Diesel that smells like sunshine

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Diesel that 'smells like sunshine'

By Dr Garth Cambray

On Christmas eve, 2005, I cycled along the highway which passes from Johannesburg to the coast. Lots of fancy, executive cars whizzed past carrying people to the sea. And that was when I noticed that the executive diesels that went by smelt much better than the executive petrol vehicles that went by. These vehicles were powered by SASOL turbodiesel a super low sulphur content diesel fuel. And they smelt clean, like sunshine just after a thunderstorm.

To find out more about how SASOL makes low sulphur diesel, it is important to take a trip back into the mists of chemical engineering history, to 1920's Germany.

The Fischer Tropsch Process

In the 1920's Germany in many ways resembled South Africa, having an abundance of low grade coal, and very little oil. Research conducted by Franz Fischer and Hans Tropsch resulted in the development of a process where a carbon source such as coal or wood is partially oxidised to produce a gas rich in carbon monoxide and hydrogen. This synthesis gas is then reacted in various ways on catalytic surfaces to produce more complex hydrocarbons - some of which are fuels.

The following shows how synthesis gas was produced in the original Fischer-Tropsch experiments in the first equation, and how this gas, reacted on a catalyst was used to produce larger molecules through a chain elongation reaction.

The second world war inspired a far greater investment in this technology and resulted in much fuel being produced for the German war effort. The increased focus on the Fischer Tropsch process resulted in much fine tuning and innovation leading to the actual field of Fischer-Tropsch chemistry being formed.

SASOL

Secunda by night showing the huge SASOL Fischer Tropsch infrastructure installed and operating at this plant since the 1980's. Image from Sasol.

In the 1950s, South Africa recruited many skills from the fuel projects of war time Europe to begin applying and developing Fischer-Tropsch technology to help South Africa gain a level of fuel independence. This led to the establishment of an entire town, Sasolberg, with the first SASOL liquid fuels being produced from coal in 1955.

Large investments in technology development around the original Fischer-Tropsch process resulted in various huge improvements in the efficiency and SASOL commissioned more reactors at SASOL 2 (1980) and 3 (1982) which resulted in the creation of the town of Secunda to service these plants. The 301 meter high chimney at the SASOL 3 plant is the largest free standing structure in the Southern Hemisphere.

The SASOL plants currently produce roughly half of the fuel supply of South Africa, with a combined output of 150 000 - 160 000 barrels per day.

Synthetic Fuels and the Global Economy

In terms of global fuel security, synthetic fuels will ease the overall demand for crude oil. In China and India, economies which are showing the worlds fastest growth in energy demand, synthetic fuels will be very important in the growth and long term stability of these countries.

In many parts of the world, huge surpluses of natural gas exist. Natural gas can be transported in liquefied form and used in a fashion similar to that of other liquid fuels. However, the danger associated with transporting gases increases the overall cost of that option.

A modification of the processes used by SASOL to produce fuels from coal, allows the production of fuels from gas. These technologies, Gas to Liquid (GTL) production systems have leapt onto the global arena with the commissioning of the Oryx GTL plant in Qatar.

In the related article on the Oryx plant, we will explore more in depth how this works. From the perspective of the global economy, the commissioning of this plant has in some ways been a light at the end of an increasingly gloomy energy tunnel.

Why Synthetic Fuels can be better for your vehicle

When crude oil is processed into diesel or petrol, a large highly varied feedstock in the form of crude oil is processed and various catalytic reactions produce a mixture of chemicals which are then separated out into broad categories of things that are roughly the same. These categories include diesel oil, paraffin, petrol and waxes. Many of the contaminants from the crude oil make it through this whole process. A common contaminant in diesel oil fractions is sulphur.

In your diesel engine, the oil in the engine protects the wear of bearings and the rings which seal the pistons in the cylinders. When fresh, a lubricating oil has a specific viscosity. If diesel containing sulphur burns in your engine, some of that sulphur can combine to form acids in much the same way acid rain forms, and these acids, acting in a hot engine with plenty of iron surfaces to act as catalysts, gradually degrade the oil, causing it to become less viscous and go black. As the oil becomes less viscous it protects your engine less from wear and thus, the presence of sulphur in your fuel is a polite way of saying your engine will not last as long as it would if there was no sulphur in the engine.

Picture of oil from a diesel engine running on GTL low sulphur diesel compared to an engine running on normal 350ppm sulphur diesel. This shows how the low sulphur diesel gives extended oil life. Image from Sasol.

Sulphur is also a pollutant - in addition to damaging your engine, vehicles belching sulphur into the environment damage things outside the engine as well. So the less sulphur in fuel, the better it is for everyone. Currently, most countries around the world are increasing the standards of diesel by stipulating that it must contain lower levels of sulphur.

As an example, a year ago, diesel in South Africa contained as much as 3000 parts per million (ppm) sulphur. With new legislation this has been reduced to either 500 ppm or in the case of ultra low sulphur diesel, 50 ppm. And that is the diesel which smells like a sunny summer's day.

More information:

Sources of information: www.sasol.com