Introduction to Gas Dehydration Methods

1. Introduction 

Gas dehydration is critical in natural gas processing to remove water vapor from natural gas. The presence of water in natural gas can lead to several operational problems, such as corrosion, hydrate formation, and freezing, especially in pipelines and gas processing equipment. To mitigate these issues, the gas dehydration process reduces the moisture content in the gas to an acceptable level before the gas is transported or further processed. 

Gas hydrates are crystals of natural gas and water that can appear far above the temperature where ice is formed. Gas hydrates are caged structures containing a gas molecule like methane, the cage is formed by water through hydrogen bonding. Because the gas hydrate crystals are similar to ice crystals, the problems with gas hydrates are similar to those with ice, although gas hydrates are more troublesome because of the higher formation temperature.

2. Purposes

• Corrosion Prevention: Water in gas can combine with carbon dioxide (CO2), hydrogen sulfide (H2S), and other impurities to form acids, which can corrode pipelines and equipment.
• Hydrate Formation Prevention: At high pressures and low temperatures, water can form gas hydrates, crystalline structures that block pipelines and valves, potentially leading to operational shutdowns.
• Ice Formation Prevention: In low-temperature environments, such as during gas expansion in pipelines, water can freeze and block gas flow.

3. Dehydration Methods

There are four main methods for gas dehydration:

• absorption,
• adsorption,
• membrane processes,
• and refrigeration.

A. Absorption

This is the most commonly used method, where glycols like Triethylene Glycol (TEG) absorb water from the gas in a contactor. Afterward, the water-laden glycol (rich glycol) is regenerated by heating, which removes the water, making the glycol reusable.

In dehydration by absorption, water is removed by a liquid with a strong affinity for water, glycols being the most common. The lean (dry) glycol removes the water from the gas in an absorption column known as a contactor. After the contactor, the rich (wet) glycol must be regenerated before it can be reused in the contactor. The regeneration is done by distilling the glycol thus removing the water. With glycol absorption, it is possible to lower the water contents down to approximately 10 ppm vol, depending on the purity of the lean glycol.

B. Adsorption

In this method, the gas is passed through a bed of solid desiccants like silica gel or molecular sieves, which trap water molecules. This method is more efficient than absorption for achieving ultra-low water content but is costlier.

Dehydration by adsorption is done with a two-bed system, where the beds are filled with adsorbents e.g. silica gel. The gas is lead through one of the adsorbers, where water is removed. Meanwhile, the other adsorber is regenerated by blowing hot dry gas through it, this gas is then cooled and the water condenses.

C. Membranes Process

Membranes selectively allow water to pass through, leaving drier gas. This method is used for smaller gas flows as it becomes uneconomical at larger scales.

In membrane processes the gas passes through a membrane that separates the water. Membrane processes yield water content between 20-100 ppm vol. The problem with membrane processes is that they only become economically viable compared to glycol absorption at flows below 1.5·106 Nm3/d (56 MMscfd).

D. Refrigeration

Gas is cooled to condense and remove water. This method is often used in combination with other processes, as it cannot remove enough water on its own.

Gas dehydration by refrigeration is a low-cost dehydration method. Water condenses when the gas is cooled; the water is then removed in a separator. The separation method can be conducted numerous times. The method is most efficient at high pressure. The amount of water removed in the refrigeration process is often insufficient. Because of the low cost, the refrigeration process is often used before the other dehydration processes.

4. Comparison of the methods

The two most efficient dehydration methods are absorption and adsorption. Absorption with glycol is the preferred dehydration method because it is more economical than adsorption. This is due to the following differences between absorption and adsorption: 

1. The adsorbent is more expensive than glycol. 
2. It requires more energy to regenerate adsorbent than glycol. 
3. Replacing glycol is much cheaper than replacing an adsorption bed. 
4. Glycol can be changed continuously while changing an adsorption bed requires a shutdown. 

5. Dry Gas

The efficiency of the dehydration is measured by the water contents in the dry gas. The dew-point temperature for the water in the gas is often a more useful parameter than the total water contents. The dew-point temperature must be below the minimum pipeline temperature, to avoid liquid in the gas pipeline.

The dew point in dry gas refers to the temperature at which water vapor present in the gas begins to condense into liquid water when the gas is cooled at a constant pressure. In other words, it is the temperature below which moisture in the gas will turn into liquid, forming droplets.

6. Reference

Christensen, Dan Laudal. Gas Dehydration: Thermodynamic Simulation of the Water/Glycol Mixture. Aalborg University Esbjerg, February 2009.