The Gas Sweetening Unit creates a stream, Acid Gas, which is rich in hydrogen sulfide (H²S) and carbon dioxide (CO²). The H²S is highly toxic and cannot be emitted directly to the atmosphere. Depending upon the quantity of H²SH, several different processes are available for safe disposal.
If the Acid Gas stream is small, then the acid gas is disposed of in an incinerator, also known as a thermal oxidizer. The H²S is burned to create sulfur oxides (SOx), which have a much lower toxicity than H²S. The air rate and residence time of the incinerator must be selected to completely destruct the H²S. The stack height of the incinerator is determined by using dispersion calculations to ensure that the SOx does not exceed allowable ground level concentrations.
For larger quantities of H²S, a Sulfur Recovery Unit is used to convert the H²S into elemental sulfur, which is a saleable product. Sulfur can be transported as a liquid or as a solid and is used in the manufacture of products, mainly fertilizer and sulfuric acid.
The Sulfur Recovery Unit uses the Modified Claus process to convert H²S to sulfur. In the Claus process, one third of the H²S is converted to SO2 in a reaction furnace, and then the following primary reaction takes place:
2H²S +SO²⇌ 3⁄n Sn+2H²O
The sulfur is removed from the gas stream by condensers which generate steam, and the gas is reheated. Further reaction is achieved over two or three Claus reactor beds
A three-bed Claus process can achieve up to 97 % recovery, producing Tail Gas which is burned in an incinerator. In order to further reduce plant emissions, a Tail Gas Treatment Unit (TGTU) can be specified. In the TGTU, more than 99.5 % recovery can be achieved. The remaining gas is then disposed of in a Thermal Oxidizer.
The Sulfur product is recovered as a liquid and stored in a drum or pit. In the pit, degassing takes place to remove dissolved H²S from the sulfur. Degassing prevents the release of H²S during storage and transport which may cause an explosive mixture of hydrogen sulfide air.
Submerged pumps are used to pump the molten sulfur to insulated tank trucks or rail cars. For long distance transportation, heating is required to prevent molten sulfur from solidifying. Alternatively, the liquid sulfur can be pumped to a unit which solidifies the sulfur.
Solidification is carried out in Pelletizing Unit which converts liquid sulfur into small pellets which can safely and easily transported across long distances, for example in ships.