Hydroesterification of crambe oil (Crambe abyssinica H.) under pressurized conditions
Introduction
In the search for alternative raw materials the technological aspects of biodiesel production should be considered along with agronomic factors, such as the oil content, yield, production system and crop cycle (Rogério et al., 2013). Since the raw material represents around 75% of the total production cost, the selection of an oilseed with lower added value is vital to ensure a competitive cost for the biofuel (Atabani and César, 2014). According to the Brazilian National Petroleum Agency (ANP, 2015), most of the biodiesel produced in Brazil is obtained from soybeans and, since this is a crop mainly grown for human consumption, research on the exploitation of other oilseed crops with the potential to provide biodiesel has been identified.
Crambe oil (Crambe abyssinica Hochst) is an attractive option for biodiesel production because it is drought tolerant, has a cycle of approximately 90 days and the total seed yield is around 1000–1500 kg per hectare (Falasca et al., 2010, Viana et al., 2013). The oil content of crambe seed is 48–60% (Silva et al., 2013, Santos et al., 2015), and it has a high content of erucic acid (50–60%), which is toxic, causing heart disease through increased cholesterol levels and lipidosis in the heart tissues, and thus the oil is unsuitable for human consumption (No et al., 2013). Crambe oil can therefore only be used for industrial purposes such as biodiesel production.
Crambe oil is obtained by extraction through pressing and is not passed through a refining process. Thus, the free fatty acids (FFA) content is above that suitable for processing by the conventional biodiesel production method, which can be applied to raw materials containing up to 1% of FFA and 0.06% of water (Dermibas, 2009). For the processing of vegetable oils with high acidity, hydroesterification can be applied, since this process permits the use of low quality raw materials, which are completely converted into esters. Therefore, instead of decreasing the acidity of the feedstock through a refining process, in the hydrolysis step the acidity is increased. Subsequently, the hydrolyzate is esterified and fatty acid esters are obtained with water as a byproduct.
The processes generally employed in hydrolysis are catalytic, via chemical or enzymatic routes. The chemical process involves acid catalysis, generally conducted in the presence of sulfuric acid (Ngaosuwan et al., 2009), and alkaline catalysis using sodium and potassium hydroxide (Asai et al., 1997). The use of such catalysts requires long reaction times (20–48 h), besides causing oxidation and dehydration of the products obtained, resulting in a dark colored product, which requires purification to remove the color and excess catalyst (Castro et al., 2004). Enzymatic catalysis has recently been used in hydrolysis reactions. However, it presents several technical difficulties, including long reaction times, loss of enzyme activity with increasing temperature and contact with certain solvents, in addition to the high cost of the catalyst (Hama and Kondo, 2013).
The esterification of the hydrolyzate, which is rich in free fatty acids, is usually carried out via a catalytic route using homogeneous acid catalysts (Brinks et al., 2013). However, the use of this type of catalysts is associated with slower reaction rates and difficulties related to the separation and purification of the final product (Christopher et al., 2014). Enzymatic catalysis can also be applied in the esterification reaction, however, as mentioned above, the high cost of the enzymes and long reaction times are significant disadvantages. Moreover, studies have shown that the enzymes are easily deactivated by the alcohol used (Hama and Kondo, 2013).
To minimize the problems associated with the use of catalysts in reactions for biodiesel production, the recent literature reports the application of technology involving fluids under sub- or supercritical conditions, without the addition of a catalyst in the process (Minami and Saka, 2006, Silva and Oliveira, 2014). The properties of a fluid under these conditions are intermediate between a gas and a liquid and one of the advantages is that many of the physical properties, such as density, dielectric constant and solubility, can be easily adjusted through slight variations in the temperature and pressure (Wen et al., 2009).
Water in the subcritical state (10–20 MPa and 270–350 °C) has recently been used in hydrolysis reactions. Since the water temperature is high there is a reduction in the polarity, which is due to a decrease in the dielectric constant. In addition, the kinetic energy of water also increases, resulting in a greater distance between the molecules. In the subcritical region, water has properties (density and dielectric constant) similar to an organic solvent at room temperature. This allows an increase in the reaction rate without using a catalyst (Carr et al., 2011). Subcritical water is a technically feasible and environmentally acceptable alternative in comparison with water in the supercritical state, which is associated with serious hazards since it is toxic, flammable, explosive and corrosive (Shin et al., 2012, Kansedo and Lee, 2014).
In the esterification step, the alcohol used under pressurized conditions shows an autocatalytic mechanism due to dissociation of the free fatty acids in the reaction medium under these conditions (Akgün et al., 2010). Thus, catalysts are not used in the process, eliminating their cost and the need to remove them from the final product (Pinnarat and Savage, 2010). The use of elevated temperatures and pressures in the method leads to a higher reaction rate compared to the conventional alkaline method due to decreased mass transfer limitations resulting from the high miscibility between the substrates (Alenezi et al., 2010a, Abdala et al., 2014a).
In this context, the aim of this study was to evaluate the continuous production of esters from the hydroesterification of crambe oil under pressurized conditions. The effects of the experimental variables were investigated in the hydrolysis step in order to maximize the production of free fatty acids, and the residence time was also studied. In the esterification step the effects of the temperature, ethanol to FFA molar ratio and residence time on the FFA conversion and esters content were investigated. Transesterification was also performed for comparison purposes. The results of this study contribute to research on the production of crambe biodiesel and the use of hydroesterification under pressurized conditions, for which there are very few reports available in the literature.
Section snippets
Materials
Crambe oil, donated by the MS Foundation, and water, from a Millipore (ZICW300UK) deionization system, were used as substrates in the hydrolysis reactions. Ethanol (JT Baker, 99.8%) was used in the esterification and transesterification reactions. n-Hexane (Anidrol) was used as a cosolvent in the hydrolysis and transesterification reactions. In the determination of the FFA content and composition and the glycerol and ethyl esters content as well as the qualitative analysis by thin layer
Oil characterization
Table 2 shows the composition of fatty acids present in the crambe oil. The FFA and water content were determined as 6.11 ± 0.34% and 0.121 ± 0.01%, respectively.
The predominant fatty acid found in crambe oil was erucic acid (61.82%), followed by oleic acids (20.58%) and linoleic and linolenic acids (4.71 and 4.21%, respectively). These fatty acids are classified as unsaturated and they represent ∼92% of the total composition of crambe oil. The concentrations obtained in this study are consistent
Conclusions
In this study, the hydroesterification of crambe oil under pressurized conditions was investigated. In the hydrolysis step the use of a high temperature and a solvent in the reaction medium favored the production of FFA at a low water to oil mass ratio. Water in the subcritical state is an effective alternative for the hydrolysis of crambe oil, since high levels of free fatty acids can be obtained without the use of catalysts in the reaction and within relatively short residence times. A
Acknowledgement
The authors are grateful to the MS Foundation for the donation of crambe oil and to CNPq and CAPES for financial support.
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