Dewar calorimeters

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 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. 

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. 

Figure 4.2 Left Dewar calorimeter equipped with stirrer and calibration heater. T thermometer, C calibration heater,...

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

Dewar calorimeters are useful for investigating isothermal semi-batch reactions — where one reactant is added to a second over a period of time — as well as batch processes. This is done by dividing the quantity of reactant to be added into a number of portions or aliquots. The size of each aliquot is chosen such that the temperature rise it produces is measurable but not so large as to change the reaction mechanism or introduce side reactions. After each aliquot has been added the reaction mixture must be cooled back to the starting temperature. 

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. 

As with the glass apparatus, the pressure Dewar calorimeter can be fitted with a stirrer, connections for additions and sampling, pressure and temperature sensors, and jackets for heating or cooling. It can also be connected to a dump tank for the investigation of tempering reactions (Figure 3.11) (Sections... 

Figure 4.15 shows examples of the temperature/time traces obtained from a batch process using the different types of Dewar calorimeters. 

Dewar calorimeters work by preventing heat from leaving the reaction mass. An isothermal semi-batch process must therefore be simulated by aliquot additions the quantity of reactant to be added is divided into a number of portions (aliquots) whose size is chosen such that each produces a measurable temperature rise but not one so large that the reaction mechanism would be changed or that side reactions might occur. The reaction mixture must be cooled back to the starting temperature after each addition. Figure 4.16 shows a typical trace. 

There are several experimental methods for obtaining calorimetry data for systems under reflux. The simplest is to carry out the reaction in a Dewar calorimeter or a reaction calorimeter, at the reflux temperature but under sufficient back pressure to just stop the mixture boiling. This ensures that heat is not lost by vaporization. Any heat generated is then measured in the normal way. 

The process was therefore run in a Dewar calorimeter. All of the reactants including the catalyst were charged to the Dewar at ambient temperature as in the plant process. The Dewar was fitted with a stirrer, thermocouple and an internal electrical heater which was used to raise the temperature of the reaction mixture to the process temperature of 80°C. The Dewar was placed in an oven at 80°C to eliminate heat losses and the electrical power used was measured. 

Although Fleischmann preferred to use the full calorimetric equation [Equation (13.2)] in order to measure the excess power as accurately as possible, he also provided useful approximations that greatly simplify the mathematics. A lower bound heat transfer coefficient, k n or k c, should first be calculated by assuming Px = 0. For the Dewar calorimeter with heat transfer mainly by radiation, Equation (13.2) becomes... 

A 5 ml volume of the 0.5 mol 1 solution of BF3 etherate in nitrobenzene is placed in the calorimetric cell under dry nitrogen or argon. The syringe pusher is filled with a sufficient volume of 2.5 mol 1 pyridine solution. Using these values, 0.100-0.200 ml injection steps allow for 5-10 additions. For an isothermal titration calorimeter, the time interval between each injection should be sufficient for the signal to return to the baseline (for a Dewar calorimeter, allowance is made for temperature stabilization after each injection). When the temperature of the system is equilibrated, the data acquisition and the injection programme are started. A preliminary experiment should be carried out to measure the blank value, corresponding to the heat of dilution of pyridine solution in the pure solvent. 

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