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Effect of Partial Charge Parameterization on the Phase Behavior of Dimethyl Ether

MaryBeth Helen Ketko, Chemical engineering and materials science, Wayne State University, 5050 Anthony Wayne Dr., Detroit, MI 48202 and Jeffrey J. Potoff, Department of Chemical Engineering, Wayne State University, 5050 Anthony Wayne Dr, Detroit, MI 48202.

Within the petrochemical industry, the conversion of methanol, from coal, to a gasoline additive or as fuel itself has sparked interest. However, the high toxicity and relatively low energy content of methanol have prompted interest in the conversion of methanol to dimethyl ether (DME) for use in automotive fuels. As part of this task, accurate vapor-liquid equilibrium data are required for neat DME as well as mixtures of DME with hydrocarbons and CO2. In this work, the effect of partial charge distribution on the predicted vapor-liquid equilibira of neat DME and DME+CO2 mixtures is investigated. Starting with parameters given by the Transferable Potentials for Phase Equilibria (TraPPE), new partial charge distributions are derived with guidance from a CHELPG analysis performed on ab initio calculations performed at the HF/6-31+g(d,p) level of theory and basis set. Lennard-Jones parameters are re-optimized such that the deviation of simulation from experimental saturated liquid densities and vapor pressures is minimized. Histogram-reweighting Monte Carlo simulations in the grand canonical ensemble are used to determine the vapor-liquid coexistence curve, critical properties and normal boiling point for DME for a series of partial charge distributions. Similar calculations are used to determine the pressure-composition behavior of DME+CO2 from 319 K to 360 K. Overall, the results of simulation for our reparameterized force fields are in close agreement with experiment for both the pure compoment and mixture calculations.