Biodiesel use has been rapidly growing in the United States due to high crude oil prices and the desire to reduce the country's dependence on foreign oil while improving the environment and providing value-added opportunities for American agricultural products like soybean oil. Biodiesel has an approved ASTM specification for B100-ASTM D 6751-intended for use in blends with petroleum diesel up to B20. The use of B20 and lower blends made with D 6751 grade B100 has been largely trouble-free since the onset of the biodiesel industry in North America over a decade ago.

Over the past 12 to 18 months, there have been a few isolated reports of unexpected filter clogging with B20 blends made with biodiesel meeting D 6751, even though the cold-flow properties (i.e., cloud point, cold filter plugging point) of the B20 would not indicate filter-clogging potential. Archer Daniels Midland Co. (ADM) has significant technical expertise in the chemistry of oils and fats, as well as experience with biodiesel in Europe, and is in the process of building biodiesel plants in the United States. Through its European and U.S. biodiesel experience, ADM has developed knowledge and know-how that can explain one possible root cause for infrequent incidents of filter clogging. As a result, ADM has identified corrective processing actions and potential test methods that can be used to control the phenomenon.

Sterol Glucosides
Sterol glucosides (SGs) occur naturally in vegetable oils and fats in the acylated form (see Figure 1). In this form, acylated SGs are very soluble in vegetable oil. During the biodiesel conversion process, they are converted to nonacylated SGs. ADM has found the presence of SGs (not to be confused with free sterols) in biodiesel samples and has determined that these SGs may contribute to flowability problems in biodiesel and biodiesel blends. Due to the high melting point of SGs 240 degrees Celsius (464 degrees Fahrenheit) and its insolubility in biodiesel or diesel fuel, SGs can essentially be considered "dispersed fine solid particles" in biodiesel. These dispersed SG particles may also promote the crystallization of other compounds.

Unlike monoglycerides or saturated methyl esters, SGs can't be simply heated to allow them to pass through a blocked diesel filter into a combustion chamber for burning. Even at relatively low levels (35 parts per million or higher), sterol glucosides may promote the formation of aggregates in biodiesel, exacerbating problems caused by saturated monoglycerides and other known cold-crystallizing components.

The presence of SGs at double-digit parts per million (ppm) levels may cause the formation of a cloud-like haze in biodiesel, even at room temperatures. Under the microscope, the cloud-like substance appears as agglomerates of various sizes composed of discrete particles of 10 to 15 microns.

While various SG compounds in vegetable oils and fats-and the biodiesel produced from it-are known, its impact on biodiesel hasn't been widely studied. In fact, the presence of some free sterols and tocopherols in finished biodiesel have been shown to act as natural antioxidants. In previous studies and testing in the United States, there have been only positive reports of enhanced storage life with biodiesel containing minor levels of these soluble-free sterols and tocopherols. However, SGs differ from free sterols and tocopherols in their solubility in biodiesel. From previous U.S. experiences, there have been no reported negative impacts of SGs in the ASTM D 6751 properties designed to address low-level compounds that might cause fuel system clogging or engine performance problems such as viscosity, cloud point, carbon residue, or water and sediment, as these tests are not sensitive enough to detect ppm levels of SGs.

In published literature, there are few references to the SG content of natural oils and fats, and almost no information exists on their levels in finished biodiesel. Depending on processing methods for soybean oil, the SG content in soybean oil can be higher than in other vegetable oils. Crude soybean oil may contain about 2,300 ppm SGs, while crude oils from corn and sunflowers contain only about 500 ppm and 300 ppm, respectively. Palm oil may contain SG levels similar to soy oil. The refining process for vegetable oils-or the pre- and post-processing done in biodiesel operations-may reduce SG levels to the point where it isn't problematic, even when the B100 is used as a pure fuel. If SGs are present at high enough levels and have sufficient residence time, however, it is likely they will settle to the bottom of storage vessels. Cold temperatures can accelerate this phenomenon, as well as increase the likelihood of SGs acting as possible seed crystals for developing of larger agglomerates, which may not have occurred otherwise. The agglomerates will also likely settle to the bottom of storage vessels. This potential settling effect may serve to "hide" the filter-clogging potential of SGs and SG-related aggregates, as the aggregates may sit undisturbed in the bottom of a tank.

These two factors-the reduction of SG by normal biodiesel processing or by natural settling-probably explain why SG-related issues haven't been widely observed in the United States to date. A nonscientific sampling of commercial biodiesel has shown SG levels ranging from 0 to 100 ppm.

Removing SGs from Biodiesel
A sample of commercial soybean biodiesel (containing 68 ppm sterol glucosides) was subjected to the so-called "filter blocking tendency test" (FBT)-ASTM Method D 2068, "Standard Test Method for Filter Blocking Tendency of Distillate Fuel Oils"-at 20 degrees Celsius (68 degrees Fahrenheit) through a 1.6 micron filter. This FBT test is especially sensitive to the presence of minute levels of particulate matter in a sample, even above the cloud point of the sample. Various biodiesel samples have failed the FBT test, even though no SGs were present. The commercial soybean-based biodiesel containing 68 ppm SG measured high (poor) on the FBT test, yielding an FBT value of 15.03 (the maximum value that can be obtained with this test). After passing the biodiesel sample (1,000 grams) through a bed of 5 grams of diatomaceous earth, the sample gave low FBT test results with a value of 1.01, having a sterol glucoside content of 20 ppm. It should be noted that SG in biodiesel from vegetable sources may also be removed by other means.

Selective Removal
Specific removal of SGs from biodiesel can be achieved without altering of the contents of monoacylglycerols (MAG), diacylglycerols (DAG) and triacylglycerols (TAG). MAGs, DAGs and TAGs are unreacted or partially reacted oils and fats from the biodiesel process. They are limited in the ASTM specification for biodiesel because they can cause problems with injector coking, emissions and stability. Commercial soybean-based biodiesel (555 grams) was treated by slurrying the soybean biodiesel with 5 grams of diatomaceous earth at 25 degrees Celsius (77 degrees Fahrenheit), stirring the slurry for 10 minutes at 500 revolutions per minute, and filtering the slurry through a 0.95-centimeter bed of diatomaceous earth on a 4 cm diameter Schleicher & Schuell "White Ribbon Filter" paper (Grade 598/2: 4-12 micron retention/Sargent Welch Scientific Co., Buffalo Grove, Ill.). The filtration was carried out at room temperature and the filtrate was analyzed to measure the FBT and the contents of MAG, DAG and TAG in the untreated and treated biodiesel (see Table 1).

Although the content of MAG, DAG and TAG was virtually unchanged by the filtration treatment at 25 degrees, the FBT value was greatly affected. The untreated biodiesel failed the FBT test by reaching the maximum pressure increase possible in the FBT test after only 20 milliliters had passed through the FBT filter, but the treated biodiesel had an excellent FBT value. Thus, the improvement of FBT wasn't dependent on the removal of the MAG, DAG and TAG.

Confirming Filter-Blocking Role
Biodiesel containing 22 ppm SG was obtained by treating commercial-grade biodiesel. Purified SGs (10, 30 and 50 ppm) were added to the treated biodiesel to produce biodiesel of various SG contents, which were subjected to the FBT test at 25 degrees Celsius (see Table 2). The purified SGs were added in such a manner to ensure homogeneity.

As the level of SG was increased, the FBT values increased, with the 72 ppm SG sample reaching the highest value that the test permits. This confirms that an SG presence at high enough levels could potentially cause filter problems on a dispensing pump or vehicle.

Conclusions
The presence of SG in biodiesel may cause filter blockage at temperatures above the cloud point. Cold temperatures are likely to exacerbate the negative impact of the presence of SG in both pure biodiesel and biodiesel blends. Biodiesel having reduced levels of SG can be obtained by filtering biodiesel through diatomaceous earth. The contribution of SG to filter blocking should be considered in addressing filter problems with biodiesel and biodiesel blends, and it may be necessary to put an appropriate filter blocking test or particulate measurement into practice. The FBT test is an especially sensitive test. Current efforts to include a particulate matter specification within ASTM should take the potential presence of SG into consideration.

The foregoing experimental results were supplied by ADM as part of its patent-pending biodiesel purification technology. In order to promote the welfare of the biodiesel industry in North America, ADM intends to provide licenses for this technology on a royalty-free basis. Additional background information was provided by arc-IV Consulting Inc.

1.Dunn, R. "Cold Weather Properties and Performance of Biodiesel" in The Biodiesel Handbook, Knothe, Van Gerpen and Krahl, eds; 2005, AOCS Press, Champaign, Ill.