TEG Dehydration Absorber & Regeneration Skid Sizer

Triethylene Glycol (\(\text{C}_6\text{H}_{14}\text{O}_4\), TEG) is the premier liquid desiccant for natural gas dehydration due to its high boiling point (285°C / 545°F), high thermal stability limit (206°C / 404°F), low vapor pressure at contactor temperatures, and superior hygroscopic affinity driven by hydrogen bonding between water molecules and ether-hydroxyl functional groups.

Water-TEG Vapor-Liquid Phase Equilibrium

In a counter-current contactor, dry lean TEG enters at the top tray while wet natural gas enters beneath the bottom tray. The equilibrium water vapor pressure above an aqueous TEG solution is governed by modified Raoult's law incorporating the liquid activity coefficient of water (\(\gamma_{w}\)):

\[ P_{w} = x_{w} \cdot \gamma_{w}(x_{w}, T) \cdot P_{w}^\circ(T) \]

Where \(x_w\) is the mole fraction of water in the liquid TEG phase, and \(P_w^\circ(T)\) is the saturation vapor pressure of pure water at contactor temperature. Because TEG and water form a highly non-ideal polar mixture, the activity coefficient \(\gamma_w\) is significantly less than unity at low water concentrations (\(\gamma_w \approx 0.3 - 0.5\)), enabling profound dew point suppression far below ideal liquid absorption limits.

Required Lean Glycol Concentration & Worley Curves

The contract water dew point depression (\(\Delta T_{dp}\)) is the difference between inlet gas temperature and the dry gas water dew point:

\[ \Delta T_{dp} = T_{inlet} - T_{dp,target} \]

Per GPSA Engineering Data Book Figure 20-53 and the Worley correlation, the required lean TEG purity entering the top of the contactor is directly determined by the target dew point depression. Standard atmospheric reboilers without stripping gas achieve 98.5 to 99.0 wt% TEG, providing a maximum dew point depression of approximately 35°C to 40°C (65°F to 75°F). To achieve cryogenic pipeline specifications (< 4 lb/MMSCF) or Arctic delivery conditions requiring depressions of 50°C to 70°C, lean TEG purity must reach 99.5 to 99.95 wt%, necessitating stripping gas or vacuum regeneration.

The gas contactor column provides intimate interfacial contact between the downward-flowing viscous TEG liquid and the upward-flowing high-pressure gas. Contactors are fabricated either with bubble-cap trays (traditional, robust against fouling) or structured packing (e.g., Mellapak, featuring lower pressure drops and reduced column diameter).

Souders-Brown Maximum Vapor Velocity

To prevent gas bubbling from carrying liquid glycol droplets upward out of the contactor (entrainment flooding), the superficial gas velocity must not exceed the terminal settling velocity derived from momentum balance and droplet drag, formalized by the Souders-Brown equation:

\[ v_{max} = K_{SB} \sqrt{\frac{\rho_L - \rho_G}{\rho_G}} \]

Where \(\rho_L\) is the liquid TEG density (\(\approx 1100 \text{ kg/m}^3\)), \(\rho_G\) is the in-situ gas density at operating pressure and temperature, and \(K_{SB}\) is the empirical Souders-Brown design factor:

• Bubble-Cap Trays (24-inch tray spacing): \(K_{SB} = 0.16 \text{ to } 0.18 \text{ ft/s } (0.049 \text{ to } 0.055 \text{ m/s})\).
• Structured Packing: \(K_{SB} = 0.20 \text{ to } 0.25 \text{ ft/s } (0.061 \text{ to } 0.076 \text{ m/s})\).

To maintain operational stability, avoid foaming entrainment, and handle feed rate surges, the conservative design operating velocity is set to 70% to 75% of maximum Souders-Brown velocity (\(v_{design} = 0.70 \cdot v_{max}\)).

Column Diameter & Height Sizing

The cross-sectional area and internal diameter of the contactor column are sized from actual flowing volumetric gas rate (\(Q_{act}\)):

\[ A_{col} = \frac{Q_{act}}{v_{design}} \qquad D_{col} = \sqrt{\frac{4 \cdot A_{col}}{\pi}} \]

Contactor internal diameter is rounded up to standard vessel fabrications (e.g., 24", 30", 36", 42", 48", 54", 60"). The column typically houses 6 to 8 bubble-cap trays (or 3 to 4.5 meters of structured packing, with HETP \(\approx 0.5 \text{ m}\)), delivering 1.5 to 2.5 theoretical equilibrium stages required for commercial dehydration.

The regeneration skid desorbs the absorbed water from the rich TEG stream to restore high lean purity for continuous reinjection into the contactor. The regeneration assembly consists of a flash separator (removes dissolved methane at 4-5 barg), particulate and activated carbon filters, a lean/rich glycol heat exchanger, a distillation still column with reflux condenser, and an atmospheric reboiler.

Thermal Decomposition Threshold & Operating Temperatures

Triethylene Glycol undergoes thermal degradation into corrosive organic acids and heavy oligomers at 206°C (404°F). Therefore, reboiler firetube or electric heating elements are strictly controlled at an operating bulk liquid temperature of 200°C to 204°C (392°F to 400°F) with maximum firetube heat flux limited to \(8,000 \text{ to } 10,000 \text{ BTU}/(\text{hr}\cdot\text{ft}^2)\) to prevent localized skin skin-film overheating.

Reboiler Thermal Duty Heat Balance

The total heat input required by the reboiler encompasses sensible liquid heating, endothermic latent heat of water vaporization, reflux condenser cooling load, and ambient insulation losses:

\[ Q_{reb} = 1.10 \cdot \left[ Q_{sensible} + Q_{latent} + Q_{reflux} \right] \]

• Sensible Heating: \(Q_{sens} = \dot{m}_{rich} \cdot C_{p,TEG} \cdot (T_{reb} - T_{feed})\), where rich glycol preheated in the lean/rich exchanger enters at \(\approx 150°\text{C}\) and \(C_p \approx 2.75 \text{ kJ}/(\text{kg}\cdot\text{K})\).
• Water Latent Heat: \(Q_{latent} = \dot{m}_{H_2O} \cdot \Delta H_{vap}\), with \(\Delta H_{vap} \approx 2260 \text{ kJ/kg}\).
• Reflux & Desolvation Allowance: \(\approx 25\%\) of latent vaporization duty to condense reflux in the still column top and prevent vaporized TEG loss.
• Heat Losses: 10% safety margin added for surface radiation and convection losses.

Stripping Gas Sizing for High Purity (>99.5 wt%)

Under atmospheric boiling at 204°C, the equilibrium concentration of TEG is constrained by Dalton's law of partial pressures to 98.7 wt%. To drive purity above 99.5 to 99.9 wt%, superheated dry stripping gas (fuel gas or sales gas) is sparged through a packed stripping column located between the reboiler and the surge accumulator. The stripping gas lowers the water vapor partial pressure according to:

\[ y_{w} = \frac{P_{w,eq}}{P_{total}} \]

GPSA design guidelines prescribe 2.0 to 8.0 scf of stripping gas per gallon of TEG circulated (\(0.015 \text{ to } 0.06 \text{ Sm}^3/\text{L}\)), reducing equilibrium water vapor partial pressure and stripping residual moisture down to 0.05 wt%.