Question

Why is it important to study the kinetics of catalytic steam

reforming of methane although the reaction proceeds close to

equilibrium? Discuss the possibility of transport limitations in view of the process conditions

Answer 1

Methane steam reforming is the key reaction to produce synthesis gas and hydrogen at the industrial scale. The kinetics of methane steam reforming over a catalyst occurs usually in the temperature range 500–800 °C based on the catalytic property and as a function of CH4, H2O and H2 partial pressures. The methane steam reforming reaction cannot be modeled without taking CO and H coverages into account. This is especially important at low temperatures and higher partial pressures of CO and H2. For methane CO2 reforming experiments, it is also necessary to consider the repulsive interaction of CO that lowers the adsorption energy at high CO coverage.

Steam reforming is a process of producing hydrogen by combining steam and hydrocarbon and reacting in a reformer at temperatures above 500oC in the presence of a metal-based catalyst. There are three types of reforming processes:
                 • Steam Reforming
                 • Partial Oxidation
                            o (Non-Catalytic) Partial Oxidation (POX)
                            o Catalytic Partial Oxidation (CPO)
                 • Autothermal Reforming

Aautothermal reforming is grouped under partial oxidation, because partial oxidation may be carried by a combination of non-catalytic oxidation and steam reforming. The advantage of partial oxidation and autothermal reforming is that these processes are self-sustaining and do not require external provision of heat. However, they
are less efficient in producing hydrogen.

Steam reforming of methane consists of three reversible reactions: the strongly endothermic reforming reactions, (a) and (c), and the moderately exothermic water-gas shift reaction (b).

CH4 + H2O CO + 3H2                                  [ΔH = +206 kJ mol-1] (a)
CO + H2O CO2 + H2                                    [ΔH = -41 kJ mol-1] (b)
CH4 + 2H2O CO2 + 4H2                              [ΔH = +165 kJ mol-1] (c)

The basic reforming reaction for a generic hydrocarbon CnHm may be written as:
CnHm + nH2O nCO + ((m+n)/2)H2         (d)

Reactions (a) and (d) have also been termed “oxygenolysis” for the corresponding “pyrolysis” and “hydrogenolysis” of hydrocarbons. CO2 is not only produced through the shift reaction (b), but also directly through the steam reforming reaction (c). In fact reaction (c) results from the combination of reaction (a) and (b). Because of the endothermic behavior of steam reforming, high temperature is favored. In addition, because volume expansion occurs, low pressure is favored. In contrast, reaction (b) the exothermic reaction is favored by low temperature, while changes in pressure have no effect. Reforming reactions (a) and the associated water gas shift reaction (b) are carried out normally over a supported nickel catalyst at elevated temperatures, typically above 500oC. Reactions (a) and (c) are reversible and normally reach equilibrium over an active catalyst, at high temperatures. The overall product gas is a mixture of carbon monoxide, carbon dioxide, hydrogen, and unconverted methane and steam. The temperature of the reactor, the operating pressure, the composition of the feed gas, and the proportion of steam fed to the reactor governs the product from the reformer.

The amount of carbon monoxide produced through steam reforming of methane is quite high; because the water gas shift reaction, shown in equation (b), is thermodynamically favorable at higher temperatures. The amount of carbon monoxide in the final product from the steam reforming of methane is determined by the thermodynamics and kinetics of the reaction within the reformer. This also determines the downstream processes necessary to reduce CO concentration, which is desired by proton-exchange membrane. This is accomplished by a combination of WGS reactions at lower temperatures and the preferential oxidation reaction. For solid oxide fuel cell, the 20CO concentration has to be reduced so additional hydrogen may be produced providing high WGS activity at the anode side. Steam reforming is the most important route for large scale manufacture of synthesis gas for ammonia, methanol, and other petrochemicals and for the manufacture of hydrogen for refineries. In general, reforming reactions are catalyzed by group 8-10 metals with nickel as the preferred metal for industrial application because of its activity ready availability and low cost. Methane is activated on the nickel surface. The resulting CHx species then reacts with OH species adsorbed on the nickel or on the support. Steam reforming process is divided into two steps: a section at high temperature and pressure (typically 800-1000C and 30-40 bar) in which the reforming and shift reaction occurs, followed by an additional two-step shift section at a lower temperature (typically at 200-400C) in order to maximize the CO conversion.

Many studies have been performed to investigate the kinetics of steam reforming, and though there is general agreement on first order kinetics with respect to methane, the reported activation energies span a wide range of values. This might be explained by experimental inaccuracies due to transport restrictions in the sense of diffusion and heat transfer restrictions. The effect of diffusion limitation is that the reaction rate of methane on a conventional catalyst depends only on the partial pressure of methane, whereas on a catalyst having less diffusion restrictions, the rate depends also on the partial pressures of H2O, H2, and CO.

The reaction mechanism of the steam-reforming process strongly depends on the catalyst, i.e., on the catalytically active metal and the nature of the support. The generalized reaction mechanism is as follows:
1. H2O reacts with surface catalyst atoms, yielding adsorbed oxygen and gaseous hydrogen.
2. The H2 formed is directly released into the gas phase and/or the gaseous H2 is inequilibrium with adsorbed H and H2.
3. Methane is adsorbed on surface catalyst atoms. The adsorbed methane either reacts with the adsorbed oxygen or dissociates to form chemisorbed radicals, CHx with x = 0–3.
4. The adsorbed oxygen and the carbon-containing radicals react to form chemisorbed CH2O, CHO, CO, or CO2.
5. CO and CO2 are formed out of CHO and CH2O species.

In the past, much attention is paid to the preparation of catalysts and the evaluation of the process and equipment with little work being done on the kinetics and mechanism of the reaction. It provides a theoretical and experimental approach in unraveling the mechanism and kinetics of logistic fuel external and internal reforming for solid oxide fuel cell. The building block to this began with studying the methane steam reforming process. The overall MSR reaction CH4 + 2H2O = CO2 + 4H2 is in fact composed of two reactions, the water gas shift reaction, CO + H2O = CO2 + H2 and the first reaction CH4 + H2O = CO + 3H2. A novel theoretical methodology known as the reaction route network approach provides a detailed technique in analyzing the kinetics and thermodynamics of the reaction.