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[linda_pro]

Hi all,
I am doing control valve sizing for liquid and pump sizing and i have a bit confusing with the vapour pressure of liquid.

My question is how do you get this value from hysys simulation?

normally, I will do a copy the liquid stream, then keep the Temperature, change the vapour fraction to 1---> then i get the pressure (it is the vapour pressure)

But my colleague said it is not correct, he said vapour pressure is the pressure when appear the vapour in the liquid stream. it means, the vapour fraction can be 0.001% or something..

May I have your advice?

Thank you.

[Zauberberg]

Your colleague is confusing you, as he/she must be confused as well. He (or she) refers to the initial boiling point (for component mixture), or simply the boiling point (for a single component), when first bubbles of vapor phase start to form as the temperature is increased, or pressure reduced.

By the definition, vapor pressure is "the pressure of a vapor in thermodynamic equilibrium with its condensed phases in a closed system". Therefore, vapor pressure exists at all temperatures, and furthermore it changes by the temperature.

Calculation/estimation of vapor pressure in Hysys can be done in the way you are proposing: but by setting the vapor fraction as ZERO, not 1.

Edited by Zauberberg, 28 September 2010 - 07:46 AM.

[Propacket]

For single component system, it does not matter whether you specify 1 or 0 vapor fraction.
However, for multi-component system, it does matter a lot. First of all, try to understand what does it mean by specifying 1 or 0 vapor fraction. Today , i was working with a condensate obtained from a three phase separator. Composition of the condensate is as follows:

Comp Mole Frac (Methane) 0.1575
Comp Mole Frac (Ethane) 0.0980
Comp Mole Frac (Propane) 0.1261
Comp Mole Frac (i-Butane) 0.1447
Comp Mole Frac (n-Butane) 0.1897
Comp Mole Frac (i-Pentane) 0.1204
Comp Mole Frac (n-Pentane) 0.0764
Comp Mole Frac (n-Hexane) 0.0694
Comp Mole Frac (n-Heptane) 0.0141
Comp Mole Frac (n-Octane) 0.0016
Comp Mole Frac (H2O) 0.0020

At 14.7 psia, when you specify 1 vapor fraction, Hysys gives 77.17 F temperature. And when you specify 0 vapor fraction,Hysys gives -222.8 F. What does it mean?

As far as i know, when you specify 0 vapor fraction, Hysys will give you the temperature at which lightest component(s) of the mixture will boil (Final Boiling Point). When you specify 1 vapor fraction, Hysys will give you the temperature at which heaviest component(s) of the mixture will boil (Initial Boiling Point). Clearly, you must enter 0 vapor fraction to estimate vapor pressure.

Hope it Helps

Edited by P.Engr, 28 September 2010 - 01:17 PM.

[Zauberberg]

One also needs to take care about phase envelope of the fluid, as he could find himself in such area where this value (e.g. bubble point pressure at certain temperature) does not exist.

For liquid hydrocarbon application this shouldn't be an issue as you normally operate well below critical conditions.

[linda_pro]

okie, I understood. I will do as the same your advice.

Thank all for your answers.

Linda

Edited by linda_pro, 28 September 2010 - 08:15 PM.