Tuesday, February 10, 2009

Gas Processing Method

Gas must be treated before it can be sent on to the final customer. Three primary methods of treating gas are adsorption, absorption and catalysts. These methods are performed in closed process equipment.

Absorption is the disappearance of one substance into another so that the absorbed substance loses its identifying characteristics, while the absorbing substance retains most of its original physical aspects. Used in refining to selectively remove specific components from process streams. One simple absorption process, which is also very common, is the use of glycol to absorb water from gas.

Adsorption is the adhesion of the molecules of gases or liquids to the surface of solids, as opposed to absorption, in which the molecules actually enter the absorbing medium (see adhesion and cohesion). Certain solids have the power to adsorb great quantities of gases. Charcoal, for example, which has a great surface area because of its porous nature, adsorbs large volumes of gases, including most of the poisonous ones, and is therefore used in gas masks. Certain finely divided solids have great adsorptive properties; for example, minute particles of platinum attract and hold multitudes of hydrogen molecules on their surfaces. Its ability to adsorb other gases makes platinum very useful in the production of sulfuric acid by the contact process and in the preparation of ammonia.

Dry Gas: Natural gas with so little natural gas liquids that it is nearly all methane
Sour Gas :Natural gas that contains corrosive, sulfur bearing compounds such as hydrogen sulfide and mercaptans.
Fuel Gas :Refinery gas used for heating
Desulfurization :a process in which the principal purpose is to remove sulfur from gas.
Knockout Drum :A vessel wherein suspended liquid is separated from gas or vapor Hydrogen Sulfide
Dehydration or drying of gas is accomplished by the use of water absorption or adsorption agents to remove water from the gas.

Refer to diagram below for a simple system.

The wet gas is sent to the glycol contactor. The contactor is a drum filled with trays or a mesh to maximize the mixing of its input products. Glycol is a liquid with a great affinity for water. Glycol does not absorb natural gas.
The glycol is pumped into the upper section of the contactor drum where it cascades down the inside of the drum coming into contact with the gas, which is bubbling upward. At this point the glycol absorbs any water that is in the gas. The dry gas goes out the top and the liquid glycol, now combined with water, goes out the bottom. The glycol is regenerated and then sent to the contactor drum again.
Not all of the glycol goes out the bottom on the contactor. A small mist of glycol will go out the top with the gas. Therefore a glycol knockout drum is used to get the last of the glycol out of the stream.
Desulfurization: Sour gases are dried and sweetened. Sweetening removes sulfur and also reduces concentrations of carbon dioxide.
Note, this section is about recovering sulfur from natural gas, keep in mind that sulfur is also removed from liquids.
Sulfur Recovery from hydrogen sulfide. Sulfur recovery converts hydrogen sulfide in sour gases and hydrocarbon streams to elemental sulfur. The most widely used recovery system is the Claus process, which uses both thermal and catalytic-conversion reactions. Sulfur is separated from natural gas as hydrogen sulfide and then converted to elemental sulfur by the Claus process, which involves the partial burning of hydrogen sulfide to sulfur dioxide

Hydrogen sulfide or h2s is an extremely toxic gas that smells like rotten eggs. It commonly occurs in natural gas and must be removed.

First, the hydrogen sulfide is absorbed from the natural gas at ambient temperature in a scrubber, either in alkanolamine-glycol solution or in aqueous alkaline carbonate solution. This scrubber works much like the glycol concentrator mentioned above/

Second, the hydrogen sulfide is partially oxidized to SO2 with considerable evolution of heat:

Third, this resulting sulfur dioxide is then reacted with hydrogen sulfide in the presence of Fe2O3 as a catalyst to yield the more conveniently handled elemental sulfur:

Removal of free sulfur: Sulfur also occurs in natural gas as a free element. This material is removed in a vessel by running the gas through an aluminum sieve. The free sulfur will adsorb on the aluminum. The aluminum then catalyzes or burns the sulfur to eliminate it. In cases of large amounts the sulfur is recovered and sold as a marketable by product.

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