Theoretical Computation of the Isothermal Flow Through the Reverse Screw Element of a Twin Screw Extrusion Cooker

A mathematical model of the flow of molten starch through the reverse screw element of a twin screw extrusion-cooker was proposed. The assumptions included isothermicity, Newtonian flow in each part of the reverse screw element and quasi-steady geometry. The theoretical approach is based on solving Stokes equations in the direct screw element and then on writing flow rate conservation in each part of the reverse screw element. The various flow rates, the pressure drop and the residence time distribution through the reverse screw element were computed for a CLEXTAL BC 45 twin screw extruder under different operating and geometrical conditions; good agreement was found when comparing these results with previous experimental work. The pressure drop through the reverse screw element was low (0.5–1.0 MPa); the flow rates in the different channels of the reverse screw element were important compared to the total feed rate of the extruder (two to three times larger) which might explain the broadening of the residence time distribution in the reverse screw element.