Cold flow properties of Biodiesel

Regardless of origin, all diesel fuels are susceptible to start-up and operability problems when vehicles and fuel systems are exposed to cold temperatures. As ambient temperatures cool toward their crystallization temperature, high-molecular weight paraffins (C₁₈-C₃₀ n-alkanes) in petrodiesel nucleate and form wax crystals suspended in a liquid phase composed of shorter-chain n-alkanes and aromatics. Left unattended overnight, solid wax crystals may plug or restrict flow through filters causing start-up and operability problems the next morning.

At low temperatures, higher-melting point (MP) components in the fuel nucleate and grow to form solid crystals.

- The cloud point (CP) of a fuel is defined as the temperature where crystals become visible (diameter exceeds 0.5 m) forming a hazy or cloudy suspension. Prolonged exposure of the fuel to temperatures at or below CP causes crystals to grow and cling together forming agglomerates that restrict flow.

- The Pour point (PP) is defined as the lowest temperature where the fuel flows or can be pumped.

Both CP and PP are easily measured in the laboratory. However, neither parameter efficiently predicts how diesel fuels will perform in tanks and fuel systems during cold weather. Consequently, data from field trials were correlated to develop bench-scale tests that more effectively predict temperature limits where start-up or operability problems may be expected to occur in the fuel after prolonged exposure. The first such test, cold filter plugging point (CFPP), is accepted nearly world-wide and listed among the limiting fuel parameters in the aforementioned European biodiesel fuel standard EN 14214.

- The CFPP is defined as the lowest temperature where 20 mL of fuel passes safely through a 45 μm wire mesh filter under 200 mm H₂O (0.019 atm) vacuum within 60 s.

Cold flow properties of biodiesel generally depend on fatty acid composition. Straightforward transesterification to biodiesel does not greatly alter the fatty acid composition based in the parent feedstock.

Blending with petrodiesel at relatively low blend ratio mitigates most performance-related issues with cold flow properties of neat biodiesel depending on its original feedstock. Splash blending biodiesel and petrodiesel may also present problems during cold weather.

Approaches for improving the performance of biodiesel and its blends include:

1.    Treating with commercial petrodiesel cold flow improver additives

2.    Developing new additives for biodiesel

3. Mixing FAME with alkyl esters made from transesterification with medium- and branched-chain alcohols

4. Decreasing crystallization temperature (T_f) by reducing total saturated FAME concentration.

Cold flow properties and performance continue to influence the development of biodiesel as an alternative diesel fuel or extender. On-road transportation, power generation, heaters and boilers, locomotives, farm vehicles and aviation applications may provide incentives for development of commercial-scale processes to improve cold flow properties of biodiesel.

The fatty acid composition of biodiesel is the main factor in determining their CP, PP, CFPP and LTFT. Development of feedstocks with inherently higher total concentration of saturated fatty acid alkyl esters, such as animal fats or used cooking oils, will direct research efforts in development of processing technologies to improve their cold flow properties. In some cases, the influences of total concentration of saturated fatty acid alkyl esters composition may be linearly correlated to CP or CFPP. However, the cold flow properties of biodiesel from various feedstocks can be calculated from thermodynamic models based on freezing point theory provided the crystallization properties of each individual component in an alkyl ester mixture are known.

The most promising approaches for improving the cold flow properties of biodiesel are those that reduce CP.

Fractionation (modification of the fatty acid alkyl ester composition) of biodiesel improves cold flow properties of biodiesel by modifying its fatty acid profile to remove high-melting components resulting in reduced crystallization onset temperatures. Dry fractionation, with and without crystallization modifiers, solvent fractionation and urea fractionation may significantly reduce CP. Other fractionation technologies are vacuum distillation, adsorption, membrane separations and supercritical fluid extraction. Urea clathrates, vacuum distillation and adsorption are also applied in the removal of trace concentrations of saturated monoacylglycerols and steryl glucosides, minor constituents that may be problematic to the cold weather storage stability of biodiesel and biodiesel/petrodiesel blends.

Other adaptations to the fractionation technology may be explored in future studies for application to biodiesel. An example is surfactant fractionation. Applied mostly to fats and vegetable oils, this process is similar to dry fractionation where after the crystallization the separation of solid crystals is assisted by adding a cool aqueous solution of surfactant (sodium dodecyl sulfate) containing an electrolyte (magnesium or aluminum sulfate). The combination of surfactant wetting agent and electrolyte allows solid crystals to be suspended in the aqueous phase. After separation of oil and aqueous phases by centrifugation, fractions are heated, washed and dried to remove additives. Surfactant fractionation is more efficient than dry fractionation with respect to separation efficiency and yield of liquid fractions. Its main disadvantages are high operating costs and decontamination of end products.

Another example may be to inject a low-boiling point coolant such as ammonia, CO₂ or halogenated hydrocarbon into the alkyl ester mixture. Applying this approach to fatty acid mixtures with or without solvent improves separation efficiency of solid and liquid phases.