Continuing improvement through new technology offers the potential to meet such challenges with a cost-effective, easy-to-apply solution. Shell Global Solutions profile two projects that helped a TGT unit to improve its operability and lower operating costs while meeting tough emission requirements.

The Shell Claus off-gas treating (SCOT) process is the industry’s most widely selected tail gas clean up process with 250+ references. It can achieve sulphur recovery levels of up to 99.98%, thereby realising very low levels of sulphur dioxide (SO2) emissions. However, operators’ requirements are always changing. For instance, refiners are keen to process sourer crude slates, plants today often have to process highly contaminated and complex gases containing organic sulphur compounds, while SO2 emission regulations are tightening around the globe.

Consequently, many operators find that the TGT unit is a key constraint and are having to compromise on reliability, capacity, operating costs or type of feedstocks.

New technology steps up to the challenges

So, as part of its commitment to innovation and continuous improvement, Shell Global Solutions recently developed the SCOT ULTRA process, which offers a step change in the performance of the well-established SCOT process. It features Shell and Huntsman Corporation’s jointly developed highly-selective JEFFTREAT** ULTRA family of solvents, which can achieve deep decreases in hydrogen sulphide (H2S) emissions and improved selectivity for H2S over carbon dioxide (CO2) (see boxed text “About the solvents”).

In addition, it also features Criterion Catalysts & Technologies’ (Criterion)*** C-834 high-activity, low-temperature SCOT catalyst, which adds further value by increasing the destruction of organic sulphur compounds at low operating temperatures (see boxed text “About the catalyst”).

The new SCOT ULTRA process could, therefore, be particularly valuable for operators tasked with economically meeting stringent SO2 emission regulations, including World Bank standards ([150 mg/Nm3), or those that require deep removal of carbonyl sulphide and mercaptans. While the highly selective JEFFTREAT ULTRA family of solvents bring H2S levels down to extremely low values, the C-834 catalyst also destroys the carbonyl sulphide. This is where the integration of the catalyst with the amine brings the added value of the SCOT ULTRA process.

The key components that make up the SCOT ULTRA process have already been successfully deployed and operated commercially, and offer performance advantages in both green- (see boxed text “Greenfield applications”) and brownfield applications. In the latter, Shell can evaluate systems for brownfield conversion potential; a simple swap of solvent and/or a catalyst change can improve performance when the original plant design supports the operation of the technology.

By making this change, in addition to meeting more-stringent emission regulations and having enhanced destruction of organic sulphur compounds, gas plant operators can also benefit from lower operating costs. This is because the circulation rates can be reduced dramatically, which means much-lower energy requirements.

Meanwhile, in refining, many sites are keen to exploit the use of cheaper, opportunity crudes. However, these are often higher in sulphur, so some refineries would need to increase their sulphur handling capacity. For this scenario, debottlenecking the existing SCOT unit could require extensive hardware modifications. In contrast, the new SCOT ULTRA process offers a far more cost-effective opportunity to increase capacity without hardware changes.

In some cases TGT unit operation can influence the reliability of the upstream sulphur recovery unit (SRU). A common cause of this is a low flame temperature, which may result from high CO2 in the recycle to the SRU and can lead to fouling in the condensers. The SCOT ULTRA process, however, is designed to minimise the absorption of CO2, which helps to maximise the flame temperature and, therefore, improve contaminant destruction and reliability. This issue is discussed in Case Study 1, which follows.

About the solvents

The JEFFTREAT ULTRA family of solvents offers several advantages over conventional methyl diethanolamine (MDEA) or diisopropanolamine (DIPA) solvents in revamp applications.

For example, its higher selectivity means that it can achieve deeper H2S removal specifications. In Case Study 1 in the main article, for example, the H2S in the treated gas is just 39% of that in the base case. Not only that, the solvent circulation rate is also reduced to 57%, which can translate into reduced steam, cooling and power costs.

About the catalyst

C-834 catalyst is designed to provide exceptionally high activity in low-temperature operations. Running a TGT unit at a lower temperature gives operators the opportunity to prolong cycle length. They can also reduce energy consumption by using indirect heating instead of line burners.

Crucially, through its superior hydrolysis and hydrogenation performance, the catalyst offers increased destruction of the organic sulphur compounds (carbonyl sulphide and mercaptans) that gas projects are encountering increasingly often. For example, Case Study 2 in the main article shows that the customer could benefit from a 76% reduction in carbonyl sulphide content in the SCOT offgas compared with Criterion’s conventional low-temperature TGT catalyst.

In addition, the catalyst also offers a low pressure drop, which is a key parameter for TGT units: the catalyst bed represents the area with the largest pressure drop in a SCOT unit and there is often not a lot of excess pressure drop for these units.

Greenfield applications

Although this article’s focus is brownfield applications, the SCOT ULTRA process also offers advantages over other technologies in grassroots developments.

It offers lower capital costs compared with conventional SCOT technology. This is chiefly because the absorber column height can shorter and its improved performance at higher temperatures requires no chiller. The operating costs are also less, mainly because of the lower circulation rate, which cuts the system’s energy requirements. This impact is typically much higher for greenfield applications because the process design will be customized to best suit the solvent properties.

In addition, just like for brownfield applications, it enables exacting emission regulations to be met even when organic sulphur is present.

Meeting stringent H2S targets

A recent evaluation of the SCOT ULTRA process at a Shell refinery demonstrates that this simple solution can have a major impact on performance.

The site had suffered several trips in its SRU caused by a build-up of high pressure owing to the deposition of ammonium salts on the condenser tubes. Generally, operators target a flame temperature of at least 1,250°C in the main burner to ensure sufficient destruction of ammonia and to prevent the build-up of these salts. However, in this case, the flame temperature was consistently below this target by 100°C or more.

The site technologists identified the root cause: Like all SCOT systems, this unit uses a recycle (Figure 1). However, the solvent has high levels of CO2 co-absorption and recycling this back to the SRU quenches the flame temperature, which was what the site technologists observed.

To resolve this, the team evaluated various potential solutions. Short-term solutions, such as co-firing fuel gas, are possible but they come with negative consequences such as soot build-up and reduced throughput. Shell Global Solutions compared the current solvent, DIPA, with MDEA, which is the standard solvent for these applications, and with the highly selective JEFFTREAT ULTRA family of solvents used in the SCOT ULTRA process.

As shown in Figure 2, of the three solvents, only the solvent used in the SCOT ULTRA process could meet the H2S target within the specific parameters. Figure 2 also shows that the lowest solvent circulation rate would be with the SCOT ULTRA process; this is an important parameter, as it determines the system’s energy requirements. Key to this was the SCOT ULTRA process’s enhanced CO2 slip performance while maintaining high H2S removal (Figure 3).

A more detailed look at the comparative evaluations (Table 1) shows that the site has to over circulate the DIPA dramatically in order to manage the H2S emissions (Column 1). This is due to the solvent’s high co-absorption and means running the system too close to its hydraulic limit.

The MDEA solvent (Column 2) is a weaker base, and, although it would achieve improvements in CO2 slip, recycle flow and flame temperature, the H2S levels would remain very high.

The SCOT ULTRA process would achieve a substantial decrease in H2S levels (Column 3) because its more powerful solvent can maintain superior CO2 slip. In fact, as shown, significant solvent reductions could be made and the site could still achieve superior H2S absorption and CO2 slip.

Table 1: The overall results of the comparative evaluation.
DIPA (current operation) MDEA (at design flow) SCOT ULTRA (at reduced flow)
H2S in treated gas (% relative to current operation base case)1 100 218 39
Solvent circulation rate (% relative to current operation base case) 100 81 57
Recycle gas flow (% relative to current operation base case) 100 41 39
H2S in recycle gas flow (% relative to current operation base case) 100 216 235
CO2 slip, % 47 84 88
SRU 7 flame temperature, °C 1,180 1,255 1,265


Controlling H2S, destroying COS

A gas plant needed to meet extremely stringent emissions limits: its target was 35 mg/Nm3 of SO2 emissions, which equates to about 20 ppmv of total sulphur components from the SCOT absorber overhead.

To help achieve these stringent specifications, Shell Global Solutions proposed Shell’s high-pressure degassing technology to recycle the vent gas to the front of the SRU. In addition, because the tail gas to the SCOT unit was highly contaminated with high levels of carbonyl sulphide, it also deployed the SCOT ULTRA process.

Criterion’s C-834 catalyst therefore played an essential role in meeting its targeted emissions limits. Figure 4 shows the total sulphur emissions. Although both cases use the JEFFTREAT ULTRA family of solvents to bring the H2S levels down to an extremely low level, the C-834 catalyst also destroys the carbonyl sulphide to a sufficient level to achieve the specifications.

Conclusion: A new level of performance

As the industry continues to develop natural gas reserves that contain organic sulphur and the limits for SO2 emissions continue to tighten, gas plant operators face major challenges in meeting their environmental mandates cost-effectively and without compromising on reliability. The same is true for refiners, many of whom are bringing more sulphur into the facility by using crude oils containing elevated sulphur levels.

Fortunately, technology developers such as Shell Global Solutions continue to push the boundaries of what is achievable and to help operators to achieve their objectives. Whereas conventional SCOT achieves sulphur recovery levels of up to 99.98%, SCOT ULTRA is demonstrating a significantly higher capability. For instance, in Case Study 1 above, the evaluations show that the refiner could achieve a sulphur recovery level of 99.99%.

It can be concluded, therefore, that the SCOT ULTRA process provides TGT unit operators with an inexpensive solution for meeting stringent emission regulations, including World Bank standards.

Key takeaways

  • With the SCOT ULTRA process, Shell has unlocked a step change in conventional SCOT performance.
  • It can help operators to meet stringent emission regulations, even with highly challenging, highly contaminated feeds.
  • In most situations, no hardware changes are necessary. This is a low-cost solution requiring only a solvent swap and/or a new catalyst in the reactor. Even a water wash is unnecessary.
  • This technology is de-risked: it is an improvement of an existing process and has already been deployed commercially.

*SCOT and SCOT ULTRA are trademarks owned by the Shell group of companies.

** JEFFTREAT is a registered trademark of Huntsman Corporation or an affiliate thereof in one or more, but not all countries.

*** Criterion Catalysts & Technologies LP (Criterion) is a wholly owned affiliate of CRI/Criterion Inc. and an affiliate of the Shell Global Solutions network of companies

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