Recently, biodiesel has attracted a great deal of attention because of its nontoxic character and biodegradability [1
]. Furthermore, biodiesel is considered carbon neutral and renewable. The utilization of biodiesel can contribute to the reduction of the carbon dioxide emissions. However, the use of edible oils for biodiesel production may be contrary to current social movements and energy policies. Non-edible vegetable oils, which are mostly produced by seed-bearing trees and shrubs, could be an alternative oil source.
Biodiesel generally has a higher cold filter plugging point (CFPP) and an inferior oxidation stability compared with petroleum diesel (a mixture of paraffinic, naphthenic, and aromatic hydrocarbons) [2
]. The fuel can form solids of sufficient size to result in the filter plugging at the CFPP. Thus biodiesel with poor low-temperature flow properties could cause the clogging of the fuel lines and filters in the fuel system that limits the application of biodiesel in cold-weather climates [3
]. In addition, the long term storage of biodiesel is related to its oxidation stability, which presently causes high concern of the major technical issues in the use of biodiesel. Oxidative stability is one of the major technical concerns with biodiesel [4
spp.) trees are generally found at altitudes ranging from 200 to 1500 m in tropical and sub-tropical climate areas including Asia, the Americas, and Europe [5
]. Soapnut tree plantations have great potential to massively produce soapnut seeds and the oil content in the kernel of the soapnut seeds is approximately 42.7 wt.% [6
]. The high content of monounsaturated fatty acids including oleic (9Z
-octadecenoic, C18:1) and eicosenoic (11Z
-eicosenoic, C20:1) acids in the soapnut oil is distinct compared with the fatty acid profiles of common vegetable oils [8
]. According to the previous study [9
], the soapnut oil methyl esters (SNME) exhibit superior oxidation stability. On the other hand, the CFPP of the SNME is 6 °C, which exceeds the limits of the current biodiesel standards.
In addition to the transesterification of vegetable oils (triacylglycerols), biodiesel composed of fatty acid methyl esters is also obtained by the esterification of free fatty acids (FFAs) with low-molecular-weight alcohols. In some cases, FFAs are byproducts of the food-processing industry, such as the edible palm oil-based oleochemical industry, which releases 4%–8% of the total fatty acids as FFAs in crude palm oil during the physical refining process [10
]. By converting the FFAs into biodiesel, a profitable system could be established to provide economic opportunities and benefit the environment without competing with the edible-oil market [12
]. The FFA-based biodiesel has a low CFPP of −6 °C and presents poor oxidation stability of an induction period (IP) of 0.2 h which can be attributed to the lack of natural antioxidants in the FFA-based biodiesel [13
The auto-oxidation of fatty acid methyl esters (FAMEs) is a chain reaction involving three basic steps: initiation, propagation, and termination [14
]. The time from the initial period to the stage with the maximum increase rate of conductivity has been defined as the IP. In general, the IP is experimentally determined by the Rancimat instrument based on EN 14,112, which is a standard method published by the European Committee in 2008 to evaluate the storage time of biodiesel. EN 14,214 standard requires that biodiesel must reach a minimum IP of 6 h at 110 °C. ASTM D6751-07, a standard specification for biodiesel in America, includes an oxidation stability requirement of a minimum IP of 3 h. Furthermore, a modified European method EN 15,751 for biodiesel and diesel blends has been published in 2009 where a number of parameters, including a larger amount of fuel sample, an elongated reaction vessel, and a higher amount of distilled water, were changed compared with EN 14,112. The modifications are mainly due to higher volatility of diesel compared with biodiesel that may lead to higher sample evaporation [15
]. To improve the oxidation stability of biodiesel, the addition of antioxidants is a promising approach to suppress auto-oxidation. If the antioxidant is active and its concentration is high enough, the transfer of hydrogen atom from the antioxidant to oxidative intermediates such as peroxyl radicals can break the chain propagation of the autoxidation process [16
The first objective of this study was to study an optimum blending ratio of the SNME and high-oleic FFA-based biodiesel according to their complementary properties. Meanwhile, EN 14,214 and EN 15,751 tests were performed for comparison with respect of the measurement of oxidation stability at different test temperatures. As a result, the biodiesel blend can meet the biodiesel specifications except for the marginal oxidation stability. Thus N,N′-di-sec-butyl-p-phenylenediamine (PDA), which is an aminic antioxidant, was used to improve the oxidation stability of the biodiesel blend. Furthermore, the consumption of the PDA in the biodiesel blend was described by first-order reaction rate kinetics. In addition, a linear relation between the natural logarithm of IP (ln IP) and the test temperature (T, °C) for the biodiesel blend stabilized with PDA was proposed. Based on the temperature dependence relation, the storage life of the biodiesel blend at an ambient temperature can be predicted by using an extrapolation method.