Colorado-based global specialty chemicals company Innospec Fuel Specialties in December published a technical bulletin entitled Cold Flow – A Point on Pour Point. The document has been shared with Singapore bunkering publication Manifold Times:
Considerations when heating VLSFO
Heating VLSFO to prevent cold flow issues will cause unintentional issues related to distillate ageing i.e. the rapid change in fuel quality due to chemical reactions in the lighter fractions of the fuel.
A study into the storage stability of VLSFO from global ports shows how the sediment present in VLSFO increases over time when a fuel is stored at slightly elevated temperatures (50°C). The Total Sediment Existent (TSE) increases with time across most fuels, and rapidly increases in most cases to exceed the ISO 8217 limits within weeks.
The increased sedimentation would manifest as excessive sludge, filter blocking and centrifugal purifier maintenance. However, these are only symptoms of ageing and other issues besides might be expected, such as reduced fuel efficiency, poor combustion and contamination of future bunkers.
There is clearly a gentle balance to be maintained when handling VLSFO that did not previously exist when using HSFO. If temperatures are too low there is the risk of cold flow issues, poor separation efficiently etc. Too high and fuel will age, leading to excess sludge and deposits. Diligent monitoring and housekeeping practices are one of the best ways to combat this, ensuring that storage temperatures are maintained high enough to prevent wax but not so high that the ageing process is unduly accelerated.
At this time there is little information on how low is too low, or how high is too high. Chemical treatments are available that retard the chemical ageing process and mitigate against the consequences of aged fuel.
Case Story 1: Wax sludge experienced in cold climate
Bunker Grade, Quantity: VLSFO IFO380, 2000MT
ISO 8217: Met
Viscosity, Pour Point: 28cSt, 6°C
WAT, WDT: Not tested
Issue: Sludge clogging backwash filters and overloading purifier. Heavy sludge left behind at bottom of bunker tank unable to be dissolved.
It was during the winter season and the container ship was transiting through a cold climate region. There was no issue faced when the first 200MT of fuel was consumed but soon after, the vessel started to experience sludge issues. Some fluctuation was also observed at the viscometer.
Why did this happen? Where heating is insufficient, especially when the vessel transits through cold climates, the fuel can separate into layers. Initially, lighter components will enter the fuel system and leaves behind the heavier components. Even with the highest possible temperature that can be achieved in bunker tanks, the sludge that remains at the bottom of the tank will not dissolve.
Case Story 2: Wax sludge experienced with on spec pour point fuel, WAT/WDT unknown
Port: South Korea
Bunker Grade, Quantity: VLSFO IFO380, 400MT
ISO 8217: Met
Viscosity, Pour Point: 33cSt, 15°C
WAT, WDT: Not tested
Issue: High generation of sludge overloading the purifier.
In this case, the sludge appeared to have both the consistency of waxes and asphaltene sludge. Hence, to eliminate the possibilities, the vessel carried out a trial and error at the purifier by increasing the separation temperature in steps of 5 °C from the recommended separation temperature which was 60 °C. When the separation temperature was maintained at 75 °C, the sludge generated was observed to be minimal and therefore, the sludge is likely waxes.
Sometimes, purifying at the recommended temperature is insufficient when the fuel has a higher risk of experiencing cold flow issues. At any point of time, if the temperature falls below WAT, wax crystals will form and they can only be dissolved by increasing the heating temperature to above WDT.
However, excessive heating either by high temperature or long-term heating can accelerate the distillate ageing process. Hence, correct heating must be applied.
Case Story 3: Wax sludge experienced with low pour point fuel, high WAT/WDT
Bunker Grade, Quantity: VLSFO IFO380, 800MT
ISO 8217: Met
Viscosity, Pour Point: 57cSt, 3°C
WAT, WDT: 74 °C, >75 °C
Issue: High generation of sludge overloading the purifier
The vessel suspected waxes due to the softer appearance of sludge that was clogging the purifier and hence, requested for the fuel lab to carry out WAT, WDT testing with the fuel. From the results, it is likely that waxes have formed in the bunker tank as the fuel’s temperature at storage fell below its’ WAT result. In an attempt to dissolve the waxes before consuming the fuel, the vessel increased the storage temperature to as close to WDT as possible, managing to achieve only 50 °C inside the bunker tanks. The sludge issue did improve for the next 48hrs before it took a turn and worsened.
Why did this happen? This is an example where issues and solutions overlap. In this case, the temperatures required to dissolve the waxes (WDT) were extremely high which brought a new problem into the equation- an irreversible distillate ageing process through thermal instability. In addition, distillate sludge can similarly clog filters and purifiers.
The fuel’s resistance to oxidation and thermal instability can be improved significantly with the help of fuel additives. However, if the distillate ageing process has already started then it cannot be reversed. Its’ effectiveness will only be to stabilise the fuel and the chances is dependent on the extent the fuel has aged. Hence, pre-combustion additives are always recommended to be dosed at an early stage- into the storage tanks before bunkering.
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Photo credit: Innospec Fuel Specialties
Published: 12 January, 2022
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