Key points
Many process industries suffer problems as a result of inconsistencies in the flow properties of the particulate materials that they handle which adversely affects operating costs and process efficiencies.
Process problems can range from extremes of; poor flowing materials that occasionally form stable obstructions within storage vessels (see fig 1) or stick in chutes/conveyors, requiring operator intervention with a hammer to reinitiate flow; to materials that fluidise readily and flow uncontrollably, requiring plant shut downs to unblocking over-filled feeders and cleaning up spillage.
In the current market place there are numerous relatively low cost automated powder flow testers (shear cells) available for undertaking characterisation tests on the bulk flow properties that allow process operators to better understand their powders.
Traditionally bulk flow properties have been used for the equipment manufactures to specify and design of new storage and handling equipment, but these measurements can also be used by powder manufactures, or manufactures of products with powdered ingredients to meet a range of objectives from; general bench marking of material and quality control; comparing new versus current ingredients; to assist reformulation and reverse engineering of equipment.
However, before looking at these different potential uses, it is useful to review the principal flow properties and what they mean as many Engineers will be unfamiliar with them.
What are the key bulk flow properties and how do they relate to process problems?
The primary flow property is the flow function which is best explained using the concept of the ‘sand castle’ experiment shown in fig 2. In the first stage a powder is compacted into a cylindrical mould to a controlled consolidation stress.
In the second stage, the cylindrical mould is removed to reveal the unconfined powder sample ‘or sand castle’. An increasing consolidation stress is reapplied to the sample ‘or sand castle’ until it fails.
The peak stress represents the unconfined failure strength of the powder, the strength of the powder at a stress free surface, i.e. at an arch over the outlet of a storage vessel. The test is then repeated using a range of larger consolidation stresses which results in a proportional increases in the unconfined failure strength.
The results of these tests are presented a flow function (shown in fig 3), a plot of the consolidation stresses (load used to compact the sand castle) on the horizontal axis versus the unconfined failure strength (strength of the sand castle) on the vertical axes. The flowability of a material can be loosely classified using the flow indices demarked by dashed lines in figure 3, highlighting the free-flowing, easy flowing, cohesive, very cohesive and non-flowing regions.
An important secondary flow property is the bulk density, i.e. the weight of particulates that can be stored in a known volume. This can be measured in a shear cell by tracking the reduction in volume of the sample of known weight as a function of consolidation stress, to give a compaction or compressibility curve as show in fig 4.
Alternative simpler techniques commonly used are the poured and tapped bulk density which gives a similar indication by does not give reference to the consolidation stress.
What is the significance of the bulk flow properties
The potential for a material to bridge or arch over the outlet of a storage vessel and thus cause a processing problem as shown in fig 1 is dictated by both the unconfined failure strength of the material and the bulk density. It is the strength of the material that provides resistance to flow, while it is the materials self-weight under gravity that generates stress to break the obstruction and ensure flow. Thus a material with a high strength and low bulk density is likely to give flow problems while a material with a low strength and high bulk density will be free flowing. The flow properties can thus be used to determine the outlet size required for reliable gravity flow.
Common factors effecting flowability
While the flow properties are very much material dependent, for a given particulate material there are two factors that significantly effect the flowability, namely the particle size and the presence or not of a surface liquid.
Assuming a dry particle, above a diameter of 100mm most materials are free flowing (as shown by a typical coarse particulate in fig 3). As the particle size is reduced below this value the material becomes more difficult flowing and flow function gets steeper as the self-weight of the particle reduces relative to cohesive inter particle forces (Van der Waals forces), as shown for the very cohesive ultra-fine powder in fig.3.
In the case of wet particulates, liquid bridges connect neighbouring particles and it is the surface tension of the liquid that generates the cohesive strength.
Thus as the moisture content increases the material becomes more difficult flowing (flow function gets steeper) until the moisture level approaches saturation and the material becomes a slurry. Thus for wet materials with particle sizes of the order of 20mm diameter can be cohesive.
The bulk density can also be used to give a good indication of flowability as shown in fig 4. E.g. free flowing materials have a tendency to be incompressible as their particles naturally settle into close packing structures, while particles of a poor flowing material (where gravity forces are low relative to interparticle forces) settle in a very open particle packing structure and therefore tend to be highly compressible when stored under the action of their self-weight.
Uses of the bulk flow properties
To conclude, a knowledge of the flow properties of your material can be used in a number ways to improve process efficiency and minimise the chance of problems as outlined below.
Bench marking tests to check the consistency of the flow properties of the product over time, as a method of inferring quality or looking for problem batches. As examples, these tests could be used to identify; differences due to variations in the particle properties of the input materials (seasonal variations in particle size and shapes of grains); variations in the mix ratio of a blended product and drifts in the ambient temperature and humidity.
Comparing potential new versus current materials – When changing the supplier of input material, or reformulating a blend, a thorough evaluation of the flow properties of new and current materials should be undertaken. This will high light any significant differences in their relative flow behaviour so that the potential for process problems can be factored into any purchasing decisions along with cost and other processing characteristics.
Research & Development of new products – When formulating new products flow property measurements can be used to systematically evaluate the effects of changes to the particle size, the blend additive (such as free flow additives, liquid binders, fillers) type and level on the materials flow and handling characteristics. Thus the flow properties can be optimised to minimise the tendencies for flow problems.
Reverse Engineering of Equipment – If different materials are handled on the same processing line with different levels of success i.e. some flow acceptably while others do not, then flow property measurements could be used to determine an operating window that the flow properties must fall within for successful processing.
The Wolfson Centre for Bulk Solids Handling Technology University of Greenwich, Chatham, Kent
Tel: +44 (0)208 331 8646 / Fax: +44(0)208 331 8647
E-mail: r.j.berry@gre.ac.uk
Web: www.bulksolids.com
