Sulfur or sulfur which is an abundant, multivalent nonmetal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. At room temperature, elemental sulfur is shining yellow, crystalline, and solid. Chemically, sulfur could react with all elements except gold, platinum, iridium, tellurium and the noble gases.
Elemental sulfur predominantly occurs naturally as the element (native sulfur), but it most commonly occurs in combined forms as sulfide and sulfate minerals. Being abundant in native form, sulfur was known in the ancient era, as it has been mentioned for its applications in ancient Greece, ancient India, Egypt, and China. In the Bible, sulfur is called brimstone. Nowadays, almost all elemental sulfur is produced as a byproduct of removing sulfur-containing contaminants from natural gas and petroleum. The most prominent commercial application of the element is to produce sulfuric acid for sulfate and phosphate fertilizers and other chemical procedures. The element sulfur is used in matches, fungicides, and insecticides. Many sulfur compounds are odoriferous, and the smells of odorized natural gas, grapefruit, garlic, and skunk scent are due to organosulfur compounds. Hydrogen sulfide emits rotting eggs odor.
TYPES OF PLANTS
Claus Sulfur Recovery Units are generally classified according to the method used for the production of SO2 and the method used to reheat the catalyst bed feeds. The various reheat methods can be used with any SO2 production method, whereas the technique used for the production of SO2 is determined by the H2S content of the acid gas feedstock.
Most sulfur recovery plants utilize one of three basic variations of the Modified Claus Process “straight-through,” “split-flow,” or “direct-oxidation.”
“Acid gas enrichment” can be applied ahead of the SRU to produce a richer acid gas stream and “oxygen enrichment” may be used in combination with any of these variations.
These three varieties of the Modified Claus Process differ in the method used to oxidize H2S and produce SO2 ahead of the first catalytic reactor. The first two processes use a flame reaction furnace ahead of the catalytic stages. The third process reacts oxygen directly with the H2S in the first catalytic reactor to produce the SO2.
A “straight-through” unit (shown in Figure 4) passes all the acid gas through the combustion burner and reaction furnace. The initial free-flame reaction usually converts more than half of the incoming sulfur to elemental sulfur. This reduces the amount which must be handled by the catalytic sections and thus leads to the highest overall sulfur recovery.
The amount of heat generated in the reaction depends on the amount of H2S available to the burner. With rich acid gas (60% - 100% H2S), the reaction heat keeps the flame temperature above 2200°F. When the gas is leaner, the flame temperature is reduced; the greater mass is heated to a lower temperature. If the temperature falls below a critical point, approximately
1800°F to 2000°F, the flame becomes unstable and cannot be maintained. This point is usually reached when the acid gas has an H2S content of 50% or less. The problem can be overcome, within limits, by preheating the acid gas and/or air before it enters the burner. However, the lower the H2S content, the higher the preheat requirement becomes; when the gas composition falls below about 40% H2S, this approach ceases to be practical.
The second method of SO2 production, known as the “split-flow” technique, is used to process leaner acid gases with 15% to 50% H2S content. In these units, at least one-third of the acid gas flows into the combustion burner and the balance usually bypasses the furnace entirely. Enough H2S is burned to provide the necessary 2:1 ratio of H2S to SO2 in the catalyst beds. The flame
temperature is kept above the minimum since the constant amount of heat supplied is absorbed by a lower mass of gas. The free-flame Claus reaction is reduced or eliminated entirely by this approach since little or no H2S is available to react in the furnace. This results in a slight reduction in the overall sulfur recovery.
Sulfur dioxide has numerous commercial applications which are based on its function as an acid, as an oxidizing or reducing agent, or as a catalyst. Sulfur dioxide is used in massive quantities in the production of sulfuric acid as a captive intermediate and in the paper and pulp industry. Moreover, sulfur is widely used in bleach, preservative, fumigant, and steeping agent for grain in food processing; extraction solvent or catalyst in the petroleum industry; intermediate for bleach production; flotation depressant for sulfide ores in the mining industry; and reducing agent in several industrial procedures.
Sulfur is used in the vulcanization of black rubber, as a fungicide and in black gunpowder. Most sulfur is, however, used in the production of sulfuric acid, which is perhaps the most important chemical manufactured by western civilizations. The most prominent application of sulfuric acid is in the manufacture of phosphoric acid, to make phosphates for fertilizers.
Mercaptans are a family member of organosulfur compounds. Some are added to natural gas supplies because of their distinctive smell so that gas leaks can be detected feasibly. Others are used in silver polish, and in the production of herbicides and pesticides.
Sulfites are used as preservatives for many foodstuffs and also bleaching paper. Many detergents and surfactants are sulfate derivatives. Calcium sulfate (Gypsum) is mined on the scale of more than 100 million tons each year to use in cement and plaster industries.
Different methods of sulfur recovery and tail gas cleanup are mentioned below:
- Straight-Through Claus
- Split-Flow Claus
- Direct Oxidation
- Acid Gas Enrichment
- Oxygen Enrichment
- Cold Bed Adsorption
- Shell Claus Off-Gas Treating (SCOT)
Catalytic Reaction Completes the Process
Any further conversion of the sulfur gases must be done by catalytic reaction. The gas is reheated by one of several means and is then introduced to the catalyst bed. The catalytic Claus reaction releases more energy and converts more than half of the remaining sulfur gases to sulfur vapor.
This vapor is condensed by generating low-pressure steam and is removed from the gas stream. The remaining gases are reheated and enter the next catalytic bed.
This cycle of reheating, catalytic conversion and sulfur condensation are repeated in two to four catalytic steps. A typical SRU has one free-flame reaction and three catalytic reaction stages. Each reaction step converts a smaller fraction of the remaining sulfur gases to sulfur vapor, but the combined effect of the entire unit is to reduce the hydrogen sulfide content to an acceptable level.
High Yields Plus Energy Claus sulfur plants can normally achieve high sulfur recovery efficiencies.
For lean acid gas streams, the recovery typically ranges from 93% for two-stage units (two catalytic reactor beds) up to 96% for three-stage units.
For richer acid gas streams, the recovery typically ranges from 95% for two-stage units up to 97% for three-stage units. Since the Claus reaction is an equilibrium reaction, complete H2S and SO2 conversion are not practical in a conventional Claus plant. The concentration of contaminants in the acid gas can also limit recovery. For facilities where higher sulfur recovery levels are required, the Claus plant is usually equipped with a tail gas cleanup unit to either extend the Claus reaction or capture the unconverted sulfur compounds and recycle them to the Claus plant.
All Claus SRU’s produce more heat energy as steam than they consume. This is particularly true for those plants equipped with waste heat boilers on the incinerator. The steam produced can be used for driving blowers or pumps, reboiler heat in the gas treating or sour water stripping (SWS) plants, heat tracing, or any of a number of other plants energy requirements.