Online analysis boosts purity for Huntsman

Raman laser-based technology in Rosemount Analytical unit helps plant deliver 99.7% purity by reducing feedstock variability in paraxylene purification.

Huntsman Petrochemicals produces up to 360kt per year of paraxylene at its Wilton site in the UK, a plant where control was recently transferred to a DeltaV digital automation system, a technology of Emerson Process Management.

Further investment, in a Rosemount Analytical Raman online laser spectrometer from Emerson, has resulted in added process optimisation and performance improvement, and allowed site engineers more insight into their production process.

Paraxylene is a key starting material in the creation of 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 centrifuging the resulting suspension.

Optimum efficiency is maintained by controlling the composition of the incoming feed stream to the purification plant.

The previous method of monitoring the process stream composition used online 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 2-3% in feed composition.

Early in 2003, a Rosemount Analytical Raman analyser was installed, allowing full online 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 0.25% with a consequent improvement in plant stability.

"The Raman analyser is definitely helping the plant performance", says Tom Liddle, Plant Manager for the paraxylene plant at Wilton.

"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 99.7% quality required first time, all the time".

Steve Gill, Process Engineer at Huntsman Petrochemicals, who pioneered this first use of a Raman spectrometer online at Wilton, says: "Considering that we are doing inline 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".

Tom Liddle is also pleased to see the plant running smoother: "Without the online 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".

Raman spectrometry uses single wavelength laser light to probe the sample stream.

On the molecular level, a very small fraction of the light intensity is scattered.

Although most of this scattered light occurs at the same laser wavelength (Rayleigh scattering) an even smaller fraction of the incident light is shifted to longer wavelengths (Raman scattering).

The shift in wavelength from that of the laser source represents an exchange of energy with sample molecules.

From the pattern of wavelength shifts and intensity of Raman scattering, both qualitative (molecular species) and quantitative (concentration) information can be determined.

In practice a multivariate calibration model is developed for the application, allowing multicomponent analysis to be performed.

The process Raman analyser at Wilton will be used for simultaneous measurements at four separate process locations on the plant - feed and recycled material as described above, plus two final product streams to monitor paraxylene purity.

"We are still learning what the analyser can do", added Tom 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 Rosemount Raman analyser is monitored remotely by Emerson engineers in Ohio, to allow on-line tuning and remote performance optimisation of the calibration model used.

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