Modeling and simulation of twin-screw extrusion

Twin-screw extruders offer improved control of the residence time distribution and mixing in materials such as plastics, rubber and food. The fluid flow and heat transfer in a fully intermeshing, corotating twin-screw extruder are investigated numerically. The control volume technique is used for numerical modeling and simulation, considering both Newtonian and non-Newtonian fluids. The velocity distributions in the screw channel are compared with experimental results and good agreement is obtained. Due to the limitations on the physical aspects of the problem, the numerical results show that not all desired operating conditions are feasible. A feasible domain, in terms of screw speed and mass flow rate, in which the extruder operation is satisfactory, is obtained for pure starch. To improve the applicable range of this model, an axial formulation is adopted for the translation region. This model yields consistent results with the earlier down-channel model while the feasibility region is extended towards lower mass flow rates. For the upper limit, a physical restriction arises in terms of the maximum flow rate. The model can be used for simulating a wide range of operating conditions while retaining the appropriate physical behavior of the process.; Based on the flow and the heat transfer characteristics, the residence time and chemical reaction are studied by tracking the particles. For normally starve-fed twin-screw extruders, the length of the completely filled section is calculated as function of the process variables using the coupling of the flow with the die. With a model of the solid conveying section, the Residence Time Distribution (RTD) for the whole extruder is calculated for corn meal at different screw speeds and flow rates. The calculated variation of RTD with the screw speed and the flow rate yields good agreement with observations from many experiments. The variation of the fully filled section length, chemical conversion and mixing effectiveness are also obtained under different operation conditions. Most of the results are in qualitative agreement with experimental results and may be used as guidelines for extruder design and determination of optimal operating condition.; For the design and operation of screw extruder control systems, a numerical investigation of the system transient behavior is carried out. A transient model for single- and twin-screw extruders is developed. The transient behavior of extruders, including response time, variation of system variables, and instability of operation is studied numerically for the rheological zones of both single- and twin-screw extruders. Conjugate heat transfer with the screw barrel is taken into account for single-screw extruders. Due to the complexity of screw and barrel geometry, and of the flow and thermal fields, the transient process with conjugate heat transfer is not considered for twin-screw extruders. However, the similarity between the transient behavior of single- and twin-screw extruders, makes it possible to predict the results for the transient processes of a twinscrew extruder with conjugate heat transfer using the model of a single-screw extruder.