Catalytic reforming coke formation

The first reaction is the isomerization from a zero-octane molecule to an alkane with 100 octane the second is the dehydrocyclization of heptane to toluene with 120 octane, while the third is the rmdesired formation of coke. To reduce the rate of cracking and coke formation, the reactor is run with a high partial pressure of H2 that promotes the reverse reactions, especially the coke removal reaction. Modem catalytic reforming reactors operate at 500 to 550°C in typically a 20 1 mole excess of H2 at pressures of 20-50 atm. These reactions are fairly endothermic, and interstage heating between fixed-bed reactors or periodic withdrawal and heating of feed are used to maintain the desired temperatures as reaction proceeds. These reactors are sketched in Figure 2-16. 

The aim of this study is to convert as much as the hydrogen in the fuel into hydrogen gas while decreasing CO and CH4 formation. Process parameters of fuel preparation steps have been determined considering the limitations set by the catalysts and hydrocarbons involved. Lower S/C (steam to carbon) ratios favor soot and coke formation, which is not desired in catalytic steam and autothermal reforming processes. A considerably wide S/C ratio range has been selected to see the effect on hydrogen yield and CO formation. 

Acidity of the support, which accelerates coke formation, is poisoned with alkali addition. The active component, promoted molybdenum sulfide, is active for dehydogenation and also has acid sites, so the general behavior discussed for catalytic reforming is the same, except for reduced activity and complications from heavier molecules. 

Secondly, for the regenerative catalytic reforming reaction (with Pt-Sn/ A1203 catalyst), and, in this case, prediction of coke formation is essential for the design and dimensioning of the reactors at the centre of the process. 

The objectives of the catalytic reforming of naphtha are to increase the naphtha octane number (petroleum refination) or to produce aromatic hydrocarbons (petrochemistry). Bifunctional catalysts that promote hydrocarbon dehydrogenation, isomerization, cracking and dehydrocyclization are used to accomplish such purposes. Together with these reactions, a carbon deposition which deactivates the catalyst takes place. This deactivation limits the industrial operation to a time which depends on the operational conditions. As this time may be very long, to study catalyst stability in laboratory, accelerated deactivation tests are required. The knowledge of the influence of operational conditions on coke deposition and on its nature, may help in the efforts to avoid its formation. 

Both nickel and platinum based catalysts are active for the C02-reforming reaction. The disadvantage of nickel based catalysts is their tendency to form coke and deactivate rapidly [6]. We have shown earlier that Pt/ZrOj is a stable (by virtue of low rate of coke formation) and active catalyst for CO2/CH4 reforming [8]. In this contribution we address the characteristics of Pt/Zr02 catalysts that influence its catalytic behaviour in order to be able understand the mechanism of the catalysed reaction and help in to optimising catalyst. 

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