The effect of large solid particles on the residence time distribution of the fluid in two-phase flow was studied in a pilot scale continuous tubular system. The residence time distribution was characterized with four parameters: the normalized minimum and mean residence time, the normalized standard deviation and the skewness. The effect of particle concentration (1 and 3%) and particle diameter (diameters approximately 0.3 and 0.4 of the tube diameter) were analysed for fluids with different viscosities and at different flow rates. It was observed that the presence of particles, even at very low concentration, has a significant effect, increasing the mean residence time and decreasing the standardized dispersion. This effect was attributed to particles breaking the velocity profile of the fluid and causing some localized mixing due to their rotational velocity. Although the presence of particles affected the results, their concentration (or number) was not relevant, in the range tested, which is very low for particulate foods.
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