Dehydration of Natural Gas
Natural Gas usually contains significant quantities of water vapor. Changes in temperature and pressure condense this vapor altering the physical state from gas to liquid to solid. This water must be removed in order to protect the system from corrosion and hydrate formation.
In 1810, an English scientist by the name of John Dalton stated that the total pressure of a gaseous mixture is equal to the sum of the partial pressures of the components. This statement, now known as Dalton’s Law of Partial Pressures, allows us to compute the maximum volume of water vapor that natural gas can hold for a given temperature and pressure.
The wet inlet gas temperature and supply pressures are the most important factors in the accurate design of a gas dehydration system. Without this basic information the sizing of an adequate dehydrator is impossible.
As an example, one MMSCF (million standard cubic feet) of natural gas saturated @ 80 degree F. and 600 PSIG (pound per square inch gauge) will hold 49 pounds of water. At the same pressure (600 PSIG) one MMSCF @ 120 degree F will hold 155 pounds of water.
Common allowable water content of transmission gas ranges from 4 to 7 pounds per MMSCF. Based upon the above examples, we would have two very different dehydration problems as a result of temperature alone.
There are many other important pieces of design information required to accurately size a dehydration system. These include pressures, flow rates and volumes.
All gasses have the capacity to hold water in a vapor state. This water vapor must be removed from the gas stream in order to prevent the formation of solid ice-like crystals called hydrates. Hydrates can block pipelines, valves and other process equipment. The dehydration of natural gas must begin at the source of the gas in order to protect the transmission system.
The source of the gas moved through the transmission lines may be producing wells or developed storage pools. Pipeline drips installed near well heads and at strategic locations along gathering and trunk lines will eliminate most of the free water lifted from the wells in the gas stream. Multi stage separators can also be deployed to insure the reduction of free water that may be present.
Water vapor moved through the system must be reduced to acceptable industry levels. Typically, the allowable water content in gas transmission lines ranges from 4 lb. to 7 lb. per MMSCF. There are basically three methods employed to reduce this water content. These are:
1. Joule-Thomson Expansion
2. Solid Desiccant Dehydration
3. Liquid Desiccant Dehydration
Joule-Thomson Expansion utilizes temperature drop to remove condensed water to yield dehydrated natural gas. The principal is the same as the removal of humidity from outside air as a result of air conditioning in your house. In some cases glycol may be injected into the gas stream ahead of the heat exchanger to achieve lower temperatures before expansion into a low temperature separator.
Solid desiccant dehydration, also known as solid bed, employs the principal of adsorption to remove water vapor. Adsorbents used include silica gel (most commonly used), molecular sieve (common in NGV dryers), activated alumina and activated carbon. The wet gas enters into an inlet separator to insure removal of contaminants and free water. The gas stream is then directed into an adsorption tower where the water is adsorbed by the desiccant. When the adsorption tower approaches maximum loading, the gas stream is automatically switched to another tower allowing the first tower to be regenerated.
Heating a portion of the mainstream gas flow and passing it through the desiccant bed regenerates the loaded adsorbent bed. The regeneration gas is typically heated in an indirect heater. This undersaturated regeneration gas is passed through the bed removing water and liquid hydrocarbons.
The regeneration gas exits the top of the tower and is cooled most commonly with an air-cooled heat exchanger. Condensed water and hydrocarbons are separated and the gas is recycled back into the wet gas inlet for processing. The third method of dehydration is via liquid desiccant and is most common in the Northeast United States. This method removes water from the gas stream by counter current contact in a tray type contactor tower with tri-ethylene glycol (TEG). Natural gas enters the unit at the bottom of the adsorber tower and rises through the tower were it intimately contacted with the TEG solution flowing downward across bubble trays. Through this contact, the gas gives up its water vapor to the TEG.
The water laden TEG is circulated in a closed system, where the water is boiled from the TEG. The regenerated TEG then is recirculated to the contacting tower.