At the Institute for Thermodynamics and Thermal Process Engineering (ITT) about 100 molecular models for pure substances have been developed so far. These models reproduce vapor pressure, saturated liquid density, and enthalpy of vaporization with a technical accuracy of less than 5%, less than 1%, and less than 5% deviation compared to experimental data, respectively. Simulation results are outlined for the pure substances ethylene oxide and ammonia, demonstrating the outstanding predictive power of the applied methods. Secondly, nucleation processes are discussed. Underlying effects on are studied on the molecular level.
Deep Dive into Molecular modeling and simulation of thermophysical properties: Application to pure substances and mixtures.
At the Institute for Thermodynamics and Thermal Process Engineering (ITT) about 100 molecular models for pure substances have been developed so far. These models reproduce vapor pressure, saturated liquid density, and enthalpy of vaporization with a technical accuracy of less than 5%, less than 1%, and less than 5% deviation compared to experimental data, respectively. Simulation results are outlined for the pure substances ethylene oxide and ammonia, demonstrating the outstanding predictive power of the applied methods. Secondly, nucleation processes are discussed. Underlying effects on are studied on the molecular level.
For development of new processes and optimization of existing plants, the knowledge of reliable thermophysical data is crucial. Classical methods, next to the often expensive and time consuming experimental determination, are based on models of Gibbs excess enthalpy (G E -models) or equations of state. These phenomenological approaches usually include a large number of adjustable parameters to describe the real behavior quantitatively correct. As in most cases these parameters have no physical meaning, their methodic deduction is rarely possible and a fit to extensive experimental data is unavoidable. Furthermore, extrapolations may be too unreliable for engineering applictions.
Molecular modeling and simulation offers another approach to thermophysical properties on a much better physical basis. First simulations of this type were done by Metropolis et al. in 1953 [1]. By solely defining the interactions between molecules, it is possible to calculate the relevant thermophysical properties with statistical methods. There are two basic techniques which are fundamentally different: molecular dynamics (MD) and Monte Carlo (MC). The goal of both approaches is to sample the phase space, i.e. all available positions and orientations of the molecules under the given boundaries, and an estimation of derivatives of the partition function. Macroscopic properties like pressure or enthalpy are calculated with statistical thermodynamic methods. Detailed descriptions are given, e.g. by Allen and Tildesley [2].
In the present project, the molecular interactions are described by effective pair potentials. These potentials distinguish dispersive, repulsive, and electrostatic interactions as well as interactions resulting from hydrogen bonding. The dispersive and repulsive part is approximated by Lennard-Jones sites due to their computational efficiency. Electrostatics is modeled by partial charges, ideal point dipoles and ideal point quadrupoles. For hydrogen bonding good results were obtained using eccentric partial charges.
Due to a consistent modeling of the pure substances, an application to mixtures is straightforward. Mixed electrostatic potentials are given directly by the underlying laws of electrostatics. The parameters of the unlike Lennard-Jones interactions are taken from combining rules, where good results are obtained with the Lorentz-Berthelot rule [3,4]. If needed, one adjustable state-independent parameter can be introduced to fit the binary simulation results to experimental data. No further parameters were to be used for the description of ternary or higher mixtures.
In the present work, different applications of molecular modeling and simulation are presented. Within the project MMSTP, two new molecular models for ethylene oxide and ammonia were developed and subsequently used for prediction of different pure substance properties. Next to this, molecular modeling and simulation was applied on the prediction of nucleation processes. Furthermore, within the present project a comprehensive study on the determination of vapor-liquid equilibria of binary and ternary mixtures is performed. This is a still ongoing task and thus is not included in the present report.
Results of this work are consistently published in peer-reviewed international journals. The following publications contribute to the present project: The present status report briefly shows results of the pure substances ethylene oxide and ammonia, demonstrating the outstanding predictive power of the applied methods. Secondly, the application to nucleation processes is presented. Next to a pure description, the authors were able to further study the underlying effects on the molecular level and to deduce a modification to classical nucleation theory. Futher details are given in the references mentioned above.
The predictive and extrapolative power of molecular modeling and simulation is of particular interest for industrial applications, where a broad variety of properties is needed but often not available. To discuss this issue, the Industrial Simulation Collective [5] has organized the Fourth Industrial Fluid Properties Simulation Challenge as an international contest. The task in 2007 was to calculate for ethylene oxide on the basis of a single molecular model a total of 17 different properties from three categories. It should be noted that our contribution was awarded with the first price [5].
Ethylene oxide (C 2 H 4 O) is a widely used intermediate in the chemical industry. In 2006, 18 million metric tons were produced mostly by direct oxidation of ethylene, over 75 % of which were used for ethylene glycols production. Despite its technical and economical importance, experimental data on thermophysical properties of ethylene oxide are rare, apart from basic properties at standard conditions [6]. This lack of data is mainly due to the hazardous nature of ethylene oxide. It is highly flammable, reactive, explosive at elevated temperatur
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