RESIDENCE TIME DISTRIBUTION OF FOOD AND SIMULATED PARTICLES IN A HOLDING TUBE

Residence time distribution (RTD) was determined for food and simulate d 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 x 10(-4) m(3)/s) and SSHE shaft speed (30, 60, and 90 r pm). Residence times of tracer particles were recorded by a video came ra equipped with a timer. Cylindrical particles remained in the holdin g tube longer than cubic particles. Increasing particle concentration and bulk flow rate tended to decrease mean normalized particle residen ce time (MNPRT). Increasing fluid viscosity decreased MNPRT significan tly, apparently due to the strong lift action of the fluid on the part icles. Although turkey particles were denser than other food particles , they had smaller residence time than the carrot and potato particles . The MNPRT of green peas was found to be significantly higher than th at of polystyrene spheres. Swept-surface heat exchanger shaft speed di d not influence MNPRT in the holding tube. The residence time distribu tion curves were narrowed by decreasing particle concentration and inc reasing bulk flow rate, and by using high viscosity fluids and cubic p articles. The longest particle residence time was 5.3 times the averag e bulk residence time. The fastest particle did not exceed 1.8 times t he average bulk velocity. This finding is within the conservative valu e of 2.0 considered safe in sizing holding tubes in aseptic processing systems. (C) 1998 Elsevier Science Limited. All rights reserved.