Dry Sweetening Processes

While the sweetening of sour gas is predominantly done with regenerative solvents, there are

also some dry processes that can be used for this purpose. Because these processes are batch

processes, two or more towers are usually used, so that one tower can be taken out of service for

chemical charge replacement without interruption of gas flow.

Iron Sponge (Iron Oxide)

Iron sponge consists of wood chips that have been impregnated with a hydrated form of iron

oxide. The material is placed in a pressure vessel through which the sour gas is flowed. Because

this is a batch process, usually two vessels are installed—one in service and the other on standby.

The H2S reacts with the iron oxide to form iron sulfide. In due course, the iron oxide is

consumed. While it is possible to regenerate the iron sulfide with air to restore the iron oxide, in

practice this is not done. Instead, the tower containing the spent iron sponge is taken out of

service, and the standby tower is placed in service. The spent iron sponge is moistened with

water, removed, and disposed of at an approved disposal site, and the tower is filled with a new

charge of iron sponge. Care has to be exercised in handling the spent material in the dry state, as

it is pyrophoric. When dry iron sulfide is exposed to air, a spontaneous chemical reaction

between the iron sulfide and oxygen takes place—oxidizing the iron sulfide to iron oxide and

emitting sulfur dioxide into the air.

SulfaTreat (Iron Oxide)

Several years ago, a new iron-oxide-based dry product with the trade name of SulfaTreat was

introduced for sweetening sour gas. The product is placed in towers, as illustrated in Fig. 5.6,

through which the sour gas is flowed. The gas stream should have a superficial gas velocity of no

more than 10 ft/minute, and the temperature of the gas should be between 70 and 110°F. The gas

must be water saturated at the tower conditions of temperature and pressure. SulfaTreat has a

different molecular structure from that of iron sponge and, upon reaction with H2S, forms iron

pyrite instead of iron sulfide. The charge of SulfaTreat is replaced when consumed.

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