Granular iron oxide-based H2S adsorbents
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The THIOPAQ biodesulfurization system uses nonhazardous, naturally occurring, and self-regulating bacteria to convert H2S in a gas stream to manageable solid elemental sulfur that can be used in a variety of agricultural applications or disposed of in a landfill.
The process removes H2S from low- and medium-pressure streams in a direct-treatment operation. The THIOPAQ system produces treated gas that typically meets an H2S outlet specification of 25 ppm or less.
The THIOPAQ biodesulfurization system converts sour landfill gas or biogas containing H2S into a sweetened gas stream. The process also includes extraction of elemental sulfur from the system.
To begin, a pH-controlled aqueous soda solution is sprayed into a contactor vessel containing plastic packing. The sour gas flows in a countercurrent direction within the vessel. The aqueous solution absorbs H2S from the sour gas stream, forming hydrogen sulfide ions (HS–). The sweetened gas stream exits the contactor with as much as 99% of the H2S removed. The contactor is the only place where H2S is present in the process other than in some gas inlet scrubbing vessels that are required upstream to better clean the gas for improved H2S removal.
The second part of the process is the conversion of the hydrogen sulfide ions that formed in the process solution. The conversion occurs when the solution is pumped into bioreactors where the haloalkaliphilic strains of the Thioalkalivibrio bacteria are stimulated by oxygen delivered by variable-speed air blowers. The bacteria oxidize and convert the sulfide ions into elemental sulfur. An enzyme produced during the biological conversion covers the sulfur particles, making them nonsticky or hydrophilic. This is unlike other liquid processes, which require chemicals for conversion and produce sticky or hydrophobic sulfur. Because sulfur particles are present throughout the process wherever the liquid solution exists, it follows that nonsticky sulfur particles lead to significantly less downtime for plant maintenance.
The process liquid that is home to the bacteria is regulated by a programmable logic controller (PLC), which monitors pH, conductivity, temperature, oxygen demand, and many other parameters. Should any of these parameters fall outside its operational range due to changes in gas flow rates, total sulfur loading, or both, the PLC automatically adjusts the parameter to ensure that the bacteria continue their highly efficient conversion cycles.
The process system is approximately 95% efficient in regenerating. Compared with typical caustic scrubber systems, only modest amounts of caustic addition are required to maintain system alkalinity and pH for absorption. No heating equipment is used for regeneration, and no catalyst is needed for conversions. This approach successfully reduces system cost. Small amounts of nutrients are added to the system to keep the bacteria in good health. After the one-time addition of bacteria to the system, the colony expands or contracts based on the feeding of H2S into the system. This enables the biological system to be self regulating, supporting significant process turndown if conditions demand.
The final step is the removal of the converted sulfur particles. Testing and experience have shown that compared with vacuum filters and filter presses, the decanting centrifuge is the most efficient and cost-effective method of removing sulfur particles. The supernatant liquid is returned to the system to minimize requirements for additional fluids.