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Cold Soak Filter Blocking Tendency

An improved test method to help predict diesel winter performance – IP PM-EA/13

Fatty Acid Methyl Ester (FAME) or biodiesel is commonly added to refined diesel to meet environmental targets in concentrations up to 7% volume in Europe for EN 590 diesel. In the very early days of FAME blending to produce EN 590 specification fuel there were some incidents of poor winter operability for road transport users and fuel dispensers, with blocked fuel line filters causing vehicle breakdowns. Subsequent analysis of deposits on blocked filters showed they were enriched with saturated monoglycerides (SMG) and/or sterol glucosides (SG). Both of these components are contaminants that can be present in FAME. These contaminants can form precipitates at low temperatures (but above the cloud point temperature of the fuel) which do not re-dissolve when the fuel is subsequently warmed.

Note that this issue is not the same as ‘waxing’ whereby low temperature causes wax crystals to form, as these materials will normally re-dissolve on warming. Cold flow improver additives are added to diesel during the cooler months to control wax formation.

After a focused industry consultation across Europe, and a number of Energy Institute (EI) workshops and meetings, a cold soak filterability test was developed and in 2008 the EI published IP PM EA/08 Determination of the filter blocking tendency of fatty acid methyl esters – Cold Soak Filtration method. This proposed method (PM) combined a cold soak step (cooling the sample for 16 hours at 5oC) and a subsequent filterability step, using IP 387 Procedure B, to determine filter blocking tendency (FBT). During the test up to 300ml of fuel sample is passed at 20ml per minute through a disposable filter and the pressure differential across the filter is measured. The test terminates when the pressure reaches 105kPa or when 300ml of fuel has passed through the filter.

Once IP PM EA was published work was also initiated by the CEN TC19 Working Group 31. WG 31 evaluated IP PM EA along with cold soak filtration methods developed by the Canadian General Standards Board and ASTM (who developed D7501).

A joint CEN WG 31/EI SC B-7 workshop was held at the Energy Institute in February 2011 to follow up and discuss progress. An overview of work carried out by WG31 members concluded that the proposed cold soak filtration tests do not distinguish all so called “failing fuels” and highlighted that there was only a limited correlation between IP PM EA and the presence of SMG and/or SG.

There was a caveat, that the data was gathered by different labs, with different methodologies on different kinds of samples with varying sample history and therefore could not be regarded as totally conclusive. It was agreed to jointly develop an improved method compared to IP PM EA, using a new approach which included an appearance rating. CEN WG 31 carried out a round robin on 10 FAME samples in 2011 which concluded that the draft CEN method had potential but still required further development. A cold soak study group was set up to optimise the method, lead by Wenzel Strojek (BP Global Fuels Technology Group, Bochum).

Sources of variability in the procedure were targeted by the group. In parallel, in support of WG31 Stanhope Seta started the development of a filter blocking verification material (to determine whether the apparatus needed for the FBT step was in good order) and an automated cold soak bath. The bath automates the warm up from 5 to 20oC after the 16 hour cold soak and an alarm sounds to notify the user.

A mini round robin in WG 31 took place towards the end of 2012 using the new verification standards which indicated that this was a promising potential improvement. Further differences were highlighted between cooling devices, such as baths and refrigerators, in terms of how long the sample takes to cool and warm up again. The importance of controlling other procedural variations was also highlighted as an area of further focus and studies in this area are ongoing.

At the request of UKPIA, EI SC B-7 has updated IP PM EA/08 to incorporate method enhancements from the new CEN WG 31 draft method and as such IP PM-EA/13 was published in November 2013.

This winter the new proposed method is being used by the fuels supply industry and other interested parties to feed back performance and user experiences to WG31 and EI SC B-7 to ensure the method is robust and fit for purpose. This will enable WG31 to further optimise the draft CEN test method. A precision study could then take place in 2014.

Copyright Stanhope–Seta Limited 2014