1. Overview
Natural gas produced from reservoirs is saturated with water vapor at reservoir conditions. As the gas cools and depressurizes through gathering and processing, this water can condense, forming liquid water that creates corrosion, hydrate plugging, flow restriction, and equipment damage. Gas dewatering (dehydration) removes water vapor to prevent these problems and meet pipeline quality specifications.
Hydrate Prevention
Freeze Protection
Water + gas at high pressure forms ice-like hydrate plugs that block pipelines and equipment.
Corrosion Control
Dry Gas = Low Corrosion
Liquid water with CO2 or H2S creates carbonic or sulfuric acid that corrodes steel pipe.
Pipeline Quality
Tariff Specifications
Pipeline operators require water content below 7 lb/MMscf (typically 4-7 lb/MMscf).
Cryogenic Processing
Ultra-Dry Gas
NGL recovery at -100°F or below requires water content below 0.1 ppm to prevent ice formation.
Water is always present: Natural gas in contact with liquid water (in the reservoir, in separators, or in pipelines) is saturated with water vapor. The amount of water vapor depends on temperature, pressure, and gas composition. Dehydration is required at virtually every gas processing facility.
2. Water Content of Natural Gas
McKetta-Wehe Correlation
Water Content of Sweet Natural Gas:
The McKetta-Wehe chart (GPSA Figure 20-3) gives the
water content of natural gas in equilibrium with
liquid water as a function of temperature and pressure.
Key relationships:
- Water content INCREASES with temperature
- Water content DECREASES with pressure
- H2S and CO2 increase water content
- Heavier hydrocarbons decrease water content slightly
Bukacek Correlation (Equation Form):
W = A / P + B
Where:
W = Water content (lb/MMscf)
P = Pressure (psia)
A = exp(a0 + a1/T + a2/T²) (temperature function)
B = exp(b0 + b1/T + b2/T²) (temperature function)
T = Temperature (°R = °F + 460)
This provides the same results as the McKetta-Wehe
chart but in equation form suitable for calculation.
Typical Water Content Values
| Condition | Pressure (psia) | Temp (°F) | Water Content (lb/MMscf) |
|---|---|---|---|
| Separator outlet | 800 | 100 | ~33 |
| Separator outlet | 800 | 80 | ~22 |
| Separator outlet | 1,000 | 100 | ~28 |
| Pipeline spec | 800 | varies | ≤ 7 |
| Cryogenic inlet | 900 | varies | ≤ 0.1 |
Sour Gas Correction
Effect of H2S and CO2 on Water Content:
Sour gas (containing H2S and/or CO2) holds MORE water
than sweet gas at the same T and P.
GPSA provides correction factors:
C_H2S = correction for H2S mole fraction
C_CO2 = correction for CO2 mole fraction
W_sour = W_sweet × C_H2S × C_CO2
For gas with 5% H2S: ~10-15% more water
For gas with 10% CO2: ~5-8% more water
The corrections become significant when acid gas
content exceeds 5-10 mole percent.
3. Dewpoint Specifications
| Application | Water Content | Dewpoint | Dehydration Method |
|---|---|---|---|
| Pipeline (standard) | 4-7 lb/MMscf | -15 to 0°F | TEG (standard) |
| Pipeline (cold climate) | 1-4 lb/MMscf | -30 to -15°F | TEG (enhanced) or Drierite |
| Turboexpander inlet | < 1 lb/MMscf | -40 to -60°F | Molecular sieve |
| Cryogenic (C2 recovery) | < 0.1 ppm | -100 to -150°F | Molecular sieve (4A) |
| LNG feed | < 0.1 ppm | < -150°F | Molecular sieve (3A/4A) |
Dewpoint Depression Calculation:
Dewpoint depression = Inlet dewpoint - Outlet dewpoint
Example:
Inlet gas: 800 psia, 100°F saturated
Inlet water content: ~33 lb/MMscf
Inlet dewpoint: 100°F
Pipeline spec: 7 lb/MMscf at 800 psia
Outlet dewpoint: ~32°F (from McKetta-Wehe at 800 psia)
Required dewpoint depression: 100 - 32 = 68°F
Standard TEG can achieve ~65-80°F depression
This is within TEG capability ✓
4. TEG Dehydration
Triethylene glycol (TEG) absorption is the most common dehydration method in the natural gas industry. Wet gas contacts lean (regenerated) TEG in a contactor tower. The glycol absorbs water from the gas, and the rich glycol is regenerated by heating in a reboiler to drive off the absorbed water.
TEG System Components:
1. Inlet scrubber: Removes free liquids and solids
2. Contactor tower: TEG absorbs water from gas
3. Flash tank: Removes dissolved hydrocarbons from TEG
4. Glycol-glycol heat exchanger: Recovers heat
5. Reboiler/still column: Regenerates TEG at 380-400°F
6. Surge tank: Stores lean TEG
7. Glycol pump: Circulates TEG through system
Key Design Parameters:
TEG circulation rate: 2-5 gal TEG per lb H2O removed
Contactor trays: 4-8 actual trays (typical)
Contactor temperature: 80-120°F (inlet gas temp)
Reboiler temperature: 380-400°F (max 404°F)
Lean TEG concentration: 99.0-99.95 wt%
TEG Concentration and Dewpoint
| Lean TEG (wt%) | Achievable Dewpoint Depression | Regeneration Method |
|---|---|---|
| 98.0 | ~45°F | Atmospheric reboiler |
| 99.0 | ~55°F | Atmospheric reboiler |
| 99.5 | ~65°F | Atmospheric reboiler (standard) |
| 99.9 | ~85°F | Stripping gas required |
| 99.95 | ~100°F | Enhanced stripping (Drizo, coldfinger) |
| 99.99 | ~120°F | Vacuum stripping or Drizo process |
TEG Circulation Rate
Required TEG Circulation:
Q_TEG = L × W_removed × Q_gas / (24 × 60)
Where:
Q_TEG = TEG circulation rate (gpm)
L = Lean-to-water ratio (gal TEG/lb H2O)
W_removed = Water removed (lb H2O/MMscf)
Q_gas = Gas flow rate (MMscf/d)
Typical L values:
Standard design: 3.0 gal/lb (most common)
Minimum: 2.0 gal/lb (high efficiency)
Maximum: 5.0 gal/lb (high dewpoint depression)
Higher circulation rates improve water removal
but increase reboiler duty, pump power, and
glycol losses. Diminishing returns above 4-5 gal/lb.
5. Molecular Sieve Dehydration
Molecular sieve (mol sieve) is an adsorption-based dehydration method that achieves much lower dewpoints than TEG. It uses synthetic zeolite crystals with uniform pore sizes that selectively adsorb water molecules from the gas stream.
Molecular Sieve Types:
3A (3 Angstrom pore): Adsorbs water only
Best for dehydration without co-adsorbing methanol
Used when methanol injection is upstream
4A (4 Angstrom pore): Adsorbs water + H2S + CO2
Most common for gas dehydration
Also removes mercaptans and COS
5A (5 Angstrom pore): Adsorbs water + n-paraffins
Used for combined dehydration and separation
Selective for normal vs branched paraffins
13X (10 Angstrom pore): Adsorbs water + all above + mercaptans
Broadest adsorption spectrum
Used for combined treating
Regeneration Cycle
| Phase | Duration | Conditions |
|---|---|---|
| Adsorption | 8-16 hours | Wet gas flows through bed at process conditions |
| Heating | 4-8 hours | Hot gas (450-600°F) drives off water |
| Cooling | 2-4 hours | Cool dry gas returns bed to adsorption temperature |
| Standby | 0-2 hours | Ready for next adsorption cycle |
Molecular Sieve Bed Sizing:
W_sieve = W_water × Q_gas × t_cycle / (C_dynamic)
Where:
W_sieve = Required sieve weight (lb)
W_water = Water loading (lb H2O/MMscf)
Q_gas = Gas flow rate (MMscf/hr)
t_cycle = Adsorption cycle time (hours)
C_dynamic = Dynamic capacity (lb H2O / 100 lb sieve)
Typical dynamic capacity: 8-13 lb/100 lb
(depends on inlet temperature, regeneration quality,
and sieve age/degradation)
Most mol sieve systems use 2 or 3 beds:
2-bed: One adsorbing, one regenerating
3-bed: One adsorbing, one heating, one cooling
6. Other Dehydration Methods
Method Comparison
| Method | Dewpoint | Best Application | Relative Cost |
|---|---|---|---|
| TEG absorption | -25 to -40°F | Standard pipeline dehydration | 1x (baseline) |
| Molecular sieve | -100 to -150°F | Cryogenic processing, LNG | 2-4x |
| Deliquescent desiccant | 15-25°F depression | Small wellsite, instrument gas | 0.3x (capital) + media cost |
| Refrigeration | -20 to -40°F | Simultaneous NGL recovery | 2-3x |
| Membrane | -40 to -60°F | Offshore, remote, small flow | 1.5-3x |
| Silica gel | -40 to -60°F | Moderate dewpoint, low pressure | 1-2x |
Methanol Injection (Hydrate Inhibition)
Methanol as Hydrate Prevention (Not Dehydration):
Methanol injection does not remove water from the gas.
It prevents hydrate formation by depressing the
hydrate formation temperature.
Hammerschmidt equation:
ΔT = K_H × W / (M × (100 - W))
Where:
ΔT = Hydrate temperature depression (°F)
K_H = Constant (2,335 for methanol)
W = Weight % of inhibitor in aqueous phase
M = Molecular weight of inhibitor (32 for methanol)
Methanol injection is used for:
- Short-term hydrate prevention (startup, shutdown)
- Gathering systems without dehydration
- Emergency hydrate remediation
- Not a substitute for dehydration at gas plants
7. Practical Considerations
TEG System Troubleshooting
| Problem | Symptom | Likely Cause |
|---|---|---|
| High outlet dewpoint | Water content exceeds spec | Low TEG concentration, high circulation rate, foaming |
| TEG losses | Makeup rate > 0.1 gal/MMscf | Foaming, carryover, vaporization, mechanical leaks |
| Foaming | Level fluctuation, high ΔP | Liquid hydrocarbons, corrosion inhibitors, fine solids |
| Glycol degradation | Dark color, odor, deposits | Overheating (>404°F), oxygen ingress, contaminants |
| High reboiler fuel | Excess fuel gas consumption | High water loading, glycol dilution, heat loss |
BTEX Emissions from TEG Reboilers
BTEX Emissions Concern:
TEG absorbs aromatic hydrocarbons (BTEX) from the gas.
These are released from the reboiler still column vent.
BTEX = Benzene + Toluene + Ethylbenzene + Xylenes
Emission control options:
- Condenser on still column (recovers hydrocarbons)
- Vapor recovery unit (VRU)
- Thermal oxidizer or combustor on vent
- Flash tank gas recovery (reduces BTEX loading)
Emission reporting:
- Annual emissions inventory (state air permit)
- GlyCalc software (EPA/GRI model for TEG emissions)
- Process simulation (HYSYS, ProMax)
Selection Guidelines
- TEG is the default choice for pipeline-quality gas with dewpoint depression up to 65-80°F
- Enhanced TEG (stripping gas, Drizo) for dewpoint depressions of 80-120°F
- Molecular sieve when dewpoint below -60°F is required (cryogenic processing)
- Deliquescent desiccant for small flows (< 5 MMscf/d) and moderate dewpoint needs
- Refrigeration when simultaneous NGL recovery is desired
- Membrane systems for offshore or remote unmanned locations
Economics drive the choice: TEG systems have the lowest life-cycle cost for standard pipeline dehydration (70-80% of all installations). Molecular sieve costs 2-4 times more but is required for cryogenic processing. Always evaluate whether enhanced TEG can meet the specification before specifying molecular sieve, as the operational complexity and energy requirements are significantly higher.
Ready to design a dehydration system?
→ Launch Gas Dewatering Calculator