Lube Oil Production Process: Traditional Methods and Contemporary Developments
Lube oil applications involve the use of minimizing friction or wear in mechanical systems. The manufacture of lubricating oil requires complex processing that changes raw materials into functional products of high quality.
The main feedstock to make lube oil is heavy oil provided by crude oil. Heavy oil is subjected to fractional distillation, and various fractions along with residues, at relatively low temperature and pressure, are separated by a process called vacuum distillation to make various lube oils of different viscosities. These fractions thus obtained fall into two categories: fractional lube oils and residual lube oils.
This article will review the conventional processes for producing lube oils, which this section will refer to by the usual colloquialism, the “old three-step” process comprising solvent refining, solvent dewaxing, and clay refining. Discussion of recent developments in the hydroprocessing of lube oils is also included.
Solvent Refining
Solvent refining is a vital process that enhances lubricating oil quality through the elimination of harmful substances and the removal of unwanted materials such as resins, asphaltenes, and polycyclic aromatic hydrocarbons. Among various refining methods like acid-base refining, adsorption refining, and hydroprocessing, solvent refining is extensively employed.
Solvent refining involves dissolving non-ideal components from lube oil in a selective solvent, separating these components, and subsequently evaporating and recycling the solvent. The choice of solvent is essential for successful solvent refining:
1. Selection Criteria for Solvent
- Strong selective dissolution: the solvent should dissolve nonideal components in the raw oil, while not dissolving ideal components in it. Actually, the process is just a kind of solvent extraction.
- Higher density: Higher density means that the solvent is denser than the raw oil. Solvent is introduced from the top of the extraction tower, while the introduction of oil is from the bottom. By flowing counter-currently, the solvent effectively dissolves non-ideal components from the oil.
- Low boiling point for easy recovery: The solvent must be generally recycled and, therefore, requires its boiling point to be lower than that of the lube oil for efficient recovery.
- Stability and compatibility: It must be stable at high temperatures and not react chemically with the raw materials.
2. Solvent Refining Process
The process consists of the following key steps:
The process begins when crude oil enters the extraction column through its bottom section while operators add solvent from its upper section. The two streams flow countercurrently, with the solvent dissolving non-ideal components from the oil. The mixture is then separated into two phases, which include an extract phase that contains non-ideal components and a raffinate phase that contains ideal lubricant components.
The extraction phase of the process contains solvent-rich material which needs to be heated until a sufficient temperature is reached for solvent evaporation. The vapor undergoes condensation, which enables the process to recover solvent for subsequent reuse.
The raffinate phase, which contains a small amount of residual solvent, is also heated to recover this solvent. The reclaimed solvent can be returned to the extraction step.
A small amount of water is introduced to remove traces of solvent and impurities. The water and solvent form separate layers which enable operators to easily remove the remaining solvent.
Solvent Dewaxing
Dewaxing seeks to bring down the pour point of lubricant oil and also ensures the production of by-products such as paraffin wax, which have commercial importance. Among various methods for dewaxing, solvent dewaxing finds wide applications due to its suitability to many lube oil fractions.
In solvent dewaxing, a solvent is blended with oil to lower the viscosity. Thereafter, it is subjected to cooling to promote crystallization of wax. Following the separation of oil and wax, the solvent recovered is reutilized. Generally, ketone and benzene-based solvents are used in solvent dewaxing processes. The typical operations involved are solvent mixing, heat treatment, contacting, crystallization, and recovery of solvent.
Clay Refining
After refining with a solvent, dewaxing, the lube oil contains impurities like resins, asphaltenes, etc. In clay refining, also called adsorption refining, impurities like resins, asphaltenes, and trace solvent are adsorbed from the lube oil on activated clays. This improves lube oil colors, stability, as well
Activated clay has an open structure with a high surface area. Hence, adsorption capacity is high. Impurities are selectively adsorbed without affecting the lube oil components. Mixing oil with activated clay, heating the mixture, allowing the clay to adsorb the impurities, filtering the oil, etc., are the steps followed in the process that yields the oil.
Lubricant Oil Blending
The quality of lubricant oil depends on the base oil quality as well as the types of additives that are used. Additives, of course, greatly increase lube oil performance to make it suitable for different industrial applications. Blending is one of the crucial processes for the production of lubricant oil.
That is, different viscosity base oils are blended, which are obtained through solvent-refining, dewaxing, and clay- refining processes for lubricant oils. For target performances, selected additives are mixed into the lube base oil. The selection of lube oil blending components/key additives is pretty flexible, and different points will pop up in various intended usage applications. This way, base oils of low viscosity are blended with proper additives to make the desired 20W machine oil of low viscosity.
Conclusion
The production of lubricant oil requires complicated processes, and these processes have serious effects on the quality of the lubricant oil. The typical processes used for the production of lubricant oil, such as solvent refining, solvent dewaxing, and clay refining, have generally contributed to improving the quality of the lubricant oil.
Additionally, modern technology, such as hydroprocessing, has been developed to accommodate the dynamic demands for lube oils with desirable attributes. As a result, these processes change the form of lubricating oils from their crude state to their final form, an essential commodity in efficient mechanical operations.
