Experimental methods in chemical engineering: Optical fibre probes in multiphase systems

As much as 75% of the raw materials in the chemical industry and 50% of consumer products are in the form of powders or granular solids. Gasification, pyrolysis, coating, granulation, drying, and mixing are examples of processes in which particles contact fluids. Researchers examine the hydrodynamics of these fluid–solid systems with pressure signals, acoustics, tomography, radioactive particle tracking, optical fibre measurements, and spectroscopy. Among these techniques, fibre optic probes are simple, inexpensive, and sensitive (spatial resolution of 100 μm) and have sampling frequencies of > 1x10⁷ Hz. Optical probes measure local hydrodynamic properties, including particle velocity, solids fraction, and voids, which are difficult to measure in heterogeneous systems like spouted beds, risers, and turbulent fluidized beds. Light from a fibre optic bundle illuminates a specific volume, and fibres from the same or separate bundle return the reflected or transmitted photons to a detector (visible, near-infrared spectroscopy, or Raman). Sample MATLAB codes included herein together with sample experimental data demonstrate how to process raw signals for gas/solids/and bubble holdup, particle and bubble velocity, bubble size, and solids flux.