Pyrolysis, of hydrocarbons

The chain length y is directly proportional to for the Hj/Brj as well as the Hj/Clj reactions. If we compare the activation energies of the two elementary steps 

This comparative study in a family of complex reactions illustrates the levels of complexity of the field of chemical kinetics. Even with such homologous species, minor differences in activation energies may modify drastically the reaction mechanisms of the reacting systems. Simple models, such as the ISM, that provide estimates of 3 from structural and electronic parameters, help to rationahse such complex kinetic behaviour. 

A Unear chain reaction is one in which chain carriers react with no net gain in the number of carriers. Although the rate laws obtained can be as complex as those seen earlier for the reaction between H2 and Btj, this is not necessarily the case. The thermal decomposition of hydrocarbons in the gas phase involves homolytic bond cleavage at the weakest chemical bond to produce two free radicals. In general, free radicals lead to chain reactions. However, the rate laws are often simple as was shown in 1934 by Rice and Herzfeld [15]. 

---

Of the many forms of carbon and graphite produced commercially, only pyrolytic graphite (8,9) is produced from the gas phase via the pyrolysis of hydrocarbons. The process for making pyrolytic graphite is referred to as the chemical vapor deposition (CVD) process. Deposition occurs on some suitable substrate, usually graphite, that is heated at high temperatures, usually in excess of 1000°C, in the presence of a hydrocarbon, eg, methane, propane, acetjiene, or benzene. 

In 1869 Berthelot- reported the production of styrene by dehydrogenation of ethylbenzene. This method is the basis of present day commercial methods. Over the year many other methods were developed, such as the decarboxylation of acids, dehydration of alcohols, pyrolysis of acetylene, pyrolysis of hydrocarbons and the chlorination and dehydrogenation of ethylbenzene." ... 

Vapor grown carbon fiber (VGCF) is the descriptive name of a class of carbon fiber which is distinctively different from other types of carbon fiber in its method of production, its unique physical characteristics, and the prospect of low cost fabrication. Simply stated, this type of carbon fiber is synthesized from the pyrolysis of hydrocarbons or carbon monoxide in the gaseous state, in the presence of a catalyst in contrast to a melt-spinning process common to other types of carbon fiber. 

The reaction is highly endothermic, so it is favored at higher temperatures and lower pressures. Superheated steam is used to reduce the partial pressure of the reacting hydrocarbons (in this reaction, ethane). Superheated steam also reduces carbon deposits that are formed by the pyrolysis of hydrocarbons at high temperatures. For example, pyrolysis of ethane produces carbon and hydrogen ... 

Muradov, N., C02-free production of hydrogen by catalytic pyrolysis of hydrocarbon fuel, Energy Fuel, 12, 41,1998. 

Fullerenes can also be obtained by pyrolysis of hydrocarbons, preferably aromatics. The first example was the pyrolysis of naphthalene at 1000 °C in an argon stream [58, 59], The naphthalene skeleton is a monomer of the Cjq structure. FuUerenes are formed by dehydrogenative coupUng reactions. Primary reaction products are polynaphthyls with up to seven naphthalene moieties joined together. FuU dehydrogenation leads to both Cjq as well as C7Q in yields less than 0.5%. As side products, hydrofuUerenes, for example CjqHjj, have also been observed by mass spectrometry. Next to naphthalene, the bowl-shaped corannulene and benzo[k]fluoranthene were... 

Carbon black is made by the vapour-phase incomplete pyrolysis of hydrocarbons to produce a fluffy fine powder. Worldwide, about 7 million tons a year are produced. It is used as a reinforcing agent in rubber products such as tyres (20-300 nm), as a black pigment (<20 nm) in printing inks, paints, and plastics, in photocopier toner, and in electrodes for batteries and brushes in motors. 

There are a large number of chain reactions that are significant in industrial processes or play an important role for the environment. Classes of chain reactions that are relevant industrially include hydrogen/halogen reactions and pyrolysis of hydrocarbons, which are both discussed below, and free-radical polymerization discussed in many textbooks on kinetics. As an example of a chain reaction of significant environmental consequence, we will discuss formation of nitric oxide from fixation of atmospheric nitrogen. 

Pyrolysis of Ethane Pyrolysis of hydrocarbons such as C2H6 is important in the petrochemical process industry. The overall reaction can be written as... 

Figure 17.23. Representative temperature profiles in reaction systems (see also Figs. 17.20, 17.21(d), 17.22(d), 17.30(c), 17.34, and 17.35). (a) A jacketed tubular reactor, (b) Burner and reactor for high temperature pyrolysis of hydrocarbons (Ullmann, 1973, Vol. 3, p. 355) (c) A catalytic reactor system in which the feed is preheated to starting temperature and product is properly adjusted exo- and endothermic profiles, (d) Reactor with built-in heat exchange between feed and product and with external temperature adjustment exo- and endothermic profiles.

Ethylene is made by pyrolysis of hydrocarbon vapors in tubes of 50-100 mm dia and several hundred meters long with a reaction time of several seconds heat is supplied by mixing with superheated steam and by direct contact of the tube with combustion gases. 

Demonstration of the technical feasibility of producing mixtures of acetylene and ethylene by pyrolysis of hydrocarbons (Wulff process or Kureha process) has led to the manufacture of vinyl chloride from such mixtures. The acetylene component reacts selectively with hydrogen chloride to form vinyl chloride, the residual ethylene is converted to dichloroethane, and the latter is cracked to vinyl chloride, with the resulting hydrogen chloride being recycled. However, this type of process has not achieved the industrial importance of the all-ethylene type of process. 

Pyrolytic graphite. Pyrolytic graphite is produced by pyrolysis of hydrocarbons under reduced pressure to give a deposit of highly ordered carbon crys-... 

Another process that is listed in Table 39 is the C02-free production of hydrogen via thermocatalytic decomposition of hydrocarbon fuels. The process involves a single-step decomposition (pyrolysis) of hydrocarbons over carbon catalysts in an air- and water-free environment. This approach eliminates the need for a water-gas shift reactor, CO2 removal and catalyst regeneration, which significantly simplifies the process60. 

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. 

New spatial forms of carbon - fullerenes, nanotubes, nanowires and nanofibers attract significant interest since the time of their discovery due to their unique physicochemical and mechanical properties [1-3]. There are three basic methods of manufacturing of the carbon nanomaterials (CNM) - laser evaporation, electric arc process, and catalytic pyrolysis of hydrocarbons. However, the multi-stage manufacturing process is a serious disadvantage for all of them. For example, the use of organic solvents (benzol, toluene, etc.) for separation of fullerenes from graphite soot results in delay of the synthesis process and decrease in the final product quantity. Moreover, some environmental problems can arise at this. 

In the course of the experiments conducted in parallel with the described method the carbon nanostructures have been produced by pyrolysis of hydrocarbons and by arc evaporation of graphite in the gas phase in order to compare physical and chemical peculiarities of the formation of nanostructures and morphology of their structure. 

---