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Crude desalting is a critical refinery process to prevent fouling, corrosion and other issues in downstream processes.
pH control is often ignored in desalting, but it is important for a number of reasons. Firstly, crude contains different organic acids or acid forming salts. These are major causes of corrosion in the crude tower, the overhead condenser tubing and in the desalter itself. Caustic is usually fed to the desalter to neutralize the acidity. Secondly, ex-sour water stripper water is often used to wash the crude in the desalter. This water may already have a low pH and cause corrosion. Thirdly, if the pH is too high, organic acids such as naphthenic acid in the crude ionize and produce soaps that form unbreakable oil/water emulsions. Lastly, and of most importance to this white paper, there is also the strong influence of the pH value on the speed of demulsifying and phase separation. This has to do with an electrokinetic property called the zeta potential.
The magnitude of the zeta potential is a measure of an emulsion's stability. The greatest coagulation in the emulsion, and hence most efficient desalting, is achieved when the zeta potential is zero. Adjusting the pH value of the emulsion by adding alkali or acid allows a zero zeta potential to be reached.
pH control in crude desalting has economic advantages over the use of surfactants, demulsifiers and corrosion inhibitors, as acids and alkali are much cheaper. Also, less oil is lost through under-carry. However, process conditions are very difficult for obtaining reliable pH measurements, so a highly robust sensor is required.
Highlights of the white paper:
- The influence of the zeta potential on desalter emulsion stability
- How pH control reduces the need for chemicals
- Process-tolerant pH sensor with predictive diagnostics
- Automated sensor maintenance
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