GOALI: Combining Discontinuous Molecular Dynamics and Chemical Process Simulation

University Of Akron, Akron OH

Investigators

Abstract

ABSTRACT CTS-0075883 J. R Elliott In this two-year exploratory grant prospects will be evaluated for integrating molecular modeling tools with a chemical process simulation package to provide a complete, rigorous, and accurate framework for physical property prediction and correlation. Chemical process simulators are becoming the primary interface for industry chemical knowledge. It has been recently discovered that perturbation theory is much more accurate than previously appreciated, especially for polyatomic molecules. Discontinuous molecular dynamics (DMD) simulation combines with perturbation theory and virial expansion to provide a basis for highly leveraged computational effort in all aspects of molecular modeling. An existing DMD program will run the minimal number of simulations necessary with reasonable speed on low-cost microprocessors. The resulting segmental potential models will act as molecular scale group contributions, analogous to conventional engineering group contribution models. Although these segmental potentials act essentially as first group contributions, higher order effects derived from molecular geometry will be explicitly addressed through the connectivity of the first order groups in the user-designated molecular structure. The initial goal will be to test the transferability of discontinuous potential models for the equilibrium properties of pure fluids and mixtures. A database of fluctuation populations for spheres, dimers, trimers, 6mers, 8mers, and benzene will be developed and be used to develop optimal step potentials for representing experimental values for vapor pressures, density, and internal energy for methane, ethane, n-hexane, n-octane, benzene, water, methanol and ethanol. The transferability of the resulting potentials will be evaluated in terms of the accuracy of predictions for n-butane, n-pentane, n-heptane, n-nonane, and n-decane. For mixtures, the transferability will be evaluated in terms of the accuracy of predictions for ternary mixtures and mixtures like n-propanol+methanol, for which the hydroxyl group in n-propanol was specifically regressed. The second goal will be to implement methods of calculating transport properties like viscosity, thermal conductivity, and diffusivity in addition to the equilibrium and coexistence properties that have been computed in the past. Resulting simulations will probe the extent to which potential models developed solely on the basis of equilibrium properties can be applied to estimating transport properties. It is sought to "dissect" model intermolecular potential functions in the sense of identifying which pieces of the potential correlate most strongly with specific physical properties. Also sought will be broadly applicable mappings of attractive effects on transport properties given DMD simulated properties for reference fluids, by analogy to the perturbation perspective for equilibrium properties. For example, the extent to which the diffusivity of n-decane can be predicted from correlated results for the diffusivity of n-octane and DMD simulations for purely repulsive n-decane will be examined. The work will be performed with collaboration between The University of Akron and ChemStations, Inc. If this exploratory work shows promise, the ultimate goal will be an internet site which clients can access for zero cost up front and relatively low hourly fees varying according to the intensity of the server side computation requested. Services provided will include molecular modeling of transport and equilibrium properties like vapor pressure, activity, water solubility, octanol partition coefficients, viscosity, and the ability to infer knowledge about one property from measurements of other properties through a common molecular model. Within the range of options will be a comprehensive collection of semi-empirical methods with estimates of the accuracy of each property. The scope of this project includes a thorough evaluation of the accuracy all the semi-empirical models and the molecular based models against a database of approximately 1300 compounds. Those evaluations will comprise a significant portion of the work beyond the exploratory phase. Also included in the web-accessible version will be flowsheeting and process simulation based on shortcut unit operation models. The sensitivity of the process capital and production costs to the estimated physical properties will be a menu option. The shortcut model will serve as a precursor for rigorous process simulations directly within a web-based environment.