ULSD/Biodiesel Blend and its Effect on Fuel/Water Separation

  Donaldson Company |  American Filtration & Separation Society Annual Conference, May 2008

Use of ULSD/biodiesel blend can cause potential on-vehicle filtration problems – paper presents studies of physical and chemical properties of different ULSD/biodiesel blends (including soy methyl ester, soybean, and animal fat biodiesel) and shows the effects of blended fuel on fuel/water separator’s efficiency

  • Use of biodiesel blends are increasing
  • Biodiesel’s storage ability, oxidative stability and high affinity to dissolved water could lead to shortened filter life and deteriorated fuel/water filtration efficiency
  • Biodiesel has higher cloud point and more unsaturated hydrocarbon content, which tend to lead to earlier wax precipitation and increased intensity of fuel oxidation
  • Affinity to water attributed to more polar nature of alkyl-esters and unsaturated acids, which barely exist in petrodiesel
  • Study conducted by NREL showed that 89.6% of the 70% total B100 market sampled in 2007 was on specification, but only applies to the biodiesel, not the fuel blends purchased by end users
  • Question is how to manage quality of biodiesel in storage, blending and delivery and quality of blended fuel
  • Soy methyl ester biodiesel
    • Soybean oil is major feedstock for biodiesel production – methyl, ethyl, isopropyl and other alcohols used in transesterification reaction
    • Collected biodiesel samples from Texas, Florida, Maryland and Minnesota and measured: interfacial tension (IFT) of fuel against water; micropsep (MSEP); initial water concentration; initial particulate contamination; density; surface tension; saturated water concentration
    • Surface tension was between 25-31 mN/m (similar to petrodiesel)
    • MSEP is used to characterize the level of difficulty for separating water-in-fuel emulsion with a standard coalescence material (measured 0-100; higher numbers mean easier separation) – all samples had very high MSEP (98/99 compared to 0 for ULSD), meaning they should not affect emulsified water filtration too much due to the instable nature of the water-in-fuel emulsion formed in the fuel
    • Dissolved water concentration for ULSD is typically between 30-180 ppm, but biodiesel dissolves over 300 ppm of water; when biodiesel is mixed with free water to reach phase equilibrium for water saturation in the fuel, 2.5-2 times more water is dissolved depending on the source of the original biodiesel – important because normally pay attention only to free or emulsified water filtration, but the huge increase in water solubility in neat biodiesel compared to ULSD can also be a major concern b/c dissolved water can promote biodiesel hydrolysis to break down the fuel molecules, which will raise fuel’s surfactancy due to formation of carboxylic acids, promote bacteria growth and fuel oxidations, and possibly affect fuel combustion and cause corrosion
    • Particulate contamination ranged from 5.5 to 23.4, compared 0.5 for ULSD; highest was in Florida – question of whether humidity or salty air contributed, Florida fuel also had lowest density, meaning its heat duty is lower than others
  • Soybean biodiesel blend
    • Mixed Minnesota biodiesel with ULSD from Texas to make fuel blends
    • IFT increases as more biodiesel is added to ULSD; B100 has IFT of 12 mN.m, B5 and B20’s IFT is 8.3 mM/m; so at blending ratio below 20 vol%, ULSDS’s IFT dominates the IFT of the blend – important because most common application of biodiesel as a transportation fuel will be using B2 to B20
    • MSEP: only B75 and B100 show very high MSEP ratings vs. 0 for other blends, meaning B75 and B100 should boost emulsified water filtration efficiency as compared to ULSD or other lower ration blends; under B50 MSEP is all zero, indicating additives initially residing in ULSD still function as strong surfactants to stabilize the water-in-fuel emulsion, but their effectiveness diminishes when ratio of biodiesel in the blend dominates possibly due to different polarity of ULSD and biodiesel, which could cause the additives initially designed for ULSD to be constrained somehow by the biodiesel molecules so their concentration and mobility to and from the interface of fuel and water decreases, thus reducing stability of emulsion
    • Diesel generally has good wettability to solid surfaces, but if fuel has more solvent power (like biodiesel), it will attack more aggressively the materials used in the fuel delivery line, and low surface tension will allow fuel to find its way to wet, spread and penetrate the solid surfaces for the chemical damage to occur more easily; material’s compatibility with biodiesel must be addressed in fuel delivery system
    • Amount of original water and particulate contamination increases with increased blending ratio, but relationship is non-linear – contamination concentration rises more quickly when blending ratio is under 20vol%
    • Viscosity of biodiesel blend decreases when temperature increases; within temperature range viscosity was measured, B2, B5 and B20 overlap with linear correlation between viscosity and temperature (which to some extent justifies use of blended fuel up to B20); cloud point of neat biodiesel is around 0 °C, a quick viscosity increase at temps below 0 °C takes place for B50 and B100 (particularly for B100); exponential increase in viscosity at low temperature for B100 could lead to engine operational problems
    • Oxidation stability of biodiesel blends can be measured using Rancimat test (the highly volatile organic acids produced by autoxidation are absorbed in water and used to indicate the induction time – the longer the induction time, the more stable the fuel to oxidation; fuel blend oxidation stability (induction time) drops dramatically as more biodiesel is added to ULSD for blends under B20, then it levels off; in U.S., induction time of an in-spec biodiesel should be at least 3 hours, in Europe it is 6 hours; high oxidation stability prevents deposits in the engine and filters and increases reliability and working life, but fuel’s oxidation stability does not necessarily correlate to the formation of the solid contamination that causes filter plugging
  • Animal fat biodiesel blend
    • Animal fat biodiesel has fewer double bonds and is free of multiple double bonds, so it is less susceptible to fuel oxidation and polymerization
    • B100 induction time for animal fat biodiesel is over 25 hours as contrast to 5 hours for the soy biodiesel; induction time for B20 animal fat blend is about 40 hours compared to 13 hours for B20 soy biodiesel blend; but stability trend is drastically different from that for soybean biodiesel/ULSD blend – stability initially increases with the blending ration, then decreases after reaching a maximal point (max induction time is around B50); cannot offer any in-depth explanation as to why animal fat biodiesel/ULSD blend did not behave similarly to that of soybean biodiesel/ULSD
    • Particle contamination of animal fat biodiesel/ULSD blend is around 1 mg/L, much lower than that of soybean biodiesel, independent of the biodiesel ratio blend – could be another supportive indication that animal fat biodiesel is more stable, which is why it is cleaner; but animal fat biodiesel has a much higher cloud point of 9 °C compared with 0 °C for soybean biodiesel, so it could cause more engine operational problems in cold climate; possibly solutions are to use cold-flow enhancing additives, adding heaters to fuel delivery line or storing vehicles in or near a building
    • Viscosity change: when blending ratio is above 50%, blended fuel starts to thicken quickly at temperature below 0 °C; starting temperature for fuel thickening of B100 is 10-15 °C (much higher than B50 and B75 and higher than soybean B100); when blending ratio is between 2% to 20%, viscosity increases almost linearly with decreasing temperature; the application of these blends generally should not cause fuel gelling problems
    • Surface tension of blends at different blending ratios behaves similarly to soybean biodiesel blends
    • Interfacial tension and MSEP are higher at ever blend ratio than soybean biodiesel, except at B2; indication that the interaction of the tested animal fat biodiesel with ULSD is chemically different than that of soybean biodiesel; also implied that the fuel/water separation efficiency for animal fat biodiesel/ULSD blend can be easier than that for soybean biodiesel/ULSD blend; increasing blend ratio for animal fat biodiesel may also enhance fuel/water separation efficiency due to increase in MSEP rating (vs. for soybean biodiesel, its hard to predict the trend of the fuel/water separation efficiency as the blending ratio gets higher, since the MSEP under B50 is all zero)
  • Fuel/Water Filtration
    • According to Southwest Research Institute (SwRI), as low as 5% biodiesel blend will dramatically reduce the emulsified water filtration efficiency from the blended fuel, independent of biodiesel feedstock
    • Biodiesel reduces interface tension of ULSD from above 25 mN/m to below 15 mN/m
    • Efficiency of filter does not decrease as blending ratio is increased up to 20 vol% for the animal fat biodiesel, but for soybean biodiesel, efficiency drops by about 20% when blending ratio reaches 10 vol% and then it becomes flat; filter efficiency in neat ULSD is about 70%, at a reduced fuel flow rate of 0.75 gal/min due to the high surfactancy of the fuel
    • For biodiesel/ULSD blends, a fuel/water separator’s efficiency is strongly dependent on the fuel properties, just as in ULSD; questions to ask for blended fuel are 1) how much surfactancy is additionally introduced or reduced by the biodiesel molecules derived from different feedstocks and 2) how do biodiesel molecules interact with the additives, especially those lubricity additives in ULSD
  • Conclusions
    • Biodiesel derived from different feedstocks makes the petrodiesel/biodiesel blend properties different; properties closely related to fuel filtration – IFT, MSEP, particle and water contamination as well as viscosity should be looked at carefully to understand how the blended fuel affects engine operation
    • Base petrodiesel fuel is equally as important – some interaction among the different fuel molecules and additives may take place during blending, significantly affecting fuel filtration and fuel/water separation
    • Animal fat biodiesel is more stable than soybean biodiesel and its use under B20 does not seem to affect emulsified water filtration
    • Provided biodiesel meets ASTM D6751 or EN14214, a systematic understanding of the impact of biodiesel/ULSD blend on engine filtration and fuel/water separation is still essential

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