Hydrogen sulfide cracking

Acid gases are mainly hydrogen sulfide (H2S) originating essentially from hydrotreating units off-gas. Smaller quantities are also produced in thermal and catalytic cracking units. 

Materials The damage that air pollutants can do to some materials is well known ozone in photochemical smog cracks rubber, weakens fabrics, and fades dyes hydrogen sulfide tarnishes silver smoke dirties laundry acid aerosols ruin nylon hose. Among the most important effects are discoloration, corrosion, the soiling of goods, and impairment of visibility. 

Cracking was caused by stress-corrosion cracking (see Chap. 9, Stress-Corrosion Cracking ) involving hydrogen sulfide and/or moist sulfur dioxide. The sulfur entered the cooling water stream through process leaks, which were repaired. 

In water solutions containing hydrogen sulfide, austenitic steels fail by stress corrosion cracking when they are quenched and tempered to high strength and hardness (above about Rockwell C24). 

Some of the most obvious examples of problems with gas and materials are frequently found in refining or petrochemical applications. One is the presence of hydrogen sulfide. Austenitic stainless steel, normally a premium material, cannot be used if chlorides are present due to intergranular corrosion and subsequent cracking problems. The material choice is influenced by hardness limitations as well as operating stresses that may limit certain perfonnance parameters. 

The delayed coking feed stream of residual oils from various upstream processes is first introduced to a fractionating tower where residual lighter materials are drawn off and the heavy ends are condensed. The heavy ends are removed and heated in a furnace to about 900 to 1,000 F and then fed to an insulated vessel called a coke drum where the coke is formed. When the coke drum is filled with product, the feed is switched to an empty parallel drum. Hot vapors from the coke drums, containing cracked lighter hydrocarbon products, hydrogen sulfide, and ammonia, are fed back to the fractionator where they can be treated in the sour gas treatment system or drawn off as intermediate products. 

Fluid catalytic cracking units present formidable emission control problems. Contaminants are present in both reactor product gas and regenerator flue gas. The reactor product contains hydrogen sulfide, ammonia, and cyanides, plus combined sulfur and nitrogen in the liquid products. Hydrogen sulfide, ammonia and cyanides are handled as part of the overall refinery waste water cleanup. The combined sulfur and nitrogen may be removed by hydrotreating. 

To prevent or reduce the possibilities of problems associated with hydrogen sulfide cracking the following measures should be considered [184] ... 

Solutions of hydrogen sulfide, because of hydrogen-induced cracking. Grades of steel are available for certain ranges of pH and hydrogen sulfide partial pressure. 

Aluminum and silicon bronzes are very popular in the process industries because they combine good strength with corrosion resistance. Copper-beryllium alloys offer the greatest strength and excellent corrosion resistance in seawater and are resistant to stress-corrosion cracking in hydrogen sulfide. 

Sulfur is removed from a number of refinery process off-gas streams (sour gas) to meet the sulfur oxide emissions limits of the Clean Air Act and to recover salable elemental sulfur. Process off-gas streams, or sour gas, from the coker, catalytic cracking unit, hydrotreating units, and hydroprocessing units can contain high concentrations of hydrogen sulfide mixed with light refinery fuel gases. 

The hydrogen sulfide and ammonia can be removed by amine extraction and acid washes respectively. Hydrotreating also removes metals from the feed that would otherwise poison the reforming and cracking catalysts. 

Hydrogen sulfide At low levels, hydrogen sulfide can inhibit aromatic ring saturation. This results in higher-octane gasoline and low-smoke-point jet fuel. At high concentrations, cracking catalyst activity is adversely affected. 

Hydrogen sulfide and mercaptans Sour crudes formed by decomposition of sulfur compounds during distillation, cracking, reforming, and hydroprocessing... 

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