Production Processes of Lubricant Base Oils

Traditional Production Processes

Until the 1930s, the production of lubricant base oils primarily relied on physical refining methods, known as the "traditional three-step process," which includes solvent refining, solvent dewaxing, and clay finishing.

1. Solvent Refining

   Solvent refining is a key step in lubricant production, mainly aimed at removing polycyclic aromatics, resins, asphaltenes, and other undesirable components from the oil. This process improves properties such as viscosity-temperature characteristics, oxidation stability, carbon residue value, and color. The technology is well-established, with common solvents including furfural, phenol, and N-methylpyrrolidone (NMP).

2. Solvent Dewaxing
   The solvent dewaxing process consists of crystallization, filtration, solvent recovery, and refrigeration. Its purpose is to remove wax from the oil, thereby reducing the pour point of the base oil. This method offers technical advantages when processing lighter feedstocks, yielding a high recovery of dewaxed oil with a higher viscosity index. To reduce energy consumption, operational costs, and capital investment, most base oil producers now integrate solvent dewaxing with wax deoiling. Significant improvements in energy efficiency, oil yield, and wax recovery have been achieved in recent years.

3. Clay Finishing
   Clay finishing involves mixing oil with activated clay at elevated temperatures. By leveraging the adsorption capacity of the clay, impurities such as nitrogen compounds, resins, asphaltenes, naphthenic acid soaps, unsaturated hydrocarbons, residual solvents, water, and mechanical contaminants are removed through heating, evaporation, and filtration. This step enhances oil color, reduces carbon residue, and improves oxidation stability and demulsibility.

Hydroprocessing Technology

Ideal components in base oils include branched-chain alkanes (isoparaffins) and monocyclic naphthenes with long alkyl side chains, while undesirable components are polycyclic aromatics and polycyclic naphthenes. Traditional solvent refining selectively extracts low-viscosity-index polycyclic hydrocarbons and other heterocyclic compounds (e.g., sulfur, nitrogen compounds, resins), thereby improving viscosity index, color, and oxidation stability. However, as a physical separation method, it only retains the ideal components originally present in the feedstock, offering limited improvements in yield, viscosity index, and other properties.

In contrast, hydroprocessing deeply converts polycyclic hydrocarbons into desirable components through hydrogenation, while nearly completely removing heterocyclic compounds. This results in higher base oil yields and more significant enhancements in quality parameters.

Key chemical reactions during hydroprocessing include:

① Removal of heterocyclic compounds;

② Aromatic saturation, ring-opening of naphthenes, and isomerization—the most critical reactions for viscosity index improvement;

③ Hydroisomerization of linear or lightly branched alkanes into highly branched isoparaffins;

④ Hydrocracking of alkanes and hydrodealkylation of naphthenes with long alkyl side chains. While the first three reactions are beneficial, the fourth leads to light oil formation and reduced base oil yield, and thus must be minimized.

Isodewaxing Technology

With increasingly stringent environmental regulations and advances in machinery industries—especially the automotive sector—higher performance is required from lubricants. To produce such high-performance lubricants, particularly wide-span multigrade engine oils, base oils must exhibit low volatility and high viscosity index (VI > 120) with saturation levels above 90%. API Group III base oils, consisting mainly of isoparaffins, meet these requirements.

Neither solvent dewaxing nor catalytic dewaxing can produce Group III base oils, as these methods only remove high-VI linear paraffins from the oil, resulting in a lower VI base oil. They do not convert these high-VI linear paraffins into high-VI, low-pour-point isoparaffins, leading to reduced yield and failure to meet high-quality base oil specifications.

Isodewaxing works by isomerizing high-pour-point linear paraffins into low-pour-point branched alkanes using specialized molecular sieve catalysts. It has become a key technology for producing API Group III base oils today. The main commercialized isodewaxing technologies include Chevron's Isodewaxing process and ExxonMobil's MSDW (Mobil Selective Dewaxing) technology.