Naphtha decomposition reactions

Nickel on an acidic support, such as that used for methane reforming, will promote the desired naphtha decomposition reaction, but it also promotes the cracking and polymerization reactions that are the basis for carbon formation. ICI has solved this problem by incorporating an alkali metal into their catalyst [7]. The alkali accelerates the reaction of carbon with steam (the primary carbon removal reaction) and at the same time neutralizes acidity in the support inhibiting the cracking and polymerization reactions (other carbon-forming reactions). The most effective alkali is K2OH (potash). Most naphtha reformers use the alkalized catalyst developed by ICI [7]. 

Derivation (1) Reaction of steam with natural gas (steam reforming) and subsequent purification (2) partial oxidation of hydrocarbons to carbon monoxide and interaction of carbon monoxide and steam (3) gasification of coal (see Note 1) (4) dissociation of ammonia (5) thermal or catalytic decomposition of hydrocarbon gases (6) catalytic reforming of naphtha (7) reaction of iron and steam (8) catalytic reaction of methanol and steam (9) electrolysis of water (see Note 2). In view of the importance of hydrogen as a major energy source of the future, development of the most promising of these methods may be expected. 

Figure 2. Dependence of the naphtha cracking decomposition reaction rate constant on temperature (a) dependence of the naphtha cracking overall kinetic order on conversion (b)...

For the higher molecular weight feedstocks such as liquefied petroleum gas (usually propane CjH8) and naphtha (q.v.), nickel catalysts with alkaline carriers or alkaline-free catalysts with magnesium oxide as additive can be used. Both types of catalyst are less active than the conventional nickel catalyst. Therefore, a less rapid decomposition of the hydrocarbons is achieved. At the same time, the reaction of water with any carbon formed is catalyzed. 

Illes et al. [44] developed a reaction model for naphtha cracking which involves an nth-order decomposition of naphtha, considered as a single constituent. 

Cyclohexane is obtained either by the hydrogenation of benzene, or from the naphtha fraction in small amounts. Its oxidation to the KA Oil dates back to 1893 and was first industrialized by DuPont in the early 1940s. Oxidation is catalyzed by Co or Mn organic salts (e.g., naphthenate), at between 150 and 180 °C and 10-20 atm. Indeed, this reaction is a two-step process (an oxidation and a deperoxidation step), and two variants are currently in use [2,3]. The oxidation step can be performed with or without a catalyst. The deperoxidation step always uses a catalyst (Co(II) or NaOH). The overall performance of both variants is almost identical, although the selectivity in the individual steps may be different. For example, in a first reactor, cyclohexane is oxidized to cyclohexylhydroperoxide the concentration of the latter is optimised by carrying out the oxidation in passivated reactors and in the absence of transition metal complexes, in order to avoid the decomposition of the hydroperoxide. In fact, the synthesis of the hydroperoxide is the rate-limiting step of the process, and, on the other hand, alcohol and ketone are more reactive than cyclohexane. The decomposition of the hydroperoxide is then carried out in a second reactor, in which the catalyst amount and reaction conditions are optimised, thus allowing the Ol/One ratio to be controlled. 

The approach to hydrocarbon cracking taken by the Froment school is to model the actual elementary steps of radicals at the various molecular configurations [38]. These are relatively few initiation hydrogen abstraction from a primary, secondary, or tertiary carbon and radical decomposition by scission of a carbon-carbon bond in /3-position to the unpaired electron. Boolean relation matrices are used to reflect the structures of the hydrocarbon reactants by indicating the existence and location of all their carbon-carbon bonds. Computer software generates reaction networks on the basis of known rate coefficients and activation energies at the various positions. Froment states the number of components in naphtha cracking as around 200, that of radicals as 40, and that of elementary radical steps... 

In reforming naphtha with a nickel-based catalyst, three phenomena occur simultaneously. The first is decomposition of naphtha into lower molecular weight unsaturated intermediates. As this is essentially a cracking process, the lower molecular weight intermediates contain double bonds or olefins. Second, there is the reaction of steam with olefinic intermediates. The third is cracking and polymerization of the olefinic fragments forming carbon. 

Mixed-gas method. In this method [148], naphtha is thermally cracked to form acetylene and ethylene. Dichloroethane is formed by reacting ethylene with chlorine, and vinyl chloride is obtained by thermal decomposition of dichloroethane with simultaneous formation of hydrogen chloride. Further, the reaction between hydrogen chloride and acetylene also gives vinyl chloride. 

Figure 11. Change of measured and calculated yields of the main products, degree of decomposition, and true reaction time of naphtha cracking with the relative length of the reactor experiment No. 44...

Methane reforming Eq. (2.36) is the simplest example of steam reforming (SR). This reaction is endothermic at MCFC temperatures and over an active solid catalyst the product of the reaction in a conventional reforming reactor is dictated by the equilibrium of Eq. (2.36) and the water-gas shift (WGS) reaction Eq. (2.37). This means that the product gas from a reformer depends only by the inlet steam/ methane ratio (or more generally steam/carbon ratio) and the reaction temperature and pressure. Similar reaction can be written for other hydrocarbons such as natural gas, naphtha, purified gasoline, and diesel. In the case of reforming oxygenates such as ethanol [125, 126], the situation is in some way more complex, as other side reactions can occur. With simple hydrocarbons, like as methane, the formation of carbon by pyrolysis of the hydrocarbon or decomposition of carbon monoxide via the Boudouard reaction Eq. (2.38) is the only unwanted product. 

The first was presented by Kumar and Kunzru (1985) for modelling of naphtha pyrolysis. In this study, it is assumed that naphtha could be represented as a pseudo-pure compound and the primary decomposition represented by a single reaction with the initial selectivities determined experimentally. Based on the experimental results, the primary reaction is represented by a first order reaction for the whole range of conversions and the initial selectivities are assumed to be constant. The secondary reactions are also represented by molecular reactions and only important secondary reactions that can occur between the various primary products have been accounted for. A sixth-order Runge-Kutta-Verner method was used by these authors to solve a set of reactions which include 22 reactions and 14 components. The predicted and experimental product yields were compared and the rate constants adjusted, by trial-and-error, to minimize the deviation between the predicted and experimental values. The major limitation of this approach is that the initial selectivities are to be determined experimentally. 

Further reduction of the number of independent kinetic parameters results from thermodynamic constraints. Initiation and termination or decomposition and addition can be considered as reversible reactions and their rate parameters are related through thermodynamics. Accounting for these constraints allowed Willems and Froment (1988) to reduce the number of rate parameters in their rigorous naphtha-cracking model from 140 to 68. This is a number that can be significantly determined from a broad data base covering the cracking of various naphthas and a number of specific components. 

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