Calcination kinetics

R.H. Borgwart, Calcination kinetics and surface area of dispersed limestone particles , AICHE Journal 31,1985,1. 

DTA-TG techniques have been applied to study calcination kinetics of raw materials, quantification of raw materials, determination of total heat for clinker formation, and prediction of material temperature profile in... 

There have been several kinetic studies of the calcination of dolomite [29], a reaction of considerable technological importance. As in many reversible reactions, the rate of carbon dioxide release is sensitive to the prevailing pressure of this product (.Pco2) in the vicinity of the reaction interfaces. At low pressures (PCo2 < 12 Torr), reaction proceeds to completion in a single stage between 900 and 950 K... 

Thermal analysis has been widely and usefully applied in the solution of technical problems concerned with the commercial exploitation of natural dolomite including, for example, the composition of material in different deposits, the influence of impurities on calcination temperatures, etc. This approach is not, however, suitable for the reliable determination of kinetic parameters for a reversible reaction (Chap. 3, Sect. 6). 

The kinetic methodology used by The University of Kentucky Center for Applied Energy Research (CAER) for analyzing the kinetic data has been reported elsewhere [10,12,17], Using the same approach, we obtained the kinetic parameters for the reduced air calcined 15% Co/Si02 and 25% Co/Si02 catalysts (see Table 3.1). 

The book focuses on three main themes catalyst preparation and activation, reaction mechanism, and process-related topics. A panel of expert contributors discusses synthesis of catalysts, carbon nanomaterials, nitric oxide calcinations, the influence of carbon, catalytic performance issues, chelating agents, and Cu and alkali promoters. They also explore Co/silica catalysts, thermodynamic control, the Two Alpha model, co-feeding experiments, internal diffusion limitations. Fe-LTFT selectivity, and the effect of co-fed water. Lastly, the book examines cross-flow filtration, kinetic studies, reduction of CO emissions, syncrude, and low-temperature water-gas shift. 

The catalysts were synthesized as films, with ceria prepared by spray pyrolysis of 0.1 M solutions of Ce(N03)3 onto nonporous alumina wafers held at 250 °C. The ceria was then calcined at 300 °C, resulting in a crystallite size of 10 nm. Pt, Pd, or Rh was vapor deposited onto the oxide film. For kinetics testing, the temperature was 300 °C. To determine the reaction order of H20, Pco was maintained constant at 0.026 atm. For the reaction order on CO, Ph2o was maintained constant at 0.02 atm. The kinetic parameters are tabulated in Table 69. 

Isomerization of jS-isophorone to a-isophorone has been represented as a model reaction for the characterization of solid bases 106,107). The reaction involves the loss of a hydrogen atom from the position a to the carbonyl group, giving an allylic carbanion stabilized by conjugation, which can isomerize to a species corresponding to the carbanion of a-isophorone (Scheme 9). In this reaction, zero-order kinetics has been observed at 308 K for many bases, and consequently the initial rate of the reaction is equal to the rate constant. The rate of isomerization has been used to measure the total number of active sites on a series of solid bases. Figueras et al. (106,107) showed that the number of basic sites determined by CO2 adsorption on various calcined double-layered hydroxides was proportional to the rate constants for S-isophorone isomerization (Fig. 3), confirming that the reaction can be used as a useful tool for the determination of acid-base characteristics of oxide catalysts. 

Figure 2 shows illustrated mechanism for acetone self-condensation over calcined hydrotalcites, where the enolate ion is formed in a first step followed by two possible kinetic pathways 1) In the first case the subtracted proton is attracted by the basic sites and transferred to the oxygen of the enolate ion to form an enol in equilibrium. 2) In the second case the enolate ion reacts with an acetone molecule in the carbonyl group, to produce the aldol (diacetone alcohol). Finally, the p carbon is deprotonated to form a ternary carbon and then loses an OH group to obtain the final products. 

Samples of HY zeolite were exhaustively treated with successive doses of tetramethylsilane in a static reactor at different temperatures in the range 250°-650°C. Rate data for methane evolution were obtained, and the kinetics were discussed. Silicon and some carbon were incorporated, giving gray materials parts of which were calcined in oxygen. Samples of the original H Y, the treated zeolite, and calcined materials were tested for their abilities to accept electrons from perylene and to isomerize cyclopropane and protoadamantane. The treated zeolite had good electron transfer properties but low and high activities for the isomerizations, respectively. However, the opposite was true for the calcined materials. These results are discussed in terms of the acidic properties of the modified zeolites. 

When the silica has been calcined a 400°C or lower, most or even all of the chromyl chloride attaches as chromate, losing two chlorides per Cr. All of the chromium remains hexavlent. These chromate catalysts exhibit nearly identical activity to Cr03 on silica activated at the same temperature. The kinetics... 

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