Calorimetric Principles

Many different types of calorimeters are commercially available and most of the calorimetric principles are summarized in Table 2.4  

The main calorimetric modes can be resumed using Fig. 2.17, where  

Heat Isothermal calorimetry In an isotheimal calorimeter, a defined exchange of heat occurs between the sample and the surroundings at a given environment temperature. The heat flow rate is determined on the basis of the temperature difference tilong a thermal resistance between the sample and its surroundings. 

Heat Isoperibolic calorimetry In an isoperibolic ctilorimeter, the temperature of the surroundings remains constant, while the temperature of the sample can differ from the surrounding temperature. 

Heat Adiabatic calorimetry An adiabatic calorimeter is used to study chemical reactions and is one in which there is no net heat gain or loss during the chemical reaction. 

---

The RC1 Reaction Calorimeter is marketed by Mettler-Toledo. The heat-flow calorimetric principle used by the RC1 relies on continuous measurement of the temperature difference between the reactor contents and the heat transfer fluid in the reactor jacket. The heat transfer coefficient is obtained through calibration, using known energy input to the reactor contents. The heat trans-... 

The RC1 is an automated laboratory batch/semi-batch reactor for calorimetric studies which has proven precision. The calorimetric principle used and the physical design of the system are sound. The application of the RC1 extends from process safety assessments including calorimetric measurements, to chemical research, to process development, and to optimization. The ability of the RC1 to generate accurate and reproducible data under simulated plant scale operating conditions may result in considerably reduced testing time and fewer small scale pilot plant runs. 

Equation 8.8 is only valid under steady-state conditions when the heat-flow rate through the reactor wall is constant. However, if a reaction is taking place, the heat-flow rate through the reactor wall might vary depending on the calorimetric principle being applied. Therefore,... 

The aim of this section is to demonstrate how reaction calorimetry in combination with IR-ATR spectroscopy can be used for the determination of kinetic and thermodynamic parameters. Several examples of chemical reactions will be discussed, each highlighting a different aspect in the application of reaction calorimetry. The reactions considered are the hydrolysis of acetic anhydride, the sequential epoxidation of 2,5-di-ferf-butyl-l,4-benzoquinone and the hydrogenation of nitrobenzene. The results discussed in this section were obtained using a new calorimetric principle presented below. 

In this chapter, some of these instruments are reviewed. A first section is a general introduction to calorimetric principles. In a second part, some methods commonly used in safety laboratories are reviewed. This is not an exhaustive review of such instruments, but based only on the experience of the author. 

The isothermal mode is the most demanding with respect to measurement and control, as has already been stated for plant scale operation. Consequently the devices that allow this mode of operation are expensive to purchase. Their big advantage is the possibility to run classical kinetic investigations in parallel. The evaluation of the isothermal measurement with respect to the power of the process depends on the chosen calorimetric principle. 

This calorimetric principle makes use of the fact that the heat produced by the process must be equivalent to the heat accumulation in the coolant circulating in the jacket in order to ensure a constant internal temperature. 

The great advantage of this kind of evaluation of the heat balance is its complete independence of changes of the physicochemical properties of the reaction mixture. Consequently it is preferred for reaction systems for which this kind of changes in properties is known or to be expected. Especially polymerization reactions belong to this group. At the same time this calorimetric principle is independent of the product of heat transfer area and coefficient... 

However, also this calorimetric principle is not without weaknesses. Equ.(4-226) shows that the exact knowledge of the coolant mass flow is pivotal to the overall accuracy of the measurement. Today s commercially available calorimeters are equipped with pumps for the coolant like those known in type from thermostats. They are not able to provide a frilly constant mass flow rate, especially not if the experiment lasts several hours. Furthermore, the preinstalled mass flow meters usually are rotameters of quite a simple type. 

In terms of the calorimetric principle, a widely used microthermoresistive flow sensor is the thermal anemometer, which typically ccmsists of a middle heater with upstream and downstream temperature sensors, relative to the flow direction [2]. Such a calorimetric sensor is based on measuring the asymmetry temperature profile around the heater, modulated by the fluid flow [1,8]. The schematic representation of a calorimetric device and the temperature distribution in the flow direction are shown in Fig. 3. The MEMS flow sensor... 

Fig. 3 Schematic diagram for calorimetric principle (a) flow microsensors, (b) temperature distribution for flow measurements (Modifled from [7])...

Miles, M.H., Bush, B.F. and Stilwell, D.E. (1994) Calorimetric principles and problems in measurements of excess power during Pd-D20 electrolysis. The Journal of Physical Chemistry, 98, 1947-1952. 

This is necessary both for process control as well as the reliabihty of the system. The integration of sensors into the microreactor or building a multisensor module for the four functions of state is easy for a microreactor made of sUicon. For the process pressure, the piezoresistive principle is used often. With diEFerential pressure measurements, the flow rate can be determined. Alternatively, calorimetric principles are used widely. These are easy to implement technically, but a calibration is needed for eatii new medium. The most robust sensors are the Coriolis mass flow sensors. In process engineering, they are very common, but in terms of micro process engineering, there is still a need for research. In Ref. [26], sensors of this type are described. Ref [25] is a good summary of other microflow sensors. For measurement of temperature, there are many equivalent principles but will not be discussed here. Substantially, it is more difficult to measure the concentration in the reactor. In addition to optical principles, the impedance spectroscopy is often used. See Ref [27-31] for more details. 

Fig. 2.17 Sample and ther-mostated jacket in calorimetric principles...

---