Oil mist: what are they? How can we treat them?

Mist is a widespread phenomenon found both in nature and in industry. Underlying its formation is a series of very interesting physical and chemical phenomena that we will examine to explain the genesis of a specific pollutant: oil mists.
In the course of this article, we will answer three important questions about oil mist nature and abatement:

• What are oil mist? To fully understand the properties of oil mists, we will analyse nebulizers and the meaning of the word ‘colloid’;
• How do oil mist form? What are the mechanisms that can lead to the formation of oil mists in industrial contexts?
• Are there technologies able to reduce oil mist dispersed in a gaseous effluent?

To speak about oil mist nature and abatement, it is necessary to introduce a few preliminary concepts which we will examine gradually, starting with an experiment. Let’s imagine we have four different containers: two of them contain distilled water, while the other two respectively contain ethanol and oil.
If we mix water and ethanol together, we obtain a homogeneous mixture in which the ethanol and water molecules mix perfectly together. If, on the other hand, we mix water and oil together, something different occurs: the two elements do not mix together and remain ‘separate’ from each other. This is chemically due to the fact that both water and ethanol have a polar character, whereas oil has a nonpolar character.

Assuming that we have more water than oil, if we mix the two substances with a mechanical whisk, this creates a set of oil droplets immersed in the aqueous solution. It is therefore a heterogeneous mixture, characterised by two distinct phases. In the experiment, two liquid phases have been mixed and the water, since it is present in greater quantity, behaves as a continuous phase, within which the oil droplets are dispersed (dispersed phase).
So, now the question is: can a similar effect be achieved by also considering solid and/or gaseous phases? The answer is affirmative, and the resulting compounds are called colloids: a type of mixture in which one phase is finely dispersed within another.
The characteristic size of the dispersed phase typically ranges between a nanometre and a micron. Based on the state of aggregation of the dispersed phase and that of the continuous phase, it is possible to classify different types of colloid systems, as illustrated in the picture.
The types of colloids of our interest in this article are aerosols and mists, i.e. fine dispersions of liquid particles within a gas phase.

Oil mists, specifically, consist of fine oily droplets dispersed in air. Now that we have a clear understanding of what oil mists are, we can examine the mechanisms that determine their formation.

Oil mists typically form in processes that use fuel oils, lubricating oils, hydraulic oils or high-temperature polymer products. The objective now is to understand how these types of substances can, from the liquid phase, form fine dispersions in the air.
The first mechanism worth considering is that of atomisation. If a fluid, held at high pressure, passes through a thin gap or special geometric configurations, it tends to atomise and form fine droplets.
This phenomenon can be used intentionally (e.g. in combustion processes) or it may occur accidentally (e.g. in the case of cracking of a high-pressure tank).

A second phenomenon, common in plant engineering, is vaporisation and nucleation as a result of thermal increases.
Thermal increases in a given substance correlate with an increase in the average kinetic energy of the molecules, leading to a greater tendency to transition into the vapour phase. If the types of oil described above increase in temperature, due to contact with hot surfaces or as a result of pronounced friction phenomena, they tend to transition into the vapour phase.
If the oil in vapour phase encounters cooler areas, the vapour pressure value of the oil decreases significantly, resulting in condensation. Condensation nuclei will then form in the gas phase, giving rise to the aforementioned mists.
Now that we have analysed what oil mists are and how they are formed, let’s try to understand together how to reduce them!

Oil mists can be reduced by exploiting a physical principle known as coalescence. This is the phenomenon through which the particles of a liquid, a gaseous substance or a solid combine to form larger droplets. This principle is employed to ‘magnify’ the droplets and make them easier to remove from the gaseous stream.

Since oil mist are a thermodynamically unstable system, they would tend, perhaps even after an infinite amount of time, to separate from the effluent in which they are dispersed by autonomously coalescing due to the spontaneous coalescence to which they are physically subjected.
In the industrial field, however, one cannot wait an infinite amount of time for this phenomenon to occur naturally; this is why Tecnosida^® has designed the OILSCREEN coalescing filter. This system uses a filtering surface that promotes the contact of oily droplets dispersed in the intercepted air stream. This facilitates the spontaneous coalescence at a kinetic level.
When aggregated, these droplets tend to fall by gravity towards the lower part of the filter where they are collected in a tank for disposal.
The treated effluent, now free of these oily substances, is then sucked in by the fan and released into the environment through the expulsion stack.
To maximise the OILSCREEN filter’s capacity, the treated oils should not have a high viscosity and the airflow should be dust-free. In fact, depending on other possible pollutants in the stream, the system may also require a pretreatment.

In addition to designing and building the air pollutant treatment systems, Tecnosida® also offers periodic maintenance services to guarantee filtration efficiency over time.
We have successfully applied OILSCREEN in a wide variety of contexts, including:

• thermal treatment of metals;
• food packaging;
• high-temperature treatment of synthetic polymers

Contact us for more information: our sales technicians will provide you with the best solution for your needs!