Figure 3. Solids are fed to the mixer with hood cover and exhaust fan for containment.
Blended powder discharges into a transfer hopper that feeds a pneumatic conveying system taking the product to a surge hopper and on to a semi-automatic auger filler. Less free-flowing material is loaded into drums and transferred via handheld pneumatic wand to a fully automatic vertical form-fill-and-seal bagging machine.
Why ribbon blending?
After testing, Munson custom-designed the 52-ft3 (1.5-m3) ribbon blender for process requirements. The ribbon was the most economical mixer able to meet the DCP’s dual objectives of efficiently blending a variety of ingredients with dissimilar properties, including both low-micron-size ingredients and minor ingredients in small amounts, and also handling a range of batch sizes. Ingredients include free-flowing granules, and powders that compact or cake. The ribbon blender provides sufficient shear for non-free flowing materials as well as moderately lumpy materials and can handle friable, shear-sensitive materials.
Ribbon blenders can efficiently mix components in batches smaller than rated capacity, although blending time increases as batch size decreases. For example, at 40% batch capacity, a typical blend can require as much as twice the blend time, compared to full capacity, to effectively disperse minor ingredients such as dyes and enzymes. The mixer was designed to fit DCP’s typical batch size of 2,000 lbs. (907 kg); through experimentation, Longo has evaluated the turndown of the mixing system. DCP has been successful mixing batches from 800 lbs. to 2,200 lbs. (360 kg to1,000 kg).
The counter-flow agitation in the blender mixes materials rapidly and completely, and is particularly efficient at blending ingredients of widely varying quantities, densities, and particle sizes, a good fit for DCP’s requirements. Most products require colors, dyes, or fragrances in very small amounts, as small as 1 to 2 lb. (0.45-0.9 kg) in a 2,000-lb. (900-kg) batch. The ribbon blender efficiently disperses these minor ingredients, according Longo, producing a homogeneous product in five to 15 minutes, depending on batch size. A five-to-10 minute batch would be a free-flowing material batch, ranging from 75% to 100% of the batch capacity with little or no liquid addition. A 15-minute mix time would be for a batch at less than 50% capacity for a non-free flowing material, with, or without, liquid addition.
This is a substantial improvement compared to the old 35-gallon planetary mixers. A typical batch size with the old mixer was 150 to 200 lb., depending on product density.
Uniform blending is a consequence of the mixer’s design and fabrication. A pilot study was completed at Munson’s laboratory to validate the new ribbon blender design. The U-shaped vessel is constructed with large-radius fillet welds, ground smooth inside the body to eliminate rough spots that might trap material and prevent complete blending. An exceptionally small clearance, 1/8-in, or 3 mm, between blade and shell, further enhances complete mixing, with no dead spots, pockets, or corners.
The ribbon blade element consists of a central shaft that rotates two spiral or helical ribbons, inner and outer, with reversed pitches that force material in opposing directions (Figure 4). For optimum results, the blade’s geometry is customized for specific feed properties, making testing critical to the engineering process. The agitator element is easy to remove, designed in an assembly that may be lifted out if necessary for cleaning or maintenance, although ribbon blenders are well known as tough designs that rarely need maintenance.
