Hydrotreater HP separator (Chao-Seader)

The high-pressure separator on a hydrotreater: reactor effluent — mostly hydrogen, with light ends and liquid product — is flashed hot and at pressure to recover recycle gas overhead, then let down to release the dissolved gas from the product. This runs on Chao-Seader, the semi-empirical K-value method refiners actually use for hydrogen-rich hydrocarbon systems, and which the app offers but no example used. It is not a relabelled cubic: K = gamma*nu/phi builds the liquid fugacity from the Curl-Pitzer corresponding-states correlation and the activity from regular-solution theory, with SRK supplying only the vapour fugacity. On this feed it lands at 89.2% vaporised and 61.7% recycle-gas hydrogen, against Peng-Robinson's 91.7% / 59.3% and SRK's 90.4% / 60.2% — a real, if modest, difference on the number a recycle-compressor is sized from. Honest about what is approximate: the package ships as Chao-Seader, and the Grayson-Streed 1963 special coefficients for hydrogen are a documented follow-up rather than transcribed, so the HYDROGEN K itself comes from the general correlation and is approximate — it saturates the solver's K bound here. Read the hydrocarbon splits and the phase fractions, which is what the method is good at; do not read the H2 K-value as rigorous. Chao-Seader is a K-value method only, so enthalpy and density come from the inherited SRK — the pairing Aspen also uses.