Adiabatic Dewar calorimeters

Adiabatic Dewar calorimeters are usually used in the closed mode. However, it is possible to incorporate a vent line to either an external containment vessel or to a burette for measuring the permanent gas evolution rate. This vent line contains an automatic valve to simulate the operation of the pressure relief system. 

Similar data can also be obtained for vapour pressure systems from the DIERS bench-scale apparatus operated in the open mode (see Figure A2.5). A high back pressure is superimposed on the containment vessel to suppress, boiling of the sample. An adiabatic Dewar calorimeter can also be operated in this mode if it has the facility to vent to an external containment vessel. 

Wright, T.K. and Rogers, R.L (1986) Adiabatic dewar calorimeter, in Hazards IX, Institution of Chemical Engineers, Manchester. 

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 ... 

In addition to the adiabatic dewar, several adiabatic calorimeters are commercially available that allow emergency pressure-relief system sizing. These include ... 

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. 

Schematic of an adiabatic solution calorimeter. The assembly may be kept in a thermostated bath. Activation is done by impaling the glass ampoule onto the spike, releasing the sample into the contents of the Dewar flask...

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 time, temperature and temperature-rate data collected from the exothermic decomposition of materials in adiabatic calorimeters such as adiabatic Dewars or ARC are handled by a single-step A B reaction model. Townsend and Tou 2 first introduced this model as a means of obtaining safety limits and simple kinetic parameters from such adiabatic data. 

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. 

These tests can also be used to evaluate the induction time for the start of an exothermic decomposition, and the compatibility with metals, additives, and contaminants. The initial part of the runaway behavior can also be investigated by Dewar tests and adiabatic storage tests. To record the complete runaway behavior and often the adibatic temperature rise, that is, the consequences of a runaway, the accelerating rate calorimeter (ARC) can be used, although it is a smaller scale test. 

Figure 3.6 Schematic representation of the bomb calorimeter for measuring the changes in internal energy that occur during combustion. The whole apparatus approximates to an adiabatic chamber, so we enclose it within a vacuum jacket (like a Dewar flask)...

The methods used for the isothermal reactor can also be used here, but must be completed by a thermal study over the total temperature range in which the reactor will be operated. Therefore, DSC in the scanning mode, or adiabatic calorimeters such as the Accelerating Rate Calorimeter or simply the Dewar flask, can be used. 

Latent heats of evaporation of liquefied gases at low temperatures have been determined by various methods. Dewar, and Behn, dropped pieces of metal of known specific heat into the liquid and measured the gas evolved. Estreicher heated the liquid in a double Dewar vessel electrically and measured the volume of gas evolved. In Donath s apparatus (Fig. 4.VIII L) the gas passed through a copper spiral in a block of lead A, so assuming a constant temperature about 2° above the temperature in the metal calorimeter B. The gas then passed to a vessel inside B connected by a thin German-silver tube. The calorimeter was in two parts, between which was a platinum heating spiral for determining the thermal capacity. Outside was an adiabatic mantle C. The whole was in a vacuous copper jacket D. The temperature differences between A and B, and B and C, were determined by thermocouples. The rise in temperature... 

Adiabatic calorimeters the thermal conductance between the calorimetric vessel and the surrounding thermostat equals 0. These calorimeters integrate all heat effects. The heat measurement is based on the measurement of the sample temperature. The most perfect adiabatic calorimeters are those for which the temperature of the thermostat is brought to follow that of the internal vessel, like those proposed by Person (1849) and Richards (1905). In this group of adiabatic calorimeters, the authors also mention the Dewar vessel calorimeter (which they call quasi-adiabatic ) and the Berthelot calorimeter. 

The thermal resistance R is supposed to increase from the isothermal to the isoperibol and then to the adiabatic type of calorimeter. It would probably be more correct and general to base the distinction between the adiabatic and the isoperibol calorimeters on the heat transfer (involving simultaneously the thermal conductance and the temperature difference) rather than on the value of the thermal resistance. For instance, a simple Dewar vessel calorimeter provides a very high thermal resistance between the central system and the surroundings, though it is simply an isoperibol calorimeter (called quasi-adiabatic in section 4.2.), whereas Swietoslawski s adiabatic calorimeters, which do not use any vacuum insulation, certainly provide a much lower thermal resistance [15]. 

Solution calorimeters are usually adiabatic calorimeters. They are mainly used for the study of rapid reactions, for example, heats of solution, heat capacity of liquids, heat capacity of solids by a method of mixtures, or the enthalpy change of rapid reactions in solution. A schematic diagram is shown as Figure 3. The temperature sensor, plus a means of electrical calibration and a device for mixing reactants are all enclosed within a Dewar flask, or other adiabatic assembly. 

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