Ternary phase behavior of water microemulsified diesel-palm biodiesel

Abstract

This paper aims to develop a new microemulsions system comprising diesel and palm oil methyl ester (PME) that have the potential to be used as alternative fuels for diesel engines. The water-in-diesel-biodiesel microemulsions were prepared by applying PME mixed with diesel, non-ionic surfactants, co-surfactants and water to make the water-in-oil (W/O) microemulsion system. This microemulsified fuel was achieved through low-energy microemulsification by using the constant composition method. The diesel used was mixed with four different concentrations of PME, i.e., 10% (w/w) (B10), 20% (w/w) (B20), 30% (w/w) (B30) and neat diesel (B0). The amount of water was fixed at 20% (w/w). The phase behavior of the water/mixed non-ionic surfactant/diesel-PME system were studied by constructing pseudoternary phase diagrams with the goal of formulating optimized systems. The results showed that the microemulsions were formed and stabilized with a mixture of non-ionic surfactants at a weight ratio of 80:20 at 20% (w/w), and with mixed co-surfactants at a weight ratio of 25:75, 20:80 and 10:90 for B0, B10, B20 and B30 respectively. The particle size, kinematic viscosity at 40°C, refractive index, density, heating value, cloud point, pour point and flash point of the selected water-in-diesel microemulsion were 19.40 nm (polydispersity of 0.12), 2.86 mm²/s, 1.435, 0.8913 g/mL, 31.87 MJ/kg, 7.15°C, 10.5°C and 46.5°C respectively. The corresponding values of the water-in-diesel-PME selected were 20.72 nm to 23.74 nm, 13.02 mm²/s to 13.29 mm²/s, 1.442, 0.8939 g/mL to 0.8990 g/mL, 31.45 MJ/kg to 27.34 MJ/kg, 7.2°C to 6.8°C, 8.5°C to 1.5°C and 47.5°C to 52.0°C. These preliminary findings were further studied as potential fuels for diesel engines.

Keywords

palm methyl ester / palm oil biodiesel / microemulsion / water microemulsified fuels

Introduction

Alternative renewable fuels have become more important as a result of the increasing crude oil prices due to the depleting hydrocarbon resources, environmental concerns, and biodegradability. Furthermore, alternative renewable fuels provide better quality exhaust gas emission which do not contribute toward the rise of carbon dioxide, carbon monoxide and nitrogen oxides (both NO and NO₂) as well as black smoke and particulate matters [1]. Thus, vegetable oils and their derivatives have been successfully used as alternative substitute for diesel fuel in times of crisis. However, direct use of vegetable oils and their derivatives poses some problems on the mixture formation process and droplet evaporation, which are derived from their much higher viscosity (up to five times the viscosity of the diesel fuel) and higher boiling temperature [2]. For prolonged use in a direct-injection diesel engine, the higher viscosity of vegetable oils needs to be reduced significantly. This problem can be minimized by the use of the microemulsified systems, which are transparent, thermodynamically stable colloidal dispersions that are spontaneously formed, with lower viscosities than macroemulsion [3].

The emulsion fuel is defined as an emulsion of water in standard diesel fuel with specific additives, surfactants and/or co-surfactant to stabilize the system [1]. There is a growing interest in the use of diesel emulsions, in which environmental issues are the main driving force. Water-in-diesel emulsions are fuels for regular diesel engines. The advantages of an emulsion fuel are the reductions in the emissions of nitrogen oxides and particulate matters which are hazardous to health, and reduction in fuel consumption due to better burning efficiency [1]. An important aspect is that diesel emulsions can be used without engine modifications [4]. This is due to the influence of water on the emissions and on the combustion efficiency where the combustion efficiency is improved when water is emulsified with diesel [1]. A typical water content of diesel emulsions is between 10% and 20% [1,5].

Microemulsions are alternatives to diesel emulsions and can be used as a means to incorporate water into fuel. The terms “microemulsion” and “emulsion” seem to imply that such systems are very similar, differing just in the size of the dispersed component, but that is not the case [1]. Microemulsion resembled nanoemulsion because of its nanodroplets in nanometric scale but differ in physical appearance which is transparent and thermodynamic stable colloid dispersion while nanoemulsion is milky white and kinetically stable. The water droplets are as small as a nanometer across, helping to stabilize the emulsion. Formation of microemulsion systems may require a slightly high amount of surfactant to stabilize the nano-metric scale size and stability at wide range temperature [6]. Microemulsionis, a thermodynamically stable colloidal dispersion of isotropic, transparent, small droplet sized (~10–100 nm) liquid in which substantial amounts of two immiscible liquids (i.e., water and oil) are brought into a mono dispersed solution by means of an appropriate surfactant(s), with or without co-surfactant(s) [7].

The formation of the microemulsion is also influenced by the hydrophilic-lipophilic properties of the surfactants which are correlated with the affinity of their polar and non-polar moieties toward the water and the oil phase, respectively [8]. A high hydrophilic-lipophilic balance (HLB) number generally indicates good surfactant solubility in water, meaning affinity to water or water loving while a low HLB number tend to produce water-in-oil emulsion, meaning affinity toward lipid or lipid loving. Literatures have shown that the mixture of surfactants can have a good synergistic effect in reducing the interfacial tension of the liquid i.e. the liquid interface to form a single phase of emulsions. An accurate determination of the hydroliphic-lipophilic nature of surfactants plays an essential role in guiding the formulation of microemulsion. Griffin [9] introduced the concept of assumption of HLB based on emulsion stability. HLB value is a measure of both the strength and conflicting sets, namely the hydrophilic moiety and lipophilic moiety in molecule emulsifiers. Although it does not show the overall efficiency emulsifier, the HLB value should indicate the type of emulsion or expected product.

(1)N_{\mathrm{HLB}}^{\mathrm{mix}}=N_{\mathrm{HLB}}^{(1)}{W}^{(1)}+N_{\mathrm{HLB}}^{(2)}{W}^{(2)},

where N€⁽ⁱ⁾_HLB is the HLB value of surfactants (i) and W€⁽ⁱ⁾ is the percentage weight (%) of surfactants.

The concept in this paper is to use water in microemulsion fuels, either blended with diesel or alkyl ester of fatty acid in the composition of microemulsion [10]. There are abundant renewable sources that can be used for the development of microemulsion as fuel for diesel engines. In this paper, palm methyl ester, a derivative of palm oil, is selected as the vegetable oil to be used. The interest in water-in-diesel emulsions is derived from the fact that water in the form of micrometer-size droplets exerts some positive effects on the combustion of fuel [1]. Moreover, theoretically the combustion of water in a diesel microemulsion fuel should be higher than that in a commercial diesel. This is because, as the microemulsion enters the hot combustion chamber, the water droplets in the microemulsion fuel instantly explode as they flash into superheated steam. The micro-explosion causes a secondary atomization of fuel droplets and increases fuel-air mixture turbulence for a more complete combustion. Moreover, as there is superheated steam in the system, there are additional forces that drive the piston instead of only the expansion energy of the combustion gases. The micro-explosion of water droplets not only helps to increase the efficiency of complete combustion, but also helps to break down any of combustion chamber deposit resulting from an incomplete combustion. This leads to a cleaner piston and engine surface [1].

Many studies have been conducted to incorporate water into fuel instead of an injection of water directly into the diesel engine. At present, diesel engine is still the most fuel-efficient combustion engine. Moreover, oil-based emulsion fuels generally do not require any modification to the diesel engine [3]. In this paper, one portion of neat diesel is replaced by palm oil based biodiesel, water, mixed nonionic surfactants and mixed co-surfactants. This paper aimed to contribute to the development of new microemulsion systems comprising of diesel and palm oil derivatived methyl ester, as alternative fuels, as well as to increase the knowledge in the field of microemulsion. The main objective was the addition of water into diesel fuel, stabilized with a mixture of surfactants and mixed co-surfactants which contribute to smaller droplet size in order to obtain the optimized single phase microemulsion. The percentage of water in the mixtures was fixed at 20% (w/w) to evaluate the ideal composition of which optimum formulation single phase microemulsions is formed.

Materials and methods

Materials

In this paper, the continuous phase was a conventional diesel bought from a local petrol station and palm methyl ester C₁₂–C₁₈ supplied by Sime Darby Biodiesel Sdn. Bhd. The compositions of the palm biodiesel are 46.07% of C₁₆ methyl ester, 4.32% of C₁₈ methyl ester, 38.82% of C_18:1 methyl ester, 9.13% of C_18:2, 1.07% of C₁₄ methyl ester and other small percentage of fatty acid methyl ester. The physical properties of these diesel and palm oil methyl ester are summarized in Table 1.

The non-ionic surfactants fatty alcohol ethoxylate with 7 mol of ethylene oxide (FAE7), fatty alcohol ethoxylate with 2 mol of ethylene oxide (FAE2) and co-surfactant Lorol C_6–8 were supplied by Emery Oleochemicals (M) Sdn. Bhd. The HLB value of the FAE7 and FAE2 are 12.1 and 6.1 respectively. The 1-butanol was bought from Fluka (M) Sdn. Bhd. with a purity of 99.5%. Distilled water was used to obtain the microemulsion systems.

Construction of pseudoternary phase diagram

Water-in-oil (W/O) microemulsions were prepared by using a low energy method via spontaneous emulsification. The preparation of the microemulsions was adopted from Broze by using the constant composition method [11]. The components used were mixed co-surfactants, Cos1/Cos2; mixed surfactants, Sa/Sb; and the water+ oil phase, W+ O. Thirty-nine samples containing various amounts of oil and water were mixed with a mixture of surfactants and mixed co-surfactants. The mixtures were prepared by increasing the percentage (w/w, %) of ethoxylated non-ionic mixed surfactants at a weight ratio of 80:20 at 10%, 15%, 20%, 30% and 40% (w/w) with various percentages of mixed co-surfactants at four different ratios of oil phase. The weight ratio of diesel to PME were 100:0 (B0), 90:10 (B10), 80:20 (B20) and 70:30 (B30) for a total weight of 10 g in screwed cap test tubes. The mixtures were vigorously shaken with a vortex and left for 2 days at ambient temperature and then at 45°C for a month to study the stability of the microemulsions. The formulated microemulsions were observed qualitatively using polarized light sheet for changes in phase behavior formation of the microemulsions and other phases (emulsions or liquid crystals). The observed phases were then constructed as partial ternary phase diagrams. The construction of pseudoternary phase diagrams was conducted using different ratios of mixed diesel to PME (depending on PME composition; B0, B10, B20 and B30) at a fixed weight ratio of mixed non-ionic surfactant (80:20) by using the Chemix School version 3.60 phase diagram plotter software.

Thermostability study

The formulation of microemulsions were left for 2 days at ambient temperature and then stored in an incubator at 45°C for a month. The phase separation and phase behavior were monitored after a day, three days, a week, 2 weeks until a month.

Droplet size measurements

The mean droplet size of the prepared microemulsions was determined using dynamic light scattering (DLS) (Zetasizer Nano-ZS 90, Malvern, UK). The refractive index of each sample was determined before analyzing the droplet sizes.

Selection of formulation

From the constructed pseudoternary phase diagram, microemulsion containing 20% (w/w) of water was selected from all formulae containing B0, B10, B20 and B30 from the isotropic liquid phase regions and the chosen compositions were subjected to a fuel physicochemical characterization.

Characterization of fuel properties

The selected microemulsions were characterized for calorific value, kinematic viscosity at 40°C, density at 15°C, and other typical physicochemical properties of fuel test.

Result and discussion

Phase behavior of water/diesel-palm methyl ester/mixed nonionic surfactants/co-surfactants system

The water-in-diesel-PME microemulsions using water/mixed nonionic surfactant/diesel-PME system was used to form the pseudoternary phase diagram. The effects of mixed nonionic surfactants and mixed co-surfactants of a medium chain alcohols and 1-alkanol were investigated at different weight ratios of 80:20, 70:30 and 60:40, and, at weight ratio of 10:90, 20:80, 25:75, 50:50, 75:25 and 0:100, respectively. After the screening processes, the optimum ratio of mixed surfactants chosen was 80:20 with HLB value of 10.9. Additionally, it is preferable to use a mixture of surfactants rather than single surfactants with HLB values that ​​are equal [12,13] due to its tremendous synergy effect in reducing the interfacial tensions in the microemulsions. The HLB value of FAE7 is 12.1 and is classified as a high HLB while the HLB value of surfactant FAE2 is 6.1 and is classified as low HLB. Nevertheless, these are distinguished from the optimum HLB value of the emulsifier blends for formulating a mediator HLB value to produce a stable emulsion [12]. By constructing a partial phase diagram, the phase behavior of the microemulsions systems was determined.

The pseudoternary phase diagrams of water/mixed non-ionic surfactant/diesel-PME systems at ambient temperature and at 45°C are shown in Figs. 1 to 4.The phase behavior of these systems was only determined for the amount of mixed surfactant and water below 40% (w/w) and 20% (w/w), respectively. The water content did not exceed 20% (w/w) of the desirable water-in-oil (W/O) microemulsion to suit the application in the diesel engines. The amount of mixed surfactant was limited to 40% (w/w) for the whole ternary phase diagram because higher content of surfactant was not industrially applicable.

An isotropic liquid region or a monophase liquid state was successfully obtained in the pseudoternary phase diagram at the weight ratio 80:20 of mixed FAE7/FAE2. Three regions appeared on the pseudoternary phase diagrams for all water microemulsified fuel systems: two or multiple phases (emulsions), microemulsion and liquid crystals (birefringent). Figures 1 to 4 demonstrate the diesel-PME/mixed nonionic surfactants/mixed co-surfactants/water pseudoternary phase diagrams of water emulsified diesel-PME [B0, B10, B20 and B30], respectively. It was observed that the emulsion (two or multiple phase) regions were formed along the apex line of W+ O. The isotropic regions were obtained starting at 20% (w/w) mixed surfactant and with the increasing percentage of mixed co-surfactants. This phase behavior was observed for all water emulsified diesel-PME B0, B10, B20 and B30 formulae at ambient temperature. However, when the temperature increased to 45°C, the isotropic microemulsion region, which was stable at 25°C, had changed to unstable emulsion region for 20 and 30% (w/w) surfactants at 10% (w/w) cosurfactants. Thus, the emulsion region become bigger when higher temperature was given. This phase behavior was similar to all water emulsified diesel-PME formulae: B0, B10, B20 and B30.

The optimum concentration of diesel-PME/mixed non-ionic surfactants/mixed co-surfactants/water for forming microemulsions of all formulae were 7.5% (wt). Lorol C₆–C₈/1-butanol, 20% (w/w) FAE7/FAE2 and 20% (w/w) water, at both the ambient temperature and at 45°C. These points were chosen as they were the minimum percentage to form single phase clear microemulsion at a maximum of 20% (w/w) water and the surfactant was relatively lower (20% w/w), thus being stable at both the ambient temperature and at 45°C over one month. These optimum formulae were then selected for physicochemical properties tests (Table 2). In phase (III) of all formulae at ambient temperature, a liquid crystal region appeared at the low percentage of cosurfactant and had no addition of cosurfactant. This phase behavior could be observed at 20% to 40% (w/w) of mixed surfactants. However, the liquid crystal region gradually disappeared and formed isotropic microemulsion region when the temperature was increased to 45°C. This made the microemulsion regions become bigger after increasing the temperature. This may be caused by the properties of mixed nonionic surfactants which were influenced by temperature changes.

The effect of the mixed co-surfactants on formation of microemulsion was slightly different for each ratio of diesel to PME. For B0, the weight ratio of mixed co-surfactant was 25:75 (Fig. 1) while for B10 was 20:80 (Fig. 2). For B20 and B30, the results indicated that the weight ratio of mixed co-surfactants were 10:90 (Fig. 3 and Fig. 4). As the percentage of PME increased, a higher percentage of 1-butanol was required. These results indicate that the solubility of PME and water (Fig. 3 and Fig. 4) was slightly higher at the weight ratio of 10:90 than at 25:75 and 20:80 weight ratios of mixed co-surfactants. This means that B20 and B30 have a slightly higher solubility capacity of 1-alkanol to solubilise the PME and water than B0 and B10, thus leading to the formation of a bigger region of microemulsions. The results also indicate that mixed co-surfactants have better synergetic effects than a single co-surfactant or mixed non-ionic surfactant system in reducing the interfacial tension of liquid-liquid miscibility and the curvature between the oil and the aqueous phase. Thus, bigger microemulsion regions with very small droplets were formed. The characteristic of the chosen co-surfactants play a great role in the possibility of forming microemulsions [13]. The chemical potential from each component make the microemulsions form spontaneously and give a smaller size without the need of external high energy force. The spontaneous forming of microemulsions may happen due to the very low interfacial tension achieved between the aqueous and oil phase, and thus the spontaneous formation of nanodroplets occur when the components are brought into contact with each other [7,14].

Physicochemical Properties of water microemulsified diesel-PME

The chosen optimum formulae with minimum percentage of mixed surfactants and co-surfactants to form single phase microemulsions were subjected to physicochemical analysis. The optimum concentration chosen were 7.5% (w/w) of mixed co-surfactants, 20% (w/w) of water and 20% (w/w) of mixed non-ionic surfactant for all formulae of water microemulsified diesel-PME based on their stability at both ambient temperature and at 45°C for a month. The physical properties of water-in-diesel-PME microemulsionsare listed in Table 2. The increase in the amount of palm methyl ester [B0, B10, B20, and B30] caused the increase in the particle size. The particle size for microemulsified fuel namely B0, B10, B20 and B30 recorded were 19.64 nm (0.028 PdI), 20.86 nm (0.032 PdI), 22.63 nm (0.076 PdI), and 23.62 nm (0.059 PdI), respectively. Figure 5 depicts the typical DLS curve for the water/mixed non-ionic surfactant/diesel-PME. The curve was a single modal and the peak is narrow, indicating that the sample has a low polydispersity. A polydispersity index of less than 0.02 is chosen because with PdI 0.02, it is indicated that the microemulsion is highly homogenous and has a narrow single peak.

The kinematic viscosity of the selected microemulsified B0, B10, B20, and B30 at 20% (w/w) water were slightly significantly increased i.e. 12.86 mm²/s, 13.02 mm²/s, 13.27 mm²/s and 13.29 mm²/s. Increasing the concentration of PME will increase the kinematic viscosity. This is in agreement with the increment of density for microemulsified B0, B10, B20 and B30. The densities of microemulsified fuel were higher than those of diesel and biodiesel PME. An explanation for this is that the addition of other compound in microemulsion system comprising of water, biodiesel PME, surfactants and co-surfactants which contribute to the addition of molecular weight than diesel solely. With diesel replacement of 52.5% to 36.75% (w/w) results in larger molecular mass of microemulsified fuel thus, leading to higher in densities and viscosities. Besides, pure diesel fuel contain high carbonaceous content where about 75% is saturated hydrocarbons (primarily paraffin including n, iso and cycloparaffin) and 25% is aromatic hydrocarbons (including naphthalenes and alkylbenzenes) [15]. In 1976, Gillberg and Friberg<FootNote>

Gillberg G, Friberg F. Microemulsions as Diesel Fuel. American Chemical Society; 1976, 221–231

</FootNote> had published a scientific paper regarding the use of water-in-diesel oil microemulsions as fuels. They found the viscosity values were 15.7 cP, 14.3 cP and 19.8 cP for microemulsion fuels containing 10% to 30% (w/w) water contents. Furthermore, Koc & Abdullah [16] reported the viscosity values for biodiesel-diesel-water nanoemulsions containing 5% to 15% (w/w) water were 13.44 mm²/s, 15.43 mm²/s and 15.85 mm²/s, which were higher than the viscosities of diesel-microemulsions. They also reported the increment of viscosity values were in agreement with densities increment. The density values obtained were 0.852 g/cm³, 0.858 g/cm³ and 0.861 g/cm³ respectively. These findings indicate that the density of oils have affected the viscosity values of microemulsions formed.

Besides, the calorific value of the microemulsified B0, B10, B20 and B30 is significantly decreased due to the fact that the carbonaceous content in the fuel is decreased where over 50% of the diesel fuel was replaced with 20% (w/w) of water, 20% (w/w) mixed surfactants and mixed cosurfactants but the energy value were not significantly lower when compared to the replacement of the diesel portion. The calorific values for all microemulsified fuel were in the range of 27.34 MJ/kg to 31.87 MJ/kg. The difference in the energy values for B0, B10, B20 and B30 and the conventional diesel and biodiesel PME is approximately 31% to 40%, and, 16% to 31%, respectively. The presence of PME, fatty alcohol ethoxylate, medium chain alcohol groups and addition of water, made the microemulsion rich in chemically bound oxygen content, which may lower 30% to 40% of the calorific value [17]. Although lower in calorific values, their presence also helped incomplete combustion with less emission of harmful NO_x, CO and CO₂.

The microemulsionfuels have good properties in terms of lower cloud points and pour points. Their cloud point and pour point were much lower than diesel fuel and biodiesel PME and ranged from 6.8°C to 7.3°C and 1.5°C to 10.5°C, respectively. The cloud point values were close to each other while the pour point decreased as the amount of methyl ester was increased. However, the flash points for all formulae were lower than those of pure diesel and biodiesel PME and were in the range of 46.5°C to 52.0°C. Ziejewski et al. [18] also demonstrated diesel engine evaluation of nonionic sunflower oil-aqueous ethanol microemulsion whose flash point value was 27°C. Goering & Fry [19] also reported the flash point diesel oil/soy oil/alcohol microemulsion fuel was 28.3°C. The results in this paper have been corroborated by the results of similar reported studies. The flash point of alcohol-diesel microemulsion for use in compression ignition engine was also reported by Chandra & Kumar [20] in the range of 8.3°C to 14.7°C where the ethanol aqueous contained 5% to 15% water content. The drop in flash point of microemulsion fuels may be attributed to the low flash point of 1-alkanol and medium chain alcohol co-surfactants and addition of water. The low flash points have no direct effect on engine performance but care must be taken of its flammability, especially during handling, transportation, storage and use [21].

Conclusions

The phase diagrams of the water microemulsified diesel-PME with mixed non-ionic surfactant and mixed co-surfactant were successfully constructed. The findings showed that water microemulsified diesel-PME microemulsions could be prepared by means of low energy (spontaneous) using the constant composition method without using the energy intensive emulsification methods such as high pressure homogenizer and microfluidizer. The monophase liquid region of the microemulsion were successfully obtained at a lower concentration of mixed co-surfactants of 7.5% (w/w) and at 20% (w/w) of the surfactants, and were stable over one month at ambient temperature and 45°C. The microemulsion solutions were transparent with nanodroplet sizes in the range of 19.64 nm to 23.62 nm with a single modal and a narrow peak.

Based on the physical characterization for B0, B10, B20 and B30 at 20% (w/w) water and 20% mixed nonionic surfactant, these microemulsified fuels had the potential to be alternative fuels to reduce dependence of petroleum based fuels. The concentrations of diesel used in this microemulsions were in the range of 36.75% to 52.5% (w/w). However, their physical properties were quite similar to the conventional diesel. The capacity for solubilisation of palm-based oils with a mixed non-ionic surfactants and a mixed co-surfactants system was mainly related to their chemical structures and hydrophilicity and to the spontaneous curvature of the surfactant layers and packing parameters of the surfactants [14].

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