During early stages of process development chemical industry needs flexible and versatile tools to assess relevant information about chemical reaction systems. Over the years, reaction calorimeters have become essential devices for data oriented research and development in chemical engineering. They are commonly used for the optimization of a chemical process considering economic factors and safety assessment. Their applications also imply the elucidation of reaction mechanisms and associated parameters (activation energies, rate constants, and heat of reaction).

For the past few years we have been developing a new fully automated small-scale reaction calorimeter. A power-compensation heater combined with a thermoelectrically regulated metal surrounding makes the reactor well suited for fast and highly exothermic reactions. As the baseline for the heat signal is measured online, time-consuming calibrations of the global heat-transfer coefficient are not required. With a working volume from 25 to 45ml the device is particularly useful for the fine and pharmaceutical chemical industry where only small amounts of test substances are available in research and development. Despite its small volume, an ATR-FTIR probe is mounted at the bottom of the reaction vessel. Under strictly isothermal conditions the calorimeter has already demonstrated its capability to investigate fast reactions in the liquid phase such as the hydrolysis of acetic anhydride [1] or the three phases palladium-catalyzed hydrogenation of nitro compounds [2].

Knowledge of the heat capacity is crucial for process development in chemical and pharmaceutical industry. This includes scale-up, thermal safety, and design of the reactor cooling system. Here, we present a method for rapid determination of heat capacity under quasi-isothermal conditions, which is perfectly suited for our small-scale calorimeter. The heat capacity of the reactor content is calculated from amplitudes and phase shifts of the reactor temperature, of the power of the electrical heater and of the cooling rate when defined temperature oscillations are enforced. The heat capacities of eight solvents (water, ethanol, methanol, acetone, 1-octanol, diethylenglycol, toluene, and 1-butanol) covering a wide range of viscosities are calculated for various experimental conditions, including filling volume and stirrer speed. Systematic deviations are detected when compared to the corresponding literature values. Straight-line calibration with the total heat transfer coefficient and a multivariate calibration technique (Partial Least Squares) are applied to correct for these deviations. The two different calibrations show similar precision and allow for an online determination of the heat capacity with an accuracy comparable to other published methods. Successful applications include the determination of the heat capacities for n-heptane, for various homogenous ethanol/water mixtures, and during the course of the hydrolysis of concentrated sulfuric acid.

A heat-flow model of the calorimeter based on the Finite Element Method allows an improved calculation of the heat-balance, particularly when the reactor is to be used under dynamic conditions. Simulation results will be presented that allow a deeper understanding of the above-mentioned systematic deviations when the heat capacity of the reactor content is determined by the temperature oscillation method.

[1] Visentin et al, (2004) Org. Proc. Res. & Dev., 8 (5), 725-737

[2] Visentin et al, (2006) Ind. & Eng. Chem. Res., 45 (13), 4544-4553