(196c) Control Strategy and Comparison of Tuning Methods for Continuous Lactide Ring-Opening Polymerization

Polylactic acid (PLA) is a 100 % bio-based polyester with strong market growth potential in commodity applications as a substitute of fossil-based polymers such as PET, PE and PS. PLA is produced industrially by ring-opening polymerization (ROP) of lactide, the cyclic dimer of lactic acid, which is the product of fermentation of carbohydrates.¹ As well known, the lactide ROP is very sensitive to trace amounts of impurities, such as substances containing hydroxyl and/or carboxylic acid groups, carbohydrates, amino acids and metals, which heavily impact reaction rate, molecular weight and thermo-mechanical properties of the produced polymer.^2-3

In continuous industrial processes, where lactides from different sources and of different qualities might be used,^1,3-4 the quantification of all such possible impurities is often impractical. Therefore, robust in-line monitoring techniques and automatic control of the process are crucial to minimize deviations from the desired set-points and to increase safe operation of polymerization plants.⁵

Results and Discussion

The viscosity of the reacting mixture, which is a function of both conversion and molecular weight, can be monitored from data of pressure drop on selected pieces of equipment on PLA plants, or using in-line viscometers. By coupling this information with an independent conversion measurement, knowledge of the system viscosity can be used as a direct indication of the molecular weight being produced. Raman spectroscopy is especially promising for the in-line monitoring of conversion, since the monomer and the polymer peaks are easily identified and are well resolved.

In this contribution it is shown by simulations that an automatic control strategy based on the in-line independent measurements of conversion and pressure drops (or viscosities) enables a substantial reduction of the amount of off-spec material produced after a perturbation in the feed. In particular, an industrially relevant process was considered where the reaction is carried out in a loop reactor followed by a plug-flow reactor.⁶ A feedback MIMO controller was implemented on gPromstogether with a detailed kinetic model previously validated,⁷ and different tuning techniques, such as the Ziegler-Nichols, the Tyreus-Luyben and the Cohen-Coon methods,⁸ were used to optimize the parameters of the independent controllers. The performances of the different controllers are analyzed in terms of amount of off-spec material and impact of oscillations on plant safety.⁹

References

[1] R. Auras et al., Poly(lactic acid) – Synthesis, Structures, Properties, Processing, and Application, John Wiley & Sons, Hoboken, New Jersey US, 2010.

[2] X. Zhang et al., Journal of Polymer Science: Part A: Polymer Chemistry, 1994, 32, 2965-2970.

[3] D. E. Henton et al., Polylactic Acid Technology, in Natural Fibers, Biopolymers and Biocomposites Vol.16, Taylor and Francis group, 2005.

[4] H. L. Bos et al., Biofuels, Bioproducts and Biorefining, 2012, 6, 146-158.

[5] J. R. Leiza, Macromolecular Reaction Engineering, 2009, 3, 324-325.

[6] L. I. Costa et al., Macromolecular Symposia, 2016, 360, 40-48.

[7] F. Codari et al., AIChE Annual Meeting, 2013.

[8] G. Stephanopoulos, Chemical Process Control, Prentice-Hall, New Jersey, 1984.

[9] L. I Costa, U. Trommsdorff, Chemical Engineering Technology, 2016, 39, 2117-2125.