The need for improved energy efficiency, longer lubricant life, engine durability and/or higher levels of catalyst protection for emission systems attest to future requirements for increased quantities of higher performing base stocks in each facet of the automotive industry. Without the proper preparation and coordination, our industries risk the issuance of specifications for which base stocks of the necessary performance in the proper qualities and quantities, and the complementary additive systems, may not be technically nor economically viable.

Why this deep concern about base stock? Simple: It is 90 percent or more of every gallon of engine oil sold. Whatever happens to engine oil requirements, the biggest impact will be on or come from base stock.

The base stock story is one of performance, process and price. When we understand the issues in each of these areas, we can begin to see what impact base stock has on engine oils and what the future might hold.

Delivering Performance

Past columns have looked at base stock performance as an integral part of overall engine oil performance. Among the properties required for passenger car engine oils that are base stock related are the following:

1. Volatility. Concerns about oil volatility and its impact on emissions have driven volatility specifications towards less oil loss. This property is controlled at the refinery level.

2. Fuel economy. Low-temperature viscometrics have a major impact on fuel economy. Friction modifiers play a part, but the viscosity of the oil is a critical part of fuel economy performance.

3. Low-temperature performance. As viscosity of engine oils goes down, the requirement for good low-temperature protection increases. Pour point and low-temperature viscometrics – pumpability and cranking properties – become more critical.

4. Oxidation resistance. As interest in longer drain intervals continues to grow, the need for oils to be more stable increases. Additives can carry the system a long way but improved base stocks can make it an easier process.

Process is Destiny

Base stocks are manufactured by any of a number of processes. Each process imparts certain benefits to the finished oil and depends on what scheme is used. Here are the major steps:

1. Fractionation. The need for lower volatility plays an important role in how base stocks are distilled. The viscosity of any single fraction is the weighted average of the viscosities of the individual molecules in that cut. To reduce the volatility, the lighter, lower viscosity and volatile materials are removed. However, to maintain the overall viscosity of the cut, some of the heavier, higher viscosity materials are also removed. The net result is less of the fraction. One way to improve the yield of any cut is to break up (crack) the heavier materials into lighter ones of the proper viscosity and volatility. This is the approach used for low volatility materials such as the so-called Group II+ base stocks.

2. Saturation. Higher saturates in the base stock is generally equated with better performance. Oxidation resistance (when properly additized) is improved and viscosity index is raised. The process used to saturate base stock cuts is either solvent extraction or hydrotreatment. Solvent extraction is limited in its ability to raise saturates, by the crude source and solvents used. It is now more common to use hydrotreating to raise the saturates level. This process is less crude-sensitive and has the added advantage of increasing yield.

3. Wax Removal or Conversion. To improve its low-temperature response, the base stock must be dewaxed – either by physically removing the wax or by modifying it. Wax removal is carried out by simply cooling a base stock/solvent mixture enough to cause wax molecules to solidify so they can be filtered out. After that, the solvent is flash-evaporated from the base stock. This process is hit-or-miss to the extent that it doesnt remove all of the waxy species in the base stock. A better process is to modify the waxy materials present by catalyst and hydrogen. Again, the advantages are reduced yield loss and less sensitivity to crude type.

There is one issue with hydro-dewaxed base stocks: They are extremely sensitive to the pour point depressant used. Because of the process, a very specific range of waxy molecules is produced which reacts uniquely with the pour point depressant (PPD). If the wrong PPD is used, the waxy materials present will not be inhibited from gelling. Therefore, the choice of PPD is more critical.

Price Always Counts

The price of base stock is driven by the processes used to make it and the viscosity of the base stock. Historically, the higher the viscosity, the higher the price for base stocks – but a new driver is what type of processing is used. That should seem obvious, but it is more complex than it appears.

Using the older processes of solvent extraction and solvent dewaxing, yield losses are greater and the refiners choice of crude is limited to those which are relatively high in paraffin and naphthenes as well as low in sulfur. This is offset to the degree that the by-products can be successfully sold. Wax enjoys a continuing market but solvent extracts are a different matter. They were used at one time for specialized base materials but health issues associated with their chemical composition have made it more feasible to simply use them as cat-cracker feed stock.

As noted above, hydroprocessing (cracking, saturating and dewaxing) provides advantages in reduced yield loss and greater flexibility in crude sourcing. The cost of new facilities to make hydroprocessed base stocks – typically hundreds of millions of dollars – has deterred all but the largest companies from investing in them.

In the end, the price of base stocks is driven by supply and demand factors, just as any other commodity based market. Although base stocks are technically not commodities but are rather fungible materials, they are driven by demand for specific cuts.

Looking Ahead

What shape will base stock refining take in the future? First, we need to understand that the reduction in pool base stock viscosity will continue. In Octobers column (Maybe you can be too thin, page 6), I pointed out that the shift to lower finished oil viscosity engine oils was driving down the average or pool viscosity of the base stocks being used. The shift to SAE 0W-20 and SAE 5W-20 viscosity engine oils will continue due to fuel economy requirements. Of course, volatility and stability will still be major needs.

Currently, refining is moving to more hydroprocessing and away from traditional solvent-refined base stocks. Not only will this mean more Group II base stocks, it will also mean Group II+ and Group III production will increase.

A new player in the base stock game will be gas-to-liquid (GTL) processing. This is the Fischer-Tropsch process that was used by Germany during World War II to meet their lubricant needs. Using natural gas as the feed, various hydrocarbon cuts can be produced from resources that may be stranded or not economical to transport to market. Both diesel fuel and lubricant cuts can be produced.

There are a number of major oil companies actively pursuing GTL processing. As yet, there is no major facility operational in the Western hemisphere. Alaska is the most likely location for a GTL plant in the United States, and BP even launched a pilot plant on the Kenai Peninsula. However, there are no current plans for the production of base stocks there, only diesel fuel.

Certainly, synthetics of the Group IV and Group V variety (polyalphaolefins, esters, alkylated naphthalenes and others) will have a place in the base stock mix. Their low viscosity, high viscosity index and low volatility play well into the growth of low-viscosity engine oils. Their cost will be a concern but they do provide the necessary properties. Whether or not these base stocks are used will be determined by the ability of oil marketers to produce finished oils with the right set of properties.

Beyond the introduction of more hydroprocessed base stocks, Group III produced by GTL processes and the use of synthetics, there dont seem to be any other available technologies which will create more low-viscosity, low-volatility and highly stable base stocks.

The next few years could fulfill the old Chinese curse, May you live in interesting times!