Reforming stoichiometric

Stoichiometric reforming according to Equations R1 and R3 in Table 1.2 at H2O/CH4 or CO2/CH4 ratios of 1 is rarely feasible [399], because it would result in incomplete conversion at the pressures that are economical for industrial S5mgas plants (20—50 bar). This is also true for mixed CO2/H2O reforming as illustrated in Figure 1.11 [415]. The low-pressure manufacture of reducing gas for direct ore reduction is one exception (refer to Section 2.4.3). 

MIDREX [Midland-Ross] A process for the direct reduction of iron ore, using a mixture of carbon monoxide and hydrogen obtained by reforming of natural gas with water vapor and carbon dioxide. The keys to the process are a special shaft furnace and a stoichiometric reformer. The shaft furnace has several... 

This excess hydrogen is normally carried forward to be compressed into the synthesis loop, from which it is ultimately purged as fuel. Addition of by-product CO2 where available may be advantageous in that it serves to adjust the reformed gas to a more stoichiometric composition gas for methanol production, which results in a decrease in natural gas consumption (8). Carbon-rich off-gases from other sources, such as acetylene units, can also be used to provide supplemental synthesis gas. Alternatively, the hydrogen-rich purge gas can be an attractive feedstock for ammonia production (9). 

In practice, this ratio is even lower than what is shown hy the stoichiometric equation because part of the methane is oxidized to carhon dioxide and water. When resids are partially oxidized hy oxygen and steam at 1400-1450°C and 55-60 atmospheres, the gas consists of equal parts of hydrogen and carhon monoxide. Table 4-2 compares products from steam reforming natural gas with products from partial oxidation of heavy fuel oil. 

In the model equations, A represents the cross sectional area of reactor, a is the mole fraction of combustor fuel gas, C is the molar concentration of component gas, Cp the heat capacity of insulation and F is the molar flow rate of feed. The AH denotes the heat of reaction, L is the reactor length, P is the reactor pressure, R is the gas constant, T represents the temperature of gas, U is the overall heat transfer coefficient, v represents velocity of gas, W is the reactor width, and z denotes the reactor distance from the inlet. The Greek letters, e is the void fraction of catalyst bed, p the molar density of gas, and rj is the stoichiometric coefficient of reaction. The subscript, c, cat, r, b and a represent the combustor, catalyst, reformer, the insulation, and ambient, respectively. The obtained PDE model is solved using Finite Difference Method (FDM). 

No other products were detected in the gas phase. The amount of H2 produced from 85 pmol of m-C16H34 was 4.14 mmol, which is close to the stoichiometric value. One can note that reaction 2.72 stoichiometry resembles that of steam reforming of hexadecane. The authors proposed the following mechanism, which involves the initial generation of active species holes (p+) in the valence band and electrons (e ) in the conduction band of... 

The reason for this lies in the relatively high cost of gas compression, which depends on the ratio of the inlet and outlet pressures. The methane feed to the reformer will probably be available at a pressure much above 1 bar and similarly with the steam. Le Chatelier s principle indicates also that excess steam, as used in practice, will favour a higher conversion of methane compared with the stoichiometric proportions of the reactants. The same conclusion follows quantitatively from Kpi for which the equation involves the total pressure P. 

In this ideahzed, stoichiometric equation (Eq. 6.36), cellulose (represented as CjHj Oj) reacts with water to produce and CO, the conunercial production of Hjfrom methane by the catalytic steam reforming process (Wagner and Froment, 1992). 

Table 6.1 lists the stoichiometric yields of hydrogen and percentage yields by weight from steam reforming of some representative model compounds present in biomass pyrolysis oils, and also several biomass and related materials. The table also shows the equilibrium yield of H2, as a percentage of the stoichiometric yield, predicted by thermodynamic calculations at 750 °C and vdth a steam-to-carbon (S/C) ratio of 5 [32]. 

Table 6.1 Stoichiometric yields of hydrogen from complete steam reforming reactions (adapted from Ref [34]).

Reforming of glucose and sorbitol to H2 and CO2 can be described according to the following stoichiometric reactions ... 

Heinzel et al. [77] compared the performance of a natural gas autothermal reformer with that of a steam reformer. The ATR reactor was loaded with a Pt catalyst on a metallic substrate followed by a fixed bed of Pt catalyst. In the start-up phase, the metallic substrate was electrically heated until the catalytic combustion of a stoichiometric methane-air mixture occurred. The reactor temperature was increased by the heat of the combustion reaction and later water was added to limit the temperature rise in the catalyst, while the air flow was reduced to sub-stoichiometric settings. With respect to the steam reformer, the behavior of the ATR reactor was more flexible regarding the start-up time and the load change, thus being more suitable for small-scale stationary applications. 

For successful operation a selective CO oxidation catalyst in a reformer-PEFC system must be operated at ca. 353-373 Kin a complex feed consisting of CO, 02, H2, C02, H20 and N2, and be capable of reducing CO concentrations from about 1% to below 50 ppm - this is equivalent to a CO conversion of at least 99.5% [4, 54, 60], In addition, this conversion must be achieved with the addition of equimolar 02 (twice the stoichiometric amount) and the competitive oxidation of H2 must be minimized. This is expressed as selectivity, which is defined as the percentage of the oxygen fed consumed in the oxidation of CO for commercial operation a selectivity of 50% is acceptable, since at this selectivity minimal H2 is oxidized to water. 

It is a combination of exothermic steam reforming and endothermic partial oxidation applying a stoichiometric feed ratio which allows for an overall zero energy balance. As the exothermic reaction is faster, a hot-spot is very common in... 

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