Dear friend ARAD,

first of all you must be more specific about your problem.

For strong mineral acid Electr-NRTL property method is the good choice.

I guess you have to concetrate a diluite HNO3 solution (about 60% w/w/) to 99%, using H2SO4 (96 - 98% w/w).

Are you using live steam injection (superheated steam at 300°C) at the bottom of your column? Or reboiler plus steam injection?

Normally for each kg of diluite nitric acid solution you must use 4 - 5 kg of concentrated sulfuric acid at the top of your column, operating near atmosferic pressure.

Let me know.

If you have your Aspen Plus file send me it, in order I can review your assumptions.

Regards
Superandy