Flux calorimetry

Three different approaches are chiefly applied micro-, flow and heat flux calorimetry. Heat flux calorimetry is certainly the best choice for bioprocess monitoring (Fig. 17) [264]. In a dynamic calorimeter, the timely change of temperature is measured and various heat fluxes (e.g. heat dissipated by stirrer, or lost due to vaporization of water) need to be known in order to calculate the heat flux from the bioreaction ... 

Raemy, A., Froelicher, I. and Loeliger, J. 1987. Oxidation of Lipids Studied by Isothermal Heat Flux Calorimetry. Thermochim. Acta, 114,159-164. 

The superiority of this technique, especially in comparison to the so-called heat flux calorimetry, ch still remains to be described, lies in the fact that the measured signal is completely independent of the size of the heat transfer area, which may change due to a feed process, or of any other substance properties of the mixture, such as density or viscosity. These properties determine tiie heat transfer on the side of the reaction mixture or, in other words, the film heat transfer coefficient, as is well known from process engineermg. 

The principle of heat-flux calorimetry is illustrated with a schematic of a classical DTA in Appendix 9. Accuracies of heat measurements by DSC range from 10% to 0.1%. Temperature can be measured to 0.1 K. Typical heating rates vary between 0.1 and 200 K min. Sample masses can be between 0.05 and 100 mg. The smaller masses are suitable for large heat effects, such as chemical reacdons (explosions), phase transidons, or when fast kinetics is studied. The larger masses are necessary for assessment of smaller heat effects as in studies of heat capacity or glass transitions. Sensitivities are hard to estimate, but effects as small as 1.0 pi s are observable. 

A Raemy, I FroeUcher, J Lohger. Oxidation of lipids studied by isothermal heat flux calorimetry. Thermochim Acta 774 159-164 (1987). 

M Riva, L Piazza, A Schiraldi. Starch gelatinization in pasta cooking Differential flux calorimetry investigations. Cereal Chem 68 622-621 (1991). 

Chapter 9, by Kiraly (Hungary), attempts to clarify the adsorption of surfactants at solid/solution interfaces by calorimetric methods. The author addresses questions related to the composition and structure of the adsorption layer, the mechanism of the adsorption, the kinetics, the thermodynamics driving forces, the nature of the solid surface and of the surfactant (ionic, nonionic, HLB, CMC), experimental conditions, etc. He describes the calorimetric methods used, to elucidate the description of thermodynamic properties of surfactants at the boundary of solid-liquid interfaces. Isotherm power-compensation calorimetry is an essential method for such measurements. Isoperibolic heat-flux calorimetry is described for the evaluation of adsorption kinetics, DSC is used for the evaluation of enthalpy measurements, and immersion microcalorimetry is recommended for the detection of enthalpic interaction between a bare surface and a solution. Batch sorption, titration sorption, and flow sorption microcalorimetry are also discussed. 

Two methods are used to compute the heat of reaction, Q, depending if only Equation 7.1 is solved, namely, the heat balance of the reactor content, or either Equations 7.1 and 7.2 are simultaneously solved [6]. The first method is known as heat flow (flux) calorimetry and the second as heat balance calorimetry. 

Practically, the technique utilizes the principle of differential heat flux calorimetry, with which it is possible to operate under four thermodynamic situations where the perfectly controlled variation (or perturbation) of one of the three state variables (p, V, or T) is simultaneously recorded with the thermal effect resulting from the generated perturbation of the system under investigation. The principle of scanning transitiometry [23] offers the possibility to scan, in the measuring calorimetric cell, one of the three independent thermodynamic variables (p, V, or T)... 

The basic principle of heat-flow calorimetry is certainly to be found in the linear equations of Onsager which relate the temperature or potential gradients across the thermoelements to the resulting flux of heat or electricity (16). Experimental verifications have been made (89-41) and they have shown that the Calvet microcalorimeter, for instance, behaves, within 0.2%, as a linear system at 25°C (41)-A. heat-flow calorimeter may be therefore considered as a transducer which produces the linear transformation of any function of time f(t), the input, i.e., the thermal phenomenon under investigation]] into another function of time ig(t), the response, i.e., the thermogram]. The problem is evidently to define the corresponding linear operator. 

The energy change associated with the process under study induces an energy change of the calorimeter proper, which can be determined by monitoring a corresponding temperature change or heat flux. In some calorimeters the reaction occurs in a closed vessel whose volume does not vary in the course of the experiment. This happens, for example, in bomb combustion calorimetry, where the reaction takes place inside a pressure vessel called the bomb, and in... 

A. Rojas-Aguilar, A. Valdes-Ordonez. Micro-combustion Calorimetry Employing a Calvet Heat Flux Calorimeter. J. Chem. Thermodynamics 2004, 36, 619-626. 

Dong, H. B. and Hunt, J. D. (2001). A numerical model for two-pan heat flux DSC. Journal of Thermal Analysis and Calorimetry. 64,167-176. 

In the CSM laboratory, Rueff et al. (1988) used a Perkin-Elmer differential scanning calorimeter (DSC-2), with sample containers modified for high pressure, to obtain methane hydrate heat capacity (245-259 K) and heat of dissociation (285 K), which were accurate to within 20%. Rueff (1985) was able to analyze his data to account for the portion of the sample that was ice, in an extension of work done earlier (Rueff and Sloan, 1985) to measure the thermal properties of hydrates in sediments. At Rice University, Lievois (1987) developed a twin-cell heat flux calorimeter and made AH measurements at 278.15 and 283.15 K to within 2.6%. More recently, at CSM a method was developed using the Setaram high pressure (heat-flux) micro-DSC VII (Gupta, 2007) to determine the heat capacity and heats of dissociation of methane hydrate at 277-283 K and at pressures of 5-20 MPa to within 2%. See Section 6.3.2 for gas hydrate heat capacity and heats of dissociation data. Figure 6.6 shows a schematic of the heat flux DSC system. In heat flux DSC, the heat flow necessary to achieve a zero temperature difference between the reference and sample cells is measured through the thermocouples linked to each of the cells. For more details on the principles of calorimetry the reader is referred to Hohne et al. (2003) and Brown (1998). 

DSC differential scanning calorimetry Differential, ideal flux, or isoperibolic Screening, secondary reactions 1-50 mg -50 + 500 °C (2)b> 10... 

Permeability changes in full-thickness skin have been associated with temperature or solvent pretreatment. The molecular basis of these permeability changes has been attributed to lipid melting or protein conformational changes. The current studies have probed the role of lipid fluidity in the permeability of lipophilic solutes, and examined the effects of temperature on the physical nature of the stratum corneum by differential scanning calorimetry and thermal perturbation infrared spectroscopy. Combining molecular level studies that probe the physical nature of the stratum corneum with permeability results, a correlation between flux of lipophilic solutes and nature of the stratum corneum barrier emerges. 

Gnaiger, E. (1990). Concepts on efficiency in biological calorimetry and metabolic flux control. Thermochim. Acta 172,31-52. 

Gradient Heat Flux Environments and Rapid Cone Calorimetry.434... 

CASE STUDY 2. HIGH THROUGHPUT POLYMER FLAMMABILITY CHARACTERIZATION USING GRADIENT HEAT FLUX ENVIRONMENTS AND RAPID CONE CALORIMETRY ... 

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