Avoid blending blunders

Selecting the correct device is crucial to successfully handling solids. It must deal with discrete pieces that have physical size, electrical properties, frictional differences and surface characteristics that can change with the environment.

By Tom Blackwood, Healthsite Associates

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Figure 5. Such equipment finds wide use on a large scale for handling pelletized solids.

In theory the variations from production are layered in the silo and the flow in each tube is identical. This assumes that there are no fines or angel hair to block or slow the flow in each tube. Circulating the contents of the silo can improve CoV but the best results occur if the silo is emptied as it is circulated. Unfortunately, the circulation process often increases angel hair and fines; however, these can be minimized though proper design of the pneumatic convey system. In years past convey velocities were always set too high as a way to prevent plugging problems, mostly due to poor maintenance procedures. With the advent of better computer-based design programs and the ability to adjust convey flows more precisely in the production areas, this is less of a problem.

More recently multi-cone hoppers and lift tube blenders have replaced flow tube blenders. The insertion of a cone within the discharge cone of the silo causes the center to flow faster, which results in blending of the solids laterally. The cone-in-cone must be carefully designed based on the flow properties of the solids and generally isn’t adjustable in production. Mass flow blenders do little lateral mixing but can be used with an external circulation loop to blend out vertical differences. They’re not very commonly used.

Fluidizing blenders. Fluidization is one of the major reasons that solids blending can be difficult. It not only increases the chance of segregation and sifting but also can result in attrition, which alters the particle size distribution and perhaps even surface and electrical properties. However, a wide variety of devices that use fluidization have proven to be very effective blenders.

Fluidized beds are exceptionally good blenders, especially for small volume blends. Attrition is minimal when designed with very small-hole fluidization grids. These are good for small variations in particle size of the components to be blended.

The lift tube blender (Figure 6) utilizes a central pneumatic-operated tube in a silo, often with a cone-in-cone hopper to increase the relative velocity of the individual solid particles. Conveying rates can greatly exceed those reached in a conventional pneumatic convey line (200,000 lb/hr isn’t unusual). Lack of impingement of the particles on elbows can minimize attrition. Dedusting of the product can occur during blending. The solids need to be free flowing.

Figure 6. Unit can provide much higher rates than possible with pneumatic conveyors.

Figure 6. Unit can provide much higher rates than possible with pneumatic conveyors.

It’s possible to blend small amounts of fine particles by introducing them into the gas that enters the bottom of the blender via a venturi. The only limitation is that these particles need to be large enough not to get blown out of the silo and small enough not to plug the ejector feed. The venturi can be used in a convey line to blend on the fly without recirculation to the tank. The pressure at the inlet of the venturi will control the rate of injection of the second solid.

To blend a wider particle-size distribution using fluidization, choose the spouted bed. In this device, the central tube is supplemented with fluidization of the outer cone. The hopper also can have dust collection, which will allow a wider range of particle sizes.

Another variation is the use of jets in the cone of the hopper (Figure 7). These blast the solids in the hopper to create fast localized mixing due to flow of solids around a bubble of gas. The gas must be of sufficient pressure to lift the entire hopper contents and high enough in volume to exceed the minimum fluidization velocity. When the particle size distribution is wide, pulsing of the nozzles in sequence (around the cone or side-to-side) rather than all at once can minimize sifting effects. The short blending time normally doesn’t lead to much attrition, but this always requires checking.

Figure 7. Blasting solids from several points with a gas fosters fast localized mixing.

Figure 7. Blasting solids from several points with a gas fosters fast localized mixing.

Achieving success

There’re several things you must do to ensure a good blend:

  1. Educate the customer. Even though your plant has produced a uniform blended product, it may not arrive at the customer in the same state. Quality controls may ensure that the ingredients, including their size and color, are correct but segregation during filling, transport and discharge can give the impression that the product has not been blended if the customer simply takes grab samples. The particle size and distribution may provide clues that this may happen. Solids flow testing can identify mixtures that are more likely to behave badly than other materials. You must do more than make the customer aware of the steps you’re taking to ensure a good blend, it’s important to pass on tips on how to avoid segregation of the product and how to obtain representative samples.
  2. Educate your operators. A good first impression is important when delivering a product. If you’re filling a drum, don’t shake out the chute onto the top of the drum. If you’re loading a cargo ship, don’t dump the baghouse catch on top of the fill. If you do, send a note to the customer explaining why it looks so bad. Sometimes a simple demonstration of sifting and segregation principles to the operators can go a long way to improve product quality. Once I was having trouble explaining why we wanted to avoid using an intermediate hopper with a slowly deaerating material. So I took a coffee can, put some material in it and shook it up. In the first minute, I tossed in a quarter. It disappeared. Three minutes later, the same thing happened. After five minutes, the quarter landed on top and did not sink. The other two quarters were at or near the bottom of the can. It convinced the operators they had to bag as they made the material.

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