11.3 Distillation Towers, Vessels, and Internals
Key Takeaways
- A distillation tower separates a mixture by repeated vaporization and condensation, with lighter components rising and heavier components falling.
- Trays provide contact stages, and the three classic tray types are bubble cap, sieve, and valve.
- A downcomer carries liquid from one tray to the next below, and the outlet weir sets the liquid depth on the tray.
- Trays must be installed level, because an out-of-level tray drains to one side and destroys the vapor-liquid contact across the whole tray.
- Entry into a tower is permit-required confined space work, and the vessel must be isolated by blinds rather than by closed valves alone.
How a tower separates a mixture
A distillation tower (fractionating column) separates a liquid mixture into products by exploiting differences in boiling point. Heat from a reboiler at the bottom generates vapor that rises through the column. Reflux — condensed liquid returned to the top — flows down against it. At every contact stage, vapor and liquid exchange components: the lighter, lower-boiling material concentrates in the rising vapor and the heavier material concentrates in the falling liquid.
| Section | What happens |
|---|---|
| Feed tray / feed zone | The feed enters, usually part vapor and part liquid |
| Rectifying section (above the feed) | Rising vapor is enriched in the light component |
| Stripping section (below the feed) | Falling liquid is stripped of the light component |
| Overhead / top | Vapor leaves to the overhead condenser; part is drawn as distillate and part returned as reflux |
| Bottom | Liquid leaves to the reboiler; part vaporizes and returns, part is drawn as bottoms product |
More contact stages, or more reflux, gives a sharper separation.
Internals
| Component | Function |
|---|---|
| Trays (plates) | The contact stages; vapor bubbles up through liquid held on each tray |
| Downcomer | The channel that carries liquid from a tray down to the tray below |
| Outlet weir | A dam at the tray edge that sets the depth of liquid held on the tray |
| Inlet weir / seal pan | Maintains a liquid seal at the bottom of the downcomer so vapor cannot blow up it |
| Manways in the trays | Removable panels that let a worker pass from one tray space to the next |
| Demister / mist eliminator pad | Removes entrained liquid droplets from leaving vapor |
| Vapor distributor / feed inlet device | Spreads the incoming feed across the cross-section |
| Liquid distributor | Spreads liquid evenly over packing in a packed section |
| Support rings and beams | Structural support for the trays |
| Vortex breaker | At the bottom outlet, prevents a vortex from drawing vapor into the pump suction |
Tray types
| Tray | How vapor passes | Character |
|---|---|---|
| Bubble cap | Vapor rises through a riser, reverses under a slotted cap, and bubbles out below the liquid level | Maintains a liquid seal even at very low vapor rates; highest cost and pressure drop |
| Sieve | Vapor passes through plain perforations | Simplest and cheapest; weeps liquid through the holes at low vapor rate |
| Valve | Vapor lifts a movable cap or disc over each hole | Good turndown between the two; the most common modern choice |
The turndown distinction is the examinable idea. A bubble cap tray keeps working at very low vapor rates because the cap maintains a seal; a sieve tray dumps its liquid straight through the holes when vapor velocity falls.
Two failure modes at the extremes:
- Weeping and dumping at low vapor rate: liquid falls through the tray openings rather than passing across the tray and down the downcomer, so contact is lost.
- Flooding at high vapor rate: liquid is entrained upward or the downcomer cannot drain, so liquid builds up and the tower's pressure drop rises sharply. Downcomer flooding and jet flooding are both signaled by a rapid rise in differential pressure across the tower.
Packed columns
Instead of trays, a column may use packing:
- Random (dumped) packing — shaped elements such as rings and saddles poured into the bed.
- Structured packing — engineered corrugated sheets stacked in layers, giving high surface area at low pressure drop.
Packed columns need a liquid distributor at the top of each bed and a redistributor between beds, because liquid tends to migrate toward the wall. A distributor that is out of level or partly plugged causes channeling, which destroys separation efficiency just as surely as a damaged tray.
Maintenance and inspection
Work inside a tower is done during a turnaround, and it is dominated by two activities: cleaning and tray repair.
Typical findings:
| Finding | Cause |
|---|---|
| Fouling, coke, or polymer on trays | Process conditions, oxygen ingress, spent inhibitor |
| Corroded or thinned tray decks and valves | Process chemistry, water dew point, chloride attack |
| Missing or stuck valve units | Erosion, vibration, fatigue of the valve legs |
| Damaged or dislodged trays | An upset, a pressure surge, or improper reinstallation |
| Plugged distributor holes in a packed bed | Solids, corrosion products, catalyst fines |
Tray installation rules:
- Install from the bottom up, working through the tray manways, and bolt each tray panel completely before moving up.
- Check levelness of each tray and especially of the outlet weir. An out-of-level weir means the liquid runs off one end of the tray and the rest of the tray runs dry, so that stage does nothing.
- Verify the downcomer clearance — the gap between the bottom of the downcomer and the tray or seal pan below. Too small and it restricts liquid flow; too large and vapor blows up the downcomer.
- Account for every tool, bolt, and gasket. A dropped wrench left in a downcomer will shut the column down.
- Inspect from below after installation where access permits, and record the final inspection before the manway is closed.
Safe entry
A distillation tower is a permit-required confined space and often contains hydrocarbons, so entry preparation is elaborate:
- Isolate by blinding. Closed valves are not isolation for entry; blinds or physical disconnection are required, and the blind list is a controlled document.
- Drain, flush, steam out, and purge. Nitrogen purging is common, and a nitrogen-purged vessel is an immediately dangerous oxygen-deficient atmosphere until it is air-freed.
- Test the atmosphere in the order established in Chapter 2: oxygen first, then flammables, then toxics. Retest at multiple levels, because vapors stratify.
- Ventilate continuously and monitor throughout the entry.
- Attendant, permit, rescue plan, and communication are all required, and a tower adds the complication of vertical rescue from a long way up.
- Control ignition sources. Any hot work inside the tower requires its own permit and its own gas testing immediately before and during the work.
Which tray type maintains a liquid seal and continues to operate correctly at very low vapor rates?
Why must a distillation tray and its outlet weir be installed level?
A tower has been drained, steamed out, and nitrogen purged in preparation for a turnaround. What is the correct understanding of its atmosphere?