In a Flow Control article, Critical Level Control Ensures Highest Efficiency for Desalter Units, Emerson’s Lydia Miller highlights the challenges in effectively, efficiently and reliably removing this salt.
Lydia describes the role of a refinery desalter unit at the front end of the refining process:
While actual equipment configurations vary, most designs inject water into the oil stream using a mixing valve. Since oil and water do not naturally mix, an emulsifier is added to help disperse the water and maximize contact to make the salt transfer as complete as possible.
Once the concentration of salt is lowered to an acceptable level, the oil, water brine, and emulsifier mixture must be separated. To maximize this separation process:
The measurement and control challenge is to make sure the electrostatic grid is immersed only in the oil layer, since:
…the desalting tank is equipped with an electrostatic grid immersed in the upper oil… When charged, this causes any small water droplets still in the oil to coalesce so they sink more quickly down into the bottom layer.
…salt-laden water is highly conductive, so if the water level in the tank is high enough to reach the grid, it can short the system or at least cause excessive power draw.
Level measurement technology must accurately measure where the top of the emulsion layer is in the tank. Lydia describes traditional measurement methods such as bleed taps, magnetostrictive instruments that float in water layer but not the oil layer, and capacitive level measurement technology. Each method has its shortcomings from messiness to problems handling fluid density changes.
Guide wave radar (GWR) level measurement technology can be very effective since each of the layers—oil, emulsion, brine water—has different dielectric characteristics. Lydia explains that readings with GWR technology:
…can be taken continuously and are unaffected by the electrostatic fields during operation as long as the grids are properly grounded. This product family has signal processing capabilities able to boost the unit’s ability to read through fluids of low dielectric strength such as oil. It is also immune to density changes, has no moving parts that could stick because of coating, and has diagnostics capabilities to alert users before any coating would affect the measurement.
Read the article for more on the use of stilling wells in this type of application and hot environment installations considerations. For more detail on this application, read the whitepaper, Improve Reliability of Desalter Interface Measurement While Reducing Maintenance with the use of Guided Wave Radar on the Emerson.com site. Solutions to other refining measurement and control challenges are in the Refining section of Emerson.com.