The SME – Specific Mechanical Energy (Wh/kg) – and Density Control Modules allow separate selection and online control of the cooking degree and bulk density of the pellets. The extruder is divided into two process sections or areas:
• The SME Control Module consists of a throttling device with two distribution cylinders. Closing of the valve cylinders increases the length of material backed up ahead of the throttle. As a result, the filling degree and dwell (retention) time increase. This in turn increases the SME. When the cylinders are opened, the opposite happens.
• The Density Control Module consists of an open housing attached to the end of the SME Module (degassing housing). It prevents product particles from being carried along by the steam discharged.
The product temperature at the die and therefore the bulk density can be selected independently of the SME introduced into the first process section. This is accomplished either by adding positive-pressure steam (low-density product), by returning neutral-pressure steam to the precooker (medium-density product) or by generating a vacuum (high-density product).
Bühler AG is Uzwil, Switzerland. www.buhlergroup.com