Filling fine powders into fixed volume packaging or containers can be a major headache.
Whether the volume to be filled should hold a few milligrams, a sachet of tens of grams, sacks holding a few kilos, big bags holding up to a tonne or road tankers accepting 25 tonnes – the challenges are fundamentally very similar in obtaining a gravimetric fill in a system controlled by the receiving volume.
By Richard Farnish – Principal Research Fellow Department of Mechanical, Manufacturing & Design Engineering of University of Greenwich
One of the main contradictions associated with dosing fine powders lies in an over reliance on aeration to initiate flow and to establish a ‘reliable’ discharge.
It is notable that for processes that are required to fill blisters, such as in the pharmaceutical sector, the accuracy of the fill and subsequent dispersal characteristics (in inhalers) is beneficially enhanced in response to a engineered level of cohesion and compaction characteristics – which would not be achievable were the powder free-flowing.
When processes scale up to sachets and beyond, an excessive cohesion is extremely undesirable for filling processes. In situations where powders or fines continuous ‘coarse’ materials need to be fed with a fair degree of accuracy, it is not only the reliability of flow that becomes important, but also the consistency and repeatability of the flow that is the key to minimal ‘give-away’ or spillage.
An unfortunate effect of the over enthusiastic use of aeration is that although flow can be initiated quite readily, the resulting discharge is likely to be over dilated by the air that penetrates between the moving particles. Thus, two effects can occur simultaneously;
Firstly the flux density of the stream of powder entering the fixed volume receptacle will be low (i.e. the incoming charge has an excessively high gas content) and secondly the velocity of the stream of particles is high and can entrain additional air. The net effect is that unless the bulk characteristics of the powder are such that rapid gas egress can occur, the charge of material is likely to be in very low bulk density form until the entrained gas is lost from the powder and exits the system through the filling point.
Unfortunately, in the case fine powders, the expulsion of entrained gas from the powder can result in the deposition of dust on the sealing faces of the bag. In the event that the type of bag/sachet being filled is sealed by welding, the resulting closure efficiency can be significantly weakened, to the extent that sachets can burst open when transported to end users operating at significant elevations above sea level.
Another mode of failure is when sacks are palletised fresh from the production line, in which case stack instability can result or, in some cases, rupturing of sacks due to applied pressure.
Aside from the issues of bag integrity, a more common concern lies in the variation of pack weight that results directly from the poorly controlled flow of powder – and in particular the inconsistency of ‘in-flight’ material (which is a major contributor to overall fill accuracy).
Clearly, the use of aeration in conjunction with fine powders addresses the inability of such materials to discharge from standard equipment types, but at the cost (literally in many cases) of fill variation and package integrity. A consideration of filling operations at the large quantity end of the spectrum, shows that the associated problems can have equally serious impacts.
For sachets, bags and sacks vibration is sometimes applied to the bag base or directly into the contents (via extended probes) to encourage bed settlement. This approach is not readily applicable to filling operations involving road or rail tankers, where the mass of the ‘container’ is substantial.
Under such circumstances filling to obtain a required weight presents greater problems. Identical issues of low bulk density (due to air retention) can exist, which result in the volume of the wagon being occupied before the target weight is reached. Under such circumstances road tankers have the option to disconnect from the filling spout and drive a short distance within the plant to settle the load, disconnect from the spout and pressurise the wagon – followed by venting, or disconnect the tanker and tip the body (if a tipping type).
All of these approaches can deliver varying levels of settlement but, of course, most of these options could not be applied to rail wagons. All of these approaches consume valuable driver tacho time and increase Health & Safety risks associated with increased vehicular movements on site or man access to loading hatches. In addition to poor time utilisation, over aerated material can flow rapidly enough to dislodge the filling spout before the level detector sensor can effectively shut down the flow of material from above – leading to spillage and local dust emissions.
Filling Fine Powders - Dust Emissions
The issue of dust emissions is of particular relevance where local dust handling systems are installed. In many cases such units tend to fall easy prey to ‘value engineering’ on plant projects, with the result that their operation is sometimes marginal even when first installed.
Under sizing or poor choice of filter fabric, can often result in premature ‘blinding’ of the filters – which in turn limits the rate at which displaced air can leave the wagon during filling operations. This is a frequent cause of loading spouts disengaging during filling. The local response to this type of problem can often be to leave another of the filling hatches open to permit air (and dust) escape – causing major dust emission problems local to the loading bay.
The range of industries that are affected by poorly controlled flow of powders where good accuracy and consistency are sought is extremely wide. In response to these common problems, The Wolfson Centre for Bulk Solids Handling Technology is developing a research rig to investigate in detail the response behaviour of powders to the interaction with gas. A particular avenue of work that is being pursued is that of monitoring and manipulating th interstitial gas pressures within a flowing powder.
The research aims to prove a technique for direct manipulation of particle packing to, in turn, influence the bulk properties of the flowing material. The objective of this approach is to deliver a densified flow of powder at the filling point and avoid the indiscriminate use of aeration to obtain discharge.
Early experiments have shown that using as little as 4% volume air during flow can result in a ~30% increase in mass flow rate of the bulk solid when applied to the test apparatus. The key point here being that minimal air volumes can be used – which could have the knock on effect of reducing compressed air consumption and total air entrainment volumes.
Many industrial applications experience issues with spillage, dust emissions and filling accuracy when handling fine powders. Resorting to aeration to overcome the one problem (non-flowing) with others (dust egress, fill accuracy, etc.) if the design of the air introduction system or air volumes to be used have not been correctly considered and configured. Developments and research in this field are continuing and will provide an improved basis for system optimisation for new and existing plant.