What is Conventional Gravity Separator–API?
The API separator is a gravity separation device designed with Stokes' law principles in mind, which define the rise velocity of oil droplets based on density, size, and water properties. The separator's design is based on the specific gravity difference between oil and wastewater, which is much smaller than the specific gravity difference between suspended solids and water.
According to that design criterion, the majority of the suspended solids will settle to the bottom of the separator as a sediment layer, the oil will rise to the top, and the wastewater will be the middle layer between the oil on top and the solids on the bottom.
When correctly applied, the API Design Standards alter the geometry, design, and size of the separator beyond simple Stokes Law principles. Allowances for water flow entrance and exit turbulence losses, as well as other factors, are included.
API Separator
In most petroleum refineries, the API separator is the first and most important wastewater treatment step. A properly designed API separator's primary function 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 replace the API separator with other technologies or treatment scenarios. However, the API separator is the technology of choice for most refineries' primary oil/solids separation step in wastewater treatment.
A refinery wastewater may contain oil in one of three forms, and all three forms may be present in the same waste stream. The three types of oil found in wastewater are:
· Free Oil
· Emulsified Oil
· Dissolved Oil
Due to buoyant forces, discrete oil globules will rise to form an oil layer on top of the water. As a result, they are relatively easy to remove in an API separator or can be skimmed from the surface in an Equalization tank.
Gravity separation, which is what the API separator is designed for, can be used to remove free oil. Removal is affected by flow conditions, differences in specific gravity, temperature, and the oil's stability.
Emulsified oil is made up of oil droplets with diameters less than 20 microns, most of which are in the 1 – 10 micron range. Because of the dominance of inter-particle forces over buoyant forces, emulsified oil is a stable suspension in water. Under quiescent conditions aided by gravity, such as those created in an API separator, emulsified oil will not separate from water. The removal of emulsified oil necessitates the addition of chemicals such as a pH adjustment to lower the pH or emulsion breakers.
Dissolved Oil in wastewater forms a true molecular solution and cannot be removed by gravity separation. The removal of dissolved oil necessitates biological treatment.
What are the different types of separators?
1: Vertical Separator
A vertical separator is commonly used when well streams have a low to moderate gas-to-oil ratio and significant slugs of liquid are expected. It can handle larger slugs of liquid without leaking into the gas outlet, and the action of the liquid level control is less critical. Because of the greater vertical distance between the liquid level and the gas outlet, there is less tendency for the liquid to re-vaporize into the gas phase. When space is limited, such as on an offshore platform, a vertical separator takes up less floor area.
2: Single horizontal tube Separator
The horizontal separator has a much larger gas-liquid interface area, which is made up of a large, long, baffled gas-separation section that allows for much higher gas velocities. The large liquid surface area allows gas to be efficiently removed from the liquid. When gas capacity is a design criterion, this vessel provides a large interface area between the liquid and gas phases, allowing for additional separation capability. The horizontal vessel is more cost-effective in high-pressure separators due to the greater wall thickness required with larger diameters.
3: Spherical Separator
Vertical separators are spherical separators. It is inexpensive and has a small vessel arrangement. These separators have a small amount of surge room and a liquid settling section. They are ineffective when a well stream contains a lot of mud or sand or is subjected to foamy surges. It is critical to keep the liquid level under control. These separators are no longer widely used due to their limitations.
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