Anybody who has worked in the oil and gas industry knows how wrong the old saying, “Oil and water don’t mix,” can be. It might be more accurate to say, “Oil and water can be darn hard to separate.” Initial processing at the well head get rids of as much water as possible, but when crude oil gets to a refinery, it usually not only still has water, but also salt. Both of these can create serious problems.
Lydia Miller discusses this situation in the January, 2018 issue of Flow Control magazine in her article, Critical Level Control Ensures Highest Efficiency for Desalter Units. She makes the point that salt is particularly problematic, hence the need for a desalter unit.
While the solubility of salt in oil is much lower than in water, enough can be carried into the refining process to cause corrosion problems for equipment. Salt can also contain metals that may poison catalysts used downstream, so it must be removed. Fortunately, it dissolves far more readily in water, so if water is mixed with the oil, the salt washes out.
So, the refinery has to add water to the incoming crude to capture and extract the salt before it moves to the next stage. Enter the desalting unit. Its job is to inject water and mix it thoroughly with the crude to capture the salt and then remove it again, all in as short a time as possible so it can keep up with the demand for feedstock from the larger refining unit.
While the 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.
Gravity will eventually cause the water and emulsifier to settle out, but the action needs to be faster and more complete, so a desalter helps the process along by using an electric field to coalesce the water/emulsifier mix. This makes it form bigger droplets which sink more quickly. Charged electrodes in the upper sections of the tank drive this droplet formation, which is why level control is so critical and tricky.
Given oil’s poor electrical conductivity, these grids can be run around 10 kV or higher, which makes for fast and effective separation. Washed and dried oil is drawn out of the top, and water, salt and emulsifiers go out the bottom. The emulsion layer remains static in the middle.
The tricky part is keeping the emulsion and water layer below the electrodes. Allowing highly conductive salt water to reach the electrodes carrying some thousands of volts would likely get the attention of anyone near the unit and hopefully trip a circuit breaker. Controlling the flow and mix of oil, water and emulsifier to maximize throughput while still meeting the salt removal requirement requires a very accurate picture of tank level. Fortunately, a guided wave radar (GWR) transmitter from the Rosemount 5300 product family can easily spot and measure how high the emulsifier and water are.
One thing working in the operator’s favor is the difference in dielectric characteristics of all three layers. Since GWR level instruments operate by finding those differences, this application takes full advantage of that capability. A level transmitter can generally identify the top of the emulsion layer with a single probe and can often accurately measure the position of all three layers depending on their thickness and dielectric characteristics. Readings can be taken continuously and are unaffected by the electrostatic fields during operation as long as the grids are properly grounded.
The ultimate benefit of a properly operating desalter is clean crude, sufficiently free of salt and water to minimize corrosion downstream, and pressure incidents caused by steam buildup. GWR level transmitters have many more applications in refineries. You can find out more about them, and meet with other people looking at the same kinds of situations in the Emerson Exchange365 community. It’s a place where you can communicate and exchange information with experts and peers in all sorts of industries around the world. Look for the Level or Refinery Group, plus other specialty areas for suggestions and answers.