please do not post on multiple forums...
(probably) you need to select a suitable model (and verify the presence of correct parameters),
there are many versions of Aspen including different packages and I am not sure which package to recommend,
I have a different software (Prode Properties) for similar mixtures I would consider the CPA electrolyte or similar (complex) thermodynamic models (there are several available in Prode),
with Aspen you may test the E-NRTL or a EOS with complex mixing rules (depending from mixture and operating conditions)
Edited by PaoloPemi, 09 April 2019 - 05:26 AM.