Calculate Bubble Temperature using DTL

[Everton Colling]

I've just found out about the amazing DWSIM Standalone Thermodynamics Library (DTL) and have been playing with it through Python in Windows using pythonnet.

I was able to reproduce the VB examples in Python without any problems.

I'm trying to figure out how to solve a bubble Temperature problem using DTL. I have a mixture of hidrocarbons at a certain liquid phase composition and pressure and using this values I would like to calculate the vapor phase composition at equilibrium and the bubble Temperature.

Is there a simple way to achieve this objective using DTL? I've noticed that there is a Phase Property called bubbleTemperature, but I haven't found out how to get to this object.

This is what I got so far is:

dtlc = Calculator()
dtlc.Initialize() 

prpp = dtlc.GetPropPackInstance('Peng-Robinson (PR)')

T = Double(30 + 273.15) #K
P = Double(10 * 101325) #Pa

compounds = Array[String]([
        "Ethane",
        "Propane",
        "Isobutane",
        "N-butane"
])

molefractions = Array[Double]([
        0.25,
        0.25,
        0.25,
        0.25
])

mix = dtlc.CreateMaterialStream(compounds, molefractions)
mix.SetPropertyPackage(prpp)

I was able to create a Mixture, but I was not able to define the pressure and determine the bubble temperature.

Best regards,

Everton Colling

[Daniel Medeiros]

You can try the CalculateEquilibrium2 function: https://github.com/DanWBR/dwsim5/blob/windows/DWSIM.Thermodynamics/Property%20Packages/PropertyPackage.vb#L1953

Call it from prpp:

result = prpp.CalculateEquilibrium2(Interfaces.Enums.FlashCalculationType.PressureVaporFraction, P, 0.0, T0)

P = pressure in Pa
T0 = initial estimate for the bubble temperature in K

result is a FlashCalculationResult object: https://github.com/DanWBR/dwsim5/blob/98156f777a965de3475e467c1be748513fe5b7ed/DWSIM.Thermodynamics/Base%20Classes/FlashAlgorithmBase.vb#L1178

Regards
Daniel

[Everton Colling]

Thanks for the quick reply Daniel!

I managed to calculate the bubble temperature correctly using the CalculateEquilibrium2 method, but not the vapor composition.
The object returns an empty array for the vapor composition.

I looked into the source code, but I do not have experience with VB, so my insights are limited. I suspect that there are some info that I am not specifing in the mix object. Something related to the expected phases (liquid, vapor) perhaps.

This is the code snippet I used to calculate the bubble temperature:

# General Options
P = 10. * 101325 #Pa
T = 30. + 273.15 #K

comps = [
        "Ethane", 
        "Propane", 
        "Isobutane", 
        "N-butane"
]

z0 = [
        0.25, 
        0.25,
        0.25, 
        0.25
]

dtlc = Calculator()
dtlc.Initialize()
dtlc.SetDebugLevel(0)

prpp = dtlc.GetPropPackInstance('Peng-Robinson (PR)')

compsn = Array[String](comps)
xn = Array[Double](z0)
Pn = Double(P)
Tn = Double(T)

calctype = Interfaces.Enums.FlashCalculationType.PressureVaporFraction

mix = dtlc.CreateMaterialStream(compsn, xn)
prpp.set_CurrentMaterialStream(mix)

res2 = prpp.CalculateEquilibrium2(calctype, Pn, 0.0, Tn)
T2 = res2.CalculatedTemperature
y2 = res2.get_VaporPhaseMoleAmounts()

print("T =",round(T2,2),"K")
print("x =",np.round(z0,3))
print("y =",np.round(y2,3))

This gives me:

T = 289.69 K
x = [0.25 0.25 0.25 0.25]
y = [0. 0. 0. 0.]

Best regards,

Everton Colling

[Daniel Medeiros]

Try

y2 = res2.get_VaporPhaseMoleFractions()

Or use the Kvalues property to calculate the vapor composition.

[Everton Colling]

The Kvalues approach worked like a charm!

res2 = prpp.CalculateEquilibrium2(calctype, Pn, 0.0, Tn)
T2 = res2.CalculatedTemperature
y2 = []
for i in range(ncomps):
    y2.append(res2.Kvalues[i]*z0[i])

print("T =",round(T2,2),"K")
print("x =",np.round(z0,3))
print("y =",np.round(y2,3))

Giving the expected result:

T = 289.69 K
x = [0.25 0.25 0.25 0.25]
y = [0.66  0.197 0.081 0.062]

Thanks for the support!

[Daniel Medeiros]

Great! get_VaporPhaseMoleAmounts is returning zeroes because it multiplies the vapor phase mole fraction (which is zero because it is a bubble point calculation) by the vapor phase composition... thankfully the Kvalues are there for you to calculate the y vector.