Wednesday, March 21, 2007

HW #2, P7- An Application of Equations of State - 8 pts

Please post any questions or discussion related to this problem as comments on this message. Feel free to answer other students' questions. I will check the blog M-F and once on the weekend.
Dr. B

16 comments:

Anonymous said...

I believe molecular weight of R-134a is needed here. If so, can i get the value for it?

Dr. B said...

jtl610:
Yes, the MW = 102.03
I got this from the NIST Webbook !

Anonymous said...

I was thinking the same thing. 84.04 g/mol according to Wikipedia.

Dr. B said...

clueless:
let's assume NIST is more reliable.
It seems odd that Wikipedia would have something so far from correct.
Hmmm.

Anonymous said...

Dr. B, is R-134a not C2H3F3, 1,1,2-Trifluoroethane?

This is what I found on Webbook...
*snip*
1,1,2-Trifluoroethane
* Formula: C2H3F3
* Molecular weight: 84.04
*snip*

or the webpage itself:
http://webbook.nist.gov/cgi/cbook.cgi?ID=C430660&Units=SI

Of course if my user name is any indication, that might explain the discrepancy...

Anonymous said...

oops... nevermind, I have the 3 and 4 transposed!

Dr. B said...

clueless:
Nope. R134a is 1,1,1,2-tetrafluoro-ethane. You can see the properties at:
http://webbook.nist.gov/cgi/cbook.cgi?ID=C811972

SushiEater said...

Hi, for the compressibility factor EOS, I was wondering if I can solve for P by setting up
(P*Vmolar)/(R*T) = Vmolar/Vmolar(of ideal gas) and Vmolar(of ideal gas) will equal to 22.415L/mole * mass/MW.
Also, I don't know how to solve for P using the table because I couldn't find 110 degreeC in the saturation temperature table for R-134a. Finally, what is the critical P and V for R-134a?

Anonymous said...

where do you find omega for R-134a for the SRK EOS?

Anonymous said...

omega is .32684

you can double check this at this page:
http://webbook.nist.gov/cgi/fluid.cgi?TLow=200&THigh=374&TInc=5&ID=C811972&Action=Load&Type=SatP&TUnit=K&PUnit=atm&DUnit=mol%2Fl&HUnit=kJ%2Fkg&WUnit=m%2Fs&VisUnit=uPa*s&STUnit=N%2Fm&RefState=DEF

or if you can't use that long url, just create any old saturation table for R-134a and that information is given near the bottom of the page under the Auxiliary Data section. Look for "Acentric factor".

Anonymous said...

Looks like the full URL didn't come through so trust the value or create another table. :-)

Anonymous said...

I can't figure out how to do ANY of these. I can't find any of the values (Tc, Pc, 110 deg C in r-134-a tables, omega r-134-a) for these in Thermo CD.

Dr. B said...

Sushi:
Yes. I think of this as
PVwiggle = Z R T
Your eqn is correct because Z = Vwigggle / VwiggleIG
But, VwiggleIG = RT/P which is usually NOT = 22.415.
The 22.415 value only applies at standard T & P.

Because T > Tc, the properties of the R-134a in the tank must be obtained from the superheated vapor table, even though the water is actually a supercritical fluid in this system.

Generally, the LAST entry in the Sat Temp and Sat Press tables are the critical point for that substance. NIST says that:
Tc = 101.06 Cand Pc) = 4.05928 MPa
This is also given in the Sat Temp table on page 303. I seem to have left off the critical point i the Sat Press table.

Vcrit is normally not tabulated or really needed. You need VcritIG = RTc/Pc, which you can calculate.

Dr. B said...

kramer:
The best source is the NIST Webbook.
I found omega = 0.32684

Dr. B said...

neuman:
Good work !

Dr. B said...

Anon:
See my last 2 comments. When in doubt look in the NIST Webbook.