Detailed modeling of polymerization process requires the ability to model complex mixing patterns within the reactors along with the detailed chemical kinetics involved in the process. For accurate prediction of molecular weight distribution and the polydispersity of LDPE, PDF transport modeling is required. To account for detailed chemical reactions, engineers typically employ the network of zones. This modeling approach has been employed in simulating chemical engineering problems [1].

In this method different equations of a physical problem are solved on separate gird sizes determined by the nature of the equations. This modeling technique integrates the detailed mixing patterns of computational fluid dynamics (CFD) along with the detailed reaction mechanism within a single software environment. The advantage of this technique is its ability to model industrial scale complex problems with significant reduction in CPU time.

The multi-zonal approach provides the ability to simulate multiple reactor operating conditions and stability analysis accurately within reasonable turn around time.

In this paper the multi-zonal approach is applied for modeling tubular and autoclave LDPE reactors. Polymerization process is modeled with six step reactions: initiator decomposition, chain initiation, propogation, chain transfer to monomer, disproportionate termination and combination termination.

Results in terms of accuracy and CPU time are compared with those obtained from single fine grid simulation, and previously published work [2]. All the simulations are performed using commercial CFD software ANSYS-FLUENT.

Keywords

LDPE, multi-zonal, mixing, PDF transport

[1] Bezzo, F., Macchietto, S. and Pantelides, C.C., “Computational Issues in Hybrid Multizonal/Computational Fluid Dynamics Models”, AIChe Journal. 51(4):1169, 2005

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[2] Tsai, K. and Fox, R.O., “PDF Modeling of Turbulent-Mixing Effects on Initiator Efficiency in a Tubular LDPE Reactor”, AIChe Journal, 42(10):2926,1996.

[3] Fred, Z. Y., Ali, L., Simant, R. U., and Ramdhane, D., “Modeling, Simulation and Optimal Control of Ethylene Polymerization in Non-Isothermal, High-Pressure Tubular Reactors”, International Journal of Chemical Reactor Engineering, 2:A16, 2004.

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