API separators are frequently used for the treatment of refinery wastewater, which has been contaminated by oil and oil-bearing sludge. These oil / water separators are based on the difference in specific gravity to allow heavier liquids to settle below the lighter liquids. The separator separates the lighter liquid from the heavier one. The wastewater may contain insoluble oil, sludge, and some soluble elements. Soluble or emulsified oil requires further treatment and cannot be removed by settling.
An API separator tank consists of two basic sections - Both these sections perform different functions. A brief description of these two sections is given below -
The inlet section consists of two sections -
- Pre-separator flume section: A pre-separator
flume section is used for the reduction of flow velocity and
collection of floating oil. The effluent end of the pre-separator
section consists of three sections of equipment, which include - a
bar screen or trash rack, an oil skimmer and an oil retention
baffle.
- Forebay section: The pre-separator flume discharges into the forebay section of API separator. This part of the separator functions as a header that distributes the influent to the separator channels. The section can also be designed with an oil skimmer for the removal of any oil trapped in this section.
The separator section generally employs the following equipment in the order.
- Velocity head diffusion devices: The basic
function of these devices is to reduce the flow turbulence and
distribute the flow equally over the cross-sectional area of
channel. These devices can be either reaction jet baffle type or
vertical slot baffle type.
- Oil & Sludge Moving Device: These devices are based on a chain and scraper type mechanism.
The equipment works on the principle of Stokes Law that defines the rise velocity of a particle of oil based on its size and density. Generally, the specific gravity difference between the oil to be separated and water is much closer than the specific gravity of water and suspended solids. Therefore, the design of the API separator is based on the specific gravity difference between the oil to be separated and wastewater. With this design criterion, a majority of suspended solids will settle in the unit. Once the suspended solids and oil are removed from the wastewater in the API separator, the middle phase, water, is then directed on for the advanced treatment in various refinery wastewater treatment plants.
Usually, the oil layer is skimmed-off and afterwards re-processed or disposed of, and the bottom sediment layer is removed by a flight and chain scraper (or a similar device) and a sludge pump. The layer of water is sent for further treatment that usually consists of a dissolved air flotation unit for the further removal of any residual oil. After this, it is also sent to some type of biological treatment unit for the removal of undesirable dissolved chemical compounds.
Process
The pH control is used at the discharge from the API separator to improve the efficiency of secondary waste treatment processes, like flocculation. However, since the emulsified oil may still be present at this point, the pH sensor can become coated, which may result in slow response and eventual failure. To achieve the benefits of an accurate pH measurement, consistent attention is necessary, either by removing or cleaning or by automating a cleaning regimen by using a retraction device or cleaning nozzle.
Applications
Some of the typical applications of a API separator include -
- Groundwater Remediation
- Bilge Water Treatment
- Mobile Separation System
- R.O. Filter Pretreatment
- Power Plant Water Treatment
- Oil Spill Recovery
- Trench Water Treatment
- Aircraft Wash Racks
- DAF / Clarifier Pretreatment
- Vehicle Washwater Treatment
- Refinery Process Water
- Hydraulic Fluid Tank Dewatering
- Machining Coolant Oil Removal
- Tank Farm Tank Bottoms