Impurities such as water and sediment must be removed before oil produced from an oil well (emulsion oil) can be sold. A simple method used to produce saleable oil is to let gravity separate the water and oil from the emulsion. The oil-water emulsion is allowed to flow directly into a tank where the water settles to the bottom and oil floats to the top. To speed up this process, the tank is often heated using an immersed fire tube. Unfortunately, it is not as easy to separate the oil produced in North America, which is why it must be processed in a unique vessel called an oil treater.
Why Should You Add a Free Water Knockout?
A Free Water Knockout, or an FWKO, is a type of pressure vessel that uses residence time to break the oil-water emulsion in an oil battery facility. If the oil emulsion stream has a high water-to-oil ratio, i.e., significantly more water than oil, a separator is added to remove any free water. This equipment is called a Free Water Knock-out. The water is considered free water because it easily separates under normal flowing pressure and temperature conditions. If heat, chemicals, or mechanical agitation is required to separate the water, it is not free water and would be considered as a treated emulsion.
Using heat, chemicals, or other mechanical methods to remove water is expensive. So it helps to remove any free water using the FWKO to reduce the cost of producing saleable oil. The FWKO is usually a three-phase separator and is located at the inlet of the oil battery. Gas is removed off the top of the separator and sent for further processing. The water from the FWKO is filtered and purified to remove contaminants so it can be reused for enhanced oil recovery (waterflood) or disposed of by injecting it back into the reservoir. Oil from the FWKO is then treated using a settling tank, heated tank, or treater depending on the difficulty in removing the entrained water from the oil.
How Does an FWKO Work?
The free water knockout works on the theory that oil gets separated from the water because of the variation in their rate of flow, densities, and residence time. The speed of separation usually depends on a variety of factors, including the difference in their gravities, adhesion, and viscosity between the liquids. While light oil typically requires a low residence time of around five minutes to separate from water, heavy oil may take as long as 20-30 minutes. Also, chemicals may be injected into the FWKO to help separate the fluids if the emulsion is tight or has strong adhesion.
The Free Water Knockout works best when the process is steady. Large swings or continuous motions may disturb the process. Baffles are usually added to the internals of the vessel to calm the flow through the FWKO. Sometimes the inlet stream is preheated to make the separation process easier and faster, typically using a shell and tube heat exchanger.
Removing Solids from a Free Water Knockout Vessel
Since the FWKO is often the first piece of processing equipment in the production process of oil and gas, it may collect solid particles that come up with the water and oil. This is why high-quality de-sanding nozzles are installed to help clean the bottom of the separator by removing the solids. The cleaning system can be either automatic or manual.
When to Add a Treater to Your Oil Battery
When gravity separation is not enough to produce high-quality oil, vessels such as an oil treater are put to use. They typically use several techniques, including electric current, heat, chemicals, and gravity segregation to break the emulsions. Oil treaters may be horizontal or vertical.
Oil produced from an oil well contains saltwater, gas, and solids, such as sand. It is essential to remove these impurities before the oil can be refined. Oil battery facilities can help eliminate them from the oil.
The addition of an oil treater to a battery depends on the following aspects:
- The difficulty level of producing saleable oil
- The condition and type of oil at the wellhead
- The specifications and terms set by the purchaser of the oil, including the configuration of the process to produce refined products such as diesel, gasoline, and motor oil
Treater & Flash Gas
Heater treaters often produce flash gas in an oil and gas production facility. Natural gas gets discharged from an oil emulsion in the following cases:
- When the temperature of the liquid increases
- When a significant pressure drop is imposed on the liquid
The higher the operating temperature and the greater the pressure drop in the equipment, the higher the amount of gas produced per barrel of oil. Thus, flashing gas from the emulsion in the oil treater helps to ensure that the clean oil that leaves the vessel does not contain any volatile gases. These vapours can be easily collected and safely processed, flared or vented.
Stock Tank Oil from the Treater
The volume of oil after flashing to a specific temperature and nominal atmospheric storage pressure is known as the Stock Tank Oil. Regardless of the process used to deliver the saleable product, it must meet the specifications of the purchaser concerning the following aspects:
- The density of the oil
- Vapour pressure
- Basic Sediment and Water or BS&W
Basic Sediment & Water Specifications
When extracted from an oil reservoir, the crude oil may contain water and suspended solids. These particulates are referred to as sediment. If they are present in excessive quantity, desalters or de-sanders are typically used to remove these contaminants.
Basic sediment and water (BS&W) refers to the specific impurities present in crude oil. In addition to the requirements regarding the density, vapour pressure, and viscosity, sediment and water cut for sulphur content, chlorides, and hydrogen sulphides are a few main requirements. The BS&W of liquid hydrocarbon refers to the sediment, water, and oil emulsion as a percentage. The maximum BS&W content should not exceed 0.50%.
Get High-Quality Free Water Knockouts & Oil Treaters
If you are looking for an oil treater or a free water knockout to produce oil, we can help. We have top-quality equipment available for sale, rent or lease. We have experts who can understand your needs and offer products and services accordingly. We can also modify a pressure vessel by changing the internals, replacing or adding nozzles, and re-rating the temperature and pressure to ensure it meets your requirements.
For more information, feel free to get in touch with our design department by calling 403-342-7579 or toll-free at 1-800-993-9958. We can work on your project and help put one of the pressure vessels in our inventory to your use.
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