(515c) Molecular Weight Distribution Control of Autoclave Ldpe/EVA Product Using CFD Modeling

In low density polyethylene (LDPE) and ethylene vinyl acetate (EVA) production through high pressure autoclave reactor, mixing dynamics has a great effect on product properties like molecular weight distribution (MWD) and melt index (MI). In commercial reactor, the inside temperature distribution is not uniform due to the difference between the reaction rate and the dispersion rate of the raw materials including the initiator. There is a slight difference between the prediction on ideal and real mixing case in physical properties due to non-uniform temperature distribution [1]. Thus, in increasingly competitive polyolefin market, accurate MWD prediction and control are very important to achieve high productivity with product quality maintained and to make quick decisions to develop highly valued products.

In previous studies, MWD prediction was performed by assuming that each of the autoclave zones is fully mixed like CSTR [2, 3]. For the consideration of the imperfect mixing effect on reactor performance, the simulation by dividing it into modules that calculate networks between flow compartments derived from CFD and modules that calculate reactions in each compartment was performed [4, 5]. These methods are mainly to predict the more detailed MWD because the flow calculation is simple with assumed to be fully mixing or small scale network of CSTRs. Another method is to calculate flow and reaction simultaneously in the CFD code. However, this method could only express a simple property average MW using the method of moment [6]. No study of full scale CFD reactor model has been applied to complex commercial reactors for detailed MWD prediction.

In this study, commercial scale of CFD model was developed to accurately predict the mixing behavior, temperature distribution and the trend of detailed MWD of autoclave LDPE/EVA products. The model performance was validated with commercial reactor products. The characteristic of autoclave LDPE MWD is the formation of a shoulder in the high molecular weight. The analysis of detailed MWD with variation of operating conditions such as pressure and temperature was conducted. Furthermore, the developed model can predict the high molecular weight shoulder, which is a key feature of autoclave LDPE products. To predict this, the method of moment for LCB-class was applied [7]. On the other hand, since EVA does not have an asymmetric shape like LDPE, the method of general moment can be applied.

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