Getting the sulfur out: Method is kind to environment and wallet.
Seeberger, A and A Jess. 2010. Desulfurization of diesel oil by selective oxidation and extraction of sulfur compounds by ionic liquids - a contribution to a competitive process design. Green Chemistry 12:602-608.
A new technology for removing toxic sulfur compounds from diesel fuel could provide a cheaper and more environmentally friendly alternative to the currently used method, according to research published in the journal Green Chemistry.
Finding an innovative way to remove the sulfer is important because clean, low-sulfur diesel is becoming more widely used in Europe and the United States. Diesel cars have 20 - 40 percent better fuel economy and also produce 10 - 20 percent less greenhouse emissions per mile than their gasoline rivals.
Yet, the United States and Europe stringently regulate diesel's sulfur content to minimize its other harmful environmental impact: acid rain. The added step of removing the sulfur increases refining costs and availability of the alternate fuel.
Diesel fuel naturally has more organosulfur compounds than gasoline. In contrast to oil-derived diesel, though, biodiesel does not contain sulfur compounds. When deisel is burned in an engine, the sulfur compounds turn into sulfur oxides, which can combine with water in the atmosphere to make acid rain. Acidic rainwater hurts the environment; it kills trees and plants and destroys water ecosystems by changing the pH balance.
Oil refineries currently remove sulfur by a process called hydrodesulfurization. Hydrogen gas in combination with high temperatures and extreme pressure forces the sulfur out of its liquid state and into a gas phase where it can be siphoned off. The method requires large amounts of energy and expensive hydrogen gas. While this process removes some sulfur compounds, others, like dibenzothiophene (DBT) are left behind.
Ideally, a process that avoids hydrogen yet provides a cleaner and cheaper diesel is needed. Researchers are attempting this with "ionic liquids" – organic salts that are liquid at room temperature. But, the process has two big drawbacks: 1) not all of the sulfur compounds can be extracted from the diesel and 2) there is no good way to remove the ionic salts from the liquid waste so they can be reused.
A research group in Germany may have solved these problems. First, they added oxygen, which improved both quantity and types of sulfur extraction. Then, they separated the sulfur from the ionic liquids by adding and evaporating water. The process extracts sulfur compounds so they can be used as a raw material in other industries, which was not possible with hydrodesulfurization. However, an energy-efficient method of removing water from the ionic liquids is still needed.
Innovative ways to remove the sulfur will lower the cost of refining and decrease the harmful exhaust gases from diesel fuels. Both will encourage a broader use of diesel as an alternative fuel, say the authors.
