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):
| Time Needed for
Oil Droplet (0.85 Sp. Gr.) in Water to Rise 3 inches |
|
Droplet size, microns
|
Rise Time
|
|
300
|
12 sec
|
|
150
|
42 sec
|
|
60
|
4 min 12 sec
|
|
30
|
17 min 24 sec
|
|
15
|
1 hour 8 min 54 sec
|
|
5
|
10 hour 2 min 9 sec
|
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 www.lantecp.com/casestudy/cs45.pdf.
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.
