Powder flow analysis - the solution

Jo Smewing, applications manager at Stable Micro Systems, looks at powder flow in the pharma industry - the need for accurate testing and its benefits to manufacturers

It has been estimated that powders account for 50% of materials used in industry. Handling and processing powders, particulates and granules is central to pharmaceutical processing, but has traditionally been fraught with problems caused by their unpredictable and irregular behaviour, specifically with respect to flowability. With so many raw materials and semi-finished products in powder form, this sector stands to gain significant manufacturing and commercial benefits from improvements in assessing powder flow. Recent technological developments are poised to deliver those advantages.

Powder handling and processing tends to be problematic because powders exhibit properties similar to both solids and liquids. Normally, they are surrounded by air and combine in many ways to change the way the powder behaves. Many common manufacturing problems are attributed to incorrect powder flow, including non-uniformity (segregation) in blending, under- or (potentially dangerous) over-dosage, inaccurate filling and obstructions.

evaluating performance

These in turn lead to excessive rejected material, machine downtime and defective end-products. Storage, handling, production, packing, distribution and end use can all be negatively affected by common powder flow problems such as ratholing, bridging/arching and flooding.

Knowing a powder's characteristics and the factors that impact on it (table 1) is essential. This allows the optimisation of formulation and equipment and incorporates quality control principles throughout production, thereby assisting in process design, performance evaluation and troubleshooting.

Even the most rudimentary methods of assessing powder flow necessitate the one resource all technologists lack - time. However, the benefits to be gained by assessment far outweigh this immediate concern. These benefits include:

• Product development teams can evaluate new ingredients and constituents and predict their behaviour prior to commencing large-scale production. They can also check how new powders interact with existing constituents. This speeds up development time and minimises 'trial and error' tactics.

• Predictable powder flow enables ingredient selection, manufacturing procedures and equipment to be optimised, which maximises production speed, reduces the risk of stoppages and improves blend quality, filling procedures and end product quality.

• As pressure mounts to contain costs, the substitution of expensive constituents with cheaper ones is an attractive prospect. Although these substitutes may be produced to the same specification as original materials, they may not necessarily store, convey and process as easily. Discovering this after production has started would incur downtime and cost.

assessing flow

The first methods of powder flow assessment are still surprisingly widespread, despite their well-documented shortcomings and more recent technological advancements. They include:

• Angle of repose. The sample is poured onto a horizontal surface and the angle of the resulting pyramid measured (figure 1). Free-flowing powders will tend to lower repose angles.

• Flow through funnel. Orifice sizes are changed to alter the rate of flow out of the funnel - the user normally selects the orifice size through which the powder flows slowly and reasonably constantly.

• Powder volume. The user creates graphs measuring time against the volume of powder discharged from a container - free-flowing powders will flow faster, thereby showing as greater volumes within a given time.

Given the critical importance of powders in the pharmaceutical industry, these early methods fall drastically short of the standards now required by business. They are subjective, influenced by the operator and, crucially, applicable only to free-flowing powders. But as powder technology advances, particulate properties are constantly evolving. The increasing use of fine powders, for example, which tend to exhibit cohesive properties, means that testing non-free-flowing samples is essential. And for this, instruments are necessary.

The first, and most common, instrument for powder flow testing is the Jenike Shear Cell. Powder is loaded into the cell and then compressed with a defined weight. After compression, the operator measures the force needed to shear through the sample. This method is commonly used for silo and storage bin design.

It eliminates most of the subjectivity to which earlier test procedures were prone, but human involvement in the loading of the sample can impact on the results obtained, making them less reliable. In addition, because minute disturbances of any kind can impact on a sample's flow rate, none of the aforementioned test procedures is suitable for repeated testing.

The past five years has seen intense development activity as instrument designers strive to provide truly accurate, repeatable and objective equipment, which can overcome the shortcomings of earlier test methods.

One of the major factors behind the most recent developments was the need for testing in environments that were as close as possible to actual processing conditions. Manufacturers are aware that some of the most common powder handling problems arise during conveying from storage silos or bins into production. Often the powder will have been motionless for hours, days or even weeks before flow is required. Over this time, most powders will compact, with the result that far more energy is needed to cause flow than in aerated samples.

stored powders

On the other hand, powders used immediately after transport, which may have involved agitation or more pronounced vibration, will have been aerated and flow more easily than others that have been stored. Purging and air permeation systems, installed to aid flow, will naturally impact on the behaviour of a stored powder. For this reason, it is desirable to condition the sample prior to testing. The latest powder flow analysers are able to compact or aerate samples - this not only reflects real storage conditions, but eliminates any interference caused by the loading of the sample. The test and data acquisition start only after conditioning is complete.

As modern powder testing equipment causes the sample to flow (rather than relying on its own ability to do so), users are able to define how it should flow. Movement of the sample is caused by a blade or rotor, which is passed through the powder according to settings defined by the user - both moving vertically and rotating. As testing is done on a relatively small scale, the design of both the sample vessel and the blade is critical. Manufacture of these components must be of the highest quality - any blemishes on surfaces, incorrect angles or imperfect edges would impact on the flow of the sample, and the results of the test.

sample movement

When the blade is rotated in the direction shown in figure 2, the sample is lifted or aerated. This procedure conditions the powder and eliminates any loading variation. After compaction, it can measure the level of particle cohesion, flow and recovery.

Figure 3 shows compaction. It provides a simple quality control test for variations between batches and/or sources. Samples being compacted will resist flow and movement.

Slice compaction, shown in figure 4, can be used to blend and mix samples. This procedure removes air from the sample and causes friction between the surfaces of granules. Users can assess the smoothness of the contact between the powder and the blade.

These movements can be carried out in any combination and in any sequence. A simple test will involve (i) conditioning, (ii) slice compaction and slice aeration (typically repeated five times), and (iii) blade shake, to remove any powder which has adhered to the upper blade surfaces. However, sophisticated instruments can be programmed to carry out far more complex tests over prolonged periods.

During testing, software gathers data relating to time, axial force and distance. Accuracy is optimised if this information is measured at 500pps (points per second) or more. Instruments like Stable Micro Systems' Powder Flow Analyser can interpret and display this information in real time, as well as perform composite or statistical calculations. Tests on different samples, and conducted under differing conditions, can quickly and simply be compared to allow grading and ranking of powders.

Being able to measure powder flow is undoubtedly of prime importance to pharmaceutical manufacturers, but as quality standards continue to rise, few can afford to stop there. Finished product testing is essential to ensure correct performance and safeguard reputation, sales and market share. Many have legacy systems to analyse end product properties such as tablet hardness, gel viscosity, etc.

financial savings

By using a powder flow analyser, manufacturers are able to change the attachments on the instrument for testing semi-finished and finished products as well. This offers considerable financial savings, as well as simplifying training and releasing valuable lab space.

Objective and repeatable testing combined with ranking of dry and wet powder samples can provide major opportunities and benefits. These include optimising batch and source selection in terms of cost and quality; developing best mix formulations; optimising scale-up and process conditions; and maintaining product quality control.

Innovative technology provides such data either by measuring and comparing products capable of flow, or by assessing sample behaviour in test conditions simulating in-process or product handling conditions. Best-practice raw material purchasing, processing efficiency, waste minimisation and product quality - before and after storage, packing and transportation - are all desirable. The application of practical powder rheology will be a valuable tool for achieving these goals.