Using powder flow measurements to predict feeder performance

Overview

• Post By : Authors: Tim Freeman, Managing Director, Freeman Technology Ralf Weinekötter, Managing Director, Gericke AG


• Source: MIcrobioz India


• Date: 12 Feb,2021

In powder handling industries screw feeders are used routinely to control the flow of material from a hopper into a process. Powder properties have a direct impact on feeder performance, making it essential to tailor system design to the product being handled. A poorly matched powder/feeder combination may be associated with low feed rates, high screw torques and the accumulation of powder on tube walls. These factors decrease operating efficiency.

This article describes a collaborative study conducted by Freeman Technology (Tewkesbury, UK) and Gericke (Zurich, Switzerland) to identify reliably measured powder properties that can be used to predict feeder performance. Experiments were completed to assess whether properties measured using the FT4 Powder Rheometer® could be correlated directly with feeder performance. The underlying aim was to assess the feasibility of predicting feeder performance from powder properties to identify optimal screw feeder solutions for any given material. The work highlights the value of dynamic powder characterisation and concludes with the development of robust models that enable screw feed rate to be predicted.

Specifying screw feeders
Screw feeders consist of one or more rotating augers/helixes mounted in an enclosed chamber. As the auger rotates, powder is transferred according to the Archimedes screw principle.

Gericke routinely supply screw feeders across many industries. There are numerous factors that directly influence the choice and specification of each feeder, including installation constraints associated with plant layout, process requirements (capacity, continuous vs batch manufacturing, etc.) and material properties (for example, cohesivity and physical stability).

Key design variables can be manipulated to meet combinations of requirements including diameter/length of feeder, geometry (e.g. auger drive and pitch) and accessories to improve flow consistency. Vibrational feeders and agitation in the feed hopper are all possible options. Feed rate may be controlled based on weight (gravimetric) or volume (volumetric).

For Gericke, specifying the optimal screw feeder is critical to operational success. A feeder that is poorly matched to the product is likely to be associated with poor long-term operation. Flow rate may be erratic or poorly controlled and/or material can accumulate within the equipment, directly impacting the overall performance and efficiency of the process.

Understanding how to characterise powders to predict their performance in different types of equipment is, therefore, extremely helpful. Within the screw feeder, powders are subjected to different environments, flowing gravitationally from the feed hopper into the forcing, potentially compacting flow regime within the rotating auger(s). The way in which a powder responds to these conditions depends directly on its properties. Powder properties consequently have a major impact on equipment choice and the need for bespoke development.

Correlating powder properties with screw feeder performance
The study investigated correlations between flow properties of five different powders (calcium hydroxide, maltodextrin, milk protein, cellulose and calcium citrate) and their performance in two different screw feeders. In the first stage of the study, samples of each powder were subjected to comprehensive testing using an FT41. Dynamic, bulk and shear properties were measured for each powder with a high degree of repeatability (RSD<5%). Samples of each powder were then run through each of two Gericke screw feeders to determine volumetric flow rate (L/hr) at an auger rotation speed equivalent to 80 Hz. This was calculated from mass flow rate (in kg/hr) and poured density.

The two screw feeders used:
• DIWE-GLD-87 VR, full flight single-screw feeder using tube No. 3. The GLD machine is a compact, versatile feeder used for high accuracy feeding of most dry solids.
• DIWE-GZD flat bottom double-screw feeder using a 12×13.5mm tube with a conical core. The GZD unit is a compact, self-cleaning twin-screw extruder used for low capacity applications and is particularly suitable for feeding materials with poor flow characteristics.

A multiple linear regression was performed to reveal correlations between average volumetric feed rate and flow parameters. This mathematical process produces an equation quantifying a dependent (y) parameter (volumetric flow rate) in terms of influential independent (x) variables (powder properties). The process generates a p value for each parameter indicating whether the probability of its contribution to the relationship is statistically significant. Higher p values suggest the parameter has less bearing on the relationship, and therefore a smaller p value is associated with a parameter that is more relevant. A p value of 0.1 was taken as the upper limit for relevance and parameters with values higher than this were eliminated to derive a robust relationship.

For the GLD feeder the following relationship was observed:

Feed Rate = 49.54 FRI – 13.81 SE + 163.8
(R2 = 0.9466)

R2 is a measure of the ‘goodness of fit’ between the model and the data, with higher values closer to 1 indicating a close fit. This relationship suggests that only two dynamic properties are needed to robustly predict feeder performance – Specific Energy (SE) and Flow Rate Index (FRI). SE reflects how a powder behaves in an unconfined state. Whereas FRI describes how the powder’s resistance to flow changes as a function of flow rate.

FRI is the ratio of the flow energy measured at a tip speed 10 mm/s to that measured at 100 mm/s. An FRI greater than 1 indicates that the resistance to flow is greater, as evidenced by a higher flow energy, when the powder is made to flow more slowly. All powders measured had an FRI above 1, therefore all exhibited this shear thinning behaviour.

Figure 2 shows the measured flow rates for the five original powders along with the values predicted by the derived model. As suggested by the R2 value for the correlation (0.9466 for original five), the predicted values very accurately describe the observed performance of the powders in the GLD feeder.

Figure 2: Predicted vs. actual feed rate data for five powders with a close relationship between predicted and measured values from the GLD model

To challenge the predictive ability of the model, two additional powders were tested. Figure 2 also includes the measured flow rates for the two new materials (red), along with the values predicted from their powder properties. A revised R2 confirms close agreement between the predicted and measured values incorporating all seven materials, and the feasibility of predicting volumetric flow rate from powder property data.

The same process was repeated to predict performance in the GZD feeder. A simpler correlation was observed with Aerated Energy (AE) the only parameter found to be highly relevant.

Feed Rate = -0.1114 AE40 + 34.82
(R2 = 0.8383)

AE is the flow energy of samples aerated by air flowing upwards at a defined linear velocity – in this case 40 mm/s (AE40). Cohesive powders tend to have higher AE, as the impact of aeration on resistance to flow is minimal, while for free-flowing powders AE can approach 0 as the powders fluidise. The materials tested here exhibit a broad range of AE values, but a robust relationship between AE and volumetric flow rate is observed.

Figure 3 shows the measured flow rates for the five powders along with the values predicted by the derived model. As suggested by the R2 value for the correlation, the predicted values once again accurately describe the true performance of the powders in the GZD feeder.

As with the other feeder, the study was extended to verify the ability of the relationship to predict the volumetric flow rate achieved with additional materials. As before, the correlation performed robustly in this predictive mode.

Figure 3: Predicted vs. actual feed rate data for seven powders illustrating the ability of the derived model to predict volumetric flow rate through the GZD feeder

Predicting performance
This study demonstrates the feasibility of developing robust correlations between measurable powder properties and volumetric flow rate delivered by different designs of screw feeder. Each screw feeder imposes different process conditions on the powder, this is reflected in the attributes of the powder and the parameters that were relevant for predicting feeder performance. However, in both cases it is dynamic powder properties, rather than shear or bulk properties, that were found to be most relevant.

The approach of this study can be broadly applied to determine correlations for predicting performance in a wide range of powder processing equipment. Multi-faceted powder characterisation provides an essential foundation for such work, supporting the identification of 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.