Adiabatic Dewar calorimetry

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. 

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. ... 

Minimum exothermic runaway temperature Establish minimum temperature Adiabatic Dewar Adiabatic calorimetry ARC... 

Consequence of runaway reaction Temperature rise rates Gas evolution rates Adiabatic Dewar Adiabatic calorimetry Pressure ARC VSP/RSST RC1 pressure vessel... 

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 ... 

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. 

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. 

Adiabatic calorimetry. Dewar tests are carried out at atmospheric and elevated pressure. Sealed ampoules, Dewars with mixing, isothermal calorimeters, etc. can be used. Temperature and pressure are measured as a function of time. From these data rates of temperature and pressure rises as well as the adiabatic temperature ri.se may be determined. If the log p versus UT graph is a straight line, this is likely to be the vapour pressure. If the graph is curved, decomposition reactions should be considered. Typical temperature-time curves obtained from Dewar flask experiments are shown in Fig. 5.4-60. The adiabatic induction time can be evaluated as a function of the initial temperature and as a function of the temperature at which the induction time, tmi, exceeds a specified value. 

VSP , the PHI-TEC II , the RSST and the Chilworth stainless-steel dewar, as well as the self-made types. On top of these there are several other measuring set-ups, which are more or less modified versions of those mentioned. The general discussion of adiabatic calorimetry shall be conducted on the basis of the general dewar technique as a comparative study of the commercially available devices is not the subject of this book. 

Early calorimetric investigations were performed on pupae of the wax moth Galleria mellonella by means of a differential adiabatic setup [92]. Galleria represents an important pest for weak honeybee colonies where it feeds on wax, honey, pollen and other organic material. Individual pupae were kept in small Dewar flasks of 5 cm for measurements of 10 to 60 min, which were repeated in regular intervals through the whole pupal metamorphosis of about 7 days. The calorimetric experiments alternated with manometry (indirect calorimetry) to evaluate the RQ and to find hints about the type of metabolism. Heat dissipation was monitored by electrical heating of the reference flask to keep the temperature difference between both Dewars close to zero. In this way composite curves for male and female moths and both direct and indirect calorimetry were established. 

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