Exploring the benefits of Raman technology

15 November 2016

Control Engineering Europe looks at Raman spectroscopy, which is still considered by many to be a rather exotic technonology. Despite this it is starting to gain fans in the process sector for in-situ sample measurement.

Endress+Hauser has recently announced that it is to expand its US-based manufacturing facility for Raman analysers. “The new facility will become the core of our future Raman-spectroscopic analyser manufacturing,” said Tim Harrison, managing director of Kaiser Optical Systems, a wholly-owned subsidiary of Endress+Hauser.

Raman spectroscopy is a technique used to observe vibrational, rotational and other low-frequency modes in a system to provide a ‘fingerprint’ to identify molecules. It utilises the interaction of an incidental laser beam with the material being examined. When the material absorbs the light, single photons excite the molecules, which in turn emit new photons at a frequency different from the original frequency. This light scattering, or frequency shift, can be measured in order to create a molecular fingerprint for each material. 

The field of Raman spectroscopy was transformed by the creation of a holographic filter, developed by Kaiser Optical Systems, to remove unwanted diffracted light appearing at the same wavelength, which would otherwise conceal the frequency shifts typical of the material being examined. This filter enables the entire Raman spectrum to be acquired simultaneously, offering real-time analysis capabilities.

Even among industry professionals, Raman spectroscopy is often considered rather exotic although it does offer some clear benefits, such as fibre-optic connectivity to remote sampling locations without the need for sample loops and a single analyser can measure up to four sample locations simultaneously which means it can lower site installation costs and reduce cost-of-ownership.

Despite the fact that most of the reasons for not using the technology were resolved long ago, such as its approval for use in hazardous areas, many process engineers have been reluctant to use the instruments until recently. However, one company that has welcomed the addition to Raman spectroscopy to its suite of process analysis tools is chemical company, Lanxess. “The advantages of Raman spectroscopy have been acknowledged for years,” said Frank Grümbel, head of process analysis technology at Lanxess. Compared to infrared and near-infrared spectroscopy the Raman method is well suited for examining aqueous solutions or analysing inorganic substances. In addition, the technology has long been used in the laboratory to examine the composition and properties of solid, liquid or gaseous substances.

Grümbel and his team are responsible for 3,000 measuring points at the company’s German plants as well as for applications across the globe. The tasks the team has to undertaken are as diverse as the measurement tools it relies on – which includes everything from pH sensors to mass spectrometers, including Raman analysers.

Proven suitability
At Lanxess, Raman spectroscopy has proven its suitability for use in day-to-day operation over the past four years. “We use it to augment our measurement tool kit, especially in the area of near-infrared spectroscopy,” continued Grümbel. Lanxess relies on the Raman Rxn4 from Kaiser Optical Systems. The Raman RXN4 gas-phase Raman analyser offered the company an alternative to typical process gas analysers. Its chemical specificity and spectral range of allows multiple gases to be independently identified without chromatographic separation over concentration ranges from ppm levels to 100%. Up to four measurement points can be connected to the rack-mounted devices. Fibre optic technology provides flexible connectivity between the analyser and remote sensors. Measurements are carried out by immersion probes directly in the process without the need for sample preparation. 

Properly understanding and operating the instruments did, however, required advice and support from Kaiser Optical Systems initially, such as how to model and interpret the measurement data, but today the analysers are helping Lanxess to control various process plants or are helping the process analysis team to become more familiar with processes that are designed to run without Raman spectroscopy at a later point.

One application example where Raman spectroscopy analysers have proven their worth is in polymer manufacturing, where they are used to analyse the molecules’ composition. This helps to determine the precise dosage of substances that react with each other. An over- or underdosage lessens the quality of the product. Non-reactive substances will have to be recycled at a later point which requires costly processing. “We searched for a long time for solutions to tackle some of our measurement jobs. In this case, we found a good answer in Raman technology,” said Grümbel.

Raman may not be the best technology for standard applications, due its cost. However, Raman instruments can certainly be justified whenever they offer a better solution than other methods at solving a measurement task. “The benefits are readily apparent. The instruments are easy to operate and maintain; they do not impact the process and they deliver reliable results even under extreme conditions.” For Frank Grümbel, therefore, one thing is clear: “Optical analysis will become increasingly important for us in the future.”

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