Hydrocracked Base Oils What Are They?
Base Oil Manufacture
Before describing the Hydrocracking process and comparing it to the Solvent Refining process, we will first review the general principles of lubricant base oil manufacture. Lubricant base oils are produced in a series of steps, which are designed to enhance certain desirable properties. These include viscosity index, oxidation resistance, thermal stability and low temperature fluidity.
Starting from petroleum crude oil, the typical process for making a lubricant base oil is as follows:
- Separation of lighter boiling materials, such as gasoline, diesel, etc
- Distillation to give desired base oil viscosity grades
- Selective removal of impurities, such as aromatics and polar compounds
- Hydrodewaxing to improve low temperature fluidity
- Finishing to improve oxidation resistance and heat stability
Generally both Solvent Refined and Hydrocracked base oils are manufactured this way, but differ in the methods employed
Solvent Refining Process
Developed over seventy years ago, this process attempts to remove the undesirable components from the feed, by solvent extraction. Initially, light oils such as gasoline, diesel, etc are separated from crude petroleum by atmospheric distillation. The resulting material is charged to a vacuum distillation tower, where lubricant fractions of specific viscosity ranges are taken off. These fractions are then treated individually in a solvent extraction tower. A solvent, e.g., furfural, is mixed with them and extracts about 80% of the aromatic material present. After reducing the aromatic content, the solvent extracted lube fraction is dewaxed by chilling to a low temperature, which removes much of the wax and so improves the low temperature fluidity of the product. Finally, the dewaxed lube fractions are sometimes finished to improve their colour and stability, depending on the application requirements. One common method of finishing is mild hydrofinishing. This step should not be confused with Hydrocracking process, as conditions of temperature and pressure in hydrofinishing are mild and less effective.
In the Hydrocracking process, the elimination of aromatics and polar compounds is achieved by chemically reacting the feedstock with hydrogen, in the presence of a catalyst, at high temperatures and pressures.
Several different reactions occur in this process, the principal ones being:
- Removal of polar compounds, containing sulphur, nitrogen and oxygen
- Conversion of aromatic hydrocarbons to saturated cyclic hydrocarbons
- Breaking up of heavy polycyclo-paraffins to lighter saturated hydrocarbons
These reactions take place at temperatures as high as 400°C, pressures around 3000 psi and in the presence of a catalyst. The hydrocarbon molecules that are formed are very stable and this makes them ideal for use as lubricant base oils.
There are three stages in the Hydrocracking process. The first one removes unwanted polar compounds and converts the aromatic components to saturated hydrocarbons. After separation into desired viscosity grades by vacuum distillation, batches of waxy lube base oil are hydrodewaxed. These are then passed through another hydrocracker for additional saturation. This final step maximizes base oil stability, by removing the last traces of aromatic and polar molecules and is called hydrofinishing.
Hydrocracking results in base oils having the following attractive features:
- Very High Viscosity Index (100 to 130)
- Low Volatility
- Superior Oxidation Resistance
- High Thermal Stability
- Excellent Low Temperature Fluidity
- Low Toxicity
These features give performance characteristics in finished lubricants very similar to synthetics, such as poly-alpha-olefins (PAO).
Comparison of the Products of Hydrocracking and Solvent Refining Base Oils
There are significant differences in certain characteristics between Hydrocracked and Solvent Refined base oils. The main reason for the difference lies in the virtual elimination of aromatic molecules (less than 0.5%) in Hydrocracking process. Hydrocracked base oils are termed “99.5+% Pure”. In comparison, the aromatics content of Solvent Refined base oils is somewhere around 20%; so Solvent Refined base oils are considered only “80% Pure”.
|COLOUR||Hydrocracked base oils are clear and colourless|
|VISCOSITY INDEX||Hydrocracked base oils have higher VIs so they ‘thin-out’ less at high temperatures than Solvent Refined oils.|
|OXIDATION RESISTANCE||Hydrocracked base oils respond very well to anti-oxidants and so have superior resistance to oxidation and a longer lubricant life.|
|THERMAL STABILITY||Hydrocracked base oils have considerably better resistance to heat than Solvent Refined oils.|
|CARBON RESIDUE||Hydrocracked base oils produce lower residues than Solvent Refined oils.|
|DEMULSIBILITY||Hydrocracked oils separate easier from water than Solvent Refined oils|
|LOW TOXICITY||Hydrocracked base oils have low toxicity, due to a virtual absence of impurities.|
|BIODEGRADABILITY||Hydrocracked base oils biodegrade faster than Solvent Refined oils – 60% vs 30%, as measured by the CEC-L33-A-93 test.|
Lubricating Base Oils Groups.
American Petroleum Institute (API) designates several types of lubricant base oil identified as:
- Group I – Saturates < 90% and/or Sulfur >0.03% and Viscosity Index >= 80 to <120
– Manufactured by solvent extraction, solvent or catalytic dewaxing. Common Group I base oil are 150SN (solvent neutral), 500SN, and 150BS (brightstok)
- Group II – Saturates >= 90% and Sulfur <=0.03% and Viscosity Index >= 80 to <120
– Manufactured by hydrocracking and solvent or catalytic dewaxing and hydro-finishing processes. Group II base oil has superior anti-oxidation properties since virtually all hydrocarbon molecules are saturated. It has water-white color.
- Group III – Saturates >= 90% Sulfur <=0.03% and Viscosity Index >= 120
– Manufactured by special processes such as isohydromerization. Can be manufactured from base oil or slax wax from dewaxing process.
- Group IV – Poly Alpha Olefins (PAO)
- Group V – All others not included above such as naphthenics, PAG and esters.
What is Oil Oxidation?
Over time, oil tends to break down by reacting with dissolved atmospheric oxygen. This oxidation starts a chain reaction that first forms hydro peroxides and then progresses to other oxidation products – all of which increase acidity and viscosity, darken colour, and leave surface deposits and varnish. By eliminating the initial hydro peroxides and by interrupting the chain sequence, oxidation-inhibiting additives slow this deterioration rate by more than a hundredfold.
Useful life continues through an induction period as the oxidation inhibitor supply is slowly depleted.
Deterioration rate depends strongly on temperature. Although adding an inhibitor delays life-ending breakdown, slow accumulation of oxidation products and contaminants such as wear particles and soot in engine oils eventually signal a need for an oil change.
The good news is that life expectancy is extended with the paraffinic structure of new hydrocracked Group II and Group III oils. Absence of aromatic hydrocarbons gives more effective oxidation inhibitor action, minimizes sludge and varnish deposits, and generally avoids related machinery problems.
Laboratory bench tests are traditionally used to evaluate oxidation life. For example, the turbine oil stability test (TOST-ASTM D943) bubbles oxygen through an oil sample in contact with water and metal catalysts at 95ºC. Because TOST time takes several thousand hours with better base oils and additives, the more aggressive rotating pressure vessel oxidation test (RPVOT) raises pressure to 90 psi at 150ºC. Both tests measure the length of an initial induction period involving only slow oxidation. This induction period typically precedes much more rapid oxidation as measured by increased oil acidity (TOST) or a drop in oxygen pressure (RPVOT).
Typical test lives in the table below indicate there is an approximate threefold increase with hydrocracked Group II base stocks compared to Group I for premium turbine grade and hydraulic mineral oils used in turbines, compressors, electric motors and generators, and a wide range of industrial applications.
|Typical Test Life|
|TOST life, hours||4000||18000|
|RPVOT life, min||500||1800|
|TOST life, hours||2000||6000|
Oxidation Test Life with Solvent-refined Group I and Hydrocracked Group II Oils
Commonly Asked Questions on Hydrocracked Base Oils
1. What are hydrocracking and hydrodewaxing?
Hydrocracking and hydrodewaxing are refining processes that use catalyst and hydrogen at high pressure to make high-quality lubricant base oils. Hydrocracking is used to improve VI (Viscosity Index) and remove impurities, while hydrodewaxing converts wax molecules into high quality lubricant components.
2. What do Group I, II, and III mean, and what’s so great about Group II and Group III?
Groups I, II, and III are broad categories of base stocks developed by the American Petroleum Institute (API) for the purpose of creating guidelines for licensing engine oils. Typically, solvent-refined base oils fall into Group I, while hydro processed base stocks fall into Group II. Unconventional Base Oils (UCBOs) or Very-High VI stocks are normally categorized as Group III.
Group I oils contain high levels of sulphur and aromatics, which are compounds that can diminish performance. Group II & III oils have lower levels of these impurities, which result in enhanced oxidation performance for fully-formulated lubricants. With hydrodewaxing technology, Group II and III base oils have low-wax composition, which delivers improved low-temperature performance compared to conventional Group I base oils.
Due to their high level of purity, Group II & III base oils provide additional benefits in crankcase applications. For example, in heavy-duty engines, motor oils made with Group II & III base oils have demonstrated a soot dispersancy markedly higher than those made with conventional base oils. They have also demonstrated potential for greater fuel economy in passenger car engine oils.
3. What makes Group II & III base oils more resistant to oxidation?
Group II & III base oils contain lower levels of reactive compounds compared to solvent-refined Group I base oils. These “impurities,” which include aromatics and sulphur compounds, are much more susceptible to oxidative attack. Once these compounds begin to oxidize, a complex chain of reactions occurs that ultimately causes both the base oil and the additive to degrade. The virtual absence of these impurities means Group II & III base oils deliver exceptional resistance to oxidation.
4. Are Group II & III Base Oils synthetic?
A recent ruling from a respected advertising self-regulatory body decided a case on the use of the term synthetic. It found that synthetic base stocks are not limited to PAOs. The decision said that the key requirement for calling a base stock synthetic is that it be the result of conversion or processing of one complex mixture. Group II & III base oils clearly meet the test.
5. Are Group II & III base oils better for the environment?
Yes. Group II & III base oils have low toxicity as measured by eye and skin irritation, inhalation, and oral and dermal toxicity tests. They are neither mutagenic nor carcinogenic, as indicated by their performance in the modified Ames test and IP346 polycyclic aromatic test. In fact, these base oils are so pure, they meet the requirements of FDA-approved mineral oils (21CFR 178.3620(c)). This means that Group II & III base oils can be used in or used to manufacture a variety of non-food articles intended for incidental contact with food.
6. Why should I use Hydrocracked base oils?
Hydrocracked base oils offer superior product performance, resulting in greater oxidation and thermal stability, soot dispersancy in Heavy Duty Motor Oils, and low temperature performance. In addition to these benefits, hydrocracked base oils also have high VI and low volatility.
Hydrocracked base oils can be formulated with a wide variety of additives either to achieve the latest industry specifications or simply to produce premium performance lubricants.
Hydrocracked base oils can help to meet challenging future lubricant specifications cost-effectively, whereas Group I oils often cannot. Hydrocracked base oils are especially valuable where they can replace traditional synthetic base oils to achieve synthetic performance.