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.