Ethylene Glycol/Water Mixture - Vapor Pressure 1

[kcme2005]

We are trying to calculate the vapor pressure of an Ethylene Gylcol/Water mixture.

We are using pure ethylene glycol (no additives), and our mixture is 60% EG and 40% H2O.

The Critical Pressure and Temperature of EG is:
Pressure - 1116.8 psia
Temperature - 733.3°F

The Critical Pressure and Temeprature of H2O is:
Pressure - 3199.2 psia
Temperature - 705.2°F

We want to know what the Critical Pressure and Temperature of the mixture will be. Does anyone know how to go about doing this?



We used Raoults Law assuming that the mixture is an ideal mixture, but we did not get the results we were expecting.
See our calculations below:

EG
Molecular Weight - 62.07 g/mol
Density - 1.11 g/cc
Boiling Point - 387.7°F

H2O
Molecular Weight - 18.02 g/mol
Density - 1.00 g/cc

At a volume of 10cc
Weight EG = 6cc*1.11 g/cc = 6.66g
Weight H2O = 4cc*1.00 g/cc = 4.0g

Number of Moles
EG = 6.66g/62.07g/mol = 0.1073 moles
H2O = 4.00g/18.02g/mol = 0.2220 moles

Mole Fractions
EG = 0.1073mol/0.3293mol = .326
H2O = 0.2220mol/0.3293mol = .674

Vapor Pressure of EG at Critical Temp of H2O
Assuming vapor pressure of EG is linear.
EG = (705.2°F-387.7°F)/(733.3°F-387.7°F)*1116.8psia = 1026psia

Combined Vapor Pressure
EG = 0.326*1026psia = 334psia
H2O = 0.674*3199.2psia = 2156psia

EG + H2O = 334 + 2156 = 2490 psia @ 705.2°F


Any help would be greatly appreciated.

 

[TD2K]

Dow chemical has tables on ethylene glycol mixtures properties, I think vapor pressure is one of the values they give. Might want to check them out.

It will give you a good idea pretty quick if water and glycol is an ideal mixture, I don't think it is.

 

[DickRussell]

1. The vapor pressure of EG would not be linear with temperature. A closer approximation is that ln(vp) varies linearly with (1/absolute temperature). But more direct is to use an established vapor pressure curve for this. The DIPPR database gives 505.7 psia, which is far lower than your 1026.

2. The temperature and pressure are so elevated that using Raoult's law for this is too simple. An equation of state will take the pressure into account, but applying a set of nonideal liquid activity parameters would be quite an extrapolation. Mixing very dissimilar materials typically gives mixture critical T and P that are removed from the mole averaged values. The API Technical Data Book gives a qualitative presentation of this for hydrocarbon binaries. I imagine the vendors of antifreeze products have at some time measured the vapor pressure curves for various strengths of EG/water mixtures. Perhaps the Internet has some data published.

 

[Latexman]

EG and water are moderately non-ideal. I know EG is used in extractive distillation columns to break azeotropes between alcohols and water, like ethanol. The EG "hydrogen bonds" with water more so than the alcohol does (since EG has 2 -OH groups instead of 1 on the alcohol) and the resulting predominate EG-water complex acts like it is higher molecular weight and lower volatility than EG or water separately.

Good luck,
Latexman

 

[JimCasey]

CHeck with DOW. They sent me the techspecs of DOW Ethylene glycols. They were kind enough to send me "Engineering and Operating Guide for DOWTHERM SR-1 and DOWTHERM 4000
Inhibited Ethylene Glycol-based Heat Transfer Fluids"

DOWTHERM SR-1 fluid is a formulation
of 95.4 percent ethylene glycol
and a specially designed package of
industrial corrosion inhibitors. The
fluid is dyed fluorescent pink for leak
detection purposes. It has an operating
temperature range of -60°F
to 250°F (-51°C to 121°C) and
solutions in water provide freeze
protection to below -60°F (-51°C)
and burst protection to below -100°F
(-73°C).

Table 26 has Pv vs temp and concentration. 60% boils at 230F at 760mmHg.