Prediction of the Solubility of Ethylene in Polyethylene Using Molecular Simulations at the Operating Conditions of LDPE Polymerization Process Equipment

Prediction Of The Solubility Of Ethylene In Polyethylene Using Molecular Simulations At The Operating Conditions Of LDPE Polymerization Process Equipment

Sudheer Gondu*, Jhumpa Adhikari**

*Department of Chemical Engineering, IIT Bombay
 (e-mail: *sudheer7g@ iitb.ac.in, ** adhikari@che.iitb.ac.in)

 

Abstract

 

The aim of the present work is to study the fluid phase behavior of ethylene and polyethylene mixtures by employing molecular simulations and theoretical methods such as equation of state (EOS) modeling (perturbed-chain statistical associating fluid theory, PC-SAFT, and SAFT) [1, 2]. Thermodynamic properties (such as vapor-liquid equilibria data) are necessary for the design and optimization of the separators, downstream of the polymerization reactor; in the high pressure process for low density polyethylene (LDPE) production.  The vapor-liquid equilibria (VLE) data reported in literature at the operating conditions of the high pressure separator (HPS) and the low pressure separator (LPS) are limited. In this work, molecular simulations have been employed to generate the necessary data in conjunction with the Nath, Escobedo and de Pablo (NERD) force field [3, 4]. We have used the Hybrid Monte Carlo (HMC) and the Gibbs Ensemble Monte Carlo (GEMC-NPT) techniques [4] to determine the VLE of the ethylene-polyethylene mixtures. First, we have performed the simulations at the conditions reported by Nath et al. [4] and observed that our results are in reasonable agreement with the literature data [4]. This has been followed by molecular simulations at the operating conditions of the HPS and LPS. The solubility data from our simulations have also been compared with that estimated from traditionally employed EOS for polymer systems, viz., PC-SAFT and SAFT. It has been observed that ethylene content in the polymer rich phase increases with pressure at constant temperature at both the HPS and LPS operating conditions; this is consistent with the trend reported by Bogdanovic [5]. In the entire temperature and pressure range of the operating conditions of the separators, the SAFT estimates are higher than the corresponding values determined from our simulations; while PC-SAFT predictions show lower solubilites as compared to molecular simulation predictions. The VLE data estimated by all the three methods have been used in sizing the HPS and LPS. The volume of the HPS computed from molecular simulation generated data is lower than that calculated using either SAFT or PC-SAFT predictions. However, for the LPS, the volume calculated from molecular simulation data is significantly higher than that from SAFT and PC-SAFT.

 

References

 

[1] Gross J., and Sadowski, G., 2001, “Perturbed-Chain SAFT: An Equation of State Based on a Perturbation Theory for Chain Molecules”, Ind. Eng. Chem. Res., 40, 1244-1260.

 

[2] Huang, S.H. and Radosz, M., 1991,“Equation of State for Small, Large, Polydisperse,and Associating Molecules: Extension to Fluid Mixtures”, Ind. Eng. Chem. Res., 30,1994-2005.

 

[3] Nath, S.K., Escobedo, F.A., de Pablo, J.J. and Patramai, I., 1998,” Simulation of Vapor-Liquid Equilibria for Alkane Mixtures”, Ind. Eng. Chem. Res., 37, 3195-3202.

[4] Nath, S.K., Banaszak, B.J. and de Pablo, J. J. “Simulation of Ternary Mixtures of Ethylene, 1-Hexene, and Polyethylene”, Macromolecules, 2001, 34, 7841-7848

[5] Bogdanovic, V., 1985,” Industrial Aspects of Phase Separation In Ethylene-Polyethylene System at Pressures of 15-30 MPa”, Ind. Eng. Chem. Process Des. Dev., 24, 576-581.