Dewar flask testing

Dewar flask tests, adiabatic storage tests (AST)—see Section 2.3.2.2, and... 

Certain equipment configurations allow for the use of Dewar flask testing at elevated pressures. Several arrangements have proved successful such as a sealed glass ampoule in the Dewar flask, a steel pressure vessel in the flask, a Dewar flask in an autoclave under inert gas pressure, and a stainless steel Dewar flask. Dewar flasks provided with an addition line can also be used to study chemical reactions. In Figure 2.21, typical temperature-time curves of Dewar flask experiments are shown. 

From the temperature-time curve, as recorded in a Dewar flask experiment or in an AST, the heat production as a function of time can be determined. Furthermore, from Dewar flask tests with an accurate internal heater or from AST experiments, the specific heat (Cp) can be determined, or in pressurized and closed vessels, the Cv as well. For the heat production, the following equation holds ... 

In addition to the thermal effects of the reaction, pressure data are acquired from an ARC experiment. As in the closed Dewar flask tests, the pressure is the result of (1) the heating of the free-board gas, (2) the vapor pressure, and (3) the reaction-produced gases. With the pressure-temperature versus time curve, the gas generation of the substance can be calculated (mol gas/mol substance). This is possible if enough knowledge of the gas solubility in the liquid and the vapor pressure of the sample are available. Such a calculation is useful for gas venting estimates for the process. 

The stability of powders can be determined by adiabatic storage tests or Dewar flask tests under an air atmosphere (Section 2.3.2.2). Several other dedicated tests have been developed [10,133-136]. 

FIGURE 2.20. Simple Test Setup for a Dewar Flask Test 67... 

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. 

FIGURE 2.21. Typical Temperature-Time Curves of Dewar Vessel Tests (after temperature equilibration between Dewar flask and oven has been reached). 

Modem Dewar flask equipment includes an adiabatic shield, a compensation heater, and a computer to provide for control and for data acquisition and analysis. An example of the application of an advanced design is the adiabatic storage test (AST) [120,121]. In the AST, the heat generated at nearly adiabatic conditions by the reacting or decomposing substances is determined as a function of time. 

Closed system tests, using an unvented test cell (see Figure A2.5) or Dewar flask, can be used for vapour pressure systems. The runaway is initiated in the way that best simulates the worst case relief scenario at plant-scale. The closed system pressure and temperature are measured as a function of time. Most commercial calorimeters include a data analysis package which will present the data in terms of rate of temperature rise, dT/dt, versus reciprocal temperature (-1 / ), and pressure versus reciprocal temperature (see Figure A2.10). However, it is important to correct the temperature data for the effects of thermal inertia. See 2.7.2. 

G 50 ml. test tube cooled with solid carbon dioxide-ether mixture in a Dewar flask. 

Dilute solutions of the materials in alcohol and ether were examined both visually and photographically with a smaller Hilger constant deviation spectrometer. In order to keep the substance at the temperature of liquid air, the solution was placed in a thin walled test tube and congealed by immersion in liquid air in an unsilvered Dewar flask. The test tube was then raised so that about 1 cm. of the gel projected above the surface of the liquid air and the beam of light was passed through this exposed layer. With care, crystallization of the solvent usually could be avoided. 

Frey s variant of the silvered vessel test has been in use in the Germany In its variant, different amounts of heat are supplied to the electric heating elements mounted inside the Dewar flask, and the temperature differences between the interior of the Dewar vessel and the furnace are measured by thermocouples. A calibration curve is plotted from the values thus obtained, and the heat of decomposition of the propellant is read off the curve. In this way, the decomposition temperature at a constant storage temperature can be determined as a function of the storage time, and the heat of decomposition of the propellants can thus be compared with each other. If the measurements are performed at different storage temperatures, the tempera-... 

Another receiver, consisting of a 3 by 30 cm. side-arm test tube, may be used. It is immersed in a Dewar flask and cooled with Dry Ice-methanol. Only a small amount of material is collected in this receiver. 

Three values of the BAM test for BPO calculated each by substituting the three pairs of heat generation data, which are each calculated based on the experimental data obtained each with the three kinds of cells, which are referred to in Fig. 42, in the test, into Eq. (72) are presented in Table 4, assuming that 400 cm each of three samples of BPO are each charged in the 500 cm Dewar flask used in the BAM heat-accumulation storage test and are each placed in the atmosphere under isothermal conditions, keeping the other conditions constant. 

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