A comprehensive study on measurement and prediction of viscosity of biodiesel-diesel-alcohol ternary blends

Highlights

• Production of waste cooking oil ethyl ester.

• Alcohol fraction effects on viscosities of ethyl ester-diesel-alcohols ternary blends.

• Temperature effects on viscosities of ethyl ester-diesel-alcohols ternary blends.

• Rational model for viscosity variations with alcohol fraction and temperature.

Abstract

Recently, biodiesel is receiving global attention as an alternative fuel for diesel engines because of its many advantages compared to petroleum-based diesel fuel. Despite many advantages, it has also some drawbacks such as: poor flow characteristics, lower energy content, higher price and viscosity. Higher viscosity leading poor atomization with larger droplet size results in bad mixing quality and incomplete combustion. Blending of biodiesel with diesel or alcohol is one of the most appropriate techniques to overcome high viscosity problem. Although viscosity data of biodiesel-diesel fuel binary blends at different temperatures are abundantly available in the literature, these of ethyl ester-diesel-alcohol ternary blends including especially higher alcohols are still insufficient. Therefore, in this study, waste cooking oil ethyl ester (biodiesel) was produced by means of transesterification, and it was blended with diesel fuel and different alcohols (methanol, ethanol, isopropanol, n-butanol and n-pentanol). Viscosities of the prepared ternary blends were measured at different temperatures accordingly DIN 53015. Finally, a rational model depending on blend temperature or alcohol fraction was proposed for estimating viscosities of them, and it was tested with the well-known other models (Arrhenius, Kendall–Monroe, Andrade and Vogel) for its predictive ability.

Introduction

As a result of the industrialization of societies and population growing, the greater consumption of energy, faster dwindling of world petroleum reserves, increase in fossil fuel prices and enactment of more stringent environmental standards could be expected in the near future [1,2]. These events have forced to find renewable clean alternatives to fossil fuels. Biodiesel has become one of the most important alternative biofuels that can be used without or little modification in diesel engines [3].

Chemically speaking, biodiesel is defined as a mixture of mono-alkyl esters obtained from (edible and non-edible) oils and animal fats [4,5]. It is usually produced by the transesterification of triglycerides in oils with a mono-hydroxyl alcohol (methanol, ethanol, propanol, etc.) in the presence of a catalyst [6]. The most important parameters affecting transesterification are catalyst amount and type, reaction temperature, reaction time, mixing intensity, alcohol to oil molar ratio and oil type [7,8]. Preference of the alcohol used in transesterification should be made by considering its cost and performance [9]. Although methanol is worldwide preferred, the use of ethanol also ensures several advantages such as reduction of toxicity and environmental concerns due to the renewable character of ethanol [10]. Moreover, ethanol, as an extraction solvent, has much higher dissolving power for oils than methanol [11]. For these reasons, recently, ethanol is taking attention as a suitable alcohol for transesterification reaction [11,12]. Ethyl esters have higher heating value and cetane number than methyl esters because of the extra carbon atom supplied by the ethanol molecule. In spite of its benefits, the use of ethanol also results in the formation of the more stable emulsion after transesterification making very difficult phase separation of esters [13]. However, the addition of extra glycerine to the mixture occurring after the transesterification reaction is found to be helpful in glycerine separation [14]. In addition, ethyl esters have poorer low-temperature properties than methyl esters [15].

As a fuel, biodiesel offers many benefits over petroleum diesel fuel as: (i) it is renewable, non-toxic, non-aromatic and biodegradable fuel, (ii) it has higher cetane number and flash point, and (iii) it significantly reduces pollutant emissions (CO, PM, HC, etc.) [16,17]. Moreover, the use of biodiesel affects the CO₂ balance by limiting its accumulation in the biosphere, which may be considered as one of the principal causes of the greenhouse effect [18].

Although these properties make biodiesel an ideal fuel in a way, biodiesel has also some shortcomings such as higher cloud and pour points because of its higher viscosity, lower heating value and volatility, and generally higher NO_x emissions [[19], [20], [21], [22]]. In addition, the high cost of biodiesel as another shortcoming is the main impediment to its commercialization. It costs about 1.5 times higher than diesel fuel depending on sources of feedstock oils [23]. Although the higher viscosity of biodiesel may eventually result in higher spray penetration into the combustion chamber [24], there are more adverse effects including poor atomization, larger droplet size, bad mixing quality and low combustion efficiency, causing to decrease engine performance and increase exhaust emissions [25,26]. Even though viscosity measurement of biodiesel is not very difficult, simple and reliable models are greatly necessary for researchers [27] because they ensure both a rapid estimation of viscosity and helps in simulation and optimisation of process equipment such as heat exchangers, reactors, mixing vessels and process piping, as well as development of better combustion models [27,28]. In fact, in literature, many studies have been performed to measure and predict viscosities of ternary blends (biodiesel-diesel fuel-alcohol or biodiesel-vegetable oil-alcohol) at different temperatures [[29], [30], [31], [32], [33], [34], [35]]. However, in these studies, (1) ethanol was generally preferred rather than higher alcohols (propanol, n-butanol and n-pentanol), thereby there is still lack of reliable measured viscosity data for ternary blends including especially ethyl ester and higher alcohols at different temperatures, (2) the well-known models (Arrhenius, Andrade, Kay, Vogel, Yuan, Ceriani, etc.) were generally evaluated to predict viscosities and there is a scarcity of new reliable models in the existing literature depending on temperature or alcohol fraction, and finally (3) in the existing literature, generally the upper limit of alcohol fraction in a ternary blend is high and the effect of alcohol fraction on viscosity is examined with a relatively large step size. However, as well-known, the cetane number of ternary mixture decreases with increasing alcohol content due to the lower cetane number of alcohol, resulting in extending the duration of ignition delay and diesel knocking. Therefore, alcohol fraction in ternary blend should be less than about 20$\%$ to avoid decreasing of the cetane number to undesirable levels, and the effect of alcohol fraction on viscosity of ternary blends should be examined in relatively smaller step size. As an example approving the above mentioned claims, Barabás and Todorut [29] measured viscosities of rapeseed oil biodiesel (methyl ester)-diesel fuel-bioethanol ternary blends at the temperature range of 273.15–343.15 $\text{K}$, and they evaluated the well-known models (Vogel-Fulcher-Tammann equation, Kay's rule, Refutas equation, etc.) for estimating the viscosity of them. In their study [29], they changed bioethanol fraction from 5$\%$ to 25$\%$ in the blend with a step size of 5$\%$.

To overcome the mentioned scarcities in the literature above, in this study, waste cooking oil biodiesel was produced by means of ethanolysis reaction. Then, it was blended with diesel fuel at a volume ratio of 20$\%$ because European strategy for the security of energy supply sets 20$\%$ substitution of conventional fuels by biodiesel in the road transport sector by 2020 according to European Directive 2003/30/EC published in 2003 [36]. The blend of 20$\%$ ethyl ester +80$\%$ diesel fuel was taken in this study as a base fuel (BF), and then the base fuel was mixed with 2, 4, 6, 8, 10, 15 and 20$\%$ volume ratios of methanol, ethanol, isopropanol, n-butanol and n-pentanol to prepare ternary fuel blend. The resulting blends were named to reflect their composition. For example, the name Pr2 indicates a blend consisting of 2% isopropanol +98% base fuel. For this blend, the real percentages of biodiesel and diesel fuel become 19.6$\%$ and 78.4$\%$, respectively. Similarly, the ternary blends including 2$\%$ of methanol, ethanol, butanol and pentanol were denoted as M2, E2, Bu2, Pe2. Similar naming and fractions are also valid for the other ternary blends. Kinematic viscosities of the prepared ternary blends were measured at 273.15, 278.15, 283.15, 288.15, 293.15, 303.15, 313.15, 323.15, 333.15, 343.15 K (i.e. 0, 5, 10, 15, 20, 30, 40, 50, 60 and 70 $\mathit{°C}$) in accordance with the international DIN standards. In other words, totally 422 viscosity points were determined at different temperatures for all test fuels including ternary blends, base fuel (BF), diesel fuel (DF), waste cooking oil biodiesel (BD) and pure alcohols. Finally, the rational model as a function of alcohol fraction ($\text{X}$) or temperature ($\text{T}$) was proposed to predict viscosities of them. The model was also tested against the experimental data reported in the other studies [29,35] and it was compared to well-known models previously suggested in the literature.