The optimal design and operation of solids handling plant rests on effectively predicting powder behaviour, before processing. State-of-the-art powder testers help engineers achieve this challenging goal in a cost- and time-effective way. Reg Freeman explains.
Defining stream properties is critically important for both plant design and troubleshooting. For liquids and gases, physical properties are mainly defined by composition, temperature and pressure alone, and may be found in the literature or modelled with some confidence. For powders the situation is very different as there is an array of variables that affect behaviour.
This dependency makes it difficult to ensure consistency in processing and in powder measurement, since reproducibility is more difficult to achieve when there are many parameters to control. Modelling is similarly complicated. Powder properties such as flowability cannot be quantified on the basis of primary variables (particle size and shape for instance) and system parameters (such as air content or stress level).
These issues often result in a failure to ensure fundamental compatibility between the material and the process plant, hampering operation on an ongoing basis. Consistent and efficient operation requires that the powder properties suit the processing step, whether this is storage in a hopper or flow through a tableting press. Ensuring compatibility is difficult but much can be learned from hard-earned experience that resides within the processing company.
Within the operational environment, powder behaviour is often described as erratic: a minor change in shut down procedure results in extensive line blockage, or a new batch of feed processes poorly despite meeting the defined, but often simplistic, specification. The root cause of these problems is a failure to appreciate the factors influencing performance and the significance of the change.
For example, de-aeration can convert a fluidised powder to a solid, so even small deviations in operational practice can be important. And specifications for a feed must include the key relevant powder properties. If a parameter affects plant performance but is not defined in the specification, because the correlation has not been recognised, then this introduces an uncontrolled source of variability.
Relevant specifications allow the detection of a poorly performing batch before it is introduced into the plant, minimising disruption and economic impact. On a longer-term basis, developing a more detailed understanding of the links between powder properties and plant performance provides a knowledge base that promotes 'right first time' process design. For each unit operation or process step there is an optimal set of properties that defines a powder fundamentally compatible with it. Identifying these key properties is extremely valuable for both operation and design. Modern powder testers that deliver multi-faceted powder characterisation are important tools for realising this goal. They provide data that rationalises existing experience, drawing from it knowledge that is more generally applicable.
How to characterise powders
Relevant powder characterisation requires the reproducible measurement of properties that can be directly correlated with aspects of process or product performance. The number of traditional tests that have been devised to describe just one very important aspect of powder behaviour - flowability - highlights the demand for data, and the difficulties of measurement. Many of the tests used are clearly based on specific aspects of processing, flow through an orifice being an obvious example, and each gives some insight into the characteristics of a powder.
To be useful in solving process problems, however, a technique must deliver reproducible data that relates to in-process behaviour. Historically this link has proved elusive, for a number of reasons. These include the tendency to rely on single number measurement methods, the focus on consolidated powders (promoted by shear cell analysis), the inability to measure all powders and a lack of reproducibility.
Many methods measure only a single variable: angle of repose, compressibility, flow/no flow through an orifice, for example, whereas the likelihood is that a combination of factors will influence process behaviour. Even when performance can be correlated with just a single parameter it may not be the one that the selected technique measures.
In addition, many conventional powder measurement methodologies are not rigorous and lack reproducibility, making it difficult to differentiate sensitively between samples. The exact method used for angle of repose testing, for example, may differ from company to company, and in the worst case from operator to operator.
This problem is exacerbated by the failure of many techniques to adequately define the state of the powder, with air content being a key factor.
Well-defined sampling and measurement methodologies that incorporate conditioning address these issues ensuring that a powder is always measured in the same baseline state. This greatly improves both reproducibility and sensitivity.
Modern, sophisticated powders testers, such as the FT4 Powder Rheometer from Freeman Technology, provide fully automated, multi faceted measurements in a time- and cost-effective way. Dynamic measurements, of the powder in motion are particularly relevant and especially useful for characterising the material's response to air.
With these instruments engineers can investigate what will happen when a powder is stored, consolidated, vibrated, forced to flow by applied pressure, aerated or even fluidised. Precise measurements of bulk density, compressibility, permeability, shear properties and flowability characteristics can all be correlated with plant performance. Measuring and comparing formulations that process well with those that do not allows the identification of the specific combination of properties needed for optimum processing, whether the operation is hopper discharge, blending, die filling, extrusion or fluidisation.
Rationalising operational experience, using this approach, gives insight for troubleshooting and design. Instead of simply identifying that powder A processes well but powder B is problematic it is possible to determine that powder A performs well because, for example, it has a basic flowability energy between 1000mJ and 1500mJ, an aeration index greater than 20 and a permeability of 5x109cm2.
This information is the foundation of a knowledge base that supports new process design and modification. It improves the chances of successfully matching powder and equipment, from the outset, and where this is not possible provides a basis for intelligent adjustment of plant and operational practice.
Meeting the demands of the process
Each unit operation places different demands on a powder and subjects it to a unique set of conditions. Recognising this is the key to developing a good powder/plant match, whether the task is designing a new process, developing a new formulation or modifying existing plant.
During storage and transport a powder may be compressed or vibrated, release any entrained air, cake and solidify. On the other hand, when pneumatically conveyed or fluidised the same material may flow like a liquid, perhaps uncontrollably. Often powders flow under gravity, but under forced flow conditions behaviour can change markedly, with the powder exerting significant resistance to flow.
It is worth considering just a few of the process-related questions surrounding flowability that a designer or formulator may need to take into account, for some commonly encountered unit operations:
- Storage. How critical are storage conditions: Is flow behaviour very sensitive to moisture level? What is the impact of vibration or compressive force? Does flowability change with storage time?
- Discharge from a hopper. Will the material flow freely and controllably: How cohesive is the powder? How easily does the material aerate? What impact does aeration have on flow behaviour? Does the blend segregate? Is the powder permeable enough to prevent surging in the flow rate? How sensitive is the powder to shear against the container surfaces?
- Filling. Will the material flow freely into the bag or die: Will it rapidly de-aerate to ensure adequate fill? Does the powder compact well during the following compression step? Is permeability high enough to allow the air to escape? Can we fill this powder at high speed?
With a universal powder tester all these aspects of behaviour can be investigated systematically. For example, the effect of air and fluidisation characteristics is assessed through dynamic measurements of flow energy as a function of air flow through the sample. The tendency to segregate and attrite are evaluated using multiple tests, by looking at whether and how flow energy changes following repeated attempts to promote separation or particle breakdown respectively.
A comprehensive testing approach gives the fullest description of the powder. The relevance of the information is uncovered through process analysis and by correlation with experience. For example, tests may show that a powder flows freely when unconfined but poorly when subjected to an applied pressure. Whether this is important or not depends on the process but clearly extrusion is going to be problematic. If a powder has a low conditioned flow energy and is easily aerated then flooding is more likely to be an issue, if the plant design encourages it.
Experience of which formulations do or do not process well, combined with comprehensive powder characterisation, enables the processing behaviour of new powders to be predicted before they are introduced into the plant. Rationalising experience in terms of reproducible, easily measured parameters, defines optimal powder properties for each process step.
This information builds into a knowledge base that encourages 'right first time' process development, the design of processes with underlying compatibility between the powder and plant. It also promotes intelligent and effective troubleshooting and plant modification.
Reg Freeman is Managing Director of Freeman Technology, Welland, Worcestershire, England. www.freemantech.co.uk