Recently our DCU heaters have suddenly increased coking. We observed an increase in skins points. After analysis we found an increase in the sodium content in the feed (+/- 50 ppm). Analyzes of sodium in the feed are carried out every two days. How can we anticipate this problem? Daily analysis of sodium in the feed? Daily analyze the sodium in the distillation? Strict control of skins point at DCU Heaters ?
The primary control of sodium in the feed to the Coker is the performance of the desalter in the Crude unit. Caustic addition in the Crude unit due to poor desalter performance or high salt content in the incoming crude oil all contribute to the sodium content of the Coker feed. This leads to monitoring the desalter and caustic addition will provide some insight into the sodium content in the feed to the Coker.
As you know, sodium in the feed will catalyze coke formation. At sodium concentrations below 10 ppm, this effect is not significant. Even up to concentrations of 15 ppm the increased fouling rate is not very noticeable. Above that concentration, the fouling rate can be excessive.
I recall a presentation many years ago from a Brazilian refiner that was contaminating the crude oil with sodium by using desalter wash water from a sour water stripper that was processing spent caustic. The caustic in the desalter wash water was entrained with the crude oil and ended up in the feed to the Coker. Once that source was removed, the Coker heater fouling rate decreased.
Monitoring the desalter performance and caustic addition in the Crude unit are probably the most direct methods of preventing sodium in the feed to the Coker.
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