One of the major problems associated with the use of biodiesel is poor low-temperature flow properties, documented by relatively high cloud points (CPs) and pour points (PPs) [1, 2]. The CP, which usually occurs at a higher temperature than the PP, is the temperature at which a fatty material becomes cloudy due to the formation of crystals and solidifi­cation of saturates. Solids and crystals rapidly grow and agglomerate, clogging fuel lines and filters and causing major operability problems. With decreasing temperature, more solids form and the material approaches the PP, the lowest temperature at which the material will still flow. Saturated fatty compounds have significantly higher melting points than unsaturated fatty compounds (Table 5.1), and in a mixture, they crystallize at higher temperatures than the unsaturates. Thus, biodiesel fuels derived from fats or oils with significant amounts of sat­urated fatty compounds will display higher CPs and PPs.

Besides the CP (ASTM D2500) and PP (ASTM D97) tests, two test methods for the low-temperature flow properties of petrodiesel exist, namely, the low-temperature flow test (LTFT) (used in North America; e. g., ASTM D4539) and cold filter plugging point (CFPP) (used outside North America; e. g., the European standard EN 116). These methods have also been used to evaluate biodiesel and its blends with No. 1 and 2 petrodiesel. Low-temperature filterability tests were stated to be nec­essary because of better correlation with operability tests than the CP or PP test [31]. However, for fuel formulations containing at least 10 vol% methyl esters, both LTFT and CFPP are linear functions of the CP [32]. Additional statistical analysis have shown a strong 1:1 correlation between LTFT and CP [32].

Several approaches to low-temperature problems of esters have been investigated, including blending with petrodiesel, winterization, additives, branched-chain esters, and bulky substituents in the chain. The latter approach may be considered a variation of the additive approach, as the corresponding compounds have been investigated in biodiesel at additive levels. Blending of esters with petrodiesel will not be discussed here.

Numerous, usually polymeric, additives were synthesized and reported mainly in the patent literature to have the effect of lowering the PP or sometimes even the CP. A brief overview of such additives has been presented [33]. Similarly, the use of fatty compound-derived mate­rials with bulky moieties in the chain [34] at additive levels has been investigated. The idea associated with these materials is that the bulky moieties in these additives would destroy the harmony of the crystal­lizing solids. The effect of some additives appears to be limited because they more strongly affect the PP than the CP or they have only a slight influence on the CP. The CP, however, is more important than the PP for improving low-temperature flow properties [35].

The use of branched esters such as isopropyl, isobutyl, and 2-butyl esters instead of methyl esters [36, 37] is another approach for improv­ing the low-temperature properties of biodiesel. Branched esters have lower melting points in the neat form (Table 5.1). These esters showed a lower TCO (crystallization onset temperature), as determined by dif­ferential scanning calorimetry (DSC) for the isopropyl esters of soybean oil (SBO) by 7-11°C and for the 2-butyl esters of SBO by 12-14°C [36]. The CPs and PPs were also lowered by the branched-chain esters. For example, the CP of isopropyl soyate was given as —9°C [7] and that of 2-butyl soyate as — 12°C [36]. In comparison, the CP of methyl soyate is around 0°C [32]. However, in terms of economics, only isopropyl esters appear attractive as branched-chain esters, although even they are more expensive than methyl esters. Branching in the ester chain does not have any negative effect on the CN of these compounds, as dis­cussed above.

Winterization [35, 38, 39] is based on the lower melting points of unsaturated fatty compounds than saturated compounds (Table 5.1). This method removes by filtration the solids formed during the cooling of the vegetable oil esters, leaving a mixture with a higher content of unsaturated fatty esters and thus with lower CP and PP. This procedure can be repeated to further reduce the CPs and PPs. Saturated fatty compounds, which have higher CNs (Table 5.1) than unsaturated fatty compounds, are among the major compounds removed by winterization. Thus the CN of biodiesel decreases during winterization. Loss of mate­rial was reduced when winterization was carried out in presence of cold — flow improvers or solvents such as hexane and isopropanol [39].

In other work [40], tertiary fatty amines and amides have been reported to be effective in enhancing the ignition quality of biodiesel without negatively affecting the low-temperature properties. Also, sat­urated fatty alcohols of chain lengths >C12 increased the PP substan­tially. Ethyl laurate weakly decreased the PP.