Perfecting Powder Storage & Discharge

The efficiency of plants has improved significantly in the post-war period, to the point that, in many cases, equipment and processes have changed almost beyond recognition. Unfortunately, the same cannot be said for the most stalwart of process equipment, which underpins the whole plant operation ," the humble silo (hopper, bunker, etc.).

Very few engineers have a solid grasp of the fundamentals of particulate handling. This situation gives rise to "in-house" storage and handling solutions or a dependence on hardware suppliers to resolve handling issues. Both of these approaches can be time-consuming and costly.

Vessels used to store and discharge particulates into the plant process or packaging equipment must be designed and constructed correctly. Plant engineers should gain an understanding of two basic discharge patterns before purchasing a vessel: core flow discharge and mass flow discharge. See Fig. 1 and Fig. 2.

The vessel shown in Fig. 1 offers the type of discharge pattern seen most frequently in standard "off-the-shelf" equipment. This type of vessel typically will feature convergent wall angles between 45 degrees and 30 degrees from the vertical, and will have an outlet dimension that has been determined by the inlet requirements of the feeder to be installed under the vessel. Once discharge from the vessel commences ," assuming flow can be initiated ," a preferential flow channel forms that extends from the outlet to the top surface of the stored material (in the case of moderately fine materials).

When free-flowing materials are present, the flow channel can expand to intersect with the vertical section of the vessel. This type of discharge pattern gives a "first in, last out" material discharge, which might not present an issue with many materials. However, in situations involving the storage of time-dependent materials, this discharge pattern can be a significant issue, particularly if the vessel is continually "topped up."

Contamination is an ever-present issue when materials that could decay and spoil freshly introduced product are handled. Over the years, this type of vessel has spawned its own industry, which produces various types of discharge "aids" to compensate for poor flow characteristics.

In contrast, a mass flow vessel is designed to encourage material to shear at the walls during discharge. The vessel typically features steeper wall angles of greater than 25 degrees from the vertical. Material flows from this type of vessel by gravity alone, on a "first in, first out" basis. Therefore, a mass flow vessel is intrinsically better suited to time-dependent materials.

Because the material is discharged evenly across the cross sectional area of the inventory, this vessel also can reduce the effects of radial segregation, which might otherwise create variations in blend ratios or bulk density values. This type of vessel requires outlet sizes that are typically one-third the size of those required for reliable operation of a core flow vessel, and has the added benefit of operating reliably by gravity alone.

The even draw-down of material also permits a degree of deaeration, or maturing, of product before the product reaches the outlet. A core flow vessel, in contrast, draws the fresh material down the preferential channel, giving rise to flushing or severe bulk density variations.

Both types of vessels have a place in industry, despite their significantly different operational characteristics. For many types of non-time-dependent particulates, in fact, a core flow vessel can be quite adequate.

Plant operators must spend a little time and effort learning about the characteristics of the bulk material to be handled. Information that should be obtained includes particle size distribution, flow property measurements (internal/wall friction) and compaction curves. Supplementary information relating to moisture sorption isotherms, permeability, segregability and degradation characteristics also might be required.

Many plant budgets do not allow for this level of investigative work, however, primarily because many engineers do not have an adequate understanding of the importance or existence of these types of tests. Instead, some plants transfer responsibility for correct equipment design to the equipment suppliers, based on the understanding that performance guarantees will be forthcoming.

When an investigation into powder properties is undertaken, it usually is limited to the determination of angle of repose, bulk density and median particle size. Or, in some more ambitious cases, a timed discharge through a fixed aperture is undertaken as an index of flowability.

These measurements are adopted almost universally by industry, the main reason being the simplicity of the tests. Although a crude index of handling ability can result from the data, these types of tests cannot yield information that can be used in the design of vessels for reliable operation. All too often "standard" vessel design will be employed in situations that require more careful consideration of the properties of the bulk solid, with poor operational characteristics often resulting.