The simplest form of primary treating equipment is a settling (skim) tank or vessel. These items are normally designed to provide long residence times during which coalescence and gravity separation can occur. If the desired outlet oil concentration is known, the theoretical dimensions of the vessel can be determined. Unlike the case of separation, with skim vessels one cannot ignore the effects of vibration, turbulence, short circuiting, etc. American Petroleum Institute (API) Publication 421, Management of Water Discharges: Design and Operation of Oil-Water Separators, uses short-circuit factors as high as 1.75 and is the basis upon which many of the sizing formulas in this chapter were derived.
Skimmers can be either vertical or horizontal in configuration. In vertical skimmers the oil droplets must rise upward counter current to the downward flow of the water. Some vertical skimmers have inlet spreaders and outlet collectors to help even the distribution of the flow, as shown in Figure 7-2. The inlet directs the flow below the oil-water interface. Small amounts of gas liberated from the water help to “float” the oil droplets. In the quiet zone between the spreader and the water collector, some coalescence can occur and the buoyancy of the oil droplets causes them to rise counter to the water flow. Oil will be collected and skimmed off the surface.
The thickness of the oil pad depends on the relative heights of the oil weir and the water leg, and the difference in specific gravity of the two liquids. Often, an interface lever controller is used in place of the water leg.
In horizontal skimmers the oil droplets rise perpendicular to the flow of the water, as shown in Figure 7-3. The inlet enters below the oil pad. The water then turns and flows horizontally for most of the length of the vessel. Baffles could be installed to straighten the flow. Oil droplets coalesce in this section of the vessel and rise to the oil-water surface where they are captured and eventually skimmed over the oil weir. The height of the oil can be controlled by interface control, by a water leg similar to that shown in Figure 7-2, or by a bucket and weir arrangement.
Horizontal vessels are more efficient at water treating because the oil droplets do not have to flow countercurrent to the water flow. However, vertical skimmers are used in instances where:
1. Sand and other solid particles must be handled. This can be done in vertical vessels with either the water outlet or a sand drain off the bottom. Experience with elaborately designed sand drains in large horizontal vessels has not been very satisfactory.
2. Liquid surges are expected. Vertical vessels are less susceptible to high level shutdowns due to liquid surges. Internal waves due to surging in horizontal vessels can trigger a level float even though the volume of liquid between the normal operating level and the high level shutdown is equal to or larger than that in a vertical vessel. This possibility can be minimized through the installation of stilling baffles in the vessel.
The choice of pressure versus atmospheric vessel for the skimmer tank is not determined solely by the water treating requirements. The overall needs of the system need to be considered in this decision. Pressure vessels are more expensive. However, they are recommended where:
1. Potential gas blowby through the upstream vessel dump system could create too much back-pressure in an atmospheric vent system.
2. The water must be dumped to a higher level for further treating and a pump would be needed if an atmospheric vessel were installed.
Due to the potential danger from overpressure and potential gas venting problems associated with atmospheric vessels, pressure vessels are preferred. However, an individual cost/benefit decision must be made.
A minimum residence time of 10 to 30 minutes should be provided to assure that surges do not upset the system and to provide for some coalescence. As previously discussed, the potential benefits of providing much more residence time will probably not be cost efficient beyond this point. Skimmers with long residence times require baffles to attempt to distribute the flow and eliminate short circuiting. Tracer studies have shown that skimmer tanks, even those with carefully designed spreaders and baffles, exhibit poor flow behavior and short circuiting. This is probably due to density and temperature differences, deposition of solids, corrosion of spreaders, etc.