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Get to Know Glycol Dehydration Unit


Natural gas dehydration unit is one of important facilities in natural gas processing. Purpose of water vapor removal in natural gas are to prevent gas hydrates formation and to reduce corrossion in pipeline. There are several methods of gas dehydration, which are absorption with liquid desiccant, adsorption with solid desiccant, and cooling below initial dew point. Selection of the method is depend on water content in feed gas and required dew-point in dry gas.

One of the most popular methods of natural gas dehydration is absorption process employing diethylene glycol or triethylene glycol as dehydration medium or desiccant. It is a mature process having been used for over fifty years. Triethylene glycol has emerged as the most popular chemicals in recent years due to several favorable desiccant characteristics. Those characteristics are high affinity for water vapor, non-corrosive, ease of generation, and low chemical losses.

[Read more: Several Natural Gas Dehydration Methods and Range of Application]

Purpose of this post is to give you a brief process description of glycol dehydration system.

Process Description of Natural Gas Dehydration Unit

Figure below shows typical schematic of triethylene glycol (TEG) dehydration unit.

Typical schematic diagram of glycol dehydration unit
Typical schematic diagram of glycol dehydration unit

First, wet inlet gas stream enters Inlet Gas Scrubber (vertical/horizontal). In this scrubber, accumulations of any liquid in the gas stream are removed. The wet gas passes out of the top of scrubber through wire mesh mist eliminator which allows for virtually no liquid carry over. Inlet gas scrubber can be two-phase scrubber (oil and gas) or three-phase scrubber (oil, gas, and water). If there is any liquid water in the inlet gas stream, it is more favorable to use three-phase gas scrubber to remove the liquid water before gas enters glycol-gas contactor (absorber).

After leaving top of inlet gas scrubber, gas enters bottom of glycol-gas contactor (absorber) and flows upward through contact medium countercurrently with flow of triethylene glycol. Contact medium in glycol absorber may be bubble trays, valve type trays, structured packing, or dumped packing in smaller capacity.

Several types of packing or tray

Schematic above shows that absorber utilizing bubble cap trays. Above the top trays in the contactor is an open space for entrainment settling where the entrained glycol particles in gas stream will settle out. Any entrained glycol droplets from the gas stream will be removed by mist eliminator in the top of absorber. Then the dry gas leaves absorber column at the top.

Dry gas flows down through a vertical glycol cooler, where in this cooler occurs heat exchanged between dry gas and hot generated glycol before it enters the absorber. Then dry gas leaves from the bottom of glycol cooler. Some design will also employ a dried gas knockout drum to recover a small amount of TEG.

Dry concentrated (lean) glycol enters the top of glycol-gas contactor (absorber) from glycol cooler and flows across each tray and down through a downcomer pipe onto the next tray. The bottom tray downcomer is fitted with a seal pot to hold a liquid seal on the trays. By this countercurrent flow of gas and TEG, the driest incoming glycol on the top is in contact with the driest outgoing gas for maximum dehydration of gas stream.

[Read more: Natural Gas Dehydration using Solid Desiccant and Free Bonus Spreadsheet for Design of Natural Gas Dehydration]

The wet (rich) glycol which has absorbed water vapor from gas stream leaves the bottom of glycol-gas absorber column. The wet glycol then enters regeneration unit.

The cool wet glycol flows through a coil in the top of regenerator still column where it is preheated by exchanging heat with hot vapors coming from reboiler. This exchange condenses reflux for the still while it begins to heat the wet glycol.

The wet glycol from the reflux condenser then flow to cooler part of glycol-glycol exchanger. The warmed wet glycol stream flows from the heat exchanger to a flash separator. Flash separator allows the release of the entrained or absorbed gas. Gas flashed in the flashed separator may be used as supplement of fuel gas for reboiler or is discharged through backpressure valve.

Normally flash separator is equipped with a liquid level control which discharges warmed wet glycol stream through solid filter unit and carbon filter unit and then to the hot section of glycol-glycol exchanger to the inlet connection of glycol stripping still column. If wet glycol streams absorbs any liquid hydrocarbons in the absorber, it is favorable to equip the flash separator for three phase operation to separate glycol from liquid hydrocarbon (condensate) before it enters reboiler. Liquid hydrocarbon present in the reboiler will cause excessive losses from stripping still vent.

After leaving filters unit, the heated wet glycol enters the stripping still column which is typically packed with ceramic saddles (or other random packing) and is insulated. The reflux created by the cool wet glycol exchange condenses glycol vapors rising from the reboiler section in the upper part of the still column. Uncondensed water vapor then leaves from
the top of the condenser section and may either be vented to the atmosphere or be additionally processed by condensing or burning for environmental control if required.

After entering the stripping still column, the wet glycol will flow downward toward the reboiler contacting hot rising glycol vapors, water vapors, and stripping gas (if required). The water vapor has a lower boiling point than glycol; therefore, any rising glycol vapors will be condensed in the stripping still and returned to the reboiler section. In the reboiler the
glycol is heated to between 350oF to 400oF to remove water vapor to reconcentrate it to desired levels. For extra dry glycol (98.5-99 weight% purity) it may be necessary to add some stripping gas to the reboiler.

Many units including standard field glycol dehydration units are equipped with a direct natural gas fired firebox in the reboiler utilizing a portion of the natural gas stream and vent gas from the flash separator for fuel. The reboiler may be fitted with a hot oil heated coil or steam coil in place of the direct fired firebox. Hot dry glycol from the surge tank flows through both sections of the glycol-glycol exchanger, into the glycol pump, and back to the gas-glycol exchanger

References:

[1] Laurance Reid Gas Conditioning Conference 2008 Gas Dehydration Fundamentals.

[2] Fundamentals of Gas Dehydration Design and Operation with Glycol Solutions

[3] Glycol Dehydrator Design Manual

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