Controlling Volatile Organic Compound (VOC) emissions, resulting from the storage of petrol and its distribution through vapour recovery systems, such as those employing carbon adsorption, was cemented into legislation in 1996. As most units are now 16 to 25 years old, owners are likely to be looking to upgrade or decommission them.
However, with the need to diversify your energy infrastructure in the race to net-zero, it can be difficult to justify the cost of managing your aging hydrocarbon infrastructure. In this article, Jeff Robson from Flotech, an Adler and Allan company, discusses how to effectively manage your existing hydrocarbon vapour recovery systems safely and compliantly; until such times as they become obsolete while avoiding costly replacements.
It was estimated that in 1988, the evaporation of hydrocarbon vapour during the transportation of petrol from refineries to service stations resulted in 128,000 tonnes of VOC emissions in the UK alone. As a result of this EU Directive 94/63/EC, and under the BAT conclusions for the refining of mineral oil and gas under Directive 2010/75/EU, controls were placed on VOC emissions resulting from the storage of petrol and its distribution from terminals to filling stations.
The guidelines have been revised, to incorporate good practice learnt during the implementation of EU Directive 94/63/EC, both in the UK, and elsewhere in Europe.
How vapour recovery units work
A basic Vapour Recovery Units (VRU) system comprises a pressure swing absorption process with two activated carbon bed vessels, alternating on a 15-minute time cycle.
Adsorption, is the process where the hydrocarbons in the air form a weak bond and form a liquid on the surface of the activated carbon, allowing the air to pass through the bed to atmosphere. The bond formed becomes stronger the larger the molecule, so high carbon number hydrocarbons are more strongly adsorbed. Because the carbon has a very high surface area (in the region of 1250m2/g before pre-conditioning) it has a very high working capacity for the vapours and, because the gasoline is forming a liquid, the energy is released in the form of heat. Hence the carbon temperature increases.
Regeneration of the carbon reactors is accomplished primarily by vessel evacuation, with a vacuum pump to a pressure level of around 80mBar(a) (approximately -0.92 Bar(g)). As the pressure falls, the hydrocarbons will begin to boil, taking the heat from the carbon, and be removed in the vapour stream through the pump. Not all hydrocarbons, particularly the small amount of heavier molecules (high Carbon numbers) will be removed with vacuum alone, so a small amount of ambient air purge is admitted into the bed during the last 25% of the regeneration cycle. The changes in partial pressures help to break the stronger adsorption bonds, to fully clean the carbon.
The desorbed hydrocarbons are recovered by absorption in a packed absorber tower using a counter flow of gasoline absorbent pumped from the supply tank. This gasoline is also used as a heat exchange fluid for removing compression heat from the vacuum pump(s).
All product recovered by the system is pumped back to the absorbent supply tank mentioned. The enriched recovered liquid product is allowed to disperse throughout the supply tank environment.
Many different types of VRUs
There were a number of Vapour Recovery Units (VRUs) in use in the UK before the legislation came into effect, however the first units installed to comply with the new legislation began to be built in 1996, with further requirements by 2000 and then finally by 2004 for smaller terminals. By far the majority of installations in the UK are carbon adsorption units. The technology required is quite simple, however to meet the legislation, they had to be reliable, while operating for many motor running hours and valve operations per day. For example, a typical unit with a 15-minute cycle which is required to operate for 10 hours per day will have valves operating nearly 15,000 times a year.
After around 25 Years of controlling VOC emissions resulting from the storage of petrol and its distribution, a lot has been learned about maintaining and improving efficiency.
What are the risks?
In most cases there is very little redundancy of equipment, meaning any defect can potentially result in a loss of recovered product and failure to meet legislation. For owners and operators, this means they can be susceptible to potentially dangerous faults if not operated correctly. Also as units become older, equipment becomes obsolete, and replacement parts are not always compatible. This is a particular issue for units built before ATEX.
Current regulations for emissions at 35g/Nm3, Non Methane VOC’s are currently easily achievable, and in the UK we have been policed fairly rigorously to achieve them. Legislation will only get tougher, and it is important to be ready to do what is needed.
Legislation does not give freedom to continue to operate in the event of breakdowns, so it is essential the availability of spares is continually monitored. Where equipment begins to become obsolete, then preparations and plans should be considered for methods of overcoming the changes.
What the guidelines state
The Energy Institute (EI) Working Group for Vapour Recovery set out the ‘Guidelines for the Design and Operation of Petrol Vapour Emission Controls at Distribution Terminals’ and regularly update them. This document focuses on key points covering initial design of vapour containment systems and VRUs, as well as system maintenance techniques, safety considerations, and testing procedures.
The document is a good source of information, not just during initial design, but also when carrying out modifications or upgrading existing systems, to ensure all safety and legal aspects are reviewed and considered. It can also be a good reference when carrying out HazOp reviews.
Maintaining an efficient VRU
VRU’s are complex mechanical and electrical systems which operate in hazardous atmospheres. Failure to service these units correctly can lead to financial inefficiencies, operational issues, safety complications and environmental emission problems.
The benefits of maintaining an efficient VRU are reduction in VOC’s, improved safety through reduced risk of flammable atmosphere and reduced losses of valuable product vapour. They are also very economic to run. Testing in 2000, showed that the cost of recovering gasoline was in the region of 1.5p/litre, and at current prices this is around 4p/litre. Typical recovery rates over a year for an efficient VRU are in the region of 1.5 litres per 1000 litres of gasoline loaded (0.15% gasoline throughput).
Maintenance can take multiple forms from preventative maintenance, gas analyser and instrument calibration, system operation checks as per OEM data on site repairs and replacement and performance testing including compliance testing, HM Revenue and Customs tests and capacity testing, carbon replacement and programmable logic controller replacement and modifications.
Upgrading the carbon
The working life of the carbon can expire after 10 years, although with good maintenance and operation, in excess of 20 years has been achieved. Replacement of carbon is an expensive operation, which also requires the VRU to be out of service for several days. The EI Working Group carried out some early research into suitable types of carbon and conditioning procedures, as well as looking at ways to test carbon for remaining operational life. Understanding the remaining lifespan of your carbon can help you to develop a strategy for migrating to new energy infrastructures, reinvesting the expense associated with carbon replacement with new technologies.
The biofuel conundrum
VRUs are associated with traditional hydrocarbon energy and will no longer be required when these fuels are no longer in use. However, there is consideration to be made with biofuel blends as we transition through greener fuel types, the process in the VRU will be required to be altered to account for varying biological elements in the fuel.
Currently the existing VRU’s are expected to be able to continue operations with up to E10 blends. E5 has been in use for some time with very little impact on the systems, and E10 is beginning to roll out. Older VRU’s employing liquid ring vacuum pumps, with Mono Ethylene Seal systems, are expected to be the first concern when E15 comes in. Mainly this is to do with the ability of the seal fluid to separate from gasoline, but there are also more complex issues around vapour pressures above 10% Ethanol as well.
Being prepared for these problems in advance will help make transitions much easier and smoother.
Balancing the Race to Zero with managing an ageing infrastructure
There are a lot of uncertainties with net-zero, but what we do know is that the world won’t change overnight. Businesses are trying to keep legacy infrastructure going to see them through to new technologies. VRUs for hydrocarbon fuel infrastructures are no exception. You need a partner that can help you maintain the units that you have safely and compliantly, while supporting you to build a strategy for transitioning to new technology with the least cost and operational impact for your unique decarbonisation journey.