In this post we will present different methods of oil/water separation and will analyze feasibility of using them to improve the efficiency of skimmers and pumping units.
Gravity Differential Separation: The oldest and more extended separation method. It’s usually the first step in the treatment of oily water. Ruled by the Stokes’ law is based on the different density of two immiscible fluids:
  •         The more different densities are, the better separation.
  •         To reach higher efficiency a long residence times and large oil drops are required.

Normally, separation efficiency ranges between 20% and 60%. However, to accelerate the process can make use of coalescence.
Separator Types:
  •         API separators
  •         Circular separators
  •         Plate separators (parallel and corrugated)
  •         Curved-Plate Finger Separators

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Rotational Separation: It was conceived to accelerate the gravity differential separation processes, since centrifugal forces are much greater than gravity. A difference of 5% between densities is enough to separate fluids, but the larger is such difference the faster the separation and the lower the energy consumption. The separation efficiency ranges between 77% and 91%. Using coalescence these values may increase.

Types:
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  • Centrifuges: Oily water is moved along a circular path by the rotational motion of the device.
  • Cyclones: The liquid is forced into circular motion due to tangential injection of the oil/water misture.
  • Vortex: Separation of oil/water mixtures is accomplished by imparting relatively large rotational motion to the mixture, in a cylindrical vessel.

Filtering is one of the oldest methods used. It is very effective for removing suspended matter especially hydrocarbons.
  •       Multimedia: consists of passing the oil/water mixture through a set of layers of different solid materials of different size and density. Its biggest advantage is the lower pressure drop for high flow rates to be treated without reducing the quality of the effluent.
  •     Absorption adsorption filters: The activated carbon and graphene, during the recent years, have demonstrated a high adsorption capacity. Saturation of cartridges and regeneration of materials are the main problems that researchers are facing today to overcome the barrier of feasibility. Efficiency between 95 and 100%.

Membrane: In recent decades they have developed water-repellent membranes and hydrophilic materials. To be effective, The membranes must be chosen according to the characteristics of the substance to be separated and find difficulties to separate viscous hydrocarbons. It is still a very expensive technology and can be justified as the last filtration step to remove the last traces of hydrocarbon from water.
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Electrodialysis: Separation takes place through the application of an electric current. It has found greater application in the field of desalination plants.
 
Reverse Osmosis: Applying a pressure to the substance to treat oil and water are separated on the membrane surface since the former have a molecular dimension greater than water, that can pass through the membrane.
 
Ultrafiltration: Similar to the reverse osmosis works at much lower pressures.
 
Coalescence: Used to group dispersed hydrocarbons in aqueous solution. Generally it helps speed the aforementioned separation processes (gravitational, rotational, filtration). In some cases you can reach efficiency values of 90-98%
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Other technologies:
  •         Electromagnetic separation
  •         Thermal separation
  •         Ultrasonic separation
  •         Chromatographic separation

To achieve an almost perfect separation we will need the succession of different methods used in cascade with the right combination to meet the needs of each case.
Reverse osmosis, ultrafiltration and adsorption by active carbon or graphene are the most potential methods but entail high operating costs, mainly energetic, and auxiliary equipment. Meanwhile gravity separation needs major work volumes and long residence times is used as a pretreatment. While filters, because of their ease of saturation and high costs in some cases are suitable for a final stage of separation in which well reach 100% efficiency.

 

This analysis of separation technologies follows that the most appropriate to improve skimmers performance are coalescence and rotational separation.