Gas Dehydration

TEG Dehydration & Dewpoint Control

Design triethylene glycol dehydration systems using GPSA equilibrium dewpoint curves and McKetta-Wehe water content charts to meet pipeline specifications.

Launch Calculator GPSA Equilibrium Data

Pipeline Spec

≤7 lb H₂O/MMscf

Typical US transmission; 4 lb/MMscf for cold climate.

TEG Concentration

98.5-99.5 wt%

Achievable dewpoint depends on lean TEG purity.

Circulation Rate

3-5 gal/lb H₂O

Standard design per GPSA; higher for low dewpoint.

Contents

Use this guide when you need to:

  • Determine achievable water dewpoint
  • Select TEG concentration for target spec
  • Choose regeneration method

1. Process Overview

TEG (triethylene glycol) dehydration removes water vapor from natural gas to prevent hydrate formation and meet pipeline water content specifications. TEG preferentially absorbs water from gas in a counter-current contactor.

Hydrate Prevention

Dewpoint Depression

Lower water dewpoint below minimum pipeline temperature to prevent ice/hydrate formation.

Pipeline Spec

Water Content Limit

FERC tariffs typically require ≤7 lb H₂O/MMscf for interstate transmission.

Pre-Treatment

Upstream Processing

Remove water before cryogenic plants, amine systems, or compression.

Why TEG?

Optimal Properties

Low vapor pressure, high water affinity, thermally stable to 400°F.

TEG Dehydration Process Flow

IMAGE: TEG Dehydration Process Flow Diagram

Schematic showing: inlet separator → glycol contactor (gas up, TEG down) → flash tank → glycol/glycol exchanger → still column → reboiler → surge tank → glycol pump → back to contactor. Show lean/rich glycol streams with labels.

Key Process Steps: 1. Inlet Separator: Remove free liquids before contactor 2. Glycol Contactor: Counter-current absorption (gas up, TEG down) 3. Flash Tank: Release dissolved hydrocarbons from rich glycol 4. Glycol/Glycol Exchanger: Preheat rich glycol, cool lean glycol 5. Still Column + Reboiler: Strip water, regenerate lean TEG 6. Glycol Pump: Return lean TEG to top of contactor

Why TEG Over Other Glycols?

Property MEG DEG TEG
Boiling Point (°F) 387 473 545
Decomposition (°F) ~329 ~328 ~404
Vapor Pressure Higher Medium Very Low
Glycol Losses High Medium Low
Typical Use Hydrate inhibitor Low-P dehy Standard dehy

TEG's high boiling point and low vapor pressure minimize glycol losses to the gas phase while allowing efficient regeneration at 380-400°F.

2. GPSA Equilibrium Dewpoint Curves

GPSA Figure 20-63 provides equilibrium water dewpoint data for TEG at various concentrations and contact temperatures. This represents the minimum achievable dewpoint when lean TEG is in perfect equilibrium with gas.

IMAGE: TEG Equilibrium Dewpoint Curves (GPSA Figure 20-63 Style)

X-axis: Contact Temperature (60-160°F), Y-axis: Equilibrium Water Dewpoint (-80 to +80°F). Curves for TEG concentrations: 95%, 97%, 98%, 98.5%, 99%, 99.5%, 99.9%. Show linear relationship with ~0.7°F dewpoint rise per 1°F contact temp increase.

Reading the Equilibrium Curves

Equilibrium Dewpoint vs Contact Temperature The relationship is approximately linear within normal operating ranges: T_dewpoint ≈ T_contact - ΔT_depression Where dewpoint depression (ΔT) depends on TEG concentration: | TEG wt% | Approx. Depression at 100°F | |----------|----------------------------| | 95.0% | 52°F (dewpoint = 48°F) | | 97.0% | 68°F (dewpoint = 32°F) | | 98.0% | 78°F (dewpoint = 22°F) | | 98.5% | 87°F (dewpoint = 13°F) | | 99.0% | 100°F (dewpoint = 0°F) | | 99.5% | 118°F (dewpoint = -18°F) | | 99.9% | 142°F (dewpoint = -42°F) | Key Insight: Each 0.5% increase in TEG purity above 98% provides ~15°F additional dewpoint depression.

Practical Equilibrium vs Actual Dewpoint

Real contactors don't achieve perfect equilibrium. An "approach temperature" accounts for:

  • Finite number of trays (typically 4-8)
  • Tray efficiency (25-35% for bubble cap)
  • Gas-liquid contact limitations
Actual Dewpoint Calculation: T_actual = T_equilibrium + T_approach Typical approach temperatures: • 6+ trays, good efficiency: +5 to +8°F • 4-6 trays, standard: +8 to +12°F • <4 trays, marginal: +12 to +15°F Example: Contact temp: 100°F TEG: 99.0 wt% Equilibrium dewpoint: 0°F (from GPSA) Approach: +8°F (6 trays) Actual dewpoint: 0 + 8 = 8°F

TEG Concentration Selection

Target Dewpoint Min TEG Required Water Content* Regeneration Method
+30°F 97.0% ~15 lb/MMscf Atmospheric reboiler
+15°F 98.0% ~8 lb/MMscf Atmospheric reboiler
0°F 98.7% ~5 lb/MMscf Atmospheric + small stripping
-20°F 99.2% ~2 lb/MMscf Stripping gas (2-5 scf/gal)
-40°F 99.5% ~1 lb/MMscf DRIZO® or vacuum

*Approximate water content at 1000 psia operating pressure

3. McKetta-Wehe Water Content Charts

The McKetta-Wehe chart (GPSA Figure 20-3) relates water content in natural gas to temperature and pressure at saturation. Once dewpoint is known, water content is determined from this chart.

IMAGE: McKetta-Wehe Water Content Chart (GPSA Figure 20-3 Style)

Log-log chart. X-axis: Temperature (-60 to +200°F), Y-axis: Water Content (0.01 to 1000 lb/MMscf). Multiple curves for pressures: 14.7, 100, 300, 500, 800, 1000, 1500, 2000 psia. Show water content decreasing with increasing pressure at constant temperature.

Using McKetta-Wehe Data

Water Content at Saturation Water content decreases with: • Decreasing temperature (exponential effect) • Increasing pressure (approximately inverse) Representative Values at Selected Pressures (lb H₂O/MMscf): | Dewpoint | 300 psia | 500 psia | 1000 psia | 1500 psia | |----------|----------|----------|-----------|-----------| | +60°F | 3.1 | 1.9 | 1.0 | 0.68 | | +32°F | 1.0 | 0.63 | 0.33 | 0.22 | | 0°F | 0.28 | 0.17 | 0.09 | 0.06 | | -20°F | 0.11 | 0.07 | 0.036 | 0.025 | | -40°F | 0.041 | 0.025 | 0.013 | 0.009 | Key Point: At 1000 psia, achieving 7 lb/MMscf pipeline spec requires dewpoint of approximately +25°F or lower.

Pressure Effect on Water Content

Higher operating pressure reduces equilibrium water content at the same dewpoint. This means:

  • High-pressure systems (>800 psia) more easily meet pipeline specs
  • Low-pressure systems (<300 psia) need lower dewpoints for same water content
  • Pressure changes during transport affect where liquid may condense
Design Consideration: Always evaluate water content at the minimum pressure point in the pipeline system (typically at delivery or after pressure reduction). Liquid water may condense downstream even if gas was "dry" at higher pressures.

4. Glycol Regeneration Methods

TEG purity is limited by the regeneration method. Higher purities require more sophisticated systems to overcome the TEG-water azeotrope.

IMAGE: TEG Regeneration Methods Comparison Diagram

Side-by-side schematics of: (1) Atmospheric reboiler only, (2) Stripping gas injection, (3) DRIZO® process with coldfinger. Label achievable TEG purities for each: 98.7%, 99.2%, 99.5%+.

Regeneration Methods and Achievable Purity

Method Max TEG % Reboiler Temp Notes
Atmospheric Reboiler Only 98.7% 380-390°F Simplest, lowest cost
Stripping Gas (1-2 scf/gal) 99.2% 390-400°F Sales gas or N₂ injection
Stripping Gas (5-10 scf/gal) 99.5% 400°F Higher gas consumption
DRIZO® (Coldfinger) 99.9% 390-400°F Patented, high purity
Vacuum Regeneration 99.5% 350-380°F Lower temp, added complexity

Thermal Degradation Limits

TEG Thermal Stability: Decomposition onset: ~404°F (207°C) Maximum safe reboiler: 400°F (204°C) Recommended operating: 380-395°F Degradation products: • Acetaldehyde, formic acid, acetic acid • Causes corrosion and glycol darkening • Monitor glycol pH (should be 7.0-8.0) Warning Signs of Degradation: • Dark/black glycol color • pH below 6.0 • Increased foaming • Reboiler fouling

Stripping Gas Injection

Dry gas injected into the reboiler reduces water partial pressure, shifting equilibrium toward higher TEG purity without increasing temperature.

Stripping Gas Rate: Typical: 2-5 scf per gallon TEG circulated High purity: 5-10 scf/gal For 6 GPM circulation at 3 scf/gal: Q_strip = 6 × 60 × 3 = 1,080 scfh Stripping gas source: Sales gas or nitrogen Disposition: Vent to atmosphere or fuel gas system

5. Contactor Design Fundamentals

The glycol contactor is where gas-liquid mass transfer occurs. Design must balance capital cost against dewpoint achievement.

Number of Trays

Trays Theoretical Stages* Typical Application
4 1.0-1.4 Simple dehy, high TEG circulation
6 1.5-2.1 Standard pipeline spec (7 lb/MMscf)
8 2.0-2.8 Low dewpoint applications
10-12 2.5-4.0 Very low dewpoint, cryogenic pre-treatment

*Assuming 25-35% tray efficiency for bubble cap trays in glycol service

Circulation Rate Selection

GPSA Recommended Circulation: Standard design: 3.0 gal TEG per lb H₂O removed Range: 2-5 gal/lb for most applications High circulation (5-7 gal/lb): Very low dewpoint targets Circulation Rate Calculation: C (gal/hr) = W × R Where: W = Water removed (lb/hr) = Q × ΔW / 24 Q = Gas flow (MMscfd) ΔW = Water content reduction (lb/MMscf) R = Specific circulation (gal/lb) Example: Gas: 50 MMscfd, Water reduction: 40 lb/MMscf W = 50 × 40 / 24 = 83.3 lb/hr At R = 3 gal/lb: C = 83.3 × 3 = 250 gal/hr = 4.2 GPM

Operating Temperature Guidelines

  • Optimal contact temp: 80-110°F
  • Minimum: 60°F (TEG viscosity increases, poor mass transfer)
  • Maximum: 130°F (reduced dehydration, higher glycol losses)
  • Lean TEG: Cool to within 10°F of gas temperature
Temperature Management: If inlet gas is too hot (>130°F), install a gas cooler upstream. If too cold (<70°F), the TEG viscosity limits mass transfer efficiency—consider heating the gas or accepting slightly higher dewpoint.

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