Carbon capture heat exchangers handle a fluid that changes character from one end of the plant to the other. Captured CO2 typically leaves a solvent stripper as a warm, water-saturated gas at low pressure. It may leave the site as a dense fluid above 2,000 psig, or as a refrigerated liquid held only about 40 psi (3 bar) above its triple-point pressure. Between those points, water condenses, impurities concentrate and react, and phase boundaries move.
Exchanger selection and mechanical design therefore depend on the specific CO2 processing duty. Flue gas cooling, solvent regeneration, compression intercooling, dehydration, liquefaction and cryogenic separation each bring different pressures, phases and corrosion risks, in transients as well as normal operation.
IN BRIEF
• Capture and conditioning are different duties. Solvent capture equipment handles hot amine and wet gas near atmospheric pressure; downstream compression coolers may see CO2 at 2,200 psig (about 152 bar gauge), NETL's pipeline basis.
• Phase behavior sets the envelope. Pure CO2 has a triple point at −69.8°F (−56.6°C) and 75.1 psia (5.18 bar absolute), and a critical point at 87.8°F (31.0°C) and 1,070 psia (73.8 bar absolute). Impurities can shift phase boundaries, making actual stream composition essential to equipment design.
• Water and impurities decide corrosion. Rigorously dry CO2 does not corrode carbon steel, but in laboratory tests SO2, NO2, O2, H2S and water at well under 100 ppmv each formed a concentrated acid phase.
• Depressurization often sets the coldest case. Liquid CO2 vented to atmosphere flashes to solid and vapor at about −109°F (−78°C), so MDMT and relief design must cover transients as well as steady operation.
• Shell-and-tube is one option, not the default. It suits high-pressure gas cooling, reboilers, condensers and cleanable duties; plate designs often win close-approach solvent duties, and plate-fin or coil-wound designs are typical for multi-stream cryogenic duties.
On this page: process routes · phase behavior · equipment table · engineering challenges · shell-and-tube selection · pressure vessels · RFQ checklist · Harris Thermal · FAQs
Where heat exchangers fit in carbon capture and CO2 processing
A carbon capture project has two thermal halves. The capture step separates CO2 from a gas mixture. Conditioning then compresses, dries, purifies and, in some chains, liquefies it for transport and storage. The two halves handle very different fluids, and the capture route determines which conditioning steps follow.
Post-combustion solvent capture
Solvent-based post-combustion designs cool and saturate flue gas, often in a direct contact cooler, then scrub it with an aqueous amine solvent. University of Texas research on piperazine describes solvent cycling between an absorber at roughly 104 to 131°F (40 to 55°C) and a stripper at 248 to 302°F (120 to 150°C). The upper end depends on the solvent's thermal stability; less stable amines such as MEA are regenerated at lower stripper temperatures.
Its exchangers, all within a licensor's process design, include the lean/rich exchanger, lean solvent cooler, steam reboiler and stripper overhead condenser. Reboiler conditions matter most for solvent life: one University of Texas study attributed about three-quarters of stripper thermal degradation of MEA to the reboiler's higher temperature and long residence time.
Pre-combustion and high-pressure separation
In gasification and reforming, CO2 is removed from shifted syngas at elevated pressure. NETL notes that physical solvents such as Selexol and Rectisol depend on high pressure, high acid-gas partial pressure or low absorber temperature; Rectisol uses methanol chilled below freezing. Exchangers handle solvent chilling, refrigeration and cross exchange at pressure.
Cryogenic separation: condensation and desublimation
Cryogenic capture separates CO2 by phase change, and a review by Font-Palma and colleagues distinguishes two mechanisms:
Partial condensation liquefies CO2 from a compressed, CO2-rich stream, such as oxy-combustion flue gas compressed in stages to roughly 15 and 30 bar (about 220 and 435 psi). It suits streams above about 50% CO2, with water removed to trace levels to prevent ice plugging and temperatures controlled to keep solid CO2 off exchanger surfaces.
Desublimation freezes CO2 out of the gas as frost. In atmospheric flue gas, CO2 partial pressure is far below the triple-point pressure, so cooling alone cannot produce liquid. One published frosting-exchanger design operates at about −148 to −184°F (−100 to −120°C) and cycles through defrost as frost cuts heat transfer and raises pressure drop.
Neither is the same as liquefying captured CO2 for transport. Equipment selection follows the separation mechanism and process configuration. Partial condensation of a compressed, dried stream generally relies on multi-stream, close-approach exchangers such as plate-fin or coil-wound designs. Desublimation must manage frost growth and removal, using alternating frosting exchangers, cold packed beds or direct contact with a cryogenic liquid.
Downstream compression, dehydration and liquefaction
Whatever the capture route, CO2 is compressed in stages with intercoolers and knockout drums, dehydrated by glycol or solid desiccant, and purified where required. Pipeline delivery ends in dense phase. Ship transport typically liquefies it against a refrigerant. The conventional range is about 15 to 19 bar (about 220 to 275 psi), and a lower-pressure option near 7 bar gauge (about 8 bar absolute) is under study (Trædal and colleagues). Direct air capture feeds the same chain; one liquid-solvent DAC design regenerates in a high-temperature calciner and delivers dry CO2 at 15 MPa (about 2,175 psi).
Why CO2 phase behavior drives exchanger and vessel design
The pure-CO2 landmarks
The NIST-published reference equation of state for CO2 places the triple point at −69.8°F (−56.6°C) and 75.1 psia (5.18 bar absolute) and the critical point at 87.8°F (31.0°C) and 1,070 psia (73.8 bar absolute). At atmospheric pressure, solid CO2 sublimes at about −109°F (−78°C). Three regions follow:
Below 75.1 psia, CO2 exists only as gas or solid; cooling produces frost, never liquid.
Between the triple-point and critical pressures, liquid–vapor equilibrium is possible for pure CO2. Whether condensation occurs depends on both pressure and temperature.
Above 1,070 psia, cooling from above 87.8°F takes CO2 from a supercritical fluid to a dense, liquid-like state without a distinct condensation step. Specific heat peaks sharply near the pseudocritical temperature, so heat-transfer coefficients vary strongly along a cooler, as a review of supercritical CO2 cooling describes.
"Dense phase" is the transport term for liquid or supercritical CO2 held at high pressure.
Impurities move the boundaries
The pure-fluid diagram does not predict an impure stream. Impurities open a two-phase envelope between bubble and dew curves, so an impure stream can split into gas and liquid where pure CO2 would be single-phase. Measurements by Okoro and colleagues on binary CO2 mixtures between about −49 and 32°F (−45 and 0°C) found that nitrogen, oxygen, methane and argon all raise bubble-point pressure, nitrogen most strongly. Impurity levels as low as 0.5 mol% increased the risk of two-phase flow.
Water behaves differently. Morland and colleagues show its solubility dropping sharply as CO2 passes from liquid to gas, so water dissolved in dense CO2 can drop out across a letdown valve. Phase checks for impure streams need a mixture equation of state and the actual composition range.
Equipment applications in CO2 capture and processing
The table maps common duties to their role and design considerations. Not every project includes every item, and many duties sit inside licensor or OEM packages.
| Equipment / Duty | Role in CO2 Processing | Key Design Considerations |
|---|---|---|
| Flue gas coolers and direct-contact-cooler water coolers | Cool and saturate flue gas before capture; reject heat from circulating cooler water | Acidic condensate, sulfate aerosols and particulate; low gas-side pressure drop; plate designs often favored for clean cooler-water duty, shell-and-tube where fouling or cleaning governs |
| Lean/rich exchanger and lean solvent cooler | Recover heat from hot lean solvent into rich solvent; trim-cool lean solvent | Close temperature approach favors plate designs; amine cracking of carbon steel and PWHT; CO2 release from heated rich solvent; degradation-product fouling |
| Reboiler and stripper overhead condenser | Supply regeneration heat; condense water for reflux while CO2 leaves as gas | Wall temperature and residence time drive solvent degradation; kettle or thermosyphon shell-and-tube common; wet, CO2-saturated condensate |
| Compression intercoolers and aftercoolers | Remove heat of compression; condense water between stages; set delivery temperature | Pressures up to pipeline level; acidic condensate before drying; pseudocritical property swings; pressure drop costs compressor power; tube rupture into cooling water; often in compressor package scope |
| Dehydration coolers and knockout drums | Remove condensed water; cool glycol; heat and cool adsorbent regeneration gas | Interstage pressure set near the CO2 water-content minimum; liquid carryover; regeneration thermal cycling; stainless steel in wet glycol sections |
| Liquefaction condensers and coolers | Condense dried CO2 against ammonia, propane or another refrigerant | Margin above the triple point at low pressure; hydrate or dry-ice solids if water exceeds specification; venting non-condensables; refrigerant leakage; low-temperature toughness |
| Cryogenic separation exchangers | Partially condense or frost CO2 out of gas mixtures | Multi-stream, close-approach duties usually plate-fin or coil-wound; frost and defrost cycling; trace water removal; mercury attack on aluminum; shell-and-tube limited to selected duties |
| Knockout drums and phase separators | Separate water or liquid CO2 from gas | MDMT from blowdown; impurities partitioning into liquid CO2; corrosion allowance in wet sections; relief and drainage |
| Conditioning and purification vessels | Dehydration contactors and adsorbers, liquid CO2 receivers, purification column shells | Cyclic regeneration loads; dryness at startup; low-temperature materials; internals by the licensor |
Shell-and-tube exchangers and ASME pressure vessels can serve many of these duties. Brazed aluminum plate-fin cores, coil-wound exchangers, printed circuit exchangers, cold boxes, and refrigeration or compressor packages are specialized products, normally supplied by their OEMs.
Engineering challenges in CO2 heat exchangers and vessels
Pressure, temperature and phase management
Phase behavior turns into equipment decisions in several places:
Thermal design: a cooler that crosses a phase boundary or the pseudocritical region needs zoned rating rather than average properties. A compression train may carry gas early and near-critical CO2 at the last stage.
Control and transients: near a boundary, small pressure changes alter phase and duty, and cooldown, warmup and settle-out can visit regions steady operation never does.
Unexpected liquid: liquid where gas was expected changes velocities and drainage, collects in low points and adds inventory that relief studies must cover.
Depressurization: blowdown cools liquid or dense CO2, and once two phases form, temperature falls with pressure along the saturation curve. If pressure falls to the triple-point pressure while liquid remains, solid forms, and solid trapped in a blocked-in section becomes high-pressure gas again as it warms.
The datasheet should list every operating case with pressure, temperature and composition, each checked against the mixture phase envelope.
Moisture, impurities and corrosion
Dry and wet CO2 are different services. NETL's impurity guidance states that rigorously dry CO2 does not corrode carbon steel, nor does SO2 without water, but that water compounds the effects of O2 and SO2. CO2 dissolving in free water also forms carbonic acid. In laboratory work at IFE, H2O, H2S, SO2, NO2 and O2 at well under 100 ppmv each reacted to form a separate phase rich in sulfuric and nitric acid. Drying alone may not remove the risk where NOx and SOx are present.
Upstream of dehydration, in the stripper overhead, early compression stages and interstage drums, water condenses by design and is acidic. Downstream, carbon steel can be acceptable if the dry specification holds. NETL's limits are conceptual-study guidelines that do not address interactions between impurities, so the design basis should state project limits and expected upset excursions.
Materials selection
Material choice answers two separate questions: will the metal resist the fluid, and will it stay tough at the lowest metal temperature?
Carbon steel suits dry CO2 and many cooling-water services with a corrosion allowance; cold service needs impact-tested grades or a Code exemption. API RP 945, written for amine treating units, covers environmental cracking of carbon steel, including postweld heat treatment; confirm its applicability against the solvent licensor's materials guidance.
Austenitic stainless steels such as 304L and 316L resist wet CO2 condensate and stay tough at very low temperatures. They remain vulnerable to chloride pitting and stress corrosion cracking in warm, chloride-bearing water.
Duplex stainless steels add strength and chloride resistance but have less low-temperature toughness than austenitic grades, so their minimum temperature must be checked against the Code and project specification.
Nickel alloys are justified by specific chemistry, such as hot acid condensate with chlorides.
An impact-tested carbon steel can still corrode in wet, acidic CO2. Alloy selection follows the same mechanism-first approach used in chemical service.
Thermal cycling and differential expansion
Adsorbent dryers regenerate with hot gas, desublimation exchangers alternate frosting and defrosting, and compressor trips swing coolers to settle-out conditions. In fixed-tubesheet exchangers, a large shell-to-tube metal temperature difference loads the tubesheet and tube joints and may require an expansion joint; U-tube and floating-head bundles absorb it. Cycling can also relax gasketed joints during cooldown and warmup, so gasket selection should account for expected cycle counts as well as design temperatures.
Pressure drop and thermal performance
Pressure drop is an operating cost. In flue gas service it adds blower power; between compressor stages, every psi lost in an intercooler and drum must be recompressed. Near the critical point, it also shifts the local pseudocritical temperature along the exchanger. No universal limit applies, so allowable pressure drop should come from the designer's energy balance and be stated per side. Temperature approach is the matching trade-off: for the lean/rich exchanger, one techno-economic study found the cost optimum at a 7 to 13°F (4 to 7°C) approach using plate exchangers.
Fouling, cleanability and maintenance
Flue gas circuits carry particulate, sulfate aerosols and acidic deposits, and solvent circuits accumulate degradation and corrosion products. Dehydrated CO2 is comparatively clean, though compressor oil, glycol carryover and elemental sulfur from impurity reactions can deposit. In many CO2 coolers the cooling-water side governs fouling, which should shape fluid allocation.
Solvent reboilers benefit from removable bundles for inspection. Water-cooled units need straight, accessible tubes on the water side, while clean, dry CO2 service can justify a fixed tubesheet. For high-pressure, leak-critical units, plan joint leak testing, a plugging margin and bundle access, and align bundle work with compressor outages, since intercooler maintenance usually requires one.
When shell-and-tube construction fits CO2 service
Shell-and-tube construction fits where pressure, phase change, fouling or cleanability governs: high-pressure CO2 intercoolers and aftercoolers, kettle and thermosyphon reboilers, overhead condensers, glycol and regeneration-gas exchangers, and refrigerant condensers.
Configuration. Fixed-tubesheet units are economical but need a differential-expansion check and give no mechanical shell-side access. U-tube bundles absorb expansion and suit high tube-side pressure, but the bends resist mechanical cleaning. Most floating-head designs allow cleaning on both sides but add an internal gasketed joint, a poor fit where streams must not mix.
Fluid allocation. Tube-side allocation can be advantageous for high-pressure CO2 because smaller-diameter pressure boundaries may reduce mechanical design and fabrication costs. Final allocation must also account for pressure drop, heat transfer, phase behavior, corrosion, cleaning access, and applicable mechanical design requirements. Placing wet, acidic CO2 in the tubes can confine corrosion-resistant alloy to tubes, tubesheets and channels, but condensate drainage or the other stream's fouling may favor the opposite arrangement. Cooling water belongs on a side that can be cleaned.
Joint integrity and leak consequences. A tube or joint leak can send high-pressure CO2 into cooling water, amine into steam condensate, or refrigerant into product. Trimeric engineers report a case in which ammonia leaking from an exchanger tube into captured CO2 formed ammonium carbamate salts. API Standard 521 treats exchanger tube failure as an internal-failure case to evaluate. As a published tube-rupture analysis shows, the evaluation defines the credible failure, then checks whether the resulting pressure would exceed what the low-pressure side and connected piping can tolerate. Only if it would does the designer select a response, such as a higher low-pressure-side design pressure or a fast-acting relief device; large pressure differences may require dynamic analysis. Where leakage is unacceptable, specify welded or welded-and-expanded tube joints and the leak test method on the purchase order.
When another technology is better. Plate exchangers often win close-approach, moderate-pressure solvent duties. Brazed aluminum plate-fin and coil-wound exchangers handle multi-stream cryogenic duties, and diffusion-bonded printed circuit exchangers offer compactness at high pressure. Air coolers avoid water use where climate allows. Where gas crosses the bundle on the shell side, the usual tube vibration and acoustic checks apply.
Pressure vessels in CO2 processing
CO2 vessels include interstage knockout drums, condensate collection vessels, phase separators, liquid CO2 receivers, and dehydration contactor and adsorber shells. Their design basis needs more than a normal operating point:
Design pressure and temperature should cover settle-out after a compressor trip, blocked outlets and the hottest regeneration step.
Minimum design metal temperature (MDMT) should reflect the coldest credible case: cold ambient, autorefrigeration during blowdown or venting of liquid CO2. ASME Section VIII, Division 1 sets impact-test requirements and exemptions by material, thickness and MDMT. Its carbon steel rules allow some credit where low temperature coincides with reduced pressure; the vessel designer should confirm whether that credit applies.
Corrosion allowance belongs where water condenses; dry sections may need little if the dehydration specification holds.
Pressure relief evaluation should consider fire, blocked discharge, tube failure in connected exchangers and thermal expansion of blocked-in liquid CO2, with relief-system design accounting for potential solid CO2 formation and blockage during depressurization.
Lethal service designation, which an H2S-bearing CO2 stream might warrant, is the user's determination under Division 1, not the fabricator's.
Code applicability is not automatic: Division 1 construction and stamping depend on design pressure, size, Code scope, jurisdiction and owner requirements. The same specification should define traceability, PMI, NDE extent, pressure and leak testing, dryness before shipment, and data reports and registration where required.
Preparing an RFQ for CO2 heat exchangers and pressure vessels
A complete inquiry lets fabricators quote like-for-like and exposes risk before geometry is fixed. It should cover:
Scope and duty: equipment list, service (for example, third-stage intercooler) and items supplied by others.
Composition and impurity limits: CO2 purity and each impurity at normal and maximum values, including SO2, NOx, O2, H2S, N2, Ar, H2, amine and glycol.
Moisture: water content or dew point for each case, and whether the equipment is upstream or downstream of drying.
Operating cases: normal, minimum, maximum, startup, shutdown, turndown, trips, regeneration and depressurization.
Phase behavior: expected phase on each side, the equation of state used, and risk of liquid, hydrate or solid CO2.
Thermal requirements: duty, terminal temperatures, allowable pressure drop per side and fouling resistances with their basis.
Mechanical design: design pressure and temperature per side, MDMT and its governing scenario, vacuum and cyclic service.
Materials: acceptable materials, corrosion allowance by component, PWHT and impact testing.
Code and jurisdiction: ASME Section VIII, Division 1 or other code, stamping, TEMA class, API 660 or owner specifications, and lethal or cyclic service.
Inspection and documentation: NDE extent, PMI, pressure and leak testing, dryness requirements and the data package.
Configuration and logistics: TEMA type, fluid allocation, bundle removal, cleaning method, shipping envelope and site access.
Design responsibility: who owns process, thermal and mechanical design, and what data the licensor, EPC or OEM supplies and when.
Common specification mistakes
Treating the stream as pure CO2 when impurities move the phase envelope.
Omitting water content, or quoting it only for dense phase when the equipment sees gas.
Providing only the normal operating case.
Ignoring metal temperature during depressurization and venting.
Assuming one exchanger technology suits every duty from flue gas to cryogenic separation.
Leaving unclear who owns the process and thermal design basis.
How Harris Thermal supports CO2 processing equipment
Harris Thermal designs and fabricates custom shell-and-tube heat exchangers and ASME Section VIII, Division 1 pressure vessels, with in-house thermal and mechanical design capabilities. Its fabrication experience with condensers, separators, and custom process vessels may be applicable to selected CO2 cooling and conditioning duties, subject to project-specific materials, temperature, pressure, and code requirements.
Equipment is built to the process design of the licensor, EPC or OEM, which retains process design responsibility. Harris also repairs, retubes and replaces existing exchangers. Harris does not supply capture process design, plate-fin cores, cold boxes, refrigeration or compressor packages, or complete plants. Any skid it builds is a Harris-fabricated assembly integrated with systems supplied by others. If you are preparing an inquiry, start with the stream composition, water content, every operating case including depressurization, and the code jurisdiction.
FAQs about carbon capture heat exchangers
What heat exchangers are used in carbon capture plants?
Solvent capture uses flue gas coolers, lean/rich exchangers, solvent coolers, reboilers and overhead condensers. Conditioning adds compressor intercoolers and aftercoolers, dehydration exchangers and, for liquefaction, refrigerated condensers. Shell-and-tube, plate, plate-fin and air-cooled designs all appear, chosen by duty.
Does CO2 condense when it is cooled?
Only under the right combination of pressure and temperature. For pure CO2, liquid–vapor equilibrium is possible between the triple-point pressure, 75.1 psia (5.18 bar absolute), and the critical pressure, 1,070 psia (73.8 bar absolute), and condensation occurs when CO2 is cooled to the saturation temperature for its pressure. Below that pressure range, cooling forms solid frost; above it, CO2 becomes denser without a distinct condensation step. In atmospheric flue gas, CO2 partial pressure is far below the triple-point pressure, so it frosts instead of condensing.
Is captured CO2 corrosive to carbon steel?
Rigorously dry CO2 is not, according to NETL guidance. With free water, CO2 forms carbonic acid. Depending on moisture content, impurity combinations, and operating conditions, contaminants such as SO2, NOx, O2, and H2S can contribute to corrosive aqueous phases. Carbon steel can suit properly dried service; wet sections upstream of dehydration usually need corrosion-resistant materials or allowances.
Can shell-and-tube heat exchangers be used for cryogenic CO2 service?
For some duties, yes, such as precoolers, condensers and refrigerant exchangers, within the fabricator's low-temperature qualifications. Multi-stream partial condensation typically uses plate-fin or coil-wound exchangers, and desublimation uses frosting exchangers, packed beds or contact liquids. Low-temperature capability does not imply corrosion resistance, so check both.
Why does depressurization matter for CO2 equipment design?
Releasing pressure from liquid or dense CO2 cools it; once two phases form, temperature follows the saturation curve down, and solid forms if pressure falls to the triple-point pressure while liquid remains. Liquid vented to atmosphere flashes to solid and vapor at about −109°F (−78°C). That can set MDMT, plug relief paths, and overpressure equipment where trapped solid later sublimes.
Do CO2 pressure vessels require an ASME stamp?
Not automatically. Requirements depend on design pressure, vessel size, Code scope, the installation jurisdiction and owner specifications. Where the jurisdiction adopts the Code and the vessel falls within its scope, Division 1 construction and stamping are typically required. Confirm this during specification.