CO₂ scrubber
Removes metabolic CO₂ from habitat atmosphere using a regenerable sorbent — most commonly zeolite molecular sieves in a 4-bed Carbon Dioxide Removal Assembly (CDRA) architecture, swung thermally between adsorption (25 °C, capture from cabin air) and desorption (200 °C, vent or recycle to Sabatier). Continuous operation since 2001 aboard ISS makes this the most flight-mature ECLSS subsystem.
Governing equations
NASA metabolic CO₂ production rate at moderate activity. Scales with crew size and activity level (peaks during EVA pre-breathe at +30 %). [1]
Equilibrium sorbent loading vs CO₂ partial pressure. q_m is the saturation capacity, b is the affinity constant. At habitat conditions (~5 mmHg CO₂) zeolite 5A loads ~ 6 wt%. [2]
NASA-STD-3001 cabin CO₂ exposure limit for 24-h average. Above this, cognitive performance degrades measurably. [3]
Regeneration energy = heat of desorption + sensible heat to ramp the bed from 25 °C to 200 °C. Bed thermal mass dominates — the CO₂ desorption ΔH is small by comparison. [2]
Key constants & quantities
| Symbol | Value | Units | Conditions | Description |
|---|---|---|---|---|
| ṁ_crew | 1.04 ±15 % | kg CO₂ / crew · day | — | NASA Baseline Values & Assumptions Document (BVAD) per-crew CO₂ production. Includes basal + activity. EVA pre-breathe peaks add ~ 30 %.[1] |
| p_CO₂,max | 5.3 | mmHg (24-h avg) | — | NASA-STD-3001 maximum allowable cabin CO₂ partial pressure for continuous habitation. ISS originally ran at ~ 7 mmHg; lowered to ~ 4 mmHg after cognitive-performance studies (2017–).[3] |
| q*_zeolite-5A | 6.5 ±0.5 wt% | wt% (CO₂ on dry sorbent) | — | Equilibrium loading of zeolite 5A at 25 °C and 5 mmHg CO₂ partial pressure — typical habitat operating point.[2] |
| m_CDRA | 7 | kg zeolite per bed (×4 beds) | — | ISS CDRA sorbent inventory. Two pairs of beds alternate adsorption/desorption on a half-hour cycle, providing continuous removal.[2] |
| T_regen | 200 ±10 °C | °C | — | Zeolite regeneration temperature. Above 250 °C accelerates hydrothermal degradation of the molecular sieve framework.[2] |
| E_specific | 1.2 ±0.3 kWh/kg | kWh / kg CO₂ removed | — | Energy budget for CDRA-class scrubbing including blowers, heaters, valve actuation, and balance-of-plant. Higher than the equilibrium minimum by ~10× due to bed thermal cycling.[4] |
Operating envelope
Mass balance
Basis: 1 kg CO₂ removed from cabin (one crew · day at nominal activity)
Inputs
| CO₂-laden cabin air | 200 | kg air (≈ 170 m³ at habitat density) | [2] |
| Electrical energy | 1.2 | kWh | [4] |
- CO₂-laden cabin air: Volume processed during one half-cycle to remove 1 kg CO₂ at the operating partial-pressure swing.
- Electrical energy: Blower + heater + valve actuation. Heater dominates (regenerating bed thermal mass).
Outputs
| CO₂-depleted cabin air | 199 | kg (returned to habitat) | [2] |
| Concentrated CO₂ | 1 | kg | [2] |
| Waste heat (rejected) | 0.9 | kWh | [4] |
- Concentrated CO₂: On ISS, vented overboard or fed to Sabatier for O₂ recovery. On Mars, always fed to Sabatier or Bosch — never vented.
- Waste heat (rejected): Most regeneration energy ends up as heat to be radiated. Mars thermal management absorbs this.
Crew of 4 generates ~ 4 kg CO₂/day → ~ 5 kWh/day electrical for scrubbing alone, before life-support water and O₂ generation are accounted for.
Variants & trade-offs
4-bed molecular sieve (CDRA, ISS heritage)
[2]Two pairs of zeolite beds: each pair has a 5A bed for CO₂ (heated regeneration) preceded by a 13X bed for water removal (regenerated by dry product gas). Pairs alternate; one adsorbs while the other desorbs.
- Capacity
- 4–10 kg CO₂/day (4-crew habitat)
- Cycle period
- 4.8–12 h full cycle
- Highest flight TRL of any space CO₂ scrubber
- Robust to a wide range of partial-pressure inlets
- Sorbent regenerable indefinitely if hydrothermal limits respected
- Direct interface to Sabatier — captured CO₂ is the Sabatier feedstock
- Zeolite sensitive to liquid water → mandatory pre-dryer
- High thermal mass → most regen energy is sensible heat, not desorption
- Sorbent dust over time → HEPA filter on outlet
Solid amine swing (Skylab RCAS heritage)
[1]Amine-functionalized polymer beads chemically bind CO₂ at low T and release it at moderate T (80–120 °C). Lower regen energy than zeolite, more tolerant of humidity.
- Capacity
- 3–8 kg CO₂/day
- Regen temperature
- 80–120 °C
- Half the regen energy of zeolite
- Tolerant of humidity — no pre-dryer needed
- Smaller, lighter, simpler than CDRA
- Amine oxidation by trace O₂ degrades sorbent — operational lifetime < zeolite
- Amine outgassing into cabin is an air-quality concern
- Lower flight heritage than CDRA
Lithium hydroxide (LiOH, Apollo-style)
[1]Single-use chemical reaction: 2 LiOH + CO₂ → Li₂CO₃ + H₂O. No regeneration, sorbent discarded after saturation. The Apollo 13 emergency.
- Capacity
- 0.8–0.95 kg CO₂ per kg LiOH (theoretical 0.92)
- Zero electrical demand — no regen energy
- No moving parts; passive operation
- Decades of crewed mission heritage (Apollo, Skylab EVA suits)
- 1.1 kg LiOH per kg CO₂ → 4-crew habitat goes through ~ 4.4 kg/day = 1.6 t/year sorbent mass
- Resupply-limited — non-regenerable means launch-mass intensive
- Suitable only for short missions or emergency backup
Failure modes
| Mode | Cause | Detection | Mitigation |
|---|---|---|---|
| Sorbent dust contamination[2] | Zeolite particles attrite from thermal cycling and bed pressure swings; dust escapes downstream of bed canister. | Particle counter spikes; downstream HEPA differential pressure rises. | Outlet HEPA filter; bed retention screens (20-µm); replacement at 5-yr inspection intervals. |
| Water co-adsorption saturation[2] | Liquid water or high humidity in cabin air saturates the 13X pre-dryer faster than zeolite 5A, allowing water through to the 5A bed → catastrophic CO₂ capacity loss. | CO₂ removal rate drops sharply; outlet humidity rises. | Tight humidity control (< 60 % RH at inlet); independent monitoring of dryer-bed cycle; emergency bed-bake-out procedure at 250 °C. |
| Heater element failure[2] | Open-circuit failure of resistance heaters disables regeneration of the off-line bed. | Regen-bed T fails to rise on schedule; outlet CO₂ partial pressure climbs. | Redundant heater elements per bed (n+1); ground spares for elements; degraded-mode operation on remaining 3 beds. |
| Blower bearing degradation[2] | Continuous-duty bearings wear under thousands of hours of operation; vibration increases until failure. | Accelerometer trend; flow rate at constant ΔP drops. | Redundant blower; magnetically suspended bearings (ISS upgrade 2008); programmed replacement every 5 yr. |
| Valve seat erosion[2] | High-temperature swing operation cycles valve seats thousands of times per year; elastomers harden, metallic seats wear. | Leak-by during desorb (CO₂ contaminates adsorb bed inlet); cycle-end pressure trace anomalies. | High-T elastomers (Kalrez); periodic seat inspection; n+1 valve redundancy at critical points. |
| Mars dust ingress to inlet[5] | Cabin air contains fine Mars regolith carried in via airlock cycles; sub-µm particles bypass HEPA filters and load the sorbent bed. | CO₂ capacity declines unexpectedly; spent-bed analysis shows iron oxide / silicate dust. | Two-stage airlock dust mitigation; HEPA + electrostatic precipitator at habitat air intake; sorbent replacement interval halved relative to ISS experience. |
Mars adjustments
Mars dust ingress to cabin air[5]
Impact: EVA cycles drag perchlorate-rich fine dust into the habitat. Submicron fraction bypasses standard HEPA and loads sorbent beds; perchlorate degrades amine-based sorbents in particular.
Mitigation: Two-stage airlock with electrostatic precipitation; HEPA + ULPA on habitat air-intake; suit-port architecture (MaRSP) eliminates cabin dust ingress entirely.
CO₂ feedstock for Sabatier[2]
Impact: On ISS, captured CO₂ is vented to space. On Mars, that CO₂ is the input to propellant production and water recovery via Sabatier. Venting destroys mission economics.
Mitigation: Pressurized intermediate storage between scrubber and Sabatier — decouples cycles. CO₂ compressor (1 → 30 bar) sized to match Sabatier feed rate.
Mission duration sets sorbent replenishment cycle[4]
Impact: ISS missions max out at 6 months; Mars crews stay 18–30 months between resupply. Sorbent loss to attrition + degradation scales linearly with time.
Mitigation: Conservative sizing: 50 % capacity margin over BVAD nominal; on-site sorbent regeneration via Sabatier waste heat or bake-out; spare bed inventory.
Pressure-swing options unavailable[2]
Impact: On Earth, pressure-swing adsorption (PSA) is common — desorb at low pressure rather than high T. On Mars, low-pressure side is the Mars atmosphere (600 Pa) which contains 95 % CO₂ — desorbing INTO it does nothing useful.
Mitigation: Thermal-swing only (TSA) for the CO₂ side. PSA can still be used for the H₂O pre-dryer because dewpoint targets are easy to hit.
Power-budget interaction with nuclear baseload[4]
Impact: ~ 5 kWh/day for 4-crew CO₂ scrubbing is 5 % of a 10 kWe nuclear baseload's 240 kWh/day output. Tight relative to overall ECLSS demand (~ 25 % of habitat power).
Mitigation: Schedule regen cycles during off-peak; couple Sabatier heat recovery to bed pre-warming, halving heater energy.
Alternatives & substitutes
Sabatier-integrated closed loop[2]
- Captured CO₂ becomes propellant + water feedstock (closes the carbon loop)
- Eliminates the venting question entirely
- Reduces consumable resupply
- Requires H₂ feed → linked to water-electrolysis availability
- Sabatier exotherm and water output need thermal + plumbing integration
- Loop closure is partial — Sabatier recovers H₂O but the carbon is sequestered as CH₄ (used as propellant)
When preferred: Always, on Mars. Venting CO₂ that the Sabatier wants is architectural malpractice.
Bosch reaction (carbon formation)[1]
- CO₂ + 2 H₂ → C(s) + 2 H₂O — fully recovers water, produces no methane
- Solid carbon is storable indefinitely as a recoverable resource
- Higher reaction temperatures (500–700 °C) than Sabatier
- Carbon clogs the reactor — operationally complex
- Lower TRL than Sabatier for sustained space operation
Photosynthetic CO₂ uptake (bioregenerative)[1]
- Couples life support to food production — algae or higher plants
- Produces O₂ as the co-product
- No regen energy beyond grow-light electricity
- Crop-area requirement is large (≥ 50 m²/crew for full CO₂ uptake)
- Slow response — diurnal cycle and biological lag
- Single-string biological reliability is poor vs CDRA-class hardware
When preferred: As an augmentation to chemical scrubbing once colony-scale agriculture is established; never as primary in early base.
Requires
References
- (1998). Living Together in Space: The Design and Operation of the Life Support Systems on the International Space Station. NASA Marshall Space Flight Center, NASA/TM-98-206956. NASA/TM-98-206956. — NASA Baseline Values & Assumptions (BVAD); LiOH, amine, and zeolite scrubber trade study.
- (2005). International Space Station Carbon Dioxide Removal Assembly Testing. 35th International Conference on Environmental Systems, SAE 2005-01-2864. doi:10.4271/2005-01-2864 — CDRA architecture, zeolite 13X/5A duty cycle, ISS performance history.
- (2023). NASA Space Flight Human-System Standard, Volume 2: Human Factors, Habitability, and Environmental Health. NASA. NASA-STD-3001 Vol. 2 Rev. C. — Cabin CO₂ partial-pressure limits; crew habitat environmental health standard.
- (2013). Carbon Dioxide — Our Common "Enemy". Submitted to International Conference on Environmental Systems, NASA Johnson Space Center. JSC-CN-29581. — CO₂ exposure effects on crew cognition; basis for ISS partial-pressure limit reduction.
- (2013). Perchlorate on Mars: a chemical hazard and a resource for humans. International Journal of Astrobiology, 12(4), 321-325. doi:10.1017/S1473550413000164 — Biological reduction of perchlorate as pre-treatment for ISRU water.