Recovery of ammonia from sour gases and conversion to valuable products
Sour gases generated from a two-stage sour water stripping unit (SWSU) comprise mainly two types of waste stream: an H₂S-rich stream and an NH₃-rich stream (H₂S lean).
Saptarshi Paul, Balaji Lakavath, V Kamesh JayantitI
Engineers India Ltd
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Conventionally, the H₂S-rich stream is processed in the Claus section of a sulphur recovery unit (SRU) to recover elemental sulphur. However, treating the NH₃-rich stream for the destruction of NH₃ has two routes, one of which is co-processing with the H₂S-rich stream in the main burner of the SRU, which often causes severe operational issues such as corrosion and choking. The second is processing through a reduction furnace/incinerator (i.e. Claus section bypass), which is associated with high Capex and occasional high NOx generation (environmental issues).
Recent trends indicate that ammonia demand in India is increasing every year, whereas historical data recorded the destruction of ~41,891 MTPA of NH₃ through the aforementioned routes in Indian refineries. Further, the increasing trend of bio-refining also needs an aqueous ammonia solution (20-30 wt%) for pH control of the hydrolysis reactor. Apart from the production of urea, various other applications of anhydrous ammonia include the manufacture of nitric acid, liquor ammonia, and the manufacture of explosives.
To overcome the challenges of treating an NH₃-rich waste stream and convert the same into a revenue-generating stream (waste to valuable product), EIL has developed an ammonia recovery process (Indian Patent no. 350771) to recover ammonia from NH₃-rich sour gases and convert it into valuable products such as anhydrous NH₃ or aqueous NH₃.
A brief schematic of the process is presented in Figure 1. The NH₃-rich sour gas and stripped sour water from the two-stage SWSU are the feed streams for the ammonia recovery process. H₂S content in the feed gas is minimised in two steps: water wash and caustic wash. The gas is suitably pressurised and cooled based on downstream requirements, and it can then be routed as product or further processed for the preparation of aqueous ammonia based on client requirements. The overall recovery of ammonia is >99%, while H₂S concentration in the product is <5 ppmw. The sour water generated from the process is routed to the SWSU and can be accommodated within the design margin of the SWSU or with minor hardware changes, if necessary.
Salient features and advantages of the ammonia recovery process are:
• Process eliminates operational issues related to ammonia-processing in the SRU and converts it into valuable product
• Process does not need any special chemicals/catalysts, and utilities required are already available in the refinery
• Acid gas processing capacity in the SRU can be increased
• Savings in the SRU can be achieved in terms of reduction in fuel gas consumption in the incinerator and power consumption for air blowers
• Impact on upstream two-stage SWSU can be accommodated within design margins or through minor hardware changes, if necessary.
This short article appeared in the 2022 Refining India Newspaper, which you can view HERE
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