Research Article
Ultrasonic Study on Biodiesel and Diesel Mixtures
Department of Physics, Pavai Engineering College, Namakkal, Tamil Nadu, India
L. Palaniappan
Department of Physics (DDE), Annamalai University, 608002, Tamil Nadu, India
The analysis of inter molecular interactions between the components of polar-non-polar liquid mixtures can be conveniently done by ultrasonic methods. Such studies become very significant as they find several applications in industrial and technological processes (Velusamy and Palaniappan, 2011; Rajasekar and Naidu, 1996; Oswal et al., 2004; Arul and Palaniappan, 2001). In modern trend of material characterization the analysis of liquid mixtures by ultrasonic methods has gained much importance in the investigation of the physicochemical behavior and in assessing the type and nature of existing molecular interactions. (Nikam et al., 1999; Alagar et al., 1992). A number of attempts (McClements and Povey, 1987; McClements, 1989; Cebula et al., 1992) have been made to understand the molecular interaction of esters, fats and vegetable oils by employing the ultrasonic velocity measurements.
The use of esterification technology with renewable feed stocks such as waste frying oils, vegetable oils, animal fats etc., leads to the production of biodiesel, which is an alternative diesel fuel (De Oliveira et al., 2005; Powlson et al., 2005). It is an oxygenated, non-toxic, sulphur-free, biodegradable and renewable fuel. The review of literature shows that the biodiesel fuel exhibits similar chemical and thermodynamic properties to that of petroleum diesel fuel (Canakci, 2005; Bijalwan et al., 2006). Hence, it is of particular interest to study the molecular association in biodiesel+diesel mixtures. The present work deals with the measurement of ultrasonic velocity, density, viscosity and computation of related parameters in the binary system of biodiesel and diesel at 303 K.
Diesel of density 815.6 kg m-3 at 303 K is purchased from IOC (Indian Oil Corporation) retail outlet. Biodiesel synthesized by esterification technology is obtained from Department of Bioenergy, School of Energy Sciences, Madurai Kamarajar University, Madurai, Tamil Nadu, India. The biodiesel+diesel mixture of various concentrations in mole fraction were prepared by taking purified samples at 303 K and the mixtures were analyzed for their purity as done by Farooq et al. (2008).
The Ultrasonic velocity (U) in liquid mixtures have been measured using an Ultrasonic interferometer (Mittal type) working at 2 MHz frequency with an accuracy of ±0.1 m sec-1. The density (ρ) and viscosity (η) are measured using a pycnometer and an Ostwalds viscometer, respectively, with an accuracy of 3 parts in 105 for density and 0.001 Nsm-2 for viscosity (Palaniappan, 2001).
Using the measured data, the acoustical parameters such as adiabatic compressibility (ρ) free length (Lf), free volume (Vf) and internal pressure (πi) and their excess parameters have been calculated using the following standard expressions (Kannappan and Palaniappan, 1999):
(1) |
(2) |
(3) |
(4) |
(5) |
and
(6) |
where, KT is the temperature-dependent constant (199.53x10-8), k is the temperature independent constant (4.28x109) for all liquids, AE stands for excess property of any given parameter, Aexp is the experimental value, Aid is the ideal value and Meff = Σ xi mi, with xi as the mole fraction and mi as the molecular weight of the ith component.
Though biodiesel and diesel have reported to have many similar properties, a sharp change exists in their sound velocity. For pure diesel, sound velocity is 1325.0 m sec-1 but for biodiesel it is 1389.6 m sec-1. The experimental values of density, viscosity and velocity at 303 K for the pure components and for the system diesel+biodiesel are given in Table 1. The perusal of this Table 1 indicates that all the observed parameters in general show an increasing trend with the increasing mole fraction of biodiesel. Biodiesel is denser than diesel and hence the increase in mole fraction of the biodiesel seems to increase the mixture density.
Table 1: | Values of density (ρ), viscosity (η) and ultrasonic velocity (U) at 303 K for biodiesel+diesel mixtures |
x1 and x2 are amount of constituent |
In the same way, the other observed parameters viz., the sound velocity and the viscosity are also higher for pure biodiesel than pure diesel. The observations found here reflect the existence of interactions between the components as the variations are highly non-linear (Arul and Palaniappan, 2001; Palaniappan, 2001, 2012). The increase in the coefficient of viscosity with increasing mole fraction of biodiesel indicates that the components may either be set away from each other (or) the presence of small size molecules are getting highly restricted.
The calculated values of adiabatic compressibility (β), free length (Lf), free volume (Vf) and internal pressure (πi) at 303 K for the pure components and for the mixtures are presented in Table 2. From Table 2, it is found that the adiabatic compressibility shows a reverse trend to that of velocity. The decreasing trends of compressibility with increasing mole fraction of biodiesel reveals that the medium is closely packed which leads to the possibility of the presence of small size components rather than the set away approach as predicted from viscosity variations (Srivastava et al., 2010; Sako et al., 2010).
Table 2: | Values of adiabatic compressibility (β), free length (Lf), free volume (Vf) and internal pressure (πi) at 303 K for biodiesel+diesel mixtures |
x1 and x2 are amount of constituent |
Fatty acids are readily soluble in the organic solvents (Gurdeep, 1994; Jacobs, 1951) and diesel is one of the product derived from petroleum, so it may act as solvent in biodiesel+diesel mixture. Hence the addition of solute molecules with reduction in solvent component leads to the separation of polar and non-polar components of solute and ultimately the system exhibits much more compactness (Narendra et al., 2011). Hence the free length decreases in the mixture with increasing biodiesel concentration.
As the unsaturated part is more than the other and as diesel is non-polar, the existence of more number of double bonds in biodiesel component may create an unexpected large interaction between like type as well as in unlike type components (Gopalan, 2002; Rolling and Vogt, 1960). To dissolve the non-polar solute molecule, in general, the solvent should at least have one hydrophobic or like group. This is absent in the present system and hence dispersive type interactions predominates the other. All these processes lead to a decrease in free volume with increase in πi with increasing mole fraction of biodiesel.
In order to substantiate the presence of interaction between the molecules, it is essential to study the excess parameters. Reddy et al. (1964) have commented that the deviation of any physical property from its ideal value is due to the adhesive or cohesive forces between the components and this can be taken as a measure of interaction.
The excess adiabatic compressibility (Fig. 1) and the excess free length (Fig. 2) are mostly negative. The presence of haphazard peaks indicate that the system posses high degree of non-ideality and also the presence of AB interaction. An increase in βE at 0.2 mole fraction of biodiesel reflects the strong AB interaction leading to complexation. The relative magnitudes of βE show that the strength of AB interaction decreases as the biodiesel concentration increases.
The role of AB interaction with regards to effective molecular size appears to differ from liquid to liquid (Thirumaran and Thenmozhi, 2010; Rao et al., 2000). The nature of AB interaction existing in the present system is such that it decreases the effective molecular size and thus, leads to negative LfE values. The existence of peak values in VfE (Fig. 3) again assures the formation of complexation that may be due to the existence of polar saturated components of the fatty acids.
The values of πEi (Fig. 4) are randomly fluctuating. Such random variations clearly convey that both dipole-dipole and induced dipole-dipole interactions are existing in the considered system.
Fig. 1: | Mole fraction vs. excess adiabatic compressibility (βE) |
Fig. 2: | Mole fraction vs. excess free length (LfE) |
Fig. 3: | Mole fraction vs. excess free volume (VfE) |
As there are huge fluctuations in these parameters, the chances of induced dipole-dipole interaction are more and the strong dipolar type interactions are additionally confirmed.
Fig. 4: | Mole fraction vs. excess internal pressure (πi) |
Fig. 5: | Mole fraction vs. ultrasonic velocity (U) |
Deviation of the experimentally extracted velocity from the theoretical values could indicate to the great extent the role of biodiesel and in this aspect, three theoretical models such as Free Length Theory (FLT), Nomotos Relation (NR) and Ideal Mixture Relation (IMR) have been considered. The trends of sound velocity estimated by the various methods along with the experimental are shown in Fig. 5, which clearly reveals that the NR shows a unanimous agreement with that of the experimental values. However, at higher and lower mole fraction of biodiesel, FLT offers a correlation as best as that of CFT with the experimental values whereas in intermediate mole fractions, both CFT and NR offers an equal correlation.
As intermolecular interactions are expected to be much more at equimolar concentrations, the order of merit for the prediction of sound velocity goes down as CFT, NR and FLT. The prediction obtained from the molecular interaction parameter clearly supports the previous conclusion that more number of small size components exists and this leads to close packing nature even though dispersive type interactions are predominating. The least agreement of FLT further confirms that the system is not associative one.
Presence of small size molecules is found to be highly restricted. Strong intermolecular interactions are noticed in the system and the components of the system show a high degree of dissociation.