Chemical Processing: Effectively Handle Powders

Take a practical approach to powder characterization and tap into operating experience.

By Tim Freeman, Freeman Technology

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While no one would deny the challenges involved in processing liquids and gases, powders are in a league of their own. Those charged with the design and operation of fluid processing plants have access to extensive physical property databases and well-established tools for property estimation and process simulation. In contrast, there’s a wide range of possible powder characterization techniques — from simple angle of repose to comprehensive analysis with a universal powder tester — but little published data. The process relevance of what information exists often is far from clear.

Powders are complex systems consisting of solids or particulates, a continuous gaseous phase (usually air) and, almost always, a liquid component. Their properties are a function of composition and an array of particle parameters that includes size and distribution, surface roughness and hardness. Vibration, compaction, attrition, segregation and many other factors influence behavior.

This makes measuring powders in a reproducible and meaningful way difficult; predicting flow properties from basic parameters, such as particle size, is well beyond current capabilities. So, powder processors have learned to rely on experience rather than fundamental knowledge. They tend to solve problems using a trial-and-error approach — and often can’t pin down the underlying cause or even why a solution works.

Such a trial-and-error approach can succeed but is time consuming and expensive. Furthermore, the experience gained, unless it extends underlying knowledge, has limited applicability.

Drivers for Process Improvement
As margins tighten any processing inefficiencies become less and less tolerable — many plants already have solved the easiest problems. Further progress requires manufacturers to develop in-depth understanding to push units to their efficiency limits and maximize return on investment. Typical goals may be:

• raising throughput by decreasing downtime or increasing flow rates;
• reducing waste by consistently maintaining product specification and cutting rework;
• switching to lower cost feeds; and
• automating process control.

To achieve these targets the manufacturing team must know what changes to make — and understand the impact that any change will have on the process and product. The starting point has to be the existing experience base but this, in its raw state, only can be used to improve operation within a well-mapped window.

If aspects of processing experience can be correlated with specific powder properties then a better understanding can be developed. Knowing which variables determine how a powder behaves in a given situation is the first step toward more-effective control. Using such knowledge to extend operation outside the established working envelope is the way to meet more-exacting performance standards.
Powder Rheometer
Figure 1.  Powder Rheometer: This device can
characterize powders in compacted and other states
that simulate process conditions.

Characterizing Powders
A powder rheometer (Figure 1) can serve as an excellent starting point to investigate process behavior. It determines the energy required to make a sample flow by measuring the force/torque acting on a blade rotating through the material [1]. Conditioning, which is a gentle agitation step that produces a homogeneous, loosely packed bed prior to measurement, ensures excellent reproducibility. This makes powder rheometry highly differentiating. It can measure powders in a compacted, conditioned, aerated and even fluidized state, to more accurately simulate process conditions.

Modern rheometers not only provide dynamic measurement but also shear and bulk property testing capabilities. It’s therefore possible, using just a single instrument, to gather a comprehensive set of data in a reasonably short time. This encourages the development of databases of powder properties that can include:

• flowability parameters such as Basic Flow Energy (BFE) and Specific Energy (SE) [1];
• shear properties, e.g., yield locus, unconfined yield stress (compression strength), cohesion and internal angle of friction; and
• bulk properties like bulk density, compressibility and permeability.

In addition, investigating specific aspects of powder behavior such as de-aeration, segregation, caking and the effect of moisture and attrition can improve understanding.

Using this database of properties, it’s possible to relate process behavior to the characteristics of the powder. It also becomes feasible to identify those variables that critically impact performance, as the following case study illustrates.

Changing Feedstock
A manufacturer wants to consider other suppliers for a powder feed. The operational team strongly resists, though, because feedstock has been switched before and productivity was severely compromised. The powder flow through the plant, particularly from the feed hoppers, now is easily maintained; it was erratic with the alternative material and blockages were common.

In this case, the manufacturer is paying for material with superior handling performance — but there’s limited understanding of the properties that confer this superiority. Better understanding could have predicted failure of the alternative feed. So, the starting point for assuring a new feed is acceptable is to develop a specification that defines the existing material’s success. Unless this can be done, the manufacturer is stuck with the current supplier.

Analysis of the current feed provides data that can be used to understand why it behaves well in the process. For example, the powder may have relatively low cohesion and a low compaction index (CI). A low CI means that flow energy is relatively unaffected by the application of a consolidating pressure. In a hopper, exiting material is compressed by the weight of the powder above it. Under such conditions, this powder won’t form a bridge blocking the hopper outlet and will tend to flow relatively easily. The measured data, therefore, offer insights on why this material may be so easy to handle.

Comparing this material’s properties with those of the alternative feed that proved problematic, of course, would be even more illuminating. Such an analysis might confirm that the feed should have low cohesivity and low CI — and also may indicate that it mustn’t be prone to segregation.

Specifying acceptable values for these parameters and testing prospective materials against these criteria would give some assurance of success.

Using Powder Properties
The previous example illustrates two ways in which powder characterization data can be used. One approach is to comprehensively measure the powder and then look at all the properties to see how well the material fits with the demands of the process.

For example, consider die filling, where powder is used to fill a specific volume of defined shape. Ideally the powder should flow freely into the die, settle and then flow into the required shape as filling proceeds. In this case, logic would suggest that flow behavior under compacting and unconfined conditions will be important. The way in which the powder releases air also has a bearing. A powder with low permeability may trap air, compromising filling or the effectiveness of any subsequent compression step. So, a powder that releases air easily is preferable from this point of view — however, such powders can quickly become immobile because air lubricates powder flow. Achieving optimal performance requires balancing these effects.
Investigating Aeration in Powders
Figure 2. Investigating Aeration: Results of a study of the
effect of air on two powders underscores how differently
powders can behave because of aeration. A small amount
of air can dramatically improve flow properties of some powders.
Click image to enlarge.

Figure 2 presents the results of a study of the effect of air on two powders. The data show how flow energy falls as the velocity of air travelling up through the sample increases. The free-flowing powder’s flow energy drops almost to 0, indicating that it has fluidized; the minimum fluidization velocity occurs at around 2 mm/s. This small amount of air radically improves the material’s flow properties. In contrast, the cohesive material forms channels through which the air escapes.

Shear properties also can affect processability. Figure 3 shows data for the stress required to shear a previously consolidated powder sample as a function of applied normal stress. The two materials exhibit different behavior — one presents a much greater flow resistance. This powder is quite cohesive (C1 value) and thus more likely to form stable bridges that can compromise hopper discharge and die filling than the other material. Powders with low shear strength will tend to flow freely even when compacted. C2 is the cohesion value or shear strength of the powder when not consolidated.

The alternative approach is to compare the properties of materials that process well in the operation with those that perform poorly, as in the following study.

Optimizing Equipment Use
A titanium dioxide producer makes different grades for applications in the paper, plastics, cosmetics and food industries. It uses surface treatment, milling and drying processes to produce the required properties. With some grades, operation is both easy and productive while others demand very close control. The manufacturer instigates a review with the aim of improving throughput and productivity.

The producer needs to understand how the properties of different TiO2 grades affect how they interact with the processing equipment. Existing process experience allows materials to be classified using “Processability Rankings” (PR). For example, a PR of 2 may describe a trouble-free powder that processes consistently at a fast rate. A PR of 9 may indicate a powder that can’t be processed to any degree, while an intermediate value of 6 may signify a powder that performs acceptably if the process is closely monitored or control is fine-tuned. This is a useful way of starting to quantify experience. Factors to take into account when ranking processability include:
Measuring Shear Properties
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; and
PR9 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).

Processability Issues on Tableting Line

Material

PR rating

Problems

Formulation 1        2 Potential segregation during transfer
Formulation 2        5 Bridging and adhesion to machinery
Formulation 3        8 Weight variability and high wastage
Table 1. Simple format highlights key concerns for each powder during processing.


Troubleshooting Tips to Diagnose Problems
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: tim@freemantech.co.uk.

Reference 1. 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).
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