Oct-2016

VOC recovery in crude oil loading

Facilities to reduce VOC emissions during loading are providing major environmental benefits driven by emissions standards and legislation.

GRANT JOHNSON and MIKE DODD
Costain

Article Summary

Volatile organic compounds (VOCs) are a collection 
of organic compounds displaying similar behaviour in the atmosphere, contributing to the formation of ground-level ozone and including components such as benzene that are directly hazardous to human health.

Emissions of VOCs have reduced significantly over the last 25 years. There has for example been a more than 50% reduction in non-methane VOCs (NMVOCs) in Europe following the adoption of national ceiling limits on emissions (see Figure 1).¹ The reduction has been primarily due to improvements in road transport, reducing both evaporative and exhaust emissions, and through legislative measures limiting the use and emissions of solvents in products.

VOC emissions from production and distribution of oil make up a relatively small proportion of overall global VOC emissions, but the local effects can be significant given for example the large volumes of gas displaced in marine loading of tankers, and the presence of benzene and other harmful substances. Vapour emissions control systems have already been installed at many ports and terminals to recover NMVOCs from loading of both crude oil and products such as naphtha and gasoline.²

Vapour evolution and VOC removal requirements
Proprietary simulation tools can be used to calculate the maximum hydrocarbon content (alpha value) of the vapour and also the maximum vapour rate, which depends on the oil composition (including ethane and propane content), loading rate, temperature and vapour pressure at the loading arm.

Where there are multiple loading berths, and a common vapour recovery unit (VRU) is considered, the maximum vapour rate will normally be based on co-incident loading, excluding any very improbable scenarios. A maximum vapour growth rate (the additional vapour volume rate compared with the oil loading rate) of 25% would be typical.

Ship loading will normally be completed in 12-24 hours, and rate of loading will be held steady for the majority of this time. Depending on the ship’s operational requirements, start-up and topping off will be at a reduced loading rate of say 20-30% of the normal rate, and for less than one hour. This determines the minimum turndown load on the VRU.

A hydrocarbon content in the displaced vapour of around 
30 mol% on a dry basis would be a typical maximum, mainly made up of C3-C6 hydrocarbons (see Figure 2), of which the benzene content may be greater than 1000 parts per million on a molar basis. Most crude carriers are purged with inert gas generated from the exhaust of the ship’s engines, and the inert content of the vapour can therefore contain more than 50 mol% nitrogen, with the balance made up of oxygen, carbon dioxide, carbon monoxide, water vapour, NOx and sulphur dioxide. Terminal regulations will normally limit the hydrogen sulphide, mercaptans and oxygen allowed in the vapour in the ship’s tanks at the start of loading. Particulate matter, such as rust, can also be carried with the vapour.

Selection of the technology and design of the vapour collection system and VRU needs to account for the effect of any contaminants present, considering the sensitivity of the process, and impact on equipment and materials of construction.

A typical VRU will have a target recovery of 85-95% of the NMVOC content of the displaced vapour, on top of which there may also be a focus on removing individual components, such as benzene, to a few parts per million depending on local requirements. Legislation in some jurisdictions dictates an absolute limit of NMVOC content in vented vapour.

Close to 1.9 billion tonnes (14 billion barrels) of crude oil were loaded into ships in 2015, displacing around 3 billion cu m of vapour. Each cubic metre can contain up to around 1 litre of valuable recoverable liquid. Although the drivers are primarily environmental, the vapour displaced from loading of a single very large crude carrier (VLCC) carrying 2 million barrels could yield just under 2000 barrels of recoverable oil, with a value at the time of writing of close to $100000. Depending on the cost of the site specific infrastructure needed for collection and recovery of the vapour, utilisation of the unit, operating costs, and other factors, it is conceivable that payback of around five years could be achieved.

Technology options
Various process technologies are available for reducing VOC emissions, the main ones being:
• Combustion (thermal oxidation)
• Condensation (refrigeration)
• Absorption in crude oil
• Membrane separation
• Absorption in cold liquid (lean oil)
• Pressure swing/vacuum adsorption (carbon vacuum adsorption).

Most solutions in marine loading applications are based on a combination of technologies, increasing the overall recovery and energy efficiency, as described below.

Combustion/thermal oxidation
The hydrocarbon content of the displaced vapour is oxidised to carbon dioxide and water, destroying the harmful components, and also reducing the global warming potential of the gas. Vapour combustion units can handle a wide range of vapour compositions and achieve destruction efficiencies greater than 98%.

Various types of combustion systems are available, ranging from simple enclosed flares to catalytic oxidisers with internal heat recovery. Combustion processes do not recover the valuable VOCs, and they produce carbon dioxide and other combustion products such as carbon monoxide and nitrous and sulphur oxides, emissions of which also need to be controlled to meet applicable environmental regulations and limits. Production of nitrous oxides is dependent on the design of the burner, and also the oxygen concentration at the burner, operating temperature, residence time, and the type of supplementary fuel used. Heat may be recovered from the combustion system if this can be used in the facility.

Combustion processes may be combined with other processes to allow recovery of the majority of the hydrocarbons from the displaced vapour, with only the balance combusted to achieve overall removal efficiencies of over 98%.

Condensation (refrigeration)
If the temperature of the displaced vapour is reduced sufficiently, the VOC content will condense, allowing separation from the vapour. Condensation is normally achieved by compressing the vapour and chilling against evaporating refrigerant (see Figure 3). To meet typical emissions targets by condensation alone requires cooling to very low temperatures, and therefore needs a complex refrigeration unit and heat exchange system. To avoid hydrates or freezing, dehydration of the vapour, for example using molecular sieve or injection of methanol, is normally needed prior to chilling.