Reforming kinetics

In the model development, the following strategies were employed. 

The hydrocarbon lumps and reaction network for both the start-of-cycle and the deactivation kinetics were defined. 

The start-of-cycle kinetic problem was uncoupled from the deactivation kinetics by taking advantage of their widely different time constants. 

An experimental design was developed which uncoupled the overall problem into a number of smaller parameter estimation problems. This approach reduced confounding between parameters, for both start-of-cycle and deactivation kinetics. 

For the start-of-cycle kinetics, the following assumptions were made. 

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Start-of-cycle kinetic lumps in KINPTR are summarized in Table V. A C5-light gas lump is required for mass balance. Thirteen hydrocarbon lumps are defined. The reforming kinetic behavior can be modeled without splitting the lumps into their individual isomers (e.g., isohexane and n-hexane). Also, the component distribution within the C5- lump can be described by simple correlations, as discussed later. The start-of-cycle reaction network that defines the interconversions between the 13 kinetic lumps is shown in Fig. 9. This reaction network results from kinetic studies on pure components and narrow boiling fractions of naphthas. It includes the basic reforming reactions... 

The general mathematical description for determining reforming kinetics is shown below for hydrocarbon conversion and deactivation rates, respectively,... 

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. 

The elements of range in value from 0 to 1 and are the ratio of the reformer kinetic constants at time on stream t to the values at start of cycle. At any time on stream t, the deactivation rate constant matrix K(a) is determined by modifying the start-of-cycle K with a. From the catalytic chemistry, it is known that each reaction class—dehydrogenation, isomerization, ring closure, and cracking—takes place on a different combination of metal and acid sites (see Section II). As the catalyst ages, the catalytic sites deactivate at... 

Complete reforming kinetics have been developed for several commercial catalysts, including those used in Mobil reformers. Since KINPTR affects Mobil s business strategy, the complete reforming kinetics are proprietary. However, as an example, KINPTR C6 kinetics will be presented for UOP s R16H platinum-rhenium-alumina catalyst. Both the hydrocarbon conversion and the deactivation equations [Eqs. (36), (40)] can be directly applied to the C6 system. For the C6 hydrocarbon conversion, Eq. (40) becomes... 

The most striking feature of the stress softening phenomenon of thermoelastoplastics is its complete reversibility under certain conditions, consisting in a reformation process in the stress-free state which involves healing of cracks and reaching the initial integrity of the hard phase 119,120,12 ). The reformation kinetics in stress-softened samples indicates that this process is controlled by diffusion. 

We studied methane reforming kinetics at atmospheric pressure. The utilization of circulation flow systems made it possible to use nickel foil as... 

L., Renken, A., Catalyst coating in microreactors for methanol steam reforming kinetics, in Matlosz, M., Ehrfeld, W., Baselt, J. P. (Eds.), Microreaction Technology - IMRET 5 Proc. of the 5th International Conference on Microreaction Technology, Springer-Verlag, Berlin, 2001, 322-331. 

Two papers have appeared on the kinetics of the steam reforming of heptane over nickel catalysts.205 206 The first concerns the steam reforming of heptane at low concentrations for injection into internal combustion engines the catalyst was Ni-Al203 and the reaction temperature was in the range 360-406 °C.205 The second concerned an evaluation of the intrinsic steam reforming kinetic parameters from rate measurements on full-size particles of a Ni-MgO catalyst in the temperature range 450-550 °C.206... 

To speed the operation of the preprocessor, the convergence loops in the linked reformer simulator have been modified. In the inner loop, after a certain number of iterations, the mole fractions of C4 and heavier in the recycle gas are held constant. It was found that slight variations in these components (which are assumed to react in the reforming kinetic model) slowed down the rate of convergence without materially improving the accuracy of the results. 

All steam reforming catalysts in the activated form contain metallic nickel as active component, but the composition and structure of the support and the nickel content differ considerably in the various commercial brands. Thus the theoretical picture is less uniform than for the ammonia synthesis reaction, and the number of scientific publications is much smaller. The literature on steam reforming kinetics published before 1993 is summarized by Rostrup - Nielsen [362], and a more recent review is given by K. Kochloefl [422]. There is a general agreement that the steam reforming reaction is first order with respect to methane, but for the other kinetic parameters the results from experimental investigations differ considerably for various catalysts and reaction conditions studied by a number of researchers. 

K. Atwood, C. B. Knight, Reforming Kinetics and Catalysis in V. A. Slack, G. Russel James. Ammonia, Marcel Dekker Inc, New York, 1973, 145-174. 

Achenbach E, Riensche E (1994) Methane steam reforming kinetics for solid oxide fuel-cells. J Power Sources 52 283... 

T0ttrup, P.B. Evaluation of intrinsic steam reforming kinetic parameters from rate measurements on full particle size. Applied Catalysis, 1982, 4 (4), 377. 

Table 4 Steam reforming kinetics and mechanism summary of recent results... 

In the model with two types of particles, (11.28) is solved for both of them. The catalyst particles are solved for all components, with the reforming kinetics. Like the equations for the buik transport, (11.28) is soived for four components and H2O is caicuiated from the sum of the mass fractions (11.50). The source term is then defined by ... 

A tube reactor with dimensions given in Table 11.9 was simulated. A high ratio between C02-acceptor and catalyst was used because the reforming kinetics are fast compared to the sorption rate. The reactor was filled with steam (97 mole%) and a small amount of hydrogen at the desired temperature, 848 K, at startup. The input to the reactor was methane and steam, in which the steam to methane ratio is set to 6. A high steam to carbon ratio is necesarry... 

STEAM REFORMING KINETICS, CATALYST DEACTIVATION AND REACTOR DESIGN... 

The main conclusions from this detailed anlaysis of methane steam reforming kinetics can be summarized in the following points ... 

The steam reforming kinetics developed by Xu and Froment (1989a) are of a more general form than the previous steam reforming kinetics which explains a number of contradictions existing in the literature. 

Fuel component effects are being examined to determine their effect on hydrogen generation technologies. Measurements show that the fuel composition does effect the design and operation of the fuel reformer, and fuel blends low in aromatic content show higher reforming kinetics, which result in a reduced fuel processor size (and thus cost). The fuel composition effect also effects the relative carbon formation, which will effect the fuel processor durability. 

Figure 5 Profiles of reforming kinetic relative to deactivation, k /b, as a function of S C for various temperatures...

S. Tosti, A. Basile, R. Borelli, F. Borgognoni, S. Castelli, M. Fabbricino, F. Gallucci and C. Licusati, Ethanol steam reforming kinetics of a Pd-Ag membrane reactor, Int. J. Hydrogen Energy, 2009, 34, 4747-4754. 

There are numerous reports on the reforming kinetics of CH4 on Ni catalysts, however, only few deals with the reforming over the anode of a real SOFC. Timmerman et al. studied internal reforming of methane at Ni/YSZ and Ni/CGO SOFC cermet anodes [59]. Nagakawa et.al studied steam reforming kinetics on a real operating fuel cell [19]. When the partial... 

Hecht et al. have studied the reforming kinetics based on an elementary reaction mechanism, using experiments tailored to mimic a typical fuel cell operation [64]. In their separated anode experiments, two flow channels were separated by the anode of an SOFC. One of the two channels was fed with fuel and the other was fed with H2O or CO2 diluted in Argon. The experiment was designed in a way that permitted the transport of species between the two channels through the reactive porous media. In all cases the mechanism used by Hecht et al. reasonably well reproduces the experimental observations. 

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