Heat flow experiments

Fig ure 12-27. Temperature versus time plot from a semi-batoh heat flow experiment. (Source Hazard Evaluation Laboratory Ltd.)... 

The superscript zero on a thermodynamic function (for example, AH0) indicates that the corresponding process has been carried out under standard conditions. The standard state for a substance is a precisely defined reference state. Because thermodynamic functions often depend on the concentrations (or pressures) of the substances involved, we must use a common reference state to properly compare the thermodynamic properties of two substances. This is especially important because for most thermodynamic properties, we can measure only changes in that property. For example, we have no method for determining absolute values of enthalpy. We can measure only enthalpy changes (AH values) by performing heat flow experiments. 

For this process it can be postulated that the reaction is slow at the process temperature of 80°C as there is a plant hold time of 4 hours and a catalyst is used. Therefore an isothermal heat flow experiment was carried out in which the paraformaldehyde and the mixture of glycols were charged to the calorimeter and heated to 80°C. The acid catalyst was then added when the baseline had stabilized. The resulting heat profile is shown in Figure A2.3. It can be seen that on addition of the catalyst an immediate exotherm occurred, although surprisingly with a heat output of only around 5.9 kJ kg. ... 

From an appreciation of the reaction chemistry, and as the total heat change during the process is endothermic, over- or under-charging of the reactants will not lead to a higher temperature. The delayed addition of the acid catalyst has been simulated in the isothermal heat flow experiment. This showed that there is little accumulation of reactants and this maloperation will only lead... 

For single reactions, the rate of reaction is directly proportional to the rate of heat evolution observed. The most common objections to the use of thermal methods are related to the facts that most reactions are not single and that heat is a very unspecific information. Therefore the conclusion could be drawn that thermal methods are of little value for kinetic work. However, in the authors experience on several hundred reactions of widely different kinds, this is not the case.In a surprisingly large number of cases, the main reaction is dominating thermally to such an extent, that the influence of concentrations and of temperature on the reaction rate can be obtained by heat flow experiments alone. In most other cases, thermal data provide valuable information additional to the data obtained by classical means, a fact which drastically speeds up kinetic work. Of course, thermal methods are not appropriate for selectivity determinations. 

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... 

Most polymer processing methods involve heating and cooling of the polymer melt. So far the effect of the surroundings on the melt has been assumed to be small and experience in the situations analysed has proved this to be a reasonable assumption. However, in most polymer flow studies it is preferable to consider the effect of heat transfer between the melt and its surroundings. It is not proposed to do a detailed analysis of heat transfer techniques here, since these are dealt with in many standard texts on this subject. Instead some simple methods which may be used for heat flow calculations involving plastics are demonstrated. 

Bomb calorimeter. The heat flow, q, for the reaction is calculated from the temperature change multiplied by the heat capacity of the calorimeter, which is determined in a preliminary experiment... 

Some heat flows result only in changes in temperature. Under these conditions, the amount of heat can be determined from the temperature change (Ar-Tfoal - 7 imtial) Experiments show that changes in temperature depend on four factors ... 

A calorimeter Is a device used to measure heat flows that accompany chemical processes. The basic features of a calorimeter include an Insulated container and a thermometer that monitors the temperature of the calorimeter. A block diagram of a calorimeter appears in Figure 6-15. In a calorimetry experiment, a chemical reaction takes place within the calorimeter, resulting in a heat flow between the chemicals and the calorimeter. The temperature of the calorimeter rises or falls in response to this heat flow. 

Calorimetry experiments are designed so that the heat transfer is confined to the calorimeter. Equation relates heat flow and temperature change ... 

In a calorimetry experiment, the heat flow resulting from a process is determined by measuring the temperature change of the calorimeter. Then q can be related to energy change through the first law of thermodyuamics (Equation ) A S = g + W... 

The laser we use in these experiments is an exclmer laser with a pulse width of approximately 20 nsec. In this time regime the laser heating can be treated using the differential equation for heat flow with a well defined value for the thermal diffusivity (k) and the thermal conductivity (K) (4). 

A survey of the literature shows that although very different calorimeters or microcalorimeters have been used for measuring heats of adsorption, most of them were of the adiabatic type, only a few were isothermal, and until recently (14, 15), none were typical heat-flow calorimeters. This results probably from the fact that heat-flow calorimetry was developed more recently than isothermal or adiabatic calorimetry (16, 17). We believe, however, from our experience, that heat-flow calorimeters present, for the measurement of heats of adsorption, qualities and advantages which are not met by other calorimeters. Without entering, at this point, upon a discussion of the respective merits of different adsorption calorimeters, let us indicate briefly that heat-flow calorimeters are particularly adapted to the investigation (1) of slow adsorption or reaction processes, (2) at moderate or high temperatures, and (3) on solids which present a poor thermal diffusivity. Heat-flow calorimetry appears thus to allow the study of adsorption or reaction processes which cannot be studied conveniently with the usual adiabatic or pseudoadiabatic, adsorption calorimeters. In this respect, heat-flow calorimetry should be considered, actually, as a new tool in adsorption and heterogeneous catalysis research. 

In heat-flow calorimeters, it is particularly important, as already indicated in Section II, that the heat sink remain, throughout the experiment, at a constant temperature. The construction of the heat sink and thermostat in the Calvet apparatus is shown in Fig. 3. The calorimetric element fits into a conical socket (A), cut in a cylindrical block of aluminium (B). The block is positioned between the bases of two truncated cones (C and C ), placed within a thick metal cylinder (D). The metal cylinder is, in... 

Although most heat-flow calorimeters are multipurpose instruments, it is clear that for each particular type of experiment, the inner calorimeter cell must be especially designed and carefully tested. The reliability of the calorimetric data and, thence, the precision of the results depend, to a large extent, upon the arrangement of the inner cell. Typical arrangements for adsorption studies are described in the next section (Section VI.A). 

Heat-flow calorimetry may be used also to detect the surface modifications which occur very frequently when a freshly prepared catalyst contacts the reaction mixture. Reduction of titanium oxide at 450°C by carbon monoxide for 15 hr, for instance, enhances the catalytic activity of the solid for the oxidation of carbon monoxide at 450°C (84) and creates very active sites with respect to oxygen. The differential heats of adsorption of oxygen at 450°C on the surface of reduced titanium dioxide (anatase) have been measured with a high-temperature Calvet calorimeter (67). The results of two separate experiments on different samples are presented on Fig. 34 in order to show the reproducibility of the determination of differential heats and of the sample preparation. 

In the various sections of this article, it has been attempted to show that heat-flow calorimetry does not present some of the theoretical or practical limitations which restrain the use of other calorimetric techniques in adsorption or heterogeneous catalysis studies. Provided that some relatively simple calibration tests and preliminary experiments, which have been described, are carefully made, the heat evolved during fast or slow adsorptions or surface interactions may be measured with precision in heat-flow calorimeters which are, moreover, particularly suitable for investigating surface phenomena on solids with a poor heat conductivity, as most industrial catalysts indeed are. The excellent stability of the zero reading, the high sensitivity level, and the remarkable fidelity which characterize many heat-flow microcalorimeters, and especially the Calvet microcalorimeters, permit, in most cases, the correct determination of the Q-0 curve—the energy spectrum of the adsorbent surface with respect to... 

Fire extinguishment behavior of the FRC materials using Halon 1301 was quantified with a horizontal 0.10 x 0.10 m sample with edges covered tightly with heavy duty aluminum foil. The sample surface was exposed to 60 kW/m of external heat flux. Experiments were performed under forced air flow conditions, where Halon 1301 was added to the inlet air flow such that fire remained well ventilated. 

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