Oil Water Separator Applications

Oil in Water Coalescing Media – How it Works

An oil water separator (OWS) contains a coalescing medium or media. This media provides a suitable surface for oil droplets to meet and grow, or coalesce, into larger droplets. As oil droplets grow in size the buoyancy of the droplets increases. The droplets rise towards the surface of the water due to the fact that the specific gravity of oil is less than the specific gravity of water. In this way the oil will form a layer that can be separated from the water by skimming action before the water is reused or discharged. Any heavy solids present in the water being treated, or sludge, in theory should fall into the sludge compartment of the OWS unit. As oil droplets coalesce into larger droplets, the buoyancy of the droplets increases. This is reflected in the known rise time for a given size of oil droplet. The more efficient the coalescence action of the media, the larger the oil droplets become. Larger oil droplets result in reduced rise time (1):

The importance of oil droplet size is obvious in the above table. This importance is also noted in Stoke’s Law that defines the terminal rise velocity of a given sized oil droplet:

V = (2gr2) (Dd) / 9m

g = acceleration of gravity constant r = oil droplet radius
Dd = difference in Sp. Gr. of water and oil m = viscosity of water

Note: V increases in proportion to the square of droplet radius!

Traditional coalescing media have been variations of corrugated, inclined plates. By definition half of the surface of this type of media must always be aligned upward – which is the wrong direction for support of coalescing action. To attempt, in part to overcome this problem by adding more specific surface, plate separation can be made more narrow. This solution only solves the problem partially as the ratio of incorrect to correctly oriented surface remains 50/50. But as the spacing of plates narrows, plugging of the coalescing media – and hence failure of the OWS unit – becomes more likely.

Possible plugging and fouling of the media is also supported by the ~ 45º angle of inclined plate coalescing medias. As an example, plugging of coalescing media was a major problem in the oil water separator at the car wash in Scandinavia, see Case Study #45. Heavy particles could not fall into the sludge compartment. HD Q-PAC^®, made in oleophilic polypropylene, with all rounded surfaces at a 90º angle vs. the direction of water flow and many slender rods pointing down towards the sludge compartment, offers a solution to the maintenance problems that, until now, have been accepted practice in the oil water separation industry.

HD Q-PAC^® Coalescing Media vs. Conventional Media

HD Q-PAC^® can be seen above to be a radical departure from the conventional, inclined or corrugated plate design of past OWS coalescing media. An obvious consequence of this difference in the design is that, as opposed to being forced to leave holes in the media to allow for oil droplet rise (note this aspect of traditional OWS media, supplied by Facet as shown in picture on right) – the design of HD Q-PAC^® (close-up of HD Q-PAC^® needles shown in photo on left) allows for steady, unimpeded rise of oil droplets upward and sludge downward throughout its entire structure. One advantage of using polypropylene is that the OWS can operate with water temperatures up to 200°F (93°C). The cheaper corrugated medias from Munters and Brentwood are made from PVC and begin to weaken at 120°F (49°C)

With all rounded elements, the entire surface area of 132 ft2/ft3 (433 m2/m3) of HD Q-PAC^® is available to support oil droplet coalescence. As a result, there is no need for any type of 2nd stage polishing to achieve needed oil removal. This fact is based upon HD Q-PAC^® fulfilling the European Union’s EN 858-1, Test Method for Class I Coalescing Separator.