Reaction calorimeter

Original RC1 Calorimeter

A reaction calorimeter is a calorimeter that measures the amount of energy released (in exothermic reactions) or absorbed (in endothermic reactions) by a chemical reaction.

Methods

Heat flow calorimetry

Heat flow calorimetry measures the heat flowing across the reactor wall and quantifies this in relation to the other energy flows within the reactor.

$Q=UA(T_{r}-T_{j})$

where

$Q$ = process heating (or cooling) power (W)

$U$ = overall heat transfer coefficient (W/(m²K))

$A$ = heat transfer area (m²)

$T_{r}$ = process temperature (K)

$T_{j}$ = jacket temperature (K)

Heat flow calorimetry allows the user to measure heat whilst the process temperature remains under control. While the driving force T_r − T_j is measured with a relatively high resolution, the overall heat transfer coefficient U or the calibration factor UA is determined by means of calibration before and after the reaction takes place. The calibration factor UA (or the overall heat transfer coefficient U) are affected by the product composition, process temperature, agitation rate, viscosity, and the liquid level. Using reaction calorimeters to a high degree of accuracy requires understanding their limitations and having significant experience with the process.

Heat balance calorimetry

In heat balance calorimetry, the cooling/heating jacket controls the temperature of the process. Heat is measured by monitoring the heat gained or lost by the heat transfer fluid.

$Q=m_{s}C_{ps}(T_{i}-T_{o})$

where

$Q$ = process heating (or cooling) power (W)

$m_{s}$ = mass flow of heat transfer fluid (kg/s)

$C_{ps}$ = specific heat of heat transfer fluid (J/(kg K))

$T_{i}$ = inlet temperature of heat transfer fluid (K)

$T_{o}$ = outlet temperature of heat transfer fluid (K)

Heat balance calorimetry is, in principle, the ideal method of measuring heat since the heat entering and leaving the system through the heating/cooling jacket is measured from the heat transfer fluid (which has known properties).

In heat balance calorimetry, the heat entering and leaving the system through the heating/cooling jacket is measured from the heat transfer fluid (which has known properties) - making the method a very effective way of measuring heat loss or gain. Most calibration problems encountered by heat flow and power compensation calorimetry therefore do not apply to heat balance calorimetry. However, the method does not work well in traditional batch vessels since the process heat signal is obscured by large heat shifts in the cooling/heating jacket.

Power compensation calorimetry

A variation of the 'heat flow' technique is called 'power compensation' calorimetry. This method uses a cooling jacket operating at constant flow and temperature. The process temperature is regulated by adjusting the power of the electrical heater. When the experiment is started, the electrical heat and the cooling power (of the cooling jacket) are in balance. As the process heat load changes, the electrical power is varied in order to maintain the desired process temperature. The heat liberated or absorbed by the process is determined from the difference between the initial electrical power and the demand for electrical power at the time of measurement. Power compensation calorimetry requires less preparation than heat flow calorimetry, however, it still experiences similar limitations, due to any change in product composition, liquid level, process temperature, agitation or viscosity will impact the instrument's calibration. The presence of an electrical heating element is also suboptimal for process operations. A further limitation of the method is that the largest heat it can measure is equal to the initial electrical power applied to the heater.

$Q=IV\,\,\,\,\,\mathrm {or} \,\,\,\,\,\,(I-I_{0})V$

where

$I$ = current supplied to heater

$V$ = voltage supplied to heater

$I_{0}$ = current supplied to heater at equilibrium (assuming constant voltage / resistance)

Constant flux calorimetry

Diagram of COFLUX system

Constant flux heating and cooling jackets use variable geometry cooling jackets and can operate with cooling jackets at substantially constant temperature. These reaction calorimeters tend to be much simpler to use and are much more tolerant of changes in the process conditions (which would affect calibration in heat flow or power compensation calorimeters).

An example of a Co-Flux Calorimeter

In essence, constant flux calorimetry is a highly developed temperature control mechanism which can be used to generate highly accurate calorimetry. It works by controlling the jacket area of a controlled lab reactor, while keeping the inlet temperature of the thermal fluid constant. This allows the temperature to be precisely controlled even under strongly exothermic or endothermic events as additional cooling is always available by simply increasing the area over which the heat is being exchanged.

This system is generally more accurate than heat balance calorimetry (on which it is based), as changes in the delta temperature (T_out - T_in) are magnified by keeping the fluid flow as low as possible.

One of the main advantages of constant flux calorimetry is the ability to dynamically measure heat transfer coefficient (U). We know from the heat balance equation that:

$Q=m_{f}\;C_{p}\;(T_{in}-T_{out})$

We also know that from the heat flow equation that

$Q=U\;A\;LMTD$

We can therefore rearrange this such that

$U={\frac {m_{f}\;C_{p}\;(T_{in}-T_{out})}{A\;LMTD}}$

This will allow us therefore to monitor U as a function of time.

Instrumentation

A continuous reaction calorimeter

In traditional heat flow calorimeters, one reactant is added continuously in small amounts, similar to a semi-batch process, in order to obtain a complete conversion of the reaction. In contrast to the tubular reactor, this leads to longer residence times, different substance concentrations and flatter temperature profiles. Thus, the selectivity of less well-defined reactions can be affected. This can lead to the formation of by-products or consecutive products which alter the measured heat of reaction, since other bonds are formed. The amount of by-product or secondary product can be found by calculating the yield of the desired product. A continuous reaction calorimeter is a similar type of instrument used to obtain thermodynamic information on continuous processes in tubular reactors. An axial temperature profile along the tube reactor can be recorded and the specific heat of reaction can be determined by means of heat balances and segmental dynamic parameters. The system must consist of a tubular reactor, dosing systems, preheaters, temperature sensors and flow meters.

If the heat of reaction measured in the HFC (Heat flow calorimetry) and PFR calorimeter differ, most probably some side reactions have occurred. They could for example be caused by different temperatures and residence times. The totally measured Qr is composed of partially overlapped reaction enthalpies (ΔHr) of main and side reactions, depending on their degrees of conversion (U).

See also

• Controlled Lab Reactor

References

• Moser, Marlies; Georg, Alain G.; Steinemann, Finn L.; Rütti, David P.; Meier, Daniel M. (September 2021). "Continuous milli-scale reaction calorimeter for direct scale-up of flow chemistry". Journal of Flow Chemistry. 11 (3): 691–699. doi:10.1007/s41981-021-00204-y. hdl:11475/23441. ISSN 2062-249X.
• Mortzfeld, Frederik; Polenk, Jutta; Guelat, Bertrand; Venturoni, Francesco; Schenkel, Berthold; Filipponi, Paolo (2020-10-16). "Reaction Calorimetry in Continuous Flow Mode: A New Approach for the Thermal Characterization of High Energetic and Fast Reactions". Organic Process Research & Development. 24 (10): 2004–2016. doi:10.1021/acs.oprd.0c00117. ISSN 1083-6160.