Gas Solubility in Oils 1

[tesser]

Can anybody point me in the way of data on gas solubility in lubricating oils (ISO VG 20-30 grades say). What I am particularly interested in is the solubility of Nitrogen and the relationship between solubilty and temperature. I had always assumed that as temperature increased gas solubility decreased, but apparently there are exceptions to this rule.

 

[electricpete]

Try searching for Oswalt coefficients and DGA.

 

[25362]

To tesser: you are right. When studying air solubility in aviation fuels, we learn that the solubility increases with a drop in density, surface tension and viscosity. All these actually happen with higher temperatures.

Books tabulate the Ostwald coefficient C, i.e., the ratio of volume of dissolved gas to volumen of liquid solvent at given T,P.
For nitrogen in hydrocarbon oils at 273K, C=0.075.

The temperature dependence of C can be assessed by the following formula:

C_T=0.3*e^{[(0.639(700-T)/T]ln(3.333C)}

If you take the ln on both sides it'd become easier to understand the effect of absolute temperature T(K) changes.

Formulated lubes may show a bit different behaviour due to the chemistry of the additives. Same could be said concerning some synthetic lubricants.

 

[25362]

To tesser, in addition, for your information, typical gases that behave as air are nitrogen, hydrogen, helium, oxygen, argon, krypton, and CO. Others such as CO₂, methane, ethylene, acetylene, behave quite normally, i.e., their solubility in lube oils drops as temperature increases. This list hints at gas molecular size as a possible factor.

 

[mar888]

You have to be aware that also the pressure have an impact on sollubilities. increasing pressure, lower the sollubility.

 

[25362]

mar888, could you clarify your statement on pressure effects on solubility that appears to contradict Henry's Law ?

 

[mar888]

Sorry for the delay
We are measure dependence of gas in transformer oil solubolity mainly for the headspace application. The Henry law is obeyed, in the way that increasing the pressure will lower the gas concentration in gas phase. This is not appeear in the standart (ASTM) deal with this technique.
A paper is prepared