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.