Reducing VOC emissions during cargo loading
Norway has undertaken to reduce its VOC emissions to a maximum of 195,000t by 2010. This corresponds to a reduction of 37 per cent from 1990 levels. It has done so under its European Convention on Long-range Transboundary Air Pollution obligations. VOC emissions react with bright sunlight to create ground-level ozone, which can cause ill-health and distress, particularly during brief hot spells, as well as reducing agricultural and forestry production and causing damage to various types of materials.
Measured per person, Norwegian VOC emissions are among the highest in Europe. Norwegian VOC emissions rose by 35 per cent between 1989-1996. This was mainly because of a rise in the amount of crude oil transferred from installations in the North Sea. VOCs evaporate from crude oil as it is loaded onto tankers. This source accounts for more than half of all Norway’s VOC emissions.
The Norwegian Pollution Control Authority (SFT), part of the Ministry of Environment, has legislated that system’s be installed onboard offshore facilities operating on the Norwegian Continental Shelf that recover a minimum 78 per cent of VOCs. It set three deadlines -for 40 per cent of all offshore loading facilities and equipment to be fitted with a VOC recovery system by the end of March 2003, 70 per cent by the end of 2004 and 95 per cent by the end of 2005.
Navion, which is coordinating the installations as they pertain to shuttle tankers, is now in the process of handling tenders for the second stage. Six technologies were initially investigated and it has recently pre-qualified four of these to submit tenders later this year. This second stage involves five shuttle tanker installations.
The four pre-qualified technologies are systems developed by Hamworthy KSE, ABB Gas Technology, APL and Cool Sorption (a division of AkerKvaerner). The two that failed to pre-qualify are systems developed by Venturie and Knutsen OAS. Navion has given Venturie license to compete in the bidding process through its system operating as a supplement to one of the pre-qualified technologies.
Condensation systems
ExxonMobil has recently completed acceptance tests of a Hamworthy KSE VOC recovery system. The tests of the first-generation condensation system took place on the 127,466 dwt shuttle tanker
Stena Alexita
following installation in March. Contract to installation took about one year.
Hamworthy is now also pushing a second-generation system, for VOC recycling. This second-generation system is similar to the ExxonMobil installation in that VOCs emitted from crude oil during loading are condensed in a process plant and stored in separate tanks on the vessel deck. It differs in that it contains a system to regasify some of the condensated VOCs and return them to the cargo tanks during unloading to form a hydrocarbon blanket that maintains overpressure on the cargo and ensures a non-explosive atmosphere in the tanks.
Hamworthy’s business director for VOC systems, Tore Lunde, says that this VOC recycling process reduces the release of VOCs from the crude oil during transit by between 18-40 per cent, as inert gas, normally used to keep the tank atmosphere below explosion limits, creates vapour pressure differentials that encourages the release of VOC. This is in addition to the reduction during loading of crude oil.
The process module weighs about 125t, which compares to 160t for the first-generation system, which contains a second-stage condensing and cooling plant. However the storage tank requirement for the second-generation system is about 800m3, which compares to 500m3 for the first-generation one. The extra capacity is required to ensure adequate blanket gas can be created for the tanks.
Both systems are self-contained as far as energy requirements go. Surplus gas and methane are directed to a special incinerator/boiler where they are burnt to produce steam for operation of the plant. This burning means that some CO2 is emitted; Lunde reckons about 100t per loading. But the company claims this is one seventh that of competing systems.
Both systems are a development of a prototype system, weighing 230t, that has been in operation on the 130,700 dwt shuttle tanker Navion Viking
since March 2001.
ABB’s VOC recovery system is similar in principle to the second generation Hamworthy KSE system in that it is based on condensation and the creation of a hydrocarbon blanket gas to maintain overpressure on cargo and avoid explosive conditions in the tanks when offloading. Its principal difference is that it uses a membrane system to increase the VOC recovery rate.
The system works by condensing VOC vapours as they are emitted and recovering them to two storage tanks measuring about 22m x 4.5m each and placed longitudinally on deck. However the amount condensed is not always enough to create a blanket gas for a full cargo, for example when loading oils with a low flash point, so the system contains a circuit to pump some additional oil through it. Blanket gas is boiled out of this oil to top up any shortfall from the condensation system. The requirement is for 700-800m³ says Hans-Christian Paulsen, manager of the VOC recovery system project at ABB Gas Technology.
The system uses seawater to cool the blanket gas, which means it does not require a separate cooling circuit nor removal of water when cooling down. It requires about 2MW to power it, which is taken either from the ship’s existing auxiliary generating capacity or, if the spare power capacity is not already available, through an optional industrial gas turbine generator. This can burn surplus gas or diesel.
The main recovery unit is 18m long by 15m wide and is about 4m high including structure. Paulsen reckons that with pre-installation and post-installation work done in-service off-hire time required to fit the system components can be minimised to about one week. ABB Gas Technology has cooperated with ABB Offshore Systems in the development of the system, which is based on one that has some installations on FPSOs.
Absorption system
APL added a VOC recovery system to its product portfolio when it completed the purchase of Hitec Marine in September last year. Hitec Marine had in turn acquired the technology from Kvaerner Process Systems at a time when Kvaerner (pre-Aker) was selling a lot of non-core businesses and there was no certainty that VOC recovery systems would become a required standard on shuttle tankers.
The APL system comprises a VOC gas compressor, a VOC knock-out drum, an absorber column, a crude oil pump and a crude oil cooler. This is all contained within a modular unit weighing 230t and measuring about 10 x 20m. Following a pilot installation on the shuttle tanker Anna Knutsen in 1998, five installations took place in 2002 and 2003. Two of these utilised spare power on the vessel to power the system, while the other three required the installation of additional generators to power the system.
The company is currently developing a smaller process module for future installations. As well as being smaller the module will cost less than the $10 million or so that past installations have cost. Order to operation time for the system is about one year – a schedule influenced by long-lead items such as the gas compressor. Pre-installation work on the vessel requires about one week at quayside. Installation of the module itself takes a further week if carried out at quayside, longer if done at sea.
The new module will operate on the same principle as the existing one. It requires no deck storage tanks as VOC is absorbed back into the crude oil through a packed column.
Prior to entering the column, tank gas emissions are compressed. Simultaneously a side stream of the loading oil – about 5 to 10 per cent of the loading rate – is routed to the VOC recovery unit, pressurised to 7-10 bar and directed to the absorption column. In the absorption column, counter-current absorption of VOC into the crude oil takes place. The crude oil is returned to the crude loading line, while the remaining gas is routed to the riser. Plant installations to date have shown VOC recovery meeting Norwegian requirements of 78%.
Lars Mellerud APL’s manager of the system says installations have also shown that there is no increase in VOC emissions during transport. “This is a very simple system,” he says. “It is an easy module to install, operate and maintain.”
Absorption and adsorption
Cool Sorption has in the past weeks delivered to Navion an adsorption-based system, designed for recovery of VOC on fields with a low a-regime (low hydrocarbon concentration in the gas) and a low loading rate. This will be installed on the Knutsen OAS-owned shuttle tanker Navion Europa in September.
The company, however, sees the true challenge for a VOC recovery system as being one that can handle both low and high Alpha-regimes and low and high loading rates. It believes competitors systems focus on those fields with high emissions and therefore when put into operation on fields with low emissions, low Alpha-regimes and low loading rates, the energy put into the compression and cooling systems results in disproportionately high CO2 emissions for the amount of VOC recovered.
The company has therefore put forward a proposal to Navion for its second stage installations (that Navion has pre-qualified) of a system based on adsorption and absorption that can handle the stresses of handling both low and high Alpha-regimes and low and high loading rates. Development has come through combining the retained knowledge and expertise of Kvaerner Process Systems (now integrated into Aker, Cool Sorption’s parent) with Cool Sorption.
In essence the configuration is a further development of the Navion Europa installation, but with a redefined inlet/outlet crude oil absorber and a raised operational pressure to about 2 bar. The inlet VOC can be blown through an upstream absorber under slight pressure to take out hydrocarbons before the vapour flow enters the adsorption units. This is to overcome the effect that high Alpha-values have on the required size of adsorbing carbon beds. Depending on the Alpha-value of the VOC feed stream the vapour can be fed directly to the carbon beds for adsorption or to the absorber column.
The system will be arranged with one absorber, four carbon beds and four vacuum pumps. The carbon in the beds is regenerated in two stages to avoid hydrocarbon saturation – evacuation of bed gas atmosphere to create a pressure at which hydrocarbon desorbs from the carbon is followed by the introduction of a small amount of purge gas taken from the inert outlet of the adjoining bed.
Powering the system will require an extra diesel generator, but Tomm Lund, vice president of Cool Sorption, reckons that power is only needed for the vacuum pumps so a low output generator is feasible. Peak power consumption is 2.9MW and 1.5MW under normal circumstances says the company. Lund says the carbon coal granules that adsorb VOCs make the system heavier than competitors – about 5-700t – but the weight is well within the capability of shuttle tankers. Order to full installation is comparable with competitors at about a year as is necessary off-hire time to install the process module – about one week to 10 days. The system is designed to give the carbon coal granules a 7-10 year life before they need replacing.
Balanced pressure systems
While these four pre-qualifying systems cure the VOC emissions problem to the satisfaction of Norwegian authorities, Knutsen OAS – whose system Navion is apparently unconvinced by and has therefore not pre-qualified – has looked at a system that can prevent VOC emissions.
It reached the conclusion that during cargo loading, the inflow of oil causes an extreme under-pressure to develop in the cargo tanks. The under-pressure associated with a conventional drop system – about 0.2 bar – causes gas, containing VOCs, to flash out of the oil and escape into the atmosphere.
Since 1998, Knutsen OAS has operated one vessel, Anna Knutsen, with a VOC absorption plant installed (see above). Experience with this plant led the company to question the approach of having a large process facility onboard to capture escaping VOC, both in terms of economics and science. According to Per Lothe, project manager at Knutsen OAS, the company thought a better solution was to ask why VOCs were released, in other words why under-pressure develops, and how it could prevent the situation?
Its subsequent research culminated in the installation of a new full-scale prototype system on the 129,154 dwt shuttle tanker Ragnhild Knutsen. The system is designed to balance pressure during loading, thus preventing build-up of under-pressure, and is based on the installation of a new 2.5 diameter drop-line designed to handle flow rates of 0-14,000m3/hr.
Lothe says the system has been used on almost all loadings since its installation on the Aberdeen-based Ragnhild Knutsen in March last year. He says that the system has resulted in VOC emissions reduction of up to 90 per cent (if oil is received boiling there are emissions associated with that, he points out) and H2S emissions reduction of up to 90 per cent.
He points out that the system has no rotating parts – hence no associated engineering maintenance costs – and does not require new power packs or other installations. Lothe adds to this that the system costs about 15 per cent that of the large process plants, that it can be installed in less than one week and that it has a two month contract-to-delivery schedule.
Based on this, the arguments for it seem to stack up very strongly. However not all are convinced, among them Navion. The shipowner is learning what marine equipment manufacturers have known for years. “It is not hard to introduce new equipment,” confesses Lothe. “But it is very hard to come into the industry and challenge it with something new.”
Navion’s rejection of the system has not caused Knutsen OAS to give up on it though and the company is looking at its potential beyond the Norwegian continental shelf. “We are sure we can convince the oil industry that this is the correct approach,” says Lothe.
The system developed by Venturie is in essence an extension to a system the company has designed to control VOC emissions during transit (see The Motor Ship
, October 2001). A new system implemented together with the Venturie Cargo System, enables loading of cargo using back-pressure. The guiding system already running the Venturie Cargo System maintains the optimal pressure throughout the loading operation, according to Venturie, and thus reduces vapour release by 20 to 50 per cent depending on crude oil quality.
However, this falls short of the 78 per cent required by Norwegian authorities, so Navion did not pre-qualify the system, although dispensation was given for the Venturie Cargo System to compete in the tender for installation as a supplementary system. Helge Aasen, managing director of Venturie, reckons this should be a consideration as the operational and maintenance costs accruing from using any of the four pre-qualified systems during transit will prove greater than the combined installation and running costs of the Venturie Cargo System. He further emphasises that the true benefit of the system is for use during transit.
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