Reaction kinetics Steam reforming

The activation of alkanes on transition metal surfaces is an important step in many catalytic reactions. Hydrogenolysis, steam reforming and isomerization of alkanes all involve alkane dissodation. Thus, much interest exists in the mechanistic and kinetic aspects of alkane dissociation. 

T. Numaguchi and K. Kikuchi, Intrinsic kinetics and design simulation in a complex reaction network steam reforming, Chem. Eng. Sci., 1988, 43, 2295-2301. 

Steam reforming needs a secondary fuel to provide the energy supply necessary for the reaction that occurs and a catalysts to improve the kinetic of this process. In Equation (3), the primary fuel is partially oxidised by a limited amount of oxygen. Partial oxidation produces less H2 per fuel unit than stream reforming, but the kinetic reaction is faster, it requires smaller reactors and neither catalyst nor energy supply from a secondary fuel. 

Steam reformers are used industrially to produce syngas, i.e., synthetic gas formed of CO, CO2, 11-2, and/or hydrogen. In this section we present models for both top-fired and side-fired industrial steam reformers by using three different diffusion-reaction models for the catalyst pellet. The dusty gas model gives the simplest effective method to describe the intermediate region of diffusion and reaction in the reformer, where all modes of transport are significant. This model can predict the behavior of the catalyst pellet in difficult circumstances. Two simplified models (A) and (B) can also be used, as well as a kinetic model for both steam reforming and methanation. The results obtained for these models are compared with industrial results near the thermodynamic equilibrium as well as far from it. 

All hydrocarbons in the feed higher than methane are assumed to be instantly cracked into CH4, CO2, 112, and CO. Consequently the reaction system inside a reformer tube is described by the rate expressions of the kinetics of steam reforming in the methane reactions I, II, and III. 

Dicks A.L., Pointon K.D., Siddle A., 2000. Intrinsic reaction kinetics of methane steam reforming on a nickel/zirconia anode. Journal of Power Sources 86, 523-530. 

The experiments were carried out at a pressure of 1.5 bar and a flow rate of 80-270 Ncm3 min-1. At 200 °C no deactivation of the catalyst was observed. As the rate of reaction was found to show a linear dependence on the residence time, differential conditions were assumed for the measurements. Because of the determined high activation energy of 5 6 kj mol-1, mass transport limitations were excluded. A power law kinetic expression of the following form was determined for methanol steam reforming ... 

Munster and Grabke196 have studied the kinetics of the steam reforming of methane with iron, nickel, and iron-nickel alloys. The concluded that the steam reforming of methane is a sequence of two reactions ... 

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... 

Steam reforming of hydrocarbons has become the most widely used process for producing hydrogen. One of the chief problems In the process Is the deposition of coke on the catalyst. To control coke deposition, high steam to hydrocarbon ratios, n, are used. However, excess steam must be recycled and It Is desirable to minimize the magnitude of the recycle stream for economy. Most of the research on this reaction has focused mainly on kinetic and mechanistic considerations of the steam-methane reaction at high values of n to avoid carbon deposition ( L 4). Therefore, the primary objective of this studyis to determine experimentally the minimum value of n for the coke-free operation at various temperatures for a commercial catalyst. 

Without a doubt, a complete picture of the dynamics of dissociative chemisorption and the relevant parameters which govern these mechanisms would be incredibly useful in studying and improving industrially relevant catalysis and surface reaction processes. For example, the dissociation of methane on a supported metal catalyst surface is the rate limiting step in the steam reforming of natural gas, an initial step in the production of many different industrial chemicals [1]. Precursor-mediated dissociation has been shown to play a dominant role in epitaxial silicon growth from disilane, a process employed to produce transistors and various microelectronic devices [2]. An examination of the Boltzmann distribution of kinetic energies for a gas at typical industrial catalytic reactor conditions (T 1000 K)... 

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. 

K3 is the equilibrium constant for reaction (42), which is the product of the equilibrium constants for reactions (41) and (37) K4] Kiy. For the ratio C02/C0 the authors assume only a slight deviation from the equilibrium and use an empirical relation without a kinetic term C02/C0=/(CH4 conversion, S/C ratio, K,7). Other kinetic expressions may be found in [362], [418], [422], For the reaction mechanism [422] of steam reforming of methane, the following scheme (Eqs. 51-55) was proposed ... 

---