Molecular modeling of thermophysical properties of polyatomic fluids
Jog, Prasanna Krishna
Chapman, Walter G.
Doctor of Philosophy
The Statistical Associating Fluid Theory (SAFT) equation of state is a statistical mechanics based equation of state based on Wertheim's theory of associating fluids. Since SAFT is a statistical mechanics based equation of state, it is possible to include more physics in the model to make it more predictive. In this work we show the versatility of SAFT by applying it to monomers and polymers and include dipolar interactions in SAFT. SAFT is used to model phase equilibria in long chain-short chain alkane mixtures. It is shown that SAFT accurately models phase equilibria in these systems even when there is a large disparity between the chain lengths of the two alkanes. We then use the statistical associating fluid theory (SAFT) to model liquid-liquid phase equilibria in solutions of linear low density polyethylene (LLDPE) with hexane, heptane and octane. The effect of temperature, pressure, polymer concentration and polymer molecular weight on phase separation is studied. Finally, the effect of polydispersity on cloud point is also considered. SAFT results are compared with experimental data by de Loos et al. . It is shown that SAFT can model the phase behavior of the polymer in different solvents at various state conditions with a single adjustable parameter. The applicability of SAFT to these monomers and polymers shows that it is a versatile thermodynamic model. We propose an algorithm for calculating phase equilibria of polydisperse polymer systems using the SAFT equation of state. The algorithm is formally exact and the computation time is independent of the number of pseudo-components used to represent the polymer molecular weight distribution. The algorithm makes use of the form of the SAFT equation of state and simplifications resulting from it to simplify the flash calculations. Distinctive features of the phase diagrams of polydisperse systems are illustrated by calculating the cloud point and shadow point curves of polyethylene solutions in ethylene using the algorithm proposed in this work. We present results from molecular simulation and statistical mechanics based theory for dipolar hard sphere chains and dipolar Lennard-Jones (LJ) chains. We consider the chains with dipoles on alternate segments. The equation of state is obtained by applying Wertheim's associating fluid theory in the total bonding limit to a mixture of non-polar and dipolar segments. It is shown that the theory developed here is in very good agreement with computer simulation. This theory simultaneously accounts for chain and dipolar effects and can handle multiple dipolar sites in a molecule. This is demonstrated by generalizing the theory to mixtures and applying it to dipolar monomers and copolymers. The theory accurately predicts the phase behavior of real dipolar fluids.
Polymer chemistry; Chemical engineering