This is the 2D finite-difference counterpart of the 3D scrubber model. The sorbent bed is discretized into 48 slices along the flow axis. Each slice tracks a gas-phase CO2 level Ci and a solid loading qi, coupled by a Langmuir equilibrium and a linear-driving-force (LDF) rate law — the standard textbook fixed-bed adsorption model, integrated explicitly every frame with CFL-limited sub-steps:
q_eq(C) = q_max · b·C / (1 + b·C) (Langmuir isotherm)
dq_i/dt = k_LDF · (q_eq(C_i) − q_i) (adsorption slows as q_i → q_max)
dC_i/dt = -v/dx · (C_i − C_{i−1}) − β·dq_i/dt (∂C/∂t = -v·∂C/∂x - uptake)
- Inlet CO2 concentration sets C0 at the bed's leading edge, normalized so 1.0× ≈ the ~7.6 mmHg ppCO2 design limit NASA uses for nominal EVA workload.
- Purge gas flow rate sets the interstitial velocity v — faster flow pushes the mass-transfer zone through the bed quicker.
- Bed length sets the physical slice width dx = L/48 — a longer bed holds more total sorbent capacity between inlet and outlet, so v/dx (the convective term) shrinks and the front takes longer to cross it.
- Sorbent condition scales qmax: an aged/degraded bed holds noticeably less CO2 before breakthrough.
- Start Regeneration switches the inlet to vacuum (C0 → 0) with a faster desorption rate constant, draining qi back toward zero.
The heatmap strip encodes local loading qi/qmax (blue = fresh, orange = saturated) — the moving blue→orange gradient is the mass-transfer zone. The curve below plots outlet concentration CN(t): flat near zero while the zone is still inside the bed, then rising sharply once it reaches the outlet — the classic S-shaped breakthrough curve. Shortening the bed or raising the flow rate measurably moves that rise earlier.