How to simulate 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.
- 1Open the ready-made model
Open the "Gas dehydration: why the property package decides the answer" model in the MaximaLabs workspace — no install, no license. It loads live on the canvas, ready to edit and run.
- 2Confirm the thermodynamics
This process is modeled with the CPA property package over methane, ethane, propane, water — already selected, so the phase equilibrium and enthalpy are physically consistent from the first run.
- 3Review the flowsheet
The flowsheet chains Chiller, KO DRUM, Sieve. Every block is a real, solvable unit op you can reconfigure on the canvas.
- 4Run the simulation
Click Run. The deterministic solver converges the material and energy balances (recycles included) and fills the live stream table — the AI never invents a number.
- 5Read the results and iterate
Inspect the converged streams, tweak a spec, and re-run — or ask the AI copilot to explain a result or diagnose a failed solve in plain English.
- Thermodynamics
- CPA
- Components
- methane, ethane, propane, water
- Unit operations
- ChillerKO DRUMSieve
Opens live on the canvas — free, no install.
Explore the model & flowsheetFrequently asked questions
- What does the Gas dehydration: why the property package decides the answer model simulate?
- 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.
- Which thermodynamic method does it use?
- The CPA property package, over methane, ethane, propane, water — already selected. You can switch the method on the canvas before running.
- Which unit operations are in the flowsheet?
- It chains Chiller, KO DRUM, Sieve. Every block is a real, solvable unit operation you can reconfigure, add to, or remove.
- Do I need to install software or buy a license?
- No. Gas dehydration: why the property package decides the answer runs entirely in your browser on MaximaLabs — free, no install, no license. Open the model to load it live and run the deterministic solver.
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