Dehydration Process Flowsheets & Simulations
Explore 2 validated, solved Dehydration simulation flowsheets in MaximaLabs — real components: n2, methane, ethane, propane, water. Open any one directly in your browser.
Solved via: PENG-ROBINSON, CPA.
LNG front end and storage: guard beds, tank boil-off, driver limit
The parts of a 5.2 Mtpa LNG train that are not the cold box, and the four questions they answer that no bulk unit can — at real capacity, so the vessel sizes and duties are ones an engineer can check against their own plant. Guard beds: a sulfur-carbon bed takes mercury from 200 to 0.01 µg/Nm³ — the brazed-aluminium limit, because mercury attacks the plate-fin cold box by liquid-metal embrittlement — on a 4.8 m x 6.6 m bed with 4.2 years of life from a capacity balance — inside the 3-5 year window these non-regenerable beds are actually replaced on, which is what a turnaround plan needs. A 4A molecular sieve then dries the gas to 0.1 ppmv, the only route to a cryogenic water spec (a glycol contactor tops out near a −30 °C dew point), on a 4.9 m x 5.1 m bed running a 13.8 h cycle for 1.3 MW of regeneration duty. Storage: the LNG goes to a tank whose boil-off rate comes from a 0.05%/day guarantee but whose boil-off composition comes from a real equilibrium flash — and that is the interesting part, because the vapour leaves at ~11 mol% nitrogen against 1.1% in the feed. Nitrogen and methane are far more volatile at 113 K, so the boil-off is light and the stored liquid weathers heavier: nobody specified that, the flash found it. Recondensing: the boil-off goes back into the send-out LNG rather than to a flare, and the unit reports the LNG:BOG ratio (1999) against the minimum the energy balance demands (3.8), which is the constraint a terminal is actually operated against. The driver: a gas turbine burning plant fuel gas, rated 97.5 MW at ISO conditions, delivers only 84.5 MW at 35 °C — 5.5 MW short of its 90 MW refrigeration load, and flagged as such. That derate is computed from two physical effects (a fixed-geometry compressor swallows a fixed volume so mass flow follows air density, and hotter air costs more to compress), not from a vendor curve, and it comes out at 0.66%/K — mid-band for industrial machines. It is why a tropical LNG train makes less product in summer.
17 unit ops • PENG-ROBINSON
51 0
View & openGas 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.
6 unit ops • CPA
10 0
View & open