Determination of biodiesel commercial mixer reaction kinetics

L. A. C. Matos¹, A. B. Brugnera¹, E. P. Banczek¹ and P. R. P. Rodrigues¹ 1GPEL – Grupo de Pesquisa em Eletroquímica e Biocombustíveis, Departamento de Química, Universidade do Centro

Oeste – UNICENTRO, Rua Simeão Camargo Varela de Sá, 85040-080, Guarapuava, Paraná, Brazil. *Corresponding author: e-mail: prprodrigues@unicentro.br, Phone: +55 4236298168

Abstract: Current environmental policies demand the reduction of impact caused by fossil fuels, for this reason the ANP – Brazil’s Petroleum National Agency has required the addition of a certain percentage of biodiesel to the automotive diesel since 2008. Thus, it is necessary to better know the biodiesel behavior regarding the metal oxidation present in the storage, transport and the biodiesel use. The most widely commercial used biodiesel is the mixer, a mixture of vegetal and animal originated biodiesel. The objective of this study is to promote the determination of the biodiesel mixer oxidation kinetics, in the presence and absence of carbon steel. The technique employed in this study is the determination of induction time, using Rancimat®, in which results are expressed in conductivity as a function of induction time. This technique is a continuous kinetics measurement and the reaction order obtained for the biodiesel oxidation, in the absence and presence of metal, was one of the first order, however, the biodiesel oxidation rate increases in the presence of carbon steel samples, with minimization of the activation energy around 10 kJ/mol.

Keywords: biodiesel; oxidative stability; carbon steel.

1. Introduction

Environmental and economic problems faced by the world in the mid 70s enhanced research seeking alternative fuel that could replace fossil fuels, reducing the environmental impact caused by conventional fuel and mainly offering commercial stability in case fossil reserves became temporarily unavailable, for example [1]. In this sense, research led to the development of biodiesel, an alternative, renewable, biodegradable and environmentally friendly fuel. Chemically, the biodiesel is an ester originated from the reaction of transesterification of vegetal or animal fat with a short chain alcohol, via acid or basic, homogeneous or heterogeneous catalysis, producing glycerin as a by product [2,3]. In Brazil, the great territory and climatic diversity allow the cultivation of different oily seeds for the extraction of the initial oil that will be used as raw-material to obtain the fuel. However, animal fat waste from poultry houses and slaughterhouses can also be used to extract the initial oil. Due to its high compatibility with diesel, the biodiesel can be incorporated to it, without the need to adapt the engine. For this reason, from 2008 on biodiesel was incorporated to the Brazilian motor fleet, initially in the proportion 2:98 (B2), and nowadays at the ratio 5:95, widely known as B5 mixture [4]. The use of biodiesel implied the study of its properties, as, when in contact with the engine, the fuel can have their initial properties altered, resulting in products of fuel deterioration which might cause environmental problems and damage to the machines that operate with this mixture of mineral diesel and biodiesel [5]. Another aspect to be taken into consideration is the fuel stability, defined as the fuel resistance to degradation in different conditions, generating products that might result in operational and environmental problems. In the case of biodiesel, the oxidative stability is an evaluation parameter; as it describes the fuel ability to degrade through oxidation or self-oxidation processes [6]. The presence of a metal in contact with the fuel might cause the generation of products from the metal corrosion, as well as, accelerate the fuel degradation, due to the presence of metallic ions which interfere in the process that occurs with the fuel. Amongst the most used metals in engines and storage tanks, which are potentially in direct contact with the biofuel, are the iron alloys and fibers and boards of aluminum and copper. Iron alloys such as the carbon steel are mainly used in the fuel storage system and in some cases in the exhaustion systems. Preliminary studies indicate that this alloy suffers small changes in the surface morphology when in contact with the fuel, and mass loss occurs due to the contact with the biofuel [4,5]. This study aims to determine the biodiesel mixer oxidation kinetics in the presence and absence of a metal commonly used in the car industry and in storage tanks, the carbon steel.

2. Materials and Methods

The commercial biodiesel ‘mixer’ was used, whose raw materials are methanol, 50% soybean oil, 40% animal fat, and 10% cotton oil. In order to determine the fuel oxidative stability, the norm DIN EN 14112 was used, with the Rancimat® equipment, at 90ºC, 100ºC, 110ºC, 120ºC and 130ºC.

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To evaluate the metal influence in the fuel oxidative stability, and consequently in the fuel oxidation kinetics, samples of carbon steel (SAE 1010) were employed, finely sanded with sandpaper SiC #320, #400, #600, #1200, #2400 and #3000, with an area of about 1cm², immersed in the equipment cell which contains the biofuel. The kinetics study was carried out taking into consideration that from the oxidation time, reactions occur at a significant speed to the study purposes. In this case, the conductivity of the system (Λ) is a base parameter to determine the reaction order, and it is directly related with the reagents concentration in the medium [7].

3. Results and Discussion

The conductivity curves vs. time, obtained for both systems follow the same pattern described in Figure 1, which shows the biodiesel mixer response, at 130ºC:

Figure 1 – Curve obtained by the Rancimat® for the biodiesel mixer, at 130ºC.

In order to determine the oxidation reaction order of both systems (biodiesel mixer and biodiesel mixer + carbon steel), the following kinetics equations were employed, using data obtained from the induction time:

Λ=Λo - kt (1) Where: Λ= conductivity, k=speed constant e t=time.

ln Λ = ln Λo – kt (2) Where: Λ= conductivity, k= speed constant and t=time.

For the test at 130ºC, the results are presented in Figures 2 (a) and (b).

(a)

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(b)

Figure 2 – Biodiesel mixer kinetic behavior, at 130ºC, (a) zero order; (b) first order.

The analysis of Figure 2 allows to conclude that the kinetics behavior of the first order is the most suitable to describe the biodiesel mixer reaction order, as it obtained the best correlation coefficient close to 1 (R=0.9991). The same behavior is observed at all temperatures under study. After concluding that the biodiesel mixer kinetics presents kinetics of zero order, it was possible to calculate the biodiesel mixer activation energy. It is known that the activation energy is described by the Ahrrenius equation (eq. 3) [8]: ln = ln − (3)

Where: k=speed Constant; A=Constant; R=gases constant; Ea=activation energy and T=temperature, in kelvin.

In all experimental measurements at different temperatures, different speed constants were obtained (k), with which it is possible to obtain the activation energy, by plotting 1n k vs. 1/T, Figure 3, as described in equation 3.

Figure 3 – Biodiesel mixer activation energy. Multiplying the angular coefficient of figure 3 by the gases constant (R = 8.314 J.K.mol-1), the biodiesel mixer activation energy value 71.3 kJ/mol was obtained. Figures 4 and 5 show the same kinetics treatment for the carbon steel + biodiesel mixer systems.

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(a)

(b)

Figure 4 – Kinetics behavior obtained for the carbon steel + biodiesel mixer, at 130ºC (a) zero order; (b) first order.

Figure 5 – Activation energy for the biodiesel mixer + carbon steel system.

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The biodiesel mixer activation energy value, in the presence of carbon steel, is 61.4kJ/mol. The activation energy values (Ea) reported in the literature reveal that the biodiesel activation energy oscillates between 70 and 110 kJ/mol, depending on the raw material used in the biofuel production and whether antioxidants are added or not. Ea values obtained for the biodiesel in the absence of carbon steel (in vitreo) was 71.3 kJ/mol, while in the presence of carbon steel it was 61.4 kJ/mol, which means there was minimization of approximately 10 kJ/mol, suggesting that in the presence of carbon steel the biodiesel oxidation is accelerated [9].

4. Conclusions

- The applied continuous method, oxidative stability employing the Rancimat®, promotes the indirect determination of oxidation kinetics of the systems under study: biodiesel mixer and biodiesel mixer + carbon steel 1010;

- The biodiesel mixer oxidation kinetics is of a first order, and this order is kept in the presence of a metallic substrate, which suggests that there was no change in the oxidation mechanism;

- The biodiesel mixer activation energy in the presence of carbon steel reduces around 10 kJ. mol-1, suggesting that the metal acts as a catalyst of the biofuel oxidation reaction.

Acknowledgments To CAPES, CNPq and Fundação Araucária for the financial support.

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