VOC recovery Systems (2013)

Topic last reviewed: 10 April 2013
Sectors: Downstream, Midstream, Upstream

Volatile organic compounds (VOCs), except methane, are called nmVOCs. Such compounds evaporate from crude oil. Storage, loading and unloading of oil offshore, in storage ships (FSOs and FPSOs), in onshore storage tanks and terminals, and on shuttle tankers, contribute significant emissions of nmVOCs.

It is possible to install VOC recovery units on each of these applications to capture and recover nmVOCs. On storage ships, the VOC recovery systems can reduce nmVOC emissions by more than 90%.

There are two generic approaches to VOC recovery, known as ‘active’ and ‘passive’ VOC recovery technology. Active vapour recovery unit (VRU) systems typically include a compression step followed by condensation, absorption and/or adsorption. Passive VRU systems use vapour-balanced loading/unloading with nmVOC as blanket gas for storage vessels.

Active technology for VOC recovery

Active VRU technology consists of process equipment that is designed to capture nmVOC evaporation from the crude oil storage, loading and unloading operations for beneficial use. The required compression for these systems can lead to some increase in emissions of CO₂ and NO_X due to the energy demands of the compressor.

Compression-condensation

The advantage with compression-condensation technology is that nmVOC emissions can be avoided because condensed nmVOC is stored in a separate tank. The technology provides further opportunity to use associated gas in vapour boilers and vapour turbines for beneficial use such as running compressors. The vapour boiler may be designed with a multi-fuel burner capable of burning nmVOC gases that are not condensed (ca. 20%) in addition to associated gas (methane) and condensed liquids, so that the nmVOC emissions are reduced by nearly 100%.

Absorption

Emissions of nmVOCs can also be recovered by absorption in pressurized crude oil (8–11 bar) or chilled liquid. However, this system can lead to some increase in emissions of CO₂ and NO_X due to the energy demand for pressurizing the oil in addition to compression of the nmVOC and inert gas. The typical energy demand for a plant used for loading 8,000 m³/h is 2–3 MW. These types of plants can reduce nmVOC emissions by at least 80%. Some methane will also be absorbed. For chilled liquid absorption, methanol is injected to prevent water in the vapour from freezing at reduced temperatures. A typical process flow diagram for chilled liquid absorption is shown below.

Adsorption

The main principle for an adsorption system is to separate fractions of hydrocarbons from inert gas. Several technologies are commercially available. One type of adsorption system that is used on shuttle tankers transporting oil from the Heidrun field, and on floating storage and offloading units (FSOs), for example Åsgard C and Volve in the North Sea (Norway), uses an active coal filter to separate nmVOCs from inert gas. Other technologies include carbon bed adsorption. Both use activated carbon beds, which require periodic desorption (vacuum regeneration) and have an expected service life of 7–10 years (Reference 6). The hydrocarbons (with the exception of methane) will then be recycled back to the oil with help from the adsorption plant. The reduction of the emissions of nmVOC is ca. 90%. The system is of special interest for gases with low fractions of hydrocarbons or low loading rates, where the oil produced is passed directly over to combined storage/shuttle tankers or FSOs. The system is not suitable for ordinary shuttle tankers due to the large size of the plants.

Vapour recovery units

Vapour recovery units (VRUs) can be installed on onshore oil storage tanks to recover tank nmVOC emissions. Hydrocarbon vapours from the tank are drawn out under low pressure (4 oz to 2 psi) and first piped to a separator suction scrubber (knock-out pot) to separate any liquids condensed in the piping network. From the knock-out pot/scrubber, vapours flow through a compressor that provides the low-pressure suction for the system. Compressors are generally electric. The required electricity generation capacity for remote locations may contribute to combustion emissions. VRUs are equipped with a control pilot to prevent the creation of a vacuum in the top of the tank. Vapours are then metered and removed from the system for pipeline sale or onsite fuel supply. VRUs are capable of recovering more than 95% of the hydrocarbon vapours from tanks.

Hydrocarbon gas as blanket gas

On floating production, storage and offloading (FPSO) vessels, nmVOC emissions can be reduced by using hydrocarbon gas as blanket gas, instead of inert gas, and integrating this solution with the existing production plant for oil and gas. When an inert gas is used as the blanket gas, the hydrocarbons from the storage vessel get mixed with the inert gas (up to 50–70% by volume) by the end of the loading period; therefore, the hydrocarbons are also vented to the atmosphere with the inert gas. By using hydrocarbon gas as the blanket gas in combination with a vapour recovery system, this venting of hydrocarbons to the atmosphere can be eliminated. During offloading of the vessel, the hydrocarbon gas is taken from the production process into the storage portion of the vessel to fill the vapour space as a blanket gas. During normal oil production, the hydrocarbon gas is recovered back for export or reinjection. Emissions reductions of 100% can potentially be achieved, and regularity is above 95%. Such solutions are used on FPSOs operating in the North Sea in Norway, such as Norne, Åsgard A and Skarv.

Passive technology for VOC recovery

Active technologies tend to be large, complicated and costly to install and operate. Passive technology solutions have therefore been developed as an alternative to active technology.

KVOC technology

One such passive technology is the KVOC® system, developed by Knutsen OAS Shipping AS. The KVOC® technology can be used to reduce the nmVOC emissions that evaporate from loading of the crude oil. One of the key features of this technology is a new drop line design that prevents the flashing generated by the conventional drop line since the pressure is kept at the oil’s true vapour pressure or higher during the entire loading period. This technology is simple and significantly less expensive to install than active technologies. No operational mitigation measures or costs are required. Passive VOC recovery can reduce emissions of nmVOC by 50%, with an operational regularity of 100%. No energy demand is required to operate the passive VOC recovery plants, hence emissions of CO₂, NO_X and SO_X for power generation can be avoided. Operational safety is the same as for ships without VOC recovery units. Other passive technologies include vapour balanced loading and the use of nmVOCs as a blanket gas for storage tanks. Compared to active technology, passive technology is simpler and has much lower emissions related to construction and installation.

Technology maturity

Key metrics

Decision drivers

Operational issues/risks

Active system operational issues: Active technologies are complicated and can be challenging to operate properly. Operational regularity for three absorption systems in the Norwegian VOC industry cooperative (VOCIC) was 96%; for eight condensation systems, operational regularity was 92% in 2011.

Active system risks: Safety challenges vary according to the technology implemented. Condensation with boiler control presents risks associated with the storage of VOC, and an additional ignition source. Chilled liquid absorption has the risks associated with the recovery of the absorbent through distillation, and the storage of methanol for antifreeze. Adsorption of hydrocarbons onto activated carbon is exothermic and can lead to hot spots and fires in the carbon bed if not properly managed.

Passive system operational issues: Operation of KVOC® systems is similar to conventional drop lines.

Passive system risks: Safety risks are similar to those on ships without KVOC® systems.

Opportunities/business case

Opportunities:

• Increased amount of crude oil to market
• Recovered vapour

Industry case studies

Active VOC recovery: offshore loading, Norwegian Continental Shelf (Reference 1)

In 2011, 18 facilities (both passive and active) were operating under the VOC industry cooperative (VOCIC) to reduce nmVOC emissions. The cost of nmVOC reductions for VOCIC facilities was about 200 million NOK in 2011. Of this total cost, about 125 million NOK was for capital investment while the remaining 75 million NOK was for net operating expenses. Some VOCIC shuttle tankers use active VOC recovery systems to achieve nmVOC reductions. The table below summarizes the reductions achieved by each ship.

Table 1: Achieved nmVOC reduction by Norwegian shuttle tankers using active VOC recovery systems
(Based on Tables 1 and 5 from Reference 1)

Passive VOC recovery offshore loading, Norwegian Continental Shelf (Ref. 1)

In 2011, 18 facilities (both passive and active) were operating under VOC industry cooperative (VOCIC) to reduce nmVOC emissions. The cost of nmVOC reductions for VOCIC facilities was about 200 million NOK in 2011. Of this total cost, about 125 million NOK was for capital investment while the remaining 75 million NOK was for net operating expenses. Typical unit abatement cost for vapour balancing on tankers and loading facilities is estimated at 40–300 ECU per tonne VOC (Reference 7). Some VOCIC shuttle tankers use passive VOC recovery systems to achieve nmVOC reductions. The table below summarizes the reductions achieved by each ship.

Table 2: Achieved nmVOC reduction by Norwegian shuttle tankers using passive VOC recovery systems
(Based on Tables 1 and 5 from Reference 1)

References:

1. Norwegian Petroleum Directorate (NPD) (2011). ‘Miljøteknologirapporten’ (Environmental technology report). Published March 2011, Norwegian version only.
2. Knutsen OAS Shipping AS (website).
3. Equinor (website): ‘The Mongstad facility’ (Norway).
4. Equinor (website): ‘Sture oil terminal’, (Norway).
5. Hamworthy Gas Systems AS (2008). ‘VOC Recovery and Power Generating System: Gas Systems. Ref: HGS 3017 1208/2.
6. Shipley, S. (2011). Developing an effective crude oil vapor recovery system. In ‘Port Technology International’, Edition 49, pp 80–82.
7. Klimont, Z., Amann, M. and Cofala, J. (2000). ‘Estimating Costs for Controlling Emissions of Volatile Organic Compounds (VOC) from Stationary Sources in Europe’. IIASA Interim Report IR-00-051.
8. Rudd, H.J and Hill, N.A. (2001). ‘Measures to Reduce Emissions of VOCs during Loading and Unloading of Ships in the EU’. Report no. AEAT/ENV/R/0469 Issue 2. Produced for the European Commission DG Environment.
9. U.S. EPA (2006). ‘Lessons Learned from Natural Gas STAR Partners: Installing Vapor Recovery Units on Storage Tanks’. United States Environmental Protection Agency, October 2006.
10. Hartveit, B.E. (2010). Hamworthy Presentation on VOC and FGR recovery: Technology Briefing BG Group, 15 June 2010.