Carbonyl Sulfide (COS) is a sulfur-containing compound that is a major concern for the energy and chemical industries as it causes corrosion, damaging equipment, pipelines and catalysts. COS is commonly found in natural gas, refinery gases, synthesis gas, carbon dioxide (the feedstocks used in the food or beverage processes) and other industrial gas streams as well as liquid hydrocarbon products (e.g., LPG). In addition, removing COS is necessary to meet strict specifications for total sulfur content in end-products like natural gas, liquified petroleum gas (LPG), and chemical feedstocks.
- Hydrolysis is a common method where a catalyst (often based on activated alumina) is used to for converting COS into H2S and CO2. The resulting H2S is then removed in a separate acid gas removal unit.
COS + H2O = H2S + CO2
- Absorption or Extraction involves using alkanolamine solutions (amines) or caustic wash solutions to absorb and remove COS directly from gas or liquid streams. The performance of these methods is highly dependent on the specific amine or solvent used and the composition of the feedstock (e.g., presence of CO2)
- Adsorption uses solid adsorbents that directly binds the COS (either physically or chemically).
The removal of carbonyl sulfide (COS) is a critical step in a variety of industrial processes, primarily driven by the need to protect catalysts, prevent corrosion, and meet product purity and environmental specifications. The main processes requiring COS removal include:
- Natural Gas Processing and Industrial Gas Treatment
- Natural Gas Treatment: COS is a naturally occurring sulfur impurity in the natural gas that must be removed to prevent:
- Corrosion - In the presence of water, COS can hydrolyze to form highly corrosive hydrogen sulfide (H2S) and carbon dioxide (CO2).
- Damage to Equipment - COS, and the H2S it forms can damage equipment (e.g., compressors, valves) and pipelines.
- Synthesis Gas (Syngas) Production and Utilization: Syngas (a mixture of CO and H2) often contains COS due to the presence of high CO concentration in the gas.
- Catalyst Protection: In the chemical conversion processes, like the Fischer-Tropsch synthesis or hydrogen production, COS must be reduced to very low levels (often parts per billion by volume, ppbv) to prevent poisoning and deactivation of the catalysts.
- Syngas Clean-Up: COS is often converted to H2S via catalytic hydrolysis (COS + H2O ⇌ H2S + COS), as H2S is more efficiently removed by subsequent acid gas removal (AGR) units or using zinc oxide (ZnO)-based chemical sorbents
- Refinery and Petrochemical Gas Streams: Most of the refinery off-gases (e.g., Fuel Gas Streams) need desulfurization to meet combustion and environmental standards.
- Polymer and Olefin Production
- Olefin Polymerization:
- Catalyst Poisoning - High-performance polymerization catalysts (used to make plastics like polypropylene) are extremely sensitive to trace sulfur compounds, especially COS. The product specifications for COS in polymer-grade propylene are often in the parts per billion (ppb) range.
- Hydrocarbon Streams (Propylene, LPG, NGL):
- In Fluid Catalytic Cracking (FCC) Units, COS is produced during the cracking reaction; COS concentrates in the liquified petroleum gas (LPG), especially the propylene streams. Its removal is essential to meet product purity specifications for the valuable chemical feedstocks.
- Hydrotreating Processes - COS can inhibit the activity of catalysts used in selective hydrotreating and gasoline hydrotreating.
- Food-Grade and Industrial CO2 Production:
- Purification of CO2 Streams - To produce high-purity CO2 (e.g., for food and beverage applications), COS is considered a poisonous and harmful contaminant that must be removed.
- Olefin Polymerization:
- Natural Gas Treatment: COS is a naturally occurring sulfur impurity in the natural gas that must be removed to prevent:
Disposal of COS depends on the specific removal technology used in its removal. When wet scrubbing (amine/caustic treatment) is used, COS is absorbed by the chemical solution (like an amine or caustic). In an amine unit, it is then released to the sulfur recovery unit. The remaining COS may form heat stable salts within the amine system, which requires continuous management of the liquid waste stream. The COS physically removed by solid materials (such as the SulfaTrapTM sorbents), it is often disposed of as a waste product (in the case of one-time-use expendable materials).
High purity COS does have some niche commercial uses, though it’s typically produced for these applications rather than recovered from waste streams. High purity COS is used as a chemical intermediate in small-scale chemical syntheses, particularly in the manufacture of some herbicides (e.g., thiocarbamate herbicides). High purity COS has also been explored or used as an alternative to methyl bromide as a grain fumigant in the agricultural industry.