Hydrocarbon Engineering magazine published an article by Emerson’s Sarah Parker, Using Guided Wave Radar for desalter interface measurements will improve efficiency, increase reliability and reduce maintenance.
The article is not available online to non-subscribers, so I highlight a few points from the article. For those not familiar with a refinery’s desalting process, it is the process to remove salt from crude oil, which can:
…cause significant corrosion and plugging of downstream refinery equipment. To remove these salts, water is mixed with the oil to “wash” the crude, transferring the salts to the aqueous phase.
These salts are removed through water, chemical additives, and a high-voltage electrostatic grid. Here’s the theory of Desalter operation:
The Desalter removes contaminants from crude oil by first emulsifying the crude oil with chemicals and wash water to promote thorough contact of the water and oil. The salts containing some of the metals that can poison catalysts are dissolved in the water phase. After the oil has been washed and mixed as an emulsion of oil and water, demulsifying chemicals are then added and electrostatic fields are used to break the emulsion.
Sarah notes that with this process, the measurement challenge:
…is important that the oil/water interface is maintained at the correct level. However, measuring the top and bottom of the emulsion layer presents challenges for many technologies because of variations in the properties of the fluids and the layer of emulsion. An added problem is that crude oil can contain sticky components that tend to build up on surfaces, coat probes, or cause mechanical parts to stick. This can make measurements unreliable and inaccurate, and increase the frequency of maintenance.
Knowing the interface between the oil and water is critical. If the grid is exposed to the water layer, it will short circuit the grid causing a power trip condition.
Many of the level measurement technologies’ accuracies are impacted by oil/water density fluctuations and coating caused by the crude oil and emulsion layer. Guided Wave Radar (GWR) technology is:
…immune to density changes, can handle coating and has no moving parts to maintain.
In an earlier post, Level Measurement Selection Considerations, I shared the theory of operation behind the GWR technology.
The radar signal is also unaffected by the high voltage from the electrostatic grid within the desalter unit. From the top of the vessel where the GWR unit is located, the radar signal passes down the rigid or flexible probe (in the case where the desalter has a stilling well inside the vessel). The GWR measurement identifies the level where the oil and the top of the emulsion interface is located. The operators want to make sure the electrostatic grids remain in this region for optimal operations. The operators periodically will manually check the oil/emulsion layer and emulsion/water layer levels through the vessel sample taps.
In the article, Sarah shares that some, but not all GWR installations can find the bottom emulsion/water layer. It depends on the composition of the crude oil and desalter vessel properties, primarily whether it has a stilling well area for the GWR probe. She cites an example from one refinery where both layers could be measured, and thus the thickness of the emulsion layer was known resulting in:
…a reduction of emulsifier chemical usage representing about $100,000 annual cost per unit…
Sarah sums up how GWR technology improves operations:
With reliable interface level control, a desalter is able to operate more efficiently with reduced water and salt carryover to the crude unit as well as keeping water out of the electric grid. Effective separation of the oil improves its quality and reduces oil contamination to the downstream water treatment plant.
Update: I discovered a nice picture of a desalter vessel showing a slotted stilling well and and no stilling well. There’s also a nice whitepaper with more detail on this application.