Online rheometry comes of age

Tuesday, 05 August, 2008

To date, when detailed knowledge of the viscoelastic properties of a fluid is required, test volumes must be removed from the process stream and measured using an offline laboratory rheometer. This is a time-consuming procedure that may require the process line to be shut down until the results are available to confirm, or otherwise, the quality of the product.

Alternatively, the process flow may continue with the potential loss of product. In some cases, the volume of lost product could be large and this could be particularly problematic when processing high value materials (such as pharmaceuticals), products that cannot be reprocessed or products that incur a disposal cost (such as, an environmental levy).

Thus, a genuine process control rheometer which can be operated in an inline or online configuration and measure the viscoelastic properties of a process fluid in real time has the potential to improve product quality and reduce processing costs.

The CSIRO Industrial Physics designed On-Line Rheometer (OLR) is such an instrument and uses a patented multi-frequency squeezing flow technique that allows the viscoelastic flow properties of a material to be measured in a very short time. The measured rheological parameters can be used as process controls in a feedback system to control the process, and hence the quality, of the final product. The OLR reduces wastage, saving time and money with potential environmental benefits. Its simple design and stainless steel construction make it easy to use and clean.

How does the online rheometer work?

The OLR is a micro-Fourier rheometer (MFR) that is capable of rapidly characterising the rheological properties of viscoelastic fluids simultaneously over a range of frequencies (shear rates) from 1 Hz to 100 Hz during flow.

Viscoelastic materials have elastic and viscous properties that can be highly dependent on the frequency of the applied oscillation. Therefore, it is critical to measure the rheological properties over a wide range of frequencies in order to be able to fully characterise the material.

A very small amplitude squeezing motion is applied to a volume of sample fluid placed between parallel plates. The upper plate applies the squeezing motion and the force generated within the fluid is measured using a load cell attached to the lower plate. The duration of the measurement is normally less than 60 s, after which the plates are again separated ready for the measurement of a new sample volume. The displacement and force are continuously monitored and a Fourier transform is used to extract complex viscosity and storage and loss moduli at individual frequency steps. It is possible to make real-time measurements of both the viscous and elastic properties of fluids during processing for the development of new formulations.

Other rheometers employ frequency sweep techniques that measure a single frequency at a time, hence a measurement over a wide frequency range can be very time consuming. Such a technique could not be used online or inline to measure rheological properties over a frequency range as the flowing product may be constantly changing. Hence, the sample may not be representative of the product once the measurement is finished.

The OLR has been used in both a static and flow mode and has yielded results in substantial agreement with those obtained from a controlled-strain rheometer.

Australian made

Mr Pat Griffin, managing director of Rheology Solutions, announced that after a long period of discussion and negotiation, agreement with CSIRO had been reached for taking on the licence for the OLR. CSIRO initially released an expression of interest in early 2007 to seven international companies that were known to have expertise in the appropriate markets.

The OLR will be developed by Rheology Solutions and will be taken to the world market. A world-class manufacturing facility will be established employing 20 highly skilled technical and engineering staff, adding to the region’s skill base. This is a significant development for the region with the establishment of a purpose-built manufacturing facility.

Mr Dennis Silvers of CSIRO stated that: “The initiative brought to the table a viable offer that otherwise would not have occurred, which will result in about $2 million in royalties and a small manufacturing and distribution facility to be built in Bacchus Marsh, a town west of Melbourne, that will employ between 15 and 20 people and bring in more than $16 million to the local community.” 

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