Cut it out

StatoilHydro commissioned a new vapour recovery system at its Mongstad crude oil loading and storage terminal this past June. The captured emissions are processed and the hydrocarbon components are converted back to liquid form, not only cutting air pollution but also increasing the volume of oil available for export.

The Mongstad terminal, located near Bergen in western Norway, is jointly owned by StatoilHydro and Petoro, the state company responsible for commercial aspects related to the government’s direct involvement in petroleum activities on the Norwegian continental shelf. Crude oil is delivered to the terminal by shuttle tanker and pipeline, and it provides intermediate storage for more than one-third of the oil produced by StatoilHydro.

At any one time the terminal has a capacity to store 9.4m barrels (1.5m m³) of crude oil, which it retains in six rock caverns. Exports to customers in North America, Asia, and Europe are loaded from two jetties, each capable of accommodating tankers up to 380,000 dwt, along with a ship-to-ship jetty that can handle carriers of 440,000 dwt. In an average year the terminal loads 450 oil tankers.

As a result of the high crude oil loading rates, the vapour recovery system was designed to operate at a top rate of 36,000 m³ of vapour per hour and capture at least 80 per cent of the 11,500 tonnes of VOC emissions that the terminal had previously discharged into the atmosphere every year. The total cost of the plant was NKr 615m (€77m).

In addition to designing and supplying the vapour recovery units (VRU), Aker Cool Sorption A/S of Denmark provided consultancy relating to the VOC transport system. The VRUs employ conventional carbon bed adsorption technology; as vapour from the crude oil is fed into the adsorber vessel, it enters the carbon bed and the hydrocarbons are adsorbed on the surfaces or the carbon crumbs. Methane and pure air pass through the filter and are discharged to atmosphere.

At the point where the carbon bed becomes saturated with hydrocarbon, the vapour is diverted into a parallel adsorption vessel and the first vessel is regenerated using a process that reduces the pressure in the vessel. The freed hydrocarbon vapour is reconverted to a liquid by co-mingling it in a vertical adsorber vessel into which cold crude oil has been diverted from the pipeline loading the tanker. The original working adsorption vessel is then ready for re-use and placed on standby.

To handle the high volume of vapour being displaced during the ship’s loading, four sets of dual carbon bed adsorber vessels have been installed, taking an area of 40 metres by 38 metres, and rising to a height of 15 metres. Aker Cool Sorption also supplied the two separators, two adsorber vessels and crude oil feed pumps, valves, piping, and structural platforms. To prevent damage to the carbon bed by any hydrogen sulphide in the crude oil, Aker Cool Sorption installed inlet carbon-bed guard beds.

The plant start-up was within the timetable set by the Norwegian Pollution Control Authority (SFT), which has the responsibility to regulate the discharge of hazardous emissions. SFT currently requires that 95 per cent of non-methane VOC discharges from oil stored and loaded in the Norwegian territory are eliminated. The Norwegian oil and gas industry accounts for 39 per cent of national VOC emissions, of which 90 per cent are attributed to offshore crude oil loading.