Viscosity Control in PBT Production
Viscosity Control in PBT Production
Polybutylene Terephthalate (PBT), is a thermoplastic polyester similar to PET which is primarily used for injection molding due to its favorable cooling characteristics. Worldwide production output is around 800.000 metric tons produced in roughly 100 plants. PBT exhibits very attractive stiffness, toughness, and robust wearing properties.
Polybutylene terephthalate is used for housings in electrical engineering, but also in automotive construction as plug connectors and in households for example in showerheads or irons. It is also found processed into fibers in toothbrushes, false eyelashes and is used in the keycaps of some high-end computer keyboards because the texture is highly resistant to wear and discoloration due to UV radiation [wikipedia].
A new concept for sophisticated automatic viscosity control was developed through a joint industrial research project with Technip Zimmer GmbH, a leading plant engineering and construction company.
In an innovative three-stage reactor process, PBT is produced starting from 1.4-Butanediol and Purified Terephthalic Acid. The raw materials are esterified. The final reaction step to reach the desired high viscosity is done in a proprietary Double Drive Disc Ring Reactor (DD-DDR) of Technip Zimmer GmbH. The unique reactor design enabled the development of a very efficient process for high viscosity melt. The reactor allows for flexible, cost-efficient PBT production without the need for additional equipment.
The developed design for sophisticated viscosity control utilizes two manipulated variables: (i) pressure and (ii) disc speed in the reactor in order to allow for frequent changes of the viscosity setpoint. Challenges in viscosity control include:
• Handling of very different operating conditions.
• At least eight major quantities/variables that decisively influence the product viscosity.
• A very complex system identification task modeling the MISO plant behavior, even in its linear approximation.
• Severe constraints, in particular on the reactor pressure.
• Non-minimum phase characteristics from both (individual) manipulated variables to the control variable make a high-performance feedback control design extremely difficult.
These challenges are tackled with simultaneous pressure/speed control. A key to success is the development of a flexible 8-input/single-output viscosity model using actual data. Operating data using a sampling time of every six minutes over a period of six months was taken from a PBT production plant designed by Technip Zimmer GmbH. To take into account a wide range of parameters, a partial-based model was identified. This approach allowed us to estimate independent individual dynamics from each input to the output with linear regression in an output error framework.
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