A comprehensive overview of multi-scale methods is presented with a focus on the ways in which information is communicated between scales. Liquid density computations, which are important in phase equilibrium of carbon storage, are used to illustrate multi-scale process engineering ideas. It is shown that using the Gibbs-Helmholtz equation to constrain the energy parameter in cubic equations of state leads to (1) a natural bridge between bulk and molecular length scales, (2) a new Gibbs-Helmholtz constrained equation of state, and (3) a novel mixing rule, and that the GHC equation can be coupled to NTP molecular simulations to provide more accurate predictions of density for use in phase equilibrium computations and reservoir simulations. Improvements for the next generation of multi-scale tools for analysis, visualization, simulation, and optimization of complex processes including the need to proceed with partial knowledge, multiplicity at various length scales, problems with many scales, and methods for predicting behavior over very long time and length scales are also discussed. (C) 2009 Elsevier Ltd. All rights reserved.