Making of PAO's

[MolaKule]

Gasses drawn from the refinery towers are used to make PAO, a specific Gas-to-Liquid process.

The ethylene gas C2H4 or propylene (or other similar gases) are then "oligomerized" which means that the gas is transformed to various molecular weight liquids via polymerization, or the linking together of many single molecules. The products of these gases, via catalysts, results in a crystal clear liquid. [A catalyst is a chemical, usually a metal compound in this case, that increases the rate of reaction but itself does not undergo any permanent change].

The type of catalyst used determines the final molecular weight of the liquid. Low viscosity PAO's (2- 10 cSt) are made by using boron trifluoride catalysts, and heavy PAO's (40-100 cSt) are made using the alkylaluminum catalysts.

The liquid is then hydrogenated with hydrogen gas at approx. 250 C and 500 psi pressure with a nickel catalyst to further stabilize the fluid for oxidative and thermal stability.

Different catalysts, intermediate ditillation techniques, and different starting gases or combinations of gasses can be used to taylor- make PAO's for any viscosity or property required.

A gas chromatigraph will show the narrow molecular distribution of finished PAOs. This is in contrast to minerals oils and even VHVI oils, which show a wide distribution of molecules, i.e., both high and low molecular weight distributions.

A narrow disritbution of HC moleculaes means that the PAO fluid is stable in terms of
exhibiting low volatility and high VI. VI's of 150 are very common with PAO's. Of course, the higher the viscosity, the higher the Viscosity Index.

 

[427Z06]

quote:

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Originally posted by MolaKule:
Of course, the higher the viscosity, the higher the Viscosity Index.

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This is interesting. MolaKule, why is this?

 

[Brian Reid]

quote:

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427Z06:

quote:

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Of course, the higher the viscosity, the higher the Viscosity Index.

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This is interesting. .... why is this?

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The viscosity index is a measure of the rate of change of viscosity with temperature. There's a standard test method, ASTM D 2270-64, which involves comparing the tested oil - at 40°C and at 100°C - with two standard oils having and arbitrary VI of 0 and 100, respectively.

Since it measures the rate of change, you can envision it as the slope of line with the x axis being temperature and y axis being the viscosity measurement. The flatter the line, the less the slope, the less change in the viscosity with temperature.

 

[427Z06]

Brian, thanks for the post, but that still doesn't get to my question. If the slope of the line is the same, and you just start "higher" up on the line, the VI would be the same no matter the viscosity. Something must be non-linear here, oops, I think I just answered my own question.

 

[MolaKule]

quote:

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This is interesting. MolaKule, why is this?

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The higher the viscosity, the larger the molecule, structurally speaking. Larger molecules are made by bonding more carbon atoms with the hydrogen. The more carbon atoms, the more structurally "sound" the molecule.

Let's take an 8cSt PAO; it's local carbon number of atoms is C10 or ten atoms per local molecular structure; it's viscosity index is 138. Let's take a 9 cSt PAO of C12, it's VI is 146.


The more carbon atom linkages implies that it takes more energy to rip them apart and shear the molecules. Shearing of the molecules lowers the viscosity.

So more carbon atoms = more structurally sound molcules = higher molecular weight = higher VI.

The actual skeletal structure and the resulting propterties is a bit more complex than above. Factors affecting the design of the molecules involves:

molecular chain length of raw material
temperature of processing
time for processing
pressure in processing
catalyst concentration
cocatalyst type
cocatalyst feed rate
reaction quench and recovery procedures
hydrogentation catalyst and
distillation

 

[buster]

Kule, thanks for posting this. I always wanted some detailed info on how they make PAOs.

 

[Gerret]

A great backdrop to this Interesting Article (thanks again, MolaKule) is the "How Oil Refining Works" at the Howstuffworks site:

http://science.howstuffworks.com/oil-refining.htm

It has some great illustrations of processes similar to what MolaKule is describing e.g. unification via catalytic reforming

I used to work for a supplier of catalysts and I never knew exactly what they were used for until I read this howstuffworks article!

[ August 14, 2004, 10:14 PM: Message edited by: Gerret ]

 

[Drstressor]

PAOs used as lubricants are typically branched chain molecules. As such, inter- and intra- molecular interactions can occur. Only inter-molecular interactions (between molecules) contributes significantly to viscosity (unless the molecules are very long). At lower temperatures, interactions between the chains in the same molecule are favored. This results in a lower viscosity relative to a linear paraffin of the same molecular mass. As the temperature of a PAO solution is increased, thermal energy tends to extend the chains, which favors interactions with other molecules. This reduces the effect of temperature on viscosity relative to a linear paraffin of the same molecular mass. Thus, the larger the polymer size of the PAO, the higher the VI.

I hope this makes sense.

 

[Matt89]

Do PAO basestocks require more "energy" to produce?

I ask because one of the attractive features of synthetics are several aspects of energy conservation...are we losing it up front to hopefully gain back later?

 

[MolaKule]

Once the capital investement in equipment has been returned as a wild guess I would say that 1% more energy is used to produce PAO's over pure mineral bases stocks. One has to keep in mind that the feedstocks (crude, gasses) and resulting fluids are always streaming through the refinery in a constant march. So you don't have to stop a process at the refinery to crank up PAO's.

Essentially, you are simply diverting some of the resulting gasses from the towers to make another product stream. And the cataalysts are never used up.

For esters, the cost of the actual process is low (neglecting facility capital costs). It is the orginal feedstocks of starting products such acids, alcohols, and catalysts that are expensive.