The API separator is normally the first, and most important, wastewater treatment step in petroleum refineries. The primary function of a properly designed API separator is to remove the majority of oil and suspended solids from refinery wastewater prior to subsequent downstream wastewater treatment processes. For years, refineries have attempted to use other technologies or treatment scenarios as an alternative to the API separator. But most refineries ultimately select, or return to, the API separator as the technology of choice for their wastewater treatment primary oil/solids separation step. (Note: API stands for American Petroleum Institute.) Before we look at the design and performance aspects of an API separator we’ll talk about oil.
A refinery wastewater may contain oil in three forms and all three forms may be present in a given waste stream. The three forms of oil encountered in wastewater are:
Discrete oil globules will rise due to buoyant forces to form an oil layer on top of the water. This makes them relatively easy to remove in an API separator or they can be skimmed from the surface in an Equalization tank. Free oil can be removed by gravity separation which is what the API separator is designed for. Removal is a function of flow conditions, differences in specific gravity, temperature, and the stability of the oil. Coarse oily solids with a specific gravity >1.0 will settle to the bottom of the separator which is why bottom scrapers or flights are included in along with a conical hopper to collect the settled solids.
Emulsified oil is comprised of oil droplets with a diameter of less than 20 microns, mostly in the 1 – 10 micron range. Emulsified oil is a stable suspension in the water due to inter-particle forces dominating buoyant forces. Emulsified oil will not separate from the water under quiescent conditions aided by gravity, the very conditions created in an API separator. The removal of emulsified oil requires chemical addition such a pH adjustment to drop the pH or emulsion breakers. Once treated in this way the wastewater often enters a dissolved air (or nitrogen) flotation unit to remove the emulsified oil as it begins to break from the wastewater.
Dissolved oil forms a true molecular solution with water and cannot be removed by gravity separation. Dissolved oil removal requires biological treatment.
The API separator, shown in the graphic below, has become the generic name for “conventional oil-water separators.” Conventional oil-water separators can only remove free oil. Stable emulsions and dissolved oil require additional treatment.
A simple mass balance around the API separator is shown in the graphic below. The goal is have an oil concentration leaving the separator of not more than 200 ppm, though oil concentration values closer to 50 ppm can often be obtained. When the oil removal through the API is not sufficient a secondary unit process, such as induced gas flotation (IGF) or dissolved air or nitrogen flotation (DAF or DNF) will follow the API separator to provide a “polishing” step to significantly reduce the oil loading to a biological treatment system. One other note: Running an oil & grease (O&G) is not always feasible so I have often used chemical oxygen demand (COD) as a surrogate parameter to estimate the oil reduction through an API.
The API separator is a gravity separation device that works on the principle of Stokes Law, which defines the rise velocity of an oil particle based on its density and size. Typically, the difference between the specific gravity of oil to be separated and water is much closer than the specific gravity of the suspended solids and water. Therefore, the design of the API separator is based on the difference in the specific gravity of the oil to be separated and the wastewater. If this design criterion is followed, the majority of suspended solids will settle in the unit. Once the oil and suspended solids are removed from the wastewater in the API separator, the middle phase, water, is then sent on for further treatment in most refinery wastewater treatment plants.
Key factors influencing API separator performance include water temperature, horizontal velocity, the density and size of the oil droplets, and the quantity and characteristics of the suspended solids. API separators are designed to remove oil droplets with diameters as small as 0.015 cm (150 microns). In addition, API separators are designed to maintain laminar flow. Under most operating conditions, the API separator will remove both free oil and suspended solids down to a concentration of between 50 and 200 mg/L. The removal of other contaminants, including chemical oxygen demand (COD) and total suspended solids (TSS), is variable. COD removals in the range of 16 to 45% and TSS removals in the range of 33 to 68% have been documented. Removing the bulk of free oils, greases, and suspended solids from the wastewater reduces overloading and other problems in downstream treatment processes.
The design standards for the API separator have been well documented and can be found in the current edition of API Specification 421. Some of the most important design criteria developed for API separators include:
Length to width ratio. A minimum length to width ratio of 5:1 is recommended for all API separator designs to keep operating conditions as close to plug flow as possible, minimizing the potential for short circuiting.
Depth to width ratio. A minimum depth to width ratio of 0.3 to 0.5 is recommended so that separation units are not excessively deep; minimizing the amount of time it takes for oil particles to rise to the surface.
Maximum channel width and depth. The maximum API separator channel width is 20 ft; maximum depth is 8 ft.
Horizontal velocity. Maintaining a horizontal velocity of no more than 3.0 ft/min has been shown to minimize turbulence and its effect on interfering with the separation of oil from wastewater.
Inlet distribution. To minimize the effect of high wastewater inlet velocities into the API separator, and possible short-circuiting associated with these high velocities, reaction jet baffles are recommended to diffuse influent flows across the width and depth of the API separator.
Oil particle size. Majority of oil particles in most refinery wastewaters are 150 micron in size or larger. Therefore, the design standards for API separators were developed for the removal of oil particles of this size. Particles smaller than 150 micron will normally exit an API separator and will need to be removed by downstream treatment processes, unless allowances are made in the sizing of the API separator to remove these smaller particles.
A process flow diagram for a “complete API separator” system is shown below.
Important Note: One of the key parameters for the operation of an API separator is the horizontal velocity (VH). The horizontal velocity should not exceed 3 feet per minute (fpm) or 0.91 meters per minute. Please pay attention to this number and the units: feet per MINUTE. I have found many websites where the horizontal velocity value is listed as 3 feet per second. This is incorrect!! The following is a quote from the “Design and Operation of Oil-Water Separators” API Publication 421, First Edition, February 1990: “Although some separators may be able to operate at higher velocities, 3 feet per minute has been selected as a recommended upper limit for conventional refinery oil-water separators. Most refinery process-water separators operate at horizontal velocities much less than 3 feet per minute at average flow. All separators surveyed had average horizontal velocities of less than 2 feet per minute, and more than half had average velocities less than 1 foot per minute, based on typical or average flow rates.” (pg. 4)
Doing a check on API separator performance is very easily done with the key parameter being the horizontal flow rate as shown in the Excel spreadsheet below. You can expand your analysis by adding influent and effluent COD, TSS, and turbidity data which can then be used to calculate the percent removal for each of these additional parameters.