Seven Important Chemical Separations

May 19, 2016

Good chemists supposedly make good cooks, since cooking and chemistry have many things in common. You need to select the correct ingredients, measure them carefully, choose the right utensils and containment vessels, and process your mixtures at the proper temperature for the proper time. For an analytical touch, you even sample the product at stages and adjust things accordingly.

This cooking-chemistry analogy has become more concrete of late with the emergence of molecular gastronomy. Molecular gastronomy, as defined in an early article on the topic in a publication of the Royal Society of Chemistry, is "...the application of scientific principles to the understanding and improvement of small-scale food preparation."[1]

Physicists have been more associated with molecular gastronomy than chemists. The method was first described in 1799 by physicist, Benjamin Thompson, also known as Count Rumford. It was named molecular gastronomy in 1988 by Nicholas Kurti, a University of Oxford physicist, and Hervé This, a French physical chemist.

A portrait of Benjamin Thompson, Count Rumford by Moritz Kellerhoven (1758-1830). Thompson is best known for his 1797 experiments demonstrating the mechanical equivalent of heat. In those experiments, he showed that the friction of a blunt cannon boring tool could boil water in about two and a half hours. (Accession no. 1332 of the National Portrait Gallery, London, via Wikimedia Commons.)

In 2011, physicist, Nathan Myhrvold, along with Chris Young and Maxime Bilet, published the five volume cookbook, Modernist Cuisine: The Art and Science of Cooking about scientific cooking. Myhrvold's interest in the topic was inspired by his failure to find any good information about sous-vide cooking. Myhrvold is known, also, as a former chief technology officer of Microsoft and the co-founder of Intellectual Ventures.

Reactions are the heart of chemistry. If I look beyond the oxidation reactions that sustain life, I can name the Maillard reaction as my favorite reaction. This chemical reaction between amino acids and sugars is a part of cooking chemistry, since it's the reaction that imparts so much flavor when certain foods are browned in cooking. This reaction flavors such things as breads, biscuits and French fries.

"Do you want fries with that?" (Photo by Evan-Amos, via Wikimedia Commons.)

This reaction was first described in 1912 by the French chemist, Louis-Camille Maillard.[3] The reaction happens rapidly at about 140-165 °C (284-329 °F), and it produces hundreds of different flavor compounds. An alkaline environment, such as a coating of lye on pretzels, accelerates the reaction, giving a deep brown color. Quality control is important, since acrylamide, a possible carcinogen, can be produced at high temperatures.

While food might be close to my heart (as in "the way to a man's heart is through his stomach"), there is more to chemistry than just chemical reactions. Chemical separations are methods of converting a mixture into products, called fractions, at least one of which is enriched in a chemical of interest. David S. Sholl and Ryan P. Lively, both professors in the School of Chemical and Biomolecular Engineering of the Georgia Institute of Technology (Atlanta, Georgia), have just published their opinion on the seven such chemical separations that would qualify as world changing.[4-5]

As Ryan Lively, study co-author and an assistant professor in Georgia Tech's School of Chemical & Biomolecular Engineering, explains,

"Chemical separations account for about half of all U.S. industrial energy use... Developing alternatives that don't use heat could dramatically improve the efficiency of 80 percent of the separation processes that we now use."[5]

Sholl and Lively chose the following as the seven most important chemical separations: Hydrocarbons from petroleum, uranium from sea water, alkenes from alkanes, greenhouse gases from dilute emissions, rare earth metals from ores, benzene derivatives from each other, and trace contaminants from water. They write that the present separation methods are responsible for 10-15% of the world's energy consumption.[4]

Hydrocarbons from petroleum

Not surprisingly, conversion of petroleum to useful hydrocarbons by distillation consumes nearly as much energy as all other chemical processes combined. There are 90 million barrels of crude oil processed daily, consuming 230 gigawatts of energy annually.[5] As shown in the figure, the basic process involves heating the oil to drive off vapors that condense at lower temperatures. (Modified Wikimedia Commons image. Click for larger image.)

Uranium from sea water
Nuclear power is a carbon-free energy source, but you need uranium fuel for this option. As it turns out, there are more than four billion tons of uranium dissolved in seawater. Chemical separation of uranium from seawater is complicated by the chemically-similar elements vanadium and cobalt. There have been small scale tests of improved processes, but much work is still needed to be done.[5]

Alkenes from alkanes
Alkenes such as ethylene and propene are used to produce certain polymers. The total annual production of alkenes is greater than 200 million tons, and an energy-intensive step is the separation of alkenes from their single-bonded counterparts, such as ethylene from ethane. Membranes are a potential energy-saving separation technology, but an industrial-scale plant would require a million square meters of membrane.[5]

Greenhouse gases from dilute emissions
Presently, scrubbing of carbon dioxide and hydrocarbon stack emissions is done by reaction with liquid amines, with an additional heating step required to recover the amine to its initial state. A better method is needed.[5]

Rare earth metals from ores
The rare earth elements are used in the powerful magnets that enable motors and actuators for many applications, and also for some catalysts. They tend to occur together in nature and are chemically similar, so their separation from ores is difficult.

Benzene derivatives from each other
Benzene and its derivatives are used in the production of many materials, solvents and fuel additives. Distillation columns, similar to the petroleum example above, are now used to separated these. About 50 gigawatts of energy are used for this purpose annually. Membrane and sorbents could be developed for a more energy-efficient process.

Trace contaminants from water
As the Flint, Michigan, water crisis reminds us, we sometimes must purify our water before drinking. In many parts of the world, water desalination is an important process, and it's both energy and capital intensive. Some membrane processes are now used, but research is needed to solve problems such as fouling, and to make desalination more energy efficient.

Says co-author David Sholl, chair of Georgia Tech's School of Chemical & Biomolecular Engineering,

"We wanted to highlight how much of the world's energy is used for chemical separations and point to some areas where large advances could potentially be made by expanding research in these areas... These processes are largely invisible to most people, but there are large potential rewards - to both energy and the environment - for developing improved separation processes in these areas."[5]

References:

1. Cooking with chemistry, Chemistry in Britain (Chemistry World), no. 10 Royal Society of Chemistry, October 2003.
2. Nathan Myhrvold, Chris Young, and Maxime Bilet, "Modernist Cuisine: The Art and Science of Cooking," The Cooking Lab, March 7, 2011, ISBN 978-0982761007, 2438 pp. (Modernist Cuisine Web Site).
3. L-C Maillard, "Action des acides amines sur les sucres: Formation des melanoidines par voie methodique," Comptes Rendus, vol. 154 (1912), pp. 66-68.
4. David S. Sholl and Ryan P. Lively, "Seven chemical separations to change the world," Nature, vol. 532, no. 7600 (April 28, 2016), pp. 435-437. This is an open access article with a PDF file available here.
5. John Toon, "Researchers List 'Seven Chemical Separations to Change the World'," Georgia Institute of Technology Press Release, April 27, 2016.