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Nov-2017

Meeting emission targets with potential savings

New solvent and catalyst technologies deliver a step change in tail gas treating performance for green- and brownfield sites.

RIK SCHUFFELEN and LYDIA SINGOREDJO
Shell Global Solutions International
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Article Summary
Since its invention in the 1970s, Shell Claus off-gas treating (SCOT) technology has been applied more than 300 times to meet sulphur emissions targets worldwide. It has become a generic name for industry standard tail gas treating (TGT). However, to meet today’s challenging cost and (future) emissions targets, a step change in SCOT performance is required.

This challenge has driven the development of the SCOT Ultra process, which uses new solvent and catalyst technologies to deliver significant performance and cost benefits for green- and brownfield projects. For greenfield gas plants and refineries, lifecycle costs can be reduced by up to 50% with the new process relative to conventional setups. For a brownfield refinery focusing on reducing running costs, operational expenditure can be cut by up to 50% while meeting the same treated gas specifications. For the same setup, but targeting ultra low emissions, the process is tuned to achieve.

Green-and brownfield benefits
Compared with a currently applied combination of catalyst and amine solvent in TGT line-ups, existing refineries and gas plants can benefit from reduced operating costs, lower emissions, improved operability and increased capacity. Additionally, for new facilities, capital costs are expected to be lower owing to the smaller equipment size and reduced equipment count for the enhanced process.

Smaller plants
Smaller plants cost less to build. However, if there is substantial CO2 in the feed gas to the TGT, this results in larger treatment sections in two ways. First, it means higher CO2 co-absorption in the solvent, which results in significantly more solvent circulation to meet the required H2S removal. Secondly, as a consequence a larger amount of CO2 is recycled, which means that a larger SRU is also needed. More CO2 recycled to the SRU can potentially result in a quenching effect on the SRU main burner flame temperature, which can lead to partial combustion of benzene, toluene, ethylbenzene and xylene (BTEX), resulting in catalyst deactivation and reactor plugging. This in turn can lead to unscheduled plant shut-downs, shortened catalyst life and reduction of sulphur recovery efficiency may occur.

The new Jefftreat Ultra solvent addresses both these constraints. It has less CO2 co-absorption, thereby enabling less expensive, smaller greenfield SRUs to be built. The solvent further reduces greenfield capital costs through its improved selectivity at high temperatures (for instance, 60°C), which removes the need for solvent cooling in hot regions and results in a shorter absorber column.

Greater flexibility and fewer trips
Refiners may find their TGT unit is preventing them from taking advantage of high sulphur opportunity crudes. SCOT Ultra can help to create value by enabling greater feedstock flexibility. It can also improve unit reliability and capacity by debottlenecking the TFT unit without major hardware modifications.

With more sour crudes, varying feedstocks and tighter product specifications, refineries need robust TGT units. To protect its licence to operate, the solvent must remove H2S to prevent SO2 emissions from exceeding limits. However, air demand upsets in the SRU can increase the amount of H2S that the solvent must handle to keep the SO2 emissions within mandated limits.

Figure 2 shows pilot plant test results simulating an SRU air demand upset: the H2S level triples about 10 minutes into the test. The resultant H2S spike in the treated gas is much greater with generic methyl diethanolamine (MDEA) and formulated MDEA solvent compared with Jefftreat Ultra solvent.

H2S breakthrough, which ultimately leads to excessive SO2 emissions, is prevented by the higher capacity of Jefftreat Ultra, relative to MDEA. Regulatory authorities often require a rolling average SO2 concentration to be reported. The lower steady state and lesser peak concentrations help to keep this average low. This means fewer process upsets caused by difficult and changing feedstocks, and an increased opportunity to process these feedstocks to boost refinery economics.
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