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The equipment that was previously used to monitor the compounding process was an online melting point analyzer that required frequent sample taking and sent to the laboratory for analysis and confirmation. The time lag in data transfer can cause a 2% to 3% change in feed synthesis. The Rosemount Analytical Raman Analyzer, installed at the beginning of 2003, enables comprehensive on-line monitoring of synthesis. As synthesis information is updated every minute, production changes are reduced to 0.25%, significantly improving production stability.
“The Raman analyzer does improve the quality of production,†said Tom Liddle, head of the p-xylene plant. “By reducing the changes in the synthesis process, we can achieve optimized production conditions and increase stability. This is 99.7% for the first time. The quality level."
Steve Gill, a process engineer at Huntsman Petrochemicals, first used the Raman inline spectrometer. He said, "I'm very satisfied with its effectiveness. The main benefit of purge control is to give consistent solids to the separator. In addition, due to stability, We can see the impact of the change in the previous process as a post process. We have not been able to monitor in real time."
Tom Liddle is also pleased to see the factory running smoothly: “Without Raman's online control, changes in production can sometimes cause the separator to be loaded with excess solids, causing vibration and potential bearing damage. Now we have the largest Yield production, and reduce the wear of the centrifuge."
Laser detection
The Raman spectrometer uses a single wavelength laser to detect the sample. At the molecular level, light intensity is scattered in very small segments. These scattered lights occur at the same laser wavelength (Rayleigh scattering), and a smaller portion of the incident light is transferred to a longer wavelength (Raman scattering). This laser transfer represents energy conversion with small molecules. The qualitative and quantitative (concentration) information can be obtained on the mode and intensity of the wavelength shift in Raman scattering. In practical applications, it is necessary to develop a multivariate calibration model to achieve multiple component analysis.
The Raman analyzer used at Wilton will be used at the same time to measure four separate production locations: feed, recycled materials, and monitoring of paraxylene purity in both final product lines. "We are still learning about the use of analyzers," Tom Liddle said. "I know more about online testing."
The analyzer is placed in the control room. The laser passes through the optical fiber to the four measurement locations: the optical probe provides the interface to the production line. At each probe, the scattered light produced by the sample is collected and returned to the analyzer through the return fiber. Initial analysis and concentration data are sent to the Emerson DeltaV production control system via Modbus to control feed dilution.
Installation requirements
The design criteria for Raman analyzers are very different from those for laboratory instruments. It is not only a "durable experimental instrument", the entire system from the analyzer to the probe, fiber needs to be suitable for the installation of the production line.
Taking into account the long-term stability of continuous use, the reproducibility of the analytical data, sample interface, on-line diagnostics and preventive maintenance, laser safety, appropriate data output protocols, and related cost controls are the basis of the basic design.
The excitation source uses a NIR multimode semiconductor laser to minimize fluorescence interference and operate continuously (usually two and a half years). The line shape of this laser is relatively wide and the returned spectral resolution is low. Linear instability caused by pattern skipping is compensated by using an internal reference.
Monochromatic radiation passes through the fiber optics, optical filters, and Raman probe to the sample. The probe also collects the light scattered back by the sample. Rayleigh and Raman scattering passes through different optical fibers back to the analyzer. Optical filters are used to filter the unknown wavelengths. Raman scattered light is dispersed into a spectrogram and recorded by a highly sensitive CCD camera. The Rayleigh fiber ends in a photodiode as part of the laser safety function of the analysis product.
The combination of the laser, controller and detector geometry (spectral camera and camera) allows the Raman analyzer to use a spectral camera and CCD camera to simultaneously measure four industrial production streams (and record internal reference spectra).
The patented interphase correction process produces a standard spectrum that makes the internal channels comparable. Its advantage is that the calibration can be moved to multiple monitoring points and recalibration is avoided when the optics are replaced. In addition, factor-based normalization can help remove bulk samples such as air bubbles. The combination of standardized processes and internal references ensures that the spectral results are well-reproducible.
For quantitative analysis, the spectral data is converted to the content of the components by using a multivariate calibration method (typically Partial Least Squares regression). Data processing is done by the analyzer, so real data on the content of ingredients can be obtained within one minute of the update time. The Rosemount Analytical Raman Services team works on both sides of Atlanta and provides engineers with Huntsman Petrochemicals early project guidance and advice, engineering applications, and calibrations. Emerson engineers remotely monitored Rosemount Raman analyzers in Ohio to complete in-circuit debugging and remote optimization of calibration modes.
(This article is from the world of electronic engineering: http://Test_and_measurement/2013/0911/article_7809.html)
Online Raman Analyzer Improves Quality of Petrochemical Products
Huntsman Petrochemicals produces 360 kt of paraxylene per year at its Wilton, UK facility. Paraxylene is a key raw material for the manufacture of polyesters and chemical fibres. Polyesters and chemical fibres are widely used in the production of garments, films, beverage bottles and food containers. The purity of the product consists in separating the two other xylene isomers o-xylene and m-xylene. Selective crystallization and centrifugal suspension during cooling. The best purity can be achieved by controlling the compounding of the feed.