Each sorbent has its own uptake chemistry. Lithium hydroxide reacts irreversibly: 2LiOH + CO₂ → Li₂CO₃ + H₂O, consuming the solid itself, so a canister is single-use but reaches a high theoretical capacity fast. Solid amine sorbent (used in the ISS CDRA) chemisorbs CO₂ onto amine sites at cabin temperature and is regenerated by heating/vacuum desorption, trading some capacity for indefinite reuse. 5A zeolite molecular sieve physisorbs CO₂ into its pore structure — fast at low temperature but its capacity drops sharply as bed temperature rises, and it too is regenerated by a pressure/temperature swing.
q(t) = q_max·(1 − e^(−k·t))
k ∝ [CO₂] (higher cabin CO₂ → faster uptake)
q_max, k = f(chemistry, temperature)
- Chemistry — switches the sorbent's saturation capacity, rate constant and regenerability, matching published spacecraft ECLSS behavior for each material class.
- CO₂ concentration — the partial pressure driving uptake; a leakier or more crowded cabin speeds saturation.
- Bed temperature — LiOH's reaction rate rises modestly with heat, amine tolerates a wide range, zeolite loses capacity fast above ~35 °C because physisorption is exothermic and disfavored at high temperature.
- Regenerable — LiOH is consumed and discarded after one saturation cycle; amine and zeolite beds are desorbed and reused for the mission's duration.
Real-world relevance: early Mercury/Apollo/Gemini capsules and submarines used disposable LiOH canisters; the ISS's four-bed CDRA cycles solid amine, and a molecular-sieve bed dries the air first so the amine and zeolite stages aren't fouled by humidity.