SulfaTrap's COS Hydrolysis Process
SulfaTrap’s approach to managing carbonyl sulfide (COS) in process streams centers on a combination of advanced, proprietary catalyst and sorbent technologies.
We offer highly efficient methods for deep desulfurization.
SulfaTrapTM-HC is a highly active COS hydrolysis catalyst that can achieve a very high COS conversion rate at low temperatures. The catalyst maintains its activity in the presence of very low concentrations of water (only small amounts of water addition is needed above what is required by the reaction stoichiometry). The catalyst has been successfully applied, treating gas/liquid streams with high CO2 and H2S concentration (e.g., biogas, landfill gas), including COS removal from pure CO2.
Once the COS is converted into H2S, SulfaTrapTM-R7Q and SulfaTrapTM-R7J sorbents (depending on the moisture content of the gas) are used to remove the H2S, fully desulfurizing the gas.
In a typical application, the first step is to remove the bulk H2S in the gas using the SulfaTrapTM-R7Q sorbent. Depending on the size of the application, this could also be another bulk sulfur removal system (e.g., amine scrubbing, liquid scavenger or liquid redox). We then slightly elevate the temperature of the gas to achieve high kinetics in the hydrolysis reaction. A recuperating heat exchanger is used to recover some of the energy put into the process and reduce the heat load. Following the COS hydrolysis process, an H2S removal sorbent is used to ensure a high overall sulfur removal.
SulfaTrapTM also provides a series of highly specialized sorbents, such as the SulfaTrapTM-R5 Series sorbents, to directly capture and remove COS (along with CS2) from various feedstocks like natural gas, CO2, and LPG. Our sorbent-based process is often employed as a polishing step downstream of bulk sweetening units (like amine scrubbers) to achieve ultra-pure product specifications, consistently reducing total sulfur concentrations to single-digit parts-per-billion by volume (ppbv). The SulfaTrapTM-R5 Series sorbents can be used in expendable and regenerable manner (regenerable formats, allow for long operational life and economic performance by using a mild temperature swing to recover the captured sulfur).
COS hydrolysis is a chemical reaction where Carbonyl Sulfide (COS) reacts with water (H2O) to form Carbon Dioxide (CO2) and Hydrogen Sulfide (H2S). COS + H2O ⇌ CO2 + H2S
COS is often found as an impurity in natural gas, synthesis gas, liquefied petroleum gas (LPG) and other industrial feedstocks. It is the most abundant organic sulfur compound in the atmosphere. COS can cause corrosion and is a harmful pollutant.
The hydrolysis reaction is primarily used in industrial processes as a desulfurization step to remove the COS via conversion to H2S, which is much easier to remove via conventional gas treatment processes. COS hydrolysis proceeds through a complex catalytic mechanism that depends on temperature and catalyst type. The reaction is often carried out using a catalyst (like activated alumina or metal oxides) to speed up the process. Hydrolysis is generally a base-catalyzed reaction which is an exothermic reaction, releasing heat. At low to medium temperatures, the reaction mechanism involves the following key steps:
- Adsorption of water: Water molecules adsorb onto the basic active sites of the catalyst’s surface, often in the form of hydroxyl (-OH) groups.
- Adsorption of COS: Carbonyl sulfide molecules then adsorb onto the same catalytic sites, competing with water for adsorption. At low temperatures, this step slows the overall rate of conversion.
- Formation of intermediate: An intermediate species, hydrogen thiocarbonate (HSCO-2), is formed when the adsorbed COS reacts with the surface hydroxyl groups.
- Decomposition and desorption: The thiocarbonate intermediate decomposes, releasing the products H2S and CO2. The catalyst’s basic sites are then regenerated to repeat the cycle.
- Temperature: While low temperatures favor high COS conversion (due to the exothermic reaction), higher temperatures are needed to overcome kinetic limitations and achieve a high reaction rate.
- Water concentration: An appropriate amount of water is necessary for hydrolysis. However, at high concentrations or low temperatures, water can compete with COS for active sites and inhibit the reaction.
- Inhibiting impurities: Substances like carbon dioxide (CO2), and to a lesser extent hydrogen sulfide (H2S), can inhibit the reaction by competing for active sites. This competitive adsorption is a major challenge for industrial applications.
COS is released from a variety of industrial processes, either as an intended chemical intermediate, an impurity, or a byproduct. The major anthropogenic (human-caused) sources include fossil fuel processing and combustion, chemical manufacturing and other manufacturing and industrial activities.
Fossil Fuel Process and Combustion
- Natural Gas and Petroleum Refining: COS is a common sulfur-containing impurity in raw natural gas, refinery gases, and synthesis gas. It is released during the extraction, recovery, and processing of these fuels.
- Coal-Fired Power Plants and Combustion: The combustion of sulfur-containing fuels (like coal) for heat and electricity production generates and releases COS.
- Gasification and Reforming: The gasification processes (e.g., waste, biomass and coal) and reforming (e.g., steam methane reforming, autothermal reforming) generates a carbon monoxide (CO) rich reformate gas which promotes the formation of COS. Depending on the temperature and moisture, about 1/10th of all sulfur in the gas may be in the form of COS.
Chemical Manufacturing
- Carbon Disulfide (CS2) Production: COS is a significant byproduct in the manufacturing of carbon disulfide. Historically the industrial emission of CS2 (such as from the rayon industry) has been a major source, as CS2 rapidly oxidizes in the atmosphere to form COS.
- Sulfur Recovery Processes (Claus Process and Tail Gas Treatments): The Claus process is used to recover elemental sulfur from H2 COS is easily formed as a byproduct in the thermal stage of this process, especially if the tail gas is not properly treated.
- Intermediate Chemical Use: COS is used as an intermediate in the production of certain products, such as thiocarbamate herbicides.
Other Manufacturing and Industrial Activities
- Primary Aluminum Production: Direct COS emissions are associated with aluminum smelting.
- Pulp and Paper Industry: Emissions can occur from certain processes within the pulp and paper sector.
- Tire Wear and Automobiles: Emissions can be released from vehicles and the general wear and tear of tires.
Combustion of Biomass: Large-scale or industrial biomass burning also generates COS.
- Appearance: Colorless gas.
- Odor: Has a distinct, unpleasant sulfide or rotten egg odor when impure, but is odorless in its pure state.
- Flammability: Flammable, and can form explosive mixtures with air.
- Toxicity: Poisonous and narcotic in low concentrations.
- Reactivity: Stable in acidic solution but can react with bases to produce hydrogen sulfide and carbon dioxide.