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.
API Performance Factors
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.
API Separator Design Criteria
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.