Keywords
Cold flow properties Biodiesel-diesel blends Pour point depressants Dispersants Combined pour point depressants
Introduction
Biodiesel usually contains various of fatty acid methyl esters (FAME), which are derived from vegetable oils, animal fats, or waste cooking oil (Chen et al., 2010; Joshi et al., 2007; Lv et al., 2013; Madihalli et al., 2016; Cao et al., 2014). It is an eco-friendly liquid fuel because it is biodegradable and less harmful to the environment (Altaie et al., 2015; Vedharaj et al., 2014; Chiu et al., 2004; Moser et al., 2010). The use of biodiesel from waste cooking oil (BWCO) as engine fuel can reduce the consumption of petroleum-based diesel, thereby reducing waste oil emissions and improving the environment (Xue et al., 2016; Wang et al., 2015; Cao et al., 2014 ). However, the unpleasant cold flow properties always prevent biodiesel fuels from being directly added to diesel engines (Xue et al., 2016; Zheng et al., 2014; Moser, 2014; Feng et al., 2014).
To solve this problem, many countries worldwide have used biodiesel by blending with petro-diesel. Definition and detailed specifications of biodiesel blends are outlined in many foreign countries. The United States allows the use of B6 to B20 blends, and the European Committee for Standardization specifies a maximum of 7 vol.% biodiesel mixed into petro-diesel (Moser, 2012; Sharafutdinov et al.,2012; Giraldo et al., 2013). Indonesia and Argentina have increased the allowable biodiesel blends to B30 and B20, respectively. In China, the national standards on B5 have been published, and the local standards have been raised to B20. Although the low-temperature performance of biodiesel can be improved to some extent by blending, the effect still cannot satisfy the expectations. At lower temperature, a large amount of saturated fatty acid esters causes the biodiesel blends to crystallize ansd gel more rapidly than petro-diesel, thereby limiting their application (Cao et al., 2014; Ma et al., 2015). On the other hand, due to China’s vast territory, biodiesel still has limitation in the application of cold flow performance in high latitude areas.
The method of adding pour point depressants (PPDs) are commonly used to improve the low-temperature performance of biodiesel blends (Liu et al., 2019; Xu et al., 2018; Zhou et al., 2016). Polymethyl acrylate (PMA) has recently attracted attention due to its excellent effects (Chastek, 2011; Zhao et al., 2018). In a previous research, PMA was synthesized and used to improve the cold flow ability of diesel. The results showed that PMA has an excellent depression effect on diesel (Umoruddin et al., 2018; Soliman et al., 2018; Lemos et al., 2018; Wang et al., 2014). Unfortunately, in biodiesel, the single PMA copolymer often showed poor results. PMA has different effects on petro-diesel and biodiesel because of their difference in composition. Furthermore, PMA are viscous copolymers, which lead to insufficient dissolution of biodiesel; thus, the final effect of PPD in biodiesel is influenced.
Dispersants of polar groups, characterized by strong oil solubility, low price, and low dosage, can be combined with copolymer depressants to form highly effective combined PPDs (PPDCs) to improve the cold flow ability of fuel. Thus far, some studies have mentioned the combination of PPDs and dispersants. Xue et al. (2016) added PMA with Span, Tween, and PEG dispersants to BWCO. When PMA is mixed with Span 80 at a mass ratio of 2:1, and the added concentration is 5000 ppm, the CFPP is decreased by 7 °C. However, the previous literature focused more on the mixing between copolymer PPDs. At present, the combination of dispersants and PPDs to improve the performance of biodiesel is still a challenge and needs further research. Most dispersants reported in the literatures have low molecular weight and high activity laboratory-prepared copolymers (Maithufi et al., 2011; Soni et al., 2008; Papke et al., 1991; Biswell et al., 1955; Lin et al., 2020). These dispersants not only increase the experimental workload and difficulty, but also the cost. Therefore, Tween, Span, phthalic acid esters (PAEs) and fatty alcohol polyoxyethylene ether (FAPE) were selected as dispersants to be mixed with PPDs to improve the cold flow properties of biodiesel blends in this study.
In this work, a series of methacrylate-benzyl methacrylate-N-vinyl-2-pyrrolidone (RMC-MB-NVP, R= C12, C14, C16, C18) are synthesized and used as PPDs. Some dispersants, such as Tween (40, 60, and 80), Span (40, 60, and 80), PAEs, and FAPE (FAPE 5, FAPE 7, and FAPE 9) were selected and compounded together with these terpolymers to improve the cold flow properties of biodiesel-diesel blends. In addition, the depressive effects of these PPDCs and their possible action mechanism were studied.