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Partial 2-phase properties from separate liquid vapor properties

Partial 2-phase properties from separate liquid vapor properties

Partial 2-phase properties from separate liquid vapor properties

(OP)
I need to find the properties of a stream which is in two-phase. I'm only given separate properties i.e. liquid and vapor properties. I need to find the enthalpy for the two-phase mixture from the liquid and vapor enthalpy. See the table below:



I would really like to know how do I find mixture properties i.e. density, thermal conductivity, specific heat, viscosity & enthalpy

RE: Partial 2-phase properties from separate liquid vapor properties

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...

RE: Partial 2-phase properties from separate liquid vapor properties

(OP)
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

RE: Partial 2-phase properties from separate liquid vapor properties

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...

RE: Partial 2-phase properties from separate liquid vapor properties

(OP)
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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