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United States Department of Agriculture

Agricultural Research Service

Title: Analysis of Oxidative Stability of Methyl Soyate by Pressurized Differential Scanning Calorimetry (P-Dsc)

Author
item Dunn, Robert

Submitted to: Transactions of the ASAE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 29, 2000
Publication Date: N/A

Interpretive Summary: Methyl soyate, a form of biodiesel derived from soybean oil, is very reactive to oxygen when placed in contact with air. Consequently, storage of biodiesel over extended periods may lead to degradation of fuel properties that can compromise fuel quality. Most existing test methods for conventional diesel fuel take several days or weeks to complete or are not appropriate for alternative fuels such as biodiesel. This work examines pressurized-differential scanning calorimetry (P-DSC) as a potential test method that can deliver accurate results in minutes. Methyl soyate samples with varying histories were successfully rated according to relative ability to resist oxidation. A P-DSC-based screening test was developed for evaluating the effectiveness of additives with anti-oxidizing properties. This work addresses growing concerns among biodiesel fuel producers and distributors interested in inexpensive and rapid monitoring of fuel quality during production, storage, and distribution.

Technical Abstract: Biodiesel, an alternative diesel fuel that may be derived from soybean oil, is composed of long-chain saturated and unsaturated fatty acid alkyl esters. Oxidation of unsaturated esters in biodiesel occurs by contact with air and other pro-oxidizing conditions during long-term storage. Extensive oxidative degradation can compromise viscosity, acid value, peroxide value, cetane number, and other parameters in the current ASTM biodiesel fuel guideline. This work examines pressurized- differential scanning calorimetry (P-DSC) for analysis of oxidative stability of biodiesel. Heating scans were conducted in static mode (closed system) with a ramp rate of 5C/min. Increasing pressure from 1000-5000 kPa did not greatly affect results and 2000 kPa was selected for the remainder of this work. Curves were analyzed for oxidation temperature (OT) of methyl soyate samples from four separate sources, with and without antioxidants. Antioxidants significantly increased OT with respect to unmodified methyl soyate. Mixtures treated with tert.-butylhydroquinone (TBHQ) consistently gave higher OT values than those with plus/minus alpha-tocopherol, indicating P-DSC may be useful for screening antioxidants. Pressurized-DSC results were also consistent with corresponding oil stability index (OSI) data measured at 50C; however, no correlation for predicting OSI directly from OT results was evident, with the possible exception of unmodified methyl soyate.

Last Modified: 9/23/2014
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