Reforming kinetics selectivity

To effectively determine the start-of-cycle reforming kinetics, a set of experimental isothermal data which covers a wide range of feed compositions and process conditions is needed. From these data, selectivity kinetics can be determined from Eq. (12). With the selectivity kinetics known, Eqs. (17) and (18a)-(18c) are used to determine the activity parameters. It is important to emphasize that the original definition of pseudomonomolecular kinetics allowed the transformation of a highly nonlinear problem [Eq. (5)] into two linear problems [Eqs. (12) and (15)]. Not only are the linear problems easier to solve, the results are more accurate since confounding between kinetic parameters is reduced. 

Determining the reforming kinetics may then be separated into a selectivity problem (determine K) and an activity problem (determine. This is accomplished by defining the selectivity... 

The kinetics of H-O recombination is very important in the reforming reaction of methane to produce CO and H2. When more weakly bonded O js recombines with Hads (preferred on Pt), the main product next to CO will be H2. On planar Rh with a stronger M-O bond interaction, this reaction is suppressed and therefore H2 is the main product [23]. Clearly this selectivity will be dramatically affected by the presence of surface steps. 

C5 - yield. Here r( is the selectivity time when the C5 - yield at v = 1 (the reactor outlet) is equal to the experimental value of C5 -. It is determined from Eq. (12), as discussed earlier. Integration of Eq. (15) requires that the selectivity kinetics be previously determined. For isothermal reforming reactions, combining Eq. (15) with Eq. (6) gives... 

R16H selectivity and activity kinetics were fit over a wide range of temperature and pressure. Reforming selectivity is shown in Figs. 16 and 17, where benzene and hexane are plotted against C5-, the extent of reaction parameter. The effect of pressure on reforming a 50/50 mixture of benzene and cyclohexane at 756 K is shown in Fig. 16. Selectivity to benzene improves significantly when pressure is decreased from 2620 to 1220 kPa. In fact, at 2620 kPa, hexane is favored over benzene when the C5 yield exceeds 10%. This selectivity behavior can be seen in the selectivity rate constants ... 

For commercial simulations, KINPTR s selectivity kinetics determine the reformate composition and overall yield at a target reformate octane. Reformer yield-octane behavior from pilot and commercial units are shown in Fig. 29a. The large variation in the reformate yields at a given octane, as much as 25%, results from the wide range of process conditions and naphtha feed quality used in Mobil reformers. As demonstrated in Fig. 29b, KINPTR accurately normalizes these reformate yields over a wide range of octanes, including those required for gasoline lead phaseout. 

However, the removal of carbon monoxide by water-gas shift to a low level still demands its selective oxidation to the minimum concentration possible. Much research and development has been conducted during the past decades to find a gold catalyst that can do this the target is usually described by the acronym PROX (preferential oxidation), but sometimes as SCO (selective catalytic oxidation). The task is somewhat simplified by the constraints that are externally imposed the preferred feed gas, often termed idealised reformate, has the composition 1.0% CO, 1.0% 02, 75.0% H2, balance nitrogen or other inert gas, and while of course variations to this composition can be made to explore the kinetics and mechanism, and the effects of the products water and carbon dioxide can be added to observe their effects, the successful catalyst must remove almost all the carbon monoxide (to <10 ppm) and less than 0.5% hydrogen. This requirement is expressed as a selectivity based on the percentage of the oxygen consumed that is taken by the carbon monoxide this should exceed 50%, under conditions where the conversion of carbon monoxide is above 99.5%.5... 

Table 2.6. Selected Kinetic Equations for Hydrocarbon Steam Reforming... 

The aim of this paper is to understand the influence of zinc on platinum catalytic behaviour. The added metal can either deactivate or provoke an increase in the catalytic activity of platinum either for reforming reactions or depollution reactions respectively, even when the gas atmosphere is always reductive. We shall study the influence -i) of the mode of preparation, -ii) of the zinc loading and -iii) of the kinetic parameters, on the activity of S-[Pt-Zn] catalysts in DeNOx reactions.The catalysts have been characterised by TPR, chemisorption and EXAFS and tested in the reaction of selective catalytic reduction (SCR) using diesel conditions. 

The implications of non-monotonic kinetics on the selection of feed conditions for steam reformers... 

A comparison between a conventional and a TS-PFR study of methanol reforming is contained in the paper by Asprey et al. (1999) and the associated paper by Peppley (1999). Other workers have used gas phase TS-PFRs in a number of studies carried out in industry. An example of industrial work is given in Investigation of the Kinetics of Ethylbenzene Pyrolysis Using a Temperature Scanning Reactor , Domke et al. (2001). Below we present some of the results and observations from selected studies and relate them to the issues raised above. 

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