J Am Oil Chem Soc DOI 10.1007/s11746-015-2667-7 1 3 ORIGINAL PAPER An Empirical Equation for Estimation of Kinematic Viscosity of Fatty Acid Methyl Esters and Biodiesel Suriya Phankosol 1 · Kaokanya Sudaprasert 2 · Supathra Lilitchan 3 · Kornkanok Aryusuk 4 · Kanit Krisnangkura 4 Received: 16 December 2014 / Revised: 8 May 2015 / Accepted: 21 May 2015 © AOCS 2015 proposed model would be good for estimation of viscosity of biodiesel containing normal fatty acids, generally found in biodiesel feed stocks. Keywords Biodiesel · Fatty acid composition · Fatty acid methyl esters · Viscosity Introduction Biodiesel has been widely used as an alternative diesel fuel in many countries. Vegetable oils are the major sources for biodiesel production but types of oils are varied and depend on their availabilities. Therefore, the feed stocks for biodiesel of different countries are different. Conse- quently, physical and fuel properties of biodiesel may vary not only from country to country, but they may also vary from batch to batch. Thus, a standard for commercial bio- diesel is necessary for each country. Viscosity is one of the most important properties, which dictates the atomization process and the fuel droplet size. Atomization is the first stage of combustion in the diesel engine. Oxygen in the air reacts rapidly with the fuel on the outer surface of the oil droplet and releases a tremendous amount of heat to the surroundings. Thus, higher viscous liquids, which tend to form a larger droplet size, may enhance the polymerization reaction, especially oil with a high degree of unsaturation. The use of a viscous fuel leads to poor atomization which is responsible for coke formation in the injector and poor fuel combustion. [1–4]. Hence, viscosity is included in the biodiesel specification of most countries. Although experi- mental determination of biodiesel viscosity is not difficult, a simple and accurate model is greatly desirable. A good mathematical model provides not only a rapid estimation of viscosity, but helps in the further development and tailoring Abstract Kinematic viscosity (μ) is an important physi- cal property of fatty acid methyl esters (FAME) and bio- diesel. In this work, the Martin’s rule of free energy additivity is extended to cover the kinematic viscosity of saturated and unsaturated FAME commonly found in nature. The proposed model can also be extended to esti- mate kinematic viscosity of biodiesel. The kinematic vis- cosity of a FAME or a biodiesel can be easily estimated from its carbon number (z), number of double bonds (n d ) at different temperatures (T) without a prior knowledge of the viscosity of individual FAME. Both z ave and n d(ave) can be derived from its fatty acid composition. Thus, kinematic viscosity of biodiesel at temperatures between 20 and 100 °C and at atmospheric pressure can be estimated. The average absolute deviation (AAD) estimated at 20–100 °C for saturated, unsaturated FAME, biodiesels and biodiesel blends are 4.15, 3.25, 6.95 and 2.79 %, respectively. The biodiesels collected in this study (191 data points) have the z ave and n d(ave) between 14.10 and 17.96 and 0.21– 1.54, respectively. The standard deviation was 0.249. The * Kanit Krisnangkura kanit.kri@kmutt.ac.th 1 Present Address: Department of Industrial and Technology Management, Faculty of Science and Technology, Bansomdejchaopraya Rajabhat University, Bangkok 10600, Thailand 2 Division of Energy Technology, School of Energy, Environment and Materials, King Mongkut’s University of Technology Thonburi, Bangkok 10140, Thailand 3 Department of Nutrition, Faculty of Public Health, Mahidol University, Rachathewi, Bangkok 10400, Thailand 4 Division of Biochemical Technology, School of Bioresources and Technology, King Mongkut’s University of Technology Thonburi (Bangkhuntien), Bangkok 10150, Thailand