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The need to meet or exceed changing product specifications is of great concern at a time when the global economy is lagging and refiners are focusing on minimising costs.

This pressure, combined with decreasing margins and heavier and sourer crude feedstocks, is forcing refinery management and engineers to find new solutions and develop rigorous business and technology strategies.

Dealing with sulphur is a major issue for many refinery operators and is driven by the need to manufacture low-sulphur products while minimising plant emissions.

PDVSA, the national oil company of Venezuela, is working on two projects in the north of the country that have both received World Bank funding.

As a condition of World Bank involvement, the new facilities have to achieve a very demanding level of sulphur removal (99.98%) and meet the challenge of limiting atmospheric release of sulphur dioxide to levels below 150 mg/Nm3 (approximately 100 ppm).

The first project is an expansion at the El Palito refinery, which aims to double the refinery’s capacity, enable processing of heavy and extra-heavy crudes from the Orinoco Belt, and increase the production of clean fuel.

The second is at the Centro de Refinación de Paraguaná.

This refinery complex is an amalgamation of the Amuay, Bajo Grande and Cardón refineries, and accounts for more than 70% of the available refining capacity in Venezuela.

Shell Global Solutions will contribute to the projects by helping PDVSA to optimise the mid- and deep-conversion complexes.

This is scheduled to involve adding two new hydrotreaters and two new deep-conversion units, including designing and deploying the most advanced Shell Global Solutions sulphur-removal technologies.

The two Shell licensed hydrotreating units at El Palito will deal with two distinct fractions: the heavy oil and diesel streams.

Hydrotreating removes impurities such as sulphur and nitrogen before processing begins.

Hydrotreating also produces hydrogen sulphide, which is sent to a sulphur recovery unit.

With the regulatory pressure to reduce emissions, effective sulphur recovery is a vital tool for optimising environmental performance in refinery operations.

In the sulphur recovery unit, hydrogen sulphide gas from the hydrotreating process is partly burned to form sulphur dioxide and water.

The sulphur dioxide reacts with more hydrogen sulphide to form sulphur and more water.

Two distinct streams emerge from the sulphur recovery unit: sulphur, which is cooled to form a liquid, and gases, which are sent for further catalytic conversion.

The liquid sulphur recovered from sour crudes contains dissolved hydrogen sulphide, which could be problematic if it were to leave the liquid sulphur and accumulate.

There would be the possibility of explosion or of the gas reaching a lethal (>10 ppm) concentration.

Consequently, the hydrogen sulphide content of the sulphur has to be reduced to a lower level, and this is  envisaged to be supported by using Shell Global Solutions’ degassing technology.

“PDVSA approached us for support in meeting the World Bank’s emissions target because we have global knowledge in deep sulphur recovery and technologies for hydrotreaters,” says Pankaj Desai, Licensing Sales Manager Americas, Shell Global Solutions.

“In addition, the World Bank limit for atmospheric release is less than 150 mg/Nm3 (approximately 100 ppm) compared with the United States Environmental Protection Agency limit of 250-ppm sulphur dioxide content.

This is a significant step change and a conventional Shell Claus off-gas treating (SCOT™) unit would typically be unable to meet this specification,” says Desai.

“As an alternative solution, we proposed Low-Sulphur SCOT (LS SCOT) technology as part of the sulphur recovery complex.

The complex at these refineries will consist of an integrated sulphur recovery unit, a tail gas unit, a sulphur degasser, an  acid gas removal unit and a water-stripping unit.

This is the most advanced sulphur-removal technology we can offer PDVSA to help it meet the tight World Bank limits.”

With the regulatory pressure to reduce emissions, effective sulphur recovery is a vital tool for optimising environmental performance in refinery operations.

Shell has provided high-level sulphur recovery unit designs for many years based on the SCOT process and has modified the design by considering increasingly stringent targets, as Desai explains.

“We have optimised the SCOT process to treat sulphur recovery unit tail gas to very low concentrations of hydrogen sulphide at the outlet of the amine absorber,” he says.

“Consequently, under certain favourable operating conditions and when permitted, the thermal oxidiser can be operated in a hot standby mode instead of in a continuous mode, thus saving fuel.

“In addition, to improve process economics,” Desai continues, “we have increased the process’s energy efficiency by introducing the new Low-Temperature SCOT (LT SCOT) reactor.

In this configuration, a low-temperature hydrogenation reactor typically enables us to replace the standard in-line burner/reduced gas generator with a simple steam-driven heat exchanger.

Consequently, fuel demand is likely to be reduced and costs to be lower.”

In many facilities, the hydrogen sulphide recovered by the degassing units is disposed of in an incinerator.

However, the lower limits that apply to the PDVSA projects mean that this seems not to be a possibility when the overall recovery efficiency has to be met.

Shell Global Solutions’ option involves degassing the sulphur and returning the hydrogen sulphide to the inlet of the recovery unit for a second cycle.

This approach is designed to help PDVSA to keep its atmospheric release of sulphur dioxide below the stringent 150-mg/Nm3 level.

This integrated tactic, coupled with the use of LS SCOT technology, is designed to help meet the necessary emissions levels and recovery targets.

Nestor Andara, El Palito Refinery Expansion Project Manager, PDVSA, says, “These technologies should enable PDVSA to process difficult crudes while meeting stringent emissions standards and minimising air pollution in the region.

Sulphur removal will make a key contribution to meeting our emission objectives at these facilities.”

Detailed engineering work is under way for the El Palito design, and, at Centro de Refinación de Paraguaná, Shell is working to deliver the basic design package for the same line-up.

“Sulphur recovery units play a crucial role in refinery designs: they help to maintain our licence to operate.

If our sulphur recovery units are not working, the refinery has to close down.

This makes it critical that the sulphur removal technology we choose has a strong track record for safe and reliable operations,”concludes Jose Vega, Clean Fuels Project Manager, Centro de Refinación de Paraguaná.

SCOT is a Shell trademark.

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