(262u) Volumetric and Viscometric Properties of a Binary Mixture of a Model Diesel Compound with a Biobased Additive

As a consequence of ongoing regulations to avoid the depletion of fossil fuels as well as environmental concern about their use, renewable sources have gained increasing attention during the past two decades. Among them are biofuels, having biodiesel an increasing prominence.

One of the grand challenges of the biodiesel industry is to manage the surplus of glycerol generated as a by-product of its manufacturing process, whose price has steeply decreased in the past years [1]. Among the routes to exploit such compound, its chemical valorisation is on the spot as a way to obtain value-added products [2].

Among them are acetals, which derive from the reaction of glycerol with an aldehyde or ketone. Solketal (Sk) is the acetal that has attracted the most attention in terms of research [3], for its synthesis is made through the reaction of glycerol with widely available acetone [4]. Sk has proven its relevance as additive to fuels and biofuels by improving certain performance parameters [5-7].

Data on properties of fuels is important to the design of engines, due to the effect that they have on the combustion quality. Therefore, the determination of volumetric and viscometric properties is of relevance. N-hexadecane (also known as cetane) has typically been employed as a compound to model the behavior of diesel [8], so considering that Sk is a promising additive, this work intends to present the volumetric and viscometric properties of binary mixtures consisting of cetane and solketal.

Methodology

N-hexadecane (purity > 99%) and Solketal (DL-1,2-Isopropylideneglycerol, purity > 98%) by Sigma-Aldrich Co Ltd were used for the the binary mixtures. To prepare samples of varying composition (intervals of 0.1 of molar fraction), a Kern and Sohn ABS 220-4 analytical balance with a 0.0001 g accuracy was used.

Viscosity was measured using a temperature controlled Malvern Bohlin rheometer using a cone (60 mm diameter, 2° angle, 52 μm truncation) and plate geometry. The uncertainties associated to the viscosity and temperature were ±0.001 mPa s and ±0.1 K.

The density measurements were performed in a Krüss K100 tensiometer heated by a temperature controlled Tecam TE-7 Tempette water bath, with an uncertainty in the measurement of ±0.001 mN/m and ±0.01 K, respectively.

Triplicate measurements were performed for each data point for both the properties.

Summary of results and conclusions

The values of density and viscosity of pure n-hexadecane and solketal were validated with already existing data at 293.15, 303.15, 313.15 and 323.15 K for viscosity and 298.15, 303.15, 313.15 and 323.15 K in the case of density. For solketal, whose use is novel, data are still very scarce and referred only to 298.15 K and purities around 95%. The data obtained for the pure n-hexadecane did not differ in more than 3.1% at most for viscosity 0.3% for density.

Fitting of the viscosity (μ) and density (ρ) as a function of temperature were made for the pure components according to equations 1 and 2, respectively:

 μ= μ₀ e^-k/T (1) 

ρ= a+bT (2)

The values of the fitting parameters therein included were as follows:

• n-hexadecane: μ₀= 6.24x10^-6 and k=1846.20 for equation 1; a=0.9747 and b=-6.84x10^-4 for equation 2.
• solketal: μ₀= 2.59x10^-7 and k=3141.09 for equation 1; a=1.3102 and b=-8.30x10^-4 for equation 2.

Then, viscosities and densities of the 9 binary mixtures consisting of increasing molar fractions of solketal in n-hexadecane were measured. From the observed data, the excess molar volumes (V^E ), viscosity deviations (Î?μ) and excess Gibbâ??s free energy of activation for viscous flow (Î?G^E ) were computed following equations 3 to 5:

V^E=(x₁M₁+x₂M₂)/ρ-(x₁M₁/ρ₁+x₂M₂/ρ₂₎ (3)

Î?μ=μ-[x₁μ₁+(1-x₁)μ₁] (4)

Î?G^E=RT[ln(μV)-(x₁ln(μ₁V₁)+(1-x₁)ln(μ₂V₂)] (5)

where x_i and M_i represent the molar fractions and molecular weights of n-hexadecane (1) and solketal (2) in the mixture, V are the molar volumes and R is the ideal gas constant.

Finally, from the excess properties and deviations computed, correlation to a Redlich-Kister type equation was performed.

 P=x₁(1-x₁)ΣA_i(1-x₁)ⁱ (6)

in which P is the excess property or deviation and A_i are the fitting parameters in the equation.

References

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5. Garcia, E., et al., New Class of Acetal Derived from Glycerin as a Biodiesel Fuel Component. Energy & Fuels, 2008. 22(6): p. 4274-4280.

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