Heat flow modulation

Heat Capacity = Modulated Heat Flow/Modulated Heating Rate... 

In contrast to the very simple treatment outlined above in section 2.1, one can allow for the situation that the heat flow modulation might not always follow exactly the cosine modulation in the heating rate (for reasons that will become clear in the discussions on various transitions). Thus, the heat flow may lag behind. 

It is also possible, and often helpful, to use complex notation. The ratio of the amplitudes of the modulations of the temperature rise and heat flow gives one useful piece of information C = hf/ hr- The phase lag gives another. These two bits of information are equivalent to knowing both CpPCR and CpK, or the single complex quality C = CpR — iCpK where i = the square root of —1. Since the temperature rise and heat flow modulations can be written as Re cu5e and =... 

Also, using the complex notation, the heat flow modulation is just... 

Data obtained Total heat flow Total heat flow Modulated heat flow Reversing heat flow Non-reversing heat flow Heat capacity... 

The ramp rate and heat flow modulation should be considered. It is important to establish whether the peaks are spiky or well formed and uniform. Well-formed sine-wave shaped peaks are necessary for achieving a proper modulation with no distortion showing a uniform wave pattern. Figure 2.95 shows differences between modulations with no distortion and those that are highly distorted. 

The micro heat transfer module (Figure 3.15) comprises a stack of micro structured platelets which are irreversibely bonded [29, 30]. The module is heated by external sources, e.g. by placing it in an oven or by resistance heating. The single parallel flows are all guided in the same direction on the different levels provided by the platelets. Before and after, distribution and collection zones are found, connected to inlet and outlet connectors. 

Air from the compressor is split into two streams primary air is premixed with the fuel and then fed to the catalyst, which is operated under O2 defect conditions secondary air is used first as a catalyst cooling stream and then mixed with the partially converted stream from the catalyst in a downstream homogeneous section where ignition of gas-phase combustion occurs and complete fuel burnout is readily achieved. The control of the catalyst temperature below 1000 Cis achieved by means of O2 starvation to the catalyst surface, which leads to the release of reaction heat controlled by the mass transfer rate of O2 in the fuel-rich stream and of backside cooling of the catalyst with secondary air. To handle both processes, a catalyst/heat exchanger module has been developed, which consists of a bundle of small tubes externally coated with an active catalyst layer, with cooling air and fuel-rich stream flowing in the tube and in the shell side, respectively [24]. 

Differential scanning calorimetry (DSC) compares the two different heat flows one to or from the sample to be studied, and the other to or from a substance with no phase transitions in the range to be measured, e.g. glassmaking sand. Figures 1.45.1 and 1.45.2 show artist s views of parts of a modulated DSC system, and Figure 1.46 shows a commercial apparatus for DSC measurements. 

The parallel reactor for the screening of the titer-plates consists of several modules, each of them is responsible for just a single operation (Fig. 4.12). The gas flow for example is preheated and evenly distributed within the distribution module and delivered to the wells on the titer-plate. The latter is clamped between the distribution module and the insulation module and also treated as a separate reaction module. The insulation module separates the heated section of the parallel reactor from the unheated section and is further cooled by the heat exchanger module on top of it. The last module, just above the heat exchanger module, is a multi-port valve that delivers the product gas to the gas-chromatograph. 

Figure 18.12. Example heat flow behavior of temperature-modulated differential scanning calorimetry. Reproduced from Young and LeBoeuf (2000), by permission of the American Chemical Society.

Differential Scanning Calorimetry. DSC scans were made at 20°C min"1 on a Mettler TA300O system equipped with a DSC-30 low temperature module. Temperature calibration was done with a multiple Indium-lead-nickel standard. An indium standard was used for heat flow calibration. Thin shavings (ca. 0.5 mm thick) were cut with a razor blade from the cross-sectional edge of a plaque. These sections contained both surface and center portions. 

Hence, in the simplest terms, tmDSC is a description of the heat flow into the sample resulting from the sinusoidal modulation of the temperature program. Two properties of the sample can be investigated by tmDSC, the heat capacity which is directly related to the reversing component and a kinetically hindered thermal event which is related to the nonreversing component. Conventional DSC provides only a measure of the total heat flux into a sample as a function of temperature whereas tmDSC allows the heat capacity and kinetic components to be separated. However,... 

Temperature modulated DSC (MDSC)2°2-204 jg another technique that has proved useful in the study of the glass transition " - where, it has been claimed, the approach is capable of providing better resolution and sensitivity than conventional DSC.2° In this, a modulated temperature programme is superimposed upon the conventional heating ramp and the resulting heat flows are interpreted in terms of two heat capacities an in-phase storage heat capacity and an out-of-phase kinetic heat capacity. Various theoretical procedures have been proposed for this and there is little doubt that the approach can provide information that is complementary to conventional DSC.2 ° However, the technique does involve slow temperature scans (cf. high-speed DSC above) and the authors feel that there are areas where the additional data are not, at present, easy to interpret. 

A variation of DSC is the MDSC (modulated DSC), wherein heat is applied sinusoidally, such that any thermal events are resolved into reversing and nonreversing components to allow complex and even overlapping processes to be deconvoluted. The heat flow signal in conventional DSC is a combination of... 

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