Access latent heat of evaporation within a Python Script Unit Operation

[Christian Hermsdorf]

Hello!

How do I access the latent heat of evaporation or alternatively the specific enthalpy at boiling and dew point of a material stream from withtin a Python Script Unit Operation? I could not manage to find anything in this Forum or teh API documentation.

BR
Christian

[Daniel Medeiros]

import clr
import System
import DWSIM
from System import *
clr.AddReference("System.Core")
clr.ImportExtensions(System.Linq)

ms = Flowsheet.GetFlowsheetSimulationObject("MSTR-000")

pp = ms.PropertyPackage
pp.CurrentMaterialStream = ms

# get mixture composition
Vz = ms.Phases[0].Compounds.Values.Select(lambda x: x.MoleFraction).Cast[Double]().ToArray()
# get temperature
T = ms.Phases[0].Properties.temperature
# get pressure
P = ms.Phases[0].Properties.pressure

# calculate mixture enthalpy as vapor
hv = pp.DW_CalcEnthalpy(Vz, T, P, DWSIM.Thermodynamics.PropertyPackages.State.Vapor) 
# calculate mixture enthalpy as liquid
hl = pp.DW_CalcEnthalpy(Vz, T, P, DWSIM.Thermodynamics.PropertyPackages.State.Liquid) 

# enthalpy of vaporization
hvap = hv-hl 

# kJ/kg
print str(hvap) 

[Christian Hermsdorf]

Hi Daniel,

thank you very much for the quick response. Works like charm!

However, I am still a bit confused regarding the function calls and assignments and I was wondering if you could explain a bit what is actually happening with the following code:

pp = ms.PropertyPackage
pp.CurrentMaterialStream = ms

Translated this looks like the following to me:

ms.PropertyPackage.CurrentMaterialStream = ms

So, the object 'ms', which is an instance of Class 'MaterialStream' has the Property 'PropertyPackage', which again has the property 'CurrentMaterialStream', which is then set to 'ms', the "root" object itself. I would have assumed that the MaterialStream-object 'ms' should already now that its CurrentMaterialstream is 'itself'. Somehow I am missing something ...

Another curiosity regards your temperature call

T = ms.Phases[0].Properties.temperature

because I would have used the following instead:

T = fluid.GetPhase('Liquid1').Properties.temperature

Can you explain what is the difference? And what is the '[0]' referring to behind 'Phases'?

Best regards
Christian

[Daniel Medeiros]

Hi Christian,

For the first question:

• Every object in the flowsheet has an associated property package, if not explicitly then the default one is selected. Although the object knows about the property package, the opposite is not true.
• The property package can only make calculations using only one material stream at a time, and it can be associated with another "foreign" stream when you get a reference to it from your stream.
• Before making a calculation, we must associate the material stream with the property package so it can get the properties from that stream. This is done through the CurrentMaterialStream property.

For the second question:

• Both calls are correct. DWSIM stores the phases internally using an integer reference (0 = mixture, 1 = liquid mixture, 2 = vapor, 3 = liquid 1, 4 = liquid 2, 5 = liquid 3, 6 = aqueous (not used), 7 = solid). GetPhase() will return the phase using its name, which is easier than looking for the integer ID. Phases[x] gets the phase directly from the storing collection. See https://github.com/DanWBR/dwsim5/blob/windows/DWSIM.Thermodynamics/Material%20Stream/MaterialStream.vb#L235 and https://github.com/DanWBR/dwsim5/blob/windows/DWSIM.Thermodynamics/Material%20Stream/MaterialStream.vb#L293