The pharmaceutical and fine chemicals industries are strongly concerned with the manufacture of high value-added
speciality products, often in solid form. On-line measurement of solid particle size is vital for reliable control of product
properties. The established techniques, such as laser diffraction or spectral extinction, require dilution of the process
suspension when measuring from typical manufacturing streams because of their high concentration. Dilution to
facilitate measurement can result in changes of both size and form of particles, especially during production processes
such as crystallisation. In spectral extinction, the degree of light scattering and absorption by a suspension is measured.
However, for concentrated suspensions the interpretation of light extinction measurements is difficult because of
multiple scattering and inter-particle interaction effects and at higher concentrations extinction is essentially total so the
technique can no longer be applied. At the same time, scattering by a dispersion also causes a change of phase which
affects the real component of the suspension's effective refractive index which is a function of particle size and particle
and dispersant refractive indices. In this work, a novel prototype instrument has been developed to measure particle size
distribution in concentrated suspensions in-process by measuring suspension refractive index at incidence angles near the
onset of total internal reflection. Using this technique, the light beam does not pass through the suspension being
measured so suspension turbidity does not impair the measurement.
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