(88d) The Effect of Reactor Geometry on Polymer Composition Broadening

Pradeep Jain, Ivan Konstantinov, Thomas Karjala and Carlos Villa

The Dow Chemical Company, Freeport, TX 77541

The stochastic nature of polymerization processes results in mixtures of chains that differ from each other in their molecular architecture. Thus, even under constant process conditions across both time and space, one should not expect chains with the same number of repeating units due to the interactions among the various steps of the polymerization mechanism. Other factors also play significant roles, with two of the most important ones being reactor geometry and mixing. Our work focuses on the impact of all these effects on the polymer composition distribution.

Polymer composition is a key quality characteristic. In the production of Linear Low Density Polyethylene (LLDPE), α-olefins such as butene, hexene, and octene introduce small side chains into the ethylene based polymer backbone. These side chains reduce the density and the crystallinity of the final polymer, a desirable characteristic for certain applications. Increasing levels of α-olefins can also lead to products with lower molecular weights, since α-olefins are typically not as reactive as ethylene towards chain growth. The strong dependency of key product properties on the level of incorporation of α-olefins comes at a cost, however, since missing a target by adding too much or too little of these comonomers can lead to products with unexpected properties.

Using polyolefin copolymerization as an example, we rely on computational tools to study the spread in the polymer composition and molecular weight distributions under the influence of the factors mentioned earlier. We focus on process variables such as temperature, relative monomer concentration, as well as catalyst selection, i.e. reactivity ratios, to generate a product with the desired composition distribution. Furthermore, this work uses various geometries and builds on our previous investigation to illustrate these problems and demonstrates that product quality is highly sensitive to what most would consider simple details in polymerization reactor design.