What is Natural Gas Dehydration? Different types of Natural Gas Dryers.

*written by an independant 3rd party*

Natural gas dehydration can play a crucial role in the proper operation of your gas processing application.  Whether it is an onshore or off shore gas processing facility, pipeline compressor or LNG producing system, adequate dehydration of your process gas can save a substantial amount of money and reduce operational issues.

Below are a few examples of common issues due to insufficient dehydration:

• Condensation in lines can freeze and cause blockages in pipelines
• Instrument and control gas freezing
• Restrictions in fuel gas lines
• Obstruction due to hydrate formation
• Increased rate of corrosion due to condensate in lines
• Pitting and higher rate of deterioration of valves, seals, and compression equipment
• Insufficient pipeline spec gas to end user (Due to wet gas)
• Safety hazards when liquid reaches flares or combustors

One of the most common and cost-effective ways to dehydrate natural gas is by using a natural gas dryer.  There are many types of dryers/ dehydration techniques; a brief overview of the most common ones is below:

1. Desiccant- Adsorption: Contains media that absorbs moisture but does not dissolve (i.e. alumina or silica gel).  Essentially the media inside the dryer acts like a big sponge. The media eventually needs to be replaced or re-generated.
2. Desiccant- Deliquescent: Contains absorbing media that dissolves and needs to be replaced .
3. Scrubber- A dryer which uses gravity, various baffles, and mesh mist extractors to knock out moisture.
4. Flash Drum- An empty vessel. This vessel operates at a pressure lower than the upstream gas.  When the gas takes a pressure drop, it knocks out the entrapped liquid.
5. TEG Dehydrator- Uses Triethylene glycol (TEG) to dehydrate the gas. TEG has a high affinity to water.  Gas goes through a “glycol contactor” where the TEG strips moisture from the gas stream.

It is important to consider the correct dryer for your application.  Some factors and issues to consider are the following:

• Is there a risk of freezing the dryer?
• Is an operator available to monitor and drain the dryer?
• What is the pressure of my application?
• Is electricity available at the location?
• Do I need a code vessel?
• What pressure range will I be operating in?
• What is the volume of gas that I am trying to dry?

The Van Air Systems PLD Pipleline Natural Gas Dryer is a simple and convenient solution to gas dehydration and additionally addresses all the concerns above.  The PLD pipleline dryer uses a bed of deliquescent desiccant tablets which dissolve as they absorb moisture from the gas. Often, “desiccant-adsorption” dryers can freeze as the desiccant absorbs moisture and remains stagnant.  If the dryer is not properly heat traced, the freezing can result in a large obstruction in a pipeline. Similar freezing issues can be seen in a scrubber or flash drum.  If the liquid is not drained promptly, it can be prone to freezing.  The brine solution which is produced in the Van Air Systems PLD dryers has no risk of freezing above  -20^oF.  This eliminates the need for electricity to operate the dryer.

Flash Drums and scrubbers can be useful when needing to dehydrate, but a major drawback is the pressure drop that takes place when going through these pieces of equipment.  These vessels normally operate at a lower pressure than the incoming gas and use gravity as the force of separation, the gas is forced to take a pressure drop in order to drop out the liquids.  This is not ideal if you need to achieve a certain pressure in order to get into your gas sales line. In addition, since a scrubber and flash drum are using gravity to dry the gas, the rate of dehydration is a function of retention time in the vessel.  If a large volume of gas is surging through the vessel, it may not be adequately able to dry the gas.  The Van Air Systems PLD Series Pipeline dryer uses deliquescent media so very little pressure drop or specific retention time is required.

The use of automated drains in dryers is crucial, especially in applications involving fuel gas, or in gas being sent to process flares and combustors.  Too much condensate liquid in the gas stream can result in combustible liquid “burping” out of a flare.  As one would expect, this poses a significant safety risk for people in the area.   A float and pneumatic drain valve can automatically drain the dryer vessel when a certain liquid level is reached if desired.  This eliminates the need for someone to constantly monitor the liquid level in the dryer.  Additionally, electricity to operate an actuated valve is not needed.

It can be difficult to find fuel gas scrubbers that are built to code.  This is normally a concern for oil & gas operators whose process is at high pressure.  The PLD natural gas dryer has a variety of standard options.  It is a stamped ASME code vessel and can be used in 280 PSI, 720 PSI and 1450 PSI applications, other pressures available upon request.

Lastly, you may be wondering what the difference is between a TEG Dehydrator and a Deliquescent dryer such as the Van Air Systems PLD Pipeline Natural Gas Dryer. In short both types of dryers use hydrophilic elements to “grab” liquid from the gas.  A TEG dehydrator is normally used for large volumes of process gas. These units can be very large and expensive to operate.  Gas enters the bottom of a contactor in which it is showered with the TEG.  The TEG strips the liquid from the gas and drains it to the bottom of the contactor, while the gas exits from the top.  These systems are typically used for very large volumes of gas that need to meet pipeline quality specifications. Unlike the Natural Gas PLD Dryer from Van Air Systems these systems are alot less enviromentally friendly due to their emissions and less cost effective.   Also TEG dehydrator would not normally be used for smaller applications like instrument & controls gas, or gas being sent to flare.

As described above, adequate dehydration of natural gas is crucial in proper operation of your natural gas process.  There are numerous ways to dry gas with each technique having its own pros and cons.  We believe that the Van Air Systems PLD Natural Gas Dryer addresses a majority of the limitations of other dryers and dehydration systems. It is a simple and effective solution to your Natural Gas Dehydration needs.

For more information on Natural Gas Dehydration and finding a dryer for your gas application, please call Van Air Systems at 1-800-840-9906.