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Depending on the refinery configuration and operation of the facility the wastewater can be contaminated with high concentrations of oil and suspended solids. The oil concentration can range from several hundred ppm (mg/l) to several thousand ppm (mg/l). The first step in the treatment is to separate gross quantities of free oil and solids from the water. In this process suspended solids are also separated and must be handled. The primary treatment is generally passive and depends on gravity separation.
Gravity separation utilizes the difference in specific gravity between the oil and water. Oil separates from a fluid at a rate explained by Stoke's Law. The formula predicts an oil droplet rise or settling rate through water based on the density and size of the oil droplet.
This equation relates the terminal settling or rise velocity of a smooth, rigid sphere in a viscous fluid of known density and viscosity with the diameter of the sphere when subjected to a known force field (gravity).
The equation is:
V = (2gr2) (d1-d2)/9µ
V= velocity of fall (cm sec-1),
g= acceleration of gravity (cm sec-2),
r= "equivalent" radius of particle (cm),
d1= density of particle (g cm -3),
d2= density of medium (g cm-3), and
µ= viscosity of medium (dyne sec cm-2).
There are two types of separators typically used in this application. The most common type is designed with a quiescent zone allowing for the separation of the oil and water. This type of separator is commonly referred to as an API separator. The design specification can be found in API publication No. 421. When designed with the proper length, width and depth and the correct specifications for temperature, specific gravity, flow and oil droplet size this type of unit should remove the oil down to a concentration of 200 ppm (mg/l). The oil is skimmed from the water and returned to the refinery slop oil recovery system. The water phase is discharged for additional treatment in the wastewater treatment system. Depending on the location and local regulatory requirements, it may be necessary to control the off-gases. This is done with carbon adsorption or thermal destruction. In some cases the existing refinery flare system is utilized in lieu of a dictated off gas treatment system.
The second type of unit used in this application utilizes a coalescing design. The coalescing design may include parallel plates, corrugated plate interceptors (CPI), tilted plate interceptors (TPI) or slant rib coalescers. However, the concept and design are generally the same. The coalescing concept is based on having a large surface area in contact with the waste stream. The plates provide more surface area reducing the over all dimensions. This type of treatment, if properly applied, can achieve discharge limits as low as 50 ppm (mg/l). As with the API, the water is discharged for additional treatment and the oil is recovered. Due to the design these units are more susceptible to load variation and potential plugging with solids.
The solids harvested in the separation of the oil and water may be hazardous, by regulatory definition, and thus disposal is costly. The cost of disposal frequently makes reclaiming an economically feasible alternative.