(671a) Detailed Modeling of Ldpe Autoclave Reactors

Detailed models of industrial Low-density polyethylene (LDPE) autoclave reactors have been built by integrating several advanced modeling concepts:

1. A thermodynamic model based on the Statistical Association Fluid Theory (SAFT) γ-Mie Equation of State, which represents molecules as chains of distinct functional groups [1]. Self-interactions of a specific functional group (i.e. interactions with identical functional groups) and interactions with other groups are characterized by their own Mie-potentials. This approach is ideally suitable for the modeling of polymers.
2. Kinetic modeling in perfectly stirred reactor zones that considers elementary reaction steps in chemically initiated free radical polymerization [2] and solves for the evolution of the full molecular weight distribution (MWD) using the fixed pivot technique (PFT) [3].
3. A multi-zonal approach [4] to represent the complex flow pattern in industrial-size autoclave reactors. The reactor is represented as a network of perfectly stirred reactors, and the flows among the various reactors are calculated by processing the results of a “cold-flow” Computational Fluid Dynamics (CFD) simulation of the reactor.

The integrated reactor model is built on Siemens’ gPROMS® advanced process and materials modeling platform and Siemens’ Hybrid Multizonal gPROMS-CFD tool for automatic processing of results of CFD simulations performed with Siemens Star-CCM® or ANSYS Fluent®.

Implementation on the gPROMS platform allows us to take advantage of standard features such as (i) parameter estimation to adjust kinetic parameters to match plant data, and (ii) optimization to find operational set points that achieve desired product properties, such as Melt Index and resin density.

References

1. V. Papaioannou, T. Lafitte, C. Avendaño, C.S. Adjiman, G. Jackson, J. Chem. Phys. 140 (2014), 054107.
2. M. Busch, Macromol. Theory Simul. 10 (2001), 262-274.
3. S. Kumar, D. Ramkrishna, Chem. Eng. Sci. 51 (1996), 1311.
4. F. Bezzo, S. Macchietto, C.C. Pantelides, AIChE J. 49 (2003), 2133-2148