Location: Bio-oils Research Unit
Title: Specific gravity and API gravity of biodiesel and ultra-low sulfur diesel (ULSD) blends Author
Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 14, 2011
Publication Date: April 21, 2011
Citation: Dunn, R.O. 2011. Specific gravity and API gravity of biodiesel and ultra-low sulfur diesel (ULSD) blends. Transactions of the American Society of Agricultural and Biological Engineers. 54(2):571-579. Interpretive Summary: This research developed mathematical correlations for determining the relative density of biodiesel/ultra-low sulfur petrodiesel (ULSD) blends. This work addresses the need for a better understanding of the effects of blending ULSD with biodiesel on fuel density. Accurate determination of density is necessary for conversion of measured volumes to volume or mass values in custody transfer operations. Relative density data were measured for blends of biodiesel made from soybean oil, used cooking oil and ULSD. Results were analyzed to derive correlations for predicting relative density from properties of the unblended biodiesel and ULSD components. Correlations were also derived to determine API gravity, a type of density scale used by industry when blending different fuels together. Data and results from this study will be valuable to fuel producers and terminal operators as they seek to improve blending operations. Scientists and engineers will benefit from the reporting of data for various biodiesel/ULSD blends because relative density is generally correlated against fuel performance indicators.
Technical Abstract: Biodiesel is an alternative fuel made from vegetable oils and animal fats. In 2006, the U. S. Environmental Protection Agency mandated a maximum sulfur content of 15 ppm in on-road diesel fuels. Processing to produce the new ultra-low sulfur petrodiesel (ULSD) alters specific gravity (SG) and other fuel properties. Accurate determination of SG (or density) of fuels is necessary for conversion of measured volume to volume or mass at standard temperature. Furthermore, SG is generally correlated against performance indicators such as cetane number and heating value. The present study addresses the need to develop a better understanding of how blending biodiesel with ULSD affects SG. The effects of biodiesel volumetric blend ratio on SG were evaluated for blends of fatty acid methyl esters (FAME) from soybean oil methyl esters (SME) from three different fuel producers and FAME from used cooking oil (UCOME) in ULSD. Regression analysis of results at 15.6 and 40°C indicated that SG increased linearly with respect to increasing blend ratio. Average differences between predicted and measured values were no greater than 0.00084. Results for blends of the three SME’s in ULSD were pooled to yield a combined linear correlation for SME/ULSD blends (R² = 0.999 for SG at 15.6°C). The API gravities of blends were determined from SG data at 15.6°C. Two methods were proposed to calculate API gravity of a given blend with blend ratio = BX (X vol% biodiesel). Method I was to determine SG from the appropriate linear correlation reported in this work and calculate API gravity accordingly. The accuracy of Method I depended upon precision in calculating SG (not greater than 0.00084, noted above). Method II involved determining API gravities of the unblended components and taking a volumetric average at blend ratio = BX. The accuracy of Method II was within 0.11° based on comparison of predicted and measured data. It was concluded that both methods were capable of precisely determining the API gravity of blends.