Integration of Design Scales and Its Application to Multi-component Adsorption Systems

Macroscopic simulations of chemical engineering unit processes provide a means of visualizing the relationship between different process variables through the use of constitutive equations such as the mass, energy and momentum balances. Molecular simulations on the other hand provide an insight into processes on the molecular scale. These simulations coupled with various concepts from statistical mechanics, are often used to study the properties of materials under a range of process conditions.

In a natural extension to these two fields, we use molecular simulations to obtain physico-chemical properties required for use in the macroscopic simulations of multi-component adsorption systems. The application requires properties such as transport diffusion coefficients and the adsorption equilibrium properties.

We use equilibrium Molecular Dynamics simulations to study the self-diffusion coefficients of the different components in the system. We use a diatomic model for ethane. We show that the minimum dimension of the diatomic molecule governs the transition from ordinary diffusion to single-file motion. We also show that the methane self-diffusion coefficients decrease with increasing ethane mole fraction in the mixture.

We then use Grand Canonical Monte Carlo simulations to study the adsorption equilibrium properties of the mixture. We show that in the pure component case, ethane molecules are preferentially adsorbed at low loadings and low temperatures, but an increase in either quantity causes methane molecules to be preferred instead. In the binary mixture, methane molecules fail to displace ethane molecules. We also show that an increase in temperature causes a decrease in the ethane selectivity due to a decreased importance of the energetic factors.

We then take resort to the Darken equation to obtain the transport diffusion coefficient. We show that the transport diffusion coefficient decreases with increasing total concentration and methane mole fraction. We also show that the transport diffusion coefficient increases with increasing temperature.

We then use the physico-chemical properties in the macroscopic simulation of the adsorption system. We present the operating conditions viz., temperature, feed velocity and time till regeneration. We thus achieve an integration of design scales that would add another important weapon in the arsenal available for researchers.