Analysis of the dynamic behavior of a starch foam extrusion process

The starch foam extrusion process was modeled as a multiple input multiple output (MIMO) process, and the dynamics of the process were studied as a response to step changes in the input variables such as starch feed rate, screw speed, moisture content (MC), and poly(hydroxy aminoether) (PHAE) feed rate. The responses were modeled as first-order responses with a time delay. The linearity of the process was determined over a range around the set-point, and the parameters defining the first-order system such as gain K, time constant T, and dead time t, were determined in the linear range. The transfer function models can then be used in a predictive computer control system for on-line fine-tuning of the operating conditions. This could ensure a consistently high quality product even when low frequency disturbances are present in the system. It was observed that the time constants and the dead times recorded for both the pressure and torque responses did not exhibit significant variation within each manipulated or input variable tested, indicating a dynamic linearity with respect to each manipulated variable. It was also observed that for the same step-input variations in the manipulated variables, the torque loading on the twin-screw extruder exhibited a faster response (lower dead time), and also reached a steady state sooner (lower time constant). The MC and screw speed seem to be the most destabilizing variables, as they induce rapid responses in the process variables. The MC in the extruder was, hence, determined to be the most influential factor in the stability of the process, followed by screw speed and starch feed rate. PHAE feed rate was the least significant variable. Multiple step-input tests were carried out to determine the validity of the principle of superposition. The validity of the principle of superposition implied the linearity of the process in the domain tested. (c) 2006 Wiley Periodicals, Inc.