I work with Aspen Plus at my present work and after working with all the present simulators out there at some time or the other, and after doing development work on inhouse simulators, I can quite frankly say that Aspen Plus is the worse I have worked with. Yes it's powerful, and it gets the job done, but it's archaic and requires too much work to match a simulation to plant data. A simple example is mixing aniline with HCL. There is a partial neutralization reaction, with a substantial adiabatic temperature rise. Aspen does not show that. To get that done I have to create Aniline base ions, with heat of formation and then react it with H+ from HCL, plus gives a heat of formation for the ion and use design spec to vary the heat of formation until I get the right heat of reaction. At my job, we now have Chemcad for comparison and Chemcad shows this temperature rise with mixing. No sweat. There are many other examples, where a process simulated with Chemcad, matches the real plant without much work, while you spend days chasing whats wrong with data in Aspen, or the calculation routes and methods its using.

Simply put, Aspen is over rated and overly expensive.