Retrieving Volumetric Fraction

[Luca Finardi]

Hello Daniel,

in the attached flowsheet the feed is 100 m3/h water at 5.75 % vol Ethanol, corresponding approx to 4.57 % mass fraction.
If I run the script after model run, I get both Ethanol volumetric fraction and volumetric flow 0.0 in all streams.

What am I doing wrong?

Thanks for the help.

Luca

[Daniel Medeiros]

I think that the volumetric flow property is only calculated for phases, not for the overall mixture... try getting properties from the LIquid1 phase instead.

[Luca Finardi]

Thanks Daniel, I tried this but it still did not work

ms11 = Flowsheet.GetFlowsheetSimulationObject('MSTR-001')
ph = 'Liquid1'
Overall = ms11.GetPhase(ph)
EtOH = ms11.GetPhase(ph).Compounds['Ethanol']
F = Overall.Properties.volumetric_flow * 3600.   # m3/h total
f_E = EtOH.VolumetricFlow * 3600.                # m3/h EtOH
x_E = EtOH.VolumetricFraction                    # % vol EtOH

print F, f_E, x_E

How can I retrieve the pure Ethanol liquid density (at T&P) in order to calculate the % vol from the % weight?

Thanks
Luca

[Daniel Medeiros]

The following uses the rigorous definition for the calculation of partial molar volumes. I've probably implemented it only for PR and SRK property packages... can you create a ticket so I can implement it on the next update? Thanks!

import System
from System import *
import clr
from DWSIM import *

ms11 = Flowsheet.GetFlowsheetSimulationObject('MSTR-001')
ph = 'Overall'
Overall = ms11.GetPhase(ph)

V = Overall.Properties.massflow / Overall.Properties.density # m3

ne_0 = Overall.Compounds['Ethanol'].MolarFlow
nw_0 = Overall.Compounds['Water'].MolarFlow

n_0 = Overall.Properties.molarflow

msc = ms11.Clone()

print V, ne_0, nw_0, n_0

ne_1 = ne_0 + 1
nw_1 = nw_0

n_1 = ne_1 + nw_1 # mol

xe_1 = ne_1 / n_1 # molfrac
xw_1 = nw_1 / n_1 # molfrac

print xw_1, xe_1

msc.SetOverallComposition(Array[Double]([xw_1, xe_1]))
msc.GetPhase(ph).Properties.massflow = None
msc.GetPhase(ph).Properties.volumetric_flow = None
msc.GetPhase(ph).Properties.molarflow = n_1

msc.Calculate()

Overall = msc.GetPhase(ph)

V1 = Overall.Properties.massflow / Overall.Properties.density # m3

# partial molar volume for ethanol
pmv_e = (V1 - V)/(ne_1 - ne_0) # m3/mol

# molar volume for ethanol
mv_e = xe_1 * pmv_e # m3 EtOH

# volumetric fraction ethanol
xv_e = mv_e / V1
xv_w = 1 - xv_e

print("xv_w = " + str(xv_w) + ", xv_e = " + str(xv_e))

[Luca Finardi]

Thanks Daniel for this good suggestion.
I have only a doubt: I think that the last part of your calc shoud be:

# molar volume for ethanol in the original stream
mv_e = ne_0 * pmv_e # m3 EtOH

# volumetric fraction ethanol in the original stream
xv_e = mv_e / V
xv_w = 1 - xv_e

print("xv_w = " + str(xv_w) + ", xv_e = " + str(xv_e))

do you agree?

I hand back-calculated the vol fractions between 2 known EtOH % vol and solution densities (i.e 5.75 %vol density 990.1 and 6.5 % vol density 989.1) and found an error 7.5%.
May be this deviation is due to volume contraction of the Aq-EtOH mixtures.

I will open the ticket as you mentioned.

Thanks.

Luca

[Daniel Medeiros]

You're right. The second stream (cloned) is used only to calculate pmv_e, all the other properties should be taken from the original stream.

The deviation is indeed due to mixing effects, and will vary according to the method you've used to calculate mixture density.

Thanks for the ticket!
Daniel