Suitable adiabatic calorimeters

To overcome these effects, a suitable adiabatic calorimeter, for measuring runaway reaction relief sizing data, will11,21 ... 

The PHI-TEC II adiabatic calorimeter as shown in Figure 12-17 was developed by Hazard Evaluation Laboratory Ltd. (UK). The PHI-TEC can be used both as a high sensitivity adiabatic calorimeter and as multi-purpose vent sizing device [17,18]. The PHI-TEC employs the principles established by DIERS and includes advanced features compared to the VSP. It also provides important information for storage and handling and provides useful insight into the options suitable for downstream disposal of vented material. 

This Annex describes the special requirements for adiabatic calorimeters which are suitable for obtaining runaway chemical reaction data for relief system sizing. Methods for obtaining the data required for relief system sizing and for determining the system type for relief sizing (see 4.2) are then described. 

Adiabatic calorimeter. With the adiabatic calorimeter, exchange of heat between the calorimetric vessel and the cover is suppressed. This happens so that the temperatures of the vessel and the cover are maintained at almost the same temperature. The condition (Tc-Tfi) = 0 can be attained at constant cover temperature by heating or cooling the calorimetric vessel using an internal heater or heat sink placed inside the calorimetric vessel. This compensation method is suitable for endothermic processes. For the adiabatic method, the characteristic feature is not only the equality of temperatures of the calorimetric vessel and of the cover, but also their changing value - the measurement proceeds at dynamic conditions, where the temperature of the calorimetric cover follows the temperature of the calorimetric vessel. 

The most frequently used experimental methods for the determination of heat capacities in the temperature range 10 to 400 K require the measurement of electrical energy introduced into a sample of the compound in a suitable container and the measurement of the initial and the final equilibrium temperatures of the sample. The measurements are made in an adiabatic calorimeter in which heat leakage is reduced to a minimum by... 

The thermochemical study of photochemical or photochemically activated processes is not amenable to most of the calorimeters described in this book, simply because they do not include a suitable radiation source or the necessary auxiliary equipment to monitor the electromagnetic energy absorbed by the reaction mixture. However, it is not hard to conceive how a calorimeter from any of the classes mentioned in chapter 6 (adiabatic, isoperibol, or heat flow) could be modified to accommodate the necessary hardware and be transformed into a photocalorimeter. 

The problems associated with direct reaction calorimetry are mainly associated with (1) the temperature at which reaction can occur (2) reaction of the sample with its surroundings and (3) the rate of reaction which usually takes place in an uncontrolled matmer. For low melting elements such as Zn, Pb, etc., reaction may take place quite readily below S00°C. Therefore, the materials used to construct the calorimeter are not subjected to particularly high temperatures and it is easy to select a suitably non-reactive metal to encase the sample. However, for materials such as carbides, borides and many intermetallic compounds these temperatures are insufficient to instigate reaction between the components of the compound and the materials of construction must be able to withstand high temperatures. It seems simple to construct the calorimeter from some refractory material. However, problems may arise if its thermal conductivity is very low. It is then difficult to control the heat flow within the calorimeter if some form of adiabatic or isothermal condition needs to be maintained, which is further exacerbated if the reaction rates are fast. 

One of the simplest calorimetric methods is combustion bomb calorimetry . In essence this involves the direct reaction of a sample material and a gas, such as O or F, within a sealed container and the measurement of the heat which is produced by the reaction. As the heat involved can be very large, and the rate of reaction very fast, the reaction may be explosive, hence the term combustion bomb . The calorimeter must be calibrated so that heat absorbed by the calorimeter is well characterised and the heat necessary to initiate reaction taken into account. The technique has no constraints concerning adiabatic or isothermal conditions hut is severely limited if the amount of reactants are small and/or the heat evolved is small. It is also not particularly suitable for intermetallic compounds where combustion is not part of the process during its formation. Its main use is in materials thermochemistry where it has been used in the determination of enthalpies of formation of carbides, borides, nitrides, etc. 

Any dosimeter used to determine absorbed dose in an irradiated product has to be calibrated. The adiabatic character of electron beam deposition is used in calorimetry, which is the primary absolute method of measuring the absorbed dose (energy per unit mass). An example of the instrument for this purpose is the water calorimeter developed in Ris0 National Laboratory in Denmark. " This calorimeter is reported to be suitable for electrons from a linear accelerator with energies higher than 5 MeV and... 

When using the method, it is essential that the heat release rate per unit mass due to the runaway reaction, q, is measured in an suitable calorimeter (see Annex 2) which simulates the external heat input. If this is not the case, then q can be underestimated since the external heating means that the degree of conversion of the reaction is less (and the reaction rate higher) at any given temperature compared with the adiabatic situation. 

On the other hand, for slow reactions, adiabatic and isothermal calorimeters are used and in the case of very small heat effects, heat-flow micro-calorimeters are suitable. Heat effects of thermodynamic processes lower than 1J are advantageously measured by the micro-calorimeter proposed by Tian (1923) or its modifications. For temperature measurement of the calorimetric vessel and the cover, thermoelectric batteries of thermocouples are used. At exothermic processes, the electromotive force of one battery is proportional to the heat flow between the vessel and the cover. The second battery enables us to compensate the heat evolved in the calorimetric vessel using the Peltier s effect. The endothermic heat effect is compensated using Joule heat. Calvet and Prat (1955, 1958) then improved the Tian s calorimeter, introducing the differential method of measurement using two calorimetric cells, which enabled direct determination of the reaction heat. 

The Gutmann donicity may be determined by calorimetric measurement of the heat of reaction. A solution of the reference acceptor, antimony pentachloride, in dichloroethane is mixed in a suitable calorimeter with a dichloroethane solution of the solvent under investigation. Under adiabatic conditions, the change in temperature of the reaction mixture is proportional to the heat of reaction. (Naturally, the value of the heat of dilution must also be taken into consideration.) For more detail, the reader is referred to the book by Gutmann [Gu 68]. 

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