October 18, 2016
Haver & Boecker is a specialist supplier of packing and screening machines for manufacturing industries, with customers concentrated in the food, bulk chemical, and paints sectors. A major activity is the specification and manufacture of turnkey packaging solutions for powders and granules that typically include: silos and hoppers for storage; filling stations for either intermediate bulk containers (IBCs) or 1-50kg sacks; dosing and weighing systems; and pneumatic conveying plant. Hundreds of customized solutions are supplied each year with each new product presenting a different powder processing challenge.
About a decade ago, Haver & Boecker invested in an FT4 Powder Rheometer from Freeman Technology, UK, to measure the flowability of the materials being handled with the aim of ensuring more efficient and robust equipment selection and optimization. This article explores the impact that precise powder flow measurement has made and how the data that the instrument delivers is being used by the company to refine its approach to equipment design and specification.
Flowability in the Packaging Industry
For Haver & Boecker, each new packaging solution project begins with an assessment of the physical properties of the material to be processed. Ensuring continuous material flow through a packaging line is vital and relies on selecting equipment that is well-matched to the inherent characteristics of the powder or granules being handled. In the long term, a poor match compromises profitability by reducing throughput, causing machine stoppages and excessive downtime, and increasing the need for manual intervention. Identifying which physical properties to measure to avoid these issues is therefore crucial.
Characteristics such as density, particle size distribution, moisture and fat content, compaction, and particle form have all long been measured to guide equipment selection. These parameters all affect material behavior and the flowability of the powder – a critical trait for processing and packaging. However, the company recognized that even in combination these parameters were insufficient to reliably quantify flowability in a way that correlated with plant performance - a vital piece of information was missing, for robust equipment specification.
Early attempts to address this information gap by directly measuring flowability centered on the use of a Jenike shear cell. This technique involves measuring the forces required to shear one consolidated powder plane relative to another and quantifies the cohesivity of a powder via values of Unconfined Yield Strength (UYS), and other parameters. These data are widely used in the design of hoppers, for troubleshooting hopper discharge, and also, more broadly, for ranking flowability. However, shear cell analysis can be manually intensive, time consuming, and subject to significant operator-to-operator variability, depending on the instrument employed. Haver & Boecker found that it could take up to half a day to measure just one product with the shear cell, and that the resulting classification of the powder could vary between “free-flowing” and “sticky” as a result of variability in operator technique. Clearly this was a far from ideal solution and the decision was taken to research alternatives.
Embracing Dynamic Powder Testing
Around 2005, when Haver & Boecker began to research alternative methods for powder flowability measurement, dynamic powder testing was a relatively new technique; its industrial relevance was largely unproven but the possible benefits were already clear. Dynamic testing, in contrast to the existing shear cell set-up, offered relatively quick and efficient measurement, and equally importantly, process relevant data. Dynamic testing is highly sensitive and can differentiate powders that shear cell analysis classifies as identical, highlighting differences that will impact process performance. A rigorous technical assessment led the company to conclude that an investment in dynamic powder testing capability would provide access to the very best flowability data available and it became one of the first users of the FT4 Powder Rheometer from Freeman Technology.
The initial training provided for the FT4 Powder Rheometer was comprehensive and the instrument rapidly came into routine usage, with operators finding measurement simple and straightforward. Since that time, the FT4 Powder Rheometer has provided 10 years of uninterrupted, maintenance-free operation. Over that time, about 1000 samples have been analyzed each year, a substantial work rate accomplished as a result of impressive reliability.
Capitalizing on Dynamic Data
A primary difficulty associated with specifying any form of powder processing equipment is that there are few secure design algorithms linking the performance of different pieces of equipment with measurable powder properties. Hopper design is arguably the area where a ‘scientific’ approach is most well-established, but even here robust optimization can be difficult. The pragmatic approach for those routinely faced with specifying processing equipment for powders and granules is therefore to develop a database to link measurable material properties with observed process performance. Solutions that have worked well for powders with specific properties are likely to work again for those with closely similar characteristics.
Haver & Boecker has extensively scoped the impact of flowability on equipment performance by measuring the BFE of every powder handled over the past decade. Powders with the same or similar BFE value tend to require similar packaging equipment because the impact of flow is so crucial, but other properties are also factored in to ensure the best solution. By correlating experience with dynamic measurements the company has effectively created and refined its own in-house design process. With a decade of data in place engineers are comfortably secure in their reliance on this process and can securely deliver low risk solutions with a high degree of confidence. Most especially designs can be developed that minimize the amount of air in the process – thereby maximizing throughput.
Minimizing the air content in packed powders maximizes bag fill and is therefore a defining goal for the industry. However, the amount of air present in the powder directly affects the ease with which it flows. Equipment such as aeration jets is therefore used routinely to lubricate less free-flowing powders to enhance their flow performance. By robustly quantifying flowability, dynamic testing has enabled Haver & Boecker to reduce the amount of air in their packaging solutions to an absolute minimum, thereby optimizing throughput, while at the same time de-risking the process with respect to blockages/downtime. This is an important gain that underpins the provision of highly competitive processing solutions.
Example Study: A Packaging Solution for Cocoa
Table 1 shows test data for two different cocoa powders, including moisture content, bulk density, and BFE values. These data indicate that although these are samples of the ‘same’ product they are, in fact, markedly different. Cocoa 1 has a higher bulk, loose, and tamped density, and also exhibits homogeneous aeration behavior, suggesting that the addition of air effectively lubricates each individual particle. In contrast, Cocoa 2 channels when aerated, a response typically associated with relatively strong inter-particular forces, and de-aerates more rapidly. These are highly relevant characteristics when it comes to selecting packaging solutions. The BFE of Cocoa 2 is substantially lower than that of Cocoa 1 indicating that it is likely to flow more easily.
Trials of both products were conducted in an ‘air packer’ machine (see figure 2). This consists of a pressure chamber with sloped base, for easy discharge and cleaning, and allows the aeration rate to be varied to suit the characteristics of the product. Continuous suction during filling ensures high filling rates with minimum internal bag pressure.
In the trials, it took 23 seconds to fill a single 25kg bag with Cocoa 2. This resulted in a filling machine output of 110 bags per hour. As the BFE figures suggested, Cocoa 1 did not flow as well and this powder took 30 seconds to fill the same bag; machine output fell to 90 bags per hour. These results are typical of those observed across a wide range of different products and demonstrate how BFE values can be used to predict the likely performance of packaging solutions and ensure realistic expectations with regards to filling rate and throughput.
Comprehensive physical characterization provides the basis for the design/selection of any packaging solution. For Haver & Boecker, customer satisfaction relies on defining a processing solution that will work reliably for their product, from the outset, without modification or troubleshooting so optimizing that physical characterization process is crucial. Adopting dynamic powder flow measurement with the FT4 Powder Rheometer has proved highly efficient, compared to alternative powder testing options, and provides highly relevant data for the selection of processing equipment. The company now measures the flowability of every sample they receive. Correlating the results with processing experience collected over a decade of such testing helps to identify the best packaging machine for every product. Applying this approach, the company can confidently specify high-performance packaging solutions, for a wide range of materials that maximize throughput, while also minimizing operational risk.
Dynamic Powder Testing
Dynamic powder testing, as its name suggest, involves measuring a powder in motion. Dynamic powder properties are determined from measurements of the torque and force acting on a helical blade as it rotates through a powder sample along a defined path. Samples can be measured in a consolidated, moderate stress, aerated, or even fluidized state, representing the entire stress range that exists in routine powder processing. Indeed, the technique was specifically developed to test powders in support of process design, optimization and troubleshooting studies, and is widely used to maximize understanding of how a powder will behave in any given process environment.
The parameter Basic Flowability Energy (BFE) is a baseline dynamic property which quantifies how a powder will flow under forcing conditions or when in a confined state. It is measured by subjecting the powder to a downward traverse of a blade which pushes it against the confining base of the sample cell. BFE is a highly differentiating parameter, capable of detecting differences between powder samples that other techniques would classify as identical. It can be successfully used to track relatively modest changes in powder/granule characteristics and to differentiate closely similar materials in a way that relates to their processability.
Jamie Clayton is operations director, Freeman Technology Ltd, and Thomas Hilling is R&D Centre manager, Haver & Boecker. For more information on Freeman Technology Ltd, visit www.freemantech.co.uk. For more information on Haver & Boecker, visit www.haverboecker.com.
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