Monday, January 01, 2007
HW #1, 6.54 - Enthalpy, Entropy and Molar Volume of Ideal Gases - 12 pts
Estimate the molar volume, enthalpy and entropy for n-butane as a saturated vapor and as a saturated liquid at 370 K. The enthalpy and entropy are set equal to zero for the ideal gas state at 101.33 kPa and 273.15 K. The vapor pressure of n-butane at 370 K is 1435 kPa.
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7 comments:
Okay, I can figure out the residuals of H,V and S, but don't know how to get the Hliquid and Hvapor. Any suggestions?
Claire 10:24 PM
You could use the Watson and the Reidel Eqns on pages 134 eqn 4.13 & 4.12.
But I would anticipate that you would use Hr and Sr for the sat vap and sat liq from SRK. I think you already did solved for Zvap. Just solve again for the smallest Z (Zliq). F=Fr+F{IG} . Oddly enough, when you add Hr for the LIQUID to H{IG} you get Hsatliq ! The residual takes into account the property change due to vaporization ! So, in principle, you can use SRK to estimate the heat or entropy of vaporization as well !
That makes sense, but the answer I get is no where close to your Hliq = -7.5KJ/mol, When I look up H satliq at 370 K for butane on nist I get 26KJ/mol, which is what I am getting. Does this have something to do with the statement "the enthalpy and entropy are set equal to zero for the ideal gas state at 101.33KPa and 273.15K?
claire 11:48 AM:
Yes. H requires a reference state at which it is arbitrarily set equal to zero. NIST uses some reference state and it is undoubtedly different from the one given in this problem statement.
Here are a few hints I have shared with others offline...
Hr = H{actual} - H{IG}...even for liquids !
So, Hliq = H{IG} + Hr{Zliq} and Hvap = H{IG} + Hr{Zvap}.
H{IG} = INT{Cp dT} from Tref to T
S{IG} = INT{Cp/T dT} from Tref to T - R Ln{P/Pref}
Pref = 1.0133 bar and Tref = 273.15 K
Hi Sir, I would like to ask if you are familiar with this problem (I think this is a book problem) and where can I find the book? I am a 3rd year Chemical Engineering student and this will help me a lot :))
" Estimate the molar enthalpy and molar entropy for a saturated vapor of n-hexan at 330K. The entropy and enthalpy are set to zero for the ideal gas state at 101.33 kPa and 273.15 K. Use the 2nd Virial generalized correlation for residual properties"
Thank you Sir!
A.J. Cruz 6:00 AM
I am in the process of moving and my chemical engineering thermodynamics books are packed. So, I cannot tell you which book this problem came from. I suggest you look in the textbooks by Smith, van Ness and Abbott or the book by Kyle.
You will need an equation for the ideal gas heat capacity, such as the Antoine Eqn, and an equation to express the 2nd Virial Coefficient as a function of temperature: B(T). You can calculate &DeltaS; and &DeltaH; for the ideal gas using the heat capacity. You can look up in a thermo book or online the eqns for the equations you need to calculate the residual S and H. The look like this: Hr/RT= (P/R) *(B/T-dB/dT) and Sr/RT = -(P/R)*dB/dT. The only catch is that you do not know the saturation pressure at 330K. The easy way to go is to look this up. I suggest you use the NIST webbook.
Best of luck !
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