Simulation of Polymer Molecular Weight Distributions in Industrial Reactors

Polyethylene is commonplace in the modern world; thus, manufacturers constantly search for improvements to their production processes. However, because of the exothermic nature of ethylene, process intensification poses both safety and financial risks. This encourages the use of simulation techniques for the improvement of these systems. However, due to their simplifying assumptions, software designed specifically for polymer reactors have limited usefulness. Computational Fluid Dynamics (CFD) is an attractive alternative, because it is much more rigorous. Although, implementing polymerization chemistry into CFD is not a trivial task. The Method of Moments makes this possible but necessarily removes the polymer's Molecular Weight Distribution (MWD) from the simulation. The first few moments of the distribution can be used to approximate the MWD if the shape of the distribution is assumed, but this technique often fails to reproduce the bimodal distributions typical of industrial polymers. To better reconstruct the MWD, the distribution can be divided into various classes based on their degree of branching, each of which have their own MWD. These can then be summed to approximate the overall polymer distribution. This method was implemented into a simplified CFD reactor model and tested using 5 and 10 polymer classes. The number of classes, assumed distribution shape, and summation method all had a significant impact on the subsequent distribution. Ultimately, a multimodal polymer MWD was achieved that was qualitatively like distributions found in the literature. These results need to be compared to experimental data to determine the appropriate combination of classes, assumed shape, and summation methodology for reliable results in future studies.