Particle Passage Time Distributions in Vertical Pipe Flow of Solid–Liquid Food Mixtures

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In sterilizing food particulate systems, knowledge of the residence time of the fastest moving particle in the heating and holding sections of the aseptic processing system is required. It is equally important to determine the mean and maximum particle residence times and ideally the whole distribution of residence times should be known; it is important that the fastest particle is sterilized without overcooking the slowest particle. Whilst a number of studies have reported data on horizontal food flows, few have considered vertical flows although they form a significant part of food plant. This paper reports experimental results on the vertical upflow of almost neutrally-buoyant alginate particles in carboxymethylcellulose (CMC) solutions in a 48 mm diameter pipe. The spherical particles used had 5 and 10 mm diameters, and were conveyed at solids fractions of 16−55% v/v and mean mixture velocities of 77−230 mm −1 s. The mean apparent viscosity of the carrier fluids ranged from 29 to 422 mPas, and the tube Reynolds number varied from 8.7 to 381. Particle passage times were measured by Hall effect sensors or visual tracers. The results show that four different particle passage time distributions can exist depending on the flow pattern present, and the solids concentration has the strongest effect on minimum passage times. For particle Reynolds numbers less than 0.08, an annular particle region existed which caused high values of maximum passage time.