Reforming reaction network

Fig. 11. Reforming reaction network modified by catalyst state vectors. N, cyclopentane and cyclohexane naphthenes P, C6 paraffins A, aromatics C5-, pentane and lighter.

In the example given on Fig. 20, only 16 intrinsic parameters are required to define all the kinetic constants regarding the 57,559 constitutive pathways of a catalytic reforming reaction network, including molecules with up to 11 carbon atoms. 

Figure 10. Hydrocarbon synthesis and reforming reaction network on nickel surfaces. Key a, adsorbed phase g, gas phase s, solid phase and d, bulk solution.

Kinetics and Mechanism. The isoparaffins are intermediate compounds in the reforming reaction network, as shown in (Scheme 1) for n-heptane. At very low conversion, isomers are the main products. In the reaction of n-heptane on Pt/Al203, at zero conversion the isomers are the 52% in moles of the products, whereas at high conversion (95%), cracking products are the main products and the isomers yield is 3%, which shows that after being formed, the isomers are converted by successive steps (19). There is a maximum in the formation of i-hexanes both as a function of space velocity (19) and as a function of temperature (6,20). The equilibrium between ra-C6 and methylpentanes is rapidly established, but this is not true for the dimethylbutanes (8,11). This observation indicates that there is a very low kinetic constant for the transformation of single branched into doubly branched isomers (8). 

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... 

Fig. 3 shows a simplified scheme of the bifiinctional paths proposed by several authors [4,5] for interpreting the n-hexane reforming reaction. The reaction network includes (a) the isomerization and the dehydrocyclization of n-C6 to i-C6 and MCP, respectively, through a... 

This reaction network may be employed for interpreting the values of initial activities and selectivities obtained here for different naphtha reforming... 

These results show that multiple reactions of water with the albite matrix, rupturing and reforming many network bonds (Si-Al-O), occur simultaneously with Na and A1 leaching. Casey ct al. (1988b) recently reached similar conclusions in the study of the read ion of labradoritc feldspar in acid solutions at 25°C... 

To conclude, we will give the example of a lumped network, built as part of the thesis of (Cochegrue, 2001) and including cyclic and acyclic molecules up to Cl 1, to represent a complex reaction network of catalytic reforming, Fig. 27. 

The Reaction Network. The chemisorbed carbon states are likely reforming intermediates. Chemisorbed carbon is formed by the absorption and rapid dissociation of C2H4 and is removed as CO by reaction with surface oxygen, which is produced in turn by dissociative adsorption of H2O or CO2 (Figure 10). Given the high reactivity of chemisorbed carbon, the a (including a ) state is probably an intermediate in the production of other forms of catalyst carbon, 6 and e, and at lower temperature 6, f5, and y. A similar set of equations would apply to other reactive hydrocarbons, other hydrocarbon products, and CH. ... 

Other reactions important to reforming are also considered in the reaction network in Figure 10, include the water-gas-shift reaction and its reverse, the reversible adsorption and decomposition of water, the desorption and adsorption of reforming products like CO, CO2, and H2, and the formation of hydrocarbons like CH. The formation of dissolved carbon, oxygen, and hydrogen in bulk nickel is also considered. Dissolved C, 0, or H may be important in the transport of those elements to or from interfaces with other solid phase (carbon, carbides, oxides, support). The possible formation of NiO from H2O is also shown. Finally, an important reaction to consider is the formation of a deactivating layer of carbons (6 or e carbon states). 

For more complex reaction networks, these component effectiveness factors are the correct numbers to indicate the effect of diffusion on the yield and selectivity for the different components. For this reason it is the components effectiveness factors formulation which should be used in connection with complex reaction networks. It will be shown that in these cases not only rj> I are possible but also ij <0 are possible for some intermediate components. This phenomenon will be discussed in sections 5.1.9 and 5.2.2, in connection with the highly endothermic steam reforming reaction, and the highly exothermic partial oxidation of o-Xylene to phthalic anhydride. 

Fig.9 Reaction network shape-selective reforming of C7-hydrocarbons over platinum-erionite/alumina catalysts...

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

However, in many industrial processes a large number of chemical reactions occurs simultaneously. In petroleum refining operations dealing with feeds containing hundreds of components (i.e. gas oil catalytic cracking, n htha catalytic reforming, middle distillate hydrodesulfurization), where the complete analysis is a problem, the number of reactions becomes formidable and the reaction network may also become very complicated, so that components of the feed can be lumped into a small number of groups. 

Fatsikostas, A.N. and Verykios, X.E. (2004) Reaction network of steam reforming of ethanol over Ni-based catalysts. J. Catal., 225, 439-452. 

Reforming requires a catalyst with dual functions an acidic function to catalyze isomerization and cycUzation and a dehydrogenation function that requires an active metal site. Figure 6.9.5 illustrates a simplified reaction network for the example of Cg-hydrocarbons that also identifies the catalytic sites involved. 

Peppley, B.A., Amphlett, J.C., Kearns, L.M. Mann, R.E Methanol-steam reforming on Cu Zn0 Al203 catalysts. Part 1. The reaction network, Catal. A 179 (1999a), pp. 21-29. 

Apart from nickel coarsening, additional causes of the inadmissible increase in anodic overpotentials can be thought of as blockage of the nickel network by the presence of SDC particles, poor adhesion between the anode and the electrolyte, slow In situ steam reforming reaction of methane on nickel, insufficient active length of triple-phase boundaries, and so on. To overcome these... 

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