The Online Analysis Of Impurities Concentration In PX Product Based On Raman Spectroscopy

As a fast and non-destructive quantitative analysis method, Raman spectroscopy is expected to be applied in on-line quality control for industrial processes. This thesis focus on the measurement of impurities contents in PX (p-xylene) product based on Raman spectroscopy. A special preprocessing algorithm and a PLS model are built for the Raman spectra of PX product. Furthermore, an on-line analyzer is developed and has been successfully applied in an industrial PX unit. This thesis includes:(1) A featured preprocessing algorithm is proposed and a PLS model is built. The concentration of PX in the PX samples is above99.7%, so the characteristic peak of PX can be used as the reference peak. The actual central wavelength of the laser can be calculated based on a main peak of PX, and the internal normalization of spectra is used. Based on the processed normal spectra, several PLS (partial least square) regression models are built. These models can calculate the concentration of MB(methylbenzene)、EB(ethylbenzene)、MX(m-xylene) and OX(o-xylene) in PX samples.(2) An on-line analysis system is developed for a PX industrial unit. The system hardware is composed of a sample-pretreatment device and an on-line analyzer, approved cable and optic fibers are used to connect the pretreatment device and the analyzer. Sample filtering and cooling are realized in the pretreatment device; there are spectrometer, industrial embedded computer, laser and other equipment in the analyzer. The software system consists of an analysis software and a calibration software. The analysis software obtains the Raman spectrum of PX sample periodically and calculates the impurity concentration based on the above preprocessing algorithm and PLS regression models. The analysis period is2min. The data communication between the analyzer and a distributed control system is realized with MODBUS protocol on RS485bus, and4analog outputs of4-20mA are provided as backup.(3) The above on-line analysis system has been applied in an industrial PX unit in a refinery. This analysis system has been working steadily for more than8months at the unit. The results of the industrial application show many features of the analysis system, such as fast speed, high precision and little maintenance. Compared to the off-line impurity analysis data from a gas chromatography, the mean square errors of the on-line analyzer are0.005%,0.041%,0.013%,0.003%for MB, EB, MX, OX in sample; the maximal errors of MB, EB, MX and OX are0.012%,0.070%,0.020%and0.008%.