Lubricating Oils introduction

Lubricating Oils

Oil Refining

Most lubricating oils are currently obtained from distillation of crude petroleum. Due to the wide variety of petroleum constituents, it is necessary to separate petroleum into portions (fractions) with roughly the same qualities.

The refining process can be briefly described as follows:

(1) Crudes are segregated and selected depending on the types of hydrocarbons in them.

(2) The selected crudes are distilled to produce fractions. A fraction is a portion of  the crude that falls into a specified boiling point range.

(3) Each fraction is processed to remove undesirable components.

The processing may include:

  -Solvent refining to remove undesirable compounds.

  -Solvent dewaxing to remove compounds that form crystalline materials at low

   temperature   

  -Catalytic hydrogenation to eliminate compounds that would easily oxidize.

  - Clay percolation to remove polar substances.

(4) The various fractions are blended to obtain a finished product with the specified viscosity.

Additives may be introduced to improve desired characteristics.

Separation into fractions

Separation is accomplished by a two-stage process

(1)               Crude distillation.

In the first stage the crude petroleum is mixed with water to dissolve any salt. The resulting brine is separated by settling. The remaining oil is pumped through a tubular furnace where it is partially vaporized. The components that have a low number of carbon atoms vaporize and pass into fractionating column or tower. As the vapors rise in the column, cooling causes condensation. By controlling the temperature, the volatile components may be separated into fractions that fall within particular boiling point ranges. In general, compounds with the lowest boiling points have the least carbon atoms and compounds with the highest boiling points have the greatest number of carbon atoms.This process reduces the number of compounds within each fraction and provides different qualities. The final products derived from this first-stage distillation process are raw gasoline, kerosene, and diesel fuel.

 

(2)               Residuum distillation.

The second-stage process involves distilling the portion of the first-stage

that did not volatilize. Lubricating oils are obtained from this portion, which is referred to as the residuum.

To prevent formation of undesired products, the residuum is distilled under vacuum so it will boil at a lower temperature. Distillation of the residuum produces oils of several boiling point ranges. The higher the boiling point, the higher the carbon content of the oil molecules in a given range.

Viscosity also varies with the boiling point and the number of carbon atoms in the oil molecules.

 Impurity removal.

Once the oil is separated into fractions, it must be further treated to remove

impurities, waxy resins, and asphalt. Oils that have been highly refined are usually referred to as premium grades to distinguish them from grades of lesser quality in the producer's line of products. However, there are no criteria to establish what constitutes premium grade.

Types of Oil

Oils are generally classified as refined and synthetic.

Paraffinic and naphthenic oils are refined from crude oil while synthetic oils are manufactured.

Reference  is often made to long chain molecules and ring structures in connection with paraffinic and naphthenic oils, respectively. These terms refer to the arrangement of hydrogen and carbon atoms that make up the molecular structure of the oils.

The distinguishing characteristics between these oils are noted below.

Paraffinic oils.

 Paraffinic oils are distinguished by a molecular structure composed of long chains

of hydrocarbons, i.e., the hydrogen and carbon atoms are linked in a long linear series similar to a chain

Paraffinic oils contain paraffin wax and are the most widely used base stock for lubricating oils. In comparison with naphthenic oils, paraffinic oils have:

- Excellent stability (higher resistance to oxidation).

- Higher pour point.

- Higher viscosity index.

- Low volatility and, consequently, high flash points.

- Low specific gravities.

 Naphthenic oils.

Naphthenic oils are distinguished by a molecular structure composed of “rings” of hydrocarbons, i.e., the hydrogen and carbon atoms are linked in a circular pattern. These oils do not contain wax and behave differently than paraffinic oils. Naphthenic oils have:

- Good stability.

- Lower pour point due to absence of wax.

- Lower viscosity indexes.

- Higher volatility (lower flash point).

- Higher specific gravities.

Naphthenic oils are generally reserved for applications with narrow temperature ranges and where a low pour point is required.

 Synthetic oils.

Synthetic lubricants are produced from chemical synthesis rather than from the refinement of existing petroleum or vegetable oils. These oils are generally superior to petroleum (mineral) lubricants in most circumstances. Synthetic oils perform better than mineral oils in the following respects:

- Better oxidation stability or resistance.

- Better viscosity index.

- Much lower pour point, as low as -46 °C (-50 °F).

- Lower coefficient of friction.

The advantages offered by synthetic oils are most notable at either very low or very high temperatures.

Good oxidation stability and a lower coefficient of friction permits operation at higher temperatures.

 The better viscosity index and lower pour points permit operation at lower temperatures.

The major disadvantage to synthetic oils is the initial cost, which is approximately three times higher than mineral-based oils. However, the initial premium is usually recovered over the life of the product, which is about three times longer than conventional lubricants. The higher cost makes it inadvisable to use synthetics in oil systems experiencing leakage.

 Factors to be considered when selecting synthetic oils :

- pour and flash points

 -demulsibility

- Lubricity

- Rust and corrosion protection;

-thermal and oxidation stability

-Anti-wear properties

-Compatibility with seals, paints, and other oils;

-compliance with testing and standard requirements.

 “Chart of Interchangeable Lubricants,” it is important to note that synthetic oils are as different from each other as they are from mineral oils. Their performance and applicability to any specific situation depends on the quality of the synthetic base-oil and additive package, and the synthetic oils are not necessarily interchangeable.

d. Synthetic lubricant categories.

(a) Synthesized hydrocarbons. Polyalphaolefins and di alkylated benzenes :

These lubricants provide performance characteristics closest to mineral oils and are compatible with them.

Applications include engine and turbine oils, hydraulic fluids, gear and bearing oils, and compressor oils.

(b) Organic esters. Diabasic acid and polyol esters

 The properties of  these oils are easily enhanced through additives. Applications include crankcase oils and compressor lubricants.

(c) Phosphate esters. These oils are suited for fire-resistance applications.

(d) Polyglycols. Applications include gears, bearings, and compressors for hydrocarbon gases.

 (e) Silicones. These oils are chemically inert, nontoxic, fire-resistant, and water repellant. They also have low pour points and volatility, good low-temperature fluidity, and good oxidation and thermal stability at high temperatures.