Distillation Column Flooding After Reboiler Retube
A sieve-tray column separating methanol/water at 1 atm has 30 trays, 0.6 m tray spacing, and a design vapor load of 1.8 kg/s. After a reboiler retube, operators report the column cannot reach overhead purity: the top temperature rises from 65 °C to 72 °C, the pressure drop across the column increases from 8 kPa to 14 kPa, and the reboiler duty is at its maximum. The reflux ratio is held at design (R = 2.5). The feed composition and rate are unchanged. A gamma scan suggests liquid accumulation on trays 18–24.
1. Explain the symptoms using tray hydraulics and identify the most likely cause.
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The pressure drop nearly doubling (8 to 14 kPa) with unchanged feed and reflux indicates increased vapor velocity through the tray openings or increased liquid holdup on the trays. Liquid accumulation on trays 18–24 is a classic flooding/jet-flooding signature: the downcomer cannot handle the liquid load, so liquid backs up and the trays above lose efficiency. The top temperature rising to 72 °C means the overhead is richer in water - consistent with reduced separation, not with a reboiler that is simply hotter. The reboiler being at maximum duty while the column still won't make purity suggests the reboiler is boiling more, but the extra vapor is being wasted because the trays are flooded and not contacting. The most likely cause is that the retubed reboiler now delivers a higher vapor rate than design (larger tube surface or higher ΔT), pushing the column above its flooding limit. The fix is to reduce the reboiler duty or increase reflux, not to add trays.
2. If the reboiler cannot be turned down, what design or operating change restores capacity, and how would you verify it?
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Options: (1) increase reflux ratio to condense more overhead and reduce net vapor to the trays - but this increases reboiler duty, so it only works if the reboiler has margin; (2) raise column pressure slightly to reduce vapor volumetric flow, at the cost of relative volatility; (3) increase tray spacing or use a higher-capacity tray in the flooded section; (4) reduce feed preheat so less vapor is generated in the feed zone. The cleanest short-term fix is to trim the reboiler duty to the design vapor load and re-establish the design reflux. Verify with a pressure-drop survey across trays: a healthy column shows a roughly linear ΔP profile; a flooded section shows a step change. Also check the gamma scan after the change - the liquid holdup on trays 18–24 should drop. If the retube genuinely increased capacity, the correct long-term fix is to re-rate the column hydraulics, not to run it at the new duty with the old trays.