탄소와 산소의 반응속도에 관한 연구

이건주

상지대학교 환경공학과

REACTION KINETICS OF OXYGEN FOR MICROPOROUS CARBONS WITH HIGH SURFACE AREA

Keon Joo Lee

SangJi University

Department of Environmental Engineering

41, WooSanDong, Won Ju, Kangwon-Do, Korea

INTRODUCTION

The oxidation of carbon by different gases is a reaction of major industrial importance, which to date, at least, is not adequately understood on a molecular basis.  Although numerous studies(1-13 ) of these reactions have been made and mechanistic models for the reactions have been proposed, there remains considerable uncertainty even with regard to such macroscopic features of the reactions as the reaction order, the primary CO/CO2 product ratio, and the oxygen adsorption kinetics.

The development of the present adsorption models has been the result of careful experimental studies, particularly with graphite, transient kinetic studies, and studies using labeled gas species.  Such studies have provided the basis for selecting the correct reaction pathway from possible alternative steps or for determining the importance of various postulated steps in the mechanisms; however, the results of many of these studies were based on measurements made with graphite (or graphitic carbons) rather than microporous carbons.  Significant additional work is required to bring our understanding of the kinetics of microporous chars to the level of understanding of graphite kinetics.  Specifically, it is proposed to study the aspects of these reactions: 1. the role of the surface complex in the reaction.  It is believed that these studies could lead to new insights on the carbon gasification reactions in particular and on gas-solid reactions in general.

The surface complex is the principal intermediate species in the gasification reactions.  (The reactions of interest are carbon with oxygen, water, and carbon dioxide.)  In each case, the oxygen molecule irreversibly and dissociatively adsorbs on the carbon and forms a carbon-oxygen surface complex.  Subsequent breaking of adjacent carbon-carbon bonds results in the loss of carbon from the surface as carbon oxides.  Although the surface complex is the key reaction intermediate, its exact role in the mechanism is not clearly understood, and the complex appears to have a different role in each reaction.  In the low temperature (<900K) oxygen reaction, the surface oxides are stable and remain on the carbon surface as reaction proceeds.  At higher temperatures these complexes become progressively less stable, and at about 1100K, they are present only at low concentrations as true reaction intermediates.  Above 1100K, the reaction is presumably adsorption controlled.  The reactions of CO2 and H2O with (uncatalyzed) carbon become rapid enough only above 1000 K or at temperatures where the surface complexes are no longer stable.  Yet, for these reactions, it is proposed that the rate controlling step is the desorption of the surface complex.  Since the reactant supplying the oxygen is immaterial once the intermediate is formed, one explanation for this observation is that the carbon structure has a central role in the kinetic mechanism.  In other words, the active sites are different for each reactant.

RESULTS AND DISCUSSION

Reactivities of carbon

The reaction rates of a sucrose char (88 micron<dp<105 micron) were measured using a thermogravimetric analyser (TGA) and a mass spectrometer.  From the TGA data, the reaction rate was calculated by  differentiating the carbon weight loss curve with respect to time.  From the mass spectrometer data, the mole fractions of CO and CO2 in the product gas were obtained.  The CO and CO2 mole fractions were converted to the rate of carbon weight loss as follows

-dC/dt = Q( yco + yco2 ) x P/(R x T) x MWc (g/min)	(2)

where Q is the gas flow rate (cc/min) at room temperature, yco and yco2 represent mole fractions of CO and CO2, respectively, P is pressure (1 atm), R is a gas constant (82.05 cc atm/gmol K) and T is the room temperature (K) and M Wc is molecular weight of carbon.

Figure 2 and Figure 3 show the carbon weight loss obtained from the TGA data at different reaction temperature and oxygen partial pressures as a function of time.

Figures 4 to 5 show the CO and CO2 formation rate as a function of time at different temperature for 4% oxygen partial pressure.  The data in figure 5 is for a sample first reacted at 530℃ to approximately 30% carbon weight loss (0 to 700 seconds).  From 701 to 2000 second, the sample was heated under argon to 825℃ at a heating rate of 50℃ /min, outgassed for 5 minutes and cooled back to 530℃.  A second oxidation was made in the time period of 2001 to 4450 seconds. At 2500 seconds the gas was briefty switched to Argon.   During the outgassing at 825℃ (500 to 2000 seconds) approximately 90% of the products are CO .

Weight loss data obtained from the TGA measurements are a combination of the carbon loss and oxygen chemisorption on the carbon.  The oxygen chemisorption rate can be calculated by subtracting the rate of carbon loss as obtaining from gas composition data from the total rate of weight loss.

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