Insights into Continuous Mixing

Continuous mixing is a core technology in the production of many food products. Insights into how continuous mixing can be optimized and the implications for food manufacturers have been revealed byPositron Emission Particle Tracking’ (PEPT) techniques, as part of the UK’s Project Chariot research into mixing fine powders, reports Eddie McGee, Managing Director, at solids handling equipment maker Ajax Equipment.

Ajax Equipment’s Continuous Twin Screw Mixer technology was the primary mixing methodology used in Project Chariot. A twin-screw continuous mixer comprises two rotating intermeshing screws enclosed within a profiled casing. The screw arrangement consists of a series of paddles adjusted to resemble a discontinuous helix, which mix and drive material axially as the shafts rotate. The mixer offers a great deal of flexibility in the degree of mixing through changes to the operating parameters such as speed loading as well as the screw design: ribbons and various custom profile paddles, and changes to paddle frictional characteristics and angles.

The way in which material moves through a twin-screw continuous mixer has been investigated by researchers using ‘Positron Emission Particle Tracking’ (PEPT). Radioactive tracers of a similar size and density to the powder particles were passed through the mixing system to chart the movement of particles. These were detected by radiation sensors in the final quarter of the length of the mixer (22 of the total 78 mixing paddles).

The trials consisted of mixing dry sulphate powder whilst two variables (feed rate and screw rotation speed) were set at either high or low levels. The high/ low feed rates were operated at 450 kg/hr and 240 kg/hr respectively, and the high/ low screw rotation speed at 150 and 72 RPM (or 0.6 and 0.3 m/s tip speed).