Gas dehydration: why the property package decides the answer

A wet pipeline gas at 70 bar and 40 C is chilled to 15 C, the condensed free water is knocked out, and a 4A molecular-sieve bed takes the rest down to the cryogenic 0.1 ppmv spec. It is the standard front end of every gas plant, and it is one number wide: how much water the gas holds at saturation. Everything downstream is arithmetic on that number — the knockout drum's liquid rate, the sieve's cycle time, the bed size, the regeneration duty. Here the feed carries 1500 ppmv (71 lb/MMscf). Chilling to 288 K drops the saturation limit to 313 ppmv (14.9 lb/MMscf), so 3.57 mol/s of essentially pure water (99.9 mol%) falls out in the KO drum for a 3.91 MW chilling duty, and the sieve carries the remaining 0.94 mol/s. A 4000 kg bed then sizes out at 2.69 m diameter by 0.96 m deep on a 7.8-hour cycle — the 8-hour cycle real molecular-sieve dehydrators are built around — with 108 kW of regeneration duty and 11 kPa of bed pressure drop. This example runs on CPA (Cubic-Plus-Association) and not on a cubic, deliberately. Water in a hydrocarbon gas is the case a bare cubic equation of state is worst at: its hydrogen bonding is not a small correction to be absorbed into a binary interaction parameter, it is the thing that sets the answer. CPA adds a Wertheim association term on top of SRK and switches it on only for the hydrogen-bonding species, so hydrocarbons still behave exactly as SRK. Bounded, and the bound is measured. Against the McKetta-Wehe chart's ~60 lb/MMscf for methane at 100 F and 1000 psia, CPA lands at ~55, PR at ~50 and SRK at ~45 — CPA is the closest, and all three still underpredict. None of them has a cited methane-water binary interaction parameter, and extrapolating CPA's published n-alkane correlation (kij = 0.1915 - 0.026*n_carbon, fitted C3..C10) down to methane would fabricate the number that decides the answer, so it stays absent. Good for a first-cut dehydration duty; not a guarantee-grade contract number.