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Micro-organisms that digest hydrogen sulphide and excrete elemental sulphur have proved to be a cost-effective, high-performance alternative to conventional gas-treating technologies at the Salem oilfield of Citation Oil & Gas Corp. in Illinois, USA.

The independent oil and gas operator has licensed a biological gas desulphurisation process called THIOPAQ O&G from Paqell BV, a joint-venture company of Shell Global Solutions International BV and the technology’s originator, Paques BV. This environmentally friendly desulphurisation process uses naturally occurring bacteria, Thiobacillus spp., to remove hydrogen sulphide from sour gas streams. The resultant elemental sulphur can be used for fertiliser production, among other options.

Citation installed the technology at its Salem oilfield as part of a project to eliminate flaring at the wellhead and to improve air quality. The field contains 234 production wells, each of which generates a small amount of very rich, very sour casing-head gas that was previously being flared and causing up to 1,000 tonnes of sulphur dioxide to be emitted to the local atmosphere every year.

The operator has installed seven field compressors and 64 km of gas-gathering pipeline. This collects, on average, 700,000 standard cubic feet of gas per day and delivers it to a new gasprocessing plant that uses THIOPAQ O&G technology to remove hydrogen sulphide and convert it into elemental sulphur.

Hans Wijnbelt, Business Development Manager, Paqell, explains that Citation could have chosen conventional gas-treating technologies, but says that THIOPAQ O&G is typically much less capital intensive. “Traditional processing line-ups for sour gas include an amine unit, a sulphur recovery unit, a tail-gas treating unit, an incinerator and a degasser. All Citation needed to install was a THIOPAQ O&G plant, which substantially reduced the capital required.”

This is because the process needs less equipment than conventional desulphurisation processes. It is a simple process that comprises an absorption section, an optional flash vessel, a reactor section and a sulphur recovery section. Moreover, costly equipment items such as burners and reboilers are unnecessary.

Wijnbelt continues, “You might expect capital cost savings to be offset by high running costs, but THIOPAQ O&G has advantages here too because the expensive chemicals required for liquid redox processes are unnecessary; only sodium hydroxide and nutrients are needed.

“There is no compromise on performance or operability either. More than 99.9% of the hydrogen sulphide can be removed,” he adds. Performance data from the plant confirm this. The feed gas to the Salem plant contains 4% hydrogen sulphide by volume, whereas the treated gas contains just 0–2 ppmv. Moreover, minimal human intervention is required: the Salem site is unmanned 66% of the time and plant availability is over 99%. In addition, the plant exhibits little or no foaming and no corrosion has been detected.

Another key feature of the technology is its safety. It operates at ambient temperature, there is no free hydrogen sulphide after the bioreactor, and no fired or highpressure equipment is required. This last point, Wijnbelt says, has stimulated a lot of interest from designers of floating production, storage and offloading vessels.

To date, however, it has been applied in natural gas, acid gas, associated gas and refineries around the world. Wijnbelt suggests that the technology could also be applied to shale gas, liquefied natural gas and domestic gas projects, and in ventures that produce synthetic gas, such as coal gasification projects.

Citation’s use of the technology to desulphurise casing-head gas has certainly been successful. Bobby Shufeldt, Plant Manager, Citation Oil & Gas, says, “Knowing what I do now, I would definitely choose THIOPAQ O&G again. In fact, I recommended THIOPAQ O&G to another company in the area.”

With more than 120 THIOPAQ installations operating worldwide in various industries, this technology has proven its effectiveness. In the oil and gas sector, eight commercial reference units have been commissioned and a further seven units are at the start-up, construction or design phase.

Technology Development Timeline

1993 First commercial biological desulphurisation unit starts up in the Netherlands to treat biogas in the water industry.

1997 Paques and Shell Global Solutions form an alliance to develop the technology for the oil and gas industry.

2002 First commercial unit for a natural gas application built in Canada.

2007 THIOPAQ O&G receives the Sellafield Award for Engineering Excellence at the Institution for Chemical Engineers’ Innovation and Excellence Awards.

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