Paraxylene is a key starting material in the creation of the polyester resin and fibre used in the manufacture of clothing, films, drink bottles and food containers.

The pure product is separated from the other two xylene isomers -- orthoxylene and metaxylene -- in a process that involves selective crystallisation from a chilled solution and centrifugation of the resulting suspension. Optimum efficiency is maintained by controlling the composition of the incoming feed stream to the purification plant.
At Huntsman Petrochemicals' 360000t/y paraxylene plant at Wilton, UK (Fig.1), the original method of monitoring the process stream composition used on-line melting point analysers, confirmed with frequent grab samples, delivered for laboratory analysis. The delays inherent in providing meaningful data from either of these techniques led to a variability of 2--3percent in feed composition.
Plant control was recently transferred to a DeltaV digital automation system supplied by Emerson Process Management. Further investment, in a Rosemount Analytical Raman on-line laser spectrometer from Emerson, has resulted in added process optimisation and performance improvement, and allowed site engineers more insight into their production process.
Since its installation, the analyser has allowed full on-line composition monitoring of the feed to the purification plant. With composition information being updated every minute, process variability has been dramatically reduced by an order of magnitude to 0.25percent with a consequent improvement in plant stability.
"The Raman analyser is definitely helping the plant performance," said Tom Liddle, paraxylene plant manager. "By reducing the variability of the process composition, we can run the plant at the optimum settings. We have improved plant efficiency, improved consistency, and we get the 99.7percent quality required first time, all the time," he added.
Steve Gill, process engineer at Huntsman Petrochemicals, who pioneered this first use of a Raman spectrometer on-line at Wilton, agreed: "Considering that we are doing in-line dilution, I am very pleased with the performance. While the main benefit of the purification control scheme is to give consistent solids feed to our centrifuges, an additional benefit has been the ability to see the impact of upstream changes on variability. We've never been able to see that in real time before."
Liddle is also pleased to see the plant running smoother: "Without the n-line control provided by the Raman, variability in the process would occasionally lead to excessive solids loading in the centrifuges, resulting in vibration and potential bearing damage. Now we run at maximum output, and have reduced wear on the centrifuges."

How it works

The Rosemount analytical process Raman spectrometer uses an NIR multi-mode diode laser as the excitation source, to minimise fluorescence interference and to provide a long life source (typically two and a half years) in continuous operation. This laser has a broad line shape, which in turn produces low-resolution spectra. Instability of the line shape caused by mode-hopping is compensated by the use of an internal reference.
The monochromatic radiation is transmitted to the process sample via fibre optic cables, an optical filter and the Raman probe. The probe also collects light which is scattered back, and both Rayleigh and Raman scattered radiation is returned to the analyser using separate fibre optic cables. Optical filters are used to filter out unwanted wavelengths. The Raman scattered light is detected through a spectrograph and recorded by a high sensitivity CCD camera. The Rayleigh fibre terminates at a photodiode and is used as part of the analyser's laser safety function.
The compact nature of the lasers and their controllers and the geometry of the detector, both spectrograph and camera, enables the spectrometer to measure up to four process streams continuously (as well recording the internal reference spectra) on a single spectrograph and CCD camera.
Patented software routines produce standardised spectra giving inter-channel comparability. The advantages are that it facilitates calibration transfer where multi-stream monitoring is required, and avoids the need for complete re-calibration when optical components are changed. In addition, a factor-based normalisation routine helps smooth out the effects of bulk sample variability such as bubbles. The net effect of these standardisation routines and the internal reference is extremely repeatable spectral results.
For quantitative analysis, spectral data reduction and conversion to component concentrations is achieved by using a multivariate calibration method
-- typically a partial least squares regression. This data handling is carried out on board the analyser, so that actual component concentrations can be supplied as the analyser outputs for managing the process.
Complex molecules, with many atoms, may exhibit many possible transitions that can give rise to Raman scattering effects.
The intensity of each Raman spectral line depends on the concentration of these molecules, plus a factor quantifying the inherent tendency of the particular vibration to undergo a Raman transition. This can usually only be determined empirically: some compounds and transitions are particularly strong scatterers.
In aromatic hydrocarbons, the skeletal arbon-carbon double bonds provide a source of trong Raman scattering effects, making this a valuable technique for the analysis and measurement of aromatic compounds.
The process Raman analyser at Wilton is now used for simultaneous measurements at four separate process locations on the plant -- feed and recycled material, plus two final product streams to monitor paraxylene purity.
"We are still learning what the analyser can do," explained Liddle, "and have a little more to understand to get the on-line quality measurements on the product streams fully operational."

The analyser is located in a control building. The laser light is transmitted through fibre optic cables onto the plant to the four measuring locations: optical probes provide the interface to the process streams. At each probe, light scattered by the sample is collected and transmitted back to the analyser through the return fibre. The derived analysis and concentration data are transmitted via Modbus communications to the Emerson DeltaV process control system, and the signals used to control plant feed dilution.
The Rosemount analytical Raman support team, working on both sides of the Atlantic, worked with engineers from Huntsman Petrochemicals to provide project guidance from the early consultation and application engineering through to calibration and commissioning. The analyser is monitored remotely by Emerson engineers in Ohio to allow on-line tuning and remote performance optimisation of the calibration model used.