Preventing Powder Flooding and Flushing

In pharmaceutical powder handling, flooding/flushing are defined as an uncontrolled or widely fluctuating material flow similar to that of water, caused by excess air entrainment. This action often produces process in-stabilities downstream, hazardous spills, or serious "housekeeping" problems.

Powders may flow like water through any crack or crevice (including clear-ances in rotary air locks) when powder and air interact.

There are four common causes of air entrainment in powders. The back pressure from a baghouse on a typical pressure pneumatic system is one source. This pressure (usually only about 10 in. water pressure), coupled with the air en-trained in the conveying solids, may cause severe flushing from feeders at the hopper bottom.

The most frequent cause of severe air entrainment is the rathole-type flow pattern occurring when a hopper's walls are neither smooth nor steep enough to cause flow at the walls. (Conical hoppers often require slopes of 70 to 80— to accomplish this, although some advanced hoppers manufactured by Diamondback can break ratholes with slopes as low as 40—). If the rathole caves in when partially or completely emptied or if a high rate of powder flow is introduced into a bin with a rathole, incoming solids entrain a bubble of pressurized air that fluidizes solids. Flushing is almost certain to occur through belt, vibratory or screw feeders, but is also possible in rotary valves with entrained air pressure of several psi.

Uncontrolled air injection or gas generation from a chemical reaction will also fluidize a fine powder. The only appropriate use for uncontrolled air injection is when handling powder with a pneumatic convey-ing system. Even then, wide fluctuations in the solids feed rate are likely. High volume, controlled air injection may work well when feeding a blow tank because the tank is essen-tially a closed container and a very high charge rate is required to reduce the tank cycle time.

Flushing also occurs as a result of powder free fall. Voids between the powder particles entrain air, which also accelerates along with the powder. When the powder and entrained air reach the material surface, the air is trapped within the solids where it fluidizes the product. The product then flows from the bin's outlet at an uncon-trolled rate. This is especially significant when the hopper is small relative to the flow rate out and the hopper retention time is short. A short retention time prevents air escape, thereby maintaining a fluidized condition within the material.

Controlling unwanted excess air
The typical approach for controlling excess air is to use a rotary valve at the bin outlet. Unfortunately, the rotating vanes pump air into the hopper outlet. Instead of reducing excess air, the rotary valve adds to the air supply and some-times causes flushing through the valve when wear reduces the normally close tolerances.

Screw feeders pro-vide continuous feed without introducing addi-tional air. A Diamondback Technology (DBT) compacting screw, when used as a feeder, seals against air pressure in the hopper that otherwise may cause flushing. When placed above the bin, the DBT compacting screw squeezes out excess air. Unfortunately, as the powder drops into the bin, it often reaerates unless there is an anti-aeration chute to provide surface contact for the descending powder. This open let-down chute protrudes into the surface of the powder layer, dispels entrained air, and allows the powder to discharge around the chute as the powder level changes. The chute also reduces superfines segregation by reducing entrained air.

Another way to eliminate unwanted excess air is by removing it before it causes flooding at the hopper outlet. This is accomplished by using a deaeration cylinder vented or connected to a dust collector to eliminate excess air.

Powder will not flow at a consistently high rate unless there is some en-trained air. When deaerated, flow rates may fail below the required rate. When totally fluidized with indiscriminate air injection, the rate will increase, but so will extreme flushing. The answer to this pro-blem is an air permeation system that controls both the air pressure and the rate of injected air.

The Diamondback Hopper, with its one dimensional convergence, also increases flow rates significantly, even without air injection. If equipped with a DBT air permeation system, flow rates can increase by one or two orders of magnitude without flushing.

The hopper works by increasing the solids contact pressure at the gullet. This increased pressure allows powder flow without excessive voids expansion, and thereby reduces the air vacuum created when voids expand. A reduced vacuum can increase the limiting flow rate by a fac-tor of two or more.

A vertical section between the hopper outlet and feeder further enhances the hopper's effec-tiveness because powder in the vertical section pro-vides suction at the hopper outlet and, in some cases, can eliminate the vacuum effect at the outlet.

The interaction of air and powder cannot be ignored. With a properly designed hopper, and controlled air injection system, it is possible to pre-vent flushing and control the flow rate for maximum production output.

For more information: Jerry Johnason, President and CEO, Diamondback Technology by J.R. Johanson, 1237 Archer St., St. Luis Obispo, CA 93401. Tel: 805-544-3775.