(369h) Determine Mixing Effects Onto the Polymeric Microstructure of Ldpe with a Compartmentalized Hybrid-Numerical-Stochastical Model

The free-radical polymerization of ethylene under particular conditions up to 300 °C and 3000 bar leads to low-density polyethylene (LDPE) with a highly branched molecular structure and a broad molecular weight distribution. Affected by temperature, pressure and the chosen reactor type, the molecular microstructure is mainly responsible for the unique processing properties of LDPE. While tubular reactors are most commonly used for higher productivities due to better temperature handling and creating comb like molecular branching structures, autoclave products enable important applications as extrusion coating due to its tree like molecular branching structure. To predict the molecular microstructure of LDPE produced in multi-zone autoclave reactors, a hybrid numerical-stochastical model is developed considering mixing effects in addition to the kinetic network and heat balance. The macro mixing of the autoclave reactor is described by a compartmentalization approach based on the work of G. Wells.¹ Therefore, each reactor zone is divided in multiple segments where the segments are represented by the mass and heat balance of a continuous stirred-tank reactor. The mixing of the reaction mass with the injection plume is implemented by intermediate streams, connecting the segments. In order to describe the infiltration of the injection plume into the main reaction mass, the segment size is increasing from the supply point and the flow rates from the main reaction mass into the segments are correlated to the individual segment volumes to describe the eroding injection plume. The developed model gives a fast insight of the influence of mixing and how the use of different initiators affects the temperature profile and concentration gradients inside the autoclave reactor. The Monte Carlo algorithm enables the possibility to investigate effects of mixing on the molecular microstructure and gives detailed information about the origin and distribution of branches.

Acknowledgement: SCG plc.

[1] G. Wells, H. Ray, Macromolecular Materials and Engineering, 2005, 290(4), 319-346