Mixing enthalpy measurement by flow calorimetry
Calnesis is one of the few laboratories in the world to offer this type of calorimetric measurement and to have the necessary know-how to implement this experimental technique.
What is the interest of flow calorimetry?
The technique of flow calorimetry makes it possible to measure the enthalpy of mixing of two fluids. Concretely, we speak of heat of chemical reaction, heat of dissolution of gas or enthalpy of excess of a mixture between two liquids, or between a gas and a liquid. Indirectly, this technique allows the measurement of the reaction constant as a function of temperature. It also makes it possible to determine the number of “real” equivalents of a reaction. This technique is complementary and sometimes more effective than “batch” calorimetry, in particular for carrying out measurements under pressure or in the case of the “scale-up” of a continuous process.
What is the principle of measurement by flow calorimetry?
The calorimetric signal is based on the heat flux exchanged between the mixing cell and the calorimetric block during mixing. This is carried out in a mixing unit specially designed to be integrated into calorimeters of the Calvet Setaram C80 or BT2.15 type. Fluid flow is ensured by two high-pressure pumps. The two fluids circulate in stainless steel tubes and meet at the mixing point. The mixture then circulates in a stainless steel tube wound in turns in thermal contact with the interior of the mixing cell.
One or more pre-heaters are placed along the flow line to control the temperature of the fluids entering the calorimeter. This must be kept constant and precisely at the same value as that of the mixing cell. The pressure is measured at the outlet of the cell. It is kept constant using a buffer tank filled with nitrogen. In order to protect the high pressure pumps, in particular from corrosion, an injection loop can be added between the pumps and the mixing cell. Different mixture compositions can be obtained by changing the ratio of pump flow rates.
Calibration of the calorimeter is done by adjusting a calibration constant. It can be determined by the Joule effect or by a chemical calibration by measuring the heat of mixing of a reference system.
The Benefits of Flow Calorimetry
The dynamic measurement mode has certain advantages. Indeed, it does not generate any error following the integration of peaks. It allows easy calculation of mixture compositions. It limits the effects of corrosion. The measurement time is also relatively short (20 minutes per composition) and there are no disturbances related to agitation. On the other hand, knowledge of the volume properties of the studied fluids is necessary. But they can also be measured by Calnesis…
What type of results are obtained by flow calorimetry?
The figure below shows the characteristic curves obtained. It is the result of a study concerning the capture of carbon dioxide by amine absorbent solutions. We take away a lot of information from this experience. The energy released by the reaction is measured as a function of the number of moles of reactants (plates). But we can also find the number of equivalents of the reaction which corresponds in this case to the rate of charge of CO2 which can be absorbed per quantity of solvent (here 1 mole of CO2 for 1 mole of amine).
This type of complete study is the subject of a detailed report integrating the experimental data and the conclusions drawn in order to answer your problem.