Example Pulse Heating Calorimeter

Finally, experimental procedures differing from that described in the preceding examples could also be employed for studying catalytic reactions by means of heat-flow calorimetry. In order to assess, at least qualitatively, but rapidly, the decay of the activity of a catalyst in the course of its action, the reaction mixture could be, for instance, either diluted in a carrier gas and fed continuously to the catalyst placed in the calorimeter, or injected as successive slugs in the stream of carrier gas. Calorimetric and kinetic data could therefore be recorded simultaneously, at least in favorable cases, by using flow or pulse reactors equipped with heat-flow calorimeters in place of the usual furnaces. 

Since enthalpy changes can be obtained directly from measurement of heat absorption at constant pressure, even small values of AH for chemical and biochemical reactions can be measured using a micro-calorimeter.1112 Using the technique of pulsed acoustic calorimetry, changes during biochemical processes can be followed on a timescale of fractions of a millisecond. An example is the laser-induced dissociation of a carbon monoxide-myoglobin complex.13... 

In Fig. 4.40 a stirred liquid bench-scale calorimeter is displayed. It closely duplicates laboratory reaction setups. The information aboutheat evolved or absorbed is extracted from the temperature difference between the liquid return (Tj) and the reactor (T ). This difference is calibrated with electric heat pulses to match the observed effect at the end of a chemical reaction. In a typical example, 10 W heat input gives a 1.0 K temperature difference between Tj and Tr. The sample sizes may vary from 0.3 to 2.5 liters. The overall sensitivity is about 0.5 W. The calorimeter can be operated between 250 and 475 K. Heat loss corrections must be made for the stirrer and the reflux unit. The block diagram in Fig. 4.40 gives an overview of the data handling. 

---