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World Bank standards for sulphur emissions are stringent. They specify that no more than 150 mg/Nm3 or about 53 ppmv of sulphur dioxide (SO2) must leave an incinerator (dry basis and no oxygen). This is typically equivalent to 35 ppmv of SO2 in the actual stack gas.

These limits or even tighter ones are increasingly being specified on projects around the world and are setting major challenges for operators of sulphur recovery processes. One of the most common sulphur recovery configurations uses the Shell sulphur degassing process to remove hydrogen sulphide (H2S) and polysulphides (H2Sx) from the liquid sulphur produced in a Claus unit. These sulphur compounds typically exit the degasser in the vent gas and are sent to an incinerator, but burning them increases SO2 emissions.

Now, however, Shell Global Solutions has developed a way to recycle the vent gas to the front end of the Claus unit. This small modification can substantially lower stack emissions, enable compliance with World Bank standards and enhance safety.

The conventional Shell sulphur degassing process is a well-established technology with more than 330 applications worldwide and operates at near atmospheric pressure. Traditionally, degassing took place in a concrete pit, but in recent years there has been a design shift away from the use of pits in favour of vessels.

Kees van den Brand, Senior Process Engineer, Gas Treating & Sulphur Processes, Shell Global Solutions, explains that this is an important development. “The vessel unlocked the solution,” he says. “The vessel means we can operate the process at a slightly elevated pressure of about 0.9 barg, which is just enough to recycle the vent gas from the degasser to the Claus unit.”

In principle, there is no difference in operation or effectiveness between degassing in a pit or in a vessel. However, a vessel is intrinsically safer because it can be designed for pressure containment. his approach also offers the additional flexibility of off-plot installation. The conventional  atmospheric configuration requires the degasser to be placed near the sulphur condensers to minimise the pressure drop in the rundown. In contrast, with the new pressurised lineup, the degasser vessel can be installed in another part of the Claus plot.

Shell carefully evaluated the effect of higher pressures on degassing performance. The tests confirmed that degassing is more effective at a slight overpressure. More oxygen can dissolve in the sulphur, which enhances the decomposition of H2Sx. Therefore, for the same efficiency, the residence time could be decreased or smaller units could be used.

“The pressurised Shell sulphur degassing process offers a safe, robust and cheap solution for achieving the required reductions in SO2 emissions,” says van den Brand. “The main modifications required for pressurised operations are replacing the below-ground concrete pit with a below- or above-ground vessel, if appropriate, recycling the degasser vent to the front end of the Claus unit, and installing compression systems.”

Van den Brand adds, “Refineries, gas plants and upstream facilities can all benefit, either in grassroots installations or retrofit situations, and Shell expects the pressurised degasser to become the default configuration in future projects that have highly stringent SO2 emission requirements.”

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