Scientists from the Institute of Petroleum, Chemistry and Nanotechnologies of the Kazan National Research Technological University have developed and tested the method of improving the biodiesel quality with the help of supercritical carbon dioxide. They indicated how it is possible to control the fuel composition by removing from it the most ‘heavy’ and viscous components. To this end, the researchers used computer modeling and laboratory experiments in order to understand how pressure influences the process of separation.
One of the key problems of biodiesel is high viscosity, especially in the cold. This is related to the fact that its composition contains saturated ethers, for instance, ethyl palmitate. These degrade fluidity of the fuel, hinder the normal engine start and may cause the formation of deposits. To make biodiesel more convenient for use, such components should be removed leaving lighter and more fluidal substances, for example, ethyl oleate.
Scientists suggested using for this purpose carbon dioxide in a special state – supercritical. In such a form it behaves both as a gas and as a liquid: it easily penetrates into the mixture and simultaneously dissolves substances quite well. This makes it possible to use it as an effective and, at the same time, eco-friendly ‘separator.’
To test their idea, scientists assembled a laboratory installation and passed the flow of supercritical CO₂ through the mixture of ethyl oleate and ethyl palmitate. Concurrently, they created a similar computer model of this process. Since the substances needed were not available in the standard bases, their properties – density, viscosity and so on – had to be set separately with aid of special program modules. The computations took into account all that has an effect on the result: the design of the apparatus, how the flow moves inside it and how the gas contacts the liquid. Modeling was carried out at the temperature of 40 °C and at two pressures – 11 and 14 megapascals.
It turned out that pressure plays a decisive role. At 14 MPa carbon dioxide becomes denser by approximately 15-20% and passes through the installation more uniformly, without ‘empty’ zones, where the process is almost not going on. Owing to this, it works as a more effective solvent. At the same time selectivity is enhanced, too: ‘heavy’ ethyl palmitate is extracted markedly more actively than the ethyl oleate. As the pressure grows, this difference tends to increase. While at 11 MPa its share at the outlet was about 0.69, at 14 MPa it already reached 0.76.
Experiments confirmed the computations: at pressure 14 MPa and time 30 minutes the extract received consisted by 94.3% of the ethyl palmitate namely, while the remaining mixture became lighter and more fluidal.
The next step is checking the process at larger installations when the flow structure, distribution of phases and energy costs will change. A more objective economic evaluation will also be required: a higher pressure gives better separation, but needs more energy for CO₂ compression.
In the future the results of Russian scientists may help in building industrial lines for biodiesel purification, where the supercritical CO₂ will be used as an eco-friendly alternative to traditional solvents. This will allow us to get fuel with more stable properties and to make biodiesel more competitive as compared to the usual diesel.
