Introduction

Distillation is by far the most predominant separation technique used in the chemical process industry. Distillation is a cascade of equilibrium stages at which the vapor mixture is in equilibrium with the liquid mixture. An in-depth understanding of vapor-liquid equilibrium (VLE) is therefore essential to the understanding and design of distillation processes.

VLE is taught in the chemical engineering curriculum at various levels. Generally a simple Raoult's law approach is taught early in material and energy balance classes. More rigorous equations describing the equilibrium of the vapor and liquid in terms of the equality of chemical potentials of the components are discussed later in thermodynamics or separations classes. Activity coefficients and nonideal solution behavior are introduced at this level and the student focuses on flash, bubble-point, and dew-point calculations. Sometimes lost in the mathematics of this approach is the molecular underpinnings of the equilibrium between the two phases, the relationship of the observed equilibrium phase compositions to the molecular nature of the fluids, and the molecular origins of fluid nonidealities.

The purpose of this simulation module is to provide a molecular visualization of the equilibrium vapor and liquid phases corresponding to real binary mixtures. The intent is that students at any of the levels in their curriculum can get a better feel for the relationship between the interactions between molecules and the resultant extent of the phases (mass balances) and the equilibrium compositions (equality of component chemical potentials). The simulations are designed to mimic real systems but in a two-dimensional view. Molecular dynamics (MD) simulations are used, but the equations of motion have been modified to include a strong gravitational force to quickly separate the liquid and vapor phases. Additionally, empirical molecular forces are included to drive the system toward the experimentally observed equilibrium values. These adjustments to the equations of motion permit visualization in two dimensions and at short times and are not intended to be rigorous MD simulations. Rather, the simulations in this module allow the user to explore the behavior of real systems and observe on a molecular level the relationship of macroscopic variables to the molecular nature of the system.