Corrosion in Gas Processing Units

Acid constituents such as carbon dioxide and hydrogen sulfide should be removed from natural gas in central field treating plants or in gas refiners before transmission of the gas for sale. Similarly, these constituents must be removed from plant gas streams, as in steam cracking of hydrocarbons for ethylene production, before the gases are subjected to low-temperature fractionation. 

In the production of synthesis gas for subsequent conversion to ammonia or methanol, for example, it is usually necessary to remove carbon dioxide formed either by partial combustion of hydrocarbons or by the water gas shift reactions. 

Gas treatment plants and gas refineries are bothered by corrosion problems. Much of these are caused by the breakdown of solvents, e.g. monoethanolamine, at the elevated temperatures of the reboiler regenerator. It is postulated that the breakdown products can chelate with iron and prevent the formation of an insoluble protective film at the high pH of operation, which should preclude corrosion of iron. 

In this respect, there is a similarity between the corrosion of iron in amine solutions in gas regeneration, for example, and that in the effluents from hydrocracking plants described earlier. 

Corrosion and other operational problems can be greatly reduced by proper plant operation. It is recommended that the gas loading (ratio of moles of acid gases to moles of MEA) be kept to 0.45 or less, monoethanolamine concentration be kept at 20%, and that degradation products be removed by use of a side-stream reclaimer. 

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