Residence Time Distribution
David Bigio, University of Maryland
Mark Wetzel, DuPont
From 1999 through 2005, DuPont and the University of Maryland entered into a research collaboration to study the fundamentals of Residence Time Distribution (RTD) in twin screw compounding and its influence in the control of reactive extrusion systems. Funding was secured through NSF GOALI grants. Graduate students came to the DuPont Experimental Station to conduct extrusion experiments on a highly instrumented Coperion W&P ZSK-30mm twin-screw machine. DuPont provided the equipment, materials, optical probes and an on-line melt rheometer for the research.
From the academic perspective, new fundamental insights were gained relating the effects of throughput, Q, and screw speed, N, and rheology on the RTD. New concepts were developed; the Residence Volume Distribution, RVD, and the Residence Revolution Distribution, RRD which mapped the RTDs measured over a wide range of operating conditions to master curves. The RVD concept was then applied to the time-driven, closed loop control the melt viscosity of a polyolefin undergoing peroxide cracking in the extruder. A new model was developed using the RVD that quantified the dynamic response of the extruder to a variety of disturbances and setpoint changes. The effects of melt viscosity, polymer type and viscoelasticity were also explored showing how polymer structure can alter the RTD, RVD and RRD.
From the industrial perspective, the fundamental research was of strategic importance. A predictive model for residence time and RTD was developed and used in flow simulations. An RTD and RVD-based closed loop control of reactive extrusion processes showed what can and cannot be controlled. It also showed how the RTD can dampen disturbances over a frequency range related to the RTD. The concepts developed form this research led to a manufacturing implementation of viscosity control.
This successful partnership also spawned a new collaboration between DuPont, Coperion and the University of Maryland where the RTD is measured along with the dispersive mixing stress using a real-time experimental method. The Residence Stress Distribution, RSD, concept builds upon the RTD and RVD approach. It is being developed to establish new fundamentals for dispersive mixing in a twin-screw extruder and how to scale-up a process based on mixing.
The RTD and the RVD:
What can and cannot be controlled – RVD model transfer function: