A post-combustion amine capture unit is an absorber, a stripper and the heat and pumping between them, with a compression and dehydration train on the CO2 product. Its P&IDs tag instruments to ISA 5.1 like any other process sheet, so the reading skill transfers from a gas plant or a refinery. What sets the drawing set apart is where the instrument count concentrates: on the two boundaries where CO2 is measured, on the absorber temperature profile, on the reboiler, and on the tie-ins to the host plant that the unit has been bolted onto.
This is a field guide to reading that set into an instrument index, written for the engineer who has been handed a capture package on a gas plant, a cement kiln, a refinery heater or a power boiler, and has to produce the register before the DCS scope and the SIS package can start.
Key takeaways
- The capture rate is four loops: CO2 in and out of the absorber, flue gas flow, and CO2 product flow. They are the tags everyone outside the project will ask about.
- The absorber temperature profile is the unit's operating instrument, and it can run to dozens of transmitters on one vessel.
- The reboiler carries the main trip layer; CO2 compression carries the second.
- A capture retrofit is two drawing sets in two conventions, joined at the tie-ins. The index has to hold both as printed.
- Amine is a weak fire hazard. The gas detection on the sheet is for CO2 in enclosed spaces and for amine vapour at the wash section.
The train, section by section
| Section | What it does | Characteristic instrumentation |
|---|---|---|
| Flue gas conditioning | Cools and quenches the host flue gas in a direct-contact cooler, drops particulate and SOx, and a blower overcomes the absorber pressure drop | Inlet TT and PT, DCC sump LT, circulating water FT and TT, blower suction and discharge PT, blower PDT, inlet CO2 and O2 analyzers, SO2 analyzer where the host fuel needs one |
| Absorber | Flue gas up, lean amine down through packing; CO2 leaves with the rich amine | Packing temperature profile at several elevations, PDT across each bed for flooding, sump LT, lean amine FT for the liquid-to-gas ratio, treated gas CO2 analyzer at the outlet, wash-section water FT and TT, amine or ammonia analyzer on the treated gas |
| Rich amine circuit | Pumps rich amine through the lean/rich exchanger into the stripper | Rich pump discharge PT and FT, exchanger inlet and outlet TT on both sides, filter PDT, sample points |
| Stripper and reboiler | Steam-heated regeneration releases CO2 overhead; lean amine returns from the bottom | Stripper overhead PT and TT, reflux drum LT, reflux FT, reboiler steam FT and PT, reboiler TT, stripper sump LT, lean amine TT and FT |
| Lean amine circuit | Cooled lean amine returns to the absorber; a slipstream is reclaimed and filtered | Lean cooler outlet TT, lean amine FT, pH or conductivity AT, density or Coriolis for concentration, reclaimer TT and LT, filter PDT |
| CO2 compression and dehydration | Multistage compression, interstage cooling and knockout, dehydration to pipeline or storage spec | Suction and discharge PT and TT per stage, interstage knockout LT, anti-surge FT and ZT, dew point analyzer at dehydration outlet, product FT, dense-phase PT with 2oo3 voting |
A gas plant is a special case worth stating. On a sour gas plant the CO2 is already separated in the acid gas removal unit, so the "capture" project is often compression and dehydration of an acid gas stream that exists, with H2S handling, rather than a new absorber on a flue gas. On a sweet plant, or where the target is the fired equipment, the post-combustion train above applies to the turbine or heater flue gas. The two projects have different instrument counts by a wide margin, and the first question on the drawing set is which one you are holding.
The boundary loops
Everything the project is financed on comes back to four measurements, and they deserve their own paragraph in the index.
| Loop | Tag (synthetic) | What it establishes |
|---|---|---|
| CO2 in the flue gas at the absorber inlet | AT-CL-2305 | The CO2 available for capture |
| CO2 in the treated gas at the absorber outlet | AT-ABS-1102 | The CO2 not captured; the capture fraction against the inlet |
| Flue gas flow to the absorber | FT-DC-0201 | The mass basis for the capture rate |
| CO2 product flow after compression | FT-VC-2101 | The captured mass, cross-checked against the two analyzers |
These loops are drawn with more care than the rest of the sheet. Redundant analyzers, a spare transmitter, a calibration gas connection and a heated sample line are all normal, and each one becomes rows in the index that a reader unfamiliar with capture units will not expect. When a lender's engineer or a verification body asks for "the instruments that measure the capture rate", this table is the answer, and the index should be able to filter to it.
The absorber profile
The absorption reaction is exothermic and the position of the temperature bulge in the packing is the operator's picture of where the reaction is happening. Large absorbers carry a temperature transmitter at each of several elevations, and on a column of several metres diameter there may be three or four around the circumference at each level. Add the PDT across each bed, which is how flooding is seen coming, and the absorber alone can account for a quarter of the unit's analog inputs.
On the sheet these appear as a column of TTs down the vessel outline with a common area code, often numbered in a sequence that reads top to bottom. In the index they should carry the elevation or bed number the drawing gives, because the DCS graphics and the alarm rationalization will need it.
The reboiler and the trip layer
The stripper reboiler is where the unit's energy goes and where its trips concentrate. A typical set:
- low-low stripper sump or reboiler level, tripping the steam;
- high-high reboiler or stripper bottoms temperature, protecting the amine from thermal degradation;
- low-low steam supply pressure or loss of condensate return, protecting the reboiler;
- high-high stripper overhead pressure, protecting the column.
The compression train adds its own: 2oo3 discharge pressure on the final stage (PT-CMP-2103-2oo3 on the synthetic sheet), anti-surge, high-high interstage knockout level, and, where the product goes to a dense-phase pipeline, a high-high pressure function on the export line. Each of these is a safety instrumented function whose integrity level is assigned by the project's LOPA under IEC 61511, and the index should carry the SIS flag on those rows so the SIF register can be filtered out of it.
Amine solvents are a weak fire hazard, with flash points well above ambient, so the hazardous-area callouts on a capture unit are lighter than on a gas plant. What the sheet does carry is CO2 detection in enclosed spaces, because a leak from the compression train or the stripper overhead is an asphyxiation hazard, and amine or ammonia detection at the absorber wash section and the reclaimer, where solvent vapour and degradation products leave.
The tie-ins
A capture retrofit is drawn as a new unit with a boundary, and the boundary is a list of tie-ins to the host plant:
| Tie-in | Host side | New side | What the index carries |
|---|---|---|---|
| Flue gas take-off | Stack or duct damper, existing draft measurement | Isolation damper with ZSO and ZSC, inlet TT and PT | Both dampers, both sheets, the host draft loop it modifies |
| Steam to the reboiler | Host LP steam header, existing PT | New FT, PT, control valve, isolation | The host header tag as the source, the new loop as the consumer |
| Condensate return | Host condensate system | New FT, TT, level | Same pairing |
| Cooling water | Host supply and return | New FT, TT, PDT | Same pairing |
| Power and instrument air | Host MCC and air header | New feeders and regulators | The one-line tie-in and the air header tap |
| Signals | Host DCS and SIS | New controller, serial or hardwired links | The I/O list rows that cross the boundary, marked as such |
Each tie-in belongs to two drawings, and each is a row in the tie-in register with the host sheet and the new sheet on it. The index row for a new instrument at a tie-in should carry the host tag it replaces or connects to, because the MOC on the host side is written against that.
Two conventions on one project
The capture package arrives in the licensor's and the EPC's convention. The host plant is tagged in its own, which on a plant built in the 1980s or 1990s is often a house rule with its own area codes and letter meanings. The tie-in sheets carry both, and so does the index. The rule that holds is to record every tag as its sheet prints it, with the sheet reference on the row, and to resist translating the host tags into the new convention or the reverse. A translated tag is a tag the field will not find.
What the index looks like when it is done
For a capture unit the useful columns beyond the standard instrument index are:
- Boundary loop (yes or no): the capture-rate instruments, so they can be pulled out for the verification body.
- Vessel elevation or bed: for the absorber and stripper profiles.
- SIS: the trip functions, for the SIF register.
- Tie-in number: for every instrument on the boundary, with the host sheet.
- Convention: which package the tag was drawn in, so the field and the DCS build know which rule the tag follows.
The I/O list follows from the index in the usual way, with the boundary loops and the trips typically hardwired and the profiles typically on the plant network. The equipment list is short by process-plant standards, two columns, a few exchangers, a handful of pumps and the compressor train, and the line list is dominated by the amine circuits and the CO2 product line.
Tagsight reads a capture package into these registers, in whichever conventions the sheets carry, with every row referenced to its sheet and confirmed by the engineer before it exports. The engineering that decides what the capture rate is, which trips the LOPA assigns, and what the host MOC says stays with the project.