Decreased Fractionation

Accounting for a loss in fractionation is a common troubleshooting assignment. For example, crude unit operators find that they can no longer meet furnace oil end-point specs unless they sacrifice furnace oil yield. On one unit, furnace oil production had dropped from 7,000 B/SD to 4,000 B/SD. Possible explanations for this type of problem are tray flooding, im­proper heat balance, and tray damage.

Trays can flood even below design loads because of fouling of the tray decks. Flooded trays lose fractionating efficiency (see Chapter 19). Check the pressure drop across the suspect trays. If the pressure drop per tray (in feet of liquid hydrocarbon) exceeds 25% of the tray spacing, flooding is likely.

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