Gas Dehydration Unit Definition / Meaning
A Gas Dehydration Unit (GDU) is a critical piece of process equipment used in the oil and gas industry to remove water vapor from natural gas streams. The presence of water in natural gas can cause numerous problems, including the formation of hydrates (ice-like solids) that block pipelines and valves, corrosion of equipment, and reduced heating value of the gas. Dehydration ensures the gas meets pipeline and sales specifications, typically achieving a water content below 7 pounds per million standard cubic feet (lb/MMscf) for most transmission lines.
Purpose and Function
The primary function of a gas dehydration unit is to lower the water dew point of the gas so that liquid water will not condense during transportation or storage. The unit works by contacting the wet gas with a liquid desiccant (typically triethylene glycol, or TEG) in a counter-current flow arrangement. The glycol absorbs water from the gas through physical and chemical affinity. The now water-rich glycol is then regenerated by heating to drive off the water, allowing the lean glycol to be recycled back into the system.
How a Typical Glycol Dehydration Unit Works
Most GDUs follow a similar process flow. The key steps are outlined below:
- Absorption: Wet natural gas enters the bottom of an absorption tower (contactor). Lean glycol is pumped to the top of the tower and flows downward over trays or packing. As the gas rises, it contacts the glycol, which absorbs water. The dried gas leaves the top of the tower.
- Flash Separation: The rich glycol (now containing water and some absorbed hydrocarbons) exits the bottom of the contactor and passes through a flash tank to release any dissolved gases at lower pressure.
- Filtration: Rich glycol is filtered to remove solids and degradation products that can foul equipment.
- Regeneration: The rich glycol enters a regenerator (still column and reboiler) where it is heated to approximately 400°F (204°C). Water is boiled off as steam, and the glycol becomes lean (typically 98.5–99.9% pure). The steam is condensed and separated, and the lean glycol is cooled and pumped back to the contactor.
- Recycling: Lean glycol is stored in a surge tank and circulated continuously.
Types of Gas Dehydration Units
There are several common configurations of gas dehydration units, each suited to different field conditions:
| Type | Description | Common Application |
|---|---|---|
| Glycol Dehydration (TEG) | Uses triethylene glycol as the liquid desiccant. Most common in onshore and offshore facilities. | Standard natural gas processing, pipelines |
| Molecular Sieve Dehydration | Uses solid adsorbents (zeolite) to trap water molecules. Can achieve very low dew points (< -100°F). | LNG plants, cryogenic processing |
| Deliquescent Dehydration | Uses solid salt tablets that absorb water and dissolve. Simple but requires regular media replacement. | Remote wellhead applications, small flows |
| Membrane Dehydration | Uses selective gas-permeable membranes to separate water vapor. | Small to mid-scale offshore platforms |
Key Components and Operational Considerations
Regardless of the type, a well-designed GDU includes the following major components:
- Contactor Tower: A vertical pressure vessel with trays or random packing to maximize gas-liquid contact.
- Reboiler: Heats the rich glycol to drive off absorbed water. Usually fired by natural gas or electric heaters.
- Still Column: A section above the reboiler where rising vapor is partially refluxed to recover glycol carryover.
- Flash Tank: Separates entrained hydrocarbons from rich glycol.
- Filter and Carbon Bed: Removes particulate and organic contaminants to extend glycol life.
- Glycol Pumps: Circulate lean and rich glycol, often using energy exchange (e.g., Kimray pumps) for efficiency.
Operators must monitor key parameters such as glycol circulation rate, reboiler temperature, and water content in outlet gas. Foaming, corrosion, and glycol degradation are common issues that require attention.
Usage Example
In a typical midstream gas processing plant, a gas dehydration unit is installed after the inlet separator but before the gas is compressed for pipeline transport. For instance, a 100 MMscfd glycol dehydration unit at a Texas gas plant might use a 36-inch diameter contactor with 24 valve trays, operating at 900 psig and circulating 12 gallons of TEG per pound of water removed, ensuring the sales gas meets a water dew point of 7 lb/MMscf.