Self-heating rate with calorimeters

De Haven [127] gives an overview of the results of accelerating rate calorimeter (ARC) experiments. The ARC was described in Section 2.3.2.3. As mentioned in the previous description, care must be taken in scale-up of results from experiments with relatively high phi-factors. For direct simulation of plant operating conditions, a phi-factor of 1.0 to 1.05 is required. As stated in [127], a decrease in the phi-factor from 2.0 to 1.0 increases the adiabatic temperature rise by a factor of 2, but the maximum self-heat rate increases by a factor of 20. Later in Chapter 3 (Section 3.3.4.6), an example of scale-up of ARC results is given. 

The ARC system is often operated in a stepwise heat-wait-search modus. After heating to a certain temperature, the system is stabilized for a pre-defined time until the calorimeter starts seeking for a temperature increase caused by first decomposition processes. If the temperature increase surpasses a pre-defined threshold (e.g. 0.01 K/min) the furnace temperature follows the sample temperature in the adiabatic mode and the calorimeter tracks the adiabatic temperature rise due to the self-heating of the sample. If the threshold is not surpassed after a certain period of time, the calorimeter proceeds with the next temperature step. In comparison to DSC analysis ARC measurements are significantly more sensitive, usually by a factor of 100 or more. Sensitivity is as low as 0.5mW/g and self-heating rates of 0.01 K/min can be detected. 

For experiments conducted at slower heating rates under adiabatic conditions, the accelerating-rate calorimeter (ARC) is the instrument of choice [13]. The ARC allows precise control of temperature and exposes the cell to more uniform conditions over longer periods. A typical experiment requires a few days rather than a few hours, as in the case of the heating block. Because of the adiabatic environment, the onset of self-heating due to chemical reactions in the interior of the cell can be detected with greater sensitivity. 

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