REMOVAL OF ACID GASES FROM NATURAL GAS

Natural gas wellstreams often contain hydrogen sulfide (H2S) and Carbon dioxide (CO2). These two gases are called acid gases because , in the presence of water, they form acids or acidic solutions. These gases, particularly H2S, are very undesirable contaminants; unless they are present in very small quantities, they must be removed from a natural gas wellstream.

Hydrogen sulfide must be removed for several reasons, the most important being that is is a toxic, poisonous gas and cannot be tolerated in gases that may be used for domestic fuels. Hydrogen sulfide in the presence of water is extremely corrosive and can cause premature failure of valves, pipeline, andpressure vessels. It can also cause catalyst poisoning in refinery vessels and requires expensive precautionary measures. Most pipelines specifications limit H2S content to 0.25 g/100 ft3 of gas (about 4 ppm).

Carbon dioxide removal is not always required. however, removal may be required in gas going to cryogenic plants to prevent solidification of the CO2; therefore, the volume of CO2 in the wellstream must be added to the volume of H2S to arrive at the total acid-gas volume to be removed.

The term sour gas means that the gas contains H2S in amounts above the acceptable industry limits. Sweet gas means a non-H2S-bearing gas or gas that has been sweeted by treating.

The typical reaction of acid gas with MEA are

MEA +H2S    →   MEA hydrosulfide +heat

MEA + H2O + CO2   →   MEA carbonate + heat

GLYCOL/AMINE PROCESS:

The glycol amine process uses a solution composed of 10 to 30 weight% MEA, 45 to 85% glycol, and 5 to 25% water for the simultaneous removal of water vapor, H2S, and CO2 from gas streams. The combination dehydration and sweeting unit results in lower equipment cost than would be required with the standard MEA unit followed by a separate glycol dehydrator. The main disadvantages of the glycol/amine process are increased vaporization losses of MEA due to high regeration temperatures, reclaiming must be by vacuum distillation, corrosion problems are present in operating units, and application must be for gas streams that do not require low dew points.