In our business we often get requests for skimmer or separator systems to handle removing oil from water ----or to get water out of oil for reducing disposal costs. Don't confuse the two---- they are completely separate and distinct functions. How you handle the skimming part greatly influences and determines what is necessary for dewatering the oil later.
This edition of the Oily News is going to cover skimming and simple oil water separator systems. We'll discuss oil dewatering next time.
The two rules of thumb for most of these applications is "don't mix them to begin with" and/or "remove the oil from the water ASAP before it can get mixed". While this seems simplistic and obvious----- a lot of people are not aware of how they are creating part of their own problem or at least exacerbating the problem while transferring the oil and water to begin with.
Let's talk about definitions and their differences.
Oil Skimmers are devices for removing FREE-floating oil from water----- on the water. Oil/water separators are flow thru systems that are used for removing generally small amounts of oil from larger amounts of water. Neither are oil dewatering systems.
There are two types of skimmers---- selective and non-selective:
A selective skimmer works by introducing to the oil---- a selective surface (plastic or metal), usually motor driven that attracts and holds the oil, which is then scraped, wiped or squeezed off. These surfaces do not attract and hold water. When possible, selective skimming makes all post skimming operations easier since you don't handle as much water or deal with separating it later. Types include: belt skimmers, drum skimmers, disc skimmers, rope mops and tube skimmers---- all have these oil attracting surfaces. We actually specialize in these systems. These skimmers can be hard mounted or, as in the case of drum and disc skimmers can float on the water, making installation just a matter of putting it in place on/in the water. These selective skimmers are the tools of choice for inland waters oil spills.
Non-selective skimmers are surface skimmers ----- that use water as the carrier of oil into the skimmer, so you wind up getting a goodly amount of water in the skimmed oil---- unless they are sitting in pure oil on top of the water. Usually these are weir systems that have the water containing oil ---- flowing over a weir or a dam into a reservoir that holds the oil until it is pumped out or away. Weir skimmers can and often do allow whatever is floating on the surface, to flow into or over the weir. This can include sticks, leaves, foam etc. that can cause some problems with plugging up or fouling pumping systems and downstream operations.
The non-selective weir skimmers can be a part of a structure as in wastewater plants or they can be floating devices that you connect a pump to for transferring the liquid to storage. As a floating device--- they create a low spot in the water that everything on the surface near it runs into. The floating systems are often used for oil spill work on larger spills---- where there is a lot of excess tankage for storing the mixture.
The pump out system used in these operations will greatly affect what is needed for future dewatering. Gently pumped oil and water is easier to separate later. The use of positive displacement pumps does not mechanically emulsify oil and water---- nearly as much as centrifugal pumps do. A centrifugal pump can chop up the oil into small particles that do not have enough buoyancy on their own---to rise to the surface of the water. We usually use air diaphragm pumps to pump or transfer oil and water mixtures, so as to not create theses mechanical emulsions which make all downstream separations tougher. Call us if you want more info on this.
OIL WATER SEPARATORS
Passive systems with no moving parts use gravity and coalescence to separate un-emulsified mixtures of oil & water. These passive systems run under gravity or pumped flow. Active systems can include centrifuges and membrane filtration. More on both types systems in the next edition of "The Oily News"
WE LOVE OIL!
Here are links to our different skimmers.
