Plastics monitoring falls inline

Materials World magazine
,
1 Feb 2010

Real-time monitoring of high-density polyethylene (HDPE) production is now possible, claim researchers in Germany.

Their technique uses a combination of laser reflectance measurements and video monitoring to follow ethylene polymerisation in stirred slurries under different
conditions. The results have been used to explain variations in
efficiency and product range.

‘Being highly recyclable, [the applications of HDPE] are likely to increase, so improving efficiency of production is important,’ says Professor Rolf Mulhaupt, who led the project at Albert-Ludwigs University in Freiburg. ‘If producers can monitor what is happening inside their reactors, fine-tuning is possible and product quality is easier to control. The possibility of developing new catalysts is also very real.’

A particle vision and measurement probe has been used to make microscopic video images of moving particles, with camera-mediated imaging that has a pulsed light source. This is
combined with a focused beam reflectance measurement probe.

The laser is combined with simultaneous ethylene mass flow measurements to distinguish between ideal particle growth and more complex processes involving particle fragmentation and aggregation, which occur during stirring.

The instrument can detect changes in particles of two to 1,000µm and can show them in crystal morphology at the same time that size data is recorded.

Current methods for measuring these reactions have involved offline monitoring or special cells without stirring. Stirring is necessary in commercial polyethylene production, but it plays havoc with standard particle monitoring techniques.

‘The complex interplay of polymer particle growth and catalyst particle fragmentation governs polymerisation kinetics, as well as polymer particle morphologies and bulk densities,’ explains Mulhaupt. ‘In gas-phase reactions, optical microscopy has been combined with video to monitor particle growth inline. But we have taken this idea and developed it to a new level to enable it to be applied to stirred slurries, which are far more complex environments.’

Brian Brooks, Professor of Chemical Engineering at Loughborough University in the UK, says that the technique could provide valuable information, but has some limitations.

‘Images [produced using this technique] might show how the morphology changes during growth of the polymer particles, but there is a limitation on the sizes that can be visualised’, he explains. ‘Catalyst behaviour, that involves changes of entities in the sub-micron range, would not be detected’.

He adds, ‘Particle size measurements could be correlated with polymerisation kinetics but an independent assessment of the reaction rate would be required because a material balance
cannot be obtained from size measurements alone if the number of particles is not constant.’

Results from both probes are said to have compared well with those obtained from offline
monitoring. Furthermore, although only HDPE has been investigated, Mulhaupt expects application to other polyolefin slurry systems to be straightforward.

The costs of implementing the technology on a large-scale industrial HDPE production process is being investigated.