Abstract

Residence time distribution (RTD) was determined for food and simulated particles flowing in a holding tube, as affected by particle shape (cube, cylinder, sphere), particle concentration (10, 20, and 30% w/v), particle type (potato, carrot, turkey, green peas, and polystyrene), fluid viscosity (0.0% CMC, 0.5% CMC, 1.0% CMC), bulk flow rate (5.81, 7.28, and 8.71 × 10 −4 m 3/s) and SSHE shaft speed (30, 60, and 90 rpm). Residence times of tracer particles were recorded by a video camera equipped with a timer. Cylindrical particles remained in the holding tube longer than cubic particles. Increasing particle concentration and bulk flow rate tended to decrease mean normalized particle residence time (MNPRT). Increasing fluid viscosity decreased MNPRT significantly, apparently due to the strong lift action of the fluid on the particles. Although turkey particles were denser than other food particles, they had smaller residence times than the carrot and potato particles. The MNPRT of green peas was found to be significantly higher than that of polystyrene spheres. Swept-surface heat exchanger shaft speed did not influence MNPRT in the holding tube. The residence time distribution curves were narrowed by decreasing particle concentration and increasing bulk flow rate, and by using high viscosity fluids and cubic particles. The longest particle residence time was 5.3 times the average bulk residence time. The fastest particle did not exceed 1.8 times the average bulk velocity. This finding is within the conservative value of 2.0 considered safe in sizing holding tubes in aseptic processing systems.

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