P.J. Van Den Berg

Kinetic parameters in Avrami—Erofeev type reactions from isothermal and non-isothermal experiments

Abstract The differential rate equations for some simple frequently occurring Avrami—Erofeev type transformations were solved for isothermal as well as non-isothermal reactions. It is shown that the expressions commonly used to extract kinetic parameters from non-isothermal experiments are obtained via an incorrect procedure. However, the correct kinetic parameters will result from application of these equations to certain types of transformation.

The effect of dilution on the degree of conversion in fixed bed catalytic reactors

Abstract Catalyst dilution is often used to effect isothermic kinetic measurements. It is demonstrated that this dilution influences the conversion. A stochastical model is developed quantitatively describing this influence. Furthermore a dilution criterion is introduced which makes it possible to determine the allowable degree of dilution and the minimum amount of catalyst required.

The degradation of polyurethane

Abstract The aging of polyurethane was investigated by means of the isothermal storage test, adiabatic storage test and differential thermal analysis. Experiments were done between 318 and 493 K. Depending on temperature and degree of conversion, two successive reactions played a role in the aging process. For both reactions the apparent activation energy as well as the frequency factor could be determined. The apparent activation energy was about equal, but the frequency factor differed by a factor of 10 4 . For low temperatures (below 373 K) only the first reaction is important. This reaction is strongly accelerated by oxygen and water vapour. By means of gas chromatography carbon dioxide could be detected as the gaseous product of “dry” aging, whereas carbon dioxide and hydrogen were the degradation products of “wet” aging. By means of thin layer chromatography and nuclear magnetic resonance it could be proved that “wet” aging was accompanied by hydrolysis.

The disproportionation of toluene over a HY/β-AlF3/Cu catalyst: 1. Preparation and characterization

The preparation, characterization, and testing of a catalyst consisting of 72 wt% HY zeolite, 18 wt% β -AlF 3 , and 10 wt% Cu for the conversion of toluene into benzene and xylenes are described. The effect of β -AlF 3 , copper, and the activation temperature on the activity, selectivity, and stability of the catalyst were investigated. The results demonstrate that the catalyst shows satisfactory performance and reveal that 500°C is its optimum activation temperature. Texture determinations and activity measurements suggest that disproportionation activity is localized in the transitional pores of the catalyst and that the micropores only serve to collect heavy reaction products which would otherwise lead to deactivation. The results of ammonia adsorption combined with the effect of activation temperature on activity indicate that Bronsted acid sites formed during activation are responsible for the activity. It appears that only about 10% of the surface sites on freshly activated catalyst are acidic.

Comparison between twodimensional fixed bed reactor calculations and measurements

Abstract Radial temperature profiles and the average conversion have been measured at various axial distances in a fixed bed reactor. The reaction applied was the synthesis of vinyl acetate from acetic acid and acetylene. The measurements are compared with twodimensional reactor calculations. The rate expression of the vinyl acetate synthesis and the parameters describing heat and mass transfer in packed beds were measured separately. The agreement between measurements and calculation is good at a low molar ratio of acetylene and acetic acid (1·5) but less satisfying at a molar ratio of 4. In the last case the reactor operated in a more “critical region”. It is shown that slight variations in the various parameters and/or operational conditions can easily explain the differences observed.

Gas-particle heat transfer coefficients in packed beds at low Reynolds numbers

Abstract In packed beds of silicon-copper particles values for the Nusselt number were determined in the region of Reynolds number Re p = 0·24 – 0·63. The values were a linear function of Re p : At the same time the effective thermal conductivity could be measured.

Oxidation of toluene over bismuth molybdate catalysts

Abstract The oxidation of toluene by air has been studied over a silica-supported Bi-Mo-P-O catalyst (commercial Ketjen “A” catalyst) and over pure bismuth molybdate. All experiments were performed in a continuously operated fixed-bed microreactor at temperatures between 450 and 550 °C. The main part of the study concerned the kinetics of the oxidation of toluene over pure bismuth molybdate. Conversion and product distribution were measured at various space-times, toluene and oxygen partial pressures and temperatures. Benzaldehyde, benzene and carbon oxides were the only important products formed. Except for traces of benzoic acid and anthraquinone no other carbon containing products could be detected. Introductory kinetic measurements indicated a Mars-Van Krevelen mechanism. A set of rate equations, based on a redox mechanism, was derived for the reactions involved. This set of equations gave a satisfactory fit to all the experiments performed.

Thermal decomposition of aqueous manganese nitrate solutions and anhydrous manganese nitrate. Part 1. Mechanism

Abstract The thermal decomposition of aqueous manganese nitrate solutions in air or nitrogen proceeds in three steps: 1. (i) partial evaporation of water to a concentrated solution containing equimolar amounts of water and Mn(NO3)2, 2. (ii) a first decomposition step in which the residual water evolves and part of the Mn(NO3)2 decomposes to MnO2, and 3. (iii) a second decomposition step in which the remaining Mn(NO3)2 decomposes to MnO2. Decomposition of part of the Mn(NO3)2 in the first step is caused by the presence of water (vapour) which accelerates the decomposition of anhydrous Mn(NO3)2 and lowers the temperature at which this reaction starts. Without water [anhydrous Mn(NO3)2] only one decomposition step occurs. Off-gas analysis with mass spectrometry, IR spectroscopy and chemiluminescence showed NO2 to be the main gaseous product, NO being formed in much smaller amounts. amounts.

Modelling of the reduction of manganese oxides with hydrogen

MnO2 ore, Mn2O3, and Mn3O4, were reduced in a thermobalance at various hydrogen partial pressures and in the temperature range 275–400°C. It was found that the MnO2 ore and Mn2O3, are both reduced stepwise via Mn3O4. Two models were tried to describe the reactions, the shrinking core model and a more recently published model, the crackling core model. The shrinking core model is not suitable for describing these two-step reductions, but the crackling core model gave good agreement between results of calculations and measured data. The activation energies for all the reaction steps were established. The value found for a given reaction appears to depend on the starting material and its method of preparation.

The direct synthesis of methyldichlorosilane and dimethylchlorosilane

Abstract The synthesis of methylchlorosilanes with a siliconhydrogen bond, based on the reaction of silicon and methyl chloride with copper as a catalyst, has been investigated at a temperature of 332°C and a pressure of 1 atmosphere. By adding hydrogen to the gas phase, an overall selectivity of methyldichlorosilane and dimethylchlorosilane of over 80 mol% has been achieved together with a small quantity of by-products. The action of hydrogen consists of a reaction with the CuCl reaction intermediates; reaction of hydrogen with CuCl and silicon or with CuCl and chemisorbed methyl chloride also takes place. Metal chlorides such as CdCl2 and ZnCl2, which usually are promoters in the synthesis of methylchlorosilanes, do not promote the formation of the hydrogen-containing products.