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A step change in tail gas treating unit performance at low cost with SCOT ULTRA

Two projects helped a TGT unit to improve its operability and lower operating costs while meeting tough emission requirements.

John Specht, Shell Global Solutions International
Pat Holub, Huntsman Corporation
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Article Summary
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, gas 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).

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 .

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 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.

Box: 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.
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