Fine designs for FINE FEEDING

An array of flow conditions faces the designer of a handling system for fine powders. Cohesion increases with increasing moisture content and decreasing particle size. In the case of fine powders, the product will often have significant cohesion even when `bone dry'. Yet the same powder may, without having air or gas intentionally added to it, entrain air and `flow like water'. The solution to these problems lies in understanding the properties of the powder and the flowing conditions in the handling equipment.

There are essentially two types of flow that may develop in a storage or surge bin. If the walls of the hopper are steep enough to force the powder to flow on them whenever any powder is withdrawn, the flow pattern is `mass flow'. Mass flow also requires that the feeder withdraw product from the full cross section of the outlet. It ensures a first-in-first-out flow sequence.

If the hopper walls are not at least as steep as the `critical mass flow angle' a channel will form in the otherwise stagnant material. This is called `funnel flow' (see diagram overleaf). In a funnel flow hopper, the top surface of the product sometimes appears to be in motion. However, the diameter of the flow channel is generally not much bigger than the size of the outlet and the movement on the top surface is mostly toward the flow channel and not toward the hopper outlet.

If the product has enough cohesive strength, the flow channel may empty out and be stable. This is called a `rathole'. The powder appears to defy gravity and instead of forming an angle of repose, will stand stable with a vertical exposed surface. It does this even though its `strength' is relatively small and, when disturbed, the powder may flow again. By definition, a rathole can not develop in mass flow.

As a fine powder flows in a funnel flow bin or silo, it flows at a relatively high velocity down the narrow flow channel. It often entrains air, or if it enters the bin partially fluidised, cannot deaerate. Near the outlet it will have the properties of a fluid under a head of pressure - due to the weight of the rest of the material in the flow channel. If the feeder cannot hold a fluid pressure, the powder will flush through and the flow channel will empty uncontrollably.

In a mass flow bin, the powder flows through the larger flow channel, defined by the width of the bin, at a much lower velocity. Even if it enters the bin fluidised, the powder will generally have time to deaerate. Therefore, mass flow is recommended for handling fine powders. The flow sequence is much more ordered and predictable.

There is, however, a potential drawback in the flow of fine powders in mass flow in that, without special features, it can have a limited flow rate. The reason is that in mass flow the stresses on the flowing product decrease as it approaches the hopper outlet. Consequently, the bulk density decreases, the void volume increases and the air pressure in the voids decreases - often to below atmospheric in the lower regions of the hopper. When the powder reaches the outlet at atmospheric pressure, air moves up into the powder to balance the low pressure in the voids. As the flow rate of powder is increased, this condition gets worse (the magnitude of the negative pressure increases). When the flow rate of air is high enough to balance the weight of powder flowing down, the flow rate limiting condition has been reached. The powder will either rain down from a plane of separation or flow erratically as air bubbles up while powder flows down (see `b' in mass flow diagram overleaf).

At this point, increasing the feeder speed will not increase the flow rate of the product. To increase the flow rate, some of the air that was lost through the top surface while the powder was deaerating in the top part of the bin must be replaced. This `air injection' is generally an order of magnitude lower than the airflow rate required to fluidise the powder. Air is not normally injected by aerating nozzles or air pads in the hopper, which cause local fluidisation, but by allowing the air at the correct pressure to enter the powder under a shelf or a set of beams at the transition between cylinder and hopper.

As an example, a plastic powder (terephthalic acid) was required to flow in a 4m diameter bin at 35 tonnes per hour. The conical hopper had an included angle of 48 degrees - which was steep enough for mass flow with the type of liner in the bin, and an outlet diameter of 250 mm. The limiting flow rate of the powder was found in our analysis and in practice, to be 5 tonnes per hour.

With air injected into the powder at the correct pressure and rate, the flow rate of the powder could safely be increased to 35 tonnes per hour. The relatively simple injection scheme is arranged so that it automatically accounts for the level of material in the bin and for lower powder flow rates. If more than the safe limit of air is injected into the powder, a positive air pressure gradient at the outlet drives the solid down and again it fluidises.

Achieving reliable flow, however, does not necessarily require that the powder be fluidised as such. Solids flow takes place in a converging flow channel when the bulk solid does have the ability to transmit shear forces, provided the conditions are suitable. The correct conditions can be achieved in a fine powder by making use of an `air-assisted' discharging scheme. This scheme uses air to fluidise a small percentage of the powder and thereby create conditions suitable for flow even though most of the powder flowing does not have the properties of a fluid.

A flow channel will always tend to be circular. Therefore, if a product is withdrawn from a long slot at the bottom of a storage container, the flow channel opens up at the sides of the slot at an angle approaching the angle of internal friction of the material. In a relatively small vertical distance above the slot, the flow channel will have opened up to a diameter equal to the length of the slot, which is much better from a flow point of view. Unfortunately, if we try to fluidise a powder with a fluidiser that has a long aspect ratio, the air or gas takes the path of least resistance and only part of the fluidiser is effective in moving the powder. This creates a small flow channel similar to conditions that develop in an incorrectly designed feeder. One method of overcoming the problem is to divide the plenum chamber under the fluidizing membrane into compartments and give them independent air supplies.

A second method of increasing the effective length of a fluidiser with a long aspect ratio is to slope the sides. If the membrane is sloped, it increases the effect of gravity on the moving material. Therefore, sloping the membrane further from the outlet makes it more effective and the whole length of the fluidiser can be used to expand the flow channel. Relatively small amounts of gas will get the powder to move and, in fact, a very small percentage of the moving powder is actually `fluidised'.

With a large diameter flow channel available with this technique, a reliable, cost efficient storage system for a very difficult powder can be designed. In the case on which the above solution was first used, the client was hoping to fill a one ton super-sack with kaolin clay in 10 minutes. Based on previous experience with low-density material flowing from a fluidised discharger, he was sceptical that it could be done. But when installed and tested, the new equipment could fill the sacks in less than three minutes.

One reason for the success of the new discharger is the fact that such a small area of fluidising membrane forces flow in such a large flow channel. The amount of air required to ensure flow, per unit volume of the flow channel, is relatively low. The bulk density of the discharging powder is much higher than is normally achieved with a fluidising type of discharger.

The slopes of the sections and the quantity of air or gas required to achieve flow can be obtained from the compressibility and permeability data of the powder. These parameters may be measured on relatively small samples of powder in a laboratory.

Although developed for kaolin clay, this type of air-assisted discharger has a potentially wide use in providing cost efficient reliable gravity flow for many bulk solids.