Bhautesh,
Please excuse my interruption into this dialog. I am a "Johny come lately" here and am a recent member of CheResources.com. I know nothing about Aspen HYSYS other than what I just quickly observed browsing in the Internet.
Having worked with some dynamic simulation programs (ACSL, in particular) in the past going back to the mid '60s and having written many of my own routines in Basic, Fortran and for the past 35 years in Mathematica and having seen major improvements in simulation confidence as computers have improved. While working in Mathematica, I am privileged to be able to dictate the computational accuracy whether it be 20 digits, 200, or 2000 digits. I am not limited to machine accuracy issues
For your issue, I am making an assumption that Aspen HYSYS is using a finite differences algorithm. This is common. Finite difference solutions sometimes have a personality problem in that it will not allow computational conversion because:
1. the step size is too large
2. the forcing function is too large
3. the desired computational accuracy is too small
4. because of limits in the computer computational accuracy. If the error is lost in the weeds of computing accuracy, you will absolutely never get convergence.
5. something is grossly in error with the input data. I have had the dubiously personal distinction of having created my own problems. On many occasions I have verified the expression of GIGO, Garbage-In-Garbage-Out. One must be particularly mindful that the input is accurate and that the units are compatible. Another insight gained was that I was calculating garbage with great accuracy. Unfortunately, one can be deceived in to believing that accuracy is guaranteed just because it was calculation was done on a computer.
There is another possible issue that I have run into and that is that the computational algorithm may be written with first order approximations to the fundamental differential equations. Your parameters may require a higher order approximation of the differential equations. I have definitely run into that. I have learned to write all my routines with 2nd or 3rd order approximations.
If you were to study and understand the mathematical dynamics of finite differences, you would find that there are mathematical tests such as the Von Neuman stability method to determine the maximum allowable step size for a given forcing function that will allow for efficient conversion.
I seriously recommend that the computational algorithms used in the Aspen HYSYS software be fully understood otherwise, you are just blindly using the software package with total ignorance and having no understanding of how the results are computed. That is scary.
I suggest that you generate a closed-form solution to a simplified text book problem and compare your answer to the software package output if only to generate confidence with that particular software package.
Please excuse my professorial response, I hate to see somebody flounder around in circles in frustration.
Good Luck,
Luke1939