In our refinery, we inject dilute caustic in the discharge of CDU desalter to control CDU tower corrosion. The caustic converts Cl to NaCl which goes to heavy oil unit. As we know, the sodium content in VR could cause rapid coking in DCU Furance coil. Rapid coking happened to our DCU France coil in 2012. I wonder what is the limitation of sodium content in DCU feedstock(VR).
Excessive sodium in the Coker feed will accelerate fouling of the Coker heater. Concentrations of sodium of 10 ppm or less do not impact the heater fouling rate. Concentrations of up to 15 ppm sodium have not impacted fouling rates. A sodium concentration of less than 10 ppm in Coker feed should be the target value. I have seen sodium concentrations up to 100 ppm that did impact heater fouling rates. Concentrations over 15 ppm caused increased heater fouling rates, with higher concentrations having a bigger impact.
Using caustic to control chlorides in the crude tower overhead is common. This is done due to insufficient performance of the desalters for the crude slate processed. Caustic should not be used to lower the chlorides in the crude tower overhead water to less than 10 ppm as that will lead to overfeeding caustic. Additionally, a maximum dosage rate should be set so that inadvertent overfeeding does not occur. A shut off should also be included to be interlocked with the crude feed to prevent caustic from being added during a crude unit outage.