Steam reforming Chemical equilibrium

Some research groups working on the modeling of MCFC include the reforming reactions in their process models in different ways. He and Chen [1] and Yoshiba et al. [2] only consider the water-gas shift reaction in a spatially distributed anode channel. Due to its high rate, they assume the shift reaction to be in chemical equilibrium. Lukas and Lee [3] and Park et al. [4] also describe the water-gas shift reaction in equilibrium, but in addition they include the steam reforming reaction of methane as an irreversible reaction with a finite reaction rate. In particular, Park... 

In general, the gas leaving a steam-reforming reactor is close to chemical equilibrium for Reaction (1) in Table 2. In industry, the approach to equilibrium at the outlet of the reformer tubes is expressed by a temperature difference defined by ... 

Residual methane is present at the exit of the combustion zone. In the catalytic bed, the methane steam-reforming and the water shift reactions take place. The gas leaving the ATR reactor is in chemical equilibrium. Normally, the exit temperature is above 900-1100°C. The catalyst must withstand very severe conditions when exposed to very high temperatures and steam partial pressures. One example of an ATR catalyst is nickel supported by magnesium aluminum spinel. For compact design, the catalyst size and shape is optimized for a low pressure drop and high activity. 

The composition out of the reformer is based mainly on chemical equilibrium (Marsh et al., 1994, p. 165). Determine the equilibrium composition under the following conditions using Aspen Plus. (1) 750°C, 14 bar, steam/hydrogen = 3 (2) 650 C, 14 bar, steam/hydrogen = 2. 

All these factors are functions of the concentration of the chemical species, temperature and pressure of the system. At constant diffu-sionai resistance, the increase in the rate of chemical reaction decreases the effectiveness factor while al a constant intrinsic rate of reaction, the increase of the diffusional resistances decreases the effectiveness factor. Elnashaie et al. (1989a) showed that the effect of the diffusional resistances and the intrinsic rate of reactions are not sufficient to explain the behaviour of the effectiveness factor for reversible reactions and that the effect of the equilibrium constant should be introduced. They found that the effectiveness factor increases with the increase of the equilibrium constants and hence the behaviour of the effectiveness factor should be explained by the interaction of the effective diffusivities, intrinsic rates of reaction as well as the equilibrium constants. The equations of the dusty gas model for the steam reforming of methane in the porous catalyst pellet, are solved accurately using the global orthogonal collocation technique given in Appendix B. Kinetics and other physico-chemical parameters for the steam reforming case are summarized in Appendix A. 

The reformer is a direct fired chemical reactor consisting of numerous tubes located in a firebox and filled with catalyst. Conversion of hydrocarbon and steam to an equilibrium mixture of hydrogen, carbon oxides and residual methane takes place inside the catalyst tubes. Heat for the highly endothermic reaction is provided by burners in the firebox. The heat is transferred to the catalyst filled reactor tubes by a combination of radiation and convection. 

For the production of synthesis gas with the help of the steam-reforming process, the equilibrium composition at a temperature of 1100 K and a pressure of 40 atm should be determined starting from 1 mol of methane and 3.2 mol of water (steam), taking into account the following independent chemical reactions ... 

The new concept of simultaneous generation and separation of hydrogen means that membrane reformer system can be configured more compactly and can provide higher effidenqr than conventional steam reformers. The simultaneous process of hydrogen generation and separation frees the reactions from the limitation of chemical equilibrium and thus can reduce the reaction temperature from the conventional 700-800°C to 500-550°C. This means that expensive heat-resistant metals need not be used for structural components and long-term durability increases as a result of the lower operation temperature. 

Figure 15.2 shows the flow sheet of the FP-FC system. The fuel forthe system is an aqueous solution of methanol at the molar ratio of methanol to water of 1 2 for the standard case. The fuel is evaporated in the vaporizer (VAP) at 150°C. In the reformer, the vaporized methanol and water react at 250 °C to form a hydrogen-rich gas, which contains also some CO2 and CO. The steam reformer is modeled as a Gibbs reactor assuming chemical equilibrium between the species at the outlet of the reactor. At the reforming temperature of 250 °C, the equilibrium conversion of methanol is almost 100%. The selectivity of methanol to CO2 is about 97% and to CO about 3%. In the mixer (MIX), the hydrogen-rich gas from the reformer is mixed with a small quantity of air, which is needed for the oxidation of CO present in the product gas from the reformer. The selective CO oxidation takes place in the COS reactor at 150 °C. The COS reactor is modeled as a stoichiometric reactor where 50% of the supplied O2 from the air is used for complete oxidation of CO and the remaining 50% of O2 reacts with H2. 

Palladium membrane has been used for selective hydrogen removal in the course of reaction. Such a composite type of reactor, called a membrane reactor, has shown great performance, that is, upsetting the chemical equilibrium for dehydrogenation (Itoh, 1987 Itoh et al., 1992 Matsuda, Koike, Kubo, Kikuchi, 1993 Mondal Ilias, 2001 Raich Foley, 1998 She, Han, Ma, 2001), steam reforming (Galuszka,... 

The significance of this small effectiveness factor is the following The chemical reaction rate occurring inside the catalyst pores is much faster than the rate at which the reaction components can enter and then leave the catalyst pores. This means that the composition inside the pores can be at equilibrium the bulk gas composition is quite far from equilibrium. Therefore, when checking to see whether a reforming mixture has a tendency to form carbon, it is necessary to check the bulk composition of the gas as well as the equilibrium composition of the gas. As mentioned earlier, steam reformers can be said to be heat flux-limited, which means that the reactor is usually limited by heat transfer considerations and not by reaction kinetics. In other words, once the reformer has been configured in terms of the number of tubes and their dimensions to achieve the desired heat flux profile, there should be enough catalyst volume in the tubes to achieve the desired level of conversion. 

In Eqs. (22.24) and (22.25), ideal gas law is assumed for the determination of molar flow. The desired product gas of a steam reformer for hydrocarbons C Hm consists of hydrogen (H2), carbon monoxide (CO), carbon dioxide (CO2), and steam (H2O) that is added to the mixture in excess. Partial oxidation uses air for fuel conversion, leading to nitrogen (N2) as part of the product gas. It can be assumed that oxygen (O2) reacts completely. Methane (CH4) can always be found in reformates due to chemical equilibrium. Finally, the product gas of an autothermal reformer contains H2, CO, CO2, H2O, N2, and CH4. The carbon balance (C) for an idealized reforming process of any C Hm without byproducts results in... 

Figure 32.1 Hydrogen, carbon monoxide, and methane concentrations in chemical equilibrium based on the steam reforming of natural gas and the WGS reaction for a steam-to-methane ratio of 2.5.

After desulphurisation, a process which is necessary for the protection of the catalysts, the natural gas which is mainly methane is reacted with steam over a nickel catalyst. The reaction is overall endothermic, and so, in accordance with the laws of chemical equilibrium, as high a temperature as possible is required. The reactor, known as the primary reformer, is a collection of vertical metal tubes suspended in a furnace, and the exit gases around 800°C are unreacted methane 9 per cent, steam, oxides of carbon and hydrogen. The principal reactions occurring simultaneously are ... 

Absolute values of the heat transfer parameters that ean be used for seale-up are dififieult to determine in bench-scale units due to the very high gas velocities and heat fluxes in industrial units. Attempts to determine them in pilot plants operating at industrial conditions but without reaction are also highly uncertain due to small driving forces. The steam reforming reaction is, however, strongly endothermic and limited by chemical equilibrium. This implies that for a new catalyst, the reaction will be close to chemical equilibrium in the major part of the tube, so variation in catalyst activities will only have a small impact on the temperature profile (refer to Section 3.3.7). If, however, heat transfer... 

In the catalyst particle, the steam reforming reaction approaches chemical equilibrium, where all reactions become first order in their limiting component. This implies that a simplified intrinsic rate equation may be written as the following function of the distance to equilibrium [389] ... 

Similarly, if CH4 is present and steam reforming is at equilibrium, chemical equilibrium conditions determine the rates of consumption of each fuel component uniquely for a given current production. 

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