Power compensation calorimetry

Wang et al. [26] were the first to develop a power compensation calorimeter. They assumed that heat losses are constant throughout the reaction in order to link the measured and the generated heat. This can be achieved by maintaining the outside of the autoclave at a constant temperature that is slightly lower than that of the reaction. This temperature differential is maintained by the use of a smaller internal heater whose power can be externally controlled. If an exotherm occurs, then the system automatically reduces the power supplied to the internal heater to compensate and to maintain a constant temperature. The opposite happens when an endotherm occurs. 

The set-up gives the opportunity to work at larger scale with reaction conditions close to those of industrial reactors. In fact, most of the studies related to SCF chemical reaction applications are reahzed in small-scale batch or tubular reactors (1-60 mL). So far, very few pubhcations deal with calorimetry applied to the supercritical phase. 

Generally, in classical reaction calorimetry only the liquid phase is taken into account in the heat balance. This means that the gas phase in equihbrium with it is neglected because of its small contribution in terms of heat transfer and heat capacity. The situation with supercritical fluids becomes complicated as soon as they occupy all the available volume. This implies that the whole inner reactor surface has to be thermally perfectly controlled when working with supercritical fluids. In this case, the cover and the flange temperature are adjusted on-line to the reaction temperature in order to neglect the heat accumulation term. 

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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. 

Experiments were performed in tlie SIMULAR calorimeter using the power compensation method of calorimetry (note that it can also be used in the heat flow mode). In this case, the jacket temperature was held at conditions, which always maintain a temperature difference ( 20°C) below the reactor solution. A calibration heater was used to... 

Power-compensated differential scanning calorimetry (DSC) apparatus (S = sample R = reference). 

As mentioned above, titration methods have also been adapted to calorimeters whose working principle relies on the detection of a heat flow to or from the calorimetric vessel, as a result of the phenomenon under study [195-196,206], Heat flow calorimetry was discussed in chapter 9, where two general modes of operation were presented. In some instruments, the heat flow rate between the calorimetric vessel and a heat sink is measured by use of thermopiles. Others, such as the calorimeter in figure 11.1, are based on a power compensation mechanism that enables operation under isothermal conditions. 

Differential scanning calorimetry (DSC) is a calorimetric method that finds widespread use in many fields, including protein dynamics, polymers, pharmaceuticals, and inorganic materials. DSC measures energy (heat) flow into a sample and a reference substance as a function of controlled increase or decrease of temperature. In a typical power-compensated DSC (Fig. 3.2), the sample and reference are placed on metal pans in identical furnaces each containing a platinum resistance thermometer (thermocouple) and heater. During a thermal transition (e.g., when a physical change in the sample occurs),... 

Figure 10.4 Differential scanning calorimetry (DSC) instrumentation design (a) heat flux DSC and (b) power compensation DSC. A, furnace B, separate heaters and C, sample and reference holders. (Reproduced with permission from E.L. Charsley and S.B. Warrington, Thermal Analysis Techniques and Applications, Royal Society of Chemistry, Cambridge, UK. 1992 Royal Society of Chemistry.)...

Differenhal scanning calorimetry (DSC) conshtutes one of the most widely used techniques for the study of polymers, parhcularly those systems that crystallize. Although the term DSC is used in conjunchon with many different instruments, fundamentally, these can be divided into two categories heat flow instruments based upon differenhal thermal analysis (DTA) and those which are true power compensated instruments. 

Differential scanning calorimetry was introduced in the 1960s as a means of overcoming the difficulties associated with DTA. Fundamentally, there are two different types of DSC instruments heat flux and power compensation. In common with DTA, DSC involves the measurement of the temperature difference between a... 

Figure 11.17. Power-compensated diflerential scanning calorimetry (DSQ apparatus (S - sample R reference)...

The term differential scanning calorimetry has become a source of confusion in thermal analysis. This confusion is understandable because at the present time there are several entirely different types of instruments that use the same name. These instruments are based on different designs, which are illustrated schematically in Figure 5.36 (157). In DTA. the temperature difference between the sample and reference materials is detected, Ts — Tx [a, 6, and c). In power-compensated DSC (/), the sample and reference materials are maintained isothermally by use of individual heaters. The parameter recorded is the difference in power inputs to the heaters, d /SQ /dt or dH/dt. If the sample is surrounded by a thermopile such as in the Tian-Calvet calorimeter, heat flux can be measured directly (e). The thermopiles surrounding the sample and reference material are connected in opposition (Calvet calorimeter). A simpler system, also the heat-flux type, is to measure the heat flux between the sample and reference materials (d). Hence, dqjdi is measured by having all the hot junctions in contact with the sample and all the cold junctions in contact with the reference material. Thus, there are at least three possible DSC systems, (d), (c), and (/), and three derived from DTA (a), [b), and (c), the last one also being found in DSC. Mackenzie (157) has stated that the Boersma system of DTA (c) should perhaps also be called a DSC system. 

Thermal analysis is not really one subject, because the information gained and the purposes for which it can be used are quite varied. The main truly thermal technique is differential scanning calorimetry (DSC). The heat input and temperature rise for the material under test are compared with those for a standard material, both subjected to a controlled temperature programme. In power compensation DSC the difference in heat input to maintain both test pieces at the same temperature is recorded. In heat flux DSC the difference in heat input is derived from the difference in temperature between the sample and the reference material. Heat losses to the surroundings are allowed but assumed to depend on temperature only. 

Calorific Values Obtained Using Power Compensated Differential Scanning Calorimetry... 

The heat generated by the reaction is directly proportional to the reaction rate for simple systems. The interpretation of the thermogram is more complicated in the case of multiple reactions or simultaneous enthalpic processes such as mixing, dissolution, phase transition, crystallization, etc. Two different calorimetric methods will be discussed power compensation and heat flow calorimetry. 

A more recent step to speed up DSC was taken in connection with a commercial power-compensation DSC (High Performance DSC) [6] and is now available as HyperDSC from the Perkin-Elmer Inc. It is claimed to reach 500 K min. For a pan of a diameter of 5 mm, the heating rates calculated in Fig. A. 10.1 corresponds to sample masses of 20, 2, and 0.2 mg, showing that it is the heating and cooling capacity of the DSC that limits fast calorimetry, not the properties of the sample. 

Historically, DSC is a development of differential thermal analysis (DTA) and both techniques have a common origin in the measurement of temperature. The fundamental concept of both techniques is sim-ple-to measure thermal changes in a sample relative to a thermally inert reference as both are subjected to a controlled temperature program. In classical DTA, the temperature difference between sample and reference is measured as a function of temperature in classical DSC, the energy difference between sample and reference is measured as a function of temperature. Hence, DSC is simply quantitative DTA , or more precisely, DSC is a combination of DTA and adiabatic calorimetry. DSC is the more recent technique and was developed for quantitative calorimetric measurements over a wide temperature range from subambient to 1500 C. DTA is not appropriate for such precision measurements and has been progressively replaced by DSC, even for high-temperature measurements, as the major thermal anal-ysis/calorimetric technique. DSC is a differential calorimeter that achieves a continuous power compensation between sample and reference. 

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