TEG Gas Dehydration Skid

Overview

A glycol dehydration skid removes water vapor from natural gas. Gas leaving a separator is saturated with water, and that water causes two problems downstream: it forms solid hydrate plugs that block pipelines and valves, and it combines with acid gases to corrode steel. Sales contracts therefore cap water content at around 4 to 7 pounds per million standard cubic feet. The skid meets that limit by washing the gas with triethylene glycol, a hygroscopic liquid that pulls water vapor out of the gas stream, then regenerating the glycol by boiling the water back off. TEG is the standard solvent because it absorbs water strongly, regenerates to high purity and has low vapor loss.

The skid runs a closed glycol loop between two zones. The high-pressure Glycol Contactor is where dry glycol meets wet gas and absorbs the water. The low-pressure regeneration package, built around the Glycol Reboiler and Stripping Still, boils that water out and returns lean glycol to the contactor. A skid handles 1 to 200 MMSCFD and is built compact enough to truck to a remote pad.

Absorption in the contactor

Wet gas first passes through the Inlet Scrubber, a two-phase separator that drops out free liquid and slugs so the contactor sees only vapor. Carrying free water or hydrocarbon liquid into the contactor would flood the trays and contaminate the glycol, so the scrubber and its Demister Pad are essential protection.

The scrubbed gas enters the bottom of the Glycol Contactor and rises through a stack of Bubble-Cap Tray stages. Lean glycol, pumped to contactor pressure, enters at the top and flows down across each tray. On every tray the gas bubbles up through a held level of glycol behind the Bubble Cap caps, and the glycol absorbs water vapor from the gas. The dried gas leaves the top through a Mist Eliminator that catches glycol droplets, while the now-wet rich glycol collects on the bottom chimney tray and leaves for regeneration. More trays and colder, drier glycol give a deeper dew point depression.

Flashing and heat recovery

Rich glycol leaving the contactor is at high pressure and carries dissolved hydrocarbon gas and some condensed hydrocarbon liquid. It drops into the Flash Separator, a three-phase tank that releases the dissolved gas through a Flash Gas Valve for use as fuel, skims hydrocarbon liquid off the top, and passes the rich glycol on. Removing hydrocarbons here protects the reboiler from foaming and reduces emissions from the still.

The rich glycol then flows through the Glycol Heat Exchange section. The Lean/Rich Glycol Exchanger preheats the cold rich glycol with hot lean glycol returning from the reboiler, recovering heat and reducing the reboiler's fuel use. On the return path the Lean Glycol Cooler drops the lean glycol to within a few degrees of the inlet gas temperature, because glycol fed in too hot would vaporize and be lost overhead with the gas.

Regeneration

The preheated rich glycol enters the Glycol Reboiler, where a U-Tube Firetube fired by the Gas Burner heats the glycol to about 390 to 400 degrees F. Water boils off as vapor while the glycol stays liquid, because TEG boils far higher than water. The temperature is held just below the point where TEG itself begins to decompose, since overheating darkens and degrades the glycol. The water vapor rises into the Stripping Still, a packed still that lets water vapor escape to atmosphere while a Reflux Condenser Coil condenses entrained glycol and refluxes it back, keeping glycol loss low.

To reach the highest lean glycol purity, above about 99 percent, a small flow of dry Stripping Gas Coil gas is sparged through the hot glycol in the reboiler. The stripping gas lowers the partial pressure of water over the glycol and carries off the last traces of water, which is what enables the deepest dew point depressions.

Circulation and solution care

The Glycol Circulation system keeps the loop moving and clean. A high-pressure Glycol Pump lifts lean glycol from the reboiler base back to contactor pressure. Glycol cleanliness controls foaming and heat transfer, so a Sock Filter removes solids and a slipstream through the Activated Carbon Filter adsorbs hydrocarbons and degradation products. The Glycol Surge Drum holds the lean glycol inventory and accommodates the loop's volume swings.

Control and safety

The Control System holds the three variables that set performance: reboiler temperature, glycol circulation rate and contactor level. A Moisture Analyzer confirms the dry gas dew point against the sales spec. Because the reboiler is a fired vessel, a Burner Management System burner management system to NFPA 86 proves the Flame Scanner flame before admitting fuel and trips the Fuel Solenoid Valve on loss of flame, low water level or high temperature. The vessels are protected by Relief Valve devices on the skid.

Materials follow the service. The contactor, scrubber and flash vessels are carbon steel to ASME Section VIII, while the still, firetube, internals and exchanger coils are stainless because hot glycol and the water-rich still environment attack carbon steel. Process piping follows ASME B31.3, and the whole package is sized and rated using GPSA dehydration design practice.