Materials chemistry

Gas adsorption, separation, storage

Connect molecular adsorption equilibria and transport to practical gas separations and storage, with emphasis on selectivity, working capacity, regeneration, and process constraints.

1 Lower secondary 2 Upper secondary 3 Undergraduate 4 Advanced 5 Research

IntuitionCapture, distinguish, release

A porous adsorbent can preferentially hold one gas while a mixture flows through. But strong binding is not always useful: a practical material must capture the target, release it with manageable energy, and repeat the cycle quickly without losing capacity.

Follow a feed mixture through an adsorbent bed: differential uptake delays breakthrough, and bed regeneration restores capacity. Real beds add heat and mass-transfer fronts.

SchoolEquilibrium and separation

Definition: Adsorption selectivity

For components A and B in a mixture, the ideal adsorbed solution theory (IAST) selectivity is commonly written Sₐ/ᵦ=(xₐ/xᵦ)/(yₐ/yᵦ), where x and y are adsorbed- and gas-phase mole fractions at equilibrium. It is model-based and is not automatically a process separation factor.

Equilibrium is only one part of performance. A fixed-bed process also depends on diffusion, heat release, pressure drop, feed composition, cycle time, and adsorbent shaping. Breakthrough curves show when an outlet begins to contain the captured species.

Two common process cycles
Cycle	Driving change
PSA	Pressure swing; adsorption at higher pressure, desorption at lower pressure
TSA	Temperature swing; heat to desorb, cool to reload
Hybrid	Combines pressure, temperature, purge, or vacuum changes

Example: Interpreting an ideal selectivity

Solution

S=(2)/(1/4)=8: at this composition and equilibrium state, the adsorbed phase is enriched in A relative to B. It does not alone establish purity, productivity, cyclic working capacity, or energy efficiency in a real column.

UndergraduateIsotherms and working capacity

q=\frac{q_{\mathrm{sat}}bP}{1+bP},\\qquad \Delta q_{\mathrm{work}}=q(P_{\mathrm{ads}},T_{\mathrm{ads}})-q(P_{\mathrm{des}},T_{\mathrm{des}})

The Langmuir form is a useful idealization for equivalent independent sites and monolayer saturation; heterogeneous surfaces and framework transitions can require more complex models. Working capacity is the uptake difference across adsorption and regeneration conditions, not the maximum single-point loading.

AdvancedFrom molecular affinity to process value

A high Henry coefficient can favour dilute capture, yet overly strong adsorption penalizes regeneration. Mixture non-ideality, competitive water, heat effects, diffusion, shaping binders, and mechanical degradation can reverse rankings from ideal calculations. Evaluate full cyclic performance and process energy against a relevant baseline.

ResearchResearch frontier

References

The Adsorption of Gases on Plane Surfaces of Glass, Mica and Platinum · I. Langmuir, 1918
Thermodynamics of Mixed-Gas Adsorption · A. L. Myers, J. M. Prausnitz, 1965
Post-Combustion CO2 Capture Using Solid Sorbents: A Review · A. Samanta, A. Zhao, G. K. H. Shimizu, P. Sarkar, R. Gupta, 2012