Times isothermal measurement

With the use of isothermal calorimetry, very accurate heat generation rates can be acquired as a function of time. By measurement at several temperatures, global kinetic parameters can be determined, assuming that the reaction mechanism remains the same within the temperature interval investigated. The heat production of the substance under test can be expressed as ... 

In order to find points of equal degrees of conversion (or equal Q-values) in Figure 2.18, van Geel [115] developed the method to evaluate kinetic data from the so-called isoconversion lines. A heat generating substance that follows Equation (2-11), when stored under isothermal conditions at different temperatures has generated an equal amount of heat (Q) when the product of t exp(-Ea/RT) has the same value. Thus, for two heat generation/time curves measured at Ti and T2, the same amount of heat (Q) has been generated, and thus the conversion is equal when ... 

A series of isothermal experiments are performed at different temperatures in a calorimeter, for example, in a DSC. On each curve, the maximum heat release rate, which represents the worst case, is measured (Figure 11.4). The experimental procedure must reach the desired temperature as fast as possible. For this purpose, in a DSC, the oven is preheated to the desired temperature with the reference cmcible in place. The sample cmcible is then placed into the oven and the measurement begins once thermal equilibrium is achieved, which usually takes approximately 2 minutes. During this time no measurement is possible, but 2 minutes is a very short time relative to the total experimental time of several hours. Thus, the achieved conversion before the measure really starts is negligible. It is left as an exercise for the reader to verify this point Moreover, the difference may be corrected graphically. 

Do isothermal measurements from the past exist and if yes, does the observed isothermal induction time remain constant ... 

Fig. 4 CL intensity as a function of time for LDPE, LDPE-SBR and LDPE-NR. Isothermal measurements made at 100 °C in air. Figure from [81]. Copyright (Wiley 2001). Reprinted by permission of Wiley, New York...

Differential Thermal Analysis (DTA). The differential thermal analysis test serves to examine transitions and reactions which occur on the order between seconds and minutes, and involve a measurable energy differential of less than 0.04 J/g. Usually, the measuring is done dynamically (i.e., with linear temperature variations in time). However, in some cases isothermal measurements are also done. DTA is mainly used to determine the transition temperatures. The principle is shown schematically in Fig. 2.20. Here, the sample, S, and an inert substance, /, are placed in an oven that has the ability to raise its temperature linearly. 

This type of reactor is very widely used because it is one of the simplest to construct and it is cheap. Sampling and analysis of the product streams are generally easy, but they can be difficult to obtain effectively at low conversions. Isothermality of the reactor is generally attainable, particularly at low heat release. Residence-time distribution measurements can be difficult because of channeling problems, but this can be somewhat reduced by running the reactor vertically rather than horizontally. 

T. Kato, What is the Characteristic Time of Measurement of jt-A Isotherms - Necessity of a Constant Strain Rate of Compression of Insoluble Monolayer for Ji-k Measurements , Langmuir, 6, 870 (1990)... 

Nitrogen adsorption isotherms measurements were performed at 77 K on a Micrometries ASAP 2010M and on a Fisons Sorptomatic 1990. Each isotherm point is acquired when the pressure variation, within a fixed time, is lower than a fixed pressure deviation. 

Over a long period of time experimental results on amphiphilic monolayers were limited to surface pressure-area ( r-A) isotherms only. As described in sections 3.3 and 4, from tc[A) Isotherms, measured under various conditions, it is possible to obtain 2D-compressibilities, dilation moduli, thermal expansivities, and several thermodynamic characteristics, like the Gibbs and Helmholtz energy, the energy cmd entropy per unit area. In addition, from breaks in the r(A) curves phase transitions can in principle be localized. All this information has a phenomenological nature. For Instance, notions as common as liquid-expanded or liquid-condensed cannot be given a molecular Interpretation. To penetrate further into understanding monolayers at the molecular level a variety of additional experimental techniques is now available. We will discuss these in this section. 

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