(490b) Steam Cracking of Heavy Oil Fractions: Harnessing On-Line GCxGC

Despite the long tradition and vast knowledge base about petrochemical production and petroleum conversion processes there is still room to improve their performance in order to reduce energy consumption, improve selectivity and to better protect the environment. Detailed information on chemical composition of process feed and product, allows to assess their proper economical value, while simultaneously extending our understanding of process chemistry. The latter is crucial when developing process simulation models that are based on fundamental insights in the occurring chemical reactions. These models allow to predict product yields and to determine optimal process conditions, which makes them a powerful tool to improve process performance. In the last decade comprehensive 2D gas chromatography (GC×GC) has proved to be a very powerful tool to unravel the composition of highly complex mixtures1-3. Furthermore, its superior separation power makes GC×GC potentially one of the most suited analytical methods for on-line analysis of product streams in, for example, refineries and steam cracking facilities. The implementation of such advanced characterization techniques have therefore made the industrial implementation of fundamentally developed detailed kinetic models within reach. The development of a dedicated GC×GC system, equipped with both a time-of-flight mass spectrometer (TOF-MS) and a flame ionization detector (FID), will be presented. In order to take full advantage of the setup, the employed GC×GC settings, e.g. carrier gas flow rates, were determined aiming at maximal agreement between FID and TOF-MS chromatograms. This allowed a more straightforward qualitative characterization and therefore a more accurate quantification of various types of petroleum fractions. Detailed analysis of a for example a kerosene, containing n-/iso-paraffins, mono-/di-naphthenes and mono-/naphtheno-/di-aromatics, using the GC×GC allowed identification and quantification of over 300 components. To evaluate the capabilities of two-dimensional gas chromatography as an on-line analytical tool, this GC×GC-FID/TOF-MS has been incorporated into the analysis section of the pilot plant for steam cracking and pyrolysis operated at the Laboratory for Chemical Technology (LCT)4, enabling on-line qualification and quantification of the entire product stream. Provided the selection of appropriate analytical columns, operating conditions, and modulation settings, GC×GC allows to obtain a highly detailed characterization of steam cracker feeds and effluents. Within the pressure and temperature range of industrial steam cracking, the absence of interactions between large radicals and other species in the cracking mixture allows dividing the single event microkinetic model for steam cracking and pyrolysis (CRACKSIM), developed at the LCT5, 6, in two distinct parts: the monomolecular ?μ network' and the ?β network', that contains both uni- and bimolecular reactions between smaller hydrocarbon species. The ?β network' also contains reactions that involve the formation of secondary products such as aromatics. Coupling CRACKSIM to an appropriate reactor model and a solver for the resulting set of differential algebraic equations, allows simulation of the LCT pilot plant. Careful validation of the overall kinetic model was carried out, based on the experimental results obtained by steam cracking of heavy petroleum fractions, e.g. kerosene and vacuum gas oil. Topics such as lumping of species and/or feedstock molecules, network size and network reduction based on sensitivity analysis and rate of production analysis will be discussed. The detailed analytical information allowed further validation of these concepts in particular in relation to the prediction of the product yields and selectivities of both light gasses and the so-called pyrolysis gasoline and pyrolysis fuel oil fraction.

References

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