Residence time distribution in a co-rotating, twin-screw continuous mixer by the step change method

Residence time distributions were determined for the continuous processing of chocolate in a twin-screw, co-rotating mixer, and modeled using as a series combination of piston flow and ideal mixing elements or as equal size tanks in series. Color (L-value) was measured after a step change from milk chocolate to white chocolate. Both models fit the data well, although the series combination of piston flow and ideal mixing fit better for short mean residence times, accurately predicting the observed deadtime. The series of tanks model appeared to fit data better under conditions where longer mean residence times were observed. The mean residence time was significantly influenced by feed rate, screw speed and gate opening. A high shear, low conveyance screw configuration was used that led to a high fill fraction (>0.85). Therefore, feed rate had the greatest effect on the mean residence time. The time of first appearance was affected only by the gate opening, and ranged from 0.44 to 0.68 times the mean residence time.