Heavy hydrocarbon pyrolysis

Mathematical models for the pyrolysis of naphthas, gas oils, etc. are relatively empirical. The detailed analysis of such a feedstock is essentially impossible, and all heavier feedstocks have a wide range of compositions. Such heavy hydrocarbons also contain a variety of atoms often including sulfur, nitrogen, oxygen, and even various metal atoms. Nevertheless, certain models predict the kinetics of pyrolysis, conversions, yields, etc. with reasonable accuracy and help interpret mechanistic features. 

In addition to the summary and correlations described here, important experimental information and literature exists for pyrolysis of hydrocarbon systems. The review of Poutsma (18) is quite extensive and should be consulted for the chemistry related to heavy hydrocarbon systems. 

Molecular reaction schemes have a long history of use in the design of pyrolysis coils. Since the pioneer works of Myers and Watson [46] and Schutt [47] on propane pyrolysis, improved by Snow and Schutt [48], molecular reaction schemes have been applied to the modelling of the pyrolysis of light and heavy hydrocarbons [38, 49—56]. Froment and co-workers have extensively promoted molecular reaction schemes in a series of papers [57—61] a brief account can be found in a book by Froment and Bischoff [25]. 

A consequence of this is that fresh coal falling on top of the hot coals undergoes pyrolysis and emits coal gas products - hydrogen, methane, ethylene, light and heavy hydrocarbons and coal tar products - which contaminate the synthesis gas. 

Espitalie, et al. (10) studied the pyrolysis of various kerogen types in mixtures with various minerals in addition to pyrolysis experiments on the source rocks for the kerogens. They concluded that the minerals act as a heavy hydrocarbon trap, in effect, limiting the possible yield of hydrocarbons from source rocks. They also found that carbonate minerals have a lower specific activity for the trapping phenomenon than do silicate minerals. 

The interest here is focused mainly on heavy hydrocarbons feedstocks, as in the case of certain refinery processes, and on polymer thermal degradation. A radical chain mechanism is also involved in the liquid- or condensed-phase pyrolysis. This is once again characterized by initiation, radical recombination... 

Figure A2 shows the number of components involved in the primary propagation reactions of normal alkenes. These data clearly indicate that this number rapidly becomes very large, increasing the carbon number of the initial alkene. This fact justifies the need to turn to the component lumping when dealing with detailed and mechanistic models describing the hydrocarbons pyrolysis of such heavy species.

Figure 28. Correlation of tar yield with aliphatic (hydroaromatic) hydrogen. Key %, tar yield in vacuum pyrolysis and , heavy hydrocarbon yield from hydropyrolysis. (Reproduced, with permission, from Ref. 17.)...

Typical heavy hydrocarbon feedstocks, such as naphtha, gas oil and vacuum gas oil, are composed of hundreds of compounds, ranging from n-paraffins to naphthenes to aromatics. In order to properly understand the pyrolysis of these heavy feedstocks from a scientific point of view, one must first understand the pyrolysis mechanisms of the pure components which make up the complex mixture. Following this, interactions between the major components types should be studied and only after this stage has been successfully completed, should the whole feedstock be studied. 

Crude heavy oils are composed by a large variety of compounds, mostly aromatics and alkylated aromatics with a carbon number from 14 to 25. These heavy aromatic hydrocarbons are very difficult to characterize, the heavier fractions like asphaltenes being impossible to characterize in any way. Because of these complexities, it is very difficult and inaccurate to select one or several pseudo-components to represent the original heavy oil for the purpose of kinetic modelling. A combination of theoretical and experimental methods must be considered to develop a suitable kinetic scheme for heavy oil pyrolysis. The approach followed by Tan et al. 

Rebick, C. (1983) Pyrolysis of heavy hydrocarbons in L.F. Albright, B.L. Crynes and W.H. Corcoran (eds.). Pyrolysis Theory and Industrial Practice, Academic Press, N.Y., 69-87. 

It is known that polyolefinic polymers can be readily thermally decomposed to gaseous and liquid hydrocarbons. The pyrolysis of these kinds of polymers in an inert atmosphere or under vacuum at elevated temperatures gives heavy hydrocarbons as major reaction products. Mainly light paraffins and olefins are obtained during polymer thermolysis at higher temperatures (above 700 °C). In... 

By-products from EDC pyrolysis typically include acetjiene, ethylene, methyl chloride, ethyl chloride, 1,3-butadiene, vinylacetylene, benzene, chloroprene, vinyUdene chloride, 1,1-dichloroethane, chloroform, carbon tetrachloride, 1,1,1-trichloroethane [71-55-6] and other chlorinated hydrocarbons (78). Most of these impurities remain with the unconverted EDC, and are subsequendy removed in EDC purification as light and heavy ends. The lightest compounds, ethylene and acetylene, are taken off with the HCl and end up in the oxychlorination reactor feed. The acetylene can be selectively hydrogenated to ethylene. The compounds that have boiling points near that of vinyl chloride, ie, methyl chloride and 1,3-butadiene, will codistiU with the vinyl chloride product. Chlorine or carbon tetrachloride addition to the pyrolysis reactor feed has been used to suppress methyl chloride formation, whereas 1,3-butadiene, which interferes with PVC polymerization, can be removed by treatment with chlorine or HCl, or by selective hydrogenation. 

The polyaromatic hydrocarbons in the soil sample were quantitated by using an external standard of anthracene. The results reportedly for a polluted soil and sediment sample indicate that this flash evaporation-pyrolysis technique combined with gas chromatography-mass spectrometry is a valuable tool for rapidly screening polluted samples for virtually all types of anthropogenic contaminants except for heavy metals. 

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