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This is the basic question.

When we want to get the equilibrium data from simulation.

For example..

As for the mixture of water and methanol at 100 0C and 2 barg.

We made a pipe and isothermal flash diagram in simulation and put those parameter, pressure and temperature.

And run simulation and get the vapor and liquid equilibrim composition, temperature and pressure.

But, I think, when the mixture get into the flash drum, they would expand and vaporize so the pressure of the system would be less than 2 barg.

How can we say that the equilibrim date we get form that simulation is at the condition of 100 0C and 2 barg.

[Chellani]

I believe your aim is to get VLE data i.e. T-xy / P-xy curve using Hysys, if I am right read below lines otherwise just let me know that I am wrong.
Just define a stream with abrupt composition (of two components as we are talking about VLE of binary system only). Provide temperature and vapor fraction as inputs which would estimate pressure and strema would be converged. Go to composition page and you will see mole fraction of each component for liquid and vapor phase. Liquid phase mole fraction is x and mole fraction of vapor phase is y. Change the mole fraction so that new P and x,y are estimated. Using this trick u can vary x from 0 to 1 and there you have your isothermal P-xy curve.
Rather than just looking at the stream, if you connect this stream to vessel and have a look at the composition of liquid and vapor streams; sam results would be obtained.

Aspen plus is much better compared to Hysys for this. Using Aspen Plus you can simply plot T-xy / P-xy plot directly.