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Four leading technologies are compared here
Natural gas normally contains contaminants such as water vapor, carbon dioxide (CO2), and hydrogen sulfide (H2S), which must be removed prior to transporting or selling the gas. Due to its highly toxic, corrosive and odorous nature, H2S is strictly regulated, and usually requires removal to very low levels.
When selecting an H2S-removal technology, one must consider the operating pressure of the system, the concentration of H2S in the inlet stream, the presence of other contaminants, local regulations, and economics. This article compares the four dominant approaches - the use of liquid or solid H2S scavengers, liquid redox processes, and a Claus system with a tailgas-treatment unit - from both a technical and an economic standpoint (table, below). Each can remove and recover 99.9+% of H2S from natural gas streams. With the exception of the Claus approach, each may also be used to remove H2S from other industrial gas and fluegas streams.
Although H2S is rather insoluble in water, the dilute solution that does form is acidic, and can be very corrosive to pipelines and equipment. However, it is its extreme toxicity - H2S is more lethal than hydrogen cyanide - that is often the driving force for removing H2S from natural gas.
Four methods compared
Scavengers. H2S scavengers have been used for years to remove low levels of H2S from natural gas. One of the oldest is iron sponge, which is still used today. However, it has lost some of its popularity in recent years, due to the formation of iron sulfide, a hazardous (pyrophoric) reaction product. Other liquid and solid scavengers, which produce less-hazardous reaction products have gained favor.
Liquid scavengers consist of amines, nitrites and triazine; sales of triazine currently dominate this market [1]. They can be used in two ways. After liquid hydrocarbons and water are separated from the gas, the liquid scavenger can either be injected directly into the gas stream (a continuous process; Figure 1), or the gas can be passed through a vessel filled with the scavenger (a batch process).
The efficiency of liquid scavengers - typically measured in liters of scavenger per million cubic meters of gas per ppm of H2S [L/(MMm3)(ppm)] - varies significantly, but most fall within the range...