ISBN 9783843931977

978-3-8439-3197-7, Reihe Thermodynamik

Christoph Dominik Walowski
Modeling of Polyethylene Solutions with Respect to Polymer Weight Distribution and Branching

115 Seiten, Dissertation Karlsruher Institut für Technologie (2017), Softcover, A5

Zusammenfassung / Abstract

Motivated by the technical relevance to polyethylene (PE) manufacture, this work is meant a contribution towards an improved modeling of PE solutions. The treatment of such solutions within equilibrium thermodynamics is known to be challenging for several reasons: First, polymer and solvent differ in size by several orders of magnitude. Second, polymer composition is characterized by its molecular weight distribution (MWD). Third, polymer molecules are not only described by their chain size (distribution) alone, but also by their structure. In addition, PE manufacture takes place under highly varying conditions with respect to pressure and temperature.

The goal of this work is the development of a new thermodynamic model that is capable to handle these issues. To account for polydispersity, continuous thermodynamics is applied. As thermodynamic model, the Sako-Wu-Prausnitz (SWP) equation of state is linked with the lattice-cluster theory (LCT). The polymer branching is governed in two ways—first, by the LCT itself, which inherently contains molecular structure, and second, by suitable parametrization of the SWP polymer parameters. The resulting SWP-LCT model is tested against experimental cloud-point curves adopted from the literature. Due to their relevance to PE manufacture, solutions of PE in ethylene and n-hexane are investigated, with the PE samples greatly differing in MWD and branching. Within this work, two different parametrization methods are proposed, one predictive in nature, the other meant a correlation for a better representation of the experimental cloud-points. As shown, symmetrical lognormal MWDs can thereby be approximated by a simple Schulz-Flory distribution, while for the PE samples with asymmetrical lognormal MWDs, the incorporation of the MWD function into the model allows a better description of the PE solutions.

As discussed, deviations between theory and experiment may arise from model assumptions. The continuous SWP-LCT model, however, provides a reasonable representation of the cloud-point curves that is capable to handle the impact of MWD and molecular structure. In particular, the pressure drop due to polymer branching is qualitatively, and often even quantitatively, predicted correctly.