Purified by Sunlight

Schema TU

Sunlight can be used to purify water. This process is not yet in common use, however, because it requires
large, expensive installations. Researchers at ChemE would like to make photocatalysis more attractive by increasing its efficiency.

Our ancestors were already aware of the purifying power of sunlight. They bleached their sheets by laying them on the grass. The first industrial applications of solar purification took place 30–40 years ago. These applications involved water purification. Little has changed, observes Dr Ruud van Ommen of ChemE, in the Faculty of Applied Sciences. Together with Dr Michiel Kreutzer, he supervised the Iranian-Dutch PhD candidate Dr Mahsa Motegh in her study into the design of photocatalytic reactors.

According to Kreutzer, this type of reactor can be calculated with ray-tracing. In this procedure, a computer calculates countless paths that photons can take through the reactor. Photons are reflected, bent, absorbed and occasionally used for photocatalytic transformation. This usually takes place through finely separated titanium dioxide (the material that makes paint and toothpaste white). The calculation of all sorts of light paths is highly complex.

Motegh developed an alternative, in the form of several design rules for photocatalytic reactors. Her calculations predict efficiency within a margin of 20%, estimates Kreutzer.

More importantly, engineers can use the guidelines developed by Motegh to optimise photoreactors during the design process.

In general, the efficiency of photoreactors is quite low. For example, if one in every 500 photons results in a chemical transformation, this would translate into an efficiency of 0.2%.

“The challenge is to improve this efficiency”, proposes Kreutzer. “I’m not talking about 80%; even 2% would represent a tenfold improvement. Then we could reduce the surface of the reactors, and thus the investments as well. Ultimately, there are only three or four matters that designers must take into account.”

Van Ommen adds that the use of guidelines in the design of ordinary catalytic reactors has long been common practice. It is now possible for photocatalytic reactors as well.

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