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Gas and bed
The gas flows left to right through 0.48 m of pellets. CO2 sticks to the pellets following a Langmuir isotherm whose affinity falls as the bed gets hotter and whose capacity falls with humidity. The speed slider sets how many simulated seconds pass per real second.
Mode
Model checks
The science
What is modelled
A fixed bed of porous pellets treated as a one-dimensional column: gas advects at the interstitial velocity, disperses axially, and exchanges CO2 with the pellets at a rate proportional to the distance from equilibrium (linear driving force). The pressure drop follows the Ergun equation.
Equations
- Gas:
e dC/dt = -u dC/dx + e Dax d2C/dx2 - rhop (1 - e) dq/dt. - Uptake:
dq/dt = k (qeq(C, T) - q). - Isotherm:
qeq = qmax b C / (1 + b C)withb = b0 exp[(ΔH/R)(1/T - 1/298)]. - Ergun:
ΔP/L = 150 mu (1-e)2 u / (e3 d2) + 1.75 rho (1-e) u2 / (e3 d).
Checks run in the page
- CO2 mass balance over a run.
- Langmuir isotherm against the closed formula.
- Ergun pressure drop against an independent evaluation.
- 50% breakthrough time against capacity / feed rate.
- Capacity falls with temperature as the van t Hoff factor predicts.
Honest simplifications
Isothermal bed (no heat of adsorption warm-up), a single CO2 component with no competing nitrogen or water adsorption, a linear humidity penalty, and a lumped uptake rate. Educational, not a plant design tool.
What to try
- Raise the temperature to 65 C: the capacity drops and breakthrough comes much sooner.
- Double the flow: the stoichiometric time halves and the front is steeper.
- After saturation tick Regeneration and watch the outlet CO2 spike as the bed releases.