Naphthenic base stocks do not require dewaxing because they are wax free, and this is one of the reasons they have such good low-temperature properties – at least in terms of pour point. Paraffinic base stocks – be they API Group I, II or III – do contain waxy molecular components, and it is these that ultimately allow paraffinic stocks to have much higher viscosity indices than naphthenics. But these components have to be managed with dewaxing technology.

First-generation dewaxing technology was based on the still common solvent approach. It uses solvent mixes that serve two purposes: first, to dilute the base stock so its viscosity is low enough to allow rapid wax crystallization; secondly to provide a solvency environment that drives a portion of the paraffinic wax molecules out of the base stock to reach a lower pour point. The separated wax is filtered off, and the solvent can be recovered by flashing it off for reuse in subsequent dewaxing cycles.

Solvent dewaxed base stocks still have residual waxes that remain soluble down to the base stock pour point and contribute significantly to viscosity index (VI). The pour point can be lowered further by use of pour-point depressant polymers that modify wax crystal morphology. Solvent dewaxing yields a physically separated by-product – slack waxes, which can be a feedstock for wax finishing or Group III base oil production, or a high-quality cracker feed. However, they also represent a loss in terms of final base oil yields.

The second generation of dewaxing technology was to destroy linear wax molecules by cracking them away catalytically. This catalytic dewaxing technology uses hydroprocessing with crystalline zeolite catalysts that largely crack the linear paraffinic waxes to carbon numbers significantly lower than the base oil range (around 20 to 40 carbon atoms) but with little other chemical modification.

This catalytic cracking approach has several drawbacks. There is a significant VI drop since the process removes the highest VI components (waxes) without converting significant numbers to other high VI components like iso-paraffins. In addition, this process eliminates useful by-products such as a slack waxes.

As catalytic dewaxing evolved, types of zeolite were developed that could address both of these drawbacks. The third generation dewaxing technology uses zeolites that can convert linear paraffin waxes to iso-paraffins by isomerization, rather than removing them. This means that highly flowable, low pour-point, branched molecules are being synthesized in the base oil carbon number range. The end molecules have relatively high VI – admittedly not as high as the original linear paraffin waxes, but better than all other molecular species in the base oil mix. As a result, isomerization technology gets a much better balance between base oil yield and VI. This approach is the basis of most of the licensable products on the market today.

However, as with much in life, we rarely get all that we want. Most catalytic dewaxing technology is limited to the carbon ranges that the zeolites can accommodate, since most isom-erization or catalytic cracking requires the wax molecules to penetrate the molecular pores of the zeolite. Getting very big wax molecules to diffuse into such small molecular pores is some-where between very time consuming and impossible. This means we have an upper limit to the grades we can effectively dewax using either type of catalytic dewaxing technology – cracking or isomerization. This is not the case with solvent dewaxing which can easily accommodate viscosity grades as heavy as bright stock while giving a very marketable bright stock slack wax.

This upper limit on the grades that can be catalytically dewaxed is one of the prime reasons we dont routinely see Group II bright stocks. Making Group II bright stocks requires either very careful selection of feedstock – essentially naphthenics – or contenting ourselves with appearance issues such as haze caused by the presence of an unconverted higher-carbon-number wax component. The obvious ques-tion becomes why not solvent dewax Group II bright stock? The answer is that nowadays it is just not cost-effective to put in solvent dewaxing technology for such a limited product range. Solvent dewaxing tends to require a large capital expenditure and to be a slow, labor-intensive batch process. Therefore it cannot benefit from the almost continuous base stock production processes available in certain of the Group II and Group III catalytic hydroprocessing line-ups.

What, then, is the future of dewaxing? Were very unlikely to see any new solvent dewaxing setups, although those already in place largely for Group I production will likely continue as long as Group I has a market. For catalytic dewaxing, the main developments are likely to be improvements in the isomerization-to-cracking ratios, even for nominally isomerization catalysts. Along with throughput improvements, this will ensure the economics of base stock production continue to improve.