Some heuristics for recycle system from Seider et al.:
• Introduce purge streams to provide exits for species that enter as impurities in feed or are formed in irreversible side reactions, when these are in trace amounts and/or are difficult to separate.
• Lighter species leave via vapour purge, heavier species via liquid purge.
• Do not purge valuable, toxic or hazardous species (even in small quantities for latter two). Add separators for valuable species and reactors, if possible, to eliminate toxic and hazardous species.
• By‐products that are produced in reversible reactions, in small quantities, are usually recycled to extinction rather than purged or separated.
// Example: Ammonia production from N2 and H2
The process involves the following reaction at high pressures (200‐400 atm)
N2 + 3H2 ↔ 2NH3
Trace amount of argon accompany nitrogen feed, which is recovered from air, and trance amounts of methane accompany hydrogen, which is produced by steam reforming (CH4 + H2O ↔ 3H2 + CO).
