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Test for success

Oct. 8, 2004
The best hopper is the least expensive one that consistently provides the required solids flow. A few simple indices can pinpoint potential solids flow problems and help optimize design.

 By Kristin Dudley and Lee V. Dudley, Diamondback Technology

Unfortunately, there is no way to successfully predict exactly how any one bulk solid will behave based on experience alone. Even research and development, process calculations and pilot plant studies may not prevent material handling problems. Therefore, testing to determine the characteristics of bulk materials is a key element in equipment design or selection.

The only way to effectively predict and avoid processing problems is to measure a material’;s flow properties at simulated plant conditions and use these data as a basis for improving process operations, or for designing or recommending equipment. This means duplicating temperature, storage time, chemical reactions and solids contact pressures while taking into consideration the construction materials used in the process or storage vessel.

A number of bulk solids’; properties influence equipment design and function. In general, these properties exert multiple effects, as detailed in Table 1. (To view Table 1, click on the Download File button at the end of the article.) 

Using key indices
In 1991, Dr. Jerry R. Johanson developed a set of nine bulk solids indices as an easy method for characterizing bulk-material flow properties. The indices, which are derived from measured consolidation pressures, time at rest, atmospheric conditions and test sample surfaces that simulate material-handling equipment, provide simple numerical values that relate directly to solids’; flow problems and equipment design parameters. Table 1 describes the indices and their uses. (See Chemical Procesing, February, p. 39, for a visual representation of the indices.)

The Johanson Indices correlate accurately with various processing problems and can be used to predict problems in existing equipment and to improve process conditions. Here are some examples.

  Determining demixing and segregation tendencies. The Arching Index (AI) and Flow Rate Index (FRI) correlate with common segregation and demixing tendencies such as sifting, angle of repose, fluidization and air currents. In general, if all the components in a final mixture have an AI equal to or greater than 0.2 and an FRI >100, there will be little tendency for material to segregate. Testing individual components and applying the indices will help determine the optimum blend for a particular blending or process application.

  Predicting extruder and roll press limits. The FRI provides an excellent correlation between the maximum extruder and roll-press feed rates for fine powders because the extruder feed throat and the feed box of a roll press are subjected to the air squeezed from the powder. This counterflow of air holds up the powder and causes flow rate limits.

  Preventing pneumatic-conveying-line material buildup.The Chute Index (CI) predicts material buildup on pneumatic conveying lines. Some materials, such as polymers, form thin skins on conveying lines, and when these skins loosen they can clog equipment. This phenomenon is a function of line velocity, impact pressure and temperature. CI measurements at various temperatures and impact pressures can identify those materials that will stick on pipe walls.

  Determining critical moisture content. Knowing the AI, Rathole Index (RI) and Hopper Index (HI) for a material over a range of moisture contents can help decide what moisture will work in an existing processing system, and what a flow angles a hopper needs to prevent ratholes and hang-ups. As moisture decreases, the AI and RI will also generally decrease.

The CI and FRI are also moisture-sensitive. Sometimes the CI is greater than 90° for fine, wet material. This means special chute designs are required to prevent buildup at impact points. The FRI increases with a slight increase in moisture; this means that higher flow rates may be achieved by adding a minimum amount of moisture to fine, dry powders. However, powders that are too dry and powders that are too wet can create hang-ups. The FRI helps define the acceptable moisture range for a material in an existing process system.

  Predicting caking tendencies. For the most part, caking results from crystal growth in materials with water-soluble components that are stored or transported in railcars, trucks and supersacks, which are then exposed to temperature changes. These temperature variations allow moisture migration within the storage vessel and provide an ideal condition for crystal growth and ratholing. The RI simulates these conditions and can indicate when controlled temperature conditions are required to produce a better flowing material.

Hopper selection and design
The Johanson Indices also can safely and accurately answer the most common material handling and equipment design questions, such as:

  • How large should the hopper outlet be?
  • What hopper angle is needed?
  • Is a bin liner necessary?
  • What impact do vibrators or air cannons have on material in the bin and where should they be placed?
  • Is a mass-flow (flow at the walls) hopper needed? If so, what type?
  • Can a partial-mass-flow hopper be used?
  • Is air injection required to prevent limiting-flow-rate hang-ups?
  •  What chute angles are needed?

The best hopper is the least expensive one that consistently provides the required solids flow. Assuming the only purpose of a bin or hopper is material storage, the best hopper will have the lowest headroom requirement and the smallest feeder possible. Options include:

Conical hopper. The conical or funnel-flow hopper (Figure 1) is appealing because of its simple construction. It can be the most economical solution if headroom isn’;t an issue. An engineer can use the measured Johanson Indices to design a funnel-flow hopper: HI is the angle from the vertical necessary for flow at hopper walls, AI is the minimum outlet diameter and RI is the recommended top diameter of the conical hopper to prevent ratholing in the hopper. FRI is the maximum flow rate for a 12-in.-diameter outlet and can be adjusted for different outlets by the area ratio, provided the outlet exceeds AI. If the required flow rate is greater than the adjusted FRI, air injection will be necessary.Conical Hopper
Figure 1. This hopper’;s simple constructionoften makes it the most economical choice.Chisel-shaped hopper. The advantage of the chisel-shaped hopper (Figure 2) is that the slide angle can be HI plus 10° from the vertical with the end wall angle (HI) the same as that of a cone. This can increase the chisel hopper’;s capacity while decreasing its overall height. The outlet width must be AI/2 or greater with a slot length at least 2.5 times the width. The maximum flow rate without air injection is evaluated by multiplying the FRI by the ratio of the outlet area, i.e., a 12-in.-diameter circle.Chisel Hopper
Figure 2. This design allows greater hopper capacityat a lower overall height than a conical hopper.Cone-in-cone hopper. A cone-in-cone hopper (Figure 3) saves headroom because the outside cone angle measured from the vertical is 2HI. The inside cone angle is HI. The inside cone outlet must exceed the AI; so, the outer cone outlet must be 2AI minimum. Using air injection to increase flow rates is not recommended for cone-in-cone hoppers because it creates non-symmetric flow. Often, flow will occur only in a small localized channel when the FRI limit is exceeded, even though the AI and HI criteria are met.Cone-in-Cone Hopper
Figure 3. This design saves headroom,but doesn’;t suit air injection. Diamondback Hopper. This is a one-dimensional convergence hopper that combines the advantages of a chisel hopper and the circular outlet of the simple cone. However, unlike the cone, the outlet diameter in a Diamondback Hopper can be as small as AI/2 (the same as the chisel slot width). This allows for a much smaller feeder. One set of Diamondback Hopper sides must be vertical and the other must be HI plus 20° from the vertical (Figure 4). This makes the hopper very effective for solids with small HI values. The limiting rate in a Diamondback Hopper is somewhat larger than the area-adjusted FRI. Air injection works well to accelerate flow rates.Diamondback Hopper
Figure 4. This hopper combines the advantages of a chisel hopper with a circular outlet.

Achieve solid success
Bulk materials are composed of individual particles that can vary in size, shape, surface roughness, moisture content and chemical composition. You cannot extrapolate the behavior of one material to other materials, even those that have similar chemical compositions.
The key to success is knowing a material’;s flow properties and understanding the conditions that affect them. This knowledge, coupled with the ability to easily and accurately interpret the property data, will provide a sound basis for process improvements and equipment design or selection.

Kristin Dudley is a consulting engineer for Diamondback Technology Inc., San Luis Obispo, Calif., a firm that specializes in equipment and consulting related to solids handling. E-mail her at [email protected].  Lee Dudley is president of Diamondback Technology. E-mail him at [email protected].

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