Water Dewpoint Dehydration Screening

Use this skill for public, educational gas water-content screening. It estimates the saturated water content of natural gas using the open GPSA Bukacek correlation and compares it to a sales-gas water specification so an agent can flag whether dehydration is likely required before detailed design.

When to Use

• When a user asks whether a gas stream needs dehydration to meet a water spec.
• When an agent needs a quick saturated-water-content estimate to scope a dehydration study.
• When examples must run without confidential dehydration designs, glycol data, or company gas specs.
• Before asserting any hydrate, ice, or free-water finding — run the saturation test below to establish whether free water is actually available. A hydrate finding built on an unverified water number is the single easiest way to produce a confident wrong answer.

Is the stream already dehydrated? — the saturation test

A stated water content in a datasheet or stream table tells you nothing on its own; you have to compare it against saturation at the same pressure and temperature.

ratio = stated_water_content / saturated_water_content(P, T)

ratio ~ 1.0   -> stream is AT SATURATION: wet gas, free water available.
                 Water was removed (if at all) by COOLING + KNOCK-OUT, not dehydration.
ratio << 1    -> stream is DEHYDRATED (glycol contactor or molecular sieve upstream).
                 Typical TEG: 30-50 ppm(mol); typical sales-gas spec: ~100 ppm(mol).
ratio > 1.0   -> the stated composition carries entrained/free water, or the stated
                 P/T is not the condition the composition belongs to. Resolve before use.

Dewatering is not dehydration

This is the trap the test exists to catch. A suction scrubber, knock-out drum, or cooler + separator removes free water and leaves the gas exactly at saturation at the separator outlet condition. It lowers the absolute water content but gives zero dryness margin — the stream re-condenses water on any subsequent cooling or expansion. Only a glycol contactor or a molecular sieve produces an actual water dew-point depression.

Seeing a scrubber on the P&ID and concluding "the gas is dry" is wrong. Seeing a contactor with a regeneration package is the evidence that matters.

Where to get the authoritative water number

Prefer the PFD heat & material balance over an equipment datasheet: the H&M stream table shows the whole removal path stream by stream (inlet -> cooled -> separator liquid -> separator gas), so the mechanism is visible, not inferred. A separator liquid stream at tens of mol-% water is direct proof of knock-out.

Two cautions when reading these:

• A datasheet water content that lands exactly on saturation is a design assumption marker, not a measurement — the process engineer set "water = saturation at inlet". Treat it as the conservative wet basis it is, and say so.
• The PFD case and the datasheet case are often different duties (different year, feed or rate), so their pressures, temperatures and water contents legitimately differ. Carry both; do not silently blend them.

Computing saturation rigorously

Bukacek (below) is fine for screening. For a decisive answer, or below 0 °C, or with significant CO2/H2S, use NeqSim CPA: add the dry components plus an excess of water so a free water phase is guaranteed, flash at the stated P and T, and read the water mole fraction back out of the gas phase.

from neqsim import jneqsim

SystemSrkCPAstatoil = jneqsim.thermo.system.SystemSrkCPAstatoil
ThermodynamicOperations = jneqsim.thermodynamicoperations.ThermodynamicOperations

def saturated_water_mole_fraction(dry_components, pressure_bara, temperature_c):
    """Water mole fraction in the gas when a free water phase is present."""
    s = SystemSrkCPAstatoil(273.15 + temperature_c, pressure_bara)
    total = sum(dry_components.values())
    for name, x in dry_components.items():
        s.addComponent(name, x / total)
    s.addComponent("water", 0.05)          # excess -> guarantees a free water phase
    s.setMixingRule(10)                     # CPA
    s.setMultiPhaseCheck(True)
    ThermodynamicOperations(s).TPflash()
    s.initProperties()
    return float(s.getPhase("gas").getComponent("water").getx())

Cross-check with waterDewPointTemperatureMultiphaseFlash(): if the computed water dew point of the stated composition equals the stated stream temperature, the stream is saturated.

Inputs

• pressure: gas pressure in bar absolute.
• temperature: gas temperature in kelvin.
• water_spec: sales-gas water-content specification in lb/MMscf, default 7.0 (common pipeline spec).

Outputs

• saturated_water_content_lb_mmscf: saturated water content from the Bukacek correlation.
• water_spec_lb_mmscf: the supplied water specification.
• spec_ratio: ratio of saturated water content to the spec.
• dehydration_required: whether the saturated content exceeds the spec.
• dehydration_warning: ok, watch, or dehydration-required.
• assumptions: public assumptions used by the placeholder model.

Engineering Method

The Python class WaterDewpointModel uses the open GPSA Bukacek correlation only:

• the saturated water content uses W = A / P + B, where A and B are public temperature-dependent terms and P is in psia.
• the spec ratio compares the saturated content to the supplied water spec.
• the warning is a simple rule-based label aligned with sales-gas dehydration intent.

This is educational and screening-only logic. The Bukacek correlation is for sweet natural gas in equilibrium with liquid water and is most accurate above 32 F. It does not include acid-gas corrections, salinity, hydrate suppression, or glycol contactor performance. It is not a replacement for validated water-content and dehydration design (for example GPSA and rigorous thermodynamics) and a qualified process review.

Python Usage Pattern

from water_dewpoint_dehydration_screening import WaterDewpointModel

model = WaterDewpointModel()
result = model.evaluate(
    pressure=70.0,
    temperature=305.15,
)

print(result.dehydration_warning)
print(result.saturated_water_content_lb_mmscf)
print(result.dehydration_required)

Related NeqSim Functionality

For validated water-content and dehydration calculations, redirect to existing NeqSim functionality:

• neqsim.thermo.system.SystemSrkCPAstatoil (CPA equation of state) — rigorous water content and water dew point for natural gas.
• neqsim.process.design.template.DehydrationTemplate — TEG dehydration process template.
• neqsim.process.equipment.absorber glycol contactor models — detailed dehydration performance.

This skill is a public Bukacek triage layer that decides when to invoke validated NeqSim water-content and dehydration models.

Validation Checklist

• Pressure and temperature are positive and in the stated units.
• Example inputs are public and synthetic.
• Tests cover an in-spec case, a dehydration-required case, and invalid input.
• Results are described as educational screening indicators.
• Real design is redirected to validated methods, GPSA, NeqSim CPA, and qualified review.

Common Mistakes

Limitations

• No proprietary dehydration designs, glycol data, or company gas specs are included.
• No acid-gas, salinity, or hydrate-suppression corrections are performed.
• No glycol contactor or regeneration performance is modeled.

References

• NeqSim repository: https://github.com/equinor/neqsim
• NeqSim Skills Guide: https://github.com/equinor/neqsim/blob/master/docs/integration/skills_guide.md