4-Bed Molecular Sieve CO2 Scrubber (2D)

Crew & Cabin

Crew size 4

4-Bed Molecular Sieve

Bed cycle time 8 min

Desorption heater temp 204°C

Bed Status

Cabin Readout

ppCO2 (4-bed)

3.8 mmHg

CO2 fraction

0.50%

Removal rate

— g/hr

Single-bed ppCO2

off

How it works

A real spacecraft 4-bed molecular sieve (4BMS) CO2 scrubber staggers four zeolite beds through the same adsorb/regenerate cycle, each offset by a quarter period, so two beds are always adsorbing cabin CO2 while the other two are heated and vented to vacuum to desorb what they captured last cycle. Adsorption follows Langmuir kinetics — uptake slows as a bed fills — and desorption is a temperature-driven exponential decay:

dθ/dt =  k_ads · p(CO2) · (1 − θ)        (bed in ADSORB phase, loading θ ∈ [0,1])
dθ/dt = −k_des(T) · θ                     (bed in REGEN phase, vented to space)
k_des(T) = k_des,ref · (T_heater / T_ref)^1.6

The cabin itself is a mass balance between crew metabolic production and the summed removal of every bed currently adsorbing:

d(ppCO2)/dt = [ N_crew · m_CO2 − Σ_active k_ads · p(CO2) · (1 − θ_i) · Q_bed ] / V_cabin

An adult astronaut produces roughly 1.0 kg of CO2 per day at rest. NASA's spacecraft maximum allowable concentration is about 5.3 mmHg (~0.7% at 1 atm); the nominal ISS target sits near 3–4 mmHg. Because two beds are always mid-adsorption at any instant, the staggered 4-bed schedule keeps scrubbing capacity continuously available — unlike a single bed with no regeneration, which this sim can overlay for comparison: its loading climbs toward saturation (θ→1) and its removal rate decays toward zero, so cabin ppCO2 climbs unchecked.

Crew size — scales metabolic CO2 production, the only source term in the mass balance.
Bed cycle time — how long each bed spends in ADSORB (then REGEN) before the staggered sequencer swaps it; too long and even a cycling bed nears saturation before its turn to regenerate ends.
Desorption heater temp — higher temperature drives k_des up, so a regenerating bed empties faster and is fully ready for its next adsorption phase.
Compare toggle — overlays a single, never-regenerated bed on the same cabin so you can see the difference cycling makes.

Adsorbing bed Regenerating bed 4-bed cabin ppCO2 Single-bed ppCO2

Cabin ppCO2
3.8 mmHg