The rise rate of the oil to surface is determined by Stoke’s Law. There are three main factors affecting the rise rate; oil droplet size, oil specific gravity and temperature. Other factors include oil/dirt particles and flow rate or turbulence. According to Stoke’s Law, a 100 micron size oil droplet will rise three inches in five minutes. When factoring in a flow rate, you can see how a simple oil/water separator will have to be quite large to give the oil enough time to rise to the surface. A 20 micron size oil droplet will rise three inches in 60 minutes. Large oil droplets are more buoyant and, therefore, rise faster.

In order to reduce the physical size of the oil/water separator, coalescors have been used successfully for many years. The concept of a coalescor is to use oleophillic (oil loving) media such as polypropylene or teflon. As oil and water flow through the media, oil droplets impinge on the media and coalesces on the surface. Coalescing, or binding together, makes them larger and more buoyant. As you can see from the above example, a 100 micron oil particle will rise three inches twelve times faster than a 20 micron particle.

Typical Three-Part Oil/Water Separator

Now to further enhance this process, we can use these coalescing media as incline plates thereby drastically reducing the rise or fall of a particle.

By using coalescing incline plates, the rise of an oil droplet can be reduced to 1/4" until it hits the upper surface of the plates and glides up as it coalesces with other particles. Also, solids or dirt will settle and glide downward on the plate surface. As it attaches to other dirt particles, the coalesced oil particles will gain buoyancy and rise rate, and the dirt particles will gain speed as they grow heavy and settle at a faster rate.

Example:

Solids Settling – Standard Separator vs. Inclined Plates

It takes fine sand 0.1 mm diameter 38 seconds to settle 12 inches in an open tank or 114 seconds in a typical 3 ft. deep separator. By adding ¼ inch incline 60º plates, you reduce the settling time by a factor of 72, Therefore, the addition of ¼ inch incline plates will reduce the settling time of a 0.1 mm diameter fine sand from 114 seconds in a standard separator to 1.58 seconds in a ¼ inch incline plate separator. The same calculation can be made for oil droplet sizes and rise time incline plates.

In addition to incline plate coalescing separation, RGF utilizes a micro matrix coalescor system of finely spaced poly mesh with an incline grid matrix.

This RGF system of micro matrix and HCA-3 hydrocarbon absorbers are used in our Marine Bilge Filter Systems and has achieved results of .87 ppm during a two day UL/U.S. Coast Guard Test consisting of blended 25% to 100% diesel fuel and oil under simulated sea conditions.

The addition of auto oil skimmers and the RGF Oil Accumulator makes free oil removal easy.

Note: The use of RGF's OWS Bio-Disks can reduce both free and emulsified oils by greater than 50% using natural bacteria.

Emulsified oils either chemically or mechanically emulsified will not separate in a gravity separator. Emulsion splitting methods will be necessary before the emulsion reaches the separator. Alternately, quick release detergents or cleaning chemicals can be used, which will release the emulsion and permit the gravity action to occur. Alternately, a floccing or membrane system can be used for emulsified oily water problems.

 

Water Volume vs. 15 ppm of Oil Contamination