Supercritical fluids that create and destroy

In keeping with Tom Edgar's comments, supercritical fluids were much in evidence at the research event. At the University of Bradford, Srinivas Palakodaty and colleagues are treating solutions with supercritical carbon dioxide, to crystallise out the solute as fine, dry particles. This would be extremely useful for the pharmaceutical industry, which often needs ingredients in solvent-free micron-sized particles.

The Bradford researchers' technique mixes a methanol-saturated aqueous solution of lactose with a stream of supercritical CO2 in a high-pressure nozzle (see figure). At these conditions, they explain, water and methanol are insoluble in the CO2, which acts as an `antisolvent', forcing the lactose to crystallise and simultaneously extracting the solvents. The lactose crystallises out as a `nest' of dry, wafer-like crystals with a diameter of 3micro m, says the team.

Meanwhile, Matjaz Krajnc and Janez Levec of the Slovenian National Institute of Chemistry are investigating the use of supercritical water to oxidise acetic acid - one of the most stable organic compounds, which forms a stumbling block in systems designed to destroy organic wastes.

Supercritical water is very efficient at destroying organic compounds, but acetic acid is so stable that it decomposes only slowly even under these conditions. The Slovenian team has therefore been looking for a catalyst - and has found one, based on a mixture of cobalt, copper and zinc oxides. The catalysed oxidation reaction, which requires pressure of 240bar and 420oC temperature, has an activation energy of 109.7kJ/mol, they found, compared with the 180kJ/mol required without a catalyst, giving better conversion at lower temperatures and times.