Powder characterization and particle analysis: complete guide to bulk processing and formulation
Powder characterisation: the key to predictable process behaviour
The properties of a powder determine how it behaves during transport, dosing, mixing, granulation and tableting. Powders with identical chemical composition can be processed fundamentally differently if their particle size, shape, bulk density or flowability differ. Systematic powder characterisation is therefore essential in process design, scale-up and troubleshooting in the pharmaceutical, food and chemical industry.
Particle size distribution (PSD): the most critical powder property
Particle Size Distribution (PSD) describes what percentage of the particles has a given size. PSD directly affects: API dissolution rate, mixing quality, flowability, capsule and tablet fill time, filterability and pneumatic transport behaviour.
Common measurement methods for PSD:
- Laser diffraction: fastest method, measurement range 0.1–3500 µm, wet or dry dispersion, standard in pharma and food
- Sieve analysis: simple, low-cost, suitable for particles >45 µm; less suitable for cohesive powders
- Dynamic image analysis (DIA): also measures particle shape alongside size; valuable for anisotropic particles (needles, flakes)
- Electrical sensing zone method (Coulter counter): high accuracy for narrow distributions in liquid
Bulk density: poured and tapped
Bulk density is measured as poured bulk density and tapped bulk density. The difference between the two provides information about the powder's compressibility. The Carr Index and Hausner Ratio are derived measures:
- Carr Index = (tapped - poured) / tapped × 100%. Values <15% = good flowability, >35% = poor flowability
- Hausner Ratio = tapped / poured. <1.25 = good flow, >1.4 = cohesive powder
Flowability and cohesiveness: process-relevant parameters
A powder's flowability determines whether it flows spontaneously and reproducibly from a silo, hopper or dosing device. Cohesive powders form bridges and rat holes. Measurement methods for flowability include the angle of repose test, the flow function (Jenike shear cell) and dynamic flowability (powder rheometer such as the FT4).
Flow improvers such as silica (Aerosil), magnesium stearate or talc are added to improve cohesive powders — but affect other product properties and generally require formulation validation.
Powder characterisation across the production cycle
Powder characterisation takes place at four points: at procurement (raw material specification), at process design (scale-up from lab to production), during deviation investigation (OOS results or process disruptions) and during regular trending as part of process analytical technology (PAT). Hecht Technology supplies process equipment designed for powders with widely varying characteristic properties. More information: hecht-technology.nl.