Thermal reactions - catalytic steam cracking

The thermal cracking of higher alkanes becomes significant above 650°C [374] [532] with the formation of alkenes, aromatics and coke. This is applied in steam crackers in ethylene plants, where steam is added as a diluent and for minimising coke formation. 

The product distribution in steam cracking is determined by the kinetic severity function (KSF) [374] [533], which is the integral of the rate constant and residence time, 9  

Using the cracking of n-pentane as a reference point, ks depends on temperature and hence KSF depends on the temperature profile and residence time profile of the reactor. In practice, this is difficult to determine. However, the KSF can be determined empirically if the feed contains n-pentane simply by  

In principle, the tubular reformer is a steam cracker filled with catalyst. Feed preheaters can also be considered steam crackers, each characterised by a KSF and corresponding olefin yields. As olefins are coke precursors in the steam reforming process, preheaters should be designed for a low KSF or the problem should be eliminated by installation of a prereformer (refer to Sections 1.2.3 and 5.3.4). 

There have been attempts to improve the steam cracking process by installing a catalyst in the cracking tubes [374] [547], This resulted in coproduction of syngas and light alkenes from heavy gas oil and naphtha. The catalyst was a potassium-promoted zirconia support operating at 

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Reaction of Steam on Hydrocarbons. The catalytic interaction of steam and hydrocarbons has been used commercially on a large scale. The thermal cracking of hydrocarbons is an important part of petroleum refining and produces a large amount of hydrogen. The reaction of steam on methane from natural gas at about 1100 °C is... 

Catalytic tar elimination over nickel-based catalysts mainly proceeds by steam and dry (CO2) reforming reactions, although there can be simultaneous thermal reactions of cracking and, perhaps, of hydrocracking. Therefore, the steam and COj contents in the flue gas have an important role in the overall tar elimination. Tar conversion (elimination) depends on the properties of the catalyst used, on the space-time (or space-velocity), bed temperature, H2O to carbon to be reformed ratio and on the operation variables of the upstream gasifier such as equivalence ratio and temperatures in the bed and in the freeboard. 

The current world production of ethene and propene is mainly covered by the petrochemical route based on steam cracking, that is, thermal pyrolysis of petroleum liquids (naphtha, gas oils) and natural gas condensates, that is, ethane, propane, etc. [13-15]. A schematic stoichiometry is given in Eq. (5.2). As an alternative, ethanol can be converted via catalytic dehydration to ethene, as shown in Eq. (5.3) [16]. For steam cracking of naphtha, the reaction stoichiometry gives a maximum product yield of nearly 100 wt%, whereas ethanol conversion can lead only to maximum yields of 61 wt%. 

Not all petrochemical processes are catalytic—the steam cracking of hydrocarbons to lower olefins is a thermal process at 700 to 800°C or more. However, excluding free-radical polymerization processes, this is a rare example, though severe conditions may still be required in some catalysed processes on thermodynamic grounds or to achieve acceptable rates (several mol h per litre of reaction volume). As we shall see in this and the following chapter, the major impact of catalysis is to provide a remarkably wide range of products from a small number of building blocks. 

Cracking of hydrocarbons to produce olefins can be done thermally or catalyt-ically. Thermal cracking is the most typical method. It is basically a pyrolysis step, as discussed in the section on refining. To avoid polymerization reactions with the olefins, steam can be injected to quench these side reactions. This process is commonly referred to as steam cracking. 

Reactors for Non-catalytic Single-Phase Systems Classical reactors for single-phase reactions are stirred tank reactors for liquids (Figure 4.10.3) and flow tubes for fluids in all aggregation states. Ethylene and propylene synthesis from naphtha by thermal cracking in the presence of steam is a good example for a tubular reactor (Section 6.6). The tubes of a steam cracker have an internal diameter of 10 cm and... 

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