Partial 2-phase properties from separate liquid vapor properties

• Dec 4, 2018

• #2

PaoloPemi

Mechanical

Mar 23, 2009

269

IT

for H (enthalpy) you can start from H(mixture) = H(vapor)+H(liquid)
with a similar expression for derivatives, cp = (dH/dT)p

the same for density, Density(mixture) = 1.0 / (Volume(vapor)+Volume(liquid))...

while for thermal conductivity, viscosity etc. you need mere elaborate expressions, see The Properties of Gases and Liquids or similar for details...

 

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• Dec 4, 2018

• Thread starter
• #3

takiyasamsama

Chemical

Feb 10, 2015

108

MY

Hello PaoloPemi,

The H(mixture) = H(Vapor) + H(Liquid) as you mentioned; I got the calculated duty closed to the required duty.
I also tried another method which H(mixture) = [wt. fraction (vapor) x H(vapor)] + [wt. fraction (liquid) x H(liquid)] and I get really close duty to the required duty.

At first I also tried adding up the enthalpy but for different case sometimes I got duty which is way off from the required duty.

I needed to find the enthalpy mixture so that I could find the LMTD for a two-phase for a partial two-phase heat exchanger where one side is sensible heat (single phase) and the other side is latent heat (partial phase) then plotted T-h diagram for the process

 

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• Dec 5, 2018

• #4

PaoloPemi

Mechanical

Mar 23, 2009

269

IT

it depends from the way you solve the flash operations, if you calculate specific enthalpies (for vapor and liquid phases) Ok for H(mixture) = [wt. fraction (vapor) x H(vapor)] + [wt. fraction (liquid) x H(liquid)],
I attach a (very basic) Excel page which calculates temperatures in diferent zones, but there are many similar examples...

 

 https://files.engineering.com/getfile.aspx?folder=9c09bd8b-213c-496a-a54d-182921778dce&file=htcprops.xls

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• Dec 5, 2018

• Thread starter
• #5

takiyasamsama

Chemical

Feb 10, 2015

108

MY

PaoloPemi, you sir are very helpful. Thank you, I'll try your excel sheet and see the result.

Yes, basically (I think) I'm had to look for specific enthalpy because after I find the H(mixture) at specific temperature points, I had to make a cumulative enthalpy and from there multiply each cumulative enthalpy with mass flow to obtain required duty. Afterwards the program plot the T-h diagram with both streams and find its effective LMTD.

 

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