2 replies to this topic

[Miche.So]

Hi.

 

I've question about Vapor Pressure.

 

I want to get vapor pressure from the stream by using hysys.

And, i found the way how to get result.

 

First of all, the pressure of stream was reduced to operating pressure of the vessel by using valve(adiabatic flash)

then, the stream was used as referenced stream to set "vapor fraction = 0"

So i got a new pressure, that would be vapor pressure at the composition of stream.

 

But i was wondering why "vapor fration = 0"  can be vapor pressure.

Plz tell me the reasons.

 

And there is one more question,

I think, vapor pressure of a stream greater than operating pressure of vessel just mean that the liquid is flashing.

Otherwise the liquid would flash so that the vapour pressure of the new liquid is below the operating pressure.

That means, the vapour pressure of liquid at operating temperature is equal to the operating pressure.

 

Which one is right as a vapor pressure??

 

Thank you for your time in advance.

Best regards.
 

[SPC]

Hi Soyeon,

 

Following is the answer to your technical query:

 

  Your Question : But i was wondering why "vapor fration = 0"  can be vapor pressure.

Plz tell me the reasons.

 

My Reply: Vapor fraction concept is applicable for Liquid, of course ; for vapor pressure to exist, the vapor (gas phase) MUST be in physical contact with the liquid (or solid) it came from. You CAN'T have vapor pressure without two phases being present and in contact!!!!!! In Mixture of Liquid + Vapor , Vapor Fraction = 0 in Hysys means 100% Liquid.

 

Your Question:  I think, vapor pressure of a stream greater than operating pressure of vessel just mean that the liquid is flashing.

Otherwise the liquid would flash so that the vapour pressure of the new liquid is below the operating pressure.

That means, the vapour pressure of liquid at operating temperature is equal to the operating pressure.

 

Which one is right as a vapor pressure??

 

My Reply:  Vapor pressure of liquid at operating temperature is equal to the operating pressure in Vessel as long as both Liquid & Vapour are at one equilibrium state. 

 

Please see also below link for better understanding :

 

https://www.chem.pur...ids/vpress.html

 

http://www.chemteam....orPressure.html

 

Regards,

 

SPC

[serra]

soyeon,

you may start analyzing a phase envelope (or phase diagram),

see for example

"http://www.prode.com...seenvelope.htm"

the bubble line shows the condition where all vapor is condensed (bubble pressure),

normally, for a mixture, speaking of vapor pressure we mean the bubble pressure,

almost all simulators include methods to calculate bubble pressure.