Temperature-time data calorimetry

Adiabatic calorimetry. Dewar tests are carried out at atmospheric and elevated pressure. Sealed ampoules, Dewars with mixing, isothermal calorimeters, etc. can be used. Temperature and pressure are measured as a function of time. From these data rates of temperature and pressure rises as well as the adiabatic temperature ri.se may be determined. If the log p versus UT graph is a straight line, this is likely to be the vapour pressure. If the graph is curved, decomposition reactions should be considered. Typical temperature-time curves obtained from Dewar flask experiments are shown in Fig. 5.4-60. The adiabatic induction time can be evaluated as a function of the initial temperature and as a function of the temperature at which the induction time, tmi, exceeds a specified value. 

The experiments are usually carried out at atmospheric pressure and the initial goal is the determination of the enthalpy change associated with the calorimetric process under isothermal conditions, AT/icp, usually at the reference temperature of 298.15 K. This involves (1) the determination of the corresponding adiabatic temperature change, ATad, from the temperature-time curve just mentioned, by using one of the methods discussed in section 7.1 (2) the determination of the energy equivalent of the calorimeter in a separate experiment. The obtained AT/icp value in conjunction with tabulated data or auxiliary calorimetric results is then used to calculate the enthalpy of an hypothetical reaction with all reactants and products in their standard states, Ar77°, at the chosen reference temperature. This is the equivalent of the Washburn corrections in combustion calorimetry... 

The temperature/time curves obtained from Dewar calorimetry can be analysed to yield thermodynamic and kinetic data. The procedure is much easier when the experimental data have been logged by computer. 

Figure 22 Crystallization halftime (time of peak maximum) of PBT as a function of crystallization temperature. Data from DFSC, AC chip-based calorimeter after quick cooling from the melt inside the DFSC, and DSC. The data are extended to slow crystallization measurements by using AC calorimetry and DSC at low and high temperatures, respectively. The dynamic glass-transition temperature from AC calorimetry at 40 Hz is also indicated (see Section 2.31.4.4).

Differential Scanning Calorimetry. A sample and an inert reference sample are heated separately so that they are thermally balanced, and the difference in energy input to the samples to keep them at the same temperature is recorded. Similarly to DTA analysis, DSC experiments can also be carried out isothermally. Data on heat generation rates within a short period of time are obtained. Experimental curves from DSC runs are similar in shape to DTA curves. The results are more accurate than those from DTA as far as the TMRbaiherm is concerned. 

Titration calorimetry or thermometric titration calorimetry is a technique in which one reactant is titrated continuously into the other reactant, and either the temperature change or heat produced in the system is measured as a function of titrant added. In isoperibol titration calorimetry, the temperature of a reaction vessel in a constant-temperature environment is monitored as a function of time (Figure 8.4) (Hansen et al., 1985 Winnike, 1989). A single titration calorimetric experiment yields thermal data as a function of the ratio of the concentrations of the reactants. 

Differential Scanning Calorimetry (DSC) was used for a long time in the field of process safety [21-23], This is essentially due to its versatility for screening purposes. The small amount of sample required (micro-calorimetric technique) and the fact that quantitative data are obtained, confer on this technique a number of advantages. The sample is contained in a crucible placed into a temperature controlled oven. Since it is a differential method, a second crucible is used as a reference. This may be empty or contain an inert substance. 

The induction time is the time involved between the instant where the sample reaches its initial temperature and the instant where the reaction rate reaches its maximum. In practice, two types of induction times must be considered the isothermal and the adiabatic. The isothermal induction time is the time a reaction takes to reach its maximum rate under isothermal conditions. It can typically be measured by DSC or DTA. This assumes that the heat release rate can be removed by an appropriate heat exchange system. Since the induction time is the result of a reaction producing the catalyst, the isothermal induction time is an exponential function of temperature. Thus, a plot of its natural logarithm, as a function of the inverse absolute temperature, delivers a straight line. The adiabatic induction time corresponds to the time to maximum rate under adiabatic conditions (TMRJ). It can be measured by adiabatic calorimetry or calculated from kinetic data. This time is valid if the temperature is left increasing at the instantaneous heat release rate. In general, adiabatic induction time is shorter than isothermal induction time. 

The second mechanism by which antifreeze mixtures function, acceleration of cement hydration, was confirmed through calorimetry tests [110], The data (Fig. 7.38) show that the CWA accelerated the time to reach peak temperatures by 8-10 h. It was also reported that, relative to plain cement, the high and low doses of the CWA increases the total amount of heat generated by 625 and 569%, respectively,... 

Immersion calorimetry has much to offer for the characterization of powders and porous solids or for the study of adsorption phenomena. The technique can provide both fundamental and technologically useful information, but for both purposes it is essential to undertake carefully designed experiments. Thus, it is no longer acceptable to make ill-defined heat of immersion measurements. To obtain thermodynamically valid energy, or enthalpy, or immersion data, it is necessary to employ a sensitive microcalorimeter (preferably of the heat-flow isothermal type) and adopt a technique which involves the use of sealed glass sample bulbs and allows ample time (usually one day) for outgassing and the subsequent temperature equilibration. 

This contrast between Idealized and real systems Is a recurring and natural feature and we shall not shun it. A certain emphasis on the model systems is motivated by the consideration that such studies are the basis for further understanding. At the same time a certain prudence against fitting a limited number of equations to a limited number of data is advised. Rarely do such procedures lead to unique solutions. Even a perfect fit does not imply that the underlying model applies, although It may describe the adsorption empirically. The rule remains that for a proper characterization preferably further measurements should be carried out, say adsorption at more temperatures and/or with different adsorptives, or adsorption in conjunction with (mlcro)-calorimetry. Once such experiments have been done and found to concur, the mathematical confidence limit may become as high as the physical one. 

An accelerated reaction calorimetry (ARC) test determined the time-to-maximum rate (TMR) of the decomposition from subambient conditions. Further testing on the material diluted in hexane/heptane was performed to investigate the consequences of a runaway reaction in these mixtures. ARC showed an exotherm from the first heat-wait-search step (at 31°C), which progressed to more than 110°C. A second exotherm was seen at 160°C, progressing to 200°C. The initial decomposition showed a TMR on the order of several days from the storage temperature ( 0°C) anticipated in kilo laboratory campaigns. The TMR is on the order of an hour from room temperature, however, which supports the data from the VSP test of cyclopentadiene. 

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