5 Critical Engineering Traps in TEG Gas Dehydration
1. The Reboiler Thermal Cracking Limit at 404°F (206°C)
Triethylene glycol begins thermal decomposition at 206°C (404°F), breaking down into corrosive organic acids (formic and acetic acids) and sludge polymers. Inexperienced operators attempting to drive off residual water by turning up the reboiler temperature past 400°F rapidly destroy the glycol inventory. The acidic decomposition products attack firetubes and carbon steel piping, reducing reboiler life from 20 years to under 18 months.
2. Contactor Hydrocarbon Condensation and Violent Foaming
If lean TEG enters the contactor column colder than the incoming wet gas, the cold glycol cools the gas below its hydrocarbon dew point. Heavy hydrocarbons (C6+ condensates) condense directly into the glycol stream. Hydrocarbon liquids form a tenacious chemical emulsion with TEG, triggering massive contactor foaming. Trays flood instantly, blowing hundreds of gallons of expensive glycol overhead into the export pipeline. The lean TEG temperature must strictly be controlled at 5°F to 10°F (3°C to 6°C) hotter than the inlet gas.
3. Stripping Gas Emission & BTEX Environmental Release
TEG selectively absorbs aromatic hydrocarbons—Benzene, Toluene, Ethylbenzene, and Xylenes (BTEX)—from the wet natural gas. In the regeneration still column, BTEX compounds flash off into the atmospheric steam overhead vent. Environmental EPA regulations (MACT subpart HHH) strictly limit hazardous air pollutant emissions. Venting still column gas directly to the atmosphere without a condenser, thermal oxidizer, or VRU incurs catastrophic regulatory fines.
4. Flash Tank Pressure Sizing and Glycol Pump Cavitation
Rich TEG leaves the 1,000 psig contactor saturated with dissolved methane gas. The rich glycol must enter a 3-phase flash tank operating between 50 and 75 psig to release solution gas and skim off free liquid hydrocarbons before reaching the reboiler. If flash tank pressure is set too low (<35 psig), volatile gas bubbles choke the rich/lean heat exchanger and cause severe cavitation in the downstream glycol circulation pumps.
5. Solid Particulate and Salt Saltation in Still Column Trays
Subsurface formation water carries dissolved mineral salts (NaCl, CaCl2). When wet gas carries saline mist past the inlet separator, salt dissolves in the TEG. Because salt cannot boil off in the reboiler, it precipitates as hard ceramic scale directly on the reboiler firetube bundle. Hot spots develop under the scale cake, creating localized tube metal temperatures exceeding 700°F (370°C) that cause sudden reboiler firetube rupture.
Natural gas dehydration sizing follows the thermodynamic vapor-liquid equilibria and hydrodynamic flooding limitations specified by GPSA:
W_sat = (A / P_psia) + B (lb H2O / MMSCF at 60°F and 14.7 psia)
2. Water Removal Rate:
ΔW = W_in - W_out (lb/MMSCF) ⇒ m_water = (Q_MMSCFD · ΔW) / 24 (lb/hr)
3. TEG Circulation Rate:
Q_TEG = Circulation_Ratio · m_water (gal/hr) ⇒ GPM = Q_TEG / 60
4. Operating Gas Density at Contactor Conditions:
ρ_G = (P_psia · MW_gas) / (Z · R_u · T_R) (lb/ft³)
5. Souders-Brown Maximum Superficial Velocity:
v_max = C_SB · √[ (ρ_L - ρ_G) / ρ_G ] (ft/s, where ρ_L ≈ 70.0 lb/ft³ for TEG)
6. Contactor Column Diameter:
v_design = (Design_Factor % / 100) · v_max
Q_actual_cfs = [ Q_MMSCFD · 10^6 · 14.7 · T_R · Z ] / [ 86400 · P_psia · 520 ]
A_col = Q_actual_cfs / v_design ⇒ D_col = √[ (4 · A_col) / π ]
7. Reboiler Thermal Duty:
Q_reb ≈ Q_TEG · 1,000 BTU/gal + m_water · 1,000 BTU/lb (plus heat losses η_reb)
Typical commercial columns are rounded up to the nearest standard nominal pipe or rolled shell diameter (e.g. 24", 30", 36", 42", 48", 54", 60", 66", 72", 84").