Hi, ColourfulFigsnDiags:
All Aspen Plus or Hysys could do for you is to use the UNIFAC group contribution method for estimating the VLE behavior. Unfortunately, UNIFAC is often not very good for large molecules such as decalin that have a large multiplicity of the same main group (in this case the hydro). This is because the mixture VLE data used to build the UNIFAC database contains few such instances. So, you could end up with very inaccurate predictions and not know it.
Generally, I would recommend measuring the mutual solubility experimentally in such situations, fitting temperature-dependent UNIQUAC parameters based on this LLE data, and then estimating the VLE using the same UNIQUAC parameters. If the compounds are completely miscible, you would have to (a) measure at least some VLE data, or (b) obtain the infinite dilution activity coefficients using either differential ebulliometry or gas chromatography. (In this instance I doubt the chromatography idea would work as decalin is very non-volatile). The very much simpler differential ebulliometry technique was developed by Professor Charles Eckerdt over 20 years ago.
Hope this is all clear. A good reference for understanding the theory is "Computer Calculations for Multicomponent Vapor-Liquid Equilibria" by Prausnitz et al (Prentice Hall).
might have to use HYSYS or ASPEN to produce at least a number I guess.
