Generally, if we perform thermal decomposition reaction of a material with different heating rates, fractional conversion curves for different heating rates shift towards right with increase of heating rate. But when I perform thermal decomposition of cellulose with different heating rates, it does not follow the above rule. Can anyone provide me with any explanation to it?
Dear Dr. Kashmiri Deka,
It is quite unexpected but interesting information. You have not provided the analysis details. I assume it is thermo-gravimetric analysis (as you mentioned thermal decomposition). Is it single stage or multi stage decomposition reaction ? Please provide the peak temperatures corresponding to different heating rates . Briefly provide the formula (or procedure) on how you get the fractional conversion data from a TG data.
This is really surprising. Normal (and justified) tendency is a shift towards higher isoconversional temperatures with higher heating rates as we measured for the Whatman paper in the publication attached. Please, provide experimental details.
It was DSC analysis. It is a multi stage decomposition reaction and we considered the peak at near 350oC for calculation of fractional conversion from different heating rates. The exact peak positions were 363.0, 367.8, 369.1, 343.9 for heating rates 8oC/min, 10oC/min, 12oC/min, 14oC/min respectively. The fractional conversion values were calculated based on the areas covered by the peak.
Which purge gas was applied - nitrogen, air or oxygen? Which type of DSC was employed - power compensated or heat flux? Which was the sample mass? The sample mass for the last sample does not differ substantially from the others?
Rough calculation of Ea using three data points at 8, 10, and 12 K/min provides approximately 200 kJ/mol. It looks not so bad for the thermal decomposition at the conrresponding temperature range, in general. Because the reaciton may be overlapping mutistep reaction, the peak position (fractional conversion at the major peak top) may change with heating rate due to the change in the overlapping degree or the contribution of each reaction step. You should select the appropriate range of heating rate for evaluating the shift of kinteic rate data with heating rate and the kinetic parameters. I recommned to add another measurement at 5 K/min and exclude the data at 14 K/min. In my resarch, I usually do not use the heating rate higher than 5 K/min. Visit my profile in ResarchGate.
If you cold provide the information requested by Peter, we can discuss in more details about you data. (Ahoj, Peter!)
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