When the distillation system is shut down, the condition for stopping the circulating slurry is that there is no catalyst in the settler
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When the fractionation system is shut down, the condition for stopping the circulating slurry is that there is no catalyst in the settler. Why?
If there is still catalyst in the settler, stop the slurry circulation; the catalyst that enters the fractionator cannot be washed away, and some of it will deposit on the baffle plates and even the tray plates. In fact, at this point, it is not sufficient to merely have oil slurry circulation; it is also necessary to ensure that a certain amount of oil slurry is discharged, so as to keep the solid content of the oil slurry at the bottom of the tower within acceptable limits. By the same logic, oil slurry circulation must be in place before switching the catalyst at startup, and it is preferable to have a small amount of external discharge as well.
The main concern is that the catalyst components in the settler might end up in the distillation tower. If the slurry pump is stopped too early, it means that the heat circulation at the bottom of the distillation tower is interrupted, which causes the catalyst and slurry that enter that area to condense into a gel-like substance. Such a substance can only be removed manually, and it’s a very difficult task
Prevent the catalyst from causing tray blockage in the absence of slurry washing