RTD in twin-screw food extrusion

Abstract

The geometric mean residence time (GMRT) and the RTD were measured on a twin-screw extruder using a KCl tracer and electrical conductivity measurements at the die. Extruder screw speeds (150–350 rpm) and feed rate (98.55–20.0 kg/h) were varied in a 3×3 factorial experiment with the matrix diagonal having a constant specific feeding load (SFL) of 0.057 kg/(h rpm). Feed rate had a greater effect on the mean residence time than the screw speed, with a 41% reduction in GMRT (108–64.0 s) with a 2.33-fold increase in feed rate, while screw speed had a greater influence on the normalized spreads of the RTD (mixing),with an increase in normalized spread of 21% (from 0.67 to 0.81) as screw speed increased 2.33 fold. Increasing feed rate yielded no change in normalized spread. Increasing screw speed and feed rate proportionally (constant SFL) from the minimum conditions to the maximums (a 2.33-fold increase for both) decreased the GMRT more than 50% from 123 to 56.2 s, increased the normalized spread from 0.70 to 0.82, and increased the barrel fill from 55.0% to 59.5%. Increasing feed rate increased barrel fill, die pressure, motor torque and product temperature while at the same time decreased residence time. Increasing screw speed decreased barrel fill, die pressure, motor torque and the GMRT while increasing the product temperature. Increasing feed rate and screw speed proportionally (constant SFL) yielded a slightly increasing trend for barrel fill.