Improper Heat Balance

Hot vapors, flowing up the fractionator from the flash zone, are partially condensed by contact with cooler pumparound liquid. The heat absorbed by the pumparound stream is used to preheat crude. As the pumparound circulation rate is increased, both heat removal from the fractionator and from crude preheat increase. This saves furnace fuel.

Higher pumparound rates will reduce fractionation. This is shown in Figure 1-3. In this sketch, trays 3, 4, and 5 provide the fractionation between furnace oil and FCCU feed. Trays 6 and 7 are the pumparound trays and, hence, contribute little toward separating the two products.

pumparound duty

By increasing the pumparound heat removal below the furnace oil draw (tray 2), less liquid is left to be condensed above the furnace oil drawoff. This, in turn, reduces the amount of liquid that spills over tray 2 and down to trays 3, 4, and 5. The decrease in liquid (i.e., internal reflux) flowing across these trays impairs the separation between furnace oil and FCCU feed.

Under extreme circumstances, the trays below the furnace oil draw may have no liquid at all on them. Dry trays do not fractionate. When this happens, the furnace oil end point will skyrocket. Reducing the pump­around duty will correct this problem.

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