Setting up a flash tank?

Question

Basically have to do an alternate case study for our propylene plant......

We have about 200 lb/hr accumulation of methane and decided to see the cost effectivness of flashing it off before teh recycle loop.

Not sure how to determine certain variables...i'm using hysys.

our stream is entering the flash drum at 485 psi and 15 F.

Hysys is asking we input the inlet pressure and vapor pressure.

Not sure waht to do really. Drop the pressure before hand possibly??? Lowest temp we can cool our strema too is -40 F.

No matter what we do it says we have a negative absolute pressure. and it gives us a "can't determine delta P" message.

Even if you don't hysys, if you know how to determine the proper conditions, that would be helpful

thanks

Answer 1

I am not familiar with Methane, but for any flashing liquid (like steam condensate or a refrigerant) you also need to know the pressure to be maintained in the flash chamber in order to determine the percentage (or fraction by weight) of flash of the entering fluid. You have stated what the entering fluid temp and press are.
This fraction can be determined from a Mollier Diagram for steam. The Keenan and Keyes Steam Tables can also be used.
Are there a similar diagram or tables for Methane ?
Make sure you are using absolute pressures (gage +15). Also is the "hysys" program set up for F or Absolute temp
(F + 459) ?