CASE STUDY No. 1 Reboiler Puffing (Reference 1)
INSTALLATION A 7.5 foot ID, 63-tray atmospheric tower, equipped with bubble cap trays, separating a 200°F to 240°F hydrocarbon cut from the 240°F plus material. The reboiler was a horizontal thermosyphon, split flow, taking all the liquid from the bottom tray. Heating medium was hot gas oil (Figure 8.1.1A). The reboiler had a j horizontal baffle. j
PROBLEM Column failed to meet product specifications. Measured j tray efficiency was half the expected value. A definite temperature | cycle was observed on the temperature recorder located in the J
reboiler outlet line (Figure 8.1.IB). Tests showed that cycles affected bottom liquid level, liquid level in the trap pan and reboiler pressure drop. No cycling was observed further up the column. Operation record and field tests showed that when level was raised to above the reboiler return nozzle, the cycling disappeared. The cycling also disappeared when the reboiler duty was largely made up of sensible heat when compared to other times.
ANALYSIS The problem was traced back to vapor binding in the reboiler. This produced puffs of vapor from the reboiler. The puffs blew liquid away from the caps and into the downcomer. When the puff passed, liquid rushed back to the caps and momentum carried it to the tray below through the risers. Some liquid dropped into the bottom sump by this mechanism, thus bypassing the reboiler.
SOLUTION The problem was resolved by drilling holes in the horizontal baffle. This greatly reduced the vapor binding and the cycling.
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