Adiabatic pressure Dewar

Figure 3.11 Adiabatic pressure Dewar apparatus with dump lank.

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. 

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. 

There is a high rate of pressure increase dining exothermic decomposition [ 1]. Energy of decomposition (in range 240-440°C) measured as 1.58 kJ/g by DSC, and Tait24 was determined as 185°C by adiabatic Dewar tests, with an apparent energy of activation of 117 kJ/mol [2],... 

It decomposes explosively at the m.p., 98°C [1], and shows a high rate of pressure increase on exothermic decomposition [2], The heat of decomposition was determined as 1.34 kJ/g by DSC, and Taii24 was determined as 88°C by adiabatic Dewar tests, with an apparent energy of activation of 223 kJ/mol [3], When finely divided, it also shows significant dust explosion hazards, with a maximum explosion pressure of 7.75 bar, and a maximum rate of pressure rise above 680 bar/s [4], Further work on homogeneous decomposition under confinement has been reported [5],... 

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

By replacing the glass dewar with a stainless steel vessel, reactions can be studied that generate pressure. Such equipment needs to be placed in a blast cell where it can be operated remotely. Placing the dewar in an oven whose temperature is controlled to follow that in the reaction mass allows the study of the reaction under near adiabatic conditions. 

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

One of the sessions of the Symposium was largely devoted to presentation and discussion on the use of various experimental calorimetric methods for use in assessing possible hazards in chemical processing operations. The methods described covered a wide range of sample sizes and degrees of complexity Grewer, T. Adiabatic small-scale reaction test in Dewar, simple to operate. Janin, R. Measurements of heat release by DSC and of pressure development in sealed microcapsules. 

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. 

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