Progress in tail gas treatment (TIA)

The requirement to meet ultra-low SO2 emission standards, even lower than World Bank limits, is becoming more and more prevalent for new natural gas developments and/or certain regions.

Kirsten Gesink
Shell Global Solutions

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Article Summary

The requirement to meet ultra-low SO2 emission standards, even lower than World Bank limits, is becoming more and more prevalent for new natural gas developments and/or certain regions. At the same time, more complex, sour gas fields are being developed as sweet fields become depleted. Addressing these challenges while ensuring that increasingly stringent SO2 emission limits are met in the future requires technology capable of ultra-high levels of sulphur recovery, even when dealing with difficult gas streams. Next to meeting the increasingly stringent SO2 emission limits, proposed technology should also satisfy the requirement of lowest life cycle cost. In this respect, Shell Global Solutions is continuously developing and/or improving its portfolio of gas treatment technologies.

The company has utilised its knowledge and experience of designing and operating Shell Claus off-gas treating (SCOT) units since the 1970s to hone the technology for this purpose. The company has also extended its tail gas treating portfolio with the addition of the Shell Cansolv TGT+ technology and with the current development of the next generation SCOT. These technologies are able to meet stringent SO2 emission limits for a range of emission requirements (even better than World Bank limits).

Shell Cansolv TGT+ technology can be part of an integrated sour gas treating solution, enabling ultra-low SO2 emissions and the capture of 99.9+% of the overall sulphur present in sour gas streams while minimising the complexity of the process line-up. This system sits at the back end of the gas processing line-up and serves as the final destination to capture SO2 from all the plant’s sour gas streams routed to the incinerator, as illustrated in Figure 1.

Figure 1 shows the different off-gases routed to the Shell Cansolv unit via the incinerator from the acid gas enrichment unit (AGEU) and the degasser. These gases are combined with the flash gas from the acid gas removal unit (AGRU) and disulphide oils generated in caustic treating of extracted natural gas liquids (NGL) or condensate, following treatment in the molecular sieve unit (MSU). Without the Shell Cansolv technology, each of these individual streams would have required dedicated treatment or would have been emitted to atmosphere after incineration and thus contributed to higher levels of SO2 emissions.

The process centres on the Shell Cansolv SO2 Scrubbing System, which has been used worldwide to capture post-combustion SO2 and recycle it back to the thermal stage of a Claus sulphur recovery unit.

The unit is being introduced in several new projects across the Middle East and Europe, where SO2 emission regulations are very stringent. Some of the projects are now in the construction phase and due to start up soon.

The SCOT process uses catalytic conversion and amine absorption processes, which are familiar to plant operators, downstream of a Claus unit. The conventional SCOT system can achieve sulphur recovery levels of up to 99.98%.

Shell Global Solutions has licensed more than 300 SCOT units, which has enabled the team to build up a strong background in using tail gas treating as a means of meeting SO2 emission limits.

Based on the extensive knowledge in the area of tail gas treating, Shell is currently developing the next generation SCOT, aiming for a step change performance in sulphur recovery level of 99.9%+. Figure 2 represents the SCOT line-up and the potential benefits of the next generation of SCOT technology.

Key components of the next generation SCOT are:
1.    High-performance low temperature SCOT catalyst C-834 (developed by Criterion):
    a.    Superior hydrolysis and hydrogenation performance
    b.    Low temperature, which helps to prolong cycle length and reduce energy consumption by allowing the use of indirect heating, instead of line burner
    c.    Low pressure drop
2.    Highly selective solvent, capable of:
    a.    Deep H2S treating
    b.    Selective H2S removal
    c.    Relative to MDEA, the high selective solvent can:
    •    Achieve the same performance with reduced circulation
    •    Achieve superior performance with the same circulation
    •    Achieve the same performance with a shorter absorber height.

The high selectivity of the solvent relative to MDEA potentially brings the following benefits:
•    Better equilibrium (for the H2S reaction) and deeper H2S treating
•    Reduced circulation requirements and potential opex savings due to reduced regeneration energy consumption requirements
•    Improvement in SRU operation.
Deciding on which tail gas treating unit to use for a project comes down to the specific project requirements. The heart of the Shell Cansolv TGT+ system is an absorber and a regenerator, akin to the SCOT unit (see Figures 1 and 2). 

CANSOLV and SCOT are marks of Shell. 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.

This short case study originally appeared in PTQ's Technology In Action feature - Q4 2016 issue.

For more information: Kirsten.Gesink@shell.com

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