(349c) Combining Molecular and Chemical Process Simulation for Product Design
AIChE Annual Meeting
2006
2006 Annual Meeting
Product Design
Integrated Product and Process Design
Wednesday, November 15, 2006 - 9:20am to 9:45am
When new molecules are synthesized, their target properties may be well known but their engineering properties may be completely unknown. For example, a small molecule drug candidate may be known to have high biological activity, but measuring its vapor pressure, bulk density, viscosity, and thermal conductivity would require gram quantities. Even the melting temperatures measured by millions of freshman chemistry students require purified quantities near 0.01g. Developing those quantities with that purity can be overwhelming for the large number of trial products that may be encountered in the early stages of development.
The Step Potential Equilibria And Discontinuous Molecular Dynamics (SPEADMD) model provides a basis for molecular modeling of thermodynamic and transport properties. It is based on Discontinuous Molecular Dynamics (DMD) and second order Thermodynamic Perturbation Theory (TPT). DMD simulation is applied to the repulsive part of the potential, complete with molecular details like interpenetration of the interaction sites, 110°
bond angles, branching, and rings.
Unfortunately, there is no theory comparable to TPT when treating transport properties.
5 Most theories of transport properties rely on empirical variations of correlations for spherical reference fluids. Furthermore, existing correlations are typically specific to a given range of conditions: gas, dense gas, or liquid, for example. To overcome this situation, we must leverage the dynamics from the reference fluid simulations while accurately correlating and predicting experimental data. We show how to achieve this combination of rigorous fundamentals and empirical accuracy and compare to the accuracy of existing engineering correlations for diffusivity, viscosity, and thermal conductivity.Once the engineering properties have been characterized, the results must be communicated to the chemical process simulator. This can be achieved by characterizing the results from the simulations in terms of standard forms with compound specific constants. This procedure is illustrated for the PVT properties in terms of the equation of state, and for the transport properties in terms of standard forms inspired by Enskog theory, but not limited to spherical reference fluids. Minor modifications to the data structures within the process simulator are sufficient to accommodate the new standard forms. The result is a complete bridge from the molecular scale properties to the plant scale, from nanometers to kilometers.
Keywords: Physical properties, molecular dynamics simulation, vapor pressure, density, phase equilibria, thermodynamic perturbation theory, diffusivity, shear viscosity, and thermal conductivity.
(1) Cui, J.; Elliott Jr., J. R. Phase Diagrams for Multi-Step Potential Models of n-Alkanes by Discontinuous Molecular Dynamics/Thermodynamic Perturbation Theory.J. Chem. Phys. 2002, 116, 8625.
(2) Unlu, O.; Gray, N. H.; Gerek, Z. N.; Elliott, J. R. Transferable Step Potentials for the Straight Chain Alkanes, Alkenes, Alkynes, Ethers, and Alcohols.Ind. Eng. Chem. Res. 2004, 43, 1788-1793.
(3) Baskaya, F. S.; Gray, N. H.; Gerek, Z. N.; Elliott, J. R. Transferable Step Potentials for Amines, Amides, Acetates, and Ketones.Fluid Phase Eq. 2005, 236, 42-52.
(4) Gray, N. H.; Gerek, Z. N.; Elliott, J. R. Molecular Modeling of Isomer Effects in Naphthenic and Aromatic Hydrocarbons.Fluid Phase Eq. 2005, Vol 228-229C, 147-153.
(5) Alder, B. J.; Alley, W. E.; Rigby, M. Correction to the Van Der Waals Model for Mixtures and for the Diffusion Coefficient.Physica 1974, 73, 143-155.