Compensating Calorimeters

Techniques that use analog and numerical correction of the dynamic properties ( 3.2.11.6) of the calorimeter compensate the transmittance zeros and poles and can be also applied to reconstruct the thermokinetics. In agreement with Eq. (3.41), the transmittance has the form... 

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... 

Differential scanning calorimetry (DSC) can be performed in heat compensating calorimeters (as the adiabatic calorimetry), and heat-exchanging calorimeters (Hemminger, 1989 Speyer, 1994 Brown, 1998). 

When a Joule heating or a Peltier cooling are used to compensate part of the heat absorbed or liberated in the calorimeter cell, Eq. (17) gives... 

Smoke parameter (in the Cone calorimeter) and smoke factor (in both calorimeters) are combined properties of smoke obscuration and heat release which compensate for the incomplete burning of fire retardant samples and which should predict smoke hazard in real fires. 

Adiabatic calorimeters are complex home-made instruments, and the measurements are time-consuming. Less accurate but easy to use commercial differential scanning calorimeters (DSCs) [18, 19] are a frequently used alternative. The method involves measurement of the temperature of both a sample and a reference sample and the differential emphasizes the difference between the sample and the reference. The two main types of DSC are heat flux and power-compensated instruments. In a heat flux DSC, as in the older differential thermal analyzers (DTA), the... 

The two basic types of reaction calorimeters commonly used for safety assessments are isothermal (including both heat flow and power compensation calorimeters) and adiabatic. 

In power compensation calorimeters, the jacket temperature is set slightly below the desired reaction temperature. A heater in the reaction mass maintains the set temperature. A change in electrical power to the heater compensates for any change in reaction temperature. This provides a direct measure of the heat produced by the chemical reaction. 

Ideally, the energy equivalents e and f should be measured over the same temperature range of the reaction ran, to avoid errors from their variation with temperature and to achieve maximum compensation for errors in the calibration of the temperature sensor [26,128,129], These errors are, however, frequently negligible in the temperature ranges involved, and the measurement of or f is normally performed outside the Jj -> Tf interval. This procedure saves time because there is no need to readjust the initial temperature of the calorimeter between the calibration and main experiment runs. It is therefore a common practice, even when an exothermic reaction is studied, to measure before the reaction and ef after the reaction and adjust the experimental conditions so that Jr is the midpoint between J] and Tf. In this case, the temperature of the thermostatic... 

The first heat flow calorimeter based on Seebeck, Peltier, and Joule effects was built by Tian at Marseille, France, and reported in 1923 [156-158]. The set-up included two thermopiles, one to detect the temperature difference 7) — 7) and the other to compensate for that difference by using Peltier or Joule effects in the case of exothermic or endothermic phenomena, respectively. This compensation (aiming to keep 7) = T2 during an experiment) was required because, as the thermopiles had a low heat conductivity, a significant fraction of the heat transfer would otherwise not be made through the thermopile wires and hence would not be detected. 

Tian s instrument had several important advantages over other types of calorimeter available at the time, such as isoperibol or adiabatic instruments (1) It could monitor rather small temperature changes (less than 10-4 K) and therefore minute samples could be used (2) it could be applied to investigate the thermochemistry of very slow phenomena (up to about 24 h) and (3) the use of the compensating Peltier cooling or Joule heating allowed one to investigate the... 

Another problem related to the validity of equation 9.9 is that equation 9.6 applies only to heat conduction. If T — 12 is large, some significant fraction of heat will be transferred by convection and radiation and thus will not be monitored by the thermopile. Consequently, the use of partial compensating Peltier or Joule effects was essential in the experiments involving Calvet s calorimeter, whose thermopiles had a fairly low thermal conductivity. 

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. 

Figure 12.3 Schemeofa power compensation differential scanning calorimeter. A sample furnace Ar reference furnace B temperature sensor of the sample furnace Br temperature sensor of the reference furnace C resistance heater of the sample furnace Cr resistance heater of the reference furnace D cell S sample R reference.

Adiabatic calorimeters have also been used for direct-reaction calorimetry. Kubaschewski and Walter (1939) designed a calorimeter to study intermetallic compoimds up to 700°C. The procedure involved dropping compressed powders of two metals into the calorimeter and maintaining an equal temperature between the main calorimetric block and a surrounding jacket of refractory alloy. Any rise in temperature due to the reaction of the metal powders in the calorimeter was compensated by electrically heating the surrounding jacket so that its temperature remained the same as the calorimeter. The heat of reaction was then directly a function of the electrical energy needed to maintain the jacket at the same temperature as the calorimeter. One of the main problems with this calorimeter was the low thermal conductivity of the refractory alloy which meant that it was very difficult to maintain true adiabatic conditions. 

Fig. 3.27. The Pask-Plesch reaction calorimeter, approximately to scale. A phials of reagents, B phial magazine, C cold finger (not essential), D phial breaker, E vessel of calorimeter, F heater, Gj Pt wires of the conductivity probe, Gj terminals, H vacuum jacket, thermometer probe, terminals from thermometer probe and the compensating leads, K tap for evacuation of pseudo-Dewar space or admitting air, M magnetic pusher, T main tube,...

There are two types of differential scanning calorimeters (a) heat flux (AT) and (b) power compensation (AT). Subsequent sections of this experiment will not distinguish between the two types. In either type of calorimeter, the measurement is compared to that for a reference material having a known specific heat [16,17], As AT and AT have opposite signs there is some potential for confusion [3], e.g., at the melting point, Tm, Ts < Tr, and AT < 0, whereas Ts > Tr and AT > 0 because latent heat must be supplied (subscripts s and r refer to the sample and the reference material, respectively) [3]. 

An important variation of the adiabatic principle is isoperibol calorimetry. Well-defined heat leaks, minimized by efficient calorimeter construction and experiment design, are compensated for by calculation and/or extrapolation. The isoperibol design holds the temperature of the immediate environment surrounding the calorimeter constant. The word isoperibol literally means "constant temperature environment. ... 

Now let us consider Benzotrifiuroxan (BTF), the last entry in Table 2. It produces no water and its Pj is large. Since no water is produced, the above compensating effects are inoperative. We may speculate that the computed Q is larger than the measured Q because the reaction 2CO = C02 + C j proceeds to the left, because some product expansion is unavoidable in the calorimeter. This reaction (CO formation) is endothermic. If the above suppositions are correct, one would expect closer agreement between computed and measured Q s for BTF at low packing densities (at low Pj little C02 is formed via the above reaction and obviously it cannot subsequently revert to CO upon product expansion)... 

Fig. 8.1 Standard set-up of a reaction calorimeter [4]. Left side heat-flow, heat-balance and power-compensation calorimeters. Right side Peltier calorimeters.

Fig. 8.2 Main heat-flow rates that have to be considered in heat-flow, heat-balance and power-compensation reaction calorimeters running under strictly isothermal conditions [4]. The heat-flow rates inside a Peltier calorimeter are analogous (compare with Fig. 8.1). The direction of the heat-flow arrows corresponds to a positive heat-flow rate. For explanation of the different heat-flow rates, see the text.

The calorimeter that has been used to obtain the results presented in this section basically combines the power-compensation and heat-balance principles (see Sections 8.2.2.2 and 8.2.2.3). The heat-balance principle is implemented by Peltier elements [18]. This new... 

Adiabatic conditions may be achieved either by a thermal insulation or by an active compensation of heat losses. Examples are the Dewar calorimeter, achieving a thermal insulation [2-4] or the Accelerating Rate Calorimeter (ARC) [5] or the Phitec [6], using a compensation heater avoiding the heat flow from the sample to the surroundings. These calorimeters are especially useful for the characterization of runaway reactions. 

Visentin, F., Zogg, A., Kut, O. and Hungerbtihler, K. (2004) A pressure resistant small scale reaction calorimeter that combines the principles of power compensation and heat balance (CRC.v4). Organic Process Research ej Development,... 

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