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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Now as a potential process improvement, let's consider replacing the pitched-blade turbine with a hydrofoil impeller. It could be used at the same speed and sized for the same power draw as the pitched-blade turbine. Because the hydrofoil impeller has a lower power number, it must have a larger diameter — 40 in. in this case — for the same power. The hydrofoil impeller still will require 0.69 hp. but will reduce blend time to only 41 sec. Thus, with the same power and speed, which is also the same torque, the hydrofoil gives more rapid blending. Before making a switch, though, it's important to check the natural frequency of the shaft to ensure the weight of the larger impeller doesn't cause mechanical problems.

Feed Conditions
Not all blending jobs involve adding a small quantity of liquid to a well-mixed batch. You must consider both the rate and quantity of addition in real process applications.

Figure 1 illustrates three potential feed rates. Let's look at their implications on blend time. Say a slow feed rate requires 5 min.; so uniformity can't be achieved until after 5 min. — probably more like 5 min. plus 81 sec. for our example. At an intermediate feed rate that's much less than the estimated blend time, 81 sec. may be appropriate. A high feed rate, sufficient to influence the blending flow pattern, may lead to a slightly reduced or at least different blend time.

Feed Location
Figure 2: Feed location -- Adding liquid just above the
tip of down-pumping axial-flow impeller generally
provides best results.
The quantity of addition will have effects similar to those for feed rate. If putting in that amount of liquid will take longer than estimated blend time, add addition time to estimated blend time. When feeding in a large volume, use the final liquid level to estimate blend time following the addition.

Sometimes location of the feed is more important than its rate or quantity. Published data typically come from experiments using surface feed (Figure 2). Because most tanks have bottom valves a feed location at the bottom of the tank also is possible. However, neither of these locations provides any special advantage, as both rely on adequate liquid motion between feed location and impeller region to initiate intense blending. At least part of the blend time is required to move the feed to the impeller region.

The most vigorous mixing occurs in the immediate region of the impeller. Local power-per-volume dissipation can exceed average dissipation by an order of magnitude. For fast chemical reactions, especially those with competing or consecutive reactions that produce alternative byproducts, feed near the impeller can be critical. The rate of initial blending may determine the quality and quantity of desired product. The ideal location for the feed is just above the tip of a down-pumping axial-flow impeller. At that point the feed is drawn quickly though the most-intense mixing, speeding both initial dispersion and final blend time. Velocity of flow from the dip pipe must be fast enough to prevent back-mixing at the tip and at a velocity comparable to local velocities so as not to adversely influence the flow created by the impeller.

While a wall feed location may appear to offer similar benefits to the dip-pipe location near the impeller, flow between the tank wall and impeller may follow many paths. Experience shows that wall feed causes more problems than it potentially could solve.

Physical Property Effects
Density and viscosity also can significantly impact blending and blend time. The most important influence of density appears in impeller power. For turbulent mixing impeller power is proportional to liquid density. Density differences between bulk liquid and the addition also may affect blend time. Viscosity differences can have an even bigger effect, both on bulk blending and on blend time.
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