Variable-heating-rate experiments

Moulinie and Woefle [97] have described how DSC can be used to study the kinetics of the decomposition of azodicarbonamide (AZDC) in polyolefin foams. Activation energies and rate constants were computed from the results that were obtained using variable heating rate experiments. 

Subsequently, the release of ammonia (the only degradation product which could be measured quantitatively) was measured during a series of TGA experiments with variable heating rates. These experiments were performed to show how the heating rate influences the temperature and thus the possible occurance of thermal degradation, in a (small) curing TGA sample. 

Sichina, W. J., Considerations in Variable Heating Rates Thermogravimetric Experiments, Proc. 2P North Am. Thermal Anal. Soc. Conf, Atlanta, GA, pp. 206-211 (1992)... 

In this section, the experiments have been carried out in the same way, as it is describe on the experimental section for pyrolysis experiments. The studied variable has been the heating rate. Four different values have been used p = 5, 10, 15 and 30 C/min. Fig. 8 shows the heating program followed in each case. 

Important variables in the pyrolysis of wood are temperature, heating rate and duration of pyrolysis. The reactor temperature was varied from 300 to 500 C, and the duration of pyrolysis was 80, 200, 1800 and 3600 s. After the experiment, the glassware, fluidized bed including sand, wire screen and cotton filter were washed with 20 ml of distilled acetone. 

The liquid yields calculated by using Eq. 2 are also given in Fig. 2b. Equation 2 represents the correlation obtained by means of regression analysis. The correlation coefficient is 0.9358. However, Eqs. 1 and 2 are only valid for the experiments within the extreme range of the heating rates between 5°C/min and 140°C/min while maintaining other variable parameters to be constant, including reactor type and dimensions, biomass conditions, etc. 

Wendlandt identified some 16 variables which influence the results from DTA and DSC experiments. Whilst many are attributable to the design of the equipment or to the inherent properties of the sample there remains a core of variables where the practitioner is able to exert some control. Sample preparation and containment, heating rate and atmosphere all come within this core and even small refinements in technique can often enhance the quality of the results. Two somewhat extreme examples illustrate the need for careful control of experimental technique. The first is the well-known example of the decomposition of an oxalate to carbonate and CO. This is an endothermic process but the thermal analysis curve will show an exotherm due to combustion of CO if there is a trace of air remaining in the apparatus. The second example illustrates how the use of a crucible with an ill-fitting lid may give rise to ambiguous results. Thus, in the study of emulsion explosives the vaporisation of the sample... 

In addition to these particular features of the soot combustion reactions, the reaction rate also depends on general variables, such as temperature for isothermal reactions and heating rate for ramp experiments, nature and partial pressure of gases in the stream, space velocity/residence time of gases in the solid bed, soot-to-catalyst ratio, and so on. 

Using the Hi-Res TGA technique, a simplified method has been developed by Salin, et al.,l l to extract kinetic parameters from variable heating experiments by using a mathematical function which takes into account resolution, sensitivity, and initial heating rate. These parameters affect the overall heating rate and ean be controlled by the operator. 

Controlled variables for the experiment were diameter to in.) and length (44 to 130 in.) of test section, flow rate (6 to 23 liters/min), wind velocity (0 and 15 mph), and supply dewar pressure. Other variables such as ambient temperature, humidity, and barometric pressure were not controlled but measured so that their effects on the heat transfer rate could be analyzed. Heat leak calculations were based upon measured liquid flow rates and thermodynamic properties upstream from the orifice and in the receiver. 

The improvement in sensitivity in the derivative curve is considerable, although the quantitative data presented are rather inconsistent between the four samples. Variability of final ash content has not proved a problem with 10-mg samples and the inconsistency is believed to be a function of the small sample size. Baseline subtraction has been found valuable in allowing for this, as some buoyancy effects will always be inescapable in the TGA experiment. The qualitative data are a great improvement at the higher heating rate... 

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