Separating oil or solvents from water by mechanical means involves the application of some basic physical principles. First, oil and water are not soluble in each other. Even so-called soluble oils are really an emulsion, not a true solution. Second, water is denser than oil or petroleum based solvents. These properties tend to result in natural separation unless other factors interfere.
For illustration purposes let’s examine what occurs naturally in a tank of mixed oil and water, where water is the larger component, when it is allowed to stand. The oil initially exists in the form of droplets of various sizes. Because oil is lighter than water, all these droplets will tend to rise to the surface. However, if a droplet is elow
a certain size, the forces acting on it other than gravity (molecular, electrostatic, etc.) are greater than the gravitational force and will prevent it from rising to the surface, much as a dandelion seed may be swept along by air currents for miles and may rise thousands of feet even though it is heavier than air. Above this size, droplets will rise toward the surface. If gravity were the only factor all droplets would accelerate at the same rate and all droplets from a given depth would reach the surface at the same time regardless of droplet size. However, other factors interfere with this acceleration with the result that the speed a droplet will rise to the surface is proportional to the size of the droplet. In our tank of oil/water mix, the largest droplets rise to the top fastest, the smallest (larger than a certain minimum size), slowest.
Now imagine the same process occurring in a tank of flowing mixture. The oil/water mix is introduced at one end and flows toward the other end slowly. The largest droplets separate the fastest, the smallest the slowest. If you were to look at a cross section of the flow cut parallel to the flow direction, you would observe a wedge effect. At the entrance end of the tank there is a uniform mix of oil and water, forming a layer of clean water at the bottom, increasing in height as you move toward the discharge end of the tank. This is not a clear boundary. In reality, each droplet size will have its own wedge with the angle determined by its size and the flow velocity down the tank. The largest droplets have a steeply rising wedge with the angle decreasing with decreasing droplet size. If the droplet is small enough or the flow down the tank is fast enough, the droplet will not separate before the end of the tank is reached. Separation is encouraged by making the oil droplets larger (rises faster), making the flow slower (longer time to separate), or making the tank shallower (shorter average distance to rise to the surface). All this assumes the flow in the tank is non turbulent to prevent remixing the oil and water.
Coalescence means coming together. It refers to small droplets coming into contact with other droplets and combining to form larger drops. In an oil/water mixture, if two oil droplets are brought into contact with one another there will exist a separating film of water between the droplets. If they are held in contact for a sufficient length of time, this film will thin until it ruptures and the droplets will join (coalesce).
Factors may be present such as dirt or certain chemical agents that will interfere with this process and inhibit coalescence. In this case we have what is known as an emulsion. If sufficient inhibitors are present, a stable emulsion will result and will not separate in any reasonable time. Examples are soluble oil coolants and mayonnaise. The only practical way to separate chemical stabilized emulsions is to interfere with the action of the emulsifying agent. Mechanical coalescers/separators are only effective on mechanically induced emulsions.
Coalescence can be enhanced as well as inhibited. Increasing the force holding the droplets in contact (centrifuge, fiber mat) will enhance coalescence, as will bringing the droplets into contact with a surface that will
wet with oil but not water. This latter is the coalescing action of a superior coalescer/ separator. As the oil droplets make contact with the surface of the media, they sheet out on the material and flow upwards towards the top edges. When enough oil gathers at an edge to overcome the surface tension, a droplet will be released toward the surface. This droplet will be sufficient size to separate rapidly from the water (recalling the discussion on the effect of droplet size on separation rate).
In the best coalescer/separator machines, the oil/water mixture is introduced to the tank through either a bag filter or a primary coalescer/diffuser. The bag filter is an option when solids are present and results in a non-turbulent flow into the tank volume. This is important to the process of separation. instead of the bag filter, an oil flow diffuser may be used. This serves two purposes. First, it acts as a coalescer to enhance separation rate. Second, it functions as a flow diffuser to minimize tank turbulence.
From this point, the oil/water mix begins gravity separation. Once in the coalescing media two effects are observed. First, the media surface coalescing action described above. Second, the material matrix serves to break up the tank into units with smaller height; therefore, an oil droplet has a shorter distance to rise before it encounters a surface. This greatly enhances separation for a given tank size.
At the tank discharge end an oil retaining weir is located to prevent separated oil from flowing out of the tank with the clean water. This barrier extends almost to the bottom of the tank. The separated water must flow under this weir and then over the water discharge weir to exit the tank. The purpose of this water discharge weir is to maintain a constant depth of fluid in the separator tank.
Separated oil accumulates on the tank surface until it reaches sufficient depth to overflow the adjustable oil discharge weir or be removed by a motorized wheel skimmer. Why use a wheel skimmer on a coalescer? It maintains the tank surface in an oil free condition, preventing the buildup of anaerobic bacteria. This results in a much cleaner coalescer tank than is the case if the entire surface remains covered with a film of oil. If a proper wheel skimmer is installed, the oil is removed to a secondary separation reservoir, where any water picked up by the wheel is allowed to drop back into the separation tank before the oil is discharged through an adjustable weir.
The secondary separation volume used with well designed wheel skimmer installation offers several advantages. First, when used with machine tool coolants, wheel skimmers tend to pick up some coolant once most surface oil has been removed. This secondary separation volume has an open bottom to allow the coolant to fall back out rather than be discharged with the oil. Second, this volume acts as a second coalescer stage for the oil and by restricting the surface area we are able to run a much thicker film of oil, resulting in almost no water in the oil discharge while at the same time maintaining the main volume surface almost free of oil. This improves efficiency and greatly improves system cleanliness.