Monitoring: take a direct approach

Paul Boughton

Inline process monitoring directly in the extruder by measuring colour using UV-VIS spectrophotometers. By Fuat Eker

Inline measurements of the UV-VIS spectrum not only indicate product quality but also provide an excellent insight for process monitoring in the extrusion stage. All parameters in the production process (temperature, pressure, production speed, feeders, pumps, etc.) impact directly on the observed UV-VIS spectrum. Measurements can now be made directly in the melt without the need for sampling. Immediate detection of deviation from specification allows immediate adjustment of the process parameters.

Most polymer extruders still rely on the old and exceedingly cumbersome method of off-line colour measurements (VIS), which usually involves the following time consuming steps: manually take a sample from the process; send/take the sample to the laboratory; shape the material into a measurable sample i.e. a sheet or a plate/chip; allow the sample to cool to room temperature; make the ‘off-line’ measurement with a colour spectrophotometer; report the results to production; decide on the process adjustment; and repeat the first six steps if necessary to obtain another measurement UV-VIS/colour corrections after change of process parameters.

The interval between sampling and obtaining the results from the laboratory can be several hours. Another serious disadvantage is that only a single measurement is generated in this time period and the quality of the colour during, before and after the sampling point is unknown.

The logical replacement for this process is inline UV-VIS spectroscopy for colour measurement taking place directly in the melt. Such an approach eliminates the first six steps of the offline methodology and the measured results are available on a real-time basis. Complete documentation is possible and intervention can take place immediately and the new results be seen without delay. 

Process monitoring

Extrusion is a very complex process, influenced by a range of factors that are under varying degrees of control at any point in time. Such factors include process parameters such as temperature, pressure, production speed followed by the extruder itself, for instance pumps, feeders, blenders, combination of the screws, cooling and vacuum systems. On top of this, raw material can influence the result, reflecting different lots and mixture ratios. All these factors, either singly or in combination, lead to changes in the quality and consistency of the material produced during the extrusion process.

Today, evolving and converging technologies, such as remote monitoring, advanced analytics and data management can bring major improvements to costs, productivity, quality, safety and reliability. As Accenture documents in a recent report, many chemical company executives recognise the value of these technologies, however in practice the management teams often move ahead too slowly in the adoption of these new technologies. Consequently, companies fail to gain competitive advantages that could otherwise have come from combining digital process data with other IT systems that are in place in most processing plants.

ColVisTec has designed a range of probes and UV-VIS spectrometers with great care taken to make the systems easy to install, and very reliable and robust even when operating under the harshest conditions (process temperature up to 400°C, pressure up to 200 bar). Process information can be digitally distributed and combined with other process data in real time and made available for machine operators as well as the plant managers via their control systems. With the much larger number of data points, completely new insight into the state of the operations becomes viable, and the idea of a digital process control system for the entire production flow far more achievable.

Early adopters of this new approach are, for example, found among polymer recycling companies that have been quick to identify and implement new monitoring technologies. A plausible reason is that these companies are often faced with highly complex raw material streams. Though it can also be reflecting a fresh overall approach to polymer manufacturing, not found among the incumbent players, and a strong desire to increase the value of the end product into a fully recyclable output rather than accepting that the reused polymers should end up being down-cycled.

User benefits

In summary, there are a host of benefits to the end users of this new approach – with real-time information about process stability and quality being top of the list. Other benefits include: detection of dosage elevations; detecting changes in feedstock, both for virgin as well as recycled polymers; determination of the impact of speed changes on the quality of the extrusion; and determination of optimal extruder screw configuration. Residence time measurement and fast analysis with three data points/second is another key benefit. Detection of pulsing pumps is also possible, along with the ability to detect and avoid off-specification batches and waste production. Furthermore, optimised recipe formulation and processes design are attractive features for end users. Finally, a comprehensive quality audit trail enables customer documentation of the quality of the delivered product. Overall, the end result is reduced costs and a higher ROI.

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Fuat Eker is with ColVisTec