Help defining new mixture using python wrappers

[mina19]

Hello, I am not able to find any examples on defining a new mixture using REFPROP python wrappers.
If I have a fluid composition that is 98% ethane, 1.5 % methane, and 0.5% propane on a molar basis, how do I create that composition?
I am trying to define this composition and then make calls to get its density, enthalpy, entropy given temperature and pressure.

Code that I could copy/paste with explanatory comments would be extremely helpful as I'm a first-time REFPROP user. Thank you in advance.

[ianhbell]

Some info here: https://refprop-docs.readthedocs.io/en/latest/DLL/high_level.html#f/_/REFPROPdll . There are also copious examples within the issues in this repository you should check out.

A snippet for your mixture to calculate the normal boiling point temperature of the saturated liquid state:

import os
os.environ['RPPREFIX'] = os.getenv('HOME')+'/REFPROP10'

root = os.getenv('RPPREFIX')
import ctREFPROP.ctREFPROP as ct
RP = ct.REFPROPFunctionLibrary(root)
RP.SETPATHdll(root)

p = 101325
z = [0.98, 0.015, 0.005]
q = 0
r = RP.REFPROPdll('ETHANE;METHANE;PROPANE','PQ','T',RP.MOLAR_BASE_SI,0,0,p,q, z)
print('TNBP:', r.Output[0], r.hUnits)

yielding

TNBP: 174.91159925978994 K

[mina19]

Thank you for the quick response Ian!
I have a few questions...

1. In the above example, what property is q? I see that we are passing a value of 0 for it and in the documentation it says that it means "quality".
2. If I want calculations performed with my user-defined units, what is the proper way to do that?

I tried
p = 14.7
z = [0.98, 0.015, 0.005]
q = 0
r = RP.REFPROPdll('ETHANE;METHANE;PROPANE','PQ','T',RP.MOLAR_ENGLISH,0,0,p,q, z)
print('TNBP:', r.Output[0], r.hUnits)

and this returns the correct number in Fahrenheit.

But if I do:

USER = RP.GETENUMdll(0, "USER").iEnum

p = 0
z = [0.98, 0.015, 0.005]
q = 0
r = RP.REFPROPdll('ETHANE;METHANE;PROPANE','PQ','T',USER,0,0,p,q, z)
print('TNBP:', r.Output[0], r.hUnits)
this returns TNBP: -9999990.0 T which is incorrect.

where
USER (can be changed by calling the REFPROP subroutine)
iUnits ---> 11
Temperature C
Pressure psig
Density kg/m^3
Enthalpy J/g
Entropy (J/g)/K
Speed m/s
Kinematic vis. cm^2/s
Viscosity mPa-s
Thermal cond. W/(m-K)
Surface tension N/m
Molar Mass g/mol

[ianhbell]

I cannot recommend the use of USER units. Much better to use one of the base SI variants (molar or mass). It is fragile and has some bugs not present with the use of base-SI units. Also the use of base SI units avoids unit conversions in your code.

q is the molar vapor quality, or the number of moles in the vapor phase relative to the total number of moles. It is 1 for a saturated vapor and 0 for a saturated liquid