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Author:
Anna Frank, Equipment Selection Expert
Anna Frank, Equipment Selection Expert
Gas sweetening and NH₃ capture run on the same vessel: a vertical column in which a falling liquid strips contaminants out of a rising gas. This guide covers the amine absorber tower used in natural gas processing and refining, then applies the same logic to water-fed NH₃ units. The emphasis is on figures engineers actually check: loadings, stage counts, temperature limits, and solvent cleanliness.
How an Amine Absorber Tower Works
Sour feed enters the bottom of the contactor at anywhere from near-atmospheric pressure (refinery fuel gas) to over 1,000 psig (gas plants). Lean solvent — an aqueous MEA, DEA, or MDEA solution at 15–50 wt% — is pumped onto the top tray and flows down against the rising stream. H₂S and CO₂ react with the weak base to form salts that break apart on heating, which is what makes the loop regenerative. Removal is driven by chemistry, so even a lean partial pressure of a few millibars still pulls the contaminant across the interface — the reason chemical solvents outperform plain water at low concentrations.
The rich solution leaves the sump, releases dissolved hydrocarbons in a flash drum, picks up heat in the lean/rich exchanger, and is stripped in the regenerator at a reboiler temperature of 230–260 °F. Cooled solvent returns to the top, and the sweetened stream leaves through a mist eliminator, typically at a 4 ppmv H₂S pipeline spec. In refineries the same amine absorption tower also treats fuel gas and LPG ahead of combustion or further processing. The loop scales from a skid handling a few hundred scfm to plant contactors twelve feet across. By operating principle these columns belong to wet scrubbers — the packed and tray units in our wet scrubber line work on identical gas–liquid contact.
The rich solution leaves the sump, releases dissolved hydrocarbons in a flash drum, picks up heat in the lean/rich exchanger, and is stripped in the regenerator at a reboiler temperature of 230–260 °F. Cooled solvent returns to the top, and the sweetened stream leaves through a mist eliminator, typically at a 4 ppmv H₂S pipeline spec. In refineries the same amine absorption tower also treats fuel gas and LPG ahead of combustion or further processing. The loop scales from a skid handling a few hundred scfm to plant contactors twelve feet across. By operating principle these columns belong to wet scrubbers — the packed and tray units in our wet scrubber line work on identical gas–liquid contact.
Operating Principle of a Vertical Packed Column
Tower Internals: Trays vs Packing
A high-pressure contactor traditionally carries about 20 valve or sieve trays at 24-inch spacing; that count has been the standard for H₂S service for decades. Trays tolerate suspended solids, keep working when the liquor gets dirty, and turn down to roughly 25–30% of design vapor rate before weeping starts. Valve decks hold efficiency across a wider vapor range than sieve decks, and both beat bubble caps on cost; bubble caps survive only where the liquid rate is extremely low.
Packed beds win on pressure drop — 0.2–0.6 in. w.c. per foot of bed against roughly 2–3 in. per tray — and on corrosion resistance, since random media and internals are available in polypropylene and FRP. Wetted surface of random media runs 30–80 ft²/ft³ depending on ring size, with liquid redistribution every 10–20 ft of bed. The media do the separation work here, so bed selection deserves the same rigor as vessel sizing. In our line, TORNADO columns use random media, while the TORNADO ST is a foam-tray design intended for dust-laden streams. A mist eliminator pad on top is standard in both layouts; carryover is the main route of losses, so the pad deserves the same inspection schedule as the internals.
Packed beds win on pressure drop — 0.2–0.6 in. w.c. per foot of bed against roughly 2–3 in. per tray — and on corrosion resistance, since random media and internals are available in polypropylene and FRP. Wetted surface of random media runs 30–80 ft²/ft³ depending on ring size, with liquid redistribution every 10–20 ft of bed. The media do the separation work here, so bed selection deserves the same rigor as vessel sizing. In our line, TORNADO columns use random media, while the TORNADO ST is a foam-tray design intended for dust-laden streams. A mist eliminator pad on top is standard in both layouts; carryover is the main route of losses, so the pad deserves the same inspection schedule as the internals.
Dual Packed Bed Tower
Design Parameters and Solvent Selection
Circulation rate follows from pickup: moles of acid gas entering per minute, divided by allowable net loading, fix the pumping rate, and oversized circulation burns reboiler duty in direct proportion. For MEA, rich loading is capped near 0.35 mol/mol by corrosion: pushing past it attacks carbon steel at the bottom of the vessel and in the lean/rich exchanger. MDEA earns its keep in selective service — CO₂ slips through, the recovered acid stream grows richer in H₂S, and the sulfur plant downstream runs on a stronger feed. Lean-solution temperature is held 10–15 °F above the incoming feed — a guideline published in the GPSA Engineering Data Book — because a colder liquid condenses hydrocarbons and triggers foaming. Reaction heat creates a temperature bulge in the bottom section, sometimes 50 °F and more above the feed, which is worth checking before trusting equilibrium data taken at inlet conditions. Diameter is set at 70–80% of flooding velocity, height by stage count; both calculations follow the general method for any counterflow column.
H2S Scrubber: Design, Operational Principles, Tank Solutions, and Types of Media
Ammonia Absorption Tower Design
NH₃ dissolves at about 30 g per 100 g of water at 77 °F, so a water-fed ammonia gas absorption tower looks deceptively simple to size. Dissolution releases roughly 1,800 kJ per kilogram captured; the liquor warms up, equilibrium vapor pressure over it climbs, and the driving force collapses in the lower stages. Practical ammonia absorption tower design therefore centers on heat removal — a high liquid rate, an intercooler, or both — and on the outlet target: OSHA's 50 ppm eight-hour permissible exposure limit for workplace air is the figure most vent specs trace back to.
A quick water balance shows the scale: at a 2% outlet strength of the liquor, every kilogram of captured NH₃ ties up about 50 kg of water, so once-through schemes suit only small vents, and recirculation with cooling dominates.
A quick water balance shows the scale: at a 2% outlet strength of the liquor, every kilogram of captured NH₃ ties up about 50 kg of water, so once-through schemes suit only small vents, and recirculation with cooling dominates.
Ammonia Wet Scrubber
For the absorption of ammonia in a tray tower, three to six theoretical stages at a moderate liquid rate deliver 95–99% removal; sieve and foam trays are preferred here because cooling coils fit between passes and the internals shrug off dust in the feed. Where a single-digit-ppm outlet is required, a second stage sprayed with dilute sulfuric acid converts residual NH₃ into ammonium sulfate. Torch-Air builds such duplex systems both as vertical twin columns and as the horizontal BOREAS-P3. For NH₃ duty the shell and packing are usually polypropylene or FRP; temperatures stay low enough for plastics to keep their strength.
Operating Problems and Solvent Filtration
Foaming is the classic upset: heat-stable salts, degradation products, condensed hydrocarbons, and injected well chemicals all stabilize bubbles, and the symptoms are erratic differential pressure with liquid carryover. Antifoam buys hours; cleanliness removes the cause. Corrosion adds a second problem — iron sulfide fines that erode pump seals, stabilize foam further, and plug the bed. Carbon steel handles lean service, while the hot bottom section and rich piping usually call for stainless internals.
The standard cure is a filtration train on a 10–20% slipstream of circulating solution: a mechanical cartridge stage rated 10 µm (5 µm in dirty service), an activated carbon bed that strips dissolved hydrocarbons and surfactants, and an after-filter catching carbon fines. Vessels of this type are supplied in our adsorber line. Feed quality matters equally: an inlet coalescer upstream, plus cartridge dust collection where the source stream carries particulates, keeps internals clean and the antifoam bill near zero. The carbon bed needs scheduled change-out: a shake test of a sample — persistent foam in the bottle — tells the operator the bed is spent long before the column differential pressure does.
The standard cure is a filtration train on a 10–20% slipstream of circulating solution: a mechanical cartridge stage rated 10 µm (5 µm in dirty service), an activated carbon bed that strips dissolved hydrocarbons and surfactants, and an after-filter catching carbon fines. Vessels of this type are supplied in our adsorber line. Feed quality matters equally: an inlet coalescer upstream, plus cartridge dust collection where the source stream carries particulates, keeps internals clean and the antifoam bill near zero. The carbon bed needs scheduled change-out: a shake test of a sample — persistent foam in the bottle — tells the operator the bed is spent long before the column differential pressure does.
Automation in Wet Scrubbers
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Head of Engineering,
Vladimir Nikulin
Vladimir Nikulin
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