Article updated on 8 July 2020.
Oils and lubricants are an essential part of many mechanical systems; be it a machine in the industrial sector, or the engine of an automobile. Lubricants provide a way for two surfaces to slide over each other without grinding and causing wear. However, given the environments that they are used in, many oils and lubricants are susceptible to various contaminants. One way to remove the water and other volatile components within a lubricant is through vacuum distillation. In this article, we look at lubricants, the issues of lubricant contamination, and how vacuum distillation is a useful technique for assessing the contamination levels of lubricants.
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The Importance of Oils and Lubricants
Lubricants and oils are used across many different areas of industry, with the most common use being within industrial machines and automotive engines. Lubrication is one of the fundamental aspects of tribology that helps to prevent wear of machinery. Often, it is the lubricant, or oil, within a mechanical system that provides this lubrication to avoid the wear and degradation of metal mechanical components.
Without lubricants, mechanical parts in any given system would grind each other down very quickly, and parts would need to be replaced more often than it would be economically feasible to do so. While lubricants don’t entirely stop wear from happening, they slow it down significantly so that mechanical parts can be used for years without needing to be replaced.
Lubricant Contamination
Because lubricants provide a barrier between metal surfaces, metal contaminants can enter the lubricant and reduce the lubricating properties of the lubricant/oil. The impurities within the flow can affect the laminar flow properties of the lubricant, and because these types of contaminants are made of metal, they will grind against the mechanical components and wear them down.
There are many methods which can be used to remove metals from lubricants. However, distillation is not one of them. Distillation of lubricants is concerned with the other main contaminant of lubricants – water. While a conventional mixture of oil and water is immiscible (and not a concern), the high shear environments of mechanical systems cause the water (and other low weight volatile contaminants) to become one with the oil/lubricant system. The lubricating properties of these unwanted additives are not as efficient as the lubricant itself, so fluidic impurities can cause parts of the lubricant to become ineffective (which has a knock-on effect of increased wear within the mechanical system).
The presence of water can also cause some of the iron-based components in a mechanical system to become oxidized and rust. Rust has a two-fold effect. Not only does it cause the degradation of the component itself, but it can also become easily detached from the component (and go into the lubricant), which further reduces the effectiveness of the lubricant. So, while water and other volatile components may not seem like a big issue, they in fact are and should be removed.
Vacuum Distillation
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Vacuum distillation is one of the most common methods for the analysis and removal of water, and other volatile compounds, within lubricant and oil samples. Distillation is an approach that separates out the constituents of a fluid by their boiling point (i.e., the phase change from a liquid to a vapor). Because water, oils/lubricants, and other volatile aqueous and organic constituents, all have different boiling points, it enables the various constituents within the lubricant to be removed and determined, while leaving the lubricant un-vaporized (due to the higher boiling point).
Vacuum distillation is the same process as conventional distillation, with the only difference being that the distillation occurs at pressures below one atmosphere. The reduction in pressure also means that water evaporates at a lower temperature (57 °C instead of 100 °C, or 135 °F instead of 212 °F).
There are two main reasons why a vacuum is employed in these types of distillation analysis approaches. The first is that because the constituents evaporate at lower temperatures, a high temperature is not required for the distillation, and this prevents thermal degradation of the oil and its additives. The second is that the energy requirements for the heating, and subsequent cooling, are lower, so it becomes a cheaper process (something which is vital for the industry).
Each type of lubricant needs to be analyzed separately to provide an accurate analysis of the contaminants in each system. This is particularly important in the industry to minimize both downtime and cross-contamination of lubricants which have additives that are incompatible with other lubricants.
Water in lubricants can take many forms, including dissolved water, free water and as water in oil emulsion. Vacuum distillation can be used to remove all forms of water contamination within a lubricant. Aside from water, the distillation process can also identify hydrogen disulfide, fuel, refrigerants, solvents and light hydrocarbons. Other methods can be used to remove water from lubricant systems, but the low energy consumption of vacuum distillation makes it one of the cheapest and easiest methods for the industry to implement.
Sources:
- Machinery Lubrication: https://www.machinerylubrication.com/Read/162/vacuum-distillation-water-oil
- “A review of methods for the demetallization of residual fuel oils”- Ali M. F. and Abbas S., Fuel Processing Technology, 2006, DOI: 10.1016/j.fuproc.2006.03.001
- “Vacuum Distillation Methods For Lube Oils Increase Turbomachinery Reliability”- Adams M. A. and Bloch H. P., Turbomachinery and Pump Symposia, 1988, DOI: 10.21423/R1WW9Q
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