Key points
Predicting the performance of powder processing equipment can be a significant challenge but is vital for process designers specifying new plant.
It demands a comprehensive understanding of the entire process and familiarity with the relationship between powder properties and process equipment, together with how they combine to deliver product with the intended properties and quality.
This same understanding is also required by engineers working as part of the manufacturing team, who must use the specified plant to achieve optimal output.
Sophisticated powder characterisation tools make it possible to predict powder behaviour within production processes and use this information to accelerate and optimise the selection of processing equipment. This article uses examples of two typical powder handling operations to illustrate this approach.
Hopper design and refilling protocols
Consider a blend flowing from a hopper into the feed frame of a tablet press, with the hopper being refilled every time the powder level falls below a certain point. With certain blends, the discharge flow becomes erratic shortly after the refill, resulting in inconsistent filling and plant stoppage.
Not all blends exhibit this behaviour and analysis reveals that the material at the hopper outlet, which is already compressed by the weight of the powder bed above it, is further consolidated by vibration from surrounding machinery when the flow is paused during refilling. The powder’s response to vibration is, therefore, highly relevant and problems are more likely to arise if vibration causes a major change.
Universal powder testers incorporate bulk, shear and dynamic flow measurements in a single instrument, measuring the powder in motion, and allowing the analysis of samples in a consolidated, conditioned, aerated, or even fluidised state.
This enables the flow energy of a conditioned sample to be compared with that of a sample consolidated by compaction or vibration, yielding a quantifiable consolidation index (CI), and providing the necessary insight to rationalise the processing behaviour outlined above.
Comparative studies of the die filling performance of two different powders, A and B, provide an illustration of this point. Sample A has a CI (tapped) of 1.11 while that of sample B is 2.32. This indicates that B, a relatively cohesive material with very fine (4 microns) angular particles, is significantly more affected by vibration than
A. Die filling trials confirm that the performance of B deteriorates markedly if it is consolidated, as would be expected from the results, whereas A is likely to exhibit much more robust behaviour.
A designer with access to the powder testing information has options – specify a more accommodating hopper with more steeply angled walls and/or larger outlet; pursue a policy for reducing equipment vibration; and/or install equipment for rapidly releasing a blocked hopper.
This same information leads the manufacturing team to better operational practice with respect to hopper filling and an improved response in the event of blockage, with refilling the hopper with smaller quantities more frequently likely to be one of the best solutions.
Correlating powder properties with screw feeder performance
Process designers routinely use screw feeders to control the flow of material from hoppers.
The properties of a powder will directly impact performance of the feeder and a poorly matched powder/feeder combination will typically be associated with low feed rates, high screw torques, and the accumulation of powder on the tube walls, decreasing both short and long term operating efficiency.
The key screw feeder design variables that can be manipulated include: the size of the feeder (diameter and length); the geometry, drive and pitch of the auger; and the accessories used to ensure consistent flow, such as vibrational feeders and fluidisation or agitation in the feed hopper.
Specifying the optimal screw feeder for a given application is critical to operational success, so characterising powders in order to predict is extremely useful.
A collaborative study between Gericke AG (Zurich, Switzerland) and Freeman Technology Ltd (Tewkesbury, UK) investigated the properties of five different powders and their performance in two different types of screw feeders.
The GLD is a compact, versatile feeder used for high accuracy feeding of dry solids, for pilot scale applications, and for those requiring frequent material changeover.
The GZD unit is a compact, self-cleaning, twin screw extruder used for low capacity applications and is particularly suitable for materials with poor flow characteristics.
The five powders tested were:
- Calcium Citrate
- Calcium Hydroxide
- Cellulose
- Maltodextrin
- Milk Protein
Table 1 shows the dynamic, bulk and shear powder properties for the five powders alongside the volumetric feed rate for each powder type when run through the GLD screw feeder.
Using multiple linear regression to identify correlations between these two sets of data, two dynamic flow properties were found to predict performance in the GLD feeder: Specific Energy (SE) and Flow Rate Index (FRI).
SE reflects how a powder behaves when in an unconfined state and is heavily influenced by inter-particular interlocking and friction. FRI describes how a powder’s resistance to flow changes as a function of flow or shear rate.
Table 2 shows the powder properties again but now compared to volumetric feed rates when run through the GZD feeder. In this case, a simpler correlation was observed with Aerated Energy (AE) robustly predicting the actual feed rate.
AE is the flow energy measured when the sample is aerated by air flowing up through it at a defined linear velocity – in this case 40 mm/s, hence AE40. Cohesive powders tend to have a relatively high AE, since aeration does little to reduce the resistance they present to flow, while for free-flowing powders AE can approach zero as the powders fluidise.
The five materials tested exhibited a relatively broad range of AE values, but a robust relationship between AE and volumetric feed rate holds for all materials.
Figure 1 shows the measured feed rates for the five powders along with the predicted values. The predicted values accurately reflect observed performance in the GLD feeder. To challenge the predictive ability of this approach, two additional powders were tested – cement and lactose.
Figure 2 shows the measured feed rates for each of the original five powders, and the two new materials (shown in red), along with the predicted values.
The same experiment was undertaken with the GZD feeder and Figure 3 shows the measured feed rates for all seven powders. Once again, the predicted values accurately describe the performance in the GZD feeder.
This study demonstrates the feasibility of developing robust correlations between measurable powder properties and the volumetric feed rate delivered by different designs of screw feeder. In both cases it is dynamic powder properties which were found to be most relevant.
Predicting performance
Multi-faceted powder characterisation provides an essential foundation for identifying properties that are most relevant to performance in any specific unit operation. Powder testers that enable this approach can therefore be extremely valuable for optimising a range of powder processes.
References:
1. Freeman R. “Measuring the flow properties of consolidated, conditioned and aerated powders — A comparative study using a powder rheometer and a rotational shear cell”, Powder Technology 174 (2007) 25–33.
