Solvent-based technology usually required an intermediate tank for each grade of distillate, another tank for the same grade once it became a waxy raffinate and a final storage tank for the finished, dewaxed base stock grade. Solvent extraction or solvent dewaxing units were also subject to grade switching for individual grade campaigns.

Where line-ups using wide boiling range feeds rather than individual vacuum-cut distillate grades have been processed in the past (in API Group I production), it was never possible to optimize the solvent processing to suit all grades simultaneously. On the few occasions that this wide boiling range line-up approach was used, there were usually significant quality penalties to pay. And the more widely used lighter grades tended to be affected disproportionately.

Modern catalytic hydroprocessing does not need intermediate tankage or even grade-switching for certain line-ups, allowing quasicontinuous production. In an extreme case for a modern catalytic plant, using a full boiling range feedstock rather than individual base stock grades, refiners can saturate/crack, hydroisomerize (dewax), hydrofinish, cut into grades and run down to stock tanks with no intermediate storage tanks or interventions.

This all sounds very attractive, but as with all things in this world, there is still a price to pay. Most catalysts, especially dewaxing catalysts, rely on the diffusion of hydrocarbons to active isomerization sites. When a very wide range of molecular weights or carbon numbers are present, there is a commensurate wide range of diffusion rates. And the smallest molecules get to the isomerization sites first because they diffuse the fastest and sometimes are in no rush to leave the active sites.

This means that heavier grades get crowded out, or at least get an unfair share of the dewaxing action. As a result, heavier grades may have limitations in their low-temperature performance, such as higher than necessary cloud points in relation to pour point.

In any event, when refiners run a very wide range feed with no grade selection before dewaxing, they can set only one determining pour point, and all other grades will have resultant pour points and cloud points. This often means that grades lighter than the determining grade get too much dewaxing and an unnecessarily low pour point, while grades heavier than the determining grade get too high a pour point.

Any grade that has too low a pour point carries an economic yield penalty of excessive commensurate cracking because all dewaxing catalysts crack to greater or lesser extents, as well as their main function of isomerization. The heavier grades also have limitations in their low-temperature stability.

So what are the alternatives? If waxy raffinates are cut into grades first, they have to be stored until the dewaxing unit is ready to receive them – just as with historic technology. This has both a capital expenditure implication for tankage and some small interface material yield loss at grade switch.

These costs and losses have to be balanced against producing a grade with better low-temperature characteristics, an optimized pour point and minimal yield loss from excessive cracking. The call is not easy, and essentially means refiners have to do the math over the lifetime of the base oil unit. They also have to make a decision on the minimum acceptable qualities of individual grades, which is in effect a marketing value decision.

If refiners are prepared to build increased complexity into new or upgraded plants, there is potential to solve both the optimized quality and the capacity/yield equation to the benefit of both base oil producers and lubricant marketers. But that complexity will obviously come at a cost.

So the message is that it is worth putting a lot of effort and thought into the line-up in new and upgraded base stock facilities, and not to always go for the simplest line-ups straight off. Even simplicity itself comes with a price tag in terms of money left on the table through even small yield losses or quality constraints over the lifetime of a base stock unit.

The industry has never been so well served with the range of catalyst types for base stock manufacture of all API groups – even Group I. However, there is still more to be extracted from available catalytic technologies with appropriate thought given to how precisely they are lined up and used