Remove Solid Particles. Solid particles are the most common and the most unwelcome contaminants of mineral oils. They are removed by physical, chemical and physico-chemical methods. In this article we will look at the physical processes, such as sedimentation and centrifugation.
The use of gravity is perhaps the oldest method of cleaning contaminated oil. It is not surprising, because it is relatively simple. If the water microdroplets and mechanical impurities have sufficient size and density substantially higher than the oil density, then they settle under gravity.
Settling tanks can be static and dynamic. Static settling tanks are simpler, but more time-consuming, and a sufficient degree of purification can be achieved only after heating the oil to reduce its viscosity.
Dynamic settling tanks are equipped with additional devices to reduce settling time, which complicates maintenance of these devices. This is the reason for their rare use.
The effectiveness of sedimentation is influenced by detergent additives or substances that prevent particle agglomeration and formation of particles through coagulation.
Remove Solid Particles. The main drawback of sedimentation is the duration of the process which in many cases does not provide the necessary degree of purification. Therefore, it is used mainly as a pretreatment step for cleaning waste oils.
The oil can be cleaned by running through a hydrocyclone or a centrifuge. The hydrocyclone is simpler than a centrifuge. In hydrocyclone the oil rotates inside a stationary shell, and the centrifuge shell rotates itself.
The advantage of hydrocyclones is their small size and the absence of moving parts, but their productivity reduces with increased diameter of the shell. Also, the cleaning efficiency depends on the flow rate of the fluid.
Centrifuges are generally better at cleaning oil, creating a small pressure and ensuring a constant throughput. But such devices are more complex in construction and require skilled maintenance during operation.