Testing
Blend time generally is determined by adding a small quantity of liquid to an agitated batch of similar property liquid. Laboratory tests most often add a liquid that results in a color change, from some hue to clear, and conduct tests in transparent tanks — allowing observation of the final color change indicating a degree of complete blending, no matter the location in the tank. Because low-viscosity mixing is a turbulent process the exact location and time will vary from test to test as flow patterns fluctuate. Changing flow patterns and random velocities both are mechanisms by which rapid blending may occur in turbulent conditions.

Some more-common color change methods use either a pH indicator solution or an iodine color remover. With the pH approach, a color-to-clear indicator, such as phenolphthalene, first is set to the color form by a dilute caustic solution mixed in the test tank. Then a small quantity of a more-concentrated acid is added on the surface of the liquid. The quantity and concentration of the acid suffice to take the batch from alkaline to acidic condition, making the colored indicator disappear. Repeated tests with careful addition and timing will establish a good average for blend time at the prescribed degree of uniformity.

Blend-time test results typically are correlated as a dimensionless blend time (Θ), which is expressed as measured blend time multiplied by the mixer's rotational speed. This group is dimensionless because blend time has the units of time and rotational speed has the units of reciprocal time. The value will be the same for any unit of time, so long as both variables are expressed in that unit. Sometimes Θ is affectionately known as the Betty Crocker number because the time to uniformity effectively is related to the number of impeller revolutions (beater strokes). For turbulent mixing, Θ is a constant.

Blend time also depends upon the impeller type and the impeller-to-tank diameter ratio. (We'll address the effects of fluid properties separately.) However, Θ is independent of absolute vessel size. So for geometrically similar mixers and tanks, blending in a small or large tank requires the same number of impeller revolutions. This ability to apply laboratory measurements to process vessels is essential to the practical use of blend time correlations. Scale-up holding blend time constant is impractical as power requirements quickly become excessive.

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