Dewar calorimetry

Techniques such as adiabatic calorimetry (Dewar calorimetry) were by then well established [2, 118, 119]. All these techniques can be used for obtaining data to design for the prevention of runaway reactions, that is, to design for inherent plant safety. 

There are a number of different types of adiabatic calorimeters. Dewar calorimetry is one of the simplest calorimetric techniques. Although simple, it produces accurate data on the rate and quantity of heat evolved in an essentially adiabatic process. Dewar calorimeters use a vacuum-jacketed vessel. The apparatus is readily adaptable to simulate plant configurations. They are useful for investigating isothermal semi-batch and batch reactions, and they can be used to study ... 

R L Rogers, "The Advantages and Limitations of Dewar Calorimetry in Chemical Hazard Testing", Plant/Operations Progress, 8, 109-112, 1989... 

Rogers, R.L. (1989) The advantages and limitations of adiabatic dewar calorimetry in chemical hazards testing. Plant Operation Progress, 8 (2), 109-12. 

Before describing adiabatic dewar calorimetry in detail, some more fimdamental considerations shall be made. As has been outlined, the basic requirement to such an experiment is its performance under adiabatic conditions. This can never be achieved if the demand is taken to be absolute. It is inherent to all measuring devices that they lose some heat to the environment. What differs is the degree of this loss. Grewer has compiled some data on the heat loss characteristics of different sample vessels [IS]. They are shown in Table 4-10. 

In the following, dewar calorimetry shall be discussed in greater detail. A schematic drawing of the measuring set-up is shown in Figure 4-89. 

Dewar calorimetry, which uses a vacuum-jacketed flask to measure thermal... 

ADIABATIC (PRESSURE) DEWAR CALORIMETRY The adiabatic pressure Dewar calorimeter is a development of the Dewar apparatus described in Section 3.4.2 on page 35. The traditional glass Dewar is replaced by one made of stainless steel, allowing reactions to be carried out under pressure. The apparatus is installed in a strong containment cell to protect the operators. 

Table 4.1 also gives the half life, which is the time taken for the temperature to fall to half its original value, and the time required for a 1 K temperature drop. It can be seen that the heat losses from the typical small-scale tests used are far greater than occurs in plant items. The data obtained therefore has to be extrapolated. Tests using simple glass Dewars can simulate small plant reactors, up to 12.7 m. However, to obtain data under conditions that represent larger reactors it is necessary to use adiabatic Dewar calorimetry. ... 

Figure 4.5 shows the exothermic decomposition of a distillation residue. The exotherm (255 kJ kg ) is visible on the trace as a deviation from the baseline in the range 310-500 C. On a plant scale a significant exotherm could occur at much lower temperatures. A detailed knowledge of the kinetics of the decomposition would be desirable (Dewar calorimetry, ARC or isothermal DSC). 

DATA FROM DEWAR CALORIMETRY BATCH REACTIONS... 

The temperature/time curves obtained from Dewar calorimetry can be analysed to yield thermodynamic and kinetic data. The procedure is much easier when the experimental data have been logged by computer. 

The method of analysis depends on the nature of the reaction. Adiabatic operation to obtain kinetic parameters is essentially similar to that described for Dewar calorimetry (see Section 4.4.3, page 66). Isothermal operation involves carrying out the reaction at different temperatures and analysing the resulting power data using classical kinetic theory. 

Vent sizing methods require data on both the pressure/temperature relationship and rate of heat release/temperature relationship that occur during the course of the runaway reaction. This data can be obtained using techniques such as adiabatic pressure Dewar calorimetry or other special equipment described in Chapters 3 and 4. 

The isoperibolic Dewar calorimetry and equations developed by Fleischmann and Pons remain relevant today because this accurate electrochemical calorimetry could be applied to the investigation of the thermal behavior of a wide range of electrochemical reactions, especially irreversible processes. Thus, the understanding and adoption of this electrochemical calorimeter concept remains important both for the Pd/D system and elsewhere in science. The detailed discussions of Fleischmann s calorimetry are found mainly in US Navy reports [31-33] because the length and subject matter prohibited publication in scientific journals. 

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