Video narration
How does water gas become dry hydrogen? A lab gas-washing and drying setup shows the sequence, not the chemistry. Coke, C, meets superheated steam in a converter at about 1,000 degrees Celsius: C plus H2O yields CO plus H2. Next, excess steam drives the shift: CO plus H2O yields CO2 plus H2, with Fe2O3 catalyst and Cr2O3 promoter, at 450 to 500 degrees Celsius and atmospheric pressure. The figure says hydrogen volume after this is twice the amount initially present in water gas. Now the job changes: remove CO2 and water. At 25 to 30 atmospheres, water scrubbing leaves hydrogen-rich gas with trace CO2; then a KOH scrubber removes CO2: CO2 plus 2KOH yields K2CO3 plus H2O. After KOH, water-vapour traces remain. Cooling coils condense water; a tower of fused CaCl2 dries the gas to dry hydrogen, collected over mercury. So: make water gas, shift CO, remove CO2, then dry hydrogen.