For the biodiesel producer, measurement of Free and Total Glycerin is critical. Analysis is performed in accordance with ASTM D6584. Free glycerin (or glycerol as it is sometimes referred) is the major co-product of the biodiesel production process when the transesterification reaction has proceeded to completion or near-completion. Typically glycerin is separated from the methyl (or ethyl) esters. If inadequate separation of glycerin occurs, the free glycerin component will be detected during the D6584 analysis. If the oil is not well-reacted, evidence of triglycerides will be indicated in the analysis. Additionally, detection of diglycerides and monoglycerides suggests an incomplete reaction.
ASTM D6751 Requirement: a maximum for free glycerin of 0.020% mass and a maximum for total glycerin of 0.240% mass. There is a maximum 0.40% mass Monoglyceride requirement to qualify as a Grade No. 1B biodiesel (a Cold Soak Filtration Time of less than 200 seconds as well as passing results for the remaining ASTM D6751 analyses is required for a No. 1B designation).
Recent studies and data review by task groups have lead to the development of a new addition to the ASTM standard specification for biodiesel intended to improve the performance of biodiesel-petroleum diesel blends. The quality of biodiesel is critical to performance and acceptance in the market. ASTM International has established the standards by which biodiesel fuel quality is evaluated. These standards are defined by the ASTM Standard Specification D6751. ASTM unites subject matter experts from industry, original engine manufacturers (OEMs), and the scientific and engineering community to develop standards to ensure product performance. The current D6751 standard specification for biodiesel quality designates criteria for a low sulfur (S15) blendstock and a high sulfur (S500) blendstock. A new set of criteria is expected to be released by ASTM within the next couple of months that will maintain the current criteria as the “No.2 grade” fuel while defining a new “No.1 grade” fuel. The No.1 grade fuel will be distinguished by new specification limits for two of the current test methods existing in the standard specification; adjustment of these limits is intended to support improved low temperature operability of biodiesel when mixed with petroleum diesel. Particularly, the new No.1 grade criteria will require that the Total Monoglyceride content not exceed 0.400 % mass. The Total Monoglyceride content is a component of the Total Glycerin content. Review of data suggests that a limit of 0.400 % mass will lower the presence of saturated monoglyceride molecules which have a higher melting point and more readily participate in nucleation to initiate crystal formations that can grow in size and plug filters. In addition to the revised standard for Total Monoglycerides, the new No.1 grade criteria will also require that the Cold Soak Filtration Test results not exceed 200 seconds; (the current limit is a max of 360 seconds). Analysis of monoglyceride compounds is performed by gas chromatography in accordance with ASTM standard method D6584; analysis of cold soak filtration is performed with specific temperature set points in accordance with D7501. If you would like more information about these test methods or the forthcoming new standard specification, please feel free to contact Gorge Analytical, LLC for a free consultation. Additionally, we can assist you in optimizing your process to achieve the new specification limits if producing No.1 grade biodiesel is important to your operation.