Crude oil with no added salt

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  • Published: Feb 8, 2010
  • Author: Jon Evans
  • Channels: Ion Chromatography
thumbnail image: Crude oil with no added salt

Using ion chromatography, Brazilian chemists have shown that their new method for extracting salt from crude oil is faster, greener and more flexible than existing methods.

It's no secret that, when first pumped up from the ground, crude oil is in no fit state for anything. Before it can be used as a fuel or a chemical feedstock, the complex mixture of hydrocarbons first needs to be refined, splitting it into various fractions made up of a simpler mix of hydrocarbon molecules.

What's less well known is that before it can even be refined the crude oil needs to be cleaned up. For oil tends to come up from the ground accompanied by a load of salty water, forming a water-in-oil emulsion. It's important that most of this salty water is removed from the oil before any refining takes place, because salt can cause corrosion problems in refineries. Generally, the salt concentration should be reduced to below 500µg per gram of oil.

The current standard method for removing salt from crude oil involves adding a demulsifier, which is a mixture of surfactants and dispersants, to the oil. As its name suggests, the demulsifier breaks apart the emulsion and causes the salty water and oil to form two distinct phases. The salty water is then simply filtered off.

Unfortunately, there are a number of problems with this method. For a start, it employs toxic chemicals, which is not ideal, and it is quite time-consuming, taking several hours. It also works best with light crude oil. But as reserves of light crude oil become scarcer heavy crude oil, which contains higher concentrations of asphaltenes and resins and so requires more refining steps, is increasingly taking its place.

To develop a faster, greener salt extraction method that works with heavy crude oil, a team of Brazilian chemists led by Érico Flores at the Federal University of Santa Maria turned to microwaves. Now microwaves are already widely used to speed up chemical reactions, but a number of scientists have shown that they can also break down emulsions.

They do this both by heating the individual water-in-oil droplets, shaking apart the oily film that surrounds them, and by rearranging the electrical charge distribution of the water droplets. Up to now, however, no one had tested to see whether microwaves could work their magic on crude oil emulsions.

Flores and his team quickly discovered that they could, obtaining the best results after 30 minutes of microwave heating at 800W. The chemists also found that the process could be further improved by adding extra water to the emulsion. This is because microwaves are more effective at heating water than oil, generating water vapour that helps to break down the emulsion. Like a chemical demulsifier, this process divides the salty water and oil into two phases, allowing the water to be filtered off.

After testing this extraction method on three samples of heavy crude oil with known salt concentrations, the chemists then analysed the separated water and oil with ion chromatography, in order to determine how much salt had been extracted. They found that certain samples needed more than one round of extraction, but in all cases over 95% of the salt could be extracted after at most three rounds.

In the case of one sample, the chemists managed to reduce salt concentrations from 17000µg per gram of oil to less than 20µg in just one extraction round. Furthermore, each extraction took just one hour to perform and required no toxic chemicals. At the moment, however, the method is limited to the laboratory scale.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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