Kinetic catalyst temperature

During the studies carried out on this process some unusual behavior has been observed. Such results have led some authors to the conclusion that SSP is a diffusion-controlled reaction. Despite this fact, the kinetics of SSP also depend on catalyst, temperature and time. In the later stages of polymerization, and particularly in the case of large particle sizes, diffusion becomes dominant, with the result that the removal of reaction products such as EG, water and acetaldehyde is controlled by the physics of mass transport in the solid state. This transport process is itself dependent on particle size, density, crystal structure, surface conditions and desorption of the reaction products. 

Lietti and co-workers studied the kinetics of ammonia adsorption-desorption over V-Ti-O and V-W-Ti-O model catalysts in powder form by transient response methods [37, 52, 53[. Perturbations both in the ammonia concentration at constant temperature in the range 220-400 °C and in the catalyst temperature were imposed. A typical result obtained at 280 °C with a rectangular step feed of ammonia in flowing He over a V2O5-WO3/TiO2 model catalyst followed by its shut off is presented in Figure 13.5. Eventually the catalyst temperature was increased according to a linear schedule in order to complete the desorption of ammonia. 

In this paper selectivity in partial oxidation reactions is related to the manner in which hydrocarbon intermediates (R) are bound to surface metal centers on oxides. When the bonding is through oxygen atoms (M-O-R) selective oxidation products are favored, and when the bonding is directly between metal and hydrocarbon (M-R), total oxidation is preferred. Results are presented for two redox systems ethane oxidation on supported vanadium oxide and propylene oxidation on supported molybdenum oxide. The catalysts and adsorbates are stuped by laser Raman spectroscopy, reaction kinetics, and temperature-programmed reaction. Thermochemical calculations confirm that the M-R intermediates are more stable than the M-O-R intermediates. The longer surface residence time of the M-R complexes, coupled to their lack of ready decomposition pathways, is responsible for their total oxidation. 

In concluding this part, three main points emerge from the summary of these results. First, the difficulty in achieving the preparation of these solids in a reproducible way can be solved only if a precision in the experimental parameters similar to that employed for physical or analytical chemistry measurements is used. This is a clear demonstration of the second point, which states that the textural parameters of the materials (porosity, specific surface area and surface composition) are under kinetic control. Temperature, solvent, catalyst, water/precursor ratio and concentration of reagents are the main parameters which, beside the nature of the organic subunit R, control the texture of the final material. The third point is the difficulty in rationalizing the effect of these parameters due to the numerous mechanisms involved in the sol-gel process and their interconnections. However, it must be kept in mind that all these parameters are also powerful tools that can be very useful for the development of further applications, because they allow one to tune the texture of the materials. 

A I 000 hours run was used in this case, coresponding to 11 000 cm3 GO / cm3 catalyst The following experimental data were used in the kinetic calculation temperature, hydrogen partial pressure, LHSV, sulpher content of the feed and HGO, and the volume of GO which pass through the catalytic bed. 

Recent investigations have shown that increasing the diameter of a fixedwaxy deposits. Plugging is also found at higlt temperature or low Hj/CO operation and is related with local nonuniformity in the catalyst temperatures [22. Kinetic measurements on laboratory-scale recycle reactors have been used to predict product distributions for pilot plant reactors and were in good agreement with experimental results 1231. 

Heavy Arabian and Kuwait atmospheric residue feeds were used for evaluating kinetic parameters, temperature response, and metal deposition profiles for RM-430 catalyst. The data from these experiments indicated a one and one-half order dependence for metal removal at the process conditions tested. The temperature response of RM-430 catalyst is shown in Figure 1. The activation energy for vanadium and nickel were Vanadium -36.1 kcal/mol. Nickel -27.3 kcal/mol. 

Silanisation is a heterogeneous reaction. Silanes can be in the gas or liquid phase or in solution. The reaction is carried out at elevated temperatures, depending on the volatility of the silane and solvent, in a vessel under gentle stirring or in a fluidized bed reactor. To enhance the kinetics, catalysts are added. With chlorosilanes, organic bases are added as acid scavengers acids are employed in case of alkoxysilanes as reagents. By-products must be carefully removed by extraction with solvents. 

The primary problem of chemical kinetics is the formulation of an empirical rate law which represents the rate of a reaction as a function of the concentrations or partial pressures of reactants, products, and catalysts present in a gaseous or liquid phase, structure and composition of solid catalysts, temperature, etc. A secondary problem is the ascertainment of the mechanism, which is often represented by a sequence of consecutive steps, or several sequences in parallel. The knowledge of the mechanism of a reaction in turn may be used in order to formulate a rational rate law which may represent the empirical rate data in a more logical form than an empirical rate law. 

These experiments provide a direct comparison of the initial activities of platinum and base metal catalysts. Differences in performance— produced by such variables as catalyst bed mass, exhaust gas space velocity, and catalyst temperature—are explained by the effect of converter size on warm-up rates and by the kinetic differences for oxidation reactions over the two types of catalysts. 

The FTP was not designed for kinetic studies, and thus it is difficult to perform kinetic analyses in the traditional manner. Inlet composition, exhaust gas flow rates, and catalyst temperature all vary widely during the FTP. However, it is possible to relate some of the observed experimental trends to reported kinetic data. 

The majority of published research has concentrated on the preparation of the catalyst - the effect of different supports and different metals, the addition of second metals and the effect of different preparation methods on the selectivity of the catalysts for selective hydrogenation [2,3,5,6-10]. The effects of reaction conditions on selectivity have received considerably less attention. Gallezot and Richard [4] commented on the scarcity of systematic studies on the influence of reaction parameters such as pre-reduction of the catalyst, temperature, pressure, concentration of reactant and nature of the solvent for a given catalyst and reaction. Since then Singh et al. [11] have obtained quantitative kinetic data on the liquid phase hydrogenation of citral over Pt/SiOa catalysts and have used this information to present a kinetic model which fits their data. 

The observation of oscillations in heterogeneous catalytic reactions is an indication of the complexity of catalyst kinetics and makes considerable demands on the theories of the rates of surface processes. In experimental studies the observed fluctuations may be in catalyst temperature, surface species concentrations, or most commonly because of its accessibility, in the time variation of the concentrations of reactants and products in contact with the catalyst. It is now clear that spontaneous oscillations are primarily due to non-linearities associated with the rates of surface reactions as influenced by adsorbed reactants and products, and the large number of experimental studies of the last decade have stimulated a considerable amount of theoretical kinetic modelling to attempt to account for the wide range of oscillatory behaviour observed. 

Since non-selective, deep oxidation processes are even more thermodynamically favorable than the selective ones (Table 2), it is necessary to intercept the desired products kinetically. Catalysts, therefore, must be designed which lower the activation energy of the desired reactions and thus allow the process to operate at lower temperatures than for non-catalytic equilibrium limited processes. In this manner undesirable waste formation (deep oxidation) is minimized. 

Resoles are formed in a mechanistically complex reaction dependent on the amount of catalyst, temperature, molar ratio of the starting materials, the purity of the taw materials, and the nature of the reaction vessel. A very complete study of this mechanism was published by Freeman and Lewis [75] who investigated the reaction between substituted phenols and methanal under the influence of sodium hydroxide at 30°C. The kinetics of this reaction system were exarttined by measuring the quantity of the different hydroxymethylated phenols formed as a function of concentration and reaction time. The concentration versus time profiles for the products formed in this system are shown in Figure 5. The data of Figure 5 allow the calculation of the rate constants for the formation of different methylol substituted phenols shown in equation 13. The rate constants are ... 

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