Equation-Oriented Approach for Handling the Perturbed-Chain SAFT Equation of State in Simulation and Optimization of Polymerization Processes

Abstract

The perturbed-chain SAFT (PC-SAFT) equation of state (EOS) is a very popular and promising model for fluids. Resolution of the PC-SAFT EOS is generally conducted by subroutine-based calculations that involve logical conditions. This approach can lead to nonsmoothness and convergence issues when used with gradient-based solvers in an equation-oriented (EO) framework. In this paper, we propose a novel EO approach for the PC-SAFT EOS embedded in large flowsheet optimization. First, the mathematical structure of the PC-SAFT EOS is analyzed through a digraph. It reveals that for polymerization systems, resolution of the PC-SAFT EOS is coupled not only by phase equilibrium but also by the polymerization kinetics. This feature strongly motivates implementation of both EOS resolution and process optimization in a complete EO framework. A novel EO approach is proposed for selecting the appropriate root of the PC-SAFT EOS by incorporating thermal stability criteria to eliminate the undesirable roots. Numerical exam...