Figure 3. Measuring Shear Properties: When particles flow,
they shear, i.e., slide relative to one another, and this influences
flow characteristics. Powders with greater resistance to flow may
form stable bridges.
Click image to enlarge.
• symptoms, circumstances and frequency of any unplanned shutdowns;• final product quality;• overall plant reliability — the percent of available time the plant operates; and• controllability/productivity — the percent of operational time the plant produces material that meets the defined specification.It also can be beneficial to identify periods when the plant performs poorly; for example, is performance worse on a Monday morning after a weekend shutdown? Such details can help determine the reasons for poor performance. Correlating processability rankings with powder properties will pinpoint those parameters that dominate behavior and will increase understanding of how to effectively modify equipment to improve operation. Furthermore, measuring the values of a new material prior to processing will allow prediction of how well it will perform. If done at the development stage, this may enable tailoring properties to achieve better processing behavior. Combining experience with well-defined reproducibly measured powder properties provides a route toward more effective powder management. Using this approach it becomes possible to: • define the characteristics of materials that will process well on a given line; • assess in a process-relevant way differences in materials from various suppliers;• better understand the impact of hardware modifications;• more effectively match powder with processing equipment;• establish effective quality-control criteria for both feed and product;• understand and address causes of batch-to-batch variability; and• reduce risk associated with introducing new formulations.Understanding and knowledge gained from quantifying experience lead to better decisions and actions and, therefore, to a greater likelihood of success. The Way Forward
Defining powder flow properties in terms of the array of variables that influence them — particle size and shape, hardness, moisture and air content, for example — is beyond our current capabilities. In addition, correlations between flow properties and processing behavior aren’t yet well established, although progress is being made. However, that doesn’t mean a manufacturer must resort to a trial-and-error approach to process and product development, with its largely subjective and highly specific results. Instead, it’s possible to extract more generally applicable information via powder testers to obtain the understanding required for optimization. State-of-the-art powder testers that offer shear, bulk and dynamic property measurement give the most-comprehensive insight into powder characteristics. Samples can be analyzed in a consolidated, conditioned, aerated or even fluidized state, and important phenomena such as segregation and attrition also can be thoroughly investigated. These instruments foster the relating of operating experience to variables that can be reproducibly measured and sensitively differentiate between samples in a process-relevant way. This makes it easier to define properties of a “good” powder for a specific unit operation and to identify characteristics that will cause poor performance. The key to effective processing lies in matching equipment and powder properties so both exhibit optimal performance. The data that universal powder testers provide give insight required to optimize design, operation and troubleshooting of powder processes in this way. Five Key Steps to Rank Processability
Correlating processing experience with powder flow properties is a powerful way to determine which variables critically impact performance. A plant that handles a variety of powders should consider developing Processability Rankings (PR) to support this approach. To rank the processability of different powders: 1.
Carefully define the focus of study — whether the whole process or just a single piece of equipment such as a hopper, granulator, storage bin or conveyor. Different unit operations place different demands on the powder, so it may be beneficial to individually consider them. 2.
Identify which powders process well and which are problematic.3.
Describe in detail the issues — causes and effects —associated with poor performance. These may include:• consolidation in a vibratory environment;• moisture absorption (hygroscopic materials may absorb moisture and become difficult to handle);• segregation; • bridging in hoppers;• attrition (which can remove a surface coating, change particle shape or generate fines);• caking in storage;• lack of homogeneity of the final blend; and• weight variability of bags, vials, tablets, etc., of final product.4.
Define and assign processability rankings. For example:PR1 means “trouble-free” processing;PR5 means occasional stoppages or quality non-conformance; andPR9 means frequent stoppages and significant wastage or scrap product. 5.
Combine the information in a form that clearly relates operating experience with powder properties (such as Table 1
Troubleshooting Tips to Diagnose Problems
Processability Issues on Tableting Line
|Potential segregation during transfer
|Bridging and adhesion to machinery
|Weight variability and high wastage
Table 1. Simple format highlights key concerns for each powder during processing.
Many manufacturers dedicate equipment to processing a single powder and can cope well with routine operation. However, change of any type, whether inadvertent or planned, can pose challenges. Even minor modifications to plant or procedures can have a big impact. Use the following pointers to diagnose problems and deal with change more effectively. • Startup/shutdown:
length of time of shutdown, effectiveness of clean out and vessel filling method all can impact plant reliability. Define procedures in detail based on an understanding of powder properties and make sure they are adhered to.• Operator-to-operator variability:
manual procedures and operation provide significant scope for variation in approach and technique. Identify critical steps, share best practice and enhance underlying knowledge to minimize differences in approach. • New source or batch of feed:
avoid experiments on line. Measure the characteristics of new materials first and compare with those of known feeds to confirm similar flow properties.• Dealing with processing problems:
Resolving problems with quality or processing is important but so is learning lessons on the cause and future prevention. Measure powder properties to determine which have changed and why. • Plant or process change:
new or modified plant or a revised procedure can directly lead to problems or can cause issues downstream. A new mill, for example, can change particle size and shape, resulting in very different powder behavior. Powder characterization will highlight any inadvertent changes to critical properties.• External environment:
temperature and especially humidity can affect powder properties. Storage, transportation and vibration in particular can result in serious consolidation and compromise processability. Avoid surprises by determining susceptibility to these factors.Tim Freeman is director of operations for Freeman Technology, Malvern, U.K. E-mail him at:[email protected]
Freeman, R. F., “Measuring the flow properties of consolidated, conditioned and aerated powders : a comparative study using a powder rheometer,” Powder Technology, Vol. 174, p. 25 (May 2007).