Apr-2005

Enhancing SRUs with oxygen-enriched air

As clean fuels regulations become more stringent, refiners looking for new ways to boost the capacities of their existing sulphur-recovery units are turning to oxygen-enhancement technologies

Mukesh Mehta, CB&I TPA
Aaron Stryk, CB&I

Article Summary

Most refiners have now moved toward a higher standard of sulphur-recovery efficiency to comply with clean fuels regulations that impose a reduction of sulphur in refined products. While refiners have chosen various avenues and process solutions to address these sulphur cap requirements, reliance on the refinery’s previously installed sulphur complex has presented one of the most effective and cost-efficient solutions. Comprised of a Claus sulphur-recovery unit (SRU) and a tail gas-treating unit (TGTU), the sulphur complex processes both lean and rich hydrogen sulphide (H2S) bearing acid gas streams and ammonia-bearing sour water stripper off-gas streams to recover elemental sulphur.

However, the existing sulphur complex in many refineries may not provide enough process capacity to meet the current regulatory requirements. Current trends indicate that as crudes become increasingly heavier and contain higher concentrations of sulphur, many refineries will need at least two or more Claus units — also referred to as trains — to meet current environmental regulations. This has increased the throughput of many SRUs, which may lead to the SRU reaching its hydraulic limitation (maximum air) and more frequent shutdowns for maintenance. This can force the refinery to shut down its upstream operations unless there is another way to remove sulphur from the acid gas and off-gas streams.

To prevent unscheduled shutdowns of this nature, many refineries have a parallel Claus unit. If one of the trains needs to be taken off-stream for maintenance, the other can handle at least some of the additional throughput. However, in instances where the remaining SRU is unable to provide adequate sulphur-recovery capacity, the refinery may be forced to reduce its upstream operations substantially.

To ensure that a plant’s Claus unit complex can meet the demands for increased throughput, even in instances where one of the trains is not in service, a refiner can do one of two things: either install another SRU or retrofit a unit to provide oxygen enhancement to the Claus process. Since investment in any kind of sulphur-reduction technology is regulatory driven rather than revenue and profit driven, solutions that get the job done effectively for the least amount of money are generally preferred. Oxygen enhancement has become one of the most viable, cost-effective alternatives to installing a new SRU.

Claus combustion process
The classic Claus process has been the most common method for processing H2S-rich streams and recovering sulphur from both amine acid gas and sour water stripper gas. The Claus process can be broken down into two steps. In the first step, which is called the oxidation process, about one-third of the H2S contained in the feed is oxidised with oxygen from ambient air to create sulphur dioxide. The oxidation takes place in the burner/thermal reactor of the SRU. In the second step, which is the actual Claus reaction, the remaining H2S is reacted with the sulphur dioxide to form elemental sulphur. This step takes place in the thermal and catalytic reactor of the SRU. The reactions are listed below:
H2S + 1.5 O2 Æ SO2 + H20
2H2S + SO2 Æ 3/nSn +2H20

To obtain the oxygen needed for the combustion process, ambient air is supplied to the SRU burner via an air blower. Ambient air usually contains about 21 per cent oxygen; the balance is primarily nitrogen and some water vapour, both of which are inert in the combustion process. Since these inert components comprise nearly four times as much volume as the oxygen, the unit reaches its hydraulic limitation quickly once these components are introduced, which limits the sulphur-recovery capacity of the SRU.

Sulphur-recovery capacity becomes further constrained by other contaminants in the acid gas feeds, such as ammonia, hydrocarbons, cyanides and carbon-sulphur compounds. Since sour water stripper gas feeds contain more ammonia gas than ever before, it is now necessary for refiners to increase the amount of ambient air supplied to the SRU. This ensures that not only the necessary amount of H2S is burned to keep the plant compliant with environmental regulations, but also that the ammonia gas contained in the feed is properly destroyed. However, as more air is fed into the Claus unit, the amount of inerts (nitrogen and water) added to the system increases even more rapidly, leading to the hydraulic limitation of the unit and, eventually, to a need to increase the size of the unit or invest in another SRU.

The best way a refiner can avoid this situation is to use an oxygen-enriched air supply, which not only replaces the nitrogen in the ambient air, but also provides enough oxygen to destroy the contaminants in the feed while maintaining adequate oxidisation levels. In fact, by substituting pure oxygen for most or all of the ambient air, space is made available to increase the throughput of the acid gas feeds to the SRU, which, in turn, gives the system greater sulphur-recovery capacity.

Oxygen levels in the SRU
As previously stated, in an air-based combustion system oxygen typically comprises about 21 per cent of the ambient air, with nitrogen comprising the remaining 79 per cent when the air is dry (water vapour and other small inerts can displace some of the oxygen and nitrogen in humid conditions). By not contributing to the combustion process, nitrogen does nothing more than take up additional space, dilute the Claus reactants and reduce the overall sulphur-recovery process. Thus, in order to increase the capacity in the SRU to process more acid gas, it is first necessary to reduce the nitrogen.

There are various levels of oxygen enhancement that can be applied to increase the capacity of the SRU. Most technologies, however, are grouped into three categories. The first level of oxygen enrichment encapsulates those technologies that increase the oxygen level from 21–28 per cent (low-level oxygen combustion). The second level includes the technologies that raise the oxygen level up to 40 per cent (medium-level oxygen combustion), while the third level of oxygen combustion encompasses those technologies that increase the oxygen concentration to levels in excess of 40 per cent — in some cases, even 100 per cent. It is important to note that for every level increase in the oxygen concentration, certain plant modifications must be made, for both retrofitted SRUs and newly installed units.