(33f) Steam Cracking of Heavy Oil Fractions Revisited

Currently there is an increasing tendency to use heavy hydrocarbon mixtures such as condensates, kerosenes, light gas oils, vacuum gas oils and waxes as steam cracking feeds. Large amounts of these low cost fractions remain available due to a decreasing need for heavy fuels, a global shift in demand and depleting reserves of sweet crude oils¹. Cracking of these heavier fractions requires less heat in comparison with cracking of light fractions but results in most cases to lower ethylene and propylene yields. Moreover, the use of heavy fractions can result in a drastic increase of coke deposition in both the radiant coil and the transfer line heat exchanger. Therefore proper assessment of the feed quality and estimation of the feed value is crucial for ethylene producers, in particular because the effluent also becomes more complex. For example products containing four aromatic rings have been observed experimentally.² Another difficulty is that the current generation of simulation models have been developed for cracking of ethane and naphtha fractions and are less accurate for heavier fractions. This results in huge uncertainties in the model predictions mainly caused by the lack of detailed characterization both on the feed  as  on the product side. Complete quantification of the feed and the effluent in molecular detail for fractions heavier than naphtha was assumed impossible until quite recently. However, the steady development and improvement of comprehensive 2D gas chromatography (GC×GC) during the last decade has made it possible to revisit the steam cracking of heavy fractions. GC×GC has proven to be a very powerful tool to unravel the composition of highly complex mixtures^3,4,5. Furthermore, its superior separation power makes GC×GC potentially one of the most suited analytical methods for on-line analysis of product streams in refineries and steam cracking facilities. GC×GC makes it possible to look at steam cracking of heavy fractions from a new perspective and create new insights. Therefore we have incorporated a GC×GC-FID/TOF-MS into the analysis section of the pilot plant for steam cracking operated at the Laboratory for Chemical Technology (LCT)⁶ and have studied the cracking behavior of a huge number of heavy fractions. This contribution will discuss the cracking behavior and coking tendency of kerosene, condensate, gas oil  and vacuum gas oil fractions in comparison to traditionally used naphtha fractions. One of the main advantages of our set-up is that it does not only allow off-line analysis but also on-line qualification and quantification of the entire product stream in one single run. Over 300 different products can be identified with our approach: from permanent gasses (hydrogen, carbon monoxide, ethylene, etc.) to molecules with four aromatic rings or more than 40 carbon atoms. Examples will also be given of off-line feedstock analysis of kerosenes, atmospheric gas oils, condensates with and without a heavy tail and vacuum gas oils using the same GC×GC set-up. Comprehensive two-dimensional gas chromatography can provide the desired information regarding contaminants, composition (e.g. detailed PIONA until C40) and boiling point curve in a reasonable time.

References

1.         Singh, J.; Kumar, M. M.; Saxena, A. K.; Kumar, S., Reaction pathways and product yields in mild thermal cracking of vacuum residues: A multi-lump kinetic model. Chem. Eng. J. 2005, 108 (3), 239-248.

2.         Van Geem, K. M.; Pyl, S. P.; Reyniers, M.-F.; Vercammen, J.; Beens, J.; Marin, G. B., On-line analysis of complex hydrocarbon mixtures using comprehensive two-dimensional gas chromatography. Journal of Chromatography A 2010, 1217 (43), 6623-6633.

3.         von Muhlen, C.; Zini, C. A.; Caramao, E. B.; Marriott, P. J., Applications of comprehensive two-dimensional gas chromatography to the characterization of petrochemical and related samples. Journal of Chromatography A 2006, 1105 (1-2), 39-50.

4.         Dutriez, T.; Courtiade, M.; Thiebaut, D.; Dulot, H.; Hennion, M. C., Improved hydrocarbons analysis of heavy petroleum fractions by high temperature comprehensive two-dimensional gas chromatography. Fuel 2010, 89 (9), 2338-2345.

5.         Cortes, H. J.; Winniford, B.; Luong, J.; Pursch, M., Comprehensive two dimensional gas chromatography review. Journal of Separation Science 2009, 32 (5-6), 883-904.

6.         Vandamme, P. S.; Froment, G. F., Putting computers to work - Thermal cracking computer control in pilot plants. Chem. Eng. Prog. 1982, 78 (9), 77-82.