Hydrogen sulfide extraction could stem crude storage losses

Cargo sweetening project

Bob Eden
Capcis

• The sweet cargo de-souring kit is designed to be housed in a small, pressurized tank with no moving parts in the storage vessel or tanker. [18,295 bytes]

In 1995, reports reached the Manchester, UK-based corrosion specialists Capcis of a sour crude cargo that had been shipped to Rio de Janeiro from Angola. There was nothing remarkable in that, but it was reported that on arrival, the hydrogen sulfide content had dropped to around 85% of its starting concentration.

Although corrosion was an outside candidate for the loss of the hydrogen sulfide, it became clear that the sour gas had actually been lost from the cargo by partitioning and venting. The inert gas (IG) anti-ignition blanket in the ullage space was acting as a carrier gas for hydrogen sulfide from the oil. Regular pressure releases from the cargo to atmosphere served to vent the hydrogen sulfide from the hold. In the process, volatile organic carbons (VOCs) were also lost, but that has always been a consequence of this mode of oil transport.

Capcis proposed to the industry that these observations could be used as the basis of an on-board processing plant. Deliberate forcing of the IG blanket through the crude, followed by scavenging of the carried hydrogen sulfide, trapping the VOC and then recirculating the IG back to the oil tanks, could solve the VOC problem and add value to the crude oil by sweetening it. What is more, the raw materials - exhaust gas as the IG blanket and air as the regenerant for the sweetening process - are free and can be readily harnessed. Given the loss in value the crude suffers when soured, removal of hydrogen sulfide would return some of the value to the oil by anything from $0.50 to $2.00/bbl.

"Sweet Cargo"

So the "sweet cargo" project was born. Sponsorship was secured through the UK's Department of Trade and Industry with operator funding from Petrobras and BP Oil and technical support from Axsia Serck Baker and British Maritime Technology. The project was launched in 1997 and is due for completion in early 1999.

At this stage of the project, the concept has been proven and efforts are being put into closer simulations of a moving cargo which will lose its hydrogen sulfide more readily than static simulations. In addition to long-haul tankers, the technology can be applied on any floating installation which has access to:

• Cheap supply of inert "exhaust" gas
• Oil storage for a week or more.

Offshore storage is a candidate.

Process details

The static simulation consists of a 3-meter tall column of oil, with sample points up its side. Nitrogen, rather than exhaust gas, is blown over (not through) the oil surface to simulate a forced convection as a modification of the real environment. Through-oil "sparging," although very efficient, is considered too demanding in terms of necessary modification of the ship for sea deployment. Indeed, the inefficiency of the more readily installed "blow over" method does not cause undue problems, since one thing the cargo often does have on its side is time.

The decreasing concentration of hydrogen sulfide over time is monitored against the blow-over flow rate and depth of oil. Through mathematical modeling, a predictive tool has been built that relates the starting hydrogen sulfide concentration to the journey time and the final hydrogen sulfide concentration. In the current set-up, a cargo containing 800 ppm hydrogen sulfide would sweeten over an eight-week period. This model will allow the operator to estimate the likely market value of the crude before the end of the journey.

A 4-meter moving column has now been built to assess the effects of motion on the de-souring process. It is expected that the de-souring time will drop and that higher starting concentrations of hydrogen sulfide can be handled within the journey time.

So what of the VOCs and the trapped hydrogen sulfide? Current thoughts, and economics, point to the return of the VOCs to the cargo. It is actually cheaper to return them to the crude oil than to store them separately. The scavenged hydrogen sulfide, however, is oxidized to sulfur. Again, this is returned to the cargo since storage and disposal are more expensive than a return.

This may appear to be a strange step - having gone to the trouble of removing the hydrogen sulfide. However, the hydrogen sulfide in a crude oil usually accounts for a small fraction of the total sulfur in the cargo. Typical sulfur contents are in the fraction of a percent to the low percent concentrations. Hydrogen sulfide, on the other hand, is usually present in the tens to hundreds of ppm concentration. Despite the disproportionate effect hydrogen sulfide has on the value of crude, the sulfur that comes from it has a negligible impact on the value when set against the total sulfur content.

The sweet cargo process is designed to reduce the environmental impact of oil transport. It is also designed to add value to the crude to increase the worth of the cargo. A three-week shuttle journey could generate US$412,500, enough perhaps to cover the costs of the first de-souring unit.

Given a successful completion of the current phase of the project, the team is looking to install a unit in the hold of a cargo carrier. Should the unit work well enough at sea, then the system will be commercialized for general operation.

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