In refining, we have seen over the decades the move from simple thermal cracking – where just heat is used to break heavy crude molecules into lighter carbon numbers – to catalytic cracking to give lighter products with more control over product ranges and at lower temperatures.

Finally, we have seen hydroprocessing (sometimes referred to as hydrocracking) where a catalyst is used in the presence of high-pressure hydrogen, primarily for cracking, saturating and generally upgrading feedstocks to produce high-quality, low-sulfur middle distillates, such as kerosene and gas oils, and base oil feedstocks.

Hydrocracking for diesel, or gas oil, is the principal source of the very high viscosity index feedstocks used to produce both API Group II and III base stocks. Here, a portion of the bottoms fraction is bled off after a hydrocracking pass and used for base stock production, rather than it all being recycled for more fuels production. Note that hydrocrackers optimized for gasoline do not produce a suitable bottoms fraction for base stock output.

A fundamental type of reaction in hydroprocessing is saturation, where the hydrogen to carbon ratio of the product range is increased. Hydrogen at very high pressure is partially consumed in the process by inclusion in, say, aromatics to convert them to higher V.I. cycloparaffinic saturates.

Saturation derives from the metals loaded onto the catalyst framework. This saturation function can be carried out by hydroprocessing in either a low sulfur or high sulfur feedstock environment.

In the most common case, currently a low sulfur environment, the catalyst metals loaded on to the alumino-silicate framework are the more highly active noble metals such as platinum or palladium. This will usually require some initial hydrodesulfurization pretreatment to remove excess sulfur from the base oil feedstock to prevent catalyst poisoning.

Typical hydrogen pressures in base oil reaction vessels are around 150 atmospheres. These high pressures and reaction temperatures are required not only to get adequately rapid conversions, or kinetics, but also to ensure that the equilibrium mixture of final products is correct. If the temperatures and hydrogen pressures are inappropriate for base stock production, it is possible to make feedstocks more aromatic than they started off, which is undesirable. Such production of aromatics is called reforming.

There are alternative non-noble metals that can be loaded on catalyst alumino-silicate frameworks to work with a high-sulfur feedstock that has not been desulfurized. These are typically nickel, cobalt, molybdenum or tungsten and particularly mixes of these metals. These metals need to be positively loaded, or pre-sulfided, with sulfur species to become and stay active. Such setups have been used to manufacture Group I base stocks by hydroprocessing. This results ultimately in very low-sulfur Group I stocks. But this is not necessarily a good compositional space for base stock performance in finished lubricants.

Hydrocracking is one process in hydroprocessing, which can take the form either of main chain rupture of the hydrocarbon, moving it from a high to lower boiling range. This can, for example, move very heavy feedstock molecular weights into the base stock distillate boiling range. Alternatively, hydrocracking can be just the opening of cycloparaffins to make higher V.I. isoparaffins. Hydrocracking derives from the acidic nature of the catalyst in an alumino-silicate framework, usually tuned by varying the aluminum content.

Many of the hydroprocessing catalysts, but especially the hydroisomerization dewaxing catalysts, are based on crystalline zeolite variants. However, hydrocracking in general also makes use of amorphous or non-crystalline catalysts for main cracking reactions. Both crystalline and amorphous catalysts make use of alumino-silicate molecular frameworks substituted with other elements, such as phosphorus, to fine tune performance.

Hydrocracking can take place on the macroscopic surface of a catalyst, whether amorphous or zeolitic. In fact, some zeolite hydroisomerization catalysts have their surface cracking capability positively deactivated to improve isomerization and dewaxing selectivity.

Crystalline zeolites have microscopic channels through which hydrocarbons can diffuse both in and out. This is mainly where isomerization, or branching of waxy residues, takes place to lower the pour point. The shape and size of these channels can significantly influence the selectivity of the zeolite for a given size of feedstock or type of reaction.

In hydroprocessing, however, there is no such thing as a pure cracking or a pure isomerization catalyst. Most catalysts do both to varying degrees. The trick is to maximize the required type of reaction by tuning, for example, a catalysts acidity for cracking or metal type and content for saturation and isomerization. For example, we seek to minimize unwanted reactions such as cracking in hydroisomerization dewaxing catalysts, which results in yield loss to lighter products and gases, outside the base oil range.

Hydroprocessing technology can be used to treat wide boiling ranges of base oil feedstock simultaneously in a single treatment. This can be attractive from the viewpoint of simpler and cheaper base oil plant line ups, saving on intermediate tankage. However, there is often a price to pay in terms of final product performance of specific grades, since there can be mutual interference at active catalyst sites between the extreme ends of the boiling range being processed. This is especially the case in hydroisomerization dewaxing of base stocks.

Today, there are a significant number of catalyst suppliers to the fuels and lubes hydroprocessing market that are continuously looking to improve catalyst selectivity and absolute performance. We can fortunately, therefore, look forward to better and better base stocks.