The Kinetics Of Carbon Oxidation By Oxygen In The Presence Of Carbonates Of Alkaline Metals
The kinetics of carbon oxidation by oxygen in the presence of carbonates of alkaline metals
G. G. Tsapiuk, V. Ya. Zabuga, V. L. Budarin, V. K. Yatsimirsky
Shevchenko University, Kiev, Ukraine
The carbonates of alkaline metals represent themselves the active catalysts of carbon oxidation. There are contradictory data in the literature as to the mechanisms of their catalytic action.
In this work, the studies aimed at the kinetics of carbon oxidation in the presence of potassium and sodium carbonates have been performed.
The studies have been made by the gravimetry method in the flow-trough conditions over the temperature range 653–713 K at atmospheric pressure. The air oxygen was used as the oxidizing agent. The oxidation subject was the ash-free carbon of the DG-type produced by Dashav factory of technical carbon. K2CO3, Na2CO3 and NaHCO3 of the analytical grade have been applied as the catalytic admixtures.
The initial samples have been prepared by both the mechanical blending of carbon black and admixture, and the soaking through of the corresponding carbonate solution with the following drying at 1200C for 12 h. Each sample so prepared has been set up into the metallic cylindrical class by sprinkling. The content of admixtures has been varied from 0.5 to 5%wt.
The kinetic curve has been found out after a sample was introduced into the furnace being heated preliminary to the needed trial temperature.
The treatment of experimental data has been done based on the macro-kinetic equation that has been developed in accordance with the sample model.
In a temperature range of 673 to 873 K, the kinetic curves of carbon black oxidation have a descending character. The dependence of the rate constant versus temperature is tolerably described by the Arrenius equation.
An effect of the catalytic action has been assessed according to the difference among the rate constants in the presence of the catalyst and without it, which was related to 1 g of the catalyst, and the rate constant of carbon black oxidation without a catalyst.
The catalytic effect has been found out to depend essentially upon the
mode of the catalyst introduction into the carbon sample. So, the effect of the catalytic action of K2CO3 introduced from aquatic solution by means of carbon black soaking is greater on the order of magnitude as compared to that got to at the mechanical blending of the carbon black and catalysts powders. This testifies to an essential role of the contact between the catalyst and carbon.
As distinct from the pure carbon black and that with N2CO3 admixture, the carbon oxidation in the presence of K2CO3 is characterized under the certain conditions by an anomalous form of the kinetic curve. It represents itself a convex curve. This testifies to an increase in the oxidation rate as carbon burns out. The like can be in case at the condition of an increase in the catalytic action in the course of the sample oxidation process. This associates in turn with either a growth in the activity of a catalyst or intensification in the contact degree between the catalyst and carbon. The latter argues well with the literature data showing a high volatility of potassium carbonate.
Under the certain conditions, the kinetic curves of carbon black oxidation in the presence of catalytic admixtures are characterized by the presence of a bending point. The presence of this point we have explained through the existence of two thermal regimes of this process. The first of those, upper, close to adiabatic one comes to pass at the condition when the stage of oxygen supply to the outer surface of a sample becomes limiting. The second of those, under, close to the isothermal one takes place at the condition of the absence of the process inhibition by oxygen transfer. A transition from the upper regime to under regime and vice versa occurs by a jump-type mode in the process course of a sample burning out.
The magnitude of a sample overhead in the process course in the under regime estimated by a way of comparison of the rate constants down to the bending point and after it is 50–30 K.
In the case Na2CO3 as well as at comparatively high temperatures in the case of K2CO3, a typical bending point in the kinetic curves of carbon black oxidation is such that corresponds to a transition from the upper regime to the under one. As opposed to that, in the case of K2CO3 at low temperatures, two bending points are observed. The first of those associates with a transition from the under regime to upper one, whereas the second is coupled with that from the upper to the under regime. The first transition can be due to an increase in the effect of catalytic action of K2CO3 and an increase in the oxidation rate of carbon black. The second transition is caused by a decrease in the oxidation rate due to burning out of carbon.
The catalytic activity of potassium and sodium carbonates assessed according to the magnitude of the catalytic action effect is approximately identical.
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