LIF or dye: Comparison of different tracing methods for granular solids

Otto Scheibelhofer, Julia Kruisz, Jakob Rehrl, Eva Faulhammer, Andreas Witschnigg, Johannes Khinast*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


Determination of residence time distribution is considered essential for continuous pharmaceutical unit operations, in order to enable process understanding and control. A common way for determining residence times are tracer based methods, yet at the moment most realizations are found in liquid systems whereas use of those methods on granular samples is rarely encountered.

Here, we compare two different methods for applying and detecting tracers in granular material processed in a pharmaceutical capsule-filling unit. As first method, a simple colorimetric procedure, i.e. using a dye and a camera as photometer is used. As second method, light induced fluorescence, i.e. using a fluorescent tracer material, a stimulating source and the camera as photometer again, is used.

Both methods are tested and compared for their performance and appeared to be sufficient for the use in residence time distribution determination, yet none of the two proved to be superior. Consequently, side issues rather than analytic performance determined the final preference for the colorimetric method.
Original languageEnglish
Pages (from-to)20-31
Number of pages12
JournalPowder Technology
Early online date2020
Publication statusPublished - 1 May 2020


  • Capsule filling
  • Fluorescence
  • Powder technology
  • Process analytical technology
  • Residence time distribution
  • Tracer

ASJC Scopus subject areas

  • Chemical Engineering(all)

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