Base Stocks

Base Oil Report


Residual Waxes in Base Stocks

All paraffinic crudes contain waxes, and once base oil viscosity grades have been cut, the lighter distillate grades tend to contain 1 or 2 percent more waxes, relative to the heavier grades. Exact wax content – either in the crude or the base stock – is difficult to precisely determine, since it largely depends on how low refiners drop the temperature in order to precipitate the wax content from the crude or product.

When using some arbitrary sub-ambient temperature to precipitate waxes during, say, crude characterization, there will always be some residual waxes left in the crude solution. The dewaxing step, one of the final stages in paraffinic base stock production, is usually done just before base stock finishing.

Broadly speaking, there are two types of dewaxing process: the classic solvent dewaxing (SDW) or catalytic dewaxing (CDW). But again, dewaxing is a slight overstatement, because there will always be some residual waxes in API Group I base stocks all the way through to Group III. They would, in fact, be all the poorer if they did not contain such residues since these soluble waxes are the highest viscosity index (VI) components.

The use of CDW is enabled by the development of hydrocracking technology, since it requires the same high-pressure hydrogen supply infrastructure needed for the saturation and cracking of cyclic compounds in the initial feedstock upgrading step, where the VI is set.

This hydrogen infrastructure, along with more modern catalysts made from specific zeolites, has enabled the second-generation hydro-isomerization approach (putting branches in to get rid of linears, as opposed to the removal of linears by cracking) to CDW, whereby linear normal paraffinic waxes are chemically converted to more soluble isoparaffins of around the same carbon number, usually using noble metal-loaded zeolite catalysts. There is no slack wax yield here, but the isoparaffins formed are the next best-in-class in terms of VI contribution. So there is no real VI deficit or base oil yield loss.

By contrast, there was a first generation of hydro-catalytic dewaxing that did not involve isomerization. Rather, it involved wax cracking, in which wax molecules were literally cracked in the main chain into lighter, more-soluble waxes below the base oil boiling range. The unfortunate consequence of this approach was that there was still no marketable slack wax yield, and yet the VI of the final base stock was adversely affected. High VI normal paraffins were downgraded to fuels and were not replaced with any useful high VI isoparaffin base stock components, unlike in hydro-isomerization dewaxing.

SDW, on the other hand, does not require a hydrogen infrastructure. Although it is a much more labor-intensive process – requiring solvent dilution, chilling and filtration followed by solvent recovery – it does yield a high-value saleable product in the form of slack waxes for the wax finishing business.

Waxes, or what are broadly normal paraffins chemically, are a feature of hydrocarbon products across the crude grade spectrum, both fuels and lube base stocks. The very lightest carbon number waxes are still liquids at most temperatures. But they can become more solid and problematic as the carbon number increases, becoming solid crystals in the gas oil (diesel) range at ambient winter temperatures. In fact, a lot of diesel production lately is catalytically dewaxed too, giving winter-grade fuel using the same isomerization catalyst types as for base stock production. Light ends, or tops, which are inevitably created in base stock hydro-isomerization line-ups, can be routed to the refinery diesel pool to boost diesel low-temperature performance and are good high cetane species too.

It is thus no surprise that all base stock production – apart from naphthenics, which are made from wax-free feedstocks – requires some kind of dewaxing to set sensible pour points. Even after dewaxing, base stocks still contain some soluble wax-type substructures, down to and even below the pour point. (They can be seen and quantified using carbon 13 nuclear magnetic resonance spectroscopy.) These residual wax-like molecules in Group II and Group III stocks can still cause issues, such as hazing at moderate temperatures and poorer viscosities when measured as Brookfield viscosity. This can be exacerbated by poor choice of grade cuts and more especially dewaxing unit line-ups. If the finished lube application requires a lower than natural point, then we must resort to additives to help out. There are a range of pour point depressant (PPD) additives to turn to.

In general, residual waxes in SDW base stocks have poorer responses to PPDs relative to modern hydro-isomerized CDW stocks. Relative PPD performance manifests as SDW stocks tending not have their pour points lowered as much for a given PPD treat compared with hydro-isomerized stocks. Also, a relatively higher treat is required to achieve even that poorer position for SDW stocks compared with hydro-isomerized base stocks. In both cases, PPD responses flat-line at some PPD treat. This can be rationalized in that, for solvent dewaxed stocks, the PPD must still address what is essentially an unconverted normal paraffin residual wax. In the hydro-isomerized stocks, the PPD has to address a chemically converted isoparaffin with just some remaining waxy or normal paraffin character within the isoparaffin molecule, this being the easier job.

A proportion of residual waxes are a necessary component of a quality paraffinic base stock for VI reasons, but need to be controlled through appropriate dewaxing, supported finally by the formulators appropriate use of PPD additives, when necessary.

Related Topics

Base Stocks    Other    Waxes