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13/04/2006
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Tom Shelley reports on an optical system for monitoring processes in supercritical fluids
A new family of optical sensing systems can monitor what is going on in supercritical fluids, where temperature and pressure are sufficient to ensure that they are neither liquid nor gas, but a single phase.
The most common fluids encountered in industry are supercritical carbon dioxide, increasingly used in food and pharmaceutical preparations, and supercritical water/steam, which is of importance in the improvement of efficiencies in thermal power generating systems.
A team led by Dr Ian Grimsey, Dr Boris Shekanov and Dr Yongda Sun at the University of Bradford began a project in 2001 to find a way to monitor the sizes and shapes of particles formed in a solution of supercritical carbon dioxide. Particle size is of immense importance in many medications, especially those which have to be ingested by breathing in, since particles smaller than about 1 micron across tend to be breathed in and out while those larger than 10 microns across tend to get no further than the throat.
Owing to the withdrawal of the principal supporting partner for economic reasons, the particle monitor got no further than a drawing board scheme based on optical diffraction.
However, the project did get as far as developing a completely packaged optical sensing system that can accurately monitor ultra violet and visible light absorbance and refractive index in supercritical fluids under pressure, which has been proven on solutions of paracetamol dissolved in supercritical mixtures of carbon dioxide and ethanol. The refractive index is determined from attenuated total reflection within a prism in contact with the liquid, allowing the establishment of the ratio of ethanol and carbon dioxide while the solute concentration is determined from the ultra violet/visible light absorption.
Because of window fouling, it was found to be best to mount the optical system outside the mixing vessel, monitoring the effluent stream. It has been developed as far as being a robust portable instrument package, 30cm x 25cm x 15cm, and requires only a 0.5ml sample volume. It is extremely accurate, determining refractive index to an accuracy of {{plusminus}}0.0001 over the range 1.0000 to 1.4000. The spectrometer works over the range 200 to 850nm with a bandwidth of 2nm. It has been designed to work over the temperature range 25 deg C to 100 deg C and at pressures from 1 to 500 bar. It comes with Windows 2000 based software that can be used to display full measuring scans in real time and store them in a data file. The system is suitable for monitoring what is going on in many other high pressure or hostile environments and the developers are open to offers.
Dr Ian Grimsey's web page
Pointers
* System can determine refractive index to an accuracy of plus or minus 0.0001 over the range 1.0000 to 1.4000
* The spectrometer works over the range 200 to 850nm with a bandwidth of 2nm.
* The system has been designed to work over the temperature range 25 deg C to 100 deg C and at pressures from 1 to 500 bar. It comes with Windows 2000 based software
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Author
Tom Shelley
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