In-bin blenders, also known as gravity, tube or silo blenders, play essential roles in industries where large volumes of material require homogenization to meet specifications, to minimize the effects of segregation or to blend various other components. Often, in-bin blenders are day bins or silos retrofitted with internal blending devices. They rely on the creation of retention-time differences as material passes through the blender from fill to discharge, and they require the recirculation of the "heel" or volume of material initially located below the blending device.
Recent developments have improved the efficiency of the units, and have extended the range of materials that can be processed. With proper design, an in-bin blender can be an economic investment that provides maintenance-free operation.
The units are most successful with free-flowing components that do not have fragile, degradable particles or ones that do not have low melting points.
Strategies for achieving in-bin blending include:
first in/first out (FIFO) flow patterns to address any radial composition variations;
use of internal or external vertical tubes with holes to draw material from different bin levels and recombine them at the bottom to form a blend; and
installation of blending cylinders to impart different velocities to particles at different levels, thereby causing different retention times.
The only way to design an effective in-bin blender or determine an existing design's effectiveness, without resorting to trial and error, is to test the blend components' material flow properties against key criteria.
Figure 1:These indices provide a practical way to assess a material's suitability in blending equipement.
The basic bulk properties listed in Table 1 determine a material's suitability for in-bin blending. The Johanson Indices, which are listed in Table 2 and illustrated in Figure 1, make interpretation of these properties practical.
Gravity-flow-tube blenders require very free-flowing (FRI > 100) and uniformly sized materials that have a low angle of slide on the bin wall surface (HI), such as plastic pellets. If the material shows cohesion (AI > 0.2 ft. and a RI > 0.3 ft.), the tubes will block.
In-bin blenders with cylinders require low to moderately high cohesive solids (0.2 ft. < AI < 3 ft. and 0.3 ft.< RI < 10 ft.) and perform best with materials that are somewhat free flowing and non-fluidizable (FRI > 100).
Figure 2:As the material level approaches that of the steep conical hopper, a slow-moving region develops at the sides of the hopper.
Mass flow is a condition in which all material in a hopper moves in a non-uniform pattern during discharge. FIFO is an extreme case of mass flow that can be achieved by using steep conical- or chisel-shaped hoppers. Uniform flow is a function of hopper level. As the material level approaches the steep conical hopper, a non-uniform velocity gradient occurs, with a faster flow in the center of the hopper and slower-moving material near the transition from the straight side to the cone. This is illustrated in Figure 2.
FIFO flow hoppers
Figure 3:A layer of corn mean has been placed on sand in this ABD hopper, but intermixing does not occur during discharge, a sure sign of FIFO.
A FIFO flow pattern represents true plug flow -- each horizontal slice of material exhibits a uniform velocity and thus emerges at the outlet in the same time frame. This lessens or eliminates radial composition gradients that occur as a result of some segregation mechanisms or as a result of loading different components at off-center locations.
Figure 3 illustrates FIFO flow for a layer of corn meal placed on sand. Equal volumes of material are successively removed from the outlet. The presence of uniform velocities is indicated by the lack of intermixing of corn meal and sand even to the last sample.
Figure 4: This type of hopper provides higher velocity through the inner core.
Both a cone-in-cone hopper, as depicted in Figure 4, or an Arch-Breaking Diamondback (ABD) hopper, shown Figure 5, will deliver a FIFO-like cross-sectional average of material. However, the two approaches have different advantages: the cone-in-cone is moderately priced and easily retrofitted to existing conical hoppers, while the diamondback requires no internal support structure and can handle a larger variety of materials.
Figure 5:This design features an adjustable blending cylinder that generates velocity gradients efficiently.
The cone-in-cone blender is designed to provide a higher velocity through the inner cone. The gradient is increased by the addition of blending cylinders of decreasing diameters in the upper reaches of the cylindrical section of the blender. The velocity profile is illustrated by measuring the position of markers as material is withdrawn from the blender (Figure 6).
Figure 6:The velocity profile in the cone-in-cone blender is delineated by measuring the position of the markers as material is withdrawn.
The ABD blender relies on an adjustable blending cylinder that defines the velocity gradient in the hopper. When the cylinder is lowered into the hopper, it restricts flow on the flat converging sides of the hopper, but not at the vertical end walls. The flow velocity varies all around the circumferential region between the blending cylinder and the wall (Figure 7). Material flows fastest in the center of the blending cylinder because the material on the outer walls of the cylinder adds pressure to the material at the center of the hopper. Varying the height and position of the blending cylinder can modify the velocity gradient. Figure 8 illustrates the effectiveness of this design.
Figure 7:The adjustable blending cylinder defines the velocity gradient imposed in the cylinder of the hopper.
Blending efficiency can be improved in tall silos by adding blending cylinders of decreasing diameters from the top of the hopper to the top of the material layer. Each blending cylinder should be sized to provide an increasing center velocity. Figure 9 shows a silo with an expanded cone-in-cone blender and additional blending rings in operation.
Figure 8:After charging sand and then corn meal into this ABD blender, material is removed and then recycled. A complete blend results after recycling volumes.
1. Observe the blend through the walls of the blender while it is running. This provides an excellent first subjective test to determine if further mixing is helpful, required or harmful. Unfortunately, most industrial blenders are made from metal, making this approach impractical and thus mandating an indirect measurement of blend quality.
2. Discharge the contents of the blender onto a belt conveyor or take small samples at equal intervals as the blender is emptying. This visually identifies results, including the effect of the emptying process. To assess blend quality, however, the mixture must undergo sieve analysis or measurement of flow properties, such as the Flow Rate Index (FRI) and the ratio of FRI to the bin density index (BDI) or chemical analysis. The last method quantifies the results but requires more time, cost and effort.
Figure 9:Adding cylinders of increasingly larger diameter from the top of the hopper in a tall silo can boost blending effectiveness.
The flow pattern in a bin can be evaluated by using markers positioned at an upper layer of the bin in a pattern of a center point with successive concentric rings. The markers, selected to have a density similar to the blender's contents, are located in each quadrant and can be easily identified and separated at discharge. The blender contents are then removed in small, equal volumes. The flow contours can be reconstructed from the analysis of the marker content of the volumes. This technique, using several layers of markers in equal patterns, delineated the flow patterns shown in Figure 6. This technique also can identify dead zones or regions of stagnant material.
Lee Dudley is president of Diamondback Technology Inc., San Luis Obispo, Calif., a firm that specializes in equipment and consulting related to solids handling. He is a former consulting engineer with JR Johanson Inc.