Reforming reactions reaction parameters

Phenol methylation to 2,6-xylenol has been widely studied for the past few deeades owing to the room for improvisation from the viewpoint of product selectivity. Generally during phenol methylation to 2,6-xylenol, occurs via sequential methylation of phenol to o-cresol to 2,6-xylenol, various reaction parameters mediate the selectivity between the two. For instance, when the reaetants stoichiometry of methanol to phenol molar ratio > 2, and significant residence time of o-cresol may favor 2,6-xylenol selectivity. However, excess methanol is often used, sinee some amount of methanol tend to undergo oxidation into various reformate produets [71] under vapor phase condition. Similarly, reaction temperature, catalyst acid-base property, and space velocity of the reaetant are the parameters that govern the selectivity to 2,6-xylenol. 

R16H selectivity and activity kinetics were fit over a wide range of temperature and pressure. Reforming selectivity is shown in Figs. 16 and 17, where benzene and hexane are plotted against C5-, the extent of reaction parameter. The effect of pressure on reforming a 50/50 mixture of benzene and cyclohexane at 756 K is shown in Fig. 16. Selectivity to benzene improves significantly when pressure is decreased from 2620 to 1220 kPa. In fact, at 2620 kPa, hexane is favored over benzene when the C5 yield exceeds 10%. This selectivity behavior can be seen in the selectivity rate constants ... 

The following example illustrates one particular quantitative application of compensation behavior for the comparison of levels of activity between different systems. The Arrhenius parameters for the steam reformation reaction over nickel alumina catalysts (290) are log A = 17.25 and E = 29.0. The position of this point on compensation diagrams would appear to be more realistically represented by the compensation relation found for oxidation and exchange processes on nickel oxide (Table V, G) than that for cracking on the metal (Table I, A). One possible mechanistic explanation for this distinction is that the active catalyst is an oxide phase [possibly including NiAl204 (290)1... 

Another important aspect is optimization of the process parameters. The challenge here is to control catalytic combustion, in order to attain overlapping of the combustion and reforming reaction over a sufficient interval [46]. The operating conditions are mainly determined by the fuel composition and the heat exchange... 

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. 

The kinetics of partial oxidation, ATR, and dry reforming of liquid hydrocarbons have also been reported recently.103,155 Pacheco et al.155 developed and validated a pseudo-homogeneous mathematical model for the ATR of isooctane and the subsequent WGS reaction, based on the reaction kinetics and intraparticle mass transfer resistance. They regressed the kinetic expressions from the literature for partial oxidation and steam reforming reactions to determine the kinetics parameters for the ATR of isooctane on Pt/ceria catalyst. The rate expressions used in the reformer modeling and the parameters of these rate expressions are given in Tables 2.19 and 2.20, respectively. 

The reforming reaction is endothermic, whereas the overall cell reaction is exothermic. It is estimated that for typical operating parameters, approximately 60% of the heat produced by the fuel cell can be consumed for the reforming reaction. ... 

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. 

The aim of this paper is to understand the influence of zinc on platinum catalytic behaviour. The added metal can either deactivate or provoke an increase in the catalytic activity of platinum either for reforming reactions or depollution reactions respectively, even when the gas atmosphere is always reductive. We shall study the influence -i) of the mode of preparation, -ii) of the zinc loading and -iii) of the kinetic parameters, on the activity of S-[Pt-Zn] catalysts in DeNOx reactions.The catalysts have been characterised by TPR, chemisorption and EXAFS and tested in the reaction of selective catalytic reduction (SCR) using diesel conditions. 

The monotonic decrease of and the steam reforming reactions is caused mainly by the effect of methane depletion and production of products which tend to decrease the reaction rate, despite the negative effective order of the reactions. It should be noticed that the other two important parameters affecting the rate dependence behaviour, temperature and pressure, are kept constant in this simplified model. 

The implications of the non-monotonic behaviour phenomenon were studied on a lumped parameter system (Elnashaie and Yates, 1973 Bykov and Yablonskii, 1981 Luss, 1986), and found to give a multiplicity of the steady states. However, for the steam reforming reaction, this non-monotonicity does not give rise to multiplicity of the steady states. 

Autothermal reforming may lead to high temperature and possible hot spots in the first part of the catalyst bed since the exothermic oxidation reactions are faster than the reforming reaction. The process therefore needs careful control of the operation parameters. High conversion at fairly low temperature is reported for autothermal reforming, e.g. 95% CH4 conversion at 470 °C and 7 bar which was compared to 37% for a traditional reactor. The 24 pm thick Pd-Ag membrane used showed hydrogen flux of 0.093 mol m s . ... 

The model was applied in order to investigate the influence of various parameters on the performance of FBMR with oxygen addition. Although the results showed that autothermal operation can be achieved by using approximately 0.3 O2/CH4 feed ratio, the interaction between the different parameters is quite complex. For instance, in methane reformers an important parameter is the steam/carbon ratio. However, when feeding oxygen, the steam becomes also a product of the oxidation reaction and this makes the prediction of the reactor behaviour a bit more complicated. Furthermore, an important conclusion of the work is that oxygen addition reduces the coke formation and consequently the catalyst deactivation. 

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... 

Since a major fraction of the heat added in a tubular reformer is used for reaction, heat fluxes are significantly higher in the tubular reformer with reaction compared with bench-scale measurements widiout reaction. Parameter identification in measurements without reaction is highly uncertain in a tubular reformer due to the small temperature differences, but experiments with reactions under very different operating conditions can be used to check if the reaction has any impact on the heat transfer rate. 

The parameter (0/C)Rer corresponds to the steam to carbon ratio S/C, well known for steam reforming reactions. Equation 3 and 4 show the strong endothermy of the reforming reactions taken place ... 

The design equations described by Equations (10.1)-(10.3) were integrated numerically. The kinetic parameters are given in Table 10.1, the process parameters are given in Table 10.2, the specific heats are given in Table 10.3, and the heats of reaction are given in Table 10.4. The steam reforming reactions are endothermic and it is necessary to provide additional heat to keep... 

The reforming reaction and water-gas shift reaction, on the other hand, are dependent on many parameters such as pressure, temperature, and gas composition at the anode compartment. If these were defined separately, it could add much complexity to the model. It is therefore important to simplify the reaction kinetics as much as possible. The water-gas shift reaction is assumed to be in equilibrium owing to the fact that it has a very fast reaction rate compared to that of the reforming reaction. This imposes a simple consfraint on the gas species that appear on the anode side described below. 

Chemical reactions, too, may be characterised in a similar manner, following a strategy of effective kinetic parameters. When detailed knowledge of the reaction mechanism is lacking, the effective rate constants and other reaction-kinetic parameters can be determined by fitting a simplified kinetic model to the experimental data. For example, the steam-reforming conversion rate of CH4 (reaction (15c)) may be expressed by the following empirical equation [6] ... 

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