Sulfuric acid: catalytic SO2 oxidation (contact process)

The heart of a sulfuric-acid plant: roaster/burner gas (SO2 in excess air) is oxidized to SO3 over a vanadium-pentoxide catalyst bed, then the SO3 is absorbed to make acid. The converter is a Gibbs-minimization reactor (Aspen RGibbs equivalent) that finds the SO2 ⇌ SO3 equilibrium from first-principles formation energies — no assumed conversion — so it captures the real design tension of the contact process: the reaction SO2 + ½O2 → SO3 is exothermic, so a lower bed temperature drives the equilibrium toward SO3 (higher conversion) while a higher temperature is needed for catalyst activity. At 700 K this converts ~99% of the SO2; raise the sensitivity temperature and watch the equilibrium conversion fall — the reason real plants use multiple catalyst beds with interstage cooling. The SO3-rich gas then goes to an absorption tower (modeled as SO3 capture into the acid product, leaving a spent tail gas of N2 + excess O2). Formation-energy source: CRC Handbook / NIST-JANAF (ΔHf°/ΔGf° for SO2 and SO3). Honesty notes: the converter is isothermal at the specified bed temperature (a real adiabatic bed heats up along its length, recovered between beds — not modeled here); the final H2O + SO3 → H2SO4 absorption is a highly non-ideal exothermic step represented here as SO3 capture (the acid-forming reaction and oleum thermodynamics are not modeled), the same capture simplification the carbon-capture examples use.